Experimental Studies Of Heavy Metal Wastewater Treated By Zero-valent Iron And Acid-washed Zero-valent Aluminum

Heavy metal pollution has become a worldwide environmental problem and got more and more attention. As zero-valent iron (ZVI) and zero-valent aluminum (ZVAl) have strong activity and higher reducing efficiency, the use of ZVI and ZVAl is considered as one of the most potential technologies in the removal of heavy metal wastewater. This paper investigates the influencing factors and reaction mechanisms on the removal of hexavalent chromium (Cr(â…¥)) and dye by ZVI with ligand-enhanced reactivity, and the influencing factors and reaction mechanisms on the removal of Cr(â…¥) by acid-washed ZVAl are also studied. In addition, we designes a Permeable reactive barrier (PRB) filled with the mixture of ZVI and acid-washed ZVAl for the dynamic test, and analysises the influencing factors and reaction mechanisms of the mixture of ZVI and acid-washed ZVAl in the PRB to remove five heavy metal ions (Ni2+, Cd2+, Zn2+, Cu2+ and Cr(â…¥)) from wastewater. The main research contents and conclusions are as follows:(1) EDTA and oxalate ligand can significantly increase the removal efficiencies of Cr(VI) and Acid Red 73 (AR 73). Oxalate ligand possesses better removal efficiency in simultaneous removal process than EDTA, and oxalate generates lower TOC than EDTA. The removal efficiency of Cr(â…¥) and AR 73 is affected by ZVI concentration, pH value, and initial AR 73 concentration. At optimal operational conditions (ZVI concentration 0.3 g/L, oxalate concentration 3.0 mM, pH 3.0), the removal efficiency of Cr(â’ˆ) (20.0 mg/L) and AR 73 (25.0 mg/L) can attain 98.0% and 80.0%, respectively. In addition, the removal efficiency of Cr(â…¥) and AR 73 in Fe0/N2 process is much lower than that in Fe0/air process, which is due to the generation of hydroxyl radical to enhance the decolorization of AR 73 in Fe0/air process. Fe(â…¡) is a restricting factor in the simultaneous removal reaction, and Fe(â…¡) and Fe(â…¢) is a generation-consumption-regeneration cycle during the simultaneous removal system. From XPS, FTIR spectra and UV-visible spectra analysis, Cr(â…¥) is reduced to Cr(â…¢), and ZVI particles are enclosed by a layer of Fe3O4 and Fe2O3 after reaction. Simultaneous removal of Cr(â…¥) and Acid Orange 7 (AO 7) from wastewater by ZVI with ligand-enhanced reactivity has the obtained similar results. At optimal operational conditions (ZVI concentration 0.3 g/L, oxalate concentration 3.0 mM, pH 3.0), the removal efficiencies of Cr(â…¥) (20.0 mg/L) and AO 7 (25.0 mg/L) can attain 99.0% and 78.0%, respectively. Thus, ZVI with oxalate-enhance reactivity can effectively remove heavy metal and organic pollutants from wastewater.(2) The use of acid-washed ZVAl is a new method to remove Cr(â…¥) from wastewater. The removal efficiency of Cr(â…¥) by acid-washed ZVAl is affected by ZVAl loading, initial pH, acid washing time, humic acid concentration, and Fe+ concentration. The removal efficiency of Cr(â…¥) increases with acid-washed ZVAl loading, acid washing time, and decreases with pH values. More than 98.0% Cr(â…¥) is removed from synthetic wastewater containing 20.0 mg/L Cr(â…¥) in 180 min by 0.4 g/L acid-washed ZVAl at initial pH 2.0. HA has an obvious inhibitory effect on Cr(â…¥) removal, while Fe2+ can improve the Cr(â…¥) removal. From SEM and XPS analysis, the activity of ZVAl is greatly increased by acid-washed, and Cr(â…¥) removal by acid washing ZVAl is proved by reduction, not by adsorption. Cr2O3 and Cr(OH)3 are found on the surface of ZVAl after the reaction.(3) The removal efficiency of five heavy metal ions from wastewater by PRB filled with the mixture of ZVI and acid-washed ZVAl are better than that by PRB filled with ZVI or ZVAl alone. The mass ratio of Al/Fe is also important for the PRB, and the best mass ratio of Al/Fe is 1:2. The performance of PRB is affected with initial pH, flow rate and initial concentration of heavy metal ions. The removal efficiencies of heavy metal ions by PRB increase with initial pH, and decrease with flow rate and initial concentration. At pH 5.4 and flow rates of 1.0 mL/min, the time that the removal efficiencies of five heavy metal ions are all above 99.5% can keep about 300 h using a 80 g/40 g acid-washed ZVI/ZVA1 mixture when treating wastewater containing each heavy metal ions concentration is 20.0 mg/L. From SEM, XRD and XPS analysis, the supposed compositions on the surface of the mixture of ZVI and ZVAl used in PRBs for 20 d are Fe2O3, FeOOH, Fe(â…¢), Al oxide or hydroxide, Zn(â…¡), Cr2O3, FeCr2O4 Cd(â…¡) and so on. Heavy metal ions are removed by some reaction mechanisms including redox reactions, precipitation as metal hydroxides, adsorption and co-precipitation.