Study On Removal Of Waste Water Containing Chromium Based On Zero-valent Iron Technology

Heavy metal pollution has become a hot and difficult problem in the field of environment, and the discharge of heavy metal wastewater has posed a serious theat to our living environment and human health. Chromium is extensively used in leather, textile production, pigments, printing and dyeing, chromium plating and other industries, and it is a common heavy metal contamination. Zero-valent iron (Fe0, ZVI) as a reducing agent to remove heavy metal pollution received considerable concern recently. However, in the removal process of heavy metal contamination by Fe0, iron was easily oxidized, and with the passage of time, the Fe0surface would produce corrosion products, which hindered the progress of the reaction and reduce reaction activity on iron surface. Therefore, the removal of chromium contaminated wastewater was studied in the laboratory based on Fe0technology and in combination with other technologies. These methods and techniques includes:Fe0technology, advanced Fenton-chemical precipitation process, zero-valent iron combined with ultrasound (Fe0/US), and cation exchange resin supported nanoscale zero-valent iron (R-NZVI). The effects of various factors (including initial pH, Fe0dosage and wastewater initial concentration) on the removal efficiency were investigated, and the removal mechanism of chromium were also explored. The experimental results indicate:(1) Fe0can remove hexavalent chromium (Cr(VI)) contamination in water pollution, and the mechanism is attributed to oxidation-reduction and coprecipitation. The treatment process was affected by the solution initial pH, Fe0dosage, wastewater initial concentration and other factors. The removal efficiency increased with increase of Fe0dosage, but decreased with the increasing initial pH values or initial concentration of Cr(VI). In the optimum operation conditions of Cr(VI) initial concentration of20mg/L, Fe0dosage of6.0g/L, pH=3, the speed of150rpm and reaction60min, the removal efficiency reached more than90%.(2) Advanced Fenton-chemical precipitation process can quickly and effectively remove the strong stabilized complexed heavy metal wastewater CrEDTA. Thie process can not only remove chromium, but also reduce COD values. Fe0dosage, H2O2concentration, initial pH value, reaction time, and precipitation pH are key factors for the Cr(III) removal. Under the optimal conditions, removal efficiency of the CrEDTA complex wastewater containing30mg/L Cr(III) obtained94%.(3) Fe0/US system has a synergistic effect for the Cr(VI) removal. The removal efficiency of Cr(VI) in Fe0/US system was greater than the sum of those in sole Fe0and ultrasound alone. The initial pH of the solution, the amount of Fe0, ultrasonic time, and ultrasonic power had significant effects on the removal efficiency of Cr(VI). And under optimal conditions, more than97%of Cr(VI) could be removed by Fe0/US system. The surfaces of Fe0particles were characterized by means of scanning electron microscopy (SEM).(4) Nanoscale ZVI loaded on the surface of cation exchange resin, formed R-NZVI, it can effectively prevent nanoparticles agglomeration and reduce the its loss into the water environment. R-NZVI can quickly and effectively remove Cr(VI) in the wastewater. And Cr(VI) removal efficiency increased with the decrease of initial Cr(VI) concentration, the drop of pH, and the increasing NZVI load or the amount of resin. Under the optimal conditions, Cr(VI) removal efficiency reached83.4%. The surfaces of R-NZVI particles were characterized by means of SEM.