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Electrochemical treatment of tannery effluents

Publication . Caliari, Paulo Cezar; Lopes, Ana Maria Carreira

Tannery wastewater is highly complex and contains high concentrations of organics and other toxic chemicals, such as sulfide and chromium, which inhibit the activity of microorganisms during biological oxidations and are not removed completely from wastewater. Since the biological processes are not able to fully depollute tannery effluents, other technologies, such as electrochemical oxidation processes, are being studied for the treatment of aqueous wastewaters. In this work, the study of the electrochemical oxidation of samples from tannery wastewater, clarified or not, was carried out. Assays were run under galvanostatic conditions, with different current densities, in electrochemical reactors equipped with anodes based on metal oxides, Ti/Pt/PbO2 and Ti/Pt/SnO2-Sb2O4, and of boron-doped diamond (BDD). For the pretreatment by clarification, chemical coagulation (with iron and aluminum ions) and electrocoagulation (with iron and aluminum consumable anodes) were used. The performance of each assay was followed by variations observed in the concentrations of COD, DIC, DOC, TDC, TKN,TN, N-NH4 +, N-NH3, NO3 -, NO2 -, S2- and SO4 2-. The kinetics of the conversion of S2- into SO4 2- , in aqueous solutions of S2- (60 mM), by electro-oxidation using a BDD electrode was also investigated. For the conversion of S2- to SO4 2-, assays were run at current densities from 10 to 60 mA cm-2, with a duration varying from 10 to 42 h. The results showed that the electrochemical conversion occurs in steps, via intermediate production of other sulfur species (S2- Sx 2- SnOm y-, with x, n, m and y integers) and the oxidation rate of the sulfide ion is dependent on its concentration and current density. The reaction order strongly varies with the applied current intensity, being order 2 for the lower applied current density. For higher applied current densities, where the current control is less important, the reaction order varies from 0.15 to 0.44 for the applied current densities of 20 and 60 mA cm-2, respectively. For the formation of SO4 2- from S2- electro-oxidation, the reaction order with respect to sulfide varied from 0.35 to 0.05 when the applied current densities changed from 10 to 60 mA cm-2. The electrochemical oxidation for unclarified effluent samples under a current density of 30 mA cm-2 and in a batch with stirring (100 rpm) during 8 h assays showed the best performance for BDD electrode in the removal of COD and TDC. The others two tested electrode presented similar behavior in the DOC removal; Ti/Pt/SnO2-Sb2O4 was the least efficient in the oxidation of nitrogen to NO3 -, despite providing considerable production of gaseous nitrogen compounds, which reduces the total nitrogen load present in the final samples; removal of S2- was more intense at BDD, although the other electrodes presented similar behavior. Ti/Pt/SnO2-Sb2O4 electrode was more effective to form SO4 2-. Regarding specific charge consumption, BDD showed the lowest consumption, 2.70 C g-1 of COD removed, against 3.25 and 3.21 C g-1 of COD for the Ti/Pt/PbO2 and Ti/Pt/SnO2-Sb2O4 electrodes, respectively. In all clarifications assays the best performance was verified for chemical coagulation. This appears be derived from a better operational control in chemical coagulation than in electrocoagulation. In addition, iron cation was more efficient than aluminum cation. This is important because the clarification by iron avoids the presence of aluminum in the final sludge In the case of anodic oxidation (8 h assays, current density of 30 mA cm-2 and constant stirring at a rate of 100 rpm) for effluent samples (400 mL), previously clarified by chemical coagulation or electrocoagulation by aluminum, the clarification stage strongly influenced the anodic oxidation processes and showed that electrodegradation by Ti/Pt/PbO2 and Ti/Pt/SnO2-Sb2O4 electrodes can be used as effluent polishing step. BDD electrode generally showed better performance in the removal of contaminants from the not clarified samples and was very efficient in the S2- removal but not in its conversion to SO4 2-. On the contrary, Ti/Pt/SnO2-Sb2O4 electrode showed better performance in the production of SO4 2- from intermediate sulfur species. The energy consumption of the anodic oxidation processes performed with Ti/Pt/PbO2 and Ti/Pt/SnO2-Sb2O4 anodes is strongly influenced by the contaminant concentrations. The combined process, chemical coagulation followed by anodic oxidation with metal oxide electrodes, proved to be a good alternative to the BDD electrodes for wastewater treatment. The same situation was observed in the anodic oxidation of samples previously clarified by chemical coagulation or electrocoagulation using iron cation. Finally, in the last stage, clarified samples by Fe3+ ion, 0.25 g L-1, were submitted to anodic oxidation (8 h) in two different sets with recirculation system: set A composed by Ti/Pt/SnO2- Sb2O4 + Ti/Pt/PbO2 electrodes, and set B with BDD only. The current densities in each set were 20 mA cm-2 and 40 mA cm-2 for Ti/Pt/SnO2-Sb2O4 and Ti/Pt/PbO2, respectively, in set A; and 60 mA cm-2, for BDD, set B. Both the tested sets showed similar behavior in the removal of COD, TC, DOC, TKN and ammonia nitrogen, showing the feasibility of metal oxides as electrodes in replacement to BDD electrode in anodic oxidation systems; set A was the least efficient in the oxidation of nitrogen to NO3 -, despite show similar behavior to the set B for production of gaseous nitrogen compounds, which reduces the total nitrogen load present in the final samples; the COD concentration worked as an important inhibiting factor for TKN removal. Regarding energy consumption, in a general way, the set B showed the lowest consumption.

2017-02Doctoral thesis

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