EffectsofhumicacidonZn2+adsorptionindrinkingwaterdistributionsystemswithamorphousAl(OH)3formationWangWendong1,3,ZhangXiaoni1,ZhouLichuan1,DingZhenzhen

精品论文effects of humic acid on zn2+ adsorption in drinking water distribution systems with amorphous al(oh)3 formation wang wendong1,3, zhang xiaoni1, zhou lichuan1, ding zhenzhen2,5wang xiaochang1(1. school of environmental and municipal engineering, xian university of architecture andtechnology, xian 710055;2. xian research academy of environmental sciences, xian 710002;3. department of environmental technology and ecology, yangtze delta region institute of10tsinghua university, zhejiang jiaxing 314006)abstract: zinc () commonly exists in drinking water. this paper aims to investigate the effects ofhumic acid on the adsorption of zn2+ on amorphous al(oh)3. both suspension and well scaled forms of al(oh)3 were investigated applying icp-aes determination and visual minteq calculation. it was found that in solutions without ha, ph had notable effects on the removal of zn2+. chemical15adsorption mainly took place at ph above 7.5; 1.0 g solid could accumulate about 15.9 mg zn2+ at 15.because of h+ inhibition, active reaction sites on amorphous al(oh)3 surface were much less at ph below 7.5. the adsorption of zn2+ on al(oh)3 changed gradually from chemical coordination to physical adsorption. compared with zn2+, the adsorption of ha on al(oh)3 suspension was strong. in drinking water with ha coexisting, the adsorption of zn2+ was enhanced notably; while ph had little20effects its removal. under the bridging effects of ha, zn2+ could not be detected in solution with ha above 1.0 mg/l, much lower than the standard required level. the adsorption of al(oh)3 scale for zn2+ was weak, about 0.2 mg/g. better management on sediment accumulation and zn2+ preservation was suggested to control the concentration of zn2+ residual.keywords: aluminum hydroxide; drinking water; humic acid; pipe scale250introductionzinc was commonly found in drinking water supplies and essential for human health 1-3. however, excessive concentrations of zn2+ could result in metallic tasting water, reduced water pressure and flow in the pipes from the accumulation of zinc-containing sediments and human30health problems 4-6. leone (2006) found that excessive intake of zn2+ could increase the risks ofcancer and cardiovascular mortality 7. although very few natural waters contained zn2+ at levels sufficient to cause acute health effects, some waters either treated or untreated have chemical characteristics that allow them to mobilize zn2+ from pipe materials 8, 9. many governments had developed regulations to ensure the drinking water security; the limit value in china is 1.0 mg/l35(gb5749-2006).it had been thought that the concentration of zn2+ maintains constant as water passed through water distribution systems 10, 11. regulatory monitoring of zn2+ had been required only at entry points of the distribution system, presuming that its concentration could not increase and that such monitoring was sufficient to protect public health 12, 13. in fact, because of the effects of40accumulation and sedimentation, the concentration of zn2+ was variable 8. humic acid (ha) wasthe major component of dissolved organic matters in the raw water 14, 15. as the purification process could not remove all the ha, partial of them would remain in the treated water. ha have many organic function groups as hydroxyl and carboxyl ligands which could combine with many metal ions, and thus affects their transformations 16.45poly-aluminum chloride (pacl) was commonly used in water purification plants 17, 18. as itsfoundations: changjiang scholars and innovative research team in university (pcsirt) (grant no. irt0853), the national natural science foundation of china (no. 21007050), and the natural science foundation of shaanxi (no. 2009jq7001)brief author introduction:wang wendong,（1981-）, gender（male）, professional title(associate professor),main research(security ). e-mail: - 9 -solubility was relatively low, al(oh)3 sediments were usually formed in the pipe line 19-21. large amounts of studies found that pacl showed good removing capacity to heavy metals in coagulation 22, 23. reductions in lead levels were observed during pipe rig testing in a rochester case study, which was coincided with the opening of a new filtration plant and appeared to be50related to aluminum deposition on lead piping materials 24. julien et al. (1994) studied theadsorption of organic acids on al(oh)3 and found a correlation between removal efficiency and the number and ionization of functional groups 25. while, its affections on heavy metal adsorption on al(oh)3 scale have not be considered.the objective of this paper was to investigate the effects of ha on zn2+ adsorption in the55pipes with suspended and deposited al(oh)3 formation. all the experiments were conducted in a lab scale. the reaction mechanism among al(oh)3 solid, ha and zn2+ were discussed, which would be meaningful for the management of drinking water distribution systems.1materials and methods1.1sorption test of zn2+ on al(oh)3 scale60a drinking water distribution modeling system was set up. the raw water was drawn from a tank, and then pumped into the pipe system (fig. 1). in order to exclude the effects of other metals introduced by pipe corrosion and metal element stripping, polyvinyl chloride (pvc) pipe was used. all the water used in the experiments was deionized water adjusted by putting certain amounts of0.50 mol/l naoh, 0.50 mg/l hno3, 100 mg/l al2(so4)3, and 50.0 mg/l zncl2 to the tank.65reagent grade chemicals were used except where noted.presure gaugepumpdischargewater tankpipe systemfig. 1 drinking water distribution modeling system.water temperature was controlled by putting the tank to a constant temperature incubator70from which waters were sampled at different time interval. al(oh)3 scale was scraped from the pipeline. in order to accelerate the formation of al(oh)3 scale, solution ph and aluminum concentration were maintained at 7.5 and 50.0 mg/l at initial time. a white gelatinous coating mainly composed by al(oh)3 generated in the pipe line after 5 d operation.as soon as the formation of al(oh)3 scale, 1.0 mg/l zn2+ was introduced to the system at75stage , other parameters including flow velocity, solution ph, and temperature were maintainedat 1.0 ml/s, 7.5, and 15, respectively. the effects of flow velocity and solution ph on zn2+ adsorption were investigated at stage applying single factor experiments (table 1). samples were taken at different time intervals, and filtered applying a 0.45 m polycarbonate membranefor determination.80table 1 effecting factors and their levels selected in the experiments (t=15).experiments factors units zn2+ v ph selected levels low middle highadsorption of zn2+ph - 1.0 0.5 var. 6.50 7.50 8.50mg/lvar-1.0mg/l0-7.51.05.010.0zn2+ mg/l var. 0.5 7.5 0.0 1.0 2.0on al(oh)3 scale v ml/s 1.0 var. 7.5 0.2 0.5 1.02+adsorption of znzn2+859095100105110115120on al(oh)3 suspention ha (toc)1.2sorption test of zn2+ in al(oh)3 suspensiona batch-equilibrium technique was employed to determine the sorption of zn2+ on al(oh)3.0.20 g of air-dried al(oh)3 was put into 250-ml test tubes. then 10.0 ml 25.0 mg/l zn2+ were added. the tubes were stirred by a magnetic stirrer for 5 min. the mixtures were sampled and centrifuged at 5000 rpm for 20 min. the amount of zn2+ sorbed on al(oh)3 was determined applying icp-aes method. the sorption coefficient (kd) could be calculated according to the ratio of the sorbed zn2+ to residual zn2+, as shown in equation (1):=aloh0.5- + zn2+ =alozn0.5+ + h+(1)the effects of ha on the adsorption of zn2+ were investigated applying single factor experiments. ha was filtrated with a 0.45m polycarbonate membrane to remove insoluble matters, and then filtrated with an ultrafiltration membrane with 500 dalton molecular weight cutoff. its additions were conducted for three times, corresponding to the low, middle, and high levels respectively, as shown in table 1. solution ph was adjusted by putting 0.50 mol/l naoh and 0.50 mg/l hno3 to the test tube.the amount of zn2+ adsorbed by amorphous al(oh)3 suspension was predicted applying thevisual minteq software. by defining the member of the surfaces (1), adsorption model (diffused layer model), solid concentration (0.05 mg/l), site concentration (1.0 mmol/mmol solid), and the sorption coefficient (kd), the reaction constants between zn2+ and reactive sites weredefined 20. besides, its speciation in drinking water and the amounts of zinc precipitation couldalso be calculated.1.3water quality and solid analysis methodsthe concentrations of aluminum and zinc were determined applying an iris intrepid- icp (thermo elemental, usa). sample aliquots were pretreated with nitric acid to ph 2 for 12 h before analysis. ha was determined by a toc analyzer (toc-vwp, shimazu company, japan); other water chemistry parameters such as ph, determined by a ph meter (model 828, thermo electron corporation), and temperature, determined by a thermometer (model ttm1-jm-6200im, yuan-da technology corporation, china), were analyzed according to the standard methods described in gb/t 5750.4-2006 of china.in order to observe the composition and surface characteristics of the solids formed in the pipe line, scanning electron microscopy (jsm-6490lv, jeol ltd., japan) and x ray diffractometer (ultima iv, rigaku corporation, japan) were applied in the study.2results and discussions2.1zinc () adsorption on al(oh)3 scalein order to investigate the adsorption of zinc() on amorphous al(oh)3 scale, a drinking water distribution system was built. experiments were conducted in two stages. in the first stage,125the major water quality parameters including ph, water temperature, and zn2+ content were fixed at 7.5, 15, and 1.0 mg/l, respectively. the residual concentration of zn2+ in the bulk water was low, around 1.03 mg/l (fig. 2). however, as the flow volume increase, less zn2+ was removed from drinking water. when the flow volume was 4.0 l, zn2+ residual increased to 1.25 mg/l, closed to the concentration of zn2+ addition.stage stage 1.31.2zn2+ concentration ( mg/l)1.11.00 2000 3500 4000 4500 50005500v ( ml )v=0.2 ml/s v=1.0 ml/s ph 6.5ph 8.5c = 2.1 mg/lfig. 2 zn2+ adsorption on amorphous al(oh)3 scale formed in the modeling system.130135140145150155zinc speciation under the same water quality was calculated applying the visual minteq software. it was found that more than 99% of the zinc () existed in soluble forms, indicating that most of the zn2+ removed from drinking water was absorbed by al(oh)3 scale. at the end of stage, al(oh)3 reached to its maximum adsorption capacity. 1.0 g amorphous al(oh)3 adsorbed about 0.2 mg zn2+. in order to investigate the effects of solution ph and flow velocity on zn2+ adsorption, the system were operated at different conditions in stage .the combination between zn2+ and al(oh)3 was stable. the concentration of residual zn2+ decreased to 0.0 mg/l, when the system stopped zinc addition (fig. 2). however, when zn2+ addition increased to 1.61 mg/l, residual zn2+ was 1.6 mg/l, indicating that al(oh)3 had reached its maximum adsorption capacity. when the flow velocity decreased from 0.5 to 0.2 ml/s, zn2+ varied little. when the flow velocity was 1.0 ml/s, however, residual zn2+ increased notably toabout 1.35 mg/l (fig. 2). this might be connected with the flow flushing effects; leading to partial adsorbed zn2+ entered the water body.besides flow velocity, ph had notable effects on zn2+ adsorption (fig. 2). increasing ph to8.5, more zn2+ was absorbed. its concentration decreased gradually to 0.95 mg/l; while when the solution ph decreased to 6.5, residual zn2+ varied little. considering the al(oh)3 was prepared prior to zn2+ adsorption. most of the zn2+ was adsorbed just on the surface of al(oh)3 scale. in drinking water distribution systems, however, al(oh)3 scale usually formed with the adsorption of zn2+ simultaneously. zinc not only existed on the surface but also inside of amorphous al(oh)3 scale.2.2zinc () adsorption in the suspended al(oh)3 solutionin order to simulate the adsorption process in the pipe line, static adsorption test were conducted. solution ph was found to be the major factor affecting zn2+ adsorption. at ph 6.5, residual zn2+ were stable with the reaction, indicating that the adsorption of zn2+ on al(oh)3 was a fast process (fig. 3). increasing zn2+ addition from 0.1 to 0.5 mg/l, similar experimental results were obtained; the increment of zn2+ addition was much higher than the increment of residual zn2+, 0.07 mg/l. most of the added zn2+ was adsorbed.0.3ph6.5ph7.5zn2+ concentration ( mg/l )0.2czn0.1 mg/l= 0.5 mg/lc = 1.0 mg/lzn0.10.016016517005101520adsorption time ( min )fig. 3 zinc () adsorption with reaction time in amorphous al(oh)3 suspension.when the solution ph was 7.5, zn2+ residual was low (fig. 3). in the systems with 0.1, 0.5, and 1.0 mg/l zn2+ addition, residual zn2+ was not detected. similar experimental results were obtained at ph 8.5 (not shown in fig. 3). according to the thermodynamics calculation and x-ray diffraction spectrum analysis, zinc hydroxide or the other zinc-containing solids were not formed in the system. it was concluded that the removal of zn2+ was not attributed by its precipitation but by the adsorption of al(oh)3. in the drinking water without ha, al(oh)3 showed a strong zn2+ adsorption capacity which was notably affected by ph.based on the concentrations of residual zn2+, kd was calculated (table 2). when the solutionph was above 7.5, zn2+ removed from drinking water was close to the amounts adsorbed by al(oh)3, indicating that amorphous al(oh)3 showed good adsorption capacity in alkaline solution (fig. 4). however, when the solution ph was below 7.5, zn2+ removal was much higher than the predicted value. from equation 1, it could be concluded that the adsorption of zn2+ on amorphous al(oh)3 was highly ph dependent; and it was inhibited in acidity solution. besides,there were large amounts of pores on the surface of amorphous al(oh)3 (fig. 5). because of the existence of surface energy, more zn2+ was absorbed.zn2+ adsorbed by al(oh)30.2zn2+ residual (calculated value) zn2+ residual (determined value)zn concentration (mg/l)0.10.06.0 6.5 7.0 7.5 8.08.5ph175fig. 4 theoretical concentrations of zn2+ adsorbed by amorphous al(oh)3 at different solution ph.abc180fig. 5 morphology characteristics of amorphous al(oh)3. (a) before adsorption; (2) after adsorption for zn2+; (3)after adsorption for ha.table 2 adsorption equilibrium between amorphous al(oh)3 and zn2+ at 15.total zn=alohzn2+=alozn30.86143.8426.96.5-2.4446.26004.7141.56.5-2.3361.55844.8956.66.5-2.2076.95695.2371.76.5-2.12 (mol) (mol) (mol) (mol) ph logk logk -2.27185190elements analysis showed that aluminum and oxygen were the major elements of the aluminum-containing scale (table 3). al/o molar ratio was about 1:3, agreeing well with the elements composition of amorphous al(oh)3; and zinc was not detected. while after adsorption,its molecular weight ratio was about 1.4%. every gram of al(oh)3 could adsorb 15.9 mg of zn2+,further indicated that amorphous al(oh)3 showed strong adsorption capacity for zn2+ in the suspended solution.table 3 elements composition of amorphous al(oh)3 before (a) and after (b) adsorption.elments before adsorption after adsorption weight (%) mole (%) weight (%) mole (%)o 62.84 74.33 59.24 68.17al 33.57 23.54 30.61 20.89s 3.60 2.12 2.98 1.71zn 0.00 0.00 1.40 0.401952002052.3effects of ha on zn2+ adsorption in solution with ha coexistingbesides zn2+, ha also showed good adsorption capacity on amorphous al(oh)3. in order to reveal the reaction mechanism among ha, zn2+ and amorphous al(oh)3, the adsorption of ha on al(oh)3 was investigated. it was found that in solution without zn2+, 50 mg/l al(oh)3 input could adsorb nearly all of the ha at ph 6.58.5 (fig. 6a). similar to the adsorption of zn2+, ph also had little effects on the adsorption of ha. davis (2003) found that the adsorption of organic matters on suspended al(oh)3 was the main reason leading to the surface characteristics ofsuspended matters modified 26.as the most commonly existed organic matter, ha could react with aluminum and zn2+ by the formation of stable complexes. with ha co-existing in drinking water, the mainly parameter affecting zn2+ adsorption was not solution ph but ha. in the solution with 1.0 mg/l ha, nearly all