Abstract

Removal of fluoride from drinking water by polyaluminum chloride-chitosan (PACl-Ch) composite coagulant was studied in a batch system. Two types of PACl-Ch coagulant were synthetized as PACl-Ch I and PACl-Ch II with chitosan to aluminum (Ch:Al) weight ratios of 0.5 and 1.0, respectively, and were used for defluoridation under different operating conditions. The composite coagulants were found to be more efficient than both PACl and chitosan. By an increase in the Ch:Al ratio from 0.5 to 1.0, the composite coagulant provided a little more efficiency of fluoride removal and lower residual level of Al. The optimum conditions of pH for fluoride removal by PACl-Ch I and PACl-Ch II were 8.0 and 7.5, respectively. Fluoride removal by the coagulants was not significantly affected by natural organic matter and turbidity, but was declined by high concentrations of common ions. To achieve desirable fluoride levels (lower than 1.0 mg/L) for natural water samples with fluoride levels of 2.0–2.9 mg/L, the required dosage of the composite coagulants were determined to be as low as 3–5 mgAl/L. The PACl-Ch coagulants demonstrated high efficiency for fluoride removal as well as low Al residual level (mainly lower than 0.2 mg/L) in a wide range of operating conditions.

INTRODUCTION

Among the elements, fluorine ranks 13th in the Earth's crust in terms of abundance. Fluoride, as the simplest and most abundant anion of fluorine, exists in a number of minerals, such as fluorspar, cryolite and fluorapatite. Fluoride enters into water environments as well as the food chain through both natural resources (contact with rocks and soil) and human activity (mainly wastewater of industries using fluoride-containing materials such as aluminum, phosphate fertilizer, steel, glass fiber, brick, tile, ceramic, etc.), but elevated levels of fluoride usually occur in the groundwater resources that are located in contact with fluoride-rich rocks and soil (Fawell et al. 2006; Zhao et al. 2010; Bhatnagar et al. 2011). Although fluoride is an essential micronutrient and its adequate intake protects teeth against caries, excess intake of fluoride results in adverse health effects, most notably damage to bone tissue including skeletal and dental fluorosis (Mesdaghinia et al. 2010; WHO 2011; Abtahi et al. 2016).

Drinking water is one of the main routes of fluoride absorption. In some countries, public water supply systems are fluoridated to increase the fluoride level of drinking water in the range of 0.7–1.2 mg/L, depending on the average maximum daily air temperature. On the other hand, fluoride is one of the few chemicals causing widespread health effects as a consequence of exposure to their elevated levels through drinking water. Based on the dual role of fluoride, the World Health Organization (WHO) has set a guideline value of 1.5 mg/L, as well as a recommended minimum level of 0.5 mg/L for the mineral in drinking water (WHO 2011; Mohebbi et al. 2013; Abtahi et al. 2015; Budyanto et al. 2015; He et al. 2015).

The well-known methods for fluoride removal from drinking water include adsorption by activated alumina and bone char, ion exchange, and membrane processes. These methods are not effective enough due to high capital investment and running costs and low removal efficiency. As a result, attempts are continuing to develop high efficiency processes to remove fluoride from drinking water economically (Fawell et al. 2006; Mesdaghinia et al. 2010; Crittenden et al. 2012; Chai et al. 2013; Yang et al. 2017). Coagulation (or precipitation) has been one of the most promising methods for fluoride removal from drinking water. The process changes the form of materials dissolved in water into solid particles that are easily separated through sedimentation or depth filtration. Polyaluminum chloride (PACl) as a new coagulant has been taken into more consideration for removal of a range of pollutants from water and wastewater. PACl consists of polymeric aluminum species as well as monomers with the dominant species of ( or Al13). The polymeric species have been shown to be a more effective coagulant than the traditional ones such as aluminum sulfate and ferric chloride in terms of removal efficiency and performance flexibility under a wide range of water quality conditions (Nigussie et al. 2007; Zhu et al. 2007; Kalantary et al. 2010; Ingallinella et al. 2011; Liu et al. 2015). Recent studies indicated that a combination of coagulants with polyelectrolytes, silica or chelating agents in a manner to produce innovative hybrid or composite coagulants were mainly successful attempts, so that the composite coagulants provided superior effectiveness in the removal of the target pollutants (Ng et al. 2012, 2013; Tolkou & Zouboulis 2015; Hui et al. 2016).

In this study, by incorporating chitosan into the formulation of PACl, a composite coagulant PACl-chitosan (PACl-Ch) was synthesized and then used for fluoride removal from synthetic and natural waters. The effects of water quality parameters including fluoride concentration, pH, natural organic matter (NOM), turbidity and common ions as well as coagulant dosage were determined. The composite coagulant was also compared to the initial components in terms of fluoride removal efficiency and residual chemicals.

MATERIALS AND METHODS

Preparation of coagulants

All the chemicals used for the preparation of coagulants and synthetic water samples were of analytical grade. PACl and PACl-Ch were synthesized as explained by Ng et al. (2012). In order to prepare 0.1 M PACl solution with OH:Al molar ratio of 2.0, a volume of 50 mL of 0.5 M AlCl3.6H2O was titrated by 100 mL of 0.5 M NaOH at a titration rate of 30 mL/h under rapid stirring at 75°C and the final volume was then adjusted to 250 mL using deionized water. Stock chitosan solution of 500 mg/L was prepared by dissolving 100 mg of chitosan in 10 mL of 0.1 M HCl and then adjusting the final volume to 200 mL using deionized water. Using the PACl and chitosan solutions, two types of PACl-Ch composite coagulant were synthesized: PACl-Ch I and PACl-Ch II with chitosan to aluminum (Ch:Al) weight ratios of 0.5 and 1.0, respectively. To prepare PACl-Ch I and PACl-Ch II, a volume of 100 mL of PACl was respectively titrated by 270 and 540 mL of chitosan solution at a titration rate of 30 mL/h under rapid stirring at 55 °C. Al content of PACl-Ch I and PACl-Ch II solutions were calculated (and also measured) to be 729 and 422 mg/L, respectively. In this study, Fourier transform infrared (FT-IR) analysis was used to characterize the composite coagulants. FT-IR spectra of the PACl-Ch coagulants indicated characteristics similar to those described by Ng et al. (2012) indicating chemical combination of PACl and chitosan (data not shown).

Water samples

Both synthetic and natural water samples were examined in this study. The synthetic water samples were prepared using a stock solution of 100 mg/L NaF and deionized water. Initial pH of synthetic samples was adjusted to target values with a pH meter (Metrohm, Model 827 pH laboratory, Switzerland) using 0.1–1.0 M HCl and/or 0.1–1.0 M NaOH. A number of six natural water samples with elevated fluoride levels (2.0–2.9 mg/L) were collected from groundwater resources of Bushehr Province, in the south of Iran, and used in the coagulation experiments without any modification of water quality parameters. Physico-chemical characteristics of the natural water samples are given in Table 1.

Table 1

Physico-chemical characteristics of the natural water samples

Sample no.Fluoride level (mg/L)Total dissolved solids (TDS) (mg/L)Turbidity (NTU)pH
2.0 1,798 1.5 7.5 
2.2 1,680 0.6 7.9 
2.5 2,106 0.9 7.7 
2.6 1,987 1.1 7.8 
2.8 2,219 0.7 7.9 
2.9 2,355 0.8 7.8 
Sample no.Fluoride level (mg/L)Total dissolved solids (TDS) (mg/L)Turbidity (NTU)pH
2.0 1,798 1.5 7.5 
2.2 1,680 0.6 7.9 
2.5 2,106 0.9 7.7 
2.6 1,987 1.1 7.8 
2.8 2,219 0.7 7.9 
2.9 2,355 0.8 7.8 

Coagulation experiments

Coagulation experiments were conducted using a jar test equipment (Aztec Environmental Control Limited, UK) at laboratory temperature (20 ± 2 °C) in three subsequent stages: (1) rapid mixing to disperse the coagulant and promote collisions for 3 min at 100 rpm, (2) slow mixing or flocculation to agglomerate small particles into settleable flocs for 15 min at 45 rpm and (3) quiescence to settle flocs for 30 min. In all the coagulation experiments, the target fluoride level was considered to be 1.0 mg/L due to the nutritional role of fluoride.

Effects of coagulant type (primary or composite), coagulant dosage, pH, fluoride concentration, common minerals, NOM, and turbidity on fluoride removal were studied using synthetic water samples. Most of the experiments were conducted in initial fluoride concentrations of 4 mg/L (except for the study on the effect of fluoride concentration), initial pH value of 7.0 (except for the study on the effect of pH) and coagulant dosage of 7 mgAl/L (except for the study on the effect of coagulant dosage and fluoride concentration).

The effects of coagulant type (PACl, chitosan, PACl-Ch I and PACl-Ch II) and dosage were determined in the coagulant dosage range of 1–10 mgAl/L (for chitosan 1–10 mg/L). These experiments indicated that the hybrid coagulants were much more efficient than the primary components, therefore the other experiments were only continued with the hybrid coagulants. The effect of fluoride concentration was studied at the initial concentration range of 3–20 mg/L and coagulant dosage of 10 mgAl/L. To determine the optimum condition of pH, the initial pH value was varied from 2 to 12. The effect of common minerals on fluoride removal was studied by the addition of NaCl and Na2SO4 in the concentration range of 500–4,000 mg/L. The effect of NOM on fluoride removal was evaluated by using humic acid in the initial concentration range of 1–3 mg C/L. In order to investigate the effect of turbidity on defluoridation, bentonite clay was used to adjust initial turbidity of synthetic water samples in the range of 3–30 NTU.

Defluoridation of natural water samples (without pH adjustment) was examined using both the hybrid coagulants in the dosage range of 2–7 mgAl/L to determine the effect of natural water quality on the required dosage for achievement of fluoride target level.

Analytical methods

After each experiment, an outlet sample was gently taken from the supernatant of jar test vessels and then prepared for measurement of the intended water quality parameters. All the water quality parameters including fluoride, Al, total organic carbon (TOC), pH, turbidity, and NOM (UV absorbance at 254 nm, UV254) were measured according to the instructions of Standard Methods (APHA AWWA & WEF 2005). To control analytical quality, blank and standard solutions were also examined between samples.

RESULTS AND DISCUSSION

Comparison of coagulants

Figure 1 shows fluoride removal from water samples using PACl, chitosan, PACl-Ch I and PACl-Ch II as a function of coagulant dosage. The composite coagulants provided higher defluoridation efficiency in comparison to PACl and chitosan. The highest fluoride removal efficiency was recorded by PACl-Ch II, being 94.3% at a dosage of 10 mgAl/L. The target fluoride level (lower than 1.0 mg/L) was only achieved by PACl-Ch I and PACl-Ch II at dosages of 7.0 and 5.0 mgAl/L, respectively. The defluoridation capability of chitosan was much lower than that of the other coagulants, so that at the highest dosage (10 mg/L), the fluoride level deceased from 4.0 to 3.6 mg/L which was equal to the fluoride removal efficiency of 11%.

Figure 1

Defluoridation of water samples using PACl, chitosan, PACl-Ch I and PACl-Ch II as a function of coagulant dosage (initial concentration of fluoride: 4 mg/L).

Figure 1

Defluoridation of water samples using PACl, chitosan, PACl-Ch I and PACl-Ch II as a function of coagulant dosage (initial concentration of fluoride: 4 mg/L).

One of the main concerns regarding the application of new water treatment methods is their adverse side effects on non-target water quality parameters, including residual chemicals. In this study, residual coagulants in outlet samples were measured at different coagulant dosages as effluent Al and TOC concentrations (Figure 2). From the viewpoint of residual chemicals, the composite coagulants were also superior to their primary materials, so that for all dosages of PACl-Ch II, the residual Al concentration was lower than the Iranian drinking water quality standard for Al, being 0.2 mg/L, and the increase of effluent TOC was negligible. In outlet water samples, the residual Al and TOC of PACl-Ch I were considerably lower than the residuals of PACl and chitosan, respectively, but slightly higher than those of PACl-Ch II. As can be seen in Figure 2(b), chitosan increased the TOC of outlet water samples between 0.1 and 1.4 mg C/L. Although there are not any drinking water quality standards for TOC, the elevated TOC levels from chitosan can decline drinking water quality through increased microbial regrowth in the water distribution network (WHO 2011; Crittenden et al. 2012).

Figure 2

Residual coagulants in outlet water samples at different coagulant dosages as effluent concentrations of Al (a) and TOC (b).

Figure 2

Residual coagulants in outlet water samples at different coagulant dosages as effluent concentrations of Al (a) and TOC (b).

Effect of fluoride concentration

Figure 3 shows the defluoridation efficiency of the composite coagulants as a function of initial fluoride level. By increasing the fluoride level from 3 to 20 mg/L, the fluoride removal efficiency by PACl-Ch I and PACl-Ch II drastically fell from 93 and 95% to 30 and 37%, respectively. On the contrary, fluoride removal capacity (mg removed fluoride per g used coagulant) of the coagulants directly increased by fluoride level in a nonlinear mode (Figure 3). Although adsorption is only one of the mechanisms of fluoride removal by the composite coagulants, along with some other mechanisms including Al-F complexation, Al-F-OH coprecipitation, etc., the statement of fluoride removal capacity (qe) as a function of effluent concentration (Ce) in a manner similar to adsorption isotherm provides useful information for full-scale application of the coagulants as well as an efficient measure to compare different coagulants and adsorbents for fluoride. The fluoride removal data were characterized using four adsorption isotherm models including the Freundlich, Langmuir, Redlich–Peterson and Temkin models presented below in Equations (1)–(4), respectively (Naddafi & Saeedi 2009; Abtahi et al. 2013; Dehghani et al. 2015; Naddafi et al. 2016):  
formula
(1)
 
formula
(2)
 
formula
(3)
 
formula
(4)
where qe (mg/g) is the removal capacity, Ce (mg/L) is the effluent concentration, KF and n are the Freundlich model constants indicating removal capacity and intensity, respectively, qm (mg/g) is the maximum removal capacity; b (L/mg) is the Langmuir constant as a function of the process energy, KRP (L/g), aRP ((L/mg)γ) and γ (dimensionless) are the Redlich–Peterson constants, R (8.314 J/mol·K) is the universal gas constant, T (K) is the absolute temperature, bT (J/mol) is the Temkin constant of process heat and AT (L/g) is the Temkin binding constant.
Figure 3

Defluoridation efficiency and capacity by the composite coagulants as a function of initial fluoride level.

Figure 3

Defluoridation efficiency and capacity by the composite coagulants as a function of initial fluoride level.

Figure 4 illustrates isotherm profiles of fluoride removal by PACl-Ch I and PACl-Ch II and their fitness with the isotherm models. Isotherm parameters of fluoride removal by PACl-Ch I and PACl-Ch II are given in Table 2. According to Figure 4 and Table 2, fluoride removal by both the coagulants were best described by the Redlich–Peterson model (R2 > 0.99). Based on the Redlich–Peterson model, at an outlet target level of 1.0 mg/L, the fluoride removal capacities of PACl-Ch I and PACl-Ch II were predicted to be 97.8 and 92.6 mg/g, respectively. The fluoride removal data were also a good fit with the Langmuir model (R2 > 0.98). The Langmuir parameter qm is usually applied as the primary indicator of coagulant and adsorbent capacities for the same contaminant. The parameter qm of PACl-Ch I and PACl-Ch II for fluoride were estimated to be 150.7 and 162.4 mg/g, respectively. The maximum removal capacities (qm) of fluoride by different coagulants and adsorbents and applied experimental conditions are given in Table 3. Although comparing their results are problematic due to unequal experimental conditions employed in different studies, the maximum removal capacities of fluoride by PACl-Ch I and PACl-Ch II far exceed those by most the other coagulants and adsorbents; consequently, PACl-Ch I and PACl-Ch II could be considered as a promising alternative for defluoridation of water and wastewater.

Table 2

Isotherm parameters of fluoride removal by PACl-Ch I and PACl-Ch II

Isotherm modelsParametersPACl-Ch IPACl-Ch II
Freundlich n 5.80 5.08 
Kf 93.79 91.86 
R2 0.975 0.976 
Langmuir qm 150.7 162.4 
b 1.444 1.082 
R2 0.994 0.981 
Redlich–Peterson KRP 1,673 10,400 
aRP 16.10 111.28 
γ 0.875 0.812 
R2 1.000 0.999 
Temkin AT 232.0 121.5 
bT 139.3 138.4 
R2 0.981 0.938 
Isotherm modelsParametersPACl-Ch IPACl-Ch II
Freundlich n 5.80 5.08 
Kf 93.79 91.86 
R2 0.975 0.976 
Langmuir qm 150.7 162.4 
b 1.444 1.082 
R2 0.994 0.981 
Redlich–Peterson KRP 1,673 10,400 
aRP 16.10 111.28 
γ 0.875 0.812 
R2 1.000 0.999 
Temkin AT 232.0 121.5 
bT 139.3 138.4 
R2 0.981 0.938 
Table 3

Maximum removal capacity (qm) and experimental conditions for removal of fluoride by different adsorbents and coagulants

Adsorbent/coagulantFluoride level range (mg/L)pHTemperature (°C)Maximum removal capacity (mg/g)Reference
Alumina/carbon nanotubes 50 6.0 25 28.7 Li et al. (2001)  
Activated alumina (γ-Al2O315–100 5.0–6.0 30 16.3 Ku & Chiou (2002)  
Activated alumina (Grade OA-25) 2.5–14 7.0 – 1.45 Ghorai & Pant (2004)  
Activated titanium rich bauxite 2–50 6.0 27 ± 0.5 4.1 Das et al. (2005)  
Montmorillonite clay Neutral 30 1.5 Karthikeyan et al. (2005)  
Aluminum hydroxide 5–30 7.0 ± 0.3 23 ± 2 7.0 Shimelis et al. (2006)  
Heat treated aluminum hydroxide 5–30 7.0 ± 0.3 23 ± 2 23.7 Shimelis et al. (2006)  
Alum-impregnated activated alumina 1–35 6.5 Room temperature 40.7 Tripathy et al. (2006)  
Quick lime 10–50 – 25 ± 2 16.7 Islam & Patel (2007)  
Metal ion loaded natural zeolite 1–20 – 30 4.1 Samatya et al. (2007)  
La(III) incorporated carboxylated chitosan beads 11–19 Neutral 30 22.4 Viswanathan & Meenakshi (2008)  
Zirconium ion impregnated coconut fiber carbon – 4.0 Room temperature 40.0 Sathish et al. (2008)  
Lime stone 0–100 8.0 25 43.1 Jain & Jayaram (2009)  
Aluminum hydroxide impregnated lime stone 0–100 8.0 25 84.0 Jain & Jayaram (2009)  
Granular ferric hydroxide 1–100 6.0–7.0 25 ± 2 7.0 Kumar et al. (2009)  
Iron(III)-tin(IV) mixed oxide 10–50 6.4 ± 0.3 30 ± 2 10.5 Biswas et al. (2009)  
Chemically modified bentonite clay – 7.0 30 ± 2 4.2 Kamble et al. (2009)  
Neodymium modified chitosan 10–100 7.0 30 22.4 Yao et al. (2009)  
Synthetic nano-hydroxyapatite 3–80 5.0–6.0 25 4.6 Gao et al. (2009)  
Nano-AlOOH 3–35 5.2 ± 0.2 25 3.3 Wang et al. (2009)  
Magnesia 10–23 10.1–10.4 Room temperature 2.2 Sundaram et al. (2009)  
Copper oxide coated alumina 10 – 30 ± 1 7.8 Bansiwal et al. (2010)  
Calcium oxide-modified activated alumina 1–1,000 5.5 25 101 Camacho et al. (2010)  
Manganese oxide-modified activated alumina 1–1,000 5.5 25 10.2 Camacho et al. (2010)  
Nano-geothite 5–150 5.75 30 59.0 Mohapatra et al. (2010)  
Synthetic siderite 3–20 4.0–9.0 25 1.8 Liu et al. (2010)  
Nano-magnesia 5–200 7.0 30 ± 2 267.8 Maliyekkal et al. (2010)  
Alumina/chitosan composite 10 Neutral 30 3.8 Viswanathan & Meenakshi (2010)  
Nano-alumina 1–100 6.2 25 ± 2 14.0 Kumar et al. (2011)  
Chitosan-praseodymium complex 5–30 7.0 27 15.9 Kusrini et al. (2015)  
Zirconium metal-organic framework (UiO-66-NH2– – 20 58.8 Lin et al. (2016)  
Amorphous aluminum hydroxide 5–200 7.0 ± 0.1 25 63.9 Zhang & Jia (2016)  
Monetite bundles inlaid in chitosan – – 25 50.0 Shen et al. (2016)  
Al-humic acid-La aerogel composites 10–150 7.0 35 53.8 Liu et al. (2016)  
Iron-aluminum oxide-graphene oxide composite 3–30 – 15 22.0 Kanrar et al. (2016)  
PACl-Ch I 3–20 7.0 20 ± 2 150.7 This study 
PACl-Ch II 3–20 7.0 20 ± 2 162.4 This study 
Adsorbent/coagulantFluoride level range (mg/L)pHTemperature (°C)Maximum removal capacity (mg/g)Reference
Alumina/carbon nanotubes 50 6.0 25 28.7 Li et al. (2001)  
Activated alumina (γ-Al2O315–100 5.0–6.0 30 16.3 Ku & Chiou (2002)  
Activated alumina (Grade OA-25) 2.5–14 7.0 – 1.45 Ghorai & Pant (2004)  
Activated titanium rich bauxite 2–50 6.0 27 ± 0.5 4.1 Das et al. (2005)  
Montmorillonite clay Neutral 30 1.5 Karthikeyan et al. (2005)  
Aluminum hydroxide 5–30 7.0 ± 0.3 23 ± 2 7.0 Shimelis et al. (2006)  
Heat treated aluminum hydroxide 5–30 7.0 ± 0.3 23 ± 2 23.7 Shimelis et al. (2006)  
Alum-impregnated activated alumina 1–35 6.5 Room temperature 40.7 Tripathy et al. (2006)  
Quick lime 10–50 – 25 ± 2 16.7 Islam & Patel (2007)  
Metal ion loaded natural zeolite 1–20 – 30 4.1 Samatya et al. (2007)  
La(III) incorporated carboxylated chitosan beads 11–19 Neutral 30 22.4 Viswanathan & Meenakshi (2008)  
Zirconium ion impregnated coconut fiber carbon – 4.0 Room temperature 40.0 Sathish et al. (2008)  
Lime stone 0–100 8.0 25 43.1 Jain & Jayaram (2009)  
Aluminum hydroxide impregnated lime stone 0–100 8.0 25 84.0 Jain & Jayaram (2009)  
Granular ferric hydroxide 1–100 6.0–7.0 25 ± 2 7.0 Kumar et al. (2009)  
Iron(III)-tin(IV) mixed oxide 10–50 6.4 ± 0.3 30 ± 2 10.5 Biswas et al. (2009)  
Chemically modified bentonite clay – 7.0 30 ± 2 4.2 Kamble et al. (2009)  
Neodymium modified chitosan 10–100 7.0 30 22.4 Yao et al. (2009)  
Synthetic nano-hydroxyapatite 3–80 5.0–6.0 25 4.6 Gao et al. (2009)  
Nano-AlOOH 3–35 5.2 ± 0.2 25 3.3 Wang et al. (2009)  
Magnesia 10–23 10.1–10.4 Room temperature 2.2 Sundaram et al. (2009)  
Copper oxide coated alumina 10 – 30 ± 1 7.8 Bansiwal et al. (2010)  
Calcium oxide-modified activated alumina 1–1,000 5.5 25 101 Camacho et al. (2010)  
Manganese oxide-modified activated alumina 1–1,000 5.5 25 10.2 Camacho et al. (2010)  
Nano-geothite 5–150 5.75 30 59.0 Mohapatra et al. (2010)  
Synthetic siderite 3–20 4.0–9.0 25 1.8 Liu et al. (2010)  
Nano-magnesia 5–200 7.0 30 ± 2 267.8 Maliyekkal et al. (2010)  
Alumina/chitosan composite 10 Neutral 30 3.8 Viswanathan & Meenakshi (2010)  
Nano-alumina 1–100 6.2 25 ± 2 14.0 Kumar et al. (2011)  
Chitosan-praseodymium complex 5–30 7.0 27 15.9 Kusrini et al. (2015)  
Zirconium metal-organic framework (UiO-66-NH2– – 20 58.8 Lin et al. (2016)  
Amorphous aluminum hydroxide 5–200 7.0 ± 0.1 25 63.9 Zhang & Jia (2016)  
Monetite bundles inlaid in chitosan – – 25 50.0 Shen et al. (2016)  
Al-humic acid-La aerogel composites 10–150 7.0 35 53.8 Liu et al. (2016)  
Iron-aluminum oxide-graphene oxide composite 3–30 – 15 22.0 Kanrar et al. (2016)  
PACl-Ch I 3–20 7.0 20 ± 2 150.7 This study 
PACl-Ch II 3–20 7.0 20 ± 2 162.4 This study 
Figure 4

Isotherm profiles of fluoride removal by PACl-Ch I (a) and PACl-Ch II (b) and their fitness with the isotherm models.

Figure 4

Isotherm profiles of fluoride removal by PACl-Ch I (a) and PACl-Ch II (b) and their fitness with the isotherm models.

Effect of pH

Figure 5 shows the effect of pH on fluoride removal from water samples by the composite coagulants. The optimum pH values for fluoride removal by PACl-Ch I and PACl-Ch II were determined to be 8.0 and 7.5, respectively. Similar patterns were observed for the effect of pH on fluoride removal by both the coagulants in a manner that by increasing pH from 2 to near neutral pH, fluoride removal was constantly enhanced and then the trend was reversed until pH of 12. PACl-Ch II exhibited a little more fluoride removal than PACl-Ch I over pH range of 2–12. The optimum pH values obtained in this study were near to the natural pH of water resources, therefore the coagulants would be applied for fluoride removal from natural waters with the lowest need for pH adjustment as well as the lowest corrosion or scaling in water treatment structures and equipment. The residual Al levels of outlet samples in neutral pH were also lower than those in acidic conditions (Crittenden et al. 2012; Abtahi et al. 2015). This observation can be explained by the effect of pH on the distribution of Al species of PACl, so that the near neutral pH region Al13 (polymeric Al) fraction increases, which is the most active Al species for the coagulation process with high positive charge, stable structure and strong binding affinity (Hu et al. 2005; He et al. 2016). Similarly, Samadi et al. (2009) and He et al. (2016) reported that the optimum fluoride removal by PACl was achieved near neutral pH.

Figure 5

Effect of pH on fluoride removal from water samples by the composite coagulants (initial concentration of fluoride: 4 mg/L).

Figure 5

Effect of pH on fluoride removal from water samples by the composite coagulants (initial concentration of fluoride: 4 mg/L).

Effect of common impurities

Natural waters contain impurities that may have an undesirable influence on water treatment methods, mainly through a reaction with treating agent and/or preventing reaction or contact of treating agent with target pollutants (WHO 2011; Crittenden et al. 2012; Liu et al. 2015). The effects of mineral impurities, NOM, and turbidity on fluoride removal by composite coagulants are presented in Figure 6. Among the impurities, only the minerals (NaCl and Na2SO4) caused some negative effects on fluoride removal, so that by increasing TDS from 0 to 4,000 mg/L through adding NaCl and Na2SO4, defluoridation efficiency declined from 85% to 68 and 70%, respectively, for PACl-Ch I and from 87% to 70 and 72% for PACl-Ch II, respectively. NOM and turbidity not only did not exhibit any significant effect on fluoride removal, but also were efficiently removed along with fluoride. The removal efficiencies of NOM and turbidity by PACl-Ch I were in the ranges of 60–71 and 92–95%, respectively. The corresponding values by PACl-Ch II were also determined to be 70–75 and 90–96%, respectively. NOM and turbidity removal by Al salt has been well demonstrated in the literature and practice (Ng et al. 2013; Tolkou & Zouboulis 2015; Hui et al. 2016). Liu et al. (2013) reported that at pH > 8 fluoride caused little adverse effects on turbidity removal by Al coagulation over a wide range of fluoride to Al ratios, but at pH < 8 fluoride significantly affected turbidity removal at fluoride to Al ratios higher than 2:10 mainly through decreased zeta potential, smaller flocs, and elevated residual Al levels. The simultaneous removal of fluoride, NOM, and turbidity is considered an important advantage for the coagulants, because such a multifunctional alternative by reducing the number of treatment units can deeply decline the capital and running costs of water supply systems (Crittenden et al. 2012).

Figure 6

Effects of common impurities on fluoride removal by the composite coagulants: (a) mineral impurities, (b) NOM, and (c) turbidity (initial concentration of fluoride: 4 mg/L).

Figure 6

Effects of common impurities on fluoride removal by the composite coagulants: (a) mineral impurities, (b) NOM, and (c) turbidity (initial concentration of fluoride: 4 mg/L).

Defluoridation of natural water samples

Although the effects of common impurities on fluoride removal by the composite coagulants were determined, the examination of natural water samples could reflect the potential of the coagulation process in real situations more accurately. Figure 7 shows the required dosages of the composite coagulants for defluoridation of natural water samples to reach the target fluoride level. In order to reduce the fluoride level from 2.0 to 2.9 mg/L to the treatment goal (1 mg/L), the required dosages of PACl-Ch I and PACl-Ch II were determined to be as low as 3–5 and 3–4 mgAl/L, respectively. According to Figure 7, in similar experiments, the outlet fluoride levels of the natural waters (0.47–0.95 mg/L) were slightly higher than those of the synthetic samples (0.40–0.75 mg/L). This observation was in accordance with the finding from the effect of common mineral impurities (shown above). These results indicated that the defluoridation performance of the composite coagulants was not significantly affected by the natural water impurities and thus the process could efficiently treat natural waters in a way close to synthetic ones.

Figure 7

Required dosages of the composite coagulants for defluoridation of natural water samples to reach the target fluoride level (lower than 1.0 mg/L) and comparison of the outlet levels of natural and synthetic water samples: (a) PACl-Ch I and (b) PACl-Ch II.

Figure 7

Required dosages of the composite coagulants for defluoridation of natural water samples to reach the target fluoride level (lower than 1.0 mg/L) and comparison of the outlet levels of natural and synthetic water samples: (a) PACl-Ch I and (b) PACl-Ch II.

CONCLUSIONS

Two types of PACl-Ch composite coagulant as PACl-Ch I and PACl-Ch II were synthetized and applied for the defluoridation of synthetic and natural waters. PACl-Ch II provided slightly more fluoride removal and lower residual Al level than PACl-Ch I. Based on the Langmuir model, the maximum removal capacities of PACl-Ch I and PACl-Ch II for fluoride were 150.7 and 162.4 mg/g, respectively. PACl-Ch I and PACl-Ch II exhibited the optimum fluoride removal at pH values of 8.0 and 7.5, respectively. NOM and turbidity were removed by the coagulants along with fluoride without any considerable effects on defluoridation, but the high content of common minerals reduced fluoride removal efficiency. The required dosages of PACl-Ch I and PACl-Ch II for efficient fluoride removal from natural water samples with fluoride levels of 2.0–2.9 mg/L were determined to be as low as 3–5 and 3–4 mgAl/L, respectively. The PACl-Ch coagulants provided high efficiency and flexibility for fluoride removal from drinking water.

ACKNOWLEDGEMENTS

This research was supported by funding from the Shahid Beheshti University of Medical Sciences (Grant No. 400/123).

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