关 键 词：

凝聚 除去 水溶液 中的

资源描述：

电凝聚法除去水溶液中的磷,凝聚,除去,水溶液,中的

内容简介：

Journal of Hazardous Materials 106B (2004) 101105Removal of phosphate from aqueous solutions byelectro-coagulationNihal Bektas, Hilal Akbulut, Hatice Inan, Anatoly DimogloEnvironmental Engineering Department, Gebze Institute of Technology, Gebze/Kocaeli, P.O. Box 141, 41400, TurkeyReceived 28 July 2003; received in revised form 8 October 2003; accepted 13 October 2003AbstractThe aim of this paper was to investigate the feasibility of the removal of phosphate from aqueous solution by electro-coagulation (EC).The current density (CD) between 2.5 and 10 mA cm2and duration in the limits of 520 min were tried for different concentrations. In orderto determine optimal operating conditions, the EC process used for the phosphate removal was examined in dependence with the CD, initialconcentrations and time. The results of the experimental batch processing showed high effectiveness of the EC method in removing phosphatefrom aqueous solutions. 2003 Elsevier B.V. All rights reserved.Keywords: Phosphate; Electro-coagulation; Water treatment; Nutrient removal; Al electrode1. IntroductionAs known, phosphate discharged into the surface watersaccelerates eutrophication 1,2. This can in turn disturb thebalance of organisms present in the water and affect waterquality, mainly through the depletion of oxygen level as thealgae decay. Reduced oxygen level can have harmful effectson fish and other aquatic life, causing reductions in biodi-versity. Eutrophication can also affect the recreational valueof natural resources. Generally, total phosphorus concentra-tions in excess of 100H9262g P/l provide sufficient nutrient en-richment in the lakes.The treatment of phosphate can be achieved by severalphysical, chemical, and biological methods 3. In biolog-ical treatment plant, it is necessary to transfer phosphatefrom liquid to the sludge phase and the removal efficiencyusually does not exceed 30%, which means that remainingphosphate should be removed by another techniques 4.The phosphate removal from wastewater by adsorption us-ing different materials has also been explored 5,6. Chem-ical precipitation is also widely used for phosphate removal7,8. The common precipitants used are the aluminum sul-phate and ferric chloride. Chemical precipitants imply fur-Corresponding author. Tel.: +90-262-653-84-97x1428;fax: +90-262-653-84-90.E-mail address: dimoglo.tr (A. Dimoglo).ther costs for the purchase and installation of dosing equip-ment and operating costs for power, disposal of additionalsludge, manpower, and surely chemicals used. Moreover,during the process, an alkaline condition is necessary forprecipitation by calcium and neutral to acidic environmentsare required if aluminum and iron are used. This not onlyincreases the chemicals expenditure but also is detrimentalto biological treatment processes. The removal of differentforms of phosphorous (namely, soluble phosphorous, par-ticulate phosphorous, and total phosphate) has been studied9. The results of the paper provide a basis for developingsuch schemes for phosphate removal, which may be adaptedto particular configurations of the wastewater studied.Electro-coagulation (EC), which is known as a reliableand mainly cost-effective wastewater treatment process1018, is characterized by simple and easy-to-operateequipment, short operation time, none or negligible amountof chemicals and decreased amount of sludge 19,20. Theflocs formed by EC are relatively large and contain lessbound of water. They are also more stable and thereforeamenable to filtration 21.The aim of this paper was to study the feasibilityof the removal of phosphate from aqueous solution byelectro-coagulation. The process was examined under dif-ferent values of current density (CD), pH, initial concen-trations, and time, in order to determine optimal operatingconditions.0304-3894/$ see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.jhazmat.2003.10.002102 N. Bektas et al. / Journal of Hazardous Materials 106B (2004) 101105TC pHPurified waterFilter Inlet Electro-coagulation cell Magnetic bar-stirrerDC Power supply +Fig. 1. The EC reactor used in the laboratory experiments.2. Materials and methods2.1. ChemicalsAll chemicals used were analytical grade and usedwithout further treatment. Distilled water was used in allexperiments. Stock phosphate solutions were preparedfrom KH2PO4(Carlo Erba, 99.5%). A spectrophotometer(MERCK) was used for the analysis of phosphate in ac-cordance with the standard methods (vanadomolybdophos-phoric acid calorimetric method) 22 after separating theturbidity through the standard filter designed for a widerange of laboratory applications (MF-Millipore membranefilter, mixed cellulose ester, 0.45H9262m). When necessary, pHwas adjusted by using 0.1 N HCl or 0.1 N NaOH.2.2. ExperimentalA laboratory-scale reactor (15 cm8cm8 cm, 500 ml),made of organic glass, was used in all experiments (Fig. 1).Two groups of alternating electrodes being cathodes and an-odes (by eight plates of each type) were arranged vertically.Distances between any paired anode and cathode were takenequal to 3 mm.The electrodes were made of Al and immersed, prior tothe experiments, in 1% solution of HCl for 8 h. They wereconnected to terminals of a direct current power supply char-acterized by the ranges 05 A for current and 030 V forTable 1Reactions at the electrodes and in the bulk solutionAnode In bulk solution Cathode4OH 4e= 2H2O + O2(g)Cl2(g)+ H2O = HOCl + H+ Cl2H3O+ 2e= H2(g)+ 2H2O (in acid solutions)2H2O 4e= O2(g)+ 4H+2Cl 2e= Cl2(g)Al(aq)3+ 3H2O = Al(OH)3+ 3H+2H2O+ 2e= H2(g)+ 2OH(in alkaline solutions)Al(s) 3e= Al(aq)3+Al(aq)3+ PO43= AlPO4O2+ 2H2O + 4e= 4OHvoltage. Temperature (2122C) was stable during the ex-periments. Synthetic phosphate solutions (their concentra-tion varied in the range of 10 and 200 mg l1) were takenfor the study. At the beginning of each run the solution ofphosphate (450 ml) of the desired concentration was fed intothe reactor and 50 ml of 1% solution of NaCl (as electrolyte)was added to it to increase the solution conductivity. Eachrun was timed starting with the DC power supply switchingon.As known, Al3+ions dissolve and combine with hy-droxyl ions in the water 23,24 when direct current passesthrough the Al anodes. They form metal hydroxides, whichare partly soluble in the water under definite pH values.This step results in the colloidal particles formation withmetal hydroxides as nuclei of the latter. Around a nucleus,the adsorption layer of cations and anions is being orga-nized. Taken together, the nucleus and adsorption layerform a granule of the colloidal particle, which has a smallpositive charge. To compensate the charge, a diffusion layeris being formed around the granule, which makes the parti-cle a neutral one. The main reactions that occur during theelectrolysis reactions are shown in Table 1.It is to be noted that the disengaged Al3+ions formhard-to-dissolve sediment of AlPO4in the presence ofPO43ions. The specific electrical energy consumptionof the reactor for the phosphate removal is low enough(0.81.5 kWh m3) for the values of CD in the range of2.510 mA cm2.N. Bektas et al. / Journal of Hazardous Materials 106B (2004) 101105 1033. Results and discussion3.1. pH, time and initial concentrations effectsInitially the percentage of phosphate removal (PPR) fromthe solution in dependence with its initial pH values wasstudied. As seen from Fig. 2, under low initial value of pH4, the 15 min duration of the EC process gives PPR = 80%.The initial pH increase (up to 7) causes increase of PPR forthe same duration. PPR achieves its maximum under pH 6.0.If pH increases, the PPR values are somewhat decreased. Itis worth of noting that in the EC process pH itself changes.For low initial pH (4 and 5) 15 min processing implies pHincrease up to pHfin(6.0 and 6.5, respectively). If higherinitial pH (7 and 8) is taken, pHfindecreases (6.6 and 6.9,respectively).In other words, if the solution accumulates enoughAl(OH)3and AlPO4, then pH of the solution stabilizes (pH6.5), irregardless of the initial pH. Based on the observation,further studies were carried out under natural initial pH 6.2.In Fig. 3, variations of the phosphate removal are shownin dependence with the EC duration and initial concen-tration of the solution. CD was stable in the experiments(10 mA cm2).As seen from Fig. 3, first 5 min of the EC process giveinconsiderable PPR (3550%) for 200 and 100 mg l1ofinitial phosphate concentrations. Under low phosphate con-centrations (1050 mg l1) and EC duration equals to 5 min,the removal degree is 8190%. The increase of the EC du-ration gives better results of the removal parameter. Thus,the 10 min EC process duration demonstrates the 70 and94% phosphate removal for 200 and 100 mg l1solutions,respectively. The most considerable phosphate removal isreached under some optimal values of EC duration (15 min).However, further increase of the EC duration is of small in-fluence on the degree of phosphate removal, as seen fromFig. 3. Curves in the figure become parallel and close toeach other. The solution concentration also influences the70809010045678pHinitPhosphate removal, %)PPRpHfin7.06.56.0pHfinFig. 2. Percentage of the phosphate removal vs. initial pH (C0= 100 mg l1,CD= 10 mA cm2, t = 15 min, T = 21.5C).204060801005101520Time (min)Phosphateremoval(%)10 mg/L25 mg/L50 mg/L100 mg/L200 mg/LFig. 3. Percentage of the phosphate removal vs. time and initial concen-trations (pH 6.2, CD = 10 mA cm2, T = 21.5C).time of the phosphate removal. For low concentrations (up to50 mg l1) the PPR is high enough. For high concentrations(100, 200 mg l1), 5 and 10 min duration results in low PPRvalues. Under duration of 15 and 20 min PPR increases (forthe same concentrations of the solution). It can be supposedthat the amount of Al3+ions generated at the first stagesof the EC is not sufficient for the phosphate ions binding.When duration of the EC process increases, the Al3+ionsconcentration increases as well and results in higher quan-tity of the bound phosphate.A linear dependency exists between EC duration andcharge loading, Q (see Fig. 4). By this, higher solution con-centrations imply higher values of Q. As far as conductivityalso depends on the initial concentration linearly, voltageincreases linearly under fixed CD.Analogous temporal dependency is observed for the elec-trical energy consumption, W (see Fig. 5). For low con-centrations of phosphate ions (10100 mg l1) W does notexceed 1 kWh per m3when the process duration is equalto 20 min. For the initial concentration of 200 mg l1, thevalue of W does not exceed 1.5 kWh m3for same dura-tion.104 N. Bektas et al. / Journal of Hazardous Materials 106B (2004) 10110500,511,522,533,545 10 15 20Time (min)Q (F/m3)10 mg/L25 mg/L50 mg/L100 mg/L200 mg/LFig. 4. Charge loading (Q) vs. time and initial concentrations (pH 6.2;CD = 10 mA cm2, T = 21.5C).00,40,81,21,65101520Time (min)W (kWh/m3)10 mg/L25 mg/L50 mg/L100 mg/L200 mg/LFig. 5. Electrical energy consumption (W) vs. time and initial concentra-tions, (pH 6.2; CD = 10 mA cm2, T = 21.5C).3.2. Effect of current densityPreviously it was shown that CD can influence the treat-ment efficiency of the EC process. In Fig. 6 the dependencyof PPR on the CD is shown for different initial concentra-tions of the phosphate solution.As seen from Fig. 6, the maximal percentage of PO43ions removal for low initial concentrations (1050 mg l1)4050607080901002,5 5 7,5 10Current density (mA/cm2)Phosphate anion removal (%)10 mg/L25 mg/L50 mg/L100 mg/L200 mg/LFig. 6. Percentage of the phosphate removal in dependence on the currentdensity and initial concentrations (pH 6.2, t = 15 min, T = 21.5C).happens already under low CD values (2.5 mA cm2).For high initial concentrations of the phosphate (100200mg l1), low level of CD is not so effective for the phosphateremoval. A considerable qualitative jump can be observedbeginning with CD = 7.5mAnm2. Data given in Figs. 3and 6 show that the 15 min duration and CD = 7.5 are con-sidered as optimal parameters for the effective phosphateremoval by means of EC (90%) under its high initial con-centration (100200 mg l1). For low initial concentrations(1050 mg l1), duration is 7 min and CD = 5mAcm2.4. ConclusionsThe efficiency of the EC processes applied for thephosphate-containing aqueous solutions purification is stud-ied in this work. The results show that the EC processesprovide high and stable effects as to the contaminants re-moval. The results of the EC application have shown that itis capable of removing phosphate from aqueous solutionsof high enough concentrations.The EC method does not use any chemical reagents andmakes the process of phosphate-containing wastewater treat-ment easy for regulation and automation. A constructionaloptimization of the EC apparatus for the phosphate treat-ment is offered. In our opinion, further studies can promotesuccessive passing from laboratory-scale models to the in-dustrial specimens possessing same level of technologicalcharacteristics.References1 Pollution; Causes, Effects, and Control, R.M. Harrison (Ed.), thirded. The Royal Society of Chemistry, London, 1996.2 G. Kiely, Environmental Engineering, McGraw-Hill, New York,1997.3 G.K. Morse, S.W. Brett, J.A. Guy, J.N. Lester, Sci. Tot. Environ.212 (1998) 69.4 C. Sommariva, A. Converti, M. Del Borghi, Desalination 108 (1996)255.5 M. zacar, Chemosphere 21 (2003) 321.6 A. Ugurlu, B. Salman, Environ. Int. 24 (1998) 911.7 K. Fytianos, N. Raiko, E. Voudrias, Environ. Pollut. 101 (1998) 23.8 M. zac