Herein, a nutrient water retention agent is prepared by fully mixing sludge with carboxymethyl cellulose-g-acrylic acid (CMC-g-AA) gel and nanoscale zero-valent iron (nZVI) using polymer modifying curing technology. Experimental results show that when CMC:AA = 1:12 and CMC-g-AA gel content is 50%, sludge polymer has better water absorption and retention performance and the water retention time is extended for ∼14 days. At the same time, sludge polymer can preserve the characteristics of nutrient-rich elements and organic matter and promote plant growth. The addition of nZVI has a significant impact on reducing the risk of heavy metal toxic leaching in sludge. Moreover, analysis of variance and multiple comparisons shows that sludge polymer's particle size and water absorption times have significant effects on the water absorption and retention properties of sludge polymer. Scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and 13C-nuclear magnetic resonance analyses show that the addition of an appropriate amount of gel could increase the number of hydrophilic groups and hydrophilic mineral components in sludge polymer, increase its overall porosity and improve its water absorption and retention properties.

  • A utilization of nutrients and organic matter contained in sludge to maintain sludge water content.

  • Adding an appropriate amount of CMC-g-AA gel to the sludge can increase the hydrophilic groups and hydrophilic mineral components and increase the overall porosity of the sludge polymer.

  • The sludge polymer is rich in nutrients and organic matter, which is non-toxic and promotes plant growth.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Sludge is a waste and unavoidable by-product produced during the sewage treatment process. Besides water, its main components include inorganic particles and various forms of organic matter (Deng et al. 2019; Kor-Bicakci & Eskicioglu 2019; Liang et al. 2020). Organic matter components in sludge exist mainly in the form of organic nutrients containing elements such as nitrogen, phosphorus and potassium as well as several pathogenic bacteria, parasites (eggs), bacterial remains and other toxic organic pollutants parasitised on organic matter (Sun et al. 2019; Liu et al. 2020). The presence of these refractory organic matters makes it difficult to separate solid and liquid through physical sedimentation or general mechanical dehydration in the wastewater treatment process, increasing the difficulty of subsequent sludge treatment. At present, landfilling, composting, natural drying and incineration are the methods of sludge treatment in China, among which landfilling is the main disposal method (Su et al. 2019). In foreign countries, landfilling and agriculture are mainly adopted among the United States, Britain and Northern Ireland, whereas incineration is preferred in Japan, in which sludge incineration treatment has accounted for more than 60% of the total sludge treatment methods (Cheng & Hu 2010). Therefore, how to scientifically treat the sludge with a large output and complex components and make it harmless and resourceful has become a hot environmental spot of deep concern in China and around the world.

Presently, water shortage has become a major obstacle to agriculture, so developing water-retentive materials with obvious effects and low costs has become an important research topic. Carboxymethyl cellulose (CMC) is a major low-cost, commercially available cellulose derivative, which is widely used in industries when preparing hydrogels with high water absorption (Saravanan et al. 2019). CMC has several favourable properties for synthesising gels, such as good water solubility and the existence of reactive hydroxyl and carboxymethyl groups. Additionally, CMC's good biodegradability in soil favours its application in agriculture and forestry (Nie et al. 2004). It has been reported that copolymers of CMC with other monomers such as acrylic acid (AA) or acrylamide can produce gels (Hemvichian et al. 2014). Compared with pure CMC gel, the presence of AA leads to a higher gel fraction and swelling ratio (Bajpai & Mishra 2010), resulting in better water absorption and retention performance. In recent years, some researchers have developed this type of gel, as shown in Table 1. In addition, some researchers have mixed the gel with other substances for other uses, which provides us with new ideas, as shown in Table 2.

Table 1

Summary of the paper for synthetic gels

ReferenceThe materials usedSynthetic way
Hemvichian et al. (2014)  Carboxymethyl cellulose (CMC), acrylamide (AM), crosslinking agent: N,N’-methylenebisacrylamide (MBA) The desired reagents were mixed with distilled water and the mixture was irradiated at different doses using a Co-60 gamma irradiator 
Bajpai & Mishra (2010)  CMC, acrylic acid (AA), crosslinking agent: MBA The desired reagents were added to distilled water and then transferred to Petri dish. The Petri dish was kept at 50 °C for 24 h, then cut into and dried after solidification 
Tamás & Borsa (2016)  CMC, AA The mixture was placed into a polyethylene bag and irradiated at different doses with a 60Coγ-source 
Mai et al. (2021)  AA, acryl-amide(AM), formaldehyde, formaldehyde solution, MBA The pH of the adjusted mixture was 4.5, heated to 75 °C, and maintained for some time after the addition of formaldehyde solution then hydrated with ethanol and dried at high temperature 
ReferenceThe materials usedSynthetic way
Hemvichian et al. (2014)  Carboxymethyl cellulose (CMC), acrylamide (AM), crosslinking agent: N,N’-methylenebisacrylamide (MBA) The desired reagents were mixed with distilled water and the mixture was irradiated at different doses using a Co-60 gamma irradiator 
Bajpai & Mishra (2010)  CMC, acrylic acid (AA), crosslinking agent: MBA The desired reagents were added to distilled water and then transferred to Petri dish. The Petri dish was kept at 50 °C for 24 h, then cut into and dried after solidification 
Tamás & Borsa (2016)  CMC, AA The mixture was placed into a polyethylene bag and irradiated at different doses with a 60Coγ-source 
Mai et al. (2021)  AA, acryl-amide(AM), formaldehyde, formaldehyde solution, MBA The pH of the adjusted mixture was 4.5, heated to 75 °C, and maintained for some time after the addition of formaldehyde solution then hydrated with ethanol and dried at high temperature 
Table 2

Summary of the paper for synthetic gels and other substances

ReferenceThe materials usedSynthetic wayApplication
Ren et al. (2020)  Coal mine sludge (MS), CMC, polyaluminium chloride, and citric acid The desired reagents were added to distilled water in sequence at 25 °C MS/CMC-Al3+ gel is a clean fire protection and extinguishing material. MS: CMC: AlCit (aluminum citrate): H2O = 15:1:4:35, MS/CMC-Al3+gel displayed the best compressive strength and inhibitory effect and inhibit the generation of CO and CO2 from combustion 
Bao et al. (2011)  AA, AM, CMC, potassium persulfate (KPS), 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), Montmorillonite (MMT), MBA CMC and Na-MMT were added to distilled water, the solution was heated to 60 °C, and a mixture of AA, AM, AMPS and MBA was added to the flask and heated to 70 °C for 2 h to complete the reaction AA-co-AM-co-AMPS/MMT is a novel multi-component superabsorbent by graft copolymerization of vinyl monomers along the chains of CMC in the presence of MMT. The incorporation of MMT can not only reduce production cost, but also improve the properties (such as swelling ability, gel strength, mechanical and thermal stability) of superabsorbents 
Elsaeed et al. (2021)  CMC, AA, AM, AMPS, orange peel biochar, KPS, MBA After adding CMC and biochar, other substances were added. The solution was maintained in a reaction at 70 °C, 800 W microwave reactor. The products were washed with an ethanol solution and then dried Superabsorbent biochar composite grafted on CMC is a low-cost, alternative, and biodegradable terpolymer composite (IPNCB) for soil water retention capacity 
Olad et al. (2018AA, MBA, ammonium persulfate (APS), CMC, Rice husk (RH) After the sulfonation of CMC, other substances were added at 40 ° C and mixed, raised to 60 ° C for 4 h, and then soaked in ethanol, dehydrated and dried New slow release fertilizer encapsulated by superabsorbent nanocomposite was prepared by in-situ graft polymerization of sulfonated-carboxymethyl cellulose (SCMC) with acrylic acid (AA) in the presence of polyvinylpyrrolidone (PVP), silica nanoparticles and nitrogen (N), phosphorous (P), and potassium (K) (NPK) fertilizer compound. The hydrogel nanocomposite fertilizer formulation can be practically used in agricultural and horticultural applications 
ReferenceThe materials usedSynthetic wayApplication
Ren et al. (2020)  Coal mine sludge (MS), CMC, polyaluminium chloride, and citric acid The desired reagents were added to distilled water in sequence at 25 °C MS/CMC-Al3+ gel is a clean fire protection and extinguishing material. MS: CMC: AlCit (aluminum citrate): H2O = 15:1:4:35, MS/CMC-Al3+gel displayed the best compressive strength and inhibitory effect and inhibit the generation of CO and CO2 from combustion 
Bao et al. (2011)  AA, AM, CMC, potassium persulfate (KPS), 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), Montmorillonite (MMT), MBA CMC and Na-MMT were added to distilled water, the solution was heated to 60 °C, and a mixture of AA, AM, AMPS and MBA was added to the flask and heated to 70 °C for 2 h to complete the reaction AA-co-AM-co-AMPS/MMT is a novel multi-component superabsorbent by graft copolymerization of vinyl monomers along the chains of CMC in the presence of MMT. The incorporation of MMT can not only reduce production cost, but also improve the properties (such as swelling ability, gel strength, mechanical and thermal stability) of superabsorbents 
Elsaeed et al. (2021)  CMC, AA, AM, AMPS, orange peel biochar, KPS, MBA After adding CMC and biochar, other substances were added. The solution was maintained in a reaction at 70 °C, 800 W microwave reactor. The products were washed with an ethanol solution and then dried Superabsorbent biochar composite grafted on CMC is a low-cost, alternative, and biodegradable terpolymer composite (IPNCB) for soil water retention capacity 
Olad et al. (2018AA, MBA, ammonium persulfate (APS), CMC, Rice husk (RH) After the sulfonation of CMC, other substances were added at 40 ° C and mixed, raised to 60 ° C for 4 h, and then soaked in ethanol, dehydrated and dried New slow release fertilizer encapsulated by superabsorbent nanocomposite was prepared by in-situ graft polymerization of sulfonated-carboxymethyl cellulose (SCMC) with acrylic acid (AA) in the presence of polyvinylpyrrolidone (PVP), silica nanoparticles and nitrogen (N), phosphorous (P), and potassium (K) (NPK) fertilizer compound. The hydrogel nanocomposite fertilizer formulation can be practically used in agricultural and horticultural applications 

It is a new research direction to combine sludge and hydrogel as nutrient and water-retaining material in agriculture. In this study, we propose a resource utilisation technology that makes full use of nutrients and organic matter contained in sludge while preserving the water content of sludge using polymer modification and curing technology (Britto & Jackeline 2012), a ternary graft copolymer carboxymethyl cellulose-g-AA (hereinafter CMC-g-AA), synthesised by graft copolymerisation of carboxymethyl cellulose sodium (hereinafter CMC) with monomers such as AA, mixes with Nanoscale zero-valent iron (nZVI) and sludge to prepare sludge polymer. This technology eliminates the need for organic matter degradation equipment, related costs and sludge dewatering. Using the copolymer as fertilizer for agriculture and forestry not only realises the resource utilisation of sludge but also solves the problem of water shortage and drought in farmland and forest land. In this study, the influence of several influencing factors on the water absorption property of the material is studied and the optimum synthesis process of the system is obtained. In addition, the influence of the ratio of CMC to AA (CMC:AA), gel content and particle size on water absorption, water retention of CMC-g-AA sludge polymer and the changes of nitrogen, phosphorus and organic content before and after water loss of the polymer and the effects of plant growth are investigated, and the best synthesis process of the polymer system is obtained.

Materials

The sludge in this experiment (the water content is 71.46% and the pH is 7.08–7.21) is dewatered sludge from a sewage treatment plant in Jinzhong, Shanxi Province. Sodium CMC (C8H11O5Na, analytical reagent) was produced by Tianjin Hengxing Chemical Reagent Co., Ltd. (Tianjin, China) AA (analytical reagent) was produced by Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China) N,N’ methylene bis-acrylamide (crosslinking agent) was produced by Damao Chemical Reagent Factory (Damao, China) and potassium persulfate (Initiator) was produced by Tianjin Hengxing Chemical Reagent Co., Ltd. (Tianjin, China) Nanoscale zero-valent iron (nZVI) was produced by Tianjin Hengxing Chemical Reagent Co., Ltd. (Tianjin, China).

Methods

Preparation of CMC-g-AA gel

A certain amount of AA was measured in a container, the container was placed in a cold-water bath and NaOH was added dropwise to the container. After the reaction was completed, the container was sealed and put aside. Then, a certain amount of distilled water was added into a three-necked flask, the temperature of the water bath was adjusted to 60 °C, sodium CMC was added and heating and stirring was conducted until it dissolved. Using a water bath to maintain the temperature at 70 ± 5) °C, nitrogen was poured into the system to drive oxygen, potassium persulfate was added to initiate the reaction for 5–10 min and the neutralised acrylic monomer was added and stirred to mix the two solutions thoroughly. Then, crosslinking agent N, N′ methylene bis-acrylamide was added while stirring the solution continuously, and polymer emulsion was prepared after continuous reaction for 2.5 h (Dai et al. 2017). Specifically, the CMC:AA ratios were 1:4, 1:8, 1:12 and 1:16. According to the best dosing ratio in the literature (Chen et al. 2006; Yan et al. 2011), the selected initiator dosage of CMC is 2.0%, the best dosage of the crosslinking agent of AA is 0.4% and the neutralisation degree of AA is 75%.

Preparation of CMC-g-AA sludge polymer

Nanoscale zero-valent iron (nZVI) powder with a 5% (calculated by the sludge quality) particle size of 45 μm was added to the polymer emulsion and stirred at constant temperature for 30 min (Mueller et al. 2012). Then the polymer emulsion was added to sludge and stirred at constant temperature for 30 min. Instead, the polymer CMC: AA has four ratios: 1:4, 1:8, 1:12 and 1:16, and the proportion of polymer and sludge mixing is 0, 10, 30, 50, 70 and 90%. After the sludge polymer was naturally air-dried, the sludge polymer was ground into three particle ranges: 1–2, 3–3.5 and 3.5–5 cm. It is termed ‘sludge polymer’ in the main body of this paper (Wang et al. 2019).

Analysis of the properties

Water absorption ratio and water absorption rate
Three samples were taken from each group, the mass of each sample was weighed after drying (m0), the samples were placed inside a constant temperature water tank with a water temperature of 20 ± 5 °C, the samples were spaced at not less than 20 mm and water was added at least 30 mm above the sample. The samples were removed at regular intervals within 0–24 h, the surface water was wiped off with a damp cloth and the mass (m1) of each sample was weighed immediately. The water absorption rate QA was calculated using Equation (1):
formula
(1)
where, the mass of the sample removed for the nth time is denoted as mn, the mass of the sample taken out for the n + 1th time is denoted as mn+1, the interval between two samplings is denoted as tn and the water absorption rate is calculated using Equation (2):
formula
(2)
Water retention ratio and water loss rate
The sample (m2) was measured after water absorption saturation and was drained to a constant weight at a constant temperature of 30 °C. It was weighed regularly once a day and recorded as m3. Three samples were taken from each group. The water retention ratio was calculated based on Equation (3):
formula
(3)
The water loss rate was calculated based on Equation (4):
formula
(4)
Determination of nitrogen, phosphorus and organic matter
Determination of organic matter: The crucible quality of the organic matter was termed as m1, after adding the sample it was termed as m2, and after placing it in an oven for 105 °C for 2 h, it was termed as m3. The dried sample and crucible were placed inside a muffle furnace 550 ± 50 °C for 1 h and was termed as m4 after cooling. The organic matter was calculated based on Equation (5):
formula
(5)

Total nitrogen was determined by alkaline potassium persulfate digestion UV spectrophotometric method (HJ 636-2012). Total phosphorus was determined by ammonium molybdate spectrophotometric method (GB 11893-89).

Determination of root vigour, chlorophyll content and H2O2 content

500 mL of water soaked in CMC-g-AA sludge polymer with a 1 mL plant nutrient solution was used for hydroponic Scindapsus aureus, and the following tests were conducted every 5 days.

Determination of root activity: The method of triphenyl tetrazolium chloride (TTC) was used with reference to Comas et al. (2000) and Ruf & Brunner (2003); 0.5 g of the root tip of Scindapsus aureus was immersed in 10 mL equal amounts of 0.4% TTC solution and phosphoric acid buffer. It was kept warm in the dark for 1 h. Once the root was removed, it was ground with ethyl acetate and a small amount of quartz sand. The absorbance was measured using a spectrophotometer at a wavelength of 485 nm.

Determination of chlorophyll content: Refer to the mixed extraction method presented by Lichtenthaler (1987); 0.2 g of Scindapsus aureus leaves was taken as a sample, and a small amount of quartz sand, calcium carbonate powder and 80% acetone was added and ground into a homogenate. It was filtered and diluted to 25 mL with acetone. The absorbance was measured at wavelengths of 645 and 663 nm.

Determination of H2O2 content: Refer to the method presented by Patterson et al. (1984). 1 g of Scindapsus aureus leaves was weighed, an appropriate amount of acetone was added and grinded to 10 mL, 1 mL extract was taken and 0.1 mL of 5% of TiCl4 and 0.2 mL of concentrated ammonia water were added. It was centrifuged at 5000 r/min for 10 min. The precipitate was dissolved in 2 mol/L H2SO4 and the absorbance was measured at a wavelength of 410 nm.

Determination of leaching toxicity

In this study, the toxicity identification of heavy metals in sludge polymer was performed by the Toxicity Characteristic Leaching Procedure (TCLP) of U.S. Environmental Protection Agency. The experimental procedure of TCLP was: weigh 2.5 g of sludge polymer powder and add 50 mL, pH = 4 acetic acid leaching agent to it with solid-liquid ratio of 1:20. At 25 °C, put it into a flipped oscillator with an oscillation frequency of 30 r/min, an immersion time of 18 h. After immersion, it was centrifuged and filtered with a 0.45 μm filter membrane. Then, the Cu, Cr, Zn, Cd and Pb contents were determined by flame atomic absorption spectrophotometer (TAS-986).

Degradation performance test
We purchased flower soil from the flower market as the soil for the degradation experiment; 0.400 g of sludge polymer was tightly sewn into double-layer bags with 200 mesh polyester mesh cloth and buried 10 cm deep from the surface for degradation experiment. The degradation rate was calculated based on Equation (6):
formula
(6)
where m5 is the quality of dry sludge polymer after degradation.

Characterisation method

Fourier-transform infrared spectra (FTIR): FTIR was performed using the Impact 410 Nicolet FTIR instrument (American company) with a scanning range of 500–4000 cm−1.

Scanning electron microscopy: The scanning electron microscope image of the sample was taken by the Model-JSM-6390LV SEM (JEOL Ltd, Japan).

13C-Nuclear magnetic resonance: The 13C-NMR was determined via Bruker 400M with a MAS spin rate of 10 KHz and recovery time of 4 s.

X-ray diffraction: The Bruker D8 Advance X-ray diffractometer (BRUKER AXS, Germany) was used for testing, with a scanning speed of 10°/min, step length of 0.02° and scanning range of 5–85°.

Thermogravimetric analysis: The thermogram was recorded on the NETZSCH STA 449F3 analyzer at a heating rate of 10 °C/min in an air atmosphere over a temperature range of 30–600 °C.

Characterisation and analysis of CMC-g-AA sludge polymer

Fourier-transform infrared spectra analysis

Figure 1 shows the FTIR spectra of sludge, sludge polymer in various proportions and CMC-g-AA pure gel. Figure 1 shows that there is a triple bond or accumulated double bond absorption area near 2372 cm−1 and a –CH2– asymmetric stretching vibration at 2921 cm−1. There is no obvious difference between the six curves in these two bands.

Figure 1

Fourier-transform infrared spectra of sludge, CMC-g-AA pure gel and each proportional CMC-g-AA sludge polymer.

Figure 1

Fourier-transform infrared spectra of sludge, CMC-g-AA pure gel and each proportional CMC-g-AA sludge polymer.

Close modal

In Figure 1(a)–1(e), contrary to Figure 1(f), C–H and O–H out-of-plane bending vibration peaks on aromatic rings emerge at 835 and 795 cm−1, respectively, which proves that gel addition increases hydrophilic substances in the polymer. At 1160 cm−1, the conjugated telescopic vibration of C–O–C is the characteristic absorption peak of the ring structure of cellulose ether, which indicates that CMC-g-AA generates highly polymeric ether substances during the reaction and helps to enhance the degree of crosslinking of the structure (Yeasmin & Mondal 2015). Near 1464 cm−1 is the –CH2– telescopic vibration in CMC-g-AA, where the peak positions and shapes of curves a, b, c, d and e are the same, which is different from curve f. Hence, the introduction of the network structure of CMC-g-AA when the sludge reacts with the gel is confirmed. Peaks near 1548 and 1670 cm−1 are caused by carboxylate anions and carboxamides. Because of their hydrophilicity, the introduction of COO–, –CONH helps the water absorption of the composites (Chen et al. 2019; Elsaeed et al. 2021). Near 1715 cm−1 are symmetrical and asymmetrical stretching vibration absorption peaks of C = O in the COONa group, indicating that the mixing of CMC-g-AA gel and sludge makes the C = O absorption peak move towards a low wave number.

Figure 1(c) is the stretching vibration peak of O–H near 3408 cm−1, where the peak position of curve C is lower than other curves, indicating that more hydrogen bonds are produced after synergising CMC-g-AA gel and sludge components. Figure 1(b)–1(e) have lower peaks at 1548 cm−1 as compared to Figure 1(c), which shows that Figure 1(c) contains more carboxylate anions and carboxamides and has stronger water absorption.

Scanning electron microscopy analysis

Figure 2 shows scanning electron microscopy (SEM) images of sludge, CMC-g-AA pure gel and sludge polymer in various proportions. The microstructure of the polymer is one of the most important properties that should be considered. Figure 2(a) shows that the sludge powder is irregular porous granular minerals with diameters between 200 and 3500 nm, varying in size and morphology, with poor interparticle adhesion and loose porosity, which can be considered as water-retaining materials. Figure 2(b) shows that the surface of the CMC-g-AA pure gel is evenly distributed using a layered fibrous film, and the CMC-g-AA pure gel flocculent film is distributed with tiny shallow pores showing no obvious large pores. It is speculated that the water absorption process of pure gel mainly depends on the hydrophilic groups on the surface of high polymer molecules (Chen et al. 2006). Figure 2(e) shows that the sludge particles are wrapped by a fibrous film with high cohesive force after the addition of CMC-g-AA gel, forming large particles with a diameter between 4000 and 6500 nm. These large particles are bonded to form an interpenetrating network, forming an adhesive structure inside the sludge polymer, which results in several irregular network-connected pore channels distributed in part of sludge polymer with a cross-section ranging from 400 × 4000 to 2000 × 5000 nm, increasing the water storage space of sludge polymer. These internal channels communicate with the water-absorbing film on the surface of sludge polymer, which helps sludge polymer absorb water quickly. The special spatial structure produced by the synergistic effect of CMC-g-AA gel and sludge makes the entire sludge polymer have strong water absorption and water retention characteristics. Compared with Figure 2(e), Figure 2(c)–2(f) have no obvious channel network and poor glue-linked structure; hence, the water absorption and water retention performances are not as good as those in Figure 2(e).

Figure 2

Scanning electron micrographs: (a) Sludge, (b) CMC-g-AA pure gel (c) CMC-g-AA sludge polymer (CMC:AA = 1:4), (d) CMC-g-AA sludge polymer (CMC:AA = 1:8), (e) CMC-g-AA sludge polymer (CMC:AA = 1:12) and (f) CMC-g-AA sludge polymer (CMC:AA = 1:16).

Figure 2

Scanning electron micrographs: (a) Sludge, (b) CMC-g-AA pure gel (c) CMC-g-AA sludge polymer (CMC:AA = 1:4), (d) CMC-g-AA sludge polymer (CMC:AA = 1:8), (e) CMC-g-AA sludge polymer (CMC:AA = 1:12) and (f) CMC-g-AA sludge polymer (CMC:AA = 1:16).

Close modal

Mechanistic analysis of the synthesis of CMC-g-AA sludge polymer

Figure 3 shows the mechanism diagram of the graft copolymerisation of CMC and AA. The polymer uses potassium persulfate as the initiator and N, N′ methylene bis-acrylamide as the crosslinking agent to graft AA onto CMC, forming a ternary graft copolymer CMC-g-AA and adding a certain amount of sludge in different dosages to form a highly absorbent polymer.

Figure 3

Mechanism diagram of graft copolymerisation of CMC and AA.

Figure 3

Mechanism diagram of graft copolymerisation of CMC and AA.

Close modal

Figure 3 shows that the mechanism of the graft copolymerisation of CMC and AA is that (NH4)2S2O8 is thermally decomposed to produce radicals, which extract hydrogen from the hydroxyl groups of CMC and form alkoxy groups on the substrate. Then, the polymerisation reaction is initiated and the AA is grafted onto CMC. Because of the presence of the crosslinking agent N, N′ methylene bis-acrylamide in the system, the terminal vinyl group of methylene bis-acrylamide reacts with the polymer chain during chain growth to form a cross-linked network structure (Nath & Dolui 2018). In addition, the added nZVI can efficiently remove heavy metals from the sludge through various effects such as adsorption, reduction and precipitation, reducing the risk of heavy metal leaching.

Effect of CMC-g-AA gel content on water absorption of sludge polymer

In this experiment, the influence of CMC-g-AA gel on the water absorption properties of sludge polymer is investigated under the conditions of particle size of 1–2 mm, CMC:AA = 1:4, 1:8, 1:12 and 1:16 and CMC-g-AA gel content of 0, 10, 30, 50, 70 and 90%. As shown in Figure 4, the pH of the sludge polymer with different CMC-g-AA gel content is determined.

Figure 4

Change of water absorption rate and pH of gel content in CMC-g-AA sludge polymer.

Figure 4

Change of water absorption rate and pH of gel content in CMC-g-AA sludge polymer.

Close modal

Figure 4 shows that the water absorption rate of sludge polymer increases with an increase in CMC-g-AA gel content. When CMC-g-AA gel content is increased from 0 to 90%, the water absorption rate increases by ∼23 times. Microscopic observation shows that when CMC-g-AA gel content is 0; hence, the dried sludge block becomes a small piece with a dense middle crack edge that has a relatively weak water absorption capacity. With an increase in CMC-g-AA gel content, the outer layer of the sludge polymer water-absorbing film gradually becomes denser and the porosity of the internal space increases, significantly enhancing the water absorption and storage of sludge polymer (Olad et al. 2018). When the CMC-g-AA gel content is increased to 50%, the inside of sludge polymer is evenly dispersed with ∼2 mm visible holes, which plays a good role in water storage so that the water absorption rate is ∼15 times higher than that of the original sludge. Figures 1 and 2 show that this phenomenon may be because the CMC-g-AA sludge polymer has many hydrophilic groups such as COO– and –CONH, and its highly porous structure enhances the physical properties of sludge polymer and increases water absorption (Tulain et al. 2018). When CMC-g-AA gel content exceeds 50%, the water absorption of the hydrophilic substances in the gel starts to play a leading role, rapidly increasing the water absorption rate of sludge polymer.

Additionally, Figure 4 shows that the water absorption of sludge polymer is better when CMC:AA = 1:12, moderate when the ratio is 1:8 and relatively worse when the ratio is 1:4 and 1:16. The reason may be that when the amount of AA is too low, there are relatively more reactive points on the cellulose chain caused by the initiator, shortening the grafting chain, reducing flexibility and, thus, lowering the water absorption rate. When CMC:AA = 1:12, the strong hydrophilic group –COOH introduced into the network gradually increases and the network structure can be fully stretched and swelled; at this time, the water absorption ratio reaches the maximum. Nevertheless, if an excessive amount of AA is added continuously, the homopolymer reaction between AA will be greater than the graft copolymerisation reaction between AA and CMC; the viscosity of the reaction system will increase with an increase in AA, hindering the movement of free radicals and monomer molecules; simultaneously, it increases the probability of monomer molecular chain transfer, creating a large network gap, which cannot effectively form hydrogen bonds with water molecules, resulting in a low water absorption rate (Pourjavadi et al. 2004; Pourjavadi et al. 2006).

Figure 4 shows that the pH value of sludge polymer decreases as the amount of gel increases. When CMC-g-AA gel content is 0%, the pH value of each get ratio is between 7 and 8. As CMC-g-AA gel content increases to 90%, the lowest pH value reaches 4.1. There is no significant difference in the pH of each gel ratio even though the pH of CMC:AA = 1:16 is relatively low, probably because of the high amount of AA. Generally, the pH of water retention agents used in agroforestry should be in the optimal range of 5–7. Over-acid and over-alkali water-retaining agents are likely to change the acid–base environment of the cultivation substrate after accumulation for a long time, which is not conducive to the growth of plant roots and the absorption of nutrients. Hager (2003) found that root elongation of higher plants is promoted by low pH and hindered by high pH. Kobyahashi et al. (2010) found that the main growth obstacle caused by high pH is that it inhibits the taproot elongation. Thus, water retention agents with high pH should not be used in agriculture. Considering the requirements of sludge polymer used as agricultural and forestry water-retaining agents, it is proposed that the range of CMC-g-AA gel content should be selected as 10–50% and the corresponding pH value range as 5.0–7.0.

Effect of CMC-g-AA gel content on water retention of sludge polymer

As detailed in Figure 5, the variation of the water retention rate of sludge polymer with time is investigated for each ratio of particle size from 1 to 2 mm under the conditions of 10–50% of CMC-g-AA gel doping.

Figure 5

Change of water retention rate of CMC-g-AA gel–sludge polymer in different CMC-g-AA gel content with time. (a) CMC:AA = 1:4 (b) CMC:AA = 1:8 (c) CMC:AA = 1:12 (d) CMC:AA = 1:16.

Figure 5

Change of water retention rate of CMC-g-AA gel–sludge polymer in different CMC-g-AA gel content with time. (a) CMC:AA = 1:4 (b) CMC:AA = 1:8 (c) CMC:AA = 1:12 (d) CMC:AA = 1:16.

Close modal

In Figure 5, the ability of sludge polymer to absorb water at a constant temperature of 30 °C decreased continuously with time and levelled off after the 7th day. The water retention rate of sludge polymer with a CMC-g-AA gel content of 50% is significantly higher than that of other sludge polymers with different proportions and the lowest water retention rate of sludge polymer with 0% CMC-g-AA doping. Hence, the addition of CMC-g-AA gel can improve the water retention rate of sludge. In Figure 2(d), there are irregular network-connected pores distributed inside sludge polymer. The special spatial structure of the CMC-g-AA gel produced by the synergistic effect of CMC-g-AA and sludge makes sludge polymer possess strong water absorption and water retention properties. Figure 1 shows that hydrophilic functional groups include –COOH and –CONH. The binding of these ionised groups leads to excessive repulsion between the coiled chains, which is the reason why more water is retained finally (Tulain et al. 2018).

Additionally, Figure 5 shows that the water retention performance of CMC:AA = 1:12 is better than other ratios. In Figure 5(c), the water retention rate of sludge polymer is above 50% in the first 3 days, and then it slowly decreases and sludge polymer loses water completely after ∼2 days. It can be inferred that the network structure formed by sludge polymer is more stable than that formed by other ratios of sludge polymers.

In terms of overall water absorption and retention performance, 50% is selected as the best CMC-g-AA gel content, which is used in subsequent studies.

Change of organic matter content in CMC-g-AA sludge polymer

The detailed descriptions of analysis are provided in Supplementary data, Text S1 and Fig. S1.

Effect of CMC-g-AA sludge polymer on plant growth

To explore whether the components contained in CMC-g-AA sludge polymer, especially AA, have adverse effects on plant growth, we conducted some tests on the root vigour, chlorophyll and H2O2 content of Scindapsus aureus to study the optimal composition of sludge polymer.

Effect of CMC-g-AA sludge polymer on plant root vigour

Plant roots are active organs of absorption and synthesis, and the growth and vigour level of roots directly affect the nutritional status and yield level of aboveground parts. TTC is a redox pigment with a standard oxidation potential of 80 mV, which becomes a colourless solution when dissolved in water but generates red and insoluble TTF upon reduction. The degree of TTC reduction can indicate the activity of dehydrogenase (Anika & Richter 2007). Figure 6 shows that the intensity of tetrazolium reduction of Scindapsus aureus hydroponically grown by sludge polymer gradually increases, whereas the root vigour of Scindapsus aureus hydroponically grown using pure water decreases, probably because the plant growth is promoted by nitrogen and phosphorus nutrients and organic matter contained in sludge polymer. The strongest root vigour of Scindapsus aureus is hydroponically grown by sludge polymer with CMC:AA = 1:8, followed by CMC:AA = 1:12 and finally by two ratios of 1:4 and 1:16.

Figure 6

Changes in hydroponic tetrazole reduction strength of CMC-g-AA sludge polymer in different proportions.

Figure 6

Changes in hydroponic tetrazole reduction strength of CMC-g-AA sludge polymer in different proportions.

Close modal

From the above, sludge polymer with CMC:AA = 1:8 has the second-highest water absorption and retention performance after CMC:AA = 1:12, but its AA content is relatively low and has almost no adverse effects on plants. On the 20th day, the colour of the root of Scindapsus aureus hydroponically planted by sludge polymer CMC:AA = 1:1 becomes darker and morphologically smaller, inhibiting growth development; the roots of the remaining sludge polymer hydroponics show no significant changes. It is speculated that excessive AA inhibits the activity of dehydrogenase, decreasing its root vigour.

Effect of CMC-g-AA sludge polymer on chlorophyll and H2O2 content of plants

Chlorophyll content is closely related to photosynthesis and nitrogen nutrition, and the rapid decomposition of chlorophyll is typically the first visible sign of leaf senescence, accompanied by accumulation of active oxygen and reduction of photosynthetic efficiency (Schippers et al. 2015; Woo et al. 2018). Reactive oxygen species, such as H2O2, are key signalling molecules triggering the onset and development of plant senescence (Mueller-Roeber 2011). They can oxidise nucleic acids, proteins and other biomolecules directly or indirectly, causing damage to cell membranes, thus accelerating cellular senescence and disintegration (Wang et al. 2013).

In Figure 7(a) and 7(b), chlorophyll content shows an opposite trend as compared to H2O2 content. The chlorophyll content of Scindapsus aureus hydroponically cultivated by CMC:AA = 1:4, CMC:AA = 1:8 and CMC:AA = 1:12 sludge polymer exhibits an increasing trend and H2O2 content exhibits a decreasing trend, whereas the leaves of Scindapsus aureus cultivated by CMC:AA = 1:16 sludge polymer and pure water show a decreasing trend with chlorophyll content and an increasing trend with H2O2 content, indicating that the leaves have entered the ageing trend. This may be because although a small amount of nutrient solution is added to pure water, the nutrients that can be provided to plants from this solution are less than those provided by sludge polymers. Nitrogen and phosphorus nutrients and organic compounds contained in the sludge polymer promote the growth of plants. Moreover, nitrogen and phosphorus accumulation in the leaves increases biomass on the ground, whereas nitrogen accumulation in the leaves increases underground biomass. Nitrogen accumulation can change the distribution of carbon, nitrogen and phosphorus stoichiometry in plant tissue and has a high potential to increase plant biomass (Jin et al. 2020). Excessive AA content of the sludge polymer with CMC:AA = 1:16 had an irreversible effect on plant growth because AA destroyed the activity of peroxidase in the leaves, increasing the accumulation of reactive oxygen species and leaf senescence.

Figure 7

Changes of chlorophyll content and H2O2 content of CMC-g-AA sludge polymer at different proportions. (a) Changes in the chlorophyll content. (b) Changes in the H2O2 content.

Figure 7

Changes of chlorophyll content and H2O2 content of CMC-g-AA sludge polymer at different proportions. (a) Changes in the chlorophyll content. (b) Changes in the H2O2 content.

Close modal

Drought will increase leaf peroxidase and catalase activities, whereas hydrogel can mitigate the damage caused by oxidative stress (Mohamady & El-Damhougy 2021). Thus, based on the water absorption and retention properties as well as their effects on plant growth, CMC:AA = 1:12 with 50% admixture is selected as the optimal ratio, and the following article will conduct in-depth research accordingly.

Effect of particle size of CMC-g-AA sludge polymer on its water absorption properties under repeated water absorption conditions

The particle size is a factor that must be considered when using sludge polymers. Sludge polymers of different particle sizes have different water absorption properties because of their specific surface areas and different paths for a liquid to enter the interior. In this experiment, the particle size ranges of sludge polymer are selected as 1.0–2.0, 2.0–3.5 and 3.5–5 mm to investigate the cumulative water absorption, water absorption speed and water absorption rate of different particle sizes under three repeated water absorption conditions.

Cumulative water absorption and water absorption speed

Cumulative water absorption reflects the dynamic change of sludge polymer water absorption over time in the water absorption experiment. The slope of the cumulative water absorption curve, which is the water absorption speed, is an important index to evaluate the speed of water absorption of sludge polymer. The variation of cumulative water absorption of CMC-g-AA sludge polymer with different particle sizes under three repetitions of water absorption is shown in Figure 8.

Figure 8

Cumulative water absorption of CMC-g-AA gel–sludge polymer with different particle sizes under three repeated water absorption conditions. (a) 1.0–2.0 mm. (b) 2.0–3.5 mm. (c) 3.5–5 mm.

Figure 8

Cumulative water absorption of CMC-g-AA gel–sludge polymer with different particle sizes under three repeated water absorption conditions. (a) 1.0–2.0 mm. (b) 2.0–3.5 mm. (c) 3.5–5 mm.

Close modal

In Figure 8, the water absorption speed increases rapidly within 0–60 min of the first water absorption. Because, in the initial stage, the three-dimensional (3D) network structure of sludge polymer does not contain water, the ionisation of hydrophilic groups causes high osmotic pressure inside and outside sludge polymer, at which time the water absorption speed of sludge polymer reaches its peak. As more water is absorbed and retained by sludge polymer, osmotic pressure inside and outside sludge polymer gradually decreases and the water absorption speed gradually slows down to reach the absorption equilibrium.

Sludge polymers with a smaller particle size have larger cumulative water absorption and a rapid upward trend, reaching the liquid absorption equilibrium state at ∼120 min, and the liquid absorption equilibrium time gradually increases with an increase in the particle size of sludge polymer. The reason may be that the surface area increases as the particle size decreases. The larger the contact area with water molecules, the faster water can enter its 3D network structure (Omidiana et al. 1999).

The number of repeated water absorption of the sludge polymer and the corresponding ratios are important indicators of the durability of sludge polymer. With an increase in water absorption times, the water absorption ratio of sludge polymer gradually decreases. It may be that the network space structure of sludge polymer is destroyed and becomes loose during repeated water absorption and release, reducing the binding effect on water molecules. The water absorption difference between 2.0–3.5- and 3.5–5.0-mm-diameter sludge polymer reduces, which indicates that the reuse performance of sludge polymer with a large particle size is better than that of sludge polymer with a small particle size. It may be that sludge polymer with a small particle size is more resistant to the loss during repeated drying and dissolution of water molecules.

Water absorption rate

Water absorption rate is the maximum accumulated water absorption when sludge polymer reaches the water absorption equilibrium; it is also an important index for the water absorption of sludge polymer.

The equation analysis of the water absorption multiplicity of CMC-g-AA sludge polymer with different particle sizes at the first water absorption equilibrium shows that sludge polymer particle size and the number of times of water absorption have a significant (p < 0.05) effect on water absorption multiplicity. It is shown by equation analysis and by multiple comparisons (p < 0.05) that the water absorption ratio of the same sludge polymer decreased gradually as the number of times of repeated water absorption increases. As can be seen in Table 3, the relationship with particle size from small to large is 3.5–5.0, 1.0–2.0 and 2.0–3.5 mm.

Table 3

Water absorption rate of the first water absorption balance of CMC-g-AA sludge polymer with different particle sizes

Grain size/mmWater absorption rate/(g·g−1)
First water absorptionSecond water absorptionThird water absorption
1.0–2.0 35.0258 ± 0.20 Aa 32.6025 ± 0.20 Bb 30.3321 ± 0.14 Cc 
2.0–3.5 36.6753 ± 0.22 Bb 35.0284 ± 0.21 Cc 32.8775 ± 0.13 Aa 
3.5–5.0 36.0642 ± 0.21 Cc 34.6575 ± 0.18 Aa 32.6145 ± 0.20 Bb 
Grain size/mmWater absorption rate/(g·g−1)
First water absorptionSecond water absorptionThird water absorption
1.0–2.0 35.0258 ± 0.20 Aa 32.6025 ± 0.20 Bb 30.3321 ± 0.14 Cc 
2.0–3.5 36.6753 ± 0.22 Bb 35.0284 ± 0.21 Cc 32.8775 ± 0.13 Aa 
3.5–5.0 36.0642 ± 0.21 Cc 34.6575 ± 0.18 Aa 32.6145 ± 0.20 Bb 

Note: Different uppercase letters indicate significant differences between treatments in the same row (p < 0.05), and different lowercase letters indicate significant differences between treatments in the same column (p < 0.05). The figures in the table are composed of mean standard deviations.

Effect of particle size of CMC-g-AA sludge polymer on its water retention properties under repeated water absorption conditions

Water retention performance refers to the strength of the internal hydrophilic structure and the interaction of water molecules, which reflects whether water can be stored by sludge polymer for a long time and continue to release water for plant growth (Olad et al. 2018). Examine the effect of the water retention rate and water loss rate under the condition of repeated water absorption three times, at the sludge polymer with different particle sizes of 1.0–2.0, 2.0–3.5 and 3.5–5 mm.

Water retention rate

Place each sludge polymer in a 30 °C constant temperature oven for curing and calculate the water retention rate of sludge polymers with different particle sizes based on Equation (3). The difference in the water retention rate of different particle sizes is inferred from the change trend of the water retention rate with time, and Figure 9 shows the results.

Figure 9

Change of water retention rate of CMC-g-AA sludge polymer with different particle sizes under three repeated water absorption conditions. (a) 1.0–2.0 mm. (b) 2.0–3.5 mm. (c) 3.5–5 mm.

Figure 9

Change of water retention rate of CMC-g-AA sludge polymer with different particle sizes under three repeated water absorption conditions. (a) 1.0–2.0 mm. (b) 2.0–3.5 mm. (c) 3.5–5 mm.

Close modal

In Figure 9, when CMC-g-AA gel content is the same, the smaller the particle size, the faster the water retention rate decreases. With an increase in water absorption time, the water retention rate decreased faster in the early stage and slower in the stable stage. This phenomenon is mainly caused by the high surface area or contact area between fine particles and water (Yan et al. 2011).

Water loss rate

The water loss rate of sludge polymer can reflect its water retention capacity to a certain extent. After sludge polymer fully absorbs water, the water will be lost with the extension of time and a decrease in soil water potential. In practical applications, it is necessary to consider the speed of the water loss rate or else reasonable and effective utilisation of sludge polymer will not be achieved.

Table 4 shows that the water loss rate of the same sludge polymer gradually decreases with the extension of time. Through equation analysis and multiple comparisons (p < 0.05), when the treatment time is 1–3 days, the water loss rate of sludge polymer with a particle size of 1.0–2.0 mm is the largest, which is significantly different from the other two particle size ranges. Sludge polymer with a particle size of 3.5–5.0 mm has the lowest water loss rate, and sludge polymer with a particle size of 2.0–3.5 mm is between the two sludge polymers with particle size ranges of 1.0–2.0 and 3.5–5.0 mm. This may be related to the contact area between particles and air and the initial water content.

Table 4

Water loss rate of CMC-g-AA sludge polymer with different particle sizes after the first water absorption

Grain size/ mmTime/d
1234567
1.0–2.0 0.5694 ± 0.002a 0.5342 ± 0.002a 0.3824 ± 0.002a 0.2897 ± 0.002b 0.2521 ± 0.002b 0.1280 ± 0.002c 0.1018 ± 0.002d 
2.0–3.0 0.5202 ± 0.002b 0.4212 ± 0.002b 0.3670 ± 0.002b 0.3362 ± 0.002b 0.2956 ± 0.002b 0.1625 ± 0.002c 0.1067 ± 0.002d 
3.5–5.0 0.4491 ± 0.002c 0.3596 ± 0.002c 0.3369 ± 0.002b 0.2917 ± 0.002b 0.2689 ± 0.002b 0.2532 ± 0.002c 0.1271 ± 0.002d 
Grain size/ mmTime/d
1234567
1.0–2.0 0.5694 ± 0.002a 0.5342 ± 0.002a 0.3824 ± 0.002a 0.2897 ± 0.002b 0.2521 ± 0.002b 0.1280 ± 0.002c 0.1018 ± 0.002d 
2.0–3.0 0.5202 ± 0.002b 0.4212 ± 0.002b 0.3670 ± 0.002b 0.3362 ± 0.002b 0.2956 ± 0.002b 0.1625 ± 0.002c 0.1067 ± 0.002d 
3.5–5.0 0.4491 ± 0.002c 0.3596 ± 0.002c 0.3369 ± 0.002b 0.2917 ± 0.002b 0.2689 ± 0.002b 0.2532 ± 0.002c 0.1271 ± 0.002d 

Determination of leaching toxicity of the CMC-g-AA sludge polymer

The USEPA TCLP is a widely used toxicity identification method that simulates the co-disposal scenario of hazardous waste and municipal solid waste, where hazardous waste substances leach in the organic acid environment in the landfill. In this paper, we will put the sludge polymer with and without 5% nZVI in a simulated environment of pH = 4 to detect its leaching toxicity.

Heavy metal leaching toxicity is the main indicator of the effect of nZVI treatment. As shown in Figure 10, the leaching of heavy metals concentration such as Cu, Cr, Zn, Cd and Pb in the sludge polymer without nZVI is very high, especially, the leaching amount of Cu, Zn and Cr greatly exceeds the hazardous waste leaching standard, while the leaching amount of heavy metals in the sludge polymer after adding nZVI is significantly reduced after 18 h of extraction. TCLP method can extract heavy metals in exchangeable and carbonate bound states (Sialelli et al. 2010). From the leaching concentration of heavy metals in the figure, almost all heavy metals are below the leaching toxicity standard of EPA, indicating that the potential risk of heavy metals contained in the sludge polymer is further reduced under the reduction of nZVI. In organic acid environments, a large number of heavy metals in sludge polymer dissolve and react with nZVI. Part of the metal reduces from high price to low valence state, and adsorbs in nZVI and sludge polymer in the presence of nZVI oxide. Moreover, part of the metal ions of Cu and Pb are reduced to generate single matter adsorption on the nZVI surface. Therefore, the sludge polymer greatly reduces the amount of heavy metal leaching by adding nZVI, reducing the potential impact on the environment.

Figure 10

Effect of sludge polymer with or without nZVI on leaching concentration of heavy metals.

Figure 10

Effect of sludge polymer with or without nZVI on leaching concentration of heavy metals.

Close modal

Degradation performance test of the CMC-g-AA sludge polymer

Figure 11 shows the experimental curves of the degradation properties of the CMC-g-AA sludge polymer in 32 and 40 °C soil. After 54 days of degradation, the sludge polymer was degraded by 32.78% under 32 °C conditions; at 40 °C, the degradation ratio was 48.71%. The degradation rate under 40 °C conditions was significantly higher than 32 °C, indicating that temperature has a great effect on the degradation of this sludge polymer, probably because the higher the temperature, the more favourable the reproduction and growth of microorganisms in the soil. In environments with more microbes, carboxymethyl cellulose is decomposed first, and polyacrylic branches with molar mass below 1500 g/mol after chain break are able to directly cross the microbial cell wall in contact with active enzymes within the cell and be degraded. This sludge polymer can be partially biodegradable, which facilitates the post-treatment of a sludge polymer that does no longer have water absorption capacity after multiple uses.

Figure 11

Experimental curves of the degradation properties of the CMC-g-AA sludge polymer in the soil.

Figure 11

Experimental curves of the degradation properties of the CMC-g-AA sludge polymer in the soil.

Close modal

13C-Nuclear magnetic resonance

Figure 12 shows 13C-nuclear magnetic resonance (13C-NMR) images of the sludge and CMC-g-AA sludge polymer (CMC:AA = 1:12), and Table 5 shows the main chemical shifts of each carbon-containing functional group. Both the sludge and CMC-g-AA sludge polymer mainly comprised aromatic and aliphatic carbons, with both aliphatic carbons accounting for more than 80%. The percentage of oxygen-containing functional groups of the sludge is 46.56%, whereas that of the sludge polymer increases to 65.18%, and the increase in oxygen-containing functional groups indicates the enhanced hydrophilicity of sludge polymer (Qu et al. 2018). In Figure 12 and Table 3, oxygen-containing functional groups such as carbohydrates increase from 13.43 to 21.12%, carboxy carbon from 15 to 26.58% and carbonyl carbon from 1.89 to 4.63%, all of which have a substantial increase. This shows better hydrophilicity of sludge polymer, confirming the conclusion of infrared spectral analysis in Figure 1. Additionally, the peaks of the two curves are roughly the same, as shown in Figure 12, with the difference that the curve of sludge polymer has symbolic peaks of carbonyl at 175 ppm. The reason may be that the crosslinking reaction of CMC-g-AA gel forms carbonyl groups.

Figure 12

13C-NMR spectra of sludge and CMC-g-AA sludge polymer.

Figure 12

13C-NMR spectra of sludge and CMC-g-AA sludge polymer.

Close modal
Table 5

Functional groups from the 13C-NMR spectra

SampleAliphatic carbon (0–45 ppm)Methoxyl group (45–63 ppm)Ccrnohydrates (63–93 ppm)Aromatic carbon (93–148 ppm)Oxygen substituted (148–165 ppm)Carboxyl (165–187 ppm)Carbonyl (187–220 ppm)Aromaticity %Aliphatucity %Oxygen-containing functional groups %
Sludge 42.19 13.43 13.43 11.25 2.81 15.00 1.89 14.06 85.94 46.56 
CMC-g-AA sludge polymer 23.31 10.81 21.12 11.51 2.13 26.58 4.63 13.64 86.36 65.18 
SampleAliphatic carbon (0–45 ppm)Methoxyl group (45–63 ppm)Ccrnohydrates (63–93 ppm)Aromatic carbon (93–148 ppm)Oxygen substituted (148–165 ppm)Carboxyl (165–187 ppm)Carbonyl (187–220 ppm)Aromaticity %Aliphatucity %Oxygen-containing functional groups %
Sludge 42.19 13.43 13.43 11.25 2.81 15.00 1.89 14.06 85.94 46.56 
CMC-g-AA sludge polymer 23.31 10.81 21.12 11.51 2.13 26.58 4.63 13.64 86.36 65.18 

X-ray diffraction analysis

The detailed descriptions of X-ray diffraction analysis are provided in Supplementary data, Text S2 and Fig. S2.

Thermogravimetric analysis

The detailed descriptions of Thermogravimetric analysis are provided in Supplementary data, Text S3 and Fig. S3.

Economic feasibility analyses

Table 6 shows the economic feasibility analysis of the sludge polymer. The calculation can conclude that the total cost of treatment per kg of sludge is RMB 0.68. This method reduces the cost of sludge treatment by traditional methods, and relatively reduces the cost of large mechanical equipment. Therefore, the sludge polymer is a new type of sludge treatment method which can be applied in practice.

Table 6

Economic feasibility analyses of the sludge polymer

Chemical agentsOptimal dosage (g/kg dry sludge)Reagent cost (CNY/kg dry sludge)
Cost of drug materials CMC 100 0.05 
AA 1200 0.2 
Potassium persulfate 0.1 
MBA 115 0.08 
nZVI 50 0.07 
 Add up to: 0.5 
Water charges Water quantity required per kg of sludge (kg) Cost (CNY) 
10 0.08 
Electric charges Electricity required per kg of sludge (degree) Cost (CNY) 
0.5 0.1 
Total cost 0.68 
Chemical agentsOptimal dosage (g/kg dry sludge)Reagent cost (CNY/kg dry sludge)
Cost of drug materials CMC 100 0.05 
AA 1200 0.2 
Potassium persulfate 0.1 
MBA 115 0.08 
nZVI 50 0.07 
 Add up to: 0.5 
Water charges Water quantity required per kg of sludge (kg) Cost (CNY) 
10 0.08 
Electric charges Electricity required per kg of sludge (degree) Cost (CNY) 
0.5 0.1 
Total cost 0.68 

This study draws the following conclusions: The addition of CMC-g-AA gel helps improve the water absorption and retention performance of the sludge. According to the agroforestry requirement of pH range of 5–7, 50% is the best CMC-g-AA gel content. Before water absorption and after water loss, the content of nitrogen, phosphorus and organic matter of sludge polymer changes only slightly and the sludge polymer can promote plant root vitality and decrease H2O2 content as compared. CMC: AA = 1:12 is the best ratio. Moreover, the addition of nZVI reduces the leaching concentration of heavy metals in the sludge. In the water absorption performance experiment, the smaller the particle size, the faster the water absorption rate of sludge polymer and the shorter the time to reach the liquid absorption equilibrium. The water absorption ratio decreases with an increase in the repeated water absorption times. With a decrease in the particle size, the water retention rate of sludge polymer is significantly reduced and the water loss rate is significantly increased. FTIR analysis shows that the number of hydrophilic groups such as –OH and –COOH increases and better crystalline components such as amides appear with the addition of gelatin, all of which are beneficial to the water absorption of sludge polymer. The 13C-NMR analysis shows that the gel increased sludge polymer oxygen group.

The authors gratefully acknowledge Taiyuan University of Technology for providing experimental conditions.

The research was funded by the program of the Key Research and Development Projects (social development field) of Shanxi Province, China (201803D31039).

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

The authors declare there is no conflict.

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Supplementary data