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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 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) 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 the 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 total 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 water 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 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, 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 sludge 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+) 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 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 (ha) 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 postdoctoral 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 drinking 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 species 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 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 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 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 reagents 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 chloride (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 procedures 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 china). 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 used 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 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) large 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 the al13 species.table 1 the properties of alunimum salts coagulantscoagulantsalt mol/lalmono, %al13, %alother, %alcl30.01597.42.6untestedpac-10.8430.734.834.5pac-20.1060.3 syn t he tic test water the ha stock solution was prepared as following: 1g of ha (shanghai, china) was dissolved in 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 used 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 = coagulation 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 solutions 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 (pda 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 relationship 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 efficiency, %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 except 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 total 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 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-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 for 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 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 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 balance 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 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 time-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 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-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 calculated 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 curve (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 relation of total hardness to timelag in coagulation process of alcl3 pac-1 and pac-210alcl38 pac-1pac-2grow rate, 10-3642-2 0 2 4 6 8 10 12 14 16 18total hardness, mmol l-1 (1/2ca)fig. 4 the relation of total hardness to growth rate in coagulation process of alcl3, pac-1 and pac-1.41.3ratio1.2alcl3pac-1pac-21.11.00.90.8-2 0 2 4 6 8 10 12 14 16 18total hardness, mmol l-1 (1/2ca)fig. 5 the relation of total hardness to ratio in coagulation process of alcl3, pac-1 and pac-26.56.05.55.0twv, %4.54.03.53.0alcl3pac-1pac-22.52.01.5-2 0 2 4 6 8 10 12 14 16 18total hardness, mmol l-1fig. 6 the relation of total hardness to twv in coagulation process of alcl3, pac-1 and pac-2fig.3 displays the relation of total hardness to timelag in coagulation process of alcl3 pac-1 and pac-2. increase in total hardness corresponds to an increase in timelag of all the tested aluminum salts. and at the same total hardness, timelag of pac-2 is the longest, and timelag of alcl3 is the shortest. timelag indicates that the initial reactor rate of aluminum slats with ha. from the results in fig. 3, it can seen that the initial reactor rate is slower with increasing total hardness, which means it needs longer hydraulic resident time for water with higher total hardness, and the reaction between al13 species and ha is the most difficult, which indicates that al13 species used as coagulant need longer mixer time.fig.4 displays the relation of total hardness to growth rate in coagulation process of alcl3, pac-1 andpac-2. it can be seen that the total hardness versus growth rate relationships were different for alcl3,pac-1 and pac-2. for alcl3 and pac-1, there is the largest growth rate in the studied total hardness range. as total hardness is 4 mmol l-1, growth rate of alcl3 and pac-1 peaked, which means it takes the least time for the flocs aggregation. with increasing total hardness over the studied range, growth rate of pac-2 is lower, which means the flocs aggregation needs longer time with increasing totalhardness. at the same total hardness, growth rate of alcl3 is the largest followed by pac-1 and pac-2, except in pure water, which means that monomer al species produces flocs more easily than other al species.ratio as a function of total hardness is shown in fig. 5. there are the maximum ratio in the texted total hardness range for all aluminum salts. the maximum ratio of alcl3, pac-1 and pac-2 appears at total hardness of 8, 4, 1 mmol l-1, respectively. at the same total hardness, ratio of alcl3 is the largest, and ratio of pac-2 is the smallest, except in pure water. this indicates that the flocsproduced by monomer a

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