Study On Electrochemical Disposal Of Chromium Residue Solution And Reactive Mechanism Of Electrode

Most compounds of chromium are harmful to the environment and health of human being. Among them, compounds contain Cr⁶⁺ have the maximal toxicity because Cr⁶⁺ is carcinogen and could be assimilated easily in body. The main resources of Cr⁶⁺ are waste residues and water produced by industries such as chromite smelting or plating. Therefore, it is significant that exploring methods on disposing industrial waste residues and water contain Cr⁶⁺ in order to depollute and alleviate harm for human health. As follows, an electrochemical technology on disposing industrial chromium residue solutions and the electrode reactive mechanism have been investigated in this paper. First of all, UV spectrophotometry was applied to determine the Cr⁶⁺concentration in solutions. To dissolve the chromium and obtain the solutions of Cr⁶⁺, the optimal reagental dosage, 0.05mol/lH2SO4, was chosen. Based on the qualitative and qualitative experimental results, the maximal absorbed wavelengths of Cr6+ and chelate of Cr⁶⁺ with DPC were found, and a DPC spectrophotometry standard curve which is used to determine the concentration of Cr⁶⁺ in solutions was drawn. Moreover, ε(2.98 ×10⁴) and the relative error of this spectrophotometry were calculated, respectively. According to the orthogonal experimental results, the optimal dissolve condition of Cr waste residues was found: the mass of chromium residue was 0.20g, the pH of solution was 2.00, the temperature of dissolving was 20℃and the dissolving time was 60 minutes. Content of Cr⁶⁺ (Cr6+ %=9.99%)and Cr³⁺ (Cr3+%=1.15%) in chromium residue were accurately calculated. In the next place, an indirect electrolysis with dissoluble Fe anodes was adopted to dispose chromium residue solutions. Fe²⁺ which came from the dissolution of Fe anodes in acid solution could reduce Cr⁶⁺ to Cr³⁺, which is almost harmless and could precipitate as Cr(OH)₃ in the solutions so that the aim of eliminating the harm Cr6+ reaches. Based on the orthogonal experimental results, the optimal electrolytic condition was found: voltage of electrolysis was 8.00V, temperature of electrolysis was 60℃, time of electrolysis was 60 minutes and the area of anode was 26.40cm². Under the condition above, the clearance ratio of Cr⁶⁺ was 99.74% and the finial Cr⁶⁺ concentration in the solution after the electrolysis was about 0.35mg/l, which met the requirement of the national standard. Moreover, the relationships between the finial Cr6+ concentrations and the pH of solutions, current as well as the electrolytic time were discussed. Finally, some electrochemical methods, linear sweep voltammetry (LSV) and cycilic voltammetry (CV), were employed to investigate the electrode reactive mechanism in this paper. LSV was applied to research influence of some factors such as the scanning speed of potential (v), the initial concentration of Cr6+ and pH of solutions on the electrochemical reactions. Based on the relationship of Ep, ip and v, it was found the electrochemical processes were irreversible. CV was also used to compare the electrode processes in the solutions with and without Cr6+. It is concluded that Cr6+ would be harmful to the anode dissolution of Fe because of the passivation mechanism. Moreover, the characteristic parameters of the electrode processes, α,was calculated in terms of the standard analysis of EC mechanism. All results above from the electrochemical researches are instructive to dispose Cr6+ industrial wastes in the future.