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1、精品论文大集合The Effect of To tal Hardness on the Coagulation Performance of Aluminum Salts with Different Al Species 1Wang Yan 12, Gao BaoYu 2, Xu XiuMing 2, Xu WeiYing 2, Xu GuiYing 21 Key Laboratory of Colloid and Interface Chemistry Minstry of Education Shandong University, Jinan, P.R. China (250100)2
2、 School of Environmental Science and Engineering Shandong University, Jinan, P.R. China (250100)E-mail: baoyugao_AbstractIn order to investigate the effect of total hardness on coagulation efficiencies of Al species, different Alcoagulants - conventional Al salt (AlCl3), polyaluminum chloride (PAC-1
3、) and the purified polyaluminum chloride (PAC-2) were used to treat humic acid (HA) and kaoline clay in this study. The coagulation efficiency and coagulation kinetics were investigated by treating synthetic water with different total hardness. The results suggest that total hardness can influence t
4、he coagulation efficiencies of aluminum salts. The coagulation efficiency of AlCl3, PAC-1 and PAC-2 increased when total hardness increased. The investigations on coagulation kinetics showed that total hardness influences the flocs aggregation in the coagulation process of aluminum salts. Higher tot
5、al hardness results in longer reaction time in all cases. The maximum flocs size of AlCl3, PAC-1 and PAC-2 occurswhen total hardness reaches 8, 4, 1 mmol L-1, respectively.Keywords: Al species, humic acid, coagulation, total hardness, kinetics1Introduction Coagulation is an important process in wate
6、r and wastewater treatment. It can effectively remove various contaminants such as suspended solids, inorganic and organic particles from processing stream. At present, inorganic aluminum salts are the most widely used coagulants that destabilize small particles (e.g. bacteria, viruses and inorganic
7、 particles), which subsequently combine into larger aggregates 1. On the basis of conventional aluminum salts (AlCl3 and alum), the research on prehydrolyzed aluminum coagulants (e.g. PAC and PAS) have rapidly developed in recent years. Their applications are on the rise, especially in China, Japan,
8、 Russia and Western Europe 2. PAC contains arange of pre-formed Al() hydrolysis species with superior quality and possesses structure, which arefairly stable to further hydrolysis, resulting in higher coagulation efficiency 3. It has many advantages over conventional aluminum salts including less sl
9、udge production and less dependence on temperatureand pH. Previous studies showed that the Al() hydrolysis species in PAC are heavily dependent onthe coagulation performance and the coagulation mechanism 4.Among the cationic species in PAC, the most noticeable species is Al13 (AlO4Al12(OH)24(H2O)127
10、+) 4,5. Al13 is composed of one tetrahedral center surrounded by 12 octahedral Al units, which has high positive charge and strong binding ability to aggregates 6. Previous study indicated that Al13 is the most active species in PAC and that the Al13 content of coagulants positively correlates with
11、coagulation efficiency 7. Therefore, many efforts have been done in research to identify how aluminum salts remove contamination by studying the performance of Al13.At present, PAC with different species has attracted much attention with regard to removing humic acid (HA) in raw water. Humic acid (H
12、A) is a natural organic matter, resulted from the weathering and/or the biodegradation of dead plants and animals 8. The presence of HA in water presents a yellowish or brown color. Moreover, high affinity of HA for complexation with various pollutants including heavy1 Support by the China postdocto
13、ral Science Foundation (20080431197)- 11 -metals and pesticides causes contamination of ground and surface water. In addition, HA can form very toxic disinfection (chlorinated organic compounds including trihalomethanes) by-products which exhibit mutagenic properties during chlorination stage in dri
14、nking water treatment 9.Hu et al. 10 studied the coagulation behavior of aluminum salts in eutrophic source water and found that the coagulation efficiency of Al coagulants positively correlated with the content of Al13 in the coagulation process. Shi et al. 11 used coagulants with different Al spec
15、ies to remove HA and found that PAC was less effective than conventional Al salts. Yan et al. 5 assumed that DOC removal would be well correlated to the content of polymer aluminum species. However, it is also found that the removal efficiency of aluminum salts for treating river water is lower than
16、 those for treating pure water 1.Coagulation efficiency depends not only on the characteristics of coagulants but also on various water quality parameters, such as pH, ionic strength, total hardness, and the concentrations of certain anions and cations. However, few studies, so far, have focused on
17、the effect of water quality parameters on the coagulation efficiency of Al species. It is believed that such information would help to better understand why the coagulation behavior of Al species significantly is varied with different waters/wastewaters. The purpose of this study was, therefore, to
18、elucidate the effect of total hardness on coagulation efficiencies and kinetics of aluminum salts with different Al species in HA water, and to gain new insights into the correlation of coagulation performance of Al species with total hardness.2Materials and Methods 2. 2.1 Coagulants All the reagent
19、s used to prepare each coagulant were of analytical grade and deionized water was used to produce all solutions. The procedures of preparing each coagulant are described as follows: (1) conventional Al salt (AlCl3): directly dissolving 3.7g AlCl3 6H2O to 100mL deionized water; (2)polyaluminum chlori
20、de (PAC-1): adding 14g NaCO3 slowly into AlCl3 solution (0.84 mol L-1) underintense agitation. The temperature was kept at 70 0C by using recycling water bath 12; (3) the purified polyaluminum chloride (PAC-2): it was separated and purified from PAC-l by adding ethanol and acetone. The detailed proc
21、edures of adding ethanol and acetone method can be found in previous paper 13.Total Al concentrations (AlT) were determined using ICP-AES (PerkinElmer, Optima 2000, U. K.). Basicity values (B, OH/Al molar ratios) were determined by titrimetric methods (standard method of the chemical industry of Chi
22、na). The Al species in AlCl3, PACl-1 and PACl-2 were analyzed using a JEOL FX-90 Q NMR spectrometer with a sample tube of 10mm diameter. At the center of the sample tube, a capillary of 1 mm diameter was inserted. 0.20M NaAl(OH)4 aqueous solution and an equal volume of heavy water (D2O), which was u
23、sed as the internal standard of deuterium lock andaluminum content, were added into the capillary. The scanning frequency was pulse 900, pulse delaytime 0.1s, 8K data point, and the spectrum width 200 ppm. The number of samplings depended on the aluminum content in the sample.There are three signals
24、 in the NMR spectra: the signal near 0.0 ppm represents the monomeric and dimeric aluminum species (denoted as Almono); the signal at 62.5 ppmdenotes the Al13 species; and the signal at 80.0 ppm indicates the formation of Al(OH)4- (the internalstandard). The other Al species (denoted as Alother) lar
25、ge polymer aluminum species and solid-phaseAl(OH)3 were calculated by the following equation.The properties of coagulants used are summarized in Table 1. As seen in Table 1, AlCl3 can be considered as Almono, PAC-1 can be considered as the mixer of various Al species, and PAC-2 can be considered as
26、the Al13 species.Table 1 The properties of alunimum salts coagulantsCoagulantsAlT mol/LAlmono, %Al13, %Alother, %AlCl30.01597.42.6UntestedPAC-10.8430.734.834.5PAC-20.1060.397.12.62.2 Syn t he tic test water The HA stock solution was prepared as following: 1g of HA (Shanghai, China) was dissolved in
27、1000 ml deionized water which contained 4.2g of NaHCO3. In this condition, the HA was soluble. The synthetic test water was prepared by dissolving the HA stock solution (2 ml) in 500 ml deionized water. In order to simulate actual water, kaolin clay (10 mg) was added in the HA water. And CaCl2 was u
28、sed to adjust total hardness of tested water. UV254 and TOC of the synthetic test water and the treated water were measured and used to evaluate the coagulation efficiency in this study. The properties of the synthetic test water used are as below:UV254 = 0.572, TOC = 84.3 mg L-1, pH = 7.32.2.3 Coag
29、ulation test procedures All coagulation experiments were conducted in 1.0 L plexiglass beakers using a conventional Jartest apparatus (the DC506 Laboratory Stirrer). Suspensions (500 ml) were treated by different coagulants. The dosages of different flocculants were calculated by Al2O3. The solution
30、s were stirred rapidly at 120 rpm for 3 min during coagulation addition, followed by slow stirring at 40 rpm for 12 min and sedimentation for 12 min. Throughout the coagulation periods, the suspension was continuously sampled by peristaltic pump and monitored by a photometric dispersion analyzer (PD
31、A 2000; Rank Brothers Ltd.) to obtain data. The theory and output data of PDA 2000 have been clearly defined in the literature 14.3Results 3.1 Coagulation efficiency of alum inum salts for treating w a ter w i th different tota l hardnes The subsequent experiments were designed to examine the relati
32、onship between the coagulation efficiency of different aluminum salts and total hardness of water sample. According to previous experiments (not covered in this paper), the dosage of AlCl3, PAC-1 and PAC-2 was kept at 7, 13, 8 mgL-1, respectively.9695949392 AlCl3PAC-191 PAC-2The UV-254 removal effic
33、iency, %9089-2024681012141618Total hardness, mmol L-1 (Ca)Fig. 1 Coagulation performance of aluminum salts for treating water with different total hardnessFig. 1A shows the influence of total hardness on the UV-254 removal efficiency.It can be seen that the influence of total hardness is similar exc
34、ept for PAC-1. The UV-254 removal efficiency of AlCl3 and PAC-2 increased with increasing total hardness, and when total hardness is above 8 mmol L-1, therewas a slight increase in coagulation efficiency. While the UV-254 removal efficiency of PAC-1 increased with increasing total hardness until tot
35、al hardness is 4 mmol L-1. When total hardness is lower than 1mmol L-1, the UV-254 removal efficiency of PAC-2 is highest, followed in order by PAC-1 and AlCl3. Fig. 1B shows the influence of total hardness on TOC removal efficiency. It can be seen that the influence of total hardness is similar for
36、 all tested coagulants. The TOC removal efficiency increased with increasing total hardness. When total hardness is above 8 mmol L-1, there was a slight increase in coagulation efficiency. As total hardness exceeds 1mmol L-1, the UV-254 and COD removal efficiency is in the following order: AlCl3 PAC
37、-2 PAC-1. The above results indicate that monomer Al species remove HA effectively in all cases, especially in high total hardness water. In contrast, the coagulation efficiency of Al13 species would reach the highest level in low total hardness water.3.2 Coagulation kinetics of alu m in um salts fo
38、r treating w a ter w i th different tota l hardness PDA 2000 was used in monitoring the formation process of flocs after coagulant addition to explore the effect of total hardness on coagulation kinetics of aluminum salts with different Al species.Graphical representation of the data was provided by
39、 ratio curves which display the time vs. ratio values which was obtained from the PDA2000 experiments in this study. The curves shown in Fig. 2 were plotted using the typical ratio obtained from the experiments, which can be categorized into three regions lag region, growth region, and steady state
40、region. In the lag region, coagulants come to contact and react with contamination, but flocs growth rate is so small that the ratio increases only a little in this region. The growth region describes the stage where the flocs size increases significantly. The steady-state region describes the balan
41、ce between flocs growth and breakage where the flocs size distribution no longer changes with time.0.8Growth Region0.6FlocRatio0.40.2Lag regionSteady-State Region0.0Grow rate0100200300400 time, secFig.2 The typical ratio distribution curveIn this study, four parameters were calculated to analyze the
42、 data collected during the flocculation experiment to explain coagulation kinetics. These parameters included (1) a lag time of the lag region, which indicates the reactor rate of coagulants with HA; (2) a growth rate of the growth region, which indicates the growth characteristic of flocs; (3) a ti
43、me-weighted average steady-state ratio value, which indicates the flocs size in the steady-state region; (4) a time-weighted variance (TWV) of the steady-state ratio value, which indicates the range of flocs size in the steady-state region.It was found that the lag time and the time-weighed average
44、steady state ratio value can be obtained byGrowth equation which was computed by Origin 7.0. Growth equation is:PY =A1 A 21 + (X / X 0 )+ A 2(1)The constants (A1, A2, X0 and P) can be determined by computing ratio values obtained from the experiments using Origin 7.0. A2 is the indicator of the time
45、-weighted average steady-state ratio value(denoted as ratio ), and X0 is the indicator of the lag time (Timelag), although X0 is not the true lag time.The time-weighted ratio variance of the steady-state ratio value was used as a measure of flocs size difference in steady state region. TWV was calcu
46、lated as:Ni =1(ratioii ratio) 2 time i Ni =1 timeTime-weighted variance (TWV) =100%ratio(2)In the growth region and the sedimentation region, the growth rate and the sedimentation rate, which are noted as the linear portion, reach a constant value. Best-fit lines can be constructed for the ratio cur
47、ve (Fig. 3), from which the growth rate and the sedimentation rate can be determined as:ratioGrowth rate =(3)timeThe flocs aggregation in PDA experiments was measured and the results are summarized in Fig.3 Fig.6.200190180lag170Time , sec160150AlCl3PAC-1PAC-2140-2 0 2 4 6 8 10 12 14 16 18Total hardness, mmol L-1 (1/2Ca)Fig. 3 The relat