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Treatment of tannery liming drum wastewater by electrocoagulation AbstractThe removal of COD, suphide and oilgrease from tannery liming drum wastewater was experimentally investigated using direct current (DC) electrocoagulation (EC). In the EC of the wastewater, the effects of initial pH, electrolysis time and current density were examined. The COD, sulphide and oilgrease in the aqueous phase were effectively removed when mild steel electrodes were used as sacrificial anode. The optimum operating range for each operating variable was experimentally determined. The experimental results show that COD, sulphide and oilgrease was removed effectively. The overall COD, sulphide and oilgrease removal efficiencies reached 82%, 90% and 96%, respectively. The optimum current density for removal of COD, sulphide and oilgrease in the tannery liming drum wastewater were 35mA/cm2, 35mA/cm2 and 3.5mA/cm2 at 10min electrolysis time and pH 3, respectively. Mean energy consumptions were 5.768kWh/m3 of COD, 0.524kWh/m3 of sulphide and 0.00015kWh/m3 of oilgrease. Results show that the pseudo-second-order rate equation provides the best correlation for the removal rate of the parameters.Keywords: Tannery wastewater; Liming drum wastewater; Electrocoagulation; Mild steel electrodesArticle Outline1. Introduction2. Experimental2.1. Wastewater samples 2.2. Experimental device 2.3. Experimental procedure 2.3.1. Iron electrodes2.3.2. Aluminum electrodes3. Results and discussion3.1. Effect of electrode materials 3.2. Effect of initial pH 3.3. Effect of current density 3.4. Effect of electrolysis time 3.5. Electric Energy Consumption4. ConclusionsReferences1. IntroductionThe leather industry is well known as a high consumer of water. Variety of chemicals at significant quantities are employed for leather processing. Major chemicals used for leather manufacturing are lime, sodium and ammonium salts, fatliquors, bacterial- and fungicides, tannins, dyes, etc. Wastewater from the leather industry is known to be heavily contaminated with inorganic and organic pollutants. It can create heavy pollution from effluents containing high levels of salinity, organic loading, inorganic matter, dissolved and suspended solids, ammonia, organic nitrogen and specific pollutants (sulphide, chromium and other toxic metal salt residues) 1, 2 and 3. It was reported that the total global quantity of bovine hides, sheep, goat and pigskins was nearly 8106t as wet salted weight a year and tanning workshop worldwide used 4106t of chemicals, produced over 3108t of wastewater and about 8106t of solid waste and dewatered sludge 4 J. Buljan, Salinity within tannery effluents, World Leather 18 (2005), pp. 1820.4. There are many processes for the treatment of tannery wastewater such as chemical coagulation 5, 6 and 7, reverse osmosis membrane 2 and 8, nanofiltration 9, Fenton and H2O2/UV 10 and 11, biodegradation 6, 12, 13, 14 and 15. Although biodegradation process is cheaper than other methods, it is less effective because of the toxicity of the tannery wastewater that affects the development of the bacteria 16.Due to the limitations of the primary and biological wastewater treatment processes, alternative processes have been pursued. Amongst them, electrochemical processes have been proposed and they have received increasing attention in the last years. Compared to traditional methods, electrochemical processes offer: (a) versatility, since they may be used to treat liquid and solid waste by direct and indirect organic compound oxidation, metal reduction and electrodeposition, not to mention the electrocoagulation (EC) and electroflotation processes; (b) automation, since the current and the potential are parameters that are easily acquired and controlled, facilitating the automation of the treatment process; (c) environmental compatibility, since electrochemical processes are mediated by electron exchange with the electrode surface, dismissing the need for the addition of other chemical agents 17. Oxide electrodes may be used in the electrochemical treatment of wastewaters containing high concentrations of potentially polluting species, and they can promote the oxidation of their organic constituents 17 and 18.As far as the chromium salts is concerned, the tanning process consumes only 60% of the chromium of the tanning bath and the possibility to recover the residual metal represents a main goal in process improvement. A number of articles have been reported regarding removal of Cr especially Cr(VI) from tannery wastewater in the light of its severe hazardous impact upon the environment 19, 20, 21 and 22.EC/electro-Fenton process can be another alternative process for treating tannery waste effluents 16 and 23. Compared with other methods, there are a few advantages for the treatment of wastewater by EC 24, 25 and 26. EC with aluminum and iron electrodes was patented in the US in 1909. The EC of drinking water was first applied on a large scale in the US in 1946 27 and 28. In recent years, EC has been successfully tested to treat various industrial wastewaters including decolorization of dye solutions, polishing wastewaters, removal of phenolic compounds and dairy wastewater treatment 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 and 58.The first step in the liming drum is the soaking of the hide. Using water, the raw material is cleaned of any natural hide dirt. The main objective in soaking the hide is to restore it to its natural condition as on the living animal with a water content of approximately 65%. After this, the liming takes place in the same drum. Here, substances that are not important for the leather-production process, such as natural oils and proteins, are washed out of the hide. By adding lime and sodium sulphide, the pH-value during the dispensing process is raised. By doing so, hair is chemically removed from the surface of the hide. Soaking and liming are carried out in one process, which typically lasts from 24h to 36h 59. The effluent emanating from the beamhouse of tannery contains high concentration of sulphide ions. Since these effluents are toxic to the aquatic environment, it is essential to neutralize them and bring the discharge levels of these species to below the toxic limit 1.In the present study, the EC effect and mechanisms of chemical oxygen demand (COD), sulphide, oilgrease with different current densities, pH, conductivities and different soluble electrodes (mild steel electrodes and aluminum electrodes) were compared in detail.2. Experimental2.1. Wastewater samplesWastewater was obtained from a tank containing a mixture of the liming drum solutions at the tannery factory in Turkey (Sakarya). The composition of the wastewater is shown in Table 1.Table 1. Characteristics of wastewater.pH12.03COD (mgL1)25,300BOD5 (mgL1)10,850Suspended matter (mgL1)6,130Sulphide (mgL1)3,000Oilgrease (mgL1)185Conductivity (mScm1)37.2Full-size tableView Within Article2.2. Experimental deviceThe batch experimental setup is schematically shown in Fig. 1. The EC unit consists of an electrochemical reactor, a D.C. power supply and iron electrodes. The electrodes consist of pieces of sheet mild steel or aluminum separated by a space of 2.5cm and dipped in the wastewater. The electrodes were placed into 400mL wastewater in a 1150mL plexiglass electrolytic reactor. There were four electrodes connected in a monopolar mode in the electrochemical reactor, each one with dimensions of 9.3cm7.5cm0.3cm. The submerged surface area of the electrode plates was 140cm2. The stirrer was used in the electrochemical cell to maintain an unchanged composition and avoid the association of the flocs in the solution. The D.C. source was used to power supply the system with 015V and 03A. A Sunwa Electronics multimeter, model-YX-360TR-EB, was used for measurement the current and the potential between the two electrodes. Electrodes were washed with dilute HCl between the experiments. Experiments were conducted at 20C.Full-size image (28K)Fig. 1.Bench-scale EC reactor with bipolar electrodes in parallel connection. (1) Electrocoagulation cell; (2) anode; (3) cathode (iron); (4) bipolar electrodes; (5) magnetic stirrer controller; (6) magnetic stirrer bar; (7) D.C. power supply. View Within Article2.3. Experimental procedureAt the beginning of a run the wastewater was fed into the reactor and the pH was adjusted to a desired value using HCl, NaOH solutions. The electrodes were placed into the reactor. The reaction was timed, starting when the D.C. power supply was switched on. One of the greatest operational issues with EC is electrode passivation. During EC with electrodes, an oxide layer was formed at the anode. Eliminating the oxide formation at the anode could reduce this effect. For this reason, the electrodes were rinsed in the diluted HCl solution after the each experiment. Samples were periodically taken from the reactor. The particulates of colloidal ferric oxyhydroxides gave yellowbrown color into the solution after EC. The sedimentation was filtrated with normal filter paper. Standart Methods for Examination of Water and Wastewater were adopted for quantitative estimation of pH, conductivity, COD, oilgrease and sulphide 60. All the experiments were repeated twice, and the experimental error was below 4%, the average data were reported.If iron or aluminum electrodes are used, the generated Fe(aq)3+ or Al(aq)3+ ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides. The Fe(II) ions are the common ions generated the dissolution of iron. In contrast, OH ions are produced at the cathode. By mixing the solution, hydroxide species are produced which cause the removal of matrices (dyes and cations) by adsorption and coprecipitation.2.3.1. Iron electrodesIn the study of iron anodes, two mechanisms for the production of the metal hydroxides have been proposed 24, 25 and 26: Mechanism 1 pH4(1)anode: 4Fe(s)4Fe(aq)2+8e(2)bulk of solution: 4Fe(aq)2+10H2O(l)+O2(aq)4Fe(OH)3(s)+8H(aq)+(3)cathode: 8H(aq)+8e4H2(g)(4)overall: 4Fe(s)+10H2O(l)+O2(aq)4Fe(OH)3(s)+4H2(g)4pH7(5)anode: 4Fe(s)+24H2O(l)4Fe(H2O)4(OH)2(aq)+8H(aq)+8e(6)bulk of solution: 4Fe(H2O)4(OH)2(aq)+O2(aq)4Fe(H2O)3(OH)3(s)+2H2O(l)(7)bulk of solution: 4Fe(H2O)3(OH)3(s)2Fe2O3(H2O)6(s)+6H2O(8)cathode: 8H(aq)+8e4H2(g)(9)overall: 4 Fe(s)+16H2O(l)+O2(aq)2Fe2O3(H2O)6(s)+4H2(g)6pH9Precipitation of Fe(III) hydroxide (7) continues, and Fe(II) hydroxide precipitation also occurs presenting a dark green floc.(10)bulk of solution: 4Fe(H2O)4(OH)2(aq)4Fe(H2O)4(OH)2(s)Mechanism 2 pH4(11)anode: Fe(s)Fe(aq)2+2e(12)cathode: 2H(aq)+2eH2(g)(13)overall: Fe(s)+2H+Fe(aq)2+H2(g)4pH9(14)anode: Fe(s)+6H2O(l)Fe(H2O)4(OH)2(aq)+2H(aq)+2e(15)bulk of solution: Fe(H2O)4(OH)2(aq)Fe(H2O)4(OH)2(s)(16)cathode: 2H(aq)+2eH2(g)(17)overall: Fe(s)+6H2O(l)Fe(H2O)4(OH)2(s)+H2(g)Mechanism 3 4pH9(18)anode: 2Fe(s)+12H2O(l)2Fe(H2O)3(OH)3(aq)+6H(aq)+6e(19)bulk of solution: 2Fe(H2O)3(OH)3(aq)2Fe(H2O)3(OH)3(s)(20)bulk of solution: 2Fe(H2O)3(OH)3(s)Fe2O3(H2O)6(s)+3H2O(l)(21)cathode: 6H(aq)+6e3H2(g)(22)overall: 2Fe(s)+12H2O(l)Fe2O3(H2O)6(s)+3H2(g)In the oxygenated water and at lower pH, Fe2+ is easily converted to Fe3+. The Fe(OH)n(s) formed remains in the aqueous stream as a gelatinous suspension, which can remove the waste matter from wastewater either by complexation or by electrostatic attraction followed by coagulation. Ferric ions electrogenerated may form monomeric ions, ferric hydroxo complexes with hydroxide ions and polymeric species, namely, Fe(H2O)63+ Fe(H2O)5OH2+, Fe(H2O)4(OH)2+, Fe2(H2O)8(OH)24+, Fe2(H2O)6(OH)42+ and Fe(OH)4 depending on the pH range 26. The complexes (i.e. hydrolysis products) have a pronounced tendency to polymerize at pH 3.57.0 25 and 26.2.3.2. Aluminum electrodesThe generated Al3+ and OH react with each other to form Al(OH)3 25 and 26:(23)anode: Al(s)Al(aq)3+3e(24)cathode: 3H2O(l)+3e3/2H2(g)+3OH(aq)(25)overall: Al(aq)3+3H2O(l)Al(OH)3(s)The electrolytic dissolution of the aluminum anode produces the cationic monomeric species such as Al3+ and Al(OH)2+ at low pH, which at appropriate pH-values are transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n.(26)nAl(OH)3Aln(OH)3n3. Results and discussion3.1. Effect of electrode materialsFirst of all, treatment of liming drum wastewater, by using mild steel and aluminum electrode materials was investigated. As shown from Fig. 2, there were no significant differences between mild steel electrodes and aluminum electrodes for the elimination of oilgrease under the same condition. Under the same condition, the elimination rate of COD and sulphide using mild steel electrodes was higher than that of aluminum electrodes. As a result, mild steel electrodes are superior with respect to aluminum as sacrificial electrode material for treatment liming drum wastewater.Full-size image (17K)Fig. 2.Effect of electrode materials on treatment of liming drum wastewater (pH 3; C0,COD=22,500mgL1; C0,sulphide=2100mgL1; C0,oilgrease=189mgL1, i=6.42mA/cm2; T=298K; d=2.5cm; agiation speed=200rpm; conductivity=35mScm1). View Within Article3.2. Effect of initial pHIt has been established that pH is an important parameter influencing the performance of the EC process 26. The effect of initial pH on the COD removal efficiency is presented in Fig. 3. High COD removal percent may be attained in acidic mediums, the efficiency with increasing pH at pH 35, maximum COD removal attainable is 62% with iron electrode.Full-size image (13K)Fig. 3.Effect of initial pH on the removal efficiency of liming drum wastewater (electrodes: mild steel; C0,COD=23,800mgL1; C0,sulphide=2480mgL1; C0,oilgrease=210mgL1; i=7.85mA/cm2; T=298K; d=2.5cm; agiation speed=200rpm; conductivity=37.2mScm1). View Within ArticleThe major portion of COD in wastewater may arise from sulphide since sulphide can be oxidized by Cr6+ in COD test. Therefore, there are no significant differences between COD and sulphide removal efficiency graphics as shown in Fig. 3.Depending on the pH, sulphide exists as H2S, HS and S2. Interaction between Fe2+ ions and H2S, HS and S2 species leads to the formation of FeS precipitate that is insoluble. Both, bulk and surface reactions between S2 and Fe2+/Fe3+ are expected under experimental conditions. Reactions that occur in solution phase are:(27)Fe2+H2SFeS+2H+(28)Fe2+HSFeS+H+(29)Fe2+S2FeSElemental sulphur is formed due to oxidation at anodic sites and also by Fe3+ reduction:(30)HS+OHS0+H2O+2eIn addition to monosulphides, FeS2 formation by polysulphide-pathways is also suggested according to the following equation 1:(31)FeS+S0FeS2The kinetics of Fe2+ conversion to Fe3+ are strongly affected by the pH; the surface charge of the coagulating particle also varies with pH. In general, at lower and higher pH, Fe is increasingly soluble 61.As shown in Fig. 3, the removal efficiency of oilgrease was not affected by the pH.3.3. Effect of current densityFrom Fig. 4, it is apparent that the removal efficiency was not improved continuously with increasing current density. Fig. 4 illustrates that the COD removal efficiency increased slowly from 60.8% to 82.1% by increasing the current density from 3.5mA/cm2 to 35mA/cm2 after 10min reaction time at pH 3. As the applied current density was increased from 35mA/cm2 to 70.7mA/cm2, the COD removal efficiency did not rise significantly. The graphics of the removal efficiency of COD and sulphide was analogous because the substantially reason of COD in wastewater was sulphide. After 10min of electrolysis oilgrease efficiency reached a maximum at 3.5mA/cm2 current density; a 9496% oilgrease removal was achieved under this condition.Full-size image (18K)Fig. 4.Effect of current density on the removal efficiency of liming drum wastewater (electrodes: mild steel; pH 3; C0,COD=25,300mgL1; C0,sulphide=3000mgL1; C0,oilgrease=185mgL1; T=298K; d=2.5cm; agiation speed=200rpm; duration of electrolysis=10min; conductivity=37.2mScm1). View Within ArticleAn oilwater emulsion is a colloidal dispersion in which oil constitutes the dispersed phase and water forms the continuous phase. Emulsions are normally stabilized by the presence of an emulsifying agent, such as a surfactant. The anionic head groups on the surfactant molecules prevent aggregation and coagulation of the oil droplets via electrostatic repulsion. During electrolysis in an EC procedure, the sacrificial iron anode is oxidized to polymeric ionic species. With progressive electrolysis the ionic strength of the medium increases. Ionic polymeric iron species can neutralize the surface charge of surfactant molecules. They can either generate bridges between surfactant molecules. Simultaneously, hydrogen as well as hydroxides is generated at the cathode (Eqs. (3) and (12). The pH of the medium rises as a result of this electrochemical process. The net result of the reactions is that