Enhanced Sequestration Of Cr(?) By Copper Doped Sulfidated Zerovalent Iron(SZ?-Cu):Characterization,Performance,and Mechanisms

Heavy metal pollution is harmful to human health and environment.It has attracted more and more attention from researchers.Environmental contamination and remediation has become a research hotspot in the field of environmental engineering technology.Hexavalent chromium(Cr(?))is the priority control pollutant and various ways to remove Cr(?)have been proposed.Zero-valent iron(Z?)has broad application prospects due to its rich content,environmental friendly and high reduction capacity.However,the existence of passivation film on Z? surface limits its reactivity,resulting in its low removal efficiency of pollutants removal in neutral and alkaline environments.In this study,based on zero-valent iron strengthening technology,a copper-sulfidated Z?(SZ?-Cu)material was prepared by depositing copper onto sulfidation modified Z?,which effectively improved the Cr(?)removal in a wide pH range.In this study,comparative experiments were used to demonstrate the superiority of SZ?-Cu for sequestration of Cr(?)with Z?,sulfidated Z?(SZ?)and iron-copper(Z?-Cu).Scanning electron microscope(SEM),energy dispersive X-ray energy spectrum(EDS)and X-ray photoelectron energy spectrum(XPS)were used to characterize the four materials.It was found that the surface of SZ?-Cu composites was covered with a number of micro-particles on the coarse and rough surface.It is supposed that the formation of these irregularly shaped micro-particles is mainly attributed to the deposited Cu0 reflecting by the EDS result,while the rough surface of Z? is mainly resulted from sulfidation process.By comparing the performance of the four materials in removing Cr(?),it was found that the sequestration efficiency of Cr(?)by SZ?-Cu was 98.4%after 30 min of reaction,which was higher than the Z?(12.6%),SZ?(75.0%)and Z?-Cu(87.8%).Besides,a good linear fitting demonstrated that the kinetics of Cr(?)sequestration by different Fe-based materials followed pseudo-first-order reaction.These results clearly showed that sulfidation and copper coating can achieve high activity and significantly improve sequestration of Cr(?).In addition,in this study the influence of initial pH at 4-8 by Z?,SZ?,Z?-Cu and SZ?-Cu was investigated.It was found that an insignificant performance of Cr(?)sequestration over the pH range of 4-8 by Z?,while sequestration efficiency of Cr(?)using SZ? and Z?-Cu decreased significantly as the pH increased from 4 to 8.The sequestration efficiency used SZ?-Cu was not obviously influenced by pH.Most importantly,the sequestration efficiency of SZ?-Cu attained 97.9%within 20 min at pH 8.0,proving the strong enhancement effect in this system.Based on the optimal S/Fe of SZ? determined in the preliminary study by our team,the Cu mass loading on the surface of SZ? particles could influence the sequestration efficiency of Cr(?).Sequestration of Cr(?)using SZ?-Cu with fixed ratio of S/Fe(0.056)at different copper mass loadings from 0 to 0.41 g Cu/g(SZ?)and the optimal copper mass loading was 0.05 g Cu/g(SZ?).In addition,various experimental factors,such as initial concentrations of Cr(?),SZ?-Cu dosages and mixing rate were investigated in this study and the sequestration of Cr(?)by SZ?-Cu followed the pseudo-first-order reaction kinetics.The positive correlation between the sequestration efficiency of Cr(?),the produced rate of Fe(?)and the inhibited removal of Cr(?)in the presence of 1,10-phenanthroline indicated that Fe(?)played an important role to the sequestration of Cr(?).In order to further investigate the role of Fe(?),1,10-phenanthroline was added as the Fe(?)quencher to figure out its function during the sequestration reaction of Cr(?).The mechanism analysis showed that sulfidation can facilitate the electron transfer from iron metal to contaminant and strengthen Fe(?)releasing from Z?,and loading Cu0 can also be conducive to the production of Fe(?)due to the potential difference between the two metals.