Removal Of Cr(?) In Wastewater By Zero Valent Metal And Its Composite: The Efficiency And Mechanism

Reduction of Cr(?)to Cr(?)is the key to reduce the harm of chromium pollution.The reduction and detoxification of Cr(?)with zero valent metal(ZVM)has the advantages of low cost and environmental friendliness,and ZVM has a certain adsorption capacity,which can achieve fixation of a certain amount of total chromium while Cr(?)reduction.However,the surface passivation of ZVM results in the decrease of its electron loss ability.How to improve the reaction activity of ZVM has become the focus of research.Therefore,based on the investigation of Cr(?)removal efficiency and mechanism by micro zero valent iron(mZVI)and micron zero valent aluminum(mZVAl),focusing on the issue of strengthening ZVM restoration capabilities,further developed composite materials with reduction/adsorption functions based on both,and systematically discussed the effectiveness and mechanism of typical Cr(?)pollutants removal by the new multifunctional composite materials.The main research contents and conclusions are as follows:(1)This study investigated the influence of dissolved oxygen(DO)on the removal of Cr(?)in mZVI monometallic system at different initial pH and deeply analyzed the removal mechanism of Cr(?)by mZVI.The results showed that as the initial pH value increases,the efficiency of Cr(?)removal by mZVI gradually decreased,but DO can enhance the reduction of Cr(?)by mZVI under different initial pH conditions.The presence of DO promoted the corrosion of mZVI,and the release rate of Fe²⁺was positively linearly related to the removal rate of Cr(?)(R²=0.99).After adding1,10-phenanthroline to the reaction system,the removal of Cr(?)was greatly inhibited,which proved that the secondary Fe²⁺played a key role in the reduction of Cr(?)to Cr(?).The coexistence of NO₃^-and SO₄^2-can promote the corrosion of mZVI to release more Fe²⁺,thereby promoting the reduction of Cr(?),while the coexisting Ca²⁺and Mg²⁺had a slight inhibitory effect.Related experiments and characterization results showed that a small amount of Cr(?)can be adsorbed on the surface of mZVI and reduced to Cr(?).However,Cr(?)was mainly reduced by homogeneous secondary Fe²⁺,and DO enhanced the removal of total chromium while promoting the reduction of Cr(?).(2)Based on strengthening the adsorption effect of mZVI and promoting the corrosion of mZVI to release more Fe²⁺were beneficial to the removal of Cr(?).In the second part of this study,mZVI and activated carbon(AC)were used as raw materials to prepare a new mZVI/AC composite by ball milling.The efficiency and mechanism of removing Cr(?)were discussed,and the influence of AC on the corrosion and adsorption characteristics of mZVI was analyzed.Studies had shown that under acidic conditions,the removal rate of Cr(?)can reach 94.0%for 120 min.Monitoring the change of pH and the amount of total iron released,and combined with Tafel corrosion scans,it was shown that AC and mZVI formed micro-electrolysis to promote mZVI corrosion.Compared with mZVI,AC enhanced the homogeneous reduction reaction(the role of the secondary reducing agent Fe²⁺)and enhanced the adsorption capacity in the process of mZVI/AC removing Cr(?).In the system,the removal kinetics of Cr(?)obeyed the pseudo-second-order adsorption kinetic model,its adsorption isotherm conformed to the Langmuir model,and the maximum adsorption capacity was 14.35mg/g.Combined with Cr valence analysis in solution,1,10-o-phenanthroline shielding experiment and XPS analysis of solid particles,mZVI/AC realized the reduction of Cr(?)and most of the total Cr was removed by adsorption.In addition,the reusability of mZVI/AC was limited by the accumulation of(hydroxide)oxides on its surface and the loss of surface AC,but it can be reactivated by BM again using a small amount of AC.(3)This study first discussed the effectiveness of commercial mZVAl for removing Cr(?).The results show that the commercial mZVAl was limited by the dense surface oxide layer and was difficult to effectively remove Cr(?)by adsorption and reduction.In order to strengthen the adsorption/reduction effect of mZVAl to remove Cr(?),ball milling synthesised mZVAl/Na Cl,mZVAl/AC and mZVAl/Fe₃O₄ composites,and through material characterization analysis and comparison the removal efficiency of Cr(?),the formation of mZVAl/Fe₃O₄ adsorption/reduction system,used to study the removal effectiveness of Cr(?)from water.The results showed that mZVAl/Fe₃O₄ can efficiently remove Cr(?)in a wide pH range(3.0–11.0),and the removal efficiency was related to the mass ratio of mZVAl/Fe₃O₄,BM speed and BM time.DO and coexisting anions(NO₃^-,SO₄^2-and PO₄^3-)competed with Cr(?)for adsorption,which significantly inhibited the adsorption/reduction of Cr(?)by mZVAl/Fe₃O₄.The coexistence of Ca²⁺and Mg²⁺has no effect on the adsorption/reduction removal effect of Cr(?).The adsorption process of Cr(?)on the mZVAl/Fe₃O₄ composite conformed to the pseudo first-order adsorption kinetic model and Langmuir adsorption isotherm model,and the maximum adsorption capacity of Cr(?)under neutral conditions was8.14 mg/g.Combining Cr valence analysis in solution and solid phase XPS analysis,the adsorption process of Cr(?)on the mZVAl/Fe₃O₄ compositewas dominated by chemical adsorption.In this study,a series of ZVM-based composites with both reduction/adsorption capabilities were prepared,which achieved the reduction of Cr(?)and the fixation of total chromium under a wide pH value,providing new technical ideas for the treatment of Cr pollution,and making ZVM in the field of pollution control application has important practical significance.