A study on the removal of organic substances from low-turbidity and low-alkalinity water with metal-polysilicate coagulants

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1 Available online at Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) A study on the removal of organic substances from low-turbidity and low-alkalinity water with metal-polysilicate coagulants Wen Po Cheng a,, Fung Hwa Chi b, Chun Chang Li a, Ruey Fang Yu a a Department of Safety, Health and Environmental Engineering, National United University, Miaoli 360, Taiwan, ROC b Department of Environmental Engineering, Kun Shan University of Technology, Tanian 710, Taiwan, ROC Received 30 March 2007; received in revised form 12 June 2007; accepted 29 June 2007 Available online 4 July 2007 Abstract The coagulants of poly-aluminum-chloride (PAC), poly-aluminum-silicate-chloride (PASiC) and poly-aluminum-ferric-silicate-chloride (PAF- SiC) were prepared in this study to evaluate their coagulation efficiencies and mechanisms in synthetic low-turbidity and low-alkalinity water containing organic matter. The experimental results show that PASiC and PAFSiC could remove the kaolin turbidity of the synthetic water with or without salicylic acid present. On the other hand, when the synthetic water contained both kaolin and humic acid in low turbidity and alkalinity, PAC would remove the turbidity but charge reversal of the colloidal particles would occur easily. Also, effective coagulation was limited to a very narrow dosage range. Conversely, the dosage range for the effective coagulation of both PASiC and PAFSiC was wider, although a higher dosage was required to remove the turbidity of wastewater. Therefore, the effective removal of turbidity was not only related to the kind of coagulant, but also to the types of organic matter. The coagulants PASiC and PAFSiC, particularly, proved themselves to be superior to the PAC in the treatment of low-turbidity water Elsevier B.V. All rights reserved. Keywords: Polyaluminum silicate chloride (PASiC); Low-turbidity water; Humic acid; Salicylic acid 1. Introduction The formation of disinfecting by-products (DBP) which are thought to be carcinogens might result from the chlorination of water with a low concentration of natural organic matter (NOMs). The conventional chemical coagulation process is effective in removing colloidal particles as well as NOMs, such as organic humic acid and fulvic acid [1 3]. Removing the majority of NOMs would greatly reduce the load on the subsequent activated carbon treatment and would decrease the dosage of ozone required [4]. Most domestic and international reports advocate the use of aluminum or ferric salts to remove NOMs [5 7]. The hydrolysis of metal ions is an important reaction for the destabilization of suspended particles and organic matter in coagulation. The hydrolysis of polymeric and monomeric coagulants are quite different [8]. The hydrolysis process using single nucleus metallic salt in coagulation was completed in a short time, which make the coagulation hard to control [9,10]. Corresponding author. Tel.: ; fax: address: cwp@nuu.edu.tw (W.P. Cheng). Usually, precipitates of metal hydroxide will be produced afterwards. On the other hand, the degree of hydrolysis of polymeric salts can be controlled during manufacture, therefore the complicated reactions caused by the hydrolysis of the metal salt in coagulation can be reduced. Consequently, the use of the polymeric salts provides a simpler and more precise way to control the reactions in coagulation [11 14]. However, polymeric coagulants are not effective in neutralizing the surface charges of particles in low-turbidity water, thus failing to form settleable flocs and limiting the efficiency of polymeric coagulants in water treatment [15]. Therefore, in traditional treatment process, coagulation aids such as activated SiO 2 colloid are supplemented to improve the efficiency of turbidity removal. In the 1990s, international companies were trying to develop polyaluminum silicate chloride (PASiC) and polyaluminum ferric silicate chloride (PAFSiC) [6,16 20]. The hydrolyzed products of silicon in activated silica acid and aluminum or ferric ion polymer improves the coagulating capability of the polymer. It is more effective in removing turbidity and color, eliminating NOMs as well as lowering metal concentrations in water [21,22]. Results from previous studies also show that PASiC or PAFSiC has an excellent coagulation efficiency in low-turbidity /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.colsurfa

2 W.P. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) and low-alkalinity raw water than conventional coagulants in the treatment of raw water with low turbidity [14,20]. When both concentrations and types of NOMs in raw water of low turbidity are different, the coagulation mechanism changes accordingly [23 26]. PASiC or PAFSiC, when added directly to raw water, can either be effectively adsorbed onto the surface of colloids, thus neutralizing the surface charge, or react chemically with NOMs to form larger molecules. Both mechanisms change the water treatment efficiency. Most previous studies concentrated on the removal of either NOM or particles from the raw water with higher concentrations of NOMs and turbidity than in natural water. Few studies targeted the coagulation mechanisms for water containing both particulates and NOMs. In this study, a synthetic water with low-turbidity (less than 8 NTU) and low-alkalinity (50 mg/l as CaCO 3 ) prepared with kaolin and humic acid or salicylic acid were used to investigate the removal mechanisms and the influence of turbidity or NOMs on removal efficiency. 2. Methods and experimental procedures 2.1. Preparation of PAC, PASic and PAFSiC coagulants The PASiC and PAFSiC coagulants used in this study were prepared in the laboratory using the method developed by Yang et al. [20] and Gao et al. [21]. The final PASiC and PAFSiC coagulants had a [Al + Fe]/Si ratio of 5 and B value of 1.5 (B = [OH]/[Al + Fe]). Poly-silicic acid solution (Psi) was prepared by adding 23.5 ml 1.5 M HCl into 50 ml 0.5 mol/l SiO 2 solution while mixing rapidly. The mixture ph was adjusted to 2 to yield a mol/l Psi. A ml de-ionized water was added to 40 ml 0.25 M AlCl 3 for PASiC preparation (or 0.25 mol/l AlCl 3 + FeCl 3 solution with Al/Fe = 10/3 for PAF- SiC preparation). Depending on the required [Al + Fe]/Si ratio of 5, 6.08 ml Psi solution was added to the Al or Fe + Al solution and subsequently 30 ml 0.5 M NaOH was added slowly (speed of titration was 0.05 ml/min) to reach the specified B value allowing for metal-silicate polymerization. At the completion of the NaOH addition, the final volume of the solution was 100 ml. The mixture, PASiC and PAFSiC coagulant, was set at 40 C for a 3 h aging process. For the preparation of PAC coagulant at the basicity value 2.5 (B, B = OH /Al 3+ ), 50 ml 0.5 M NaOH solution was added using a peristaltic pump at flow rates of 0.05 ml/min into 50 ml 0.2 M AlCl 3 solution Preparation of stock solutions Humic acid The commercially available humic acid powder with molecular weights ranging from 2000 to 50,000, extracted from brown coal using NaOH (Aldrich Chemical Co., Inc., USA), was used. A stock solution was prepared by dissolving 1.4 g humic acid to 1 L 0.01 mol/l NaClO 4 solution. After adjusting ph to 8.5, the solution was continuously mixed for 6 h until all solids disappeared. The final concentration of the prepared stock solution was 440 mg/l based on total organic carbon (DOC) analysis with a Tekmar-Dohrmann Apollo 9000 TOC analyzer Salicylic acid A common aromatic acid in nature water and is a strong chelating agent, like fulvic acid [27] g salicylic acid was dissolved in 5 ml methanol and placed in a 1-L volumetric flask. After adding 750 ml distilled water and mixing for 10 min, the final volume of the mixture was adjusted with distilled water to 1L The coagulation study With rapid mixing, 1 g kaolin was added to 1 L of the mixture. After resting the mixture for 30 min, 600 ml of the supernatant from each liter of the mixture was decanted. An aliquot of 10 L of the collected supernatant was mixed with g/l NaHCO 3, g/l NaClO 4 and suitable volumes of the stock humic or salicylic acid solution to yield the final synthetic raw water with 50 mg/l CaCO 3 alkalinity, 10 2 M ionic strength and 10 NTU turbidity. The coagulation was carried out with rapid mixing at 100 rpm for 40 s, slow mixing at 20 rpm for 10 min, and followed by a settling period of 10 min. Samples of the treated water were collected at a depth of 3 cm below the surface; they were analyzed for residual turbidity and particle sizes distribution with a HACH-2100 turbidimeter and a Hiac/Royco MC100S particle counter. After having been filtered through 0.45 m filter paper, the filtrate was measured using UV absorbance at 254 nm (UV 254 ) and the DOC The kaolin adsorption study One-liter de-ionized water was used to prepare samples with turbidities of 0, 10, 100, and 600 NTU. After ph adjustment, similar concentrations of either humic acid or salicylic acid were added. After shaking for 30 min, the mixture was filtered through 0.45 m filters and the filtrate was analyzed for DOC or UV 254 to study differences between humic and salicylic acid in the samples. 3. Results and discussion 3.1. Reactions between humic acid and kaolin in synthetic waster The original ph of the prepared synthetic water was 7.5. In order to understand the mutually repulsive reactions between the humic acid and kaolin particles in the synthetic water, zetapotentials of kaolin particles in the solution with or without the humic acid were measured individually in the solution ph range from 3 to 10. The results indicated that without humic acid in low ph, kaolin particles had positive charges, while an increase in ph reversed the surface charge (Fig. 1). In the presence of humic acid, the surface charge below ph 6 changed drastically from positive to negative, while the negative surface charges did not change significantly above ph 6. Hence, the observation proved that when the solution ph was lower than 6, the surface charge was changed by reacting with the humic acid. In comparison, if the solution ph was above 6, the humic acid did not have a signif-

3 240 W.P. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) From above, the result proved that the ph of the synthetic water used in this study was around 7.5, the humic acid was not adsorbed by the kaolin particles before coagulation, there was no mutual interference. Also, the small hydrophilic salicylic acid molecules were not observably adsorped by the kaolin particles. Hence, the synthetic water with low-turbidity (less than 8 NTU (prepared with kaolin)) and low-alkalinity (50 mg/l as CaCO 3 ) was sufficient to determine the influence of NOMs on the removal of turbidity in the coagulation process. Fig. 1. Influence of humic acid on the zeta-potential of kaolin particles in synthetic water with and without humic acid. Table 1 Variation of the measured UV 254 in water samples with 3 ppm humic acid under different ph levels and kaolin turbidity Turbidity (NTU) ph 4 ph 7 ph icant influence on the surface charge of the kaolin particles. Both the kaolin particles and the humic acid molecules with negative surface charges showed intensified repulsion with high ph, so the adsorption reduced and the particles existed independently in the solution. In order to clarify the influence of humic acid, turbidity and ph, as well as whether the humic acid was adsorbed by kaolin particles to change the surface charge in different phs, the values of UV 254 absorbance for the humic acid at different phs and turbidities were measured and the results are presented in Table 1. After kaolin adsorption and filtration through filter papers, the filtrates of the synthetic water showed obvious differences in UV 254 absorbance. At ph 4, higher turbidity reduced the UV 254 absorbance, however, the absorbance did not change significantly from ph 7 to 10. Thus, in acidic conditions, the humic acid adsorbed by kaolin particles resulted in a negative surface charge, while under neutral and alkaline conditions, the humic acid was not adsorbed effectively to change the surface charge The effect of salicylic acid on turbidity removal No significant turbidity removal was observed when the low-turbidity synthetic water was subjected to poly-aluminum chloride (PAC) coagulation at the Al dosage range from 0 to 150 mol/l. This is due to the low particle concentration which redused the collision rate, and thus failing to form settleable flocs. In contrast, 50 mole/l PASiC or PAFSiC effectively removed turbidity. When PASiC or PAFSiC was used, the active silicon contained in the coagulants were apparently hydrolyzed to SiO 2, which increased the collision rate, and resulted in a higher coagulation efficiency. Results shown in Fig. 2 obtained from samples containing both salicylic acid and kaolin particles also indicated that the presence of salicylic acid did not raise the efficiency of PAC in removing turbidity from the low-turbidity and low-alkalinity synthetic water. Furthermore, the presence of small hydrophilic salicylic acid molecules did not affect the high efficiency of PASiC or PAFSiC in removing turbidity or kaolin particles. The highest turbidity removal rate was above 90%, and the residual turbidity in the treated water was only 0.64 NTU Coagulation efficiencies and reaction mechanisms PAC was effective in removing turbidity from the synthetic water with low turbidity and alkalinity containing both humic acid and kaolin (Fig. 3). The observation differs from the water containing salicylic acid above. However, a high dosage of the coagulant caused the reversal of the particle surface charge. Furthermore, the concentration of humic acid also influenced the Fig. 2. Influence of salicylic acid on the turbidity of residual.

4 W.P. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) Fig. 3. Influence of humic acid concentrations on the turbidity of residual. effectiveness of PAC coagulation. When the humic acid concentration was insufficient (Fig. 3a), PAC did not effectively remove turbidity, because the Al-humic colloids formed were not enough to enable effective reaction with the kaolin particles. On the other hand, PASiC or PAFSiC, which has a smaller charge density than PAC [22], had a broader range of effective dosages to facilitate coagulation and enhance the removal of turbidity. However, the effective dosage of PASiC or PAFSiC increased as the concentration of the humic acid increased (Fig. 3), indicating that the humic acid might have consumed some of the PASiC or PAFSiC and influenced the turbidity removal. The consumption of PASiC or PAFSiC by the humic acid is also indicated by the temporary increase in turbidity before reaching the minimum dosage for effective coagulation resulting in higher turbidity than the original. Linear regression was used to analyze the results shown in Fig. 3 comparing various concentrations of humic acid and maximum turbidity difference in the synthetic water coagulated with the three coagulants (Fig. 4). The so-called maximum turbidity difference ( NTU) refers to the difference between the maximum turbidity that can be obtained during the coagulation process and the original turbidity in the untreated synthetic water. The excellent linearity shown in Fig. 4 suggests that, after adding the coagulant, aluminum or ferric first reacted with the humic acid to form M-humic flocs (M represents Al or Fe), and then collided with kaolin particles to initiate adsorption, chemical bridging and charge neutralization. When the quantity of coagulant applied was less than the effective dose, humic acid charges were effectively neutralized, formed relatively small non-settleable M-humic flocs, and caused an increase in turbidity. Therefore, the maximum turbidity difference increased Fig. 4. Relationship between the maximum turbidity difference ( NTU) and the humic acid concentrations.

5 242 W.P. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) proportionally to the acidity of the humic acid in the synthetic water. Further, the three slopes of the linear plots shown in Fig. 4 were different. When the concentration of humic acid increased, the maximum turbidity difference caused by PAC was much smaller than that caused by PASiC or PAFSiC as indicated by a milder slope for PAC. As negatively charged poly-silicates, which were introduced during the manufacturing processes of both PASiC and PAFSiC, reacted with the hydrolyzed aluminum or ferric in the coagulation process, it resulted in the reduction of positive charges [28]. The coagulant dosage must be increased to achieve a satisfactory removal of particles. Before the effective coagulant dosage was reached, the un-neutralized M-humic flocs and the extra quantity of Si(OH) 4 caused a greater maximum turbidity difference for PASiC and PAFSiC than in the case of PAC. Comparing the particle sizes (Fig. 5) with those of synthetic water containing 3 mg/l humic acid (Fig. 3c) indicated that 90 mol/l PAC was the effective dosage. This is clear from the results in Fig. 5a and b which show that after settling, the quantity of large and small flocs was greatly reduced in the supernatant. When 0.90 mol/l PASiC or PAFSiC was used, the turbidity of the treated water was NTU. Further, the particle distribution indicated that small particles of 0.5 m diameter were predominant in the untreated water sample. Since the dosage of PASiC or PAFSiC was insufficient for effective coagulation, it formed non-settleable M-humic flocs that could collide with kaolin particles but the surface charge did not decrease effectively. Hence, the flocs formed were relatively large but non-settleable. As shown in Fig. 5b, the amount of 0.5 m flocs was reduced but the medium (2 m) and the large (5 m) particles obviously increased to cause maximum turbidity. Conversely, when the concentration of coagulant exceeded the effective dosage (Fig. 5c and d), particles with diameter 2 and 5 m were greatly reduced. The concentration of humic acid after coagulation and filtration demonstrated that, before the effective particle removal (Fig. 3c), the concentrations of humic acid in the treated samples had already started declining, and there was a higher turbidity in the treated water than in the original water (Fig. 6a). This supports the suggested coagulation mechanisms. The coagulation was achieved through the formation of M-humic colloids by the applied coagulant and the humic acids; then the colloids reacted with kaolin particles to achieve charge neutralization, thus NOMs were removed through the formation of large settleable flocs. This was proven by the fact that only when the dosage of the coagulant exceeded a minimum dosage for effective coagulation, the phenomenon of effectively turbidity removal occurred. Furthermore, the maximum turbidity difference was proportional to the original concentration of the humic acid in the untreated water sample (Fig. 4). For water samples containing salicylic acid, the reaction mechanism should be different from the samples containing Fig. 5. Particle size distributions in samples containing initial 3 mg NPDOC/L coagulated with various dosages of PAC, PASiC and PAFS.

6 W.P. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) Fig. 6. The removal efficiencies of humic acid and salicylic acid at various dosages. humic acid. When the dosages of PASiC and PAFSiC reached the effective coagulation levels in the water samples containing salicylic acid and kaolin particles, the concentrations of salicylic acid were not greatly reduced along with turbidity removal. Further, the phenomenon of maximum turbidity difference was not observed with salicylic acid as shown in Fig. 2b. These observations were similar as Huang [4] and our previously studies [29] both suggested that salicylic acid did not react directly with the applied coagulant by complex reaction. It was adsorbed by M(OH) 3 before being removed, and the M(OH) 3 was produced during the coagulation process. Thus, the removal efficiency of salicylic acid was only 10 16% and the removal rate was directly related to the coagulant dosage. 4. Conclusions PASiC or PAFSiC, a new type of polymeric coagulant consisting of long-chain polymers of silicate and aluminum or ferric ions, has a larger molecular weight and carries smaller surface charge than PAC. It is capable of removing colloidal particles through adsorption, charge neutralization, and chemical bridging. Results obtained using various organic substances in this study demonstrate that the removal of kaolin particles by coagulation with PASiC or PAFSiC is related to the types of organic matter. In the presence of both humic acid and kaolin particles, PASiC or PAFSiC reacted first with the humic acid to form M-humic colloids, and then with the kaolin particles to neutralize the surface charges to form large settleable flocs. However, when the coagulant dosage was not sufficient to initiate an effective coagulation, the turbidity of the treated water increased to a higher level than the original turbidity of the untreated water. This was mainly when the dosage was insufficient; the M-humic colloids were not sufficient to neutralize the surface charge, thus forming large but non-settleable flocs. When the water to be treated contained salicylic acid and kaolin, the salicylic acid was adsorbed onto the surface of M(OH) 3 first and hence, the removal efficiency of salicylic acid is directly related to the coagulant dosage. Generally, PAC is not effective in neutralizing surface charge, thus it is not effective in coagulation. Once the water to be treated contains various types of organic substances in different concentrations, the coagulation efficiency is different. However, PASiC and PAFSiC at high dosages are effective to remove turbidity from low-turbidity and low-alkalinity water samples. Acknowledgements We thank Mr. Yi-Jui Chen and Miss Ying-Ju Hsien for assistance with laboratory work. The authors acknowledge the financial support of National Science Council, Taiwan, ROC for this work (NSC E ). References [1] G.A. Edwards, A. Amirtharajah, Removing color caused by humic acids, J. AWWA 77 (1985) [2] T.R. Hundt, C.R. O Melia, Aluminum-fulvic acid interactions: mechanisms and applications, J. AWWA 80 (1988) [3] K.E. Dennett, A. Amirtharajah, T.F. Moran, J.P. Gould, Coagulation: its effect on organic matter, J. AWWA 88 (1996) [4] C. Huang, H. Shiu, Interactions between alum and organics in coagulation, Colloids Surf. A 113 (1996) [5] D.S. Wang, L. Hong, H.X. Tang, Removal of humic acid by coagulation with nonoal 13, Water Sci. Technol.: Water Supply 6 (2006) [6] W.P. Cheng, F.H. Chi, R.F. Yu, Evaluation the ability of polyaluminum silicate chloride coagulants for turbidity removal efficiency, Sep. Sci. Technol. 41 (2006) [7] C.Z. Hu, H.J. Liu, J.H. Qu, D.S. Wang, R. Jia, Coagulation behavior of aluminum salts in eutrophic water: significance of Al 13 species and ph control, Environ. Sci Technol. 40 (2006) [8] M. Rebhum, M. Lurie, Control of organic matter by coagulation and flocs separation, Water Sci. Technol. 27 (1993) [9] H.X. Tang, W. Stumm, The coagulating behaviors of Fe(III) polymeric species I, Water Res. 21 (1987) [10] H.X. Tang, W. Stumm, The coagulating behaviors of Fe(III) polymeric species-ii, Water Res. 21 (1987) [11] A.M. Tenny, J. Derka, Hydroxylated ferric sulphate: an aluminum salt alternative, Water Supply 10 (4) (1992) [12] J.Q. Jiang, N.J.D. Graham, C. Harward, Comparison of polyferric sulfate with other coagulants for the removal of algae and algae-derived organic matter, Water Sci. Technol. 27 (1993) [13] J.Q. Jiang, N.J.D. Graham, Preliminary evaluation of the performance of new pre-polymerized inorganic coagulants for lowland surface water treatment, Water Sci. Technol. 37 (2) (1998)

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