Investigation Of The Optimizing Control Strategy And Dissolution/Passivation Mechanism During Electrocoagulation Process For Heavy Metals Wastewater Treatment

In recent ten years, the events of heavy metals pollution in surface water have occurred frequently in China. It has led to a sharp decline of the environment quality of surface water, but also a serious threat to the safety of urban drinking water and people’s health. Therefore, it is urgent to explore the economic and automatic technologies for heavy metals wastewater treatment. Fortunately, the electrocoagulation(EC) technology gradually becomes an industrial application for heavy metal wastewater treatment with the advantages of easy-to-handle, no chemical requirement, and easily combined with other technology application, etc. However, its development and application is hampered by the issues of high cost, low-level automatization and anode passivation.In this study, the optimization design, the real-time control strategy and the passivation/ depassivation mechanism of EC were investigated with perspectives of optimizing the efficiency and cost, improving the automatic level and reducing the passivation issues respectively. The analytical tools of response surface design(RSM), environmental scanning electron microscope(SEM), electrochemical analysis, X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS) and species distribution simulation(MINTEQ) design, etc., were used. The main results of this study were as follows:The results of optimization design for efficiency and cost showed that the optimization for efficiency and cost of EC was achieved based on the model of inital p H, current density, electrode distance and alternating pulsed current(APC) of heavy metals(Cr(VI) and Pb, Cd, zinc) removal, the electric power consumption model and anode consumption model. When the initial p H, current density, electrode distance and APC were 3.19, 115.6 A/m2, 12.5 mm and 691 s respectively, it could obtain the highest removal efficiency of Cr(VI) and total Cr(VI) with the least electric power and anode material consumption. Similarly, the highest removal efficiency for composite wastewater of Pb, Cd and Zn treatment was achiev ed with the least energy and anode consumption when the initial p H, current density, electrode distance and APC were 8.86, 96.7 A/m2, 12.6 mm and 437 s, respectively. Analysis of variance(ANOVA) of RSM showed that initial p H and current density have significant interaction effect on Cr(VI) removal and Pb, Cd, Zn composite wastewater treatment. Simultaneously, electrode distance and current density have significant interaction effect on electric power consumption.The results of the real-time control strategy of EC showed that Cr(VI) concentration and initial p H were key factors to final p H. There was also an interaction effect between Cr(VI) concentration and initial p H. Specifically, when p[Cr(VI)](chromic alkalinity) = p Hi, the final p H was neutral; when p[Cr(VI)] < p Hi, the final p H was alkaline; and when p[Cr(VI)] > p Hi, the final p H was acidic. Then, a real-time control strategy of iron/aluminum-electro-coagulation-floatation(Fe/AlECF)was proposed for Cr(VI) removal based on monitoring indicators of turbidity, temperature, ratio current and p H. It could save 58.8% of anode material and 58.3% of electricity power and reduce 58.7% of sludge production. For residual water from Pb, Cd, and Zn contaminated sediment dredging treatment, aluminum-electro-coagulation-floatation(Al-ECF) was conducted with the monitoring indicators of turbidity and p H and design parameter of standing time by eliminating the insignificant factor. Compared with traditional EC, real-time control strategy of Al-ECF could save 35.8% of anode material consumption and 43.4% of power consumption, and reduce 47.9% of sludge production.The results of dissolution/passivation for Cr(VI) wastewater by Fe-EC showed that the passivation behavior was influenced by initial p H, Cr(VI) concentration, and APC. A p H-Cr(VI)-dissolution/passivation diagram was constructed with galvanos-tatic measurements. In the dissolution regions II, vd was supposed vp, it could achieve a reasonable final p H for precipitation and did not suffer passivation. However, vd was < vp in the passivation region III. This findings signified that the passive film obtained under the conditions of p H 4 and Cr(VI) = 52 mg L–1 essentially contained Fe species(Fe3+/(Fe3++Fe2+) 71.0%) but negligible levels of Cr(Cr/Cr + Fe only 1.2%). Additionally, a protective passive film was formed by short T APC(10 s). Based on the behavior of dissolution/passivation and the transformation of passive films during EC, the Cl--induced pitting dissolution behavior and its optimization were investigated. Then, a novel approach to optimize Cl- for depassivation was proposed:(a) Build a database of minimum Cl- concentrations for pitting dissolution(MCPD) as a function of p H and Cr(VI) in passivation region III(MCD = f(p H, Cr(VI)));(b) Record the curve function(g) of p H and Cr(VI) evolution(g = {(p Hi, Cr(VI)i),...,(p Hf, Cr(VI)f)}) without passivation in the Fe-EC process;(c) Obtain the optimal concentration of Cl- for depassivation(OCD) by selecting the maximum MCDs by combining the curve(g) and the database of MCPD(OCD = Max{MCPD1, MCPD2, MCPDn}).In conclusion, optimization design model by RSM could realize the optimization of efficiency and cost of heavy metals removal by EC with stable water quality. The real-time control strategy could improve the automation level of EC as well as reduce the processing cost significantly under the condition of fluctuant water quality. Based on the mechanism of passivation and depassivation during EC for Cr(VI) removal, a novel approach was proposed to optimize Cl- for depassivation, which was more accurate and stable compared with RSM.