Inorganic Ions’ Effect on the Coagulation of Contaminated Drinking Water Simulated with Kaolin-HA by Al13 Species in PAC1.doc

精品论文大集合Inorganic Ions Effect on the Coagulation of Contaminated Drinking Water Simulated with Kaolin-HA by Al13 Species in PAC1Xu Xiuming1, Wang Yan12, Gao Baoyu 1, Xu Weiying 11 School of Environmental Science and Engineering Shandong University, Jinan, P.R.China(250100)2 Key Laboratory of Colloid and Interface Chemistry Minstry of Education Jinan, China(250100)AbstractTo research the effect of coagulation conditions, especially water quality on coagulation efficiency, thecoagulation of synthetic raw water containing humic acid (HA) and kaolin by different aluminum coagulants - conventional Al salts (AlCl3, Al2(SO4)3), poly-aluminum chloride (PAC) and the purified poly-aluminum chloride (Al13) - has been investigated for a wide range of conditions, including aluminum dosages in the range of 0.5 to 16 mg/L as Al2O3, pH values in the range of 3 to 10, and the possible effects of cations and anions on coagulation efficiency. Results indicate that AlCl3, Al2 (SO4)3, PAC and the Al13 species are proved to be efficient in treating the synthetic raw water, and the coagulation efficiency may be associated with the polymerization degree of aluminum species. Water quality has varieties degrees of impact on coagulation efficiency; in particular the existence of SiO32-, HPO42-, H2PO4-, and NH4+, effluent quality has declined markedly. Moreover, Al13 species, which is thought to be the most efficient species in PAC, do not exhibit peculiar advantage in coagulation performance, contrarily it is worse than the other three coagulants especially when water contains HPO42-, and H2PO4-, and NH4+. Experiment results also showed that cations mainly affect the coagulation efficiency through influencing the molecular structure of humic acid, while anions mainly impact the system pH, and thus affect the aluminum coagulants hydrolysis- polymerization process, and ultimately change the coagulation efficiency.Key Words: poly-aluminum chloride; Al13 species; humic acid; cation; anion.1. Introduction As one of the most important natural drinking water source - surface water contains large amounts of natural organic matter，such as humic acid (HA). Besides giving the waters a yellowish color 1, taste and odor, and effecting the transfer of heavy metals in water, HA can also form disinfectionby-products 2-5 in pre-chlorination process and cause fouling problems in filtration of surface waters. Consequently how to remove natural organic matter and turbidity in water effectively is taken into account 6-13. Enhanced coagulation is recognized as the best way 14, 15. poly-aluminum chloride (PAC), the widely used inorganic polymer coagulants in water industry, is the hydroxyl polynuclear complex formed through hydrolysis-polymerization reaction in aqueous aluminum solution, and one of its more commonly accepted hydrolysates is the tridecamer (Al13O4(OH)247+, ab. Al13) 16-18. Al13 andits aggregation have great difference with other species of aluminum hydroxyl complexes in behavior,effect and mechanism during water purification process 19-20. In the past work, a large number of subjects are stressed on the formation and conversion mechanism of Al13 species21, 22, the preparation methods of PAC containing high content of Al13 species 23, 24, and the separation, purification andidentification methods of Al13 species 25-30. And some studies are based largely on the Al () solutionof part hydrolysis 31-33, in which the structure and physical/chemical properties of Al13 species will inevitably be interfered by other coexistence forms. In consequence the ultimate coagulation effect is the composite functions of multiple Al species; however Al13 species is only one of the important components 34-36.1 Support by the China postdoctoral Science Foundation (20080431197)- 17 -While research such as the influence of water quality during coagulation process for Al13 species is reported rarely. In fact, the water quality (such as the coexistence of colloidal substances, ionic strength, and various cations and anions, etc.) in coagulation system is complicated and diverse. Accordingly many parameters may be effected, such as the HA molecular structure, pH values of solution and the hydrolysis-polymerization process of coagulants, finally the coagulation performance and efficiencyare inevitably influenced. This may be the reason why coagulation performance changed greatly in different water quality system for Al species.This paper describes an investigation of the coagulation behavior of Al13 species compared with other aluminum salts in different water qualities and how do these water qualities function with many controlling parameters. Through laboratory experiments with simulated contaminated drinking water, the influence of coagulants dosage, solution pH, surface charge and ion effects have been examined. By reference to the experimental results, the suitable coagulation conditions and water quality for Al13 species to achieve its best performance and whether Al13 species are the best coagulation-flocculation species during coagulation process are identified.2. Materials and methods2.1 Materials and reagentsDe-ionized water was produced from a laboratory de-ionization unit and tap water was obtained from the laboratory supply. All reagents used were analytically pure grade. The four coagulants used in this study were all produced in our laboratory to provide a precise control on the coagulation reactions. PAC (OH/Al molar ratio of 2.0) was synthesized by adding pre-determinedamount of NaCO3 into AlCl3 solution under intense agitation. The temperature was kept at 70 0Cby using recycling water bath 37. Al13 species were separated and purified from the PAC, which contains high concentration of Al13 species, using ethanol- acetone mixed precipitation method38. The detailed procedures can be found in (Gao et al., 2005; Zhao et al., 2002).Meanwhile, AlCl3 and Al2 (SO4)3 solutions were applied in this study as a special PAC with a Bvalue of 0 in this study. The four coagulants were characterized in Table 1.Table 1. Characteristics of different coagulantsCoagulantAla/ %Alb/ %Alc/ % AlCl398.5 1.50.0Al2 (SO4)398.31.70.0PAC32.531.835.7Al130.398.11.6From Table 1, it can seen that AlCl3 and Al2 (SO4)3 can be thought as monomer, PAC the mixer of various Al species, and Al13 species the high-purified Al13 species.2.2 Preparation of stock solutionsThe commercialized HA (Shanghai, China) with 94.3% of the molecular weights ranging from30,000-50,000 Da was extracted from brown coal using NaOH solution. A stock solution was preparedby adding 1.0g humic acid into 1L de-ionized water adjusting pH to 12. Stir continuously for 2h until all solids dissolved.The kaolin stock solution was prepared as following: add appropriate kaolin particles (Tianjin, China) into de-ionized water, adjust pH alkaline with NaOH solution and vigorously mix for 0.5 h. Then after resting the mixture for 3 h, supernatant solution was adopted by Siphon method.During coagulation experiment, suitable kaolin and 5ml HA stock solution were mixed into 500ml de-ionized water to yield the final synthetic test water. The properties of the synthetic test water weredisplayed in Table 2.Kaolin clay(turbidity /NTU)Table 2.Water Quality ParametersHumic acid(UV254 adsorption /cm-1)TOCpH (mgL -1)38.850.50512.87.782.3 The coagulation studyAll coagulation experiments were conducted in 1.0 L plexiglass beakers using a conventional Jartest apparatus, the DC506 Laboratory Stirrer. The coagulation was carried out with fast mixing at200r/min for 1min, slow mixing at 40 r/min for 10 min, and followed by a setting period of 15 min. Samples of the treated water were collected at a depth of 2cm below the surface; Residual HA absorbance was analyzed by a UV spectrophotometer (UV-754, China), and the residual turbidity was measured by a portable turbidimeter (HI 93703-11, Hanna, China)Humic acid removal ratio was calculated as follows:0Absorb 254R% =0 Absorb254×100%UV-254Absorb 254Where, Absorb254 is the maximum absorbance value of supernatant solution after coagulation at 254 nm, Absorb0254 is the absorbance of the initial solution. The initial pH of the solution was measured using PHS series Precision pH/mV Meter (Shanghai, China).The familiar ions in natural water and their concentrations are shown in Table 3.Table 3 Concentration of ions in ground water/ mgL -1NH4+Na+K+Mg2+Ca2+Cl-NO32-SO42-HCO3-CO32-PSiO32-010.7250.50.460.0021350.0020.2400.0061902000.010.11301001201.8In our experiment, if there is not explaination all cations were chloride salts with concentration of5mgL -1 except for the NH4 + of 1mgL -1, and all anions were sodium salts with concentration of3×10-4 molL -1.3. Results3.1 Effect of coagulants dosage on coagulation performanceIn the jar tests, the coagulants were used in concentrations 0.5-16 mgL -1 as Al2O3. A comparison ofthe effect of different coagulants dosage on the residual turbidity and RUV-254% at pH 6.5 was presented in Fig.1.As shown in Fig. 1, in lower dosage range (< 2mgL -1), four coagulants had obvious difference incoagulation efficiency. AlCl3 provided higher coagulation performance, and Al13 species were some poor. Though both AlCl3 and Al2 (SO4)3 were monomer species, the former behaved better than the later. However, for all four coagulants the maximum removal of humic acid and kaolin suspension occurred at approximately the same coagulant dosage, 4mgL -1 as Al2O3. As dosage increasing(4-16mgL -1), the removal efficiency of both HA and kaolin suspension presented well for fourcoagulants (>90%). In general for these four coagulants in dealing with the water samples studied in this paper, the distinction in coagulation performance was small. And the prefabricated Al13 species of high-charge and high-degree-of-polymerization has not shown superior coagulation properties, or even slightly lower than that of the monomer Al coagulants, that to say, the composition species ofaluminum salts have little distinction in coagulation performance for the test here.10090(a)80RUV-254%7060AlCl50 3Al (SO )2 440PAC Al1330200.5 1 2 5 10Dosage/mgL-1(Al2O3)3530Resudial Turbidity2025 AlCl3(b)Al (SO )2 4 315PAC Al1310500.5 1 2 5 10Dosage/mgL-1(Al2O3)Fig.1 Coagulation as function of dosage at neutral pH(a) RUV-254% at different dosages (b) Residual turbidity at different dosages3.2 Effect of solution pH on coagulation performanceThe removal of humic acid and kaolin by aluminum coagulants is related to the solution pH. The result was presented in Fig.2. It was observed that at low pH range(pH<6.0), for monomer AlCl3 and Al2(SO4)3, the removal of humic acid and kaolin were less than that for polymer PAC and Al13 species, what is more, PAC seems more efficient compared to Al13 species. While at pH range of 7.0 to 9.0, the monomer coagulants seemed more outstanding. Moreover, what is remarkable is that, PAC conducted well in the whole pH range discussed here, while for the other three coagulants effect efficiency has a significant reduction in strong acid and alkali conditions(pH<4 and pH>10). On the whole Al13 species,however, do not exhibit better performance on coagulation than the other three Al salts.95 (a)9085%UV-25480R75AlCl70 3Al (SO )2 4 365PAC Al1360552 3 4 5 6 7 8 9 10 11pH1614AlCl312 Al (SO )(b)2 4 310PAC Al8 136Residual turbidity/NTU420-22 3 4 5 6 7 8 9 10 11pHFig.2 Coagulation as function of pH at optimal dosage (dosage=8mg/L) (a) RUV-254% at different pH (b) Residual turbidity at different pH3.3 Effect of ions on coagulation performanceThe former studies focused on the coagulation conditions effect on coagulation performance, and the possible effect of water quality on coagulation process is offered in the following sections.Inevitably there are various inorganic ions in natural water, which have different effects on the coagulation efficiency. As shown in Fig.5(a)、(b), cations presence had little impact on the coagulation overall, except the presence of NH4+ for which the coagulation properties of Al13 species were slightlyinhibited (residual turbidity from about 1.0 to 2.5 NTU). On the contrary, for Al2 (SO4)3, the presence of Mg2+ could promote the turbidity removal in a certain extent.Anions are also so various in natural water and some are prone to hydrolysis in water, the effect of which on coagulation can not be ignored. It was shown in Fig.5(c)、(d) that the presence of SiO32-, HPO42- and H2PO4- inhibited coagulation efficiency significantly, especially for Al2 (SO4) 3 the turbidity removal was mostly affected by H2PO4- (from about 1.0 down to 3.7 NTU), and the HA removal of it was impacted by SiO32-(from 92% down to 68%). The polymer aluminum salts, PAC and Al13 species, proved more effective than, or at least as good as, monomer aluminum in resisting waterquality changes and maintaining good coagulation performances. While both as monomer form, AlCl3is more outstanding than Al2 (SO4)3.It can be deduced through the above research that, in different coagulation conditions and water qualities, different coagulation behavior and performance for different aluminum coagulants were shown. These significant differences in coagulation performance may be caused by many factors: the interaction between pollutants themselves; the influence of water quality on pollutant structure and thus impact the reaction between coagulants and pollutants; the change of charge properties for coagulant by water quality; as well as solution pH change caused by various ions in water, and so on. In the following section the above aspects were put into in-depth discussions with a view to find out how thewater quality functioned on coagulation performance.(a)95UV90AlClRemoval/%85 3Al (SO )2 4 3254PACAl80 137550no ion KCl NaClCationsCaCl2MgCl2NH Cl42.5(b)AlCl3Al (SO )2.024 3PAC Al13Turbidity/NTU1.51.00.50.0no ionKClNaCl2MgClCaClNH Cl24Cations100(c)