During the operation of the circulating cooling water system, inorganic scale deposition may cause technical problems, such as reduction of heat transfer efficiency in cooling systems and obstruction of pipes. In the industry, chemicals are often used as scale inhibitors in scale deposition control, antiscalants popular in industry are generally phosphorus and nitrogen-containing chemicals, which may lead to eutrophication. However, increasing environmental concern and discharge limitations have guided antiscalants to move toward biodegradability, nontoxicity and cost–effectiveness. This paper reviews current research on the application of using bio-materials as scale inhibitors, including proteins and amino acids, polysaccharides, plant extracts, microbial reagents, and microbiological products. The non-bioaccumulation, low cost, readily biodegradability and sustainably available characters promote the development of green-scale inhibitor chemistry.

  • Scale inhibition mechanisms, evaluation methods and traditional scale inhibitors were prepared.

  • Scale inhibition efficiency of various bio-materials including proteins, polysaccharides, plant extracts are comprehensively reviewed, critically evaluated and along with their scale inhibition mechanism thoroughly discussed.

  • Utilizing biological methods on scaling control was proposed as future research direction.

     
  • ACC

    amorphous calcium carbonate

  •  
  • AM

    acrylic amide

  •  
  • ATMP

    aminotrimethylenephosphonic acid

  •  
  • β-CD

    β-cyclodextrin

  •  
  • β-CD-PEG

    polyethylene glycol modified β-cyclodextrin

  •  
  • BACC

    borated aminated cellulose citrate

  •  
  • CA

    chronoamperometry

  •  
  • CIT

    citrate

  •  
  • CMCS

    carboxymethyl chitosan

  •  
  • CMI

    carboxymethylinulin

  •  
  • CMP

    compound microorganism preparation

  •  
  • CMS

    carboxymethyl starch

  •  
  • CTS

    chitosan

  •  
  • DETPMP

    diethylenetriaminepentamethy-lene phosphonic acid

  •  
  • EDS

    energy dispersion spectroscopies

  •  
  • EEM

    3-D excitation/emission matrix

  •  
  • EIS

    electrochemical impedance spectroscopy

  •  
  • EPS

    extracellular polymeric secretions

  •  
  • FCP

    fast controlled precipitation method

  •  
  • FESEM

    field emission scanning electron microscope

  •  
  • FTIR

    Fourier transform infrared spectroscopy

  •  
  • HEDP

    1-hydroxyethylidene-1,1-diphosphonate

  •  
  • HPCHS

    O-(hydroxyisopropyl) chitosan

  •  
  • HPMA

    maleic anhydride homopolymer

  •  
  • IA/AMPS

    2-acrylamido-2-methylpropane sulfonic acid copolymer

  •  
  • IA/SAS/SHP

    itaconic acid-sodium allysulfonate-sodium hypophosphite copolymer

  •  
  • MA

    maleic anhydride

    MAI10-methylacridinium iodide

  •  
  • MIC

    minimum inhibitor concentration

  •  
  • OS

    oxidized starch

  •  
  • PAA

    polyacrylic acid

  •  
  • PASP

    polyaspartic acid

  •  
  • PAPEMP

    polyamino polyether methylenephosphonate

  •  
  • PBTC

    2-phosphonobutane-l,2,4-tricarboxylic acid

  •  
  • PCCA

    curcumin-citric acid-aspartic acid polymer

  •  
  • PESA

    polyepoxysuccinic acid

  •  
  • PGLU

    pteroyl-l-glutamic acid

  •  
  • PIA-co-ESA

    poly(itaconic acid-co-epoxysuccinic acid)

  •  
  • PPCA

    phosphinopoly-carboxylic acid

  •  
  • PSD

    particle size distribution

  •  
  • s-EPS

    soluble EPS

  •  
  • SC

    sodium citrate

  •  
  • SCMC

    sodium carboxymethyl cellulose

  •  
  • SEM

    scanning electron microscopy

  •  
  • SG

    sodium gluconate

  •  
  • SL

    sodium lignosulfonate

  •  
  • SSS

    styrene sulfonic sodium

  •  
  • St

    starch

  •  
  • St-g-PAA

    starch-graft-poly(acrylic acid)

  •  
  • TEM

    transmission electron microscopy

  •  
  • TGA

    thermogravimetry analysis

  •  
  • Trp

    tryptophan

  •  
  • Tyr

    tyrosine

  •  
  • XC

    xanthan

  •  
  • XGP

    xanthan gum polymer

  •  
  • XRD

    X-ray diffraction.

As a fact of global water shortage, wastewater has now been recognized as a significant source of water for non-potable uses (Wade Miller 2006; Benetti, 2008), especially as makeup water for industrial cooling circuits. However, due to poor quality, secondary-treated municipal wastewater usually contains appreciable amounts of hardness, phosphate, ammonia, dissolved solids, and organic matter compared with the amounts in fresh water (Weinberger et al. 1966). In recirculating cooling systems, the water constituents become concentrated many times (typically four to eight times) due to constant water evaporation, the inorganic ions are usually four to eight times higher than the supplemental water. The scaling ions, especially Ca2+ and carbonate alkalinity, gradually concentrate to exceed the solubility and form scale deposition on heat exchange equipment surfaces, according to the following equation:
formula
(1)

In general, scale deposition is recognized as a crystallizing procedure involving four stages (Al-Rawajfeh et al. 2005; Hasson & Semiat 2006):

  • oversaturation

  • nucleation

  • crystal growth around nucleus

  • continuous growth of microcrystals along with scale layer thickening.

Adherent scale deposition on heat exchange equipment surfaces can cause severe technical problems, such as reducing heat transfer efficiency and obstructing pipelines in cooling systems, a 2-mm layer of scale in thermal pipeline can reduce 47% heat transfer efficiency, which leads to great economic pressure in industrial production, therefore, effective countermeasures must be taken to solve scaling problems (Gauthier et al. 2012; Xu et al. 2012; Liu et al. 2013).

Addition of scale inhibitors is the most common method for controlling scale deposition, using antiscalants can reduce scaling on heat exchange surface, increase cooling water concentration ratio and lift efficiency during the desalination processes (Greenlee et al. 2010; Li et al. 2011). The scale inhibition mechanisms are multiple for different varieties of antiscalants as described in Figure 1. In general, scale inhibition mechanisms involve as follows:

  • (1)

    Threshold inhibition. Threshold scale inhibitors can effectively inhibit scale formation at tiny dosage, even 1,000 times less than a balanced stoichiometric ratio of scaling cations (Cooper et al. 1979).

  • (2)

    Scale inhibitors molecules can complex with free scaling ions (e.g., Ca2+ and Mg2+), prevent them from being precipitated via strongly chelation and dispersion effects, keep them suspended in aqueous solution (Zhang et al. 2013).

  • (3)

    Scale inhibitors' molecular functional groups can adsorb onto active sites of the scale crystals' particular growth location, thereby, scale precipitation is prevented by modifying the crystal morphology and distorting the crystal lattice (Zeng et al. 2014; Amjad & Demadis 2015).

Figure 1

The schematic diagram of scale inhibition.

Figure 1

The schematic diagram of scale inhibition.

Close modal

Multitudinous materials have been proposed and successfully experimented as antiscalant scale inhibitors, these chemical agents are chosen according to the operating conditions of cooling water reservoirs and transportation pipelines. The phosphorus scale inhibitors include phosphates and phosphonates, which are the most popular commercial scale inhibitors and benefit from their low cost and high effectiveness. However, their utilization has been restricted as their phosphorus-containing property contributes to the total environmental phosphate content, which promotes eutrophication of the receiving surface water. In addition to phosphorus scale inhibitors, synthetic non-phosphorus scale inhibitors include polycarboxylic acids, polysulfonic acids, and their derivates are also widely used in industry. These synthetic polymeric scale inhibitors exhibit high scale inhibition efficiency with excellent chelation and dispersion effect, however, their scale inhibition performance is significantly affected by molecular weight. Moreover, their feasibility at high temperature conditions indicated they were non-biodegradable (Kumar et al. 2018).

Nowadays, environmental protection pressure and government legislation lead the research on scale inhibitors to move toward ‘green’ scale inhibitors, which are characterized as follows: (1) excellent antiscaling capacity; (2) nontoxicity; (3) high biodegradability after discharged; (4) non-corrosiveness; (5) thermostability; (6) cost–effectiveness; (7) free of phosphorus, nitrogen, and heavy metals (Hasson et al. 2011; Guo et al. 2014; Liu et al. 2017; Kumar et al. 2018). Under this criterion, it is significant for industry to find alternative sources for developing new corrosion and scale inhibitors with environmentally friendly, nontoxic, and high inhibition performances.

Over the past years, many studies have been devoted to applying bio-materials as scale inhibitors, this elicits a new development idea of green scale inhibitor. The bio-materials are rich in organic compounds or macromolecular components, which contain N, O, S, and other heterocyclic atoms along with multiple bonds and aromatic compounds. Besides, bio-materials possess unique advantages of abundant raw materials, are easy biodegradable, have simply extraction, and are environmentally friendly. These features make bio-materials have great potential to be used as green scale inhibitors, this is also the inevitable trend in the development of circulating water scale inhibitors. Based on this situation, we reviewed application and research progress of bio-materials scale inhibitors and provided a comprehensive understanding of ‘green’ scale inhibitors, these promising bio-materials may replace the status of traditional chemical agents. In this review, bio-materials as scale inhibitors are classified as polysaccharides, proteins and amino acids, plant extracts, and other natural microbiological products. The efforts spent on various bio-materials as scale inhibitors have been summarized, their scale inhibition performances along with mechanisms are presented, giving a comparative basis for guiding the future development of green scale inhibitor.

Calcium carbonate deposition under liquid phase is multifeatured, in the last few years various methods have been developed for evaluating scaling propensity of industrial water, according to the experimental conditions, evaluation methodology is classified as dynamic and static (Azizi et al. 2019), of which, dynamic evaluation is specifically aimed at practical performance. However, static evaluation is mainly based on the supersaturated carbonate-depositing process, by measuring or monitoring related criteria, including solution turbidity (Demadis & Panos 2005; Tantayakom et al. 2005b), conductivity (Drela 1998), pH (Gal et al. 1996; Zhang et al. 2011), induction time (Neville & Morizot 2000; Xiao et al. 2001), concentration of Ca2+ (Liu et al. 2012), precipitate mass, and precipitate morphology (Morizot & Neville 2001; Tantayakom et al. 2005a, 2005b). In this case, scaling propensity and scale-forming conditions are estimated from several aspects, the following words present a review on evaluating scale inhibitors. These methods, which are complementary with each other, assess the scale inhibition properties from different perspectives, as no single test can assess all characteristics of one scale inhibitor, therefore, it is necessary to select suitable evaluation methods after considering the actual experimental conditions.

Table 1

Schematic structures of proteins as scale inhibitors cited in this work

Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency/FunctionalityTest methodsRefs.
Pteroyl-l-glutamic acid (PGLU)  CaCO3 100% Static jar test Kumar et al. (2010)  
Thiamine (50%) and pyridoxine (50%)  CaCO3 66.78% Chronoamperometry Menzri et al. (2017)  
Cysteine-rich Mdm2 peptide  CaCO3 58% Constant composition method Dalas et al. (2006)  
Casein peptone N1 19516/tryptone N1 19553 Casein micelle BaSO4 and CaCO3 Extend the scaling time of the test tube Dynamic tube blocking tests Mady & Kelland (2017)  
Poly(citric acid)  CaSO4 98.8% Static jar test Zhao et al. (2016)  
Citrate  CaCO3 Increased induction period from 1 min at 0%CIT/Ca to 30 min at 100% CIT/Ca Turbidity measurement Montanari et al. (2017)  
Sodium citrate (SC) + 10-methylacridinium iodide (MAI)  CaCO3 98.3% Static jar test Zhang et al. (2020)  
Curcumin-citric acid-aspartic acid polymer (PCCA) 
 
CaSO4/ CaCO3 99.7%/98.8% Static jar test Yuan et al. (2020)  
Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency/FunctionalityTest methodsRefs.
Pteroyl-l-glutamic acid (PGLU)  CaCO3 100% Static jar test Kumar et al. (2010)  
Thiamine (50%) and pyridoxine (50%)  CaCO3 66.78% Chronoamperometry Menzri et al. (2017)  
Cysteine-rich Mdm2 peptide  CaCO3 58% Constant composition method Dalas et al. (2006)  
Casein peptone N1 19516/tryptone N1 19553 Casein micelle BaSO4 and CaCO3 Extend the scaling time of the test tube Dynamic tube blocking tests Mady & Kelland (2017)  
Poly(citric acid)  CaSO4 98.8% Static jar test Zhao et al. (2016)  
Citrate  CaCO3 Increased induction period from 1 min at 0%CIT/Ca to 30 min at 100% CIT/Ca Turbidity measurement Montanari et al. (2017)  
Sodium citrate (SC) + 10-methylacridinium iodide (MAI)  CaCO3 98.3% Static jar test Zhang et al. (2020)  
Curcumin-citric acid-aspartic acid polymer (PCCA) 
 
CaSO4/ CaCO3 99.7%/98.8% Static jar test Yuan et al. (2020)  
Table 2

Schematic structures of chitosan-based polysaccharides as scale inhibitors

Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency /functionalityTest MethodsRefs.
CTS–MA–SSS–AM  CaCO3 95.62% Static jar test Guo et al. (2012)  
Chitosan biguanidine hydrochloride (CG) 
 
CaCO3/CaSO4 91%/95% Membrane scaling experiments Maher et al. (2020)  
Carboxymethyl chitosan (CMCS)  CaCO3 50 ppm of CMC obviously extended the precipitation time from 18 min to 50 min Dynamic tube block tests Macedo et al. (2019)  
O-Carboxymethylchitosan  CaCO3 Reducing average particle size from 9–25 μm to 1.5–17 μVapor diffusion method Yang et al. (2010)  
O-(hydroxyisopropyl)chitosan (HPCHS)  CaCO3 Reducing crystallization degree and the average particle size Calcium carbonate crystallization Yang et al. (2009)  
Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency /functionalityTest MethodsRefs.
CTS–MA–SSS–AM  CaCO3 95.62% Static jar test Guo et al. (2012)  
Chitosan biguanidine hydrochloride (CG) 
 
CaCO3/CaSO4 91%/95% Membrane scaling experiments Maher et al. (2020)  
Carboxymethyl chitosan (CMCS)  CaCO3 50 ppm of CMC obviously extended the precipitation time from 18 min to 50 min Dynamic tube block tests Macedo et al. (2019)  
O-Carboxymethylchitosan  CaCO3 Reducing average particle size from 9–25 μm to 1.5–17 μVapor diffusion method Yang et al. (2010)  
O-(hydroxyisopropyl)chitosan (HPCHS)  CaCO3 Reducing crystallization degree and the average particle size Calcium carbonate crystallization Yang et al. (2009)  
Table 3

Schematic structures of cellulose-based polysaccharides as scale inhibitors

Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency/functionalityTest methodsRefs.
Sodium carboxymethyl cellulose (SCMC)  CaCO3 93% pH displacement method Xu et al. (2019)  
Borated aminated cellulose citrate (BACC)  CaCO3 91.57% Static jar test Gan et al. (2018)  
Scale inhibitorsSchematic structure/synthetic routeType of scaleMaximum inhibition efficiency/functionalityTest methodsRefs.
Sodium carboxymethyl cellulose (SCMC)  CaCO3 93% pH displacement method Xu et al. (2019)  
Borated aminated cellulose citrate (BACC)  CaCO3 91.57% Static jar test Gan et al. (2018)  
Table 4

Schematic structures of glucose-based polysaccharides as scale inhibitors

Scale inhibitorsSchematic Structure/Synthetic routeType of scaleMaximum inhibition efficiency /FunctionalityTest MethodsRefs.
SG  CaCO3 Lowering the CaCO3 formation kinetics for four times Light-scattering method Ou & Chiang Hsieh (2016)  
MA–SSS-OS  CaCO3 98.87% Static jar test Guo et al. (2013)  
CMS  CaCO3 89.80% Static jar test Wang et al. (2017)  
St-g-PAA  CaCO3 95.79% Static jar test Yu et al. (2018)  
B-CD–PEG  CaCO3 89.1% Static jar test Liu et al. (2016b)  
β-CD–MA–SSS  CaCO3
Ca3(PO4)2 
99.9%/95.5% Static jar test Gu et al. (2013)  
SL  CaCO3 45.2% Static jar test Ouyang et al. (2006)  
Scale inhibitorsSchematic Structure/Synthetic routeType of scaleMaximum inhibition efficiency /FunctionalityTest MethodsRefs.
SG  CaCO3 Lowering the CaCO3 formation kinetics for four times Light-scattering method Ou & Chiang Hsieh (2016)  
MA–SSS-OS  CaCO3 98.87% Static jar test Guo et al. (2013)  
CMS  CaCO3 89.80% Static jar test Wang et al. (2017)  
St-g-PAA  CaCO3 95.79% Static jar test Yu et al. (2018)  
B-CD–PEG  CaCO3 89.1% Static jar test Liu et al. (2016b)  
β-CD–MA–SSS  CaCO3
Ca3(PO4)2 
99.9%/95.5% Static jar test Gu et al. (2013)  
SL  CaCO3 45.2% Static jar test Ouyang et al. (2006)  
Table 5

Schematic structures of plant extracts as scale inhibitors

Scale inhibitorsMain constituentsFunctionalityTest methodsRefs.
Herniaria glabra Herniaria saponins I–VII, flavonoids, umbelliferone, herniarin, phenolic acids, tannins, and essential oil. Completely inhibition CaCO3 formation at 50 mg/L Fast controlled precipitation method Horner et al. (2017)  
Spergularia rubra Phytoecdysteroids, di-C-glycosyl-flavone, 36
C-glycosyl-flavone 
Completely inhibition CaCO3 formation at 30 mg/L  Cheap-Charpentier et al. (2016)  
Parietaria officinalis Mucus, potassium nitrate, tannin, sulfur, glucan, and flavonoid pigments. Completely inhibition CaCO3 formation at 100 mg/L   
Hylocereus undatus Amino acids, sugars, polysaccharides, organic acids, fatty acids, and flavonoid Completely inhibition CaCO3 formation at 24 mg/L  Lourteau et al. (2019)  
Mazuj gall 4-Methyl-3-penten-2-one
2-methoxyfuran 4-hydroxy-4-methyl-2-pentanone 1-(2-butoxyethoxy) ethanol 1,2,3-benzenetriol hexadecane 3,4,5-trihydroxybenzoic acid methyl ester n-hexadecanoic acid
heptadecane 
1,000 ppm of Mazuj gall extract provided a high inhibition efficiency of 97.2% in against 592 ppm total hardness Static beaker tests Mohammadi & Rahsepar (2018)  
Bistorta officinalis Pyridine, toluene, 2-nonanone, p-xylene, benzene, decane, undecane, tritetracontane, ethanol, tridecane, propanedioic acid, 1,2-benzenedicarboxylic acid, benzoic acid, eicosane, 1,2-benzenedicarboxylic acid, heptacosane, tetracosane, octacosane, nonacosane, heptacosane 1,000 ppm of Bistorta officinalis extract provided a high inhibition efficiency of 99.5% in against 592 ppm total hardness  Mohammadi & Rahsepar (2019)  
Gambier extract 40% of tannic
acid, 25% of catechin, and 12% of quercetin. 
200 ppm gambier extract brought 70–80% CaCO3 inhibition efficiency Bottle-roller batch method Suharso et al. (2011)  
Modified gambier extract with benzoic and citric acid Ratio of gambier extract modification
= 2:1:2 (Gambier:benzoic acid:citric acid). 
Showed an inhibition efficiency of 12–92% under the concentration range of 50–300 ppm  Suharsoa (2017)  
Curcumin Accounts for about 77%; demethoxycurcumin
for about 17% and bisdemethoxycurcumin about 3% 
Showed the highest CaSO4 scale inhibition rate of 95.0% at 10 ppm Static jar test Nayunigari et al. (2016)  
Scale inhibitorsMain constituentsFunctionalityTest methodsRefs.
Herniaria glabra Herniaria saponins I–VII, flavonoids, umbelliferone, herniarin, phenolic acids, tannins, and essential oil. Completely inhibition CaCO3 formation at 50 mg/L Fast controlled precipitation method Horner et al. (2017)  
Spergularia rubra Phytoecdysteroids, di-C-glycosyl-flavone, 36
C-glycosyl-flavone 
Completely inhibition CaCO3 formation at 30 mg/L  Cheap-Charpentier et al. (2016)  
Parietaria officinalis Mucus, potassium nitrate, tannin, sulfur, glucan, and flavonoid pigments. Completely inhibition CaCO3 formation at 100 mg/L   
Hylocereus undatus Amino acids, sugars, polysaccharides, organic acids, fatty acids, and flavonoid Completely inhibition CaCO3 formation at 24 mg/L  Lourteau et al. (2019)  
Mazuj gall 4-Methyl-3-penten-2-one
2-methoxyfuran 4-hydroxy-4-methyl-2-pentanone 1-(2-butoxyethoxy) ethanol 1,2,3-benzenetriol hexadecane 3,4,5-trihydroxybenzoic acid methyl ester n-hexadecanoic acid
heptadecane 
1,000 ppm of Mazuj gall extract provided a high inhibition efficiency of 97.2% in against 592 ppm total hardness Static beaker tests Mohammadi & Rahsepar (2018)  
Bistorta officinalis Pyridine, toluene, 2-nonanone, p-xylene, benzene, decane, undecane, tritetracontane, ethanol, tridecane, propanedioic acid, 1,2-benzenedicarboxylic acid, benzoic acid, eicosane, 1,2-benzenedicarboxylic acid, heptacosane, tetracosane, octacosane, nonacosane, heptacosane 1,000 ppm of Bistorta officinalis extract provided a high inhibition efficiency of 99.5% in against 592 ppm total hardness  Mohammadi & Rahsepar (2019)  
Gambier extract 40% of tannic
acid, 25% of catechin, and 12% of quercetin. 
200 ppm gambier extract brought 70–80% CaCO3 inhibition efficiency Bottle-roller batch method Suharso et al. (2011)  
Modified gambier extract with benzoic and citric acid Ratio of gambier extract modification
= 2:1:2 (Gambier:benzoic acid:citric acid). 
Showed an inhibition efficiency of 12–92% under the concentration range of 50–300 ppm  Suharsoa (2017)  
Curcumin Accounts for about 77%; demethoxycurcumin
for about 17% and bisdemethoxycurcumin about 3% 
Showed the highest CaSO4 scale inhibition rate of 95.0% at 10 ppm Static jar test Nayunigari et al. (2016)  
Table 6

Schematic structures of microbiological product as scale inhibitors cited in this work

Scale inhibitorsSchematic structure/main constituents/synthetic routeType of scaleMaximum inhibition efficiency Bistorta Officinalis/functionalityTest methodsRefs.
EPS of a cyanobacterium (Schizothrix sp.) Polysaccharides, proteins,
humic-like substances, nucleic acid, lipids, and glycoproteins 
CaCO3 Inhibiting CaCO3 precipitation pH recording
in CaCO3 precipitation 
Kawaguchi & Decho (2002)  
s-EPS of Bacillus cereus  CaCO3 87.60% Static jar test Li et al. (2019)  
Xanthan gum  CaCO3 Showed an inhibition impact in against 2,664 mg/L Ca2+ and 2,544 mg/L CO32− ions at pH 9 under a concentrations range of 100–1,000 mg/L CaCO3 crystallization Yang & Xu (2011)  
Compound microorganism preparation Dry powder
of nitrobacteria, Bacillus subtilis, photosynthetic bacteria and Thiobacillus denitrificans
mixed Exhibited a limit concentration ratio of 3.87 Ultimate carbonate hardness method Chen et al. (2019)  
Protein of Tepidimonas fonticaldi Proteins  Showed high calcium adsorption capacity of 1.94 mg Ca2+/g protein at pH 10, 150 ○C and 1 atm pressure. Adsorption experiments Han et al. (2017)  
 PIA-co-ESA  CaCO3 100% Static jar test Shi et al. (2017)  
IA/SAS/SHP  CaCO3 81.2% Static jar test Cui & Zhang (2019)  
IA/AMPS  CaCO3 95.1% Static jar test Liu et al. (2016c)  
Scale inhibitorsSchematic structure/main constituents/synthetic routeType of scaleMaximum inhibition efficiency Bistorta Officinalis/functionalityTest methodsRefs.
EPS of a cyanobacterium (Schizothrix sp.) Polysaccharides, proteins,
humic-like substances, nucleic acid, lipids, and glycoproteins 
CaCO3 Inhibiting CaCO3 precipitation pH recording
in CaCO3 precipitation 
Kawaguchi & Decho (2002)  
s-EPS of Bacillus cereus  CaCO3 87.60% Static jar test Li et al. (2019)  
Xanthan gum  CaCO3 Showed an inhibition impact in against 2,664 mg/L Ca2+ and 2,544 mg/L CO32− ions at pH 9 under a concentrations range of 100–1,000 mg/L CaCO3 crystallization Yang & Xu (2011)  
Compound microorganism preparation Dry powder
of nitrobacteria, Bacillus subtilis, photosynthetic bacteria and Thiobacillus denitrificans
mixed Exhibited a limit concentration ratio of 3.87 Ultimate carbonate hardness method Chen et al. (2019)  
Protein of Tepidimonas fonticaldi Proteins  Showed high calcium adsorption capacity of 1.94 mg Ca2+/g protein at pH 10, 150 ○C and 1 atm pressure. Adsorption experiments Han et al. (2017)  
 PIA-co-ESA  CaCO3 100% Static jar test Shi et al. (2017)  
IA/SAS/SHP  CaCO3 81.2% Static jar test Cui & Zhang (2019)  
IA/AMPS  CaCO3 95.1% Static jar test Liu et al. (2016c)  

The constant composition method

The constant composition method was pioneered by Tomson & Nancollas (1978) in 1978 who investigated compositions of calcium phosphate by maintaining solution pH constant during the whole reaction. In order to maintain the stability of solution composition and pH, automatic titrators are applied in against the influence of CaCO3 precipitates. The precipitation phenomenon triggers two mechanically coupled burettes to add a suitable amount of salt solutions, hence, the crystal growth rate can be accurately obtained from the recorder of titrant traces as a function of time. The antiscaling capacity of a reagent can be further derived (Chhim et al. 2017), making the constant composition method suitable for evaluating the antiscaling capacity of scale inhibitors with quickness and high accuracy (Beck et al. 2013).

Fast controlled precipitation method

The fast controlled precipitation (FCP) method (also known as the CO2 degasification method) was first developed in 1994 (Martin-Dominguez 1994) and has been used in many studies (Tlili et al. 2001; Fathi et al. 2006; Karoui et al. 2010), the principle of this technique is based on the motivation of the calco-carbonic equilibrium in the direction of CaCO3 precipitation with dissolved CO2 continually degassing, which can be achieved by either agitation or nitrogen sparge. The reaction is based on the following equation (Menzri et al. 2017):
formula
(2)

The advantage of the FCP method is simultaneously quantifying the scaling propensity and CaCO3 crystal nucleation, however, considering influence of partial pressure of CO2, the ambient temperature, and other experimental parameters, the FCP tests should be ran in parallel (Gauthier et al. 2012).

(NH4)2CO3 vapor diffusion method

Scale crystallization occurs in the millisecond range in the process of the vapor diffusion method, which shows significant attractivity in the research on CaCO3 crystallization (Addadi et al. 1987; Gehrke et al. 2005; Tourney & Ngwenya 2009). Generally, its key distinction with other methods is that CaCO3 crystals are precipitated from experimental solution which gradually supersaturates under the continuously decomposing ammonium carbonate in a closed container, according to the following equation (Sarkar et al. 2013):
formula
(3)
formula
(4)

The vapor diffusion method is appropriate for slow CaCO3 crystallization benefits from the relatively stable gas–liquid phase that contributes to eliminate the interference of other factors, this promotes well facetted calcite crystals' formation for further microscopic observations (Gehrke et al. 2005; Liu et al. 2016a).

Turbidity method

Deposition of scaling particle aggregates causes haze and further increases solution turbidity, hence, changes on turbidity index can be utilized to evaluate antiscalant performance. During the experimental processes, the antiscaling performance of the scale inhibitor can be derived from monitoring turbidity rate as a function of time, while the dispersion effect can be derived from inspecting size and quantity of the obtained precipitation particles. The turbidity method is simple and feasible to obtain antiscalant performance, especially convenient on prescreening of antiscalants, it not only provides information on scale inhibitor performance, but also is rich in information on scale formation kinetics and inhibition mechanisms (Tantayakom et al. 2005a, 2005b; East et al. 2010; Zhou et al. 2010; Al-Hamzah et al. 2014). However, since several experimental factors can interfere with the scale crystallizing procedure, this in turn affects the reproducibility of turbidity measurement.

Critical pH method

The critical pH method was first proposed by Feitler (1972) in order to detect CaCO3 deposition, with continual addition of sodium hydroxide, the solution pH value exceeded the critical supersaturation leading carbonate precipitation to occur (Sousa et al. 2016. H+ generated by precipitation reaction resulted in an immediate pH decrease, however, the stronger antiscaling performance the inhibitor possesses, the higher critical pH it can tolerate (Zhang et al. 2011; Sousa et al. 2016).

Ultimate carbonate hardness method

Ultimate carbonate hardness method has a certain guidance on field application of antiscalants running in recirculating cooling water system. In formal running periods, concentration ratio represents the concentrating situation with respect to the cooling water system, which can be calculated either by ratio of chloridion or hardness value. Ideally without scale deposition, both chloride and the hardness concentration ratio maintain numerical equality (equal in magnitude); however in real operation, the nucleation and crystallization of scaling ions results in decrements on alkaline concentration ratio. This gives rise to a numerical difference between concentration ratio of chloride ions and alkalinity, such difference become larger as the solution become concentrated with continual evaporation. The total solution carbonate hardness is considered as ultimate carbonate hardness value when the numerical difference exceeded 0.2, thus higher ultimate carbonate hardness value was attributed to superior scale inhibitors with marked scaling tolerance.

Static jar test

The static jar test is the most widely used test method for evaluating scale inhibition efficiency due to its low cost and quickness. Static assessment is based on a scale precipitation formation experiment through mixing two chemically incompatible brines (NaHCO3 and CaCl2 for CaCO3 usually) along with the scale inhibitors to form precipitation, after which, an incubation period, usually 10 h in a water bath, is implemented on mixed brines with and without scale inhibitors. Once finished, aliquots are filtered to analyze free Ca2+ ions and the scale inhibition efficiency is obtained by dividing numerical differences according to NACE Standard Testing Methods (NACE International 2002; NACE International 2015) and Chinese National Standard (Chinese National Standard 2008; Chinese National Standard 2019).

Electrochemical methods

Electrochemical techniques, including chronoamperometry (CA), chronoelectrogravimetry, and impedancimetry, have proven very useful in the study of scale processes in a range of media (Belarbi et al. 2016; Menzri et al. 2017; Piri & Arefinia 2018), with the cathodic potential applied, the reduction of dissolved dioxygen form hydroxyl ions leading the electrode interfacial pH to increase, this accelerate in CaCO3 deposition is according to the following equations:
formula
(5)
formula
(6)
formula
(7)
The non-conductive calcareous deposition on the electrode surface reduces the current density, which is simultaneously recorded by chronoamperometric measurements. It reaches to the scaling time when the electrode surface is completely covered by the insulating layer of CaCO3, which represents the scaling condition of a sampling solution. Generally, among these electrochemical techniques, chronoamperometric is more basic and commonly used to evaluate scale inhibition efficiency while electrochemical impedance measurements allow observing the nucleation, growth, and total surface coverage of the deposition (Ketrane et al. 2009; Zuo et al. 2020).

Dynamic tube blocking test

Dynamic tube blocking test is one of the main testing methodologies used to evaluate the minimum inhibitor concentration (MIC) required to prevent the formation of scale under dynamic flow condition. In principle, it involves measurements of the increasing pressure caused by scale deposition on the test tube wall. Scale inhibition efficiency is represented by the ratio of the time needed to obstruct the tube flow in the presence or the absence of the inhibitors (Macedo et al. 2019; Sanni et al. 2019).

Pilot plant test

The pilot plant tests are conducted in a recirculating cooling water systems or in a simulating device to test the field application performance of a scale inhibitor (Neveux et al. 2016; Chen et al. 2019). Compared with other methods, pilot plant tests can not only offer a flexible operation as the system can be directly controlled by computer, but also directly obtain the feasibility of the tested agent. However, such experiment conditions of pilot plant tests are difficult to achieve in general laboratories.

During the last few decades, various types of inhibition chemicals and antiscalants have been widely used to prevent mineral scale deposition. Current scale inhibitors applied in industrial cooling water treatment can be classified into the following categories.

Phosphorus-containing scale inhibitors

Inorganic phosphonates and organophosphorus are mainly phosphorous-containing scale inhibitors that have been used for many years. Inorganic phosphonates, mainly refer as sodium triphosphate (Na5P3O10) and sodium hexametaphosphate (NaPO3)6, which are effective scale inhibitors due to the presence of a phosphate group (-PO3) (Lin & Singer 2005). Their application as antiscalants has been limited because of low solubility and lower thermal stabilities. However, organophosphorus, such as aminotrimethylenephosphonicacid (ATMP), polyamino polyether methylenephosphonate (PAPEMP), and diethylenetriaminepentamethylene phosphonic acid (DETPMP), overcome this shortcoming due to C–P bonds in its molecular structure that are more stable than O–P–O bonds at higher temperatures (Ghani & Al-Deffeeri 2010; Mpelwa & Tang 2019).

Phosphorus-containing scale inhibitors display a high strong scale inhibition action by sequestering scalants through a threshold effect at sub-stoichiometric amounts. However, as phosphorus-based inhibitors serve as nutrients after discharge, which will lead to eutrophication difficulties, therefore, high levels of phosphorus-containing scale inhibitors in industrial applications are becoming increasingly restricted (Rahman 2013).

Polycarboxylates scale inhibitors

Polycarboxylates scale inhibitors possess functional carboxyl groups (–COOH) in its molecular structure, making them have strong chelating and dispersing abilities on Ca2+. During CaCO3 crystallization process, polycarboxylates can significantly inhibit CaCO3 crystal growth by adsorption onto the crystal surface, modifying the regular CaCO3 crystal morphology (Wada et al. 2001). Widely known polycarboxylates scale inhibitors are polymaleic acid, polyacrylic acid and poly(methacrylic acid) due to their high performance–price ratio. Moreover, polycarboxylate scale inhibitors are also reported to copolymerized with other functional groups for better performance (-NH2, -OH, -COOH, -SO3H and so on) (Shakkthivel & Vasudevan 2006; Yang et al. 2017; Dong et al. 2018). In the cases of monomers embedded to polymer molecules, multiple functional groups acted synergistically to enhance both dispersion and adsorption capacity when polymers prevented scale nucleation and growth.

Organic green scale inhibitors

With the severe restriction of waste water discharge to the environment, green chemistry demands scale inhibitors to move towards the direction of environmentally friendly, which is readily biodegradability, no bioaccumulation, and nontoxic. Broadly speaking, polyaspartic acid (PASP), polyepoxysuccinic acid (PESA) and carboxymethylinulin (CMI) are three typical representatives of green inhibitors, their antiscaling performances have been well documented as well as toxicity, solubility, biodegradability, and synthetic routes, their non-nitrogenous, non-phosphorus and environmental acceptability together with excellent antiscaling properties allow them to be the most promising alternatives to conventional scale inhibitors (Hasson et al. 2011; Chaussemier et al. 2015; Kumar et al. 2018; Mpelwa & Tang 2019).

Among the above-mentioned scale inhibitors, several recent studies have been undertaken on modified PASP with B-cyclodextrin (Fu et al. 2020), urea (Zhang et al. 2016b), glycine (Migahed et al. 2016), tyrosine (Tyr) and tryptophan (Trp) (Zhang et al. 2017) by grafting copolymerization. Such modifications improved its scale inhibition efficiency in different degrees, giving PASP-based copolymer inhibitors a superior performance against CaCO3, CaSO4 and Ca3(PO4)2. Moreover, molecular dynamics simulations indicated that the scale inhibition ability of sulfamic/amino-modified PASP came from preventing the growth of CaSO4 crystal planes ((040), (041) and (113)) (Zhang et al. 2017); this provided an in-depth study on its scale inhibition mechanism at the atomistic level.

PESA as a green scale inhibitor was first synthesized in the beginning of the 1990s in the USA, its nonhazardous and highly biodegradable properties enable it to displace traditional scale inhibitors. In a previous work, PESA was regarded as the alternative scale inhibitor of PASP due to its better performance against CaCO3 deposition (Liu et al. 2012). In a recent study (Huang et al. 2019), PESA with its hyper-branched structure showed a stronger antiscaling performance than linear PESA, the related scale inhibition mechanisms included prolonging the CaCO3 crystal nucleation and reducing the number of crystal nucleus.

In addition, carboxymethylinulin (CMI) is a polysaccharide-based polycarboxylate isolated from the roots of Inula helenium, which benefits from carboxylic acid groups in its molecular structure, CMI possesses marked antiscaling properties in preventing calcium carbonate crystallization. A study from Kırboga & Öner (2012) indicated negatively charged functional groups in CMI molecules, assumed to be mainly responsible for retarding CaCO3 precipitation. Moreover, CMI molecules can interact closely with the calcite crystal surfaces of (012), (104), (10) and (110) through van der Waals intermolecular interactions, electrostatic interactions, and hydrogen-bonding interaction, consequently leading to a high efficiency in inhibiting the growth of the calcite (Zhang et al. 2016a, 2016b).

Blending scale inhibitors

Specifically, a blending scale inhibitor usually functions more efficiently than any single inhibitor. For an enhanced synergistic inhibition, blended scale inhibitors should be adopted according to a certain proportion and guarantee the concentration of each component not below the MIC. It is worth noting that the synergistic action of blending scale inhibitors usually occurs under the circumstances when each component possesses different scale inhibition mechanisms. Synergistic effects between some scale inhibitors have been found in some reports, for example ATMP and DTPMPA (Zeino et al. 2018), PBTCA, HEDP and ATMP (Li et al. 2014), sodium gluconate (SG) and PBTC (Ou & Chiang Hsieh 2016).

Bio-materials, derived from the natural environment, possess potential functionalities for antiscaling effect due to the presence of polyphosphates, carboxylic acid, alcohol, and aromatic amine groups in their molecular structure. Their non-bioaccumulation, low cost, readily biodegradability and sustainably available characters help them to gain widespread concern. Most importantly, bio-materials that are non-poisonous, of low cost, and easily biodegradable, possess great potential in application as industrial scale inhibitors. In this part, bio-materials are classified into four categories, proteins and amino acids, polysaccharides, plant extracts and microbial products, their scale inhibition performance along with mechanisms are discussed separately.

Proteins and amino acids molecules as scale inhibitors

Except for polyaspartic acid, various proteins and amino acids have proven excellent antiscaling capacity. In a previous study (Kumar et al. 2010), a type of water-soluble vitamin M, pteroyl-l-glutamic acid (PGLU), was conducted as a scale inhibitor, 120 mg/L of PGLU showed 100% CaCO3 scale inhibition efficiency according to static jar tests at 70°C. Dynamic tube blocking tests also showed the MIC required for scale inhibition at 110 °C is 160 mg/L, it is therefore concluded that PGLU could be a favorable green scale inhibitor. Besides PGLU, a mixture of another two vitamins, RS1066 (thiamine and pyridoxine), was applied as scale inhibitor by (Menzri et al. 2017), they conducted CO2 degasification method and chronoamperometry experiments to test its antiscaling performance. As a result of CO2 degassing by stirring, 160 mg/L RS1600 inhibited calcite scale via delaying the nucleation time from 30 to 72 min in tested Hamma hard water. The chronoamperometry showed that the scaling time increased two times under the presence of 40 mg/L of RS1600, corresponded to a 66.78% inhibition efficiency. The FTIR, XRD and Raman spectroscopy demonstrated the vitamin blending scale inhibitor, RS1600, modified the crystalline structure and made the calcite crystalline form a metastable phase to inhibit the calcite crystal formation. In a previous study, a 48-amino-acid-long peptide, cysteine-rich Mdm2 peptide, was tested as a scale inhibitor (Dalas et al. 2006) by constant composition method to investigated the calcite inhibition by cysteine-rich Mdm2 peptide. For a total calcium concentration range of 80–120 mg/L, the Mdm2 peptide reached a maximum inhibition of 58% on calcite crystal growth. Side terminal carboxyl groups of the Mdm2 peptide fragment were regarded as binding sites which interact with the calcite surface through hydrogen bonding, making the crystal surface grow sites blocked.

Recently, (Mady & Kelland 2017) investigated the barium sulfate and calcium carbonate inhibition performance of a series natural proteins, in their dynamic tube blocking tests, variation of tube pressure indicating tube flux changes was regarded to represent antiscaling performance. The inhibitor dosage when rapid tube blocking occurs was also recorded as fail inhibitor concentration (FIC). As a result, under a pH range of 5−7 at 100 °C and 80 bar, peptone plus 19544, casein peptone N1 19516, and tryptone N1 19553 exhibited excellent performance in preventing barium sulfate scaling in heat pipes, corresponding to FIC of 50, 50 and 20 ppm, respectively. For calcium carbonate scaling inhibition, casein peptone N1 19516 and tryptone N1 19553 were superior to commercial scale inhibitors (ATMP and DTPMP), their FIC was 5 ppm for each 8 min run.

Besides, citric acid has long been proven to be a scale inhibitor (Reddy & Hoch 2001; Wada et al. 2001), however, recent antiscaling research has laid emphasis on its derivatives, such as poly(citric acid) (Zhao et al. 2016), citrate (Tobler et al. 2015) and polymerized citric acid (Yuan et al. 2020). In the study of Zhao et al. (2016), the calcium sulfate inhibition performance of poly(citric acid) was investigated by static jar tests according to Chinese standard GB/T16632–2008. With a total hardness of 2,040 mg/L, 2.5 mg/L poly(citric acid) showed an CaSO4 inhibition rate over 90%, which rose eventually up to 98.8% when the dosage reached 25 mg/L, such antiscaling performance is preeminent. Moreover, for higher concentrations of 6,000 mg/L Ca2+ and 7,000 mg/L SO42−, 25 mg/L poly(citric acid) possessed an 80% scale inhibition rate, indicated that poly(citric acid) was a suitable scale inhibitor for high hardness. The scale inhibition mechanism carried from SEM, FTIR and XRD indicated that suspended carboxylic groups on the PCA molecules could maintain Ca2+ ions free in solution by complexation, making PCA molecules easy to be absorbed on active sites of growing CaSO4 crystals. This consequently, distorted the CaSO4 crystal lattice and inhibited the CaSO4 scale deposition.

The deprotonated form of citric acid, citrate (CIT), is a well known complexing agent for dissolving calcium and inhibiting CaCO3 growth, its excellent biodegradability make it a promising green scale inhibitor. During the process of CaCO3 crystallization, the presence of CIT could effectively retard calcium carbonate nucleation, the nucleating induction period increased with an increase in CIT/Ca ratio, from 1 min at CIT/Ca = 0% (pure system) to 30 min at 100% CIT/Ca (Montanari et al. 2017), such marked scaling prevention performance was closely related to hydroxyl and carboxyl groups which absorbed on crystal surfaces, and led to calcite crystals with modified morphologies and sizes (Tobler et al. 2015). In a recent study, sodium citrate (SC) was found to cooperate with 10-methylacridinium iodide (MAI) as a blending scale inhibitor (Zhang et al. 2020), the authors conducted static jar tests to evaluate the antiscaling performance of MAI–SC mixture in guidance of GB/T 16632-2008, an enhanced CaCO3 scale inhibition effect was found as MAI–SC mixture (MAI 50 mg/L and SC 150 mg/L) reached a peak antiscaling efficiency of 98.3% at 60 °C, the morphology of CaCO3 crystals was changed and the crystal form was altered. Such scale inhibition mechanism tends to be similar to the interaction between -COOH/-OH and Ca2+ that allowed the antiscalants to be adsorbed on the calcium carbonate scale surface, a further quantum chemical calculation demonstrated that this type of absorption mainly takes place on (104) and (102) faces for calcite, and (002) and (020) faces for vaterite. Thanks to this, the MAI–SC mixture could therefore occupy the active growth sites of calcium carbonate deposition in solution and hinder the further growth of the crystals.

Another recent research reported (Yuan et al. 2020) that a new polymer of citric acid, curcumin-citric acid-aspartic acid polymer (PCCA), acted as a suitable antiscalant for both CaSO4 and CaCO3 scale. According to static jar tests, PCCA had a maximum inhibition efficiency of 99.7% with merely 4 mg/L dosage against CaSO4, even when Ca2+ ions reached up to 16,000 mg/L, the inhibition efficiency remained as 90.7% at 10 mg/L dosage. Moreover, PCCA also possessed outstanding capacity against CaCO3, a 20 mg/L dosage of PCCA exhibited 98.8% calcite inhibition efficiency under a tested water containing 500 mg/L high calcium bicarbonate at 80°C (Table 1).

Polysaccharides as scale inhibitors

Polysaccharides are widely distributed in nature, some of which constitute the cell walls, such as chitosan and cellulose, and some are stored as nutrients for plants and animals. With widespread usage in the food, chemical, pharmaceutical, and other industries, polysaccharides also have great potential to be applied as scale inhibitors. In this section, applications of polysaccharides as scale inhibitors are classified and reviewed.

Chitosan

Chitosan (CTS), a widely distributed biopolymer in nature, has been chemically modified and shown to have the potential to be a green scale inhibitor due to its nontoxic nature. CTS and its derivatives with broad areas of application had been reported in many studies as a scale inhibitor. Previously (Guo et al. 2012), chitosan was copolymerized with maleic anhydride (MA), styrene sulfonic sodium (SSS), and acrylic amide (AM) to prepare CTS-MA–SSS–AM, which was highly effective as a CaCO3 scale inhibitor, according to static jar tests (GB/T 16632-2008) at 70°, with an advanced CaCO3 inhibition efficiency of 95.62%, 0.2 g/L CTS-MA–SSS–AM was effective against Ca2+ 0.006 mol/L and HCO3 0.0045 mol/L precipitated into calcite scale.

Recent research reported on chitosan biguanidine hydrochloride (CG), obtained by modifying chitosan with a guanidine group, showed a good performance in retarding scale formation during real membrane desalinating application (with a cross-flow unit for 6 h) (Maher et al. 2020), with only flux declines of 2.6 and 5% for CaSO4 and CaCO3 in feed water under concentrations of 10 and 15 mg/L respectively. Moreover, SEM analysis showed that CG strongly changed the precipitated scale crystals morphology. The initial needle-like CaSO4 crystal structure was modified and transformed into an irregular structure while cubic CaCO3 crystals were regulated to be cracked and distorted. The authors explained that the antiscaling mechanism of CG that introducing a chitosan backbone with guanidinium group increased its cationic charges, and made CG able to strongly combine with scalants along with anionic charges on its surfaces and consequently inhibited scale growth.

Another derivate of chitosan, carboxymethyl chitosan (CMCS) was also found to mediate the crystal growth of calcium carbonate previously (Liang et al. 2004). SEM and XRD demonstrated that a high CMCS dosage of 1,000 mg/L significantly changed the crystal morphology of precipitated CaCO3 crystals and facilitated them to transfer to calcite form. In a dynamic tube block tests according to NACE International (2005) NACE TM3110, CMCS functioned as a scale inhibitor of CaCO3 under synthetic brine solution (Macedo et al. 2019). It was found that CMCS significantly reduced the scaling tendency of tested water, the precipitation time was obviously extended from 18 to 50 min under 50 mg/L of CMC, the MIC of CMCS was 170 ppm for preventing scale formation at T = 70 °C and 1,000 psi. SEM along with infrared spectroscopy analysis showed that CMCS effectively interacted with calcium ions through its carboxylate ions and lone pair of electrons on -OH and -NH2 groups, which resulted in deformation of the CaCO3 crystal morphology and made the scaling ions hard to organize and prevent the scale formation.

These results showed consistency with previously results by Yang et al. (2010), who investigated the impact of CMCS on CaCO3 crystallization by vapor diffusion method, at 40°, they found the presence of CMCS obviously reduced the size of CaCO3 particles from a particle range of 9–25 to 1.5–17 μm. From surface characterization results on obtained CaCO3 particles, the authors suggested that during CaCO3 crystallization, CMCS complexed with Ca2+ ions by –COO− groups in its molecule, generated CMCS/Ca2+ complexes to reduce Ca2+ ion participation in CaCO3 precipitation, led to lattice distortion, and inhibited the growth of CaCO3 nanoparticles.

Except CMCS, O-(hydroxyisopropyl) chitosan (HPCHS), which was obtained by modified chitosan with poly(ethyleneoxide)–poly(propyleneoxide)–poly(ethyleneoxide) (EO)20–(PO)72–(EO)20, also exhibited a regulation effect on CaCO3 particle size distribution and morphology (Yang et al. 2009), the CaCO3 precipitates were first obtained under the presence of HPCHS and then characterized by SEM, XRD and TGA methods. Results showed that the increased HPCHS dosage decreased the particle size and roughened the crystal surfaces. Based on the results of molecular dynamic simulation, the authors proposed a mechanism that under the presence of lone pair electrons of nitrogen atoms, HPCHS molecules could adsorb onto the (104) surface and the edge of the CaCO3 particles, thus achieving a regulatory effect on CaCO3 formation (Table 2).

Cellulose

Cellulose is also a widely distributed polysaccharide in nature with nontoxicity, easy degradation and biocompatibility; cellulose and its derivatives are widely used in food, medicine, detergent, paper, energy, and environmental protection. Cellulose possesses marked potential to be an effective scale inhibitor as it benefits from the abundant hydroxyl and carboxyl groups in its molecular structure. In a recent research study (Xu et al. 2019), sodium carboxymethyl cellulose (SCMC) was applied as a scale inhibitor in industrial circulating water systems. Several methods were conducted to evaluated scale inhibition performance of SCMC, including a pH displacement method (by measuring pH change of the test solution before and after scaling), electrochemical impedance spectroscopy (EIS), and real application tests, the results showed consistently that SCMC possessed a promising prospect on fouling inhibition. Under a concentration range of 50–200 mg/L, SCMC exhibited 93 and 75% inhibition efficiency by pH displacement method and EIS study, respectively. While in a real simulated dynamic circulating cooling water system, SCMC reached a scale inhibition efficiency of 93.2% under 200 mg/L (determined by fouling resistance changes). Based on these results, a further study of molecular dynamics simulation was carried out to explore the scale inhibition mechanism (Zhao et al. 2019), results indicated that there was a strong interaction between oxygen atoms in SCMC and calcium ions in calcite (104) crystal planes, such strong interaction would distort and deform the plane of calcite and further hinder the normal growth of calcite crystal.

(Gan et al. 2018) Another multifunctional cellulose derivative, borated aminated cellulose citrate (BACC), showed excellent scale inhibition performance in a simulated cooling water, in which Ca2+ was concentrated twice contrasted with original brine described in Chinese National Standard method GB/T-16632. As a result, an 91.57% CaCO3 inhibition efficiency was obtained under the presence of 150 mg/L BACC at 70 °C. The characterization results on deposited crystals suggested that the active functional groups (-C = O, − OH, N-C = O, and B − O) in BACC molecular structure played an important role in forming stable Ca2+−BACC complexes and further prevented scale deposition (Table 3).

Glucose-based polysaccharides

Glucose is one of the most widely distributed and important monosaccharides in nature, its derivatives, starch (St) and SG have been identified as effective threshold CaCO3 scale inhibitors in several recent studies. Recently, SG showed an obvious synergistic effect with 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) on preventing CaCO3 deposition (Ou & Chiang Hsieh 2016). Light-scattering experiments for investigating the scale inhibition kinetics and mechanisms were carried out in "synthetic" water which contained 600 mg/L Ca2+ under mixed SG/PBTC for 10 and 1 ppm respectively. As a result, in the presence of SG/PBTC, the formation kinetics of CaCO3 was four times lower than that of PBTC alone, while the SEM, XRD and particle size distribution (PSD) analysis identified that the scale inhibition occurred through forming a dissoluble Ca complex, stabilizing the metastable phase of aragonite and vaterite and distorting the CaCO3 crystal morphology.

Another polysaccharide with widespread availability and that was environmental friendly was starch (St), starch is a type of natural polymer which mainly composed of polyhydric glucose. Due to abundant oxygen-containing functional groups in its molecular framework, St is talented in scale inhibition, however, St is usually modified with other molecules rather than directly used as a scale inhibitor, a suitable modification can significantly improve its antiscaling performance, according to static jar tests described in National Standard of China GB/T 16632-2008, after a water bath at 70° for 10 h, MA–SSS–OS (prepared by oxidized starch (OS) copolymerized with MA-SSS showed a noteworthy increase in CaCO3 antiscaling efficiency from 65.26% to 98.87% in a dosage range of 0.2 g/L to 0.8 g/L (Guo et al. 2013), CMS (6) performed a high inhibition efficiency of 89.80% against CaCO3 at a concentration of 60 mg/L, while St-g-PAA exhibited an antiscaling performance of 95.79% inhibition efficiency at a concentration of 40 mg/L (Wang et al. 2017; Yu et al. 2018); these results proved that starch-based polymers have great potential in scale inhibition.

β-Cyclodextrin (β-CD), which is a kind of starch that consists of seven glucose units linked by α-1,4-glucosidic bonds, was regarded as effective scale inhibitor, in a previous study (Liu et al. 2016b), β-CD was modified with polyethylene glycol to obtain a novel scale inhibitor namely polyethylene glycol modified β-cyclodextrin (β-CD–PEG), the CaCO3 inhibition performance of β-CD–PEG was evaluated by static jar tests. The results showed 180 mg/L β-CD–PEG reached a peak scaling inhibition efficiency of 89.1% against 200 mg/L Ca2+ at 40 °C. SEM and XRD analyses suggested that cyclodextrin cavity, –OH and –O– in the copolymer had strong chelating capacity to disperse the Ca2+ ions, the scale inhibitor molecules could also absorb onto crystal surfaces and occupy key points of crystal structure. However, β-CD–MA–SSS (synthesized by β-CD, MA and SSS monomers) acted more effectively towards calcium carbonate and calcium phosphate formation (Gu et al. 2013). According to static jar tests, for a total Ca2+ of 200 mg/L at pH range of 5.0–6.0 and 70°C, 10.3 mg/L β-CD–MA–SSS terpolymer exhibited nearly 99.9% of scale inhibition efficiency for CaCO3 and 95.5% for Ca3(PO4)2. Transmission electron microscopy (TEM) displayed a morphologic change on CaCO3 deposits under the impact of β-CD–MA–SSS, from regular cubic shape with a compact arrangement to flower patterns with floppy accumulation, this was attributed to carboxyl, anhydride, cyclodextrins cavity and sulfonic acid groups in β-CD–MA–SSS, which could easily chelate with Ca2+ and altered oriented crystal growth.

In an earlier research, sodium lignosulfonate (SL), another glucose-based polysaccharide, was copolymerized with other monomers for scale inhibition in cooling water systems (Ouyang et al. 2006), according to static jar tests (Ca2+ concentration of 240 mg/L at 60 °C for 6 h), SL had nearly no scale inhibition capacity of which the inhibition efficiency range was 1.0 to 2.5% under a SL dosage of 4 to 50 mg/L. A limited scale inhibition efficiency was obtained after SL was modified with acrylic acid, which was 30.7 and 45.2% at the dosages of 20 and 50 mg/L, respectively, suggested an enhancement of CaCO3 inhibition capability happened after SL was modified with acrylic acid. The authors attributed this effect to the existence of both carboxyl and sulfonic groups that allowed modified SL to strongly absorb onto CaCO3 crystalline substrates and lead a lattice distortion (Table 4).

Plant extracts as scale inhibitors

In this direction, there have been many studies on applying plant extracts as scale inhibitors, although some have been reviewed before (Chaussemier et al. 2015; Kumar et al. 2018). In recent years, novel antiscalants derived from several different plant extracts have emerged, we have followed up with the latest research on applying plant extracts as antiscalants, their antiscaling performances along with the mechanisms are described in detail.

In a few studies, the fast controlled precipitation (FCP) method along with CA were conducted to assess the scale inhibition performance of several plant extracts, including Herniaria glabra (Horner et al. 2017), Spergularia rubra, Parietaria officinalis (Cheap-Charpentier et al. 2016) and Hylocereus undatus (Lourteau et al. 2019). The FCP method was conducted at 30 °C in synthetic water (Ca2+ = 100 mg/L) with a 850 rpm stirring rate. The results showed that among the four plant extracts, Hylocereus undatus was the most efficient scale inhibitor with the lowest dosage of 24 mg/L to completely inhibit CaCO3 formation (reached an efficiency of 100% as determined by FCP resistivity response), followed by 30 mg/L for Spergularia rubra and 50 mg/L for Herniaria glabra. The least effective was P. officinalis, for which a dosage up to 100 mg/L was required for complete scale inhibition. However, in CA experiments, a higher dosage of plant extracts was generally required for preventing extra CaCO3 deposition on the electrode surface. Among the four plant extracts, the n-butanolic extract of H. glabra with a minimum dose of 75 mg/L was able to totally inhibit the precipitation of CaCO3. However, to achieve the same effect using Hylocereus undatus extract, a slightly higher concentration of 180 mg/L was required, while large doses of 300 and 700 mg/L were needed by S. rubra and P. officinalis, respectively. SEM and XRD characterization indicated that the crystal morphology became irregular and the vaterite polymorph was preferentially favored to form. The authors suggested that surface complexation between the carboxylate groups of these plant extract molecules and calcium atoms on the CaCO3 surface led an adsorption effect on the calcite crystal surface, reduced the crystal growth, and modified its geometry.

In two recent reports, the scale inhibition performances of Mazuj gall and Bistorta officinalis extract were both evaluated by static beaker tests (Mohammadi & Rahsepar 2018; Mohammadi & Rahsepar 2019). The CaCO3 crystallization process was kept in a 50 °C water bath for 18 h at pH = 8.5 after the extracts were incubated; residual Ca2+ concentration within the filtrate was determined by EDTA titration. Consequently, 1,000 ppm of Mazuj gall extract provided a high inhibition efficiency of 97.2% against 592 ppm total hardness while Bistorta officinalis extract provided a slightly enhanced scale inhibition efficiency of 99.5% under the same experimental conditions. It was thus concluded that both Mazuj gall and Bistorta officinalis extracts could be used as effective antiscaling additives in cooling water solution.

Previously, gambier extract from Uncaria gambierRoxb. leaves were reported as a green inhibitor for CaCO3 formation (Suharso et al. 2011), by bottle-roller batch method. They found that 200 ppm gambier extract brought 70–80% CaCO3 inhibition efficiency at 80 °C. Subsequently, the authors modified gambier extract with benzoic and citric acid at a ratio of (gambier extract:benzoic acid:citric acid/2:1:2) to strengthen its antiscaling capability (Suharsoa 2017). Consequently, modified gambier extract showed an inhibition efficiency of 12–92% under the concentration range of 50–300 ppm. This antiscaling effect was slightly diminished compared with original gambier extract, which showed an inhibition efficiency of 60–100% under the concentration range of 50–250 ppm. SEM and PSD analyses indicated that both gambier extract and its modified product could not only induce irregular changes on CaCO3 morphology but also reduce particle sizes. It was suggested that inhibitor molecules like tannic acid, catechin and quercetin from the gambier extract could interfere with Ca2+ at the active growth sites of the CaCO3 microcrystals to block calcite crystal growth.

Nayunigari et al. (2016) reported using curcumin, which is naturally occurring product abundant in the turmeric plant (Curcuma longa), copolymerized with malic acid in order to control scaling problems in cooling water treatment. Static jar tests were conducted to evaluate the antiscaling capacity of synthetic poly(curcumin–malic acid) with 7,500 mg/L of Ca2+ and 10,200 mg/L of SO42− for 16 h, at a temperature range of 65–70 °C and pH 8.0. Poly(curcumin–malic acid) showed the highest CaSO4 scale inhibition rate of 95.0% at 10 ppm. FTIR spectroscopy, XRD analysis and SEM suggested that functional oxygen species, such as superoxide and hydroxyl radical anions brought by curcumin gave an improvement on blocking growth sites of scale crystal and haltered the precipitation process, making curcumin-based copolymer a promising alternative for CaSO4 scale inhibition in cooling water systems (Table 5).

Microbial product as inhibitors on antiscaling application

In this part, we summarize the application of using microbial product on antiscaling, either using microbial agents or microbiological products. Microorganisms and their secreta with abundant sources and broad applying prospects, have promoted the development of ‘green’ scale inhibitors.

In a previous study, (Kawaguchi & Decho 2002) reported a new idea for CaCO3 precipitation inhibition by using extracellular polymeric secretions (EPS) of a cyanobacteria, Schizothrix sp. They found the role of Schizothrix sp. EPS in inhibiting CaCO3 precipitation in marine stromatolites was possibly because acidic ionic functional groups in EPS could bind to Ca2+ and consequently inhibit calcification. Based on the previous study, a recent study provided new insight in scale inhibition on industrial water treatment using soluble EPS (s-EPS) of Bacillus cereus (Li et al. 2019), the soluble EPS (s-EPS) of Bacillus cereus (B. cereus) was extracted and investigated its CaCO3 inhibition effect by static jar tests. Results showed the CaCO3 scale inhibition efficiency reached up to 87.60% at a concentration of 80 mg/L s-EPS. The obtained morphology of precipitated CaCO3 crystals was modified from rhomboid-shaped to disorderly granular crystals. In addition, the authors found that tryptophan and protein-like substances in s-EPS might take primary responsibility in complexing with Ca2+ determined by the 3-D excitation/emission matrix (EEM), the O and N atoms in the functional groups of protein-like substances had higher negative charges and provided many Ca2+ ions binding sites, which enabled EPS to inhibit calcium scale crystal formation.

Xanthan (XC), a microbial polysaccharide produced by Xanthomonas campestris, showed an scale inhibition impact against 2,664 mg/L Ca2+ and 2,544 mg/L CO32− ions at pH 9 under a concentration range of 100–1,000 mg/L (Yang & Xu 2011). The SEM showed that, in the presence of XC, the obtained CaCO3 particles stacked into ones compared with those rhombohedral crystals without XC. The presence of XC significantly lowered the peak intensities of calcite crystals, this was attributed to XC molecules that offered an absorption function on CaCO3 particle surfaces. A deeper antiscaling mechanism of XC was revealed in a subsequent study (Elkholy et al. 2018). The authors conducted Monte Carlo (MC) simulations to investigate the adsorption tendency of xanthan on growing calcite (CaCO3) and anhydrite (CaSO4) crystals. Adsorption tendency between gums and mineral surface (CaCO3 (10), CaSO4 (001)) was expressed in terms of the values of adsorption energy calculated from the MC simulation method. They found that xanthan gum polymer (XGP) possessed a strong adsorption at low polymerization degrees; under these circumstances, XGP exhibited a higher tendency to the CaCO3 (10) and CaSO4 (001) surface at low PD values, the authors suggested that through Coulomb interactions between the negatively charged functional groups of the scale inhibitors and free Ca2+, XGP could occupy the growing points of calcite or anhydrite crystals and hinder the further scalants' aggregation.

In a recent study, a biological method successfully handled the corrosion and scaling problem in a cooling water system makeup by urban reclaimed water (Chen et al. 2019). Such biological treatment was implemented by application of compound microorganism preparation (CMP) that consists of nitrobacteria, Bacillus subtilis, photosynthetic bacteria and Thiobacillus denitrificans which might provide new thinking approaches in circulating cooling water treatment. The results from ultimate carbonate hardness method showed that such CMP successfully reduced carbonate scaling, as CMP exhibited a limit on concentration ratio of 3.87, which was higher than common chemical treatment in China (generally for 2–3) (Ma et al. 2010; Tao et al. 2011). Moreover, a decrease in pH from 8.33 to 6.66 during the 30-day trial period was observed. This phenomenon was attributed to microbial activities, the authors speculated the combination of H+ produced by bacteria and CO32− in water contributed to the further formation of HCO3 according to the following equation:
formula
(8)
formula
(9)
thus, allowing the stable existence of high concentrations of Ca2+ in cooling water systems. Besides, a microbial reagent was also used by Han et al. (2017), who found that a thermophilic bacterial strain Tepidimonas fonticaldi showed high calcium adsorption capacity of 1.94 mg calcium/g protein at pH 10, 150 °C and 1 atm pressure. Compared with intracellular fraction, calcium absorption was predominant extracellularly at a temperature of 55 °C, which indicated that Tepidimonas fonticaldi could become an effective bio-sorbent to remove calcium and reduce scaling.

Itaconic acid (IA) is a secondary metabolite produced by Aspergillus terreus, its chemical structure makes it easy to polymerize or act as co-monomer with different other components. Its copolymer has been suggested as attractive antiscaling agents in many studies, for instance, 2-acrylamido-2-methylpropane sulfonic acid copolymer (IA/AMPS) (Cui & Zhang 2019), itaconic acid–sodium allysulfonate–sodium hypophosphite copolymer (IA/SAS/SHP) (Liu et al. 2016c), and poly(itaconic acid-co-epoxysuccinic acid) (PIA-co-ESA) (Shi et al. 2017). Among these studies, the anti scaling performance of the three IA copolymers were all conducted by static jar tests according to GB/T16632, the highest scale inhibition efficiency was attributed to PIA-co-ESA, which reached 100% at a dosage level of 18 mg/L. This impact was even more effective than PBTC (Shi et al. 2017). Besides, SEM found that added PIA-co-ESA (10 mg/L) not only reduce the size of precipitated CaCO3 crystals, but also change the crystal shape to irregular floccule and loose. The scale inhibition mechanism of PIA-co-ESA was proposed according to MD simulation results. The PIA-co-ESA molecules retarded the growth of calcite through absorbing onto (104) and (10) crystal surfaces by hydrogen bond and electrovalent bond, but the binding energy showed a slightly larger binding force for the (10) surface than the (104) surface. Such consequences are in line with Cui & Zhang (2019), in which the authors suggested that the binding strength of IA/AMPS with the calcite surface (10) was firmer than the (104) surface as well. The novel IA copolymer (IA/AMPS) was a very promising scale inhibitor for both CaCO3 and CaSO4, from the results of static jar tests; 14 mg/L IA/AMPS exhibited a CaCO3 inhibition efficiency of 81.2% while 18 mg/L IA/AMPS had a CaSO4 inhibition efficiency of 80.6%. SEM and XRD analysis indicated that IA/AMPS achieved the scale inhibition effect through occupying the active sites on the crystal surface, leading to scale crystal orientation distortion and preventing the crystal growth. In another study (Liu et al. 2016c), IA/SAS/SHP was regarded as a new biodegradable scale inhibitor synthesized by introducing sulfonic acid and phosphonic acid groups to the IA monomer, it displayed an excellent antiscaling performance according to static jar tests, and showed an inhibition efficiency of 95.1% at a concentration of 24 mg/L in confronting 600 mg/L Ca2+ and 1,200 mg/L HCO3. From SEM and XRD analysis, after added IA/SAS/SHP, the morphology of calcite crystals changed from a regular rhombohedral lattice to flower-shaped and cascade-like. The peak strengths of crystal surfaces (104) and (116) were greatly reduced. The authors suggested that IA/SAS/SHP could be adsorbed onto the crystal surfaces to destroy crystal structures, resulting in a reduced scale crystal growth rate (Table 6).

Aiming at scaling problems in industrial water systems, using bio-materials as green scale inhibitors exhibit great potential. Studies that found bio-materials with scaling resistance filled the gap of interdisciplinary studies between bio-materials chemistry and carbonate deposition. In this review, the listed bio-material proteins and amino acids, polysaccharides, plant extracts and microbial products showed excellent antiscaling performance according to different evaluation methods. These functional materials displayed multiple antiscaling mechanisms of which the best was the interaction between organic functional groups in scale inhibitor molecules and Ca2+; these functional groups included carboxyl, hydroxyl, anhydride, amino and sulfonic groups. The widely acceptable scale inhibition mechanisms of bio-materials indicate that on the one hand, scale inhibitor molecules apply complexation effects to increase Ca2+ solubility, but on the other hand, scale inhibitor molecules can absorb on scale crystal surfaces, occupying the active grow sites and retarding crystal growth.

Despite bio-materials fitting the development direction of green scale inhibitor chemistry, considering multiple factors, further studies are still necessary before real application. The primary work is assessing economic affordability as some proteins and amino acids may go through complex steps of extraction, this is even more complicated for some plant extracts. Researchers also need to consider wastes and reaction products generated by extraction, which may cause other environmental threats. Besides, most scale inhibition researches were conducted using static jar tests with convenience and quickness. But these lacked validation on long-term scale inhibition performance, so extra scale inhibition tests under field operation conditions are required, which is essential for industrial cooling water application as field operation conditions differ from laboratory level. This would involve some special water conditions with high hardness or secondary water. So, in general, the feasibility of bio-materials as scale inhibitors in large-scale applications still needs to be validated.

Another future perspective emerges as the scale inhibition tests of bio-materials are usually in single formulation, for reasons that multiple inhibitors act through different scale inhibition mechanisms to impose a higher scale inhibition resistance than inhibitor using alone. Thus, it can be speculated that synergistic effected may occur when bio-materials cooperate with other scale inhibitors. In addition, modification and graft copolymerization for purposes of improving scale inhibition performance are also significant for the development of bio-materials as scale inhibitors; the carboxylic group, sulfonic group, and other groups may offer extra dispersion effects or electrostatic interactions to scaling ions as well.

Last, but not least, relative research on scale inhibition mechanisms have been mainly launched through investigation from the aspects of inorganic calcium scale formation, taking simple molecules or scale inhibitors containing single functional groups as research objects to study the influence of inhibitors on scale formation. However, their scale inhibition mechanisms are still vague, as only rarely were explanations on the relationship between the structure and the performance of scale inhibitors included from molecular and microscopic aspects. This needs further theoretical investigations like molecular dynamics simulations or computational modeling which will provide a considerable contribution to this field.

Scaling is a major challenge for industrial circulating cooling water systems. In recent years, increasing environmental concerns and discharge limitations have restricted the utilization of conventional antiscalants greatly, hence, various types of bio-materials have been screened to apply as environmentally friendly scale inhibitors; grafting modification and compound formulation were also conducted in order to obtain higher inhibition efficiency.

Here, we summarized bio-materials as scale inhibitors in recirculating cooling water treatment, including proteins, polysaccharides, plant extracts and microbiological products. Thanks to the negatively charged functional groups on the molecular backbone chain, such as (-COOH, -OH etc.), those bio-materials displayed excellent scale inhibition performances. They only had good chelation and dispersion effects on free Ca2+, but also showed a regulation on scalants crystal morphology. Moreover, scale inhibition can also be achieved by interaction between antiscalants and crystal surfaces, like CaCO3 (104) and CaSO4 (001), which provided a popular perspective for further study of scale inhibition mechanisms.

Finally, the biodegradation properties with low toxicity and low bioaccumulation make bio-materials have promising application potential for use as scale inhibitors; however, since assessing antiscalant efficiency was usually performed by individual solutions, pilot scale tests with mixed salt solution are needed for further evaluation.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All relevant data are included in the paper or its Supplementary Information.

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