Combination of anionic polyelectrolyte and novel polyaluminumferric-silicate-chloride

Transcription

1 Combination of anionic polyelectrolyte and novel polyaluminumferric-silicate-chloride coagulant and application in coagulation/flocculation (C/F) process of water or wastewater treatment A. Tolkou, A. Zouboulis*, D. Zamboulis, M. Demirtsidou Department of Chemistry, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece, *Corresponding author: Tel , Fax: Abstract Attempts were made in this study to examine the effectiveness of coagulation/flocculation (C/F) process using polyaluminum ferric-silicate-chloride coagulants and an anionic polyelectrolyte (Magnafloc-LT 25) for the treatment of water and wastewater samples. Two different types of process were examined. In the first type the polyelectrolyte was added in the sample as a flocculation aid in order to destabilize the colloidal materials and caused the small particles to agglomerate into larger settleable flocs; and in the second one the coagulant and polyelectrolyte were combined in one unique reagent in order to increase further their flocculation efficiency; as a result, the first type provided a better effectiveness of the C/F process. Turbidity and UV 254nm absorbance were measured in the treated water and/or wastewater and the removal of organic matter (expressed as COD), were investigated for wastewater samples. Keywords: coagulation, composite coagulants; poly-aluminum-ferric-silicate-chloride coagulants, anionic polyelectrolyte, water and wastewater treatment 1. INTRODUCTION The Inorganic Polymeric Flocculants (IPFs), or pre-polymerized coagulants, such as polyaluminum chloride (PACl) represent a relatively new category of coagulation reagents, which was developed in order to increase the efficiency of coagulation/flocculation (C/F) process, but there is still need for further improvement of their properties [1, 2]. The main reason is the insufficient aggregation abilities of IPFs, which usually imposes the use of a flocculant aid (polyelectrolyte) to increase the efficiency of flocculation process. In order to improve the aggregating capacity of PACl, several efforts have been made during the recent few years, towards the incorporation of silica in its structure. Hasegawa et al. [3] noticed that by introducing metal ions into polymerized silicic acid solution, the molecular weight of the product was increased and the corresponding stability and coagulation performance were further improved. Recently, several investigators [4-7] have studied the simultaneous addition of Al (III), Fe (III) and polysilicic acid solution (psi) and the coagulation efficiency of poly-aluminum-ferric-silicate-chloride is mainly affected by the Al/Fe/Si ratio and the respective preparation techniques. Additionally, synthetic polyelectrolytes have been utilized in C/F process for water purification for more than four decades. Due to their wide usage, it is not surprisingly to consider them as alternative additives in the pre-polymerized coagulants composition for the production of new modified coagulation reagents. Apart from the expected increase of components size and molecular weight in the composite coagulants, the utilization of polyelectrolytes exhibits certain specific advantages: the inorganic coagulant (e.g. PACl) and the organic polyelectrolyte will be combined in one reagent, thus avoiding the subsequent addition of a flocculant aid (polyelectrolyte) after coagulant addition (inorganic salt) in order to enhance the flocculation process [8]. 1

2 The aim of this study was the examination of the effectiveness of C/F process using polyaluminum ferric-silicate-chloride coagulants and an anionic polyelectrolyte (Magnafloc-LT 25) as the flocculant aid for the treatment of water or wastewater samples. Furthermore, the combination of coagulant and polyelectrolyte in one unique reagent was examined in order to increase further their flocculation efficiency, by replacing the inorganic polysilicate additive with the organic polyelectrolyte in their structure. Hence, providing the respective comparison results for the coagulation performance of all the prepared coagulants, applied in water or wastewater treatment. 2. MATERIALS AND METHODS All chemical reagents used were analytically pure chemicals. De-ionized water with conductivity lower than 0.5 ms/cm was used to prepare all solutions, except those used for the preparation of coagulants. In this case, de-ionized water made carbonate free by boiling, was used Procedure for the preparation of composite coagulants Composite polyaluminum ferric silicate chloride (PSiFAC) coagulants were produced in this work at room temperature, according to a procedure proposed by Tolkou et al., 2013a [9], under various experimental conditions, and using different ratios of components and (inorganic) polymerization modes. The used initial solutions were 0.5 M AlCl 3 6H 2 O (Merck), 0.5 Μ FeCl 3.6H 2 O (Merck), 0.5 M NaOH (Merck) - as the added base - and the prepared polysilicic acid solution (psi), according to Tzoupanos et al., 2009 [10]. The most effective coagulant obtained during preliminary experiments [9, 11], i.e. PSiFAC 1.5:10:15, was used in this study. A poly-acrylamide co-polymer (Magnafloc LT-25, Ciba SC LTD, commercially available) as an anionic polyelectrolyte (APE) was obtained (0.01 % w/v) and used both as the flocculant aid (in [Al]/[APE] = 10 molar ratio) and the organic additive for the synthesis of composite coagulants (PAPEFAC). The specific polyelectrolyte is commonly used as flocculant aid in water or wastewater treatment plants, especially in Greece [12, 13]. Table 1 presents the preparation conditions of all prepared coagulants. Table 1. Preparation conditions of all laboratory prepared coagulants. Coagulant type Molar ratios Procedure PSiFAC 1.5:10:15 [OH]/[Al]: 1.5 [Al+Fe]/[Si]: 15 Co-polymerization technique: Appropriate amount of FeCl 3 solution was added in AlCl 3 solution, under vigorous stirring. Then, the resulted FpA solution was added to psi solution at desired ratios of [Al+Fe]/[Si] and base solution was added slowly (under magnetic stirring) in the mixture at the desired [OH]/[Al] molar ratio. PSiCAF 1.5:10:15 [OH]/[Al]: 1.5 [Al+Fe]/[Si]: 15 PAPEFAC [OH]/[Al]: 1.5 [Al+Fe]/[APE]: 15 Composite polymerization technique: the base solution was initially added to the AlCl 3 solution, creating an intermediate PACl solution and then, FeCl 3 solution was added at the desired [Al]/[Fe] molar ratio followed by the addition of the psi solution. Co-polymerization technique: Appropriate amount of FeCl 3 solution was added in AlCl 3 solution, under vigorous stirring. Then, the resulted FpA solution was added to APE solution at desired ratios of [Al+Fe]/[APE] and base solution was added slowly (under magnetic stirring) in the mixture at the desired [OH]/[Al] molar ratio. PΑCFAPE [OH]/[Al]: 1.5 [Al+Fe]/[APE]: 15 Composite polymerization technique: the base solution was initially added to the AlCl 3 solution, creating an intermediate PACl solution and then, FeCl 3 solution was added at the desired [Al]/[Fe] molar ratio followed by the addition of the APE solution Coagulation performance 2

3 The ph was measured by using a Metrohm Herisau ph-metre, the conductivity by using a Crison CM 35 Conductivity Meter and the turbidity measurements were performed by a HACH RATIO/XR Turbidimeter. The UV absorbance at 254 nm, as a convenient indicator of natural organic matter presence, was measured with a Shimadzu UV/Vis spectrophotometer, by using a 1 cm path length quartz cuvette. The residual aluminum concentration was determined with the eriochrome cyanine R standard method [14] Jar-tests Jar tests were used for the examination of coagulants efficiency [15]. A jar-test apparatus (Aqualytic) equipped with six paddles was used, employing 1 L glass beakers. Two types of samples were used: simulated surface water and tannery wastewater. The simulated surface water (1 L) was prepared by tap water, clay (kaolin) (10 mg/l) suspension and humic acid (5 mg/l). Tannery wastewater was used as a representative industrial wastewater; the sample was collected from the influent of a tannery wastewater treatment plant. The properties of both samples were ph 7.6, 17.2 NTU, UV 254nm and ph 7.4, 668 NTU, UV 254nm, 6800 mg/l COD respectively. Water samples were collected from the supernatant of each beaker and were analyzed for the determination of: turbidity, absorption at UV 254nm that provides an indication of the amount of natural organic matter (NOM), existing in the water samples, and COD (mg/l). 3. RESULTS AND DISCUSSION 3.1. Physicochemical properties of prepared coagulants Table 2 displays the major physicochemical properties of laboratory prepared composite inorganic and organic coagulants. Table 2. Properties of laboratory prepared coagulants. Coagulant type ph Turbidity (NTU) Conductivity (ms/cm) PSiFAC 1.5:10: PSiCAF 1.5:10: PAPEFAC PΑCFAPE It can be observed that the addition of psi in an Al-Fe solution (FpA) to induce the formation of composite coagulants, results in the increase of turbidity, in comparison with the addition of APE in FpA solution. Furthermore, the increase of turbidity is higher when the co-polymerization technique is used in both types of coagulants, due to simultaneous polymerization technique of the raw materials. It is also observed that the coagulants prepared by co-polymerization technique, present a higher conductivity than those prepared with composite polymerization Comparison of the prepared coagulants in water treatment Figure 1 displays the results of coagulation experiments, related to the treatment of model water sample (simulating surface water) with all composite laboratory-prepared coagulants by the two polymerization techniques. The concentration of coagulants ranged between 1 and 6 mg/l and the experiments were conducted at the initial ph (7.4) of water sample. In the case of the addition of APE as a flocculant aid, the concentration of polyelectrolyte was equal to 1/10 of the concentration of Al, determined as the optimum ratio through preliminary experiments (data not shown). It can be observed that regarding the parameters of turbidity removal (less than 1 NTU according to the respective legislation limit, EU Directive 98/83/EC) and UV absorbance at 254 nm reduction, among the examined coagulants, the PSiFAC 1.5:10:15, prepared with the co-polymerization 3

4 technique, either with or without the addition of polyelectrolyte (as flocculant aid), provided a better effectiveness of the C/F process. Figure 1. Comparative coagulation experiments of all laboratory-prepared coagulants for water treatment; (a) residual turbidity, (b) UV absorbance at 254 nm, (c) residual aluminum concentration. Residual aluminum concentration is a very important parameter from health perspectives and should be carefully considered, when an aluminum coagulant is applied in drinking water treatment. According to Figure 1, it can be seen that the Al concentration remaining in the sample after treatment, varies significantly, depending upon the examined coagulants. The lowest residual Al concentration, was achieved, when the coagulant and polyelectrolyte were combined in one unique reagent, and/or when is used the PSiFAC 1.5:10:15 product, prepared with the co-polymerization technique, without or with the addition of the polyelectrolyte (as aid), as for almost all the applied concentrations. The residual Al concentration remains under the respective legislation limit of 200 µg Al/L (EU Directive 98/83/EC), contrary to the composite polymerization technique Comparison of the prepared coagulants in wastewater treatment 4

5 The most effective coagulants obtained by the treatment of simulated water, i.e. PSiFAC 1.5:10:15, PSiFAC 1.5:10:15 with APE and PAPEFAC 1.5:15:10, were applied for the treatment of tannery wastewater to evaluate their coagulation efficiency for industrial wastewater. Figure 2. Comparative coagulation experiments of all laboratory prepared coagulants for wastewater treatment; (a) residual turbidity, (b) UV absorbance at 254 nm, (c) COD concentration. The results of turbidity removal, UV absorbance at 254 nm and COD (mg/l) reduction during the treatment of tannery wastewater are presented in Figure 2; as shown, all three coagulants greatly reduced the aforementioned parameters for doses higher than 100 mg/l. Mostly have, the addition of polyelectrolyte as flocculant aid increases the efficiency of processing more than its use in the structure of the composite coagulant. 4. CONCLUSIONS In this study, two different types of process applications were examined. In the first type the polyelectrolyte was added in the sample as flocculation aid and in the second one the coagulant and polyelectrolyte were combined in one unique reagent in order to increase further their flocculation efficiency. It was found that the first type provided a better effectiveness of C/F process in water 5

6 and/or wastewater treatment. Particularly, regarding turbidity removal and UV absorbance at 254 nm reduction among the examined coagulants, the PSiFAC 1.5:10:15, prepared with the copolymerization technique, either with or without the addition of the flocculant aid (polyelectrolyte), provided a better effectiveness of the C/F process. Additionally, the residual Al concentration was lower than the legislation limit when the coagulant and polyelectrolyte were combined in one unique reagent. Furthermore, in the treatment of tannery wastewater the addition of polyelectrolyte as flocculant aid increases the treatment efficiency more than its use in the structure of the composite coagulant. Acknowledgments The financial support through the co - Financed by the European Union and the Greek State Program EPAN-II (OPC-II)/ ESPA (NSRF): 'SYNERGASIA II', Project (FOUL-MEM) - "New processes for fouling control in membrane bioreactors (11SYN ), is gratefully appreciated. References 1. Sinha S., Yoon Y., Amy G., Yoon J., Determining the effectiveness of conventional and alternative coagulants through effective characterization schemes. Chemosphere, 57 (9), Crittenden J.C., Trussel R.R., Hand D.W., Howe K.J. and Tchobanoglous G., Coagulation, mixing and flocculation, in Water Treatment: Principles and Design, 2nd edition, John Wiley & Sons, New Jersey, pp Hasegawa T., Hashimoto K., Onitsuka T., Goto K., Tambo N., Characteristics of metalpolysilicate coagulants. Water Sci. Technol., 23, Gao B. Y., Yue Q. Y., Wang B. J., Properties and coagulation performance of coagulant polyaluminum-ferric-silicate-chloride in Water and Wastewater Treatment. Journal of Environmental Science and Health Part A, 41, Niu X., Li X., Zhao J., Ren Y., Yang Y., Preparation and coagulation efficiency of polyaluminum ferric silicate chloride composite coagulant from wastewater of high-purity graphite production. Journal of Environmental Sciences, 23(7), Li R., He C., He Y., Preparation and characterization of poly-silicic-cation coagulants by synchronous-polymerization and co-polymerization. Chemical Engineering Journal, 223, Sun T., Sun C.H., Zhu G.L., Miao X.J., Wu C.C., Lv S.B., Li W.J., Preparation and coagulation performance of poly-ferric-aluminum-silicate-sulfate from fly ash. Desalination, 268, Zouboulis A., Tzoupanos N Polyaluminium silicate chloride A systematic study for the preparation and application of an efficient coagulant for water or wastewater treatment. Journal of Hazardous Materials 162, Tolkou A., Zouboulis A., Samaras P., 2013a. Synthesis and coagulation performance of composite polyaluminum-ferric-silicate-chloride coagulants in water and wastewater treatment and their potentially use to alleviate the membrane fouling in MBRs. Proceedings of the International Conference Win4Life (eds. Loizidou M.), September 19-21, Tinos Island, Greece. 10. Tzoupanos N., Zouboulis A., Tsoleridis C., A systematic study for the characterization of a novel coagulant (polyaluminum silicate chloride). Colloids and Surfaces A: Physicochemical and Engineering Aspects, 342, Tolkou A., Zouboulis A., Samaras P., 2013b. PSiFAC - poly-aluminum-ferric-silicate-chloride: Synthesis and coagulation performance of a novel composite coagulant in water and wastewater treatment. Proceedings of the 4th International Conference SWAT (eds. Zouboulis A., Kugolos A., Samaras P.), October 25 27, Volos, Greece, Mortimer D.A., Synthetic polyelectrolytes: A review. Polymer International, 25 (1), Bolto B.A., Soluble polymers in water treatment. Progress in Polymer Science, 20 (6), Clesceri L., Greenberg A., Trussell R., Standard Methods for the Examination of Water and Wastewater, 17th ed., APHA AWWA WEF, Washington, D.C. 15. Moussas P. A., Tzoupanos N.D., Zouboulis A.I., Advances in coagulation/flocculation field: Aland Fe based composite coagulation regents, Desalination and Water Treatment, 33,