Design And Mechanism Of Regenerable Metal Sulfide Sorbents For Highly Efficient Removal Of Hexavalent Chromium In Wastewater

Chromium is one of common contaminants in aquatic environment.Hexavalent chromium[Cr（Ⅵ）]is of great detriment to ecosystem and human health due to its high toxicity,high mobility and high tendency of bioaccumulation.Therefore,the elimination of Cr（Ⅵ）from wastewaters is the key to the chromium decontamination.Adsorption is one of the purification technologies adopted in the treatment of Cr（Ⅵ）in industrial effluents.However,the failure of sorbent regeneration and safe disposition of Cr species adsorbed still remains unsolved.To this end,a brand new concept of removing and recovering the Cr（Ⅵ）in industrial effluents with regenerable magnetic metal sulfide sorbents was proposed in this study.Titanomaghemite（Fe₂TiO₅）is one of the metal oxide materials with superparamagnetism but poor Cr（Ⅵ）removal efficiency.In this study,iron sulfides were embedded onto the surface of Fe₂TiO₅ through sulfuration（e.g.H₂S pretreatment）to improve the performance of Cr（Ⅵ）removal over Fe₂TiO₅.Furthermore,Mo was loaded before sulfuration to improve the performance of Cr（Ⅵ）removal over sulfureted Fe₂TiO₅.Sulfureted Fe₂TiO₅ and sulfureted Mo/Fe₂TiO₅ used in the removal of Cr（Ⅵ）could be separated with external magnetic field,then transformed into Cr-contained Fe₂TiO₅ and Mo/Fe₂TiO₅ through calcination in air atomosphere,and finally be regenerated through re-sulfuration.The sulfureted Fe₂TiO₅ and sulfureted Mo/Fe₂TiO₅ after regeneration could be ultilized in the next Cr（Ⅵ）removal and subsequently,the enrichment of adosorpted Cr spieces and the recovery of Cr resource could be realized through multiple removals.The main results of this research are listed as follows:（1）The performance of Fe₂TiO₅ for Cr（Ⅵ）removal was remarkably improved with an increment in removal efficiency from 11%to 79%after sulfuration.Two routines,including the physical adsorption of Cr（Ⅵ）on the sorbents principally due to electrostatic attraction and the heterogeneous reduction of Cr（Ⅵ）adsorbed by the reductants on the sorbents,were involved in the removal of Cr（Ⅵ）over sulfureted Fe₂TiO₅,while only physical adsorption was involved in that over Fe₂TiO₅.As the iron sulfides formed on the surface of sulfureted Fe₂TiO₅ not only increased the amount of surficial positive charges,resulting in a remarkable improvement in the physical adsorption of Cr（Ⅵ）,but also introduce the S spicies to reduce physically adsorpted Cr（Ⅵ）to Cr（Ⅲ）,the performance of Fe₂TiO₅ for Cr（Ⅵ）removal was remarkably improved through sulfuration.Moreover,sulfureted Fe₂TiO₅ could be well regenerated,and the recovery of Cr resource could be achieved through multiple removal.（2）The performance of sulfureted Fe₂TiO₅ for Cr（Ⅵ）removal was further improved with an increment in removal efficiency from 79%to 100%through Mo loading.As the Mo S₃ formed on the surface of sulfureted Mo/Fe₂TiO₅ was not only account for the increase of surficial positive charges,but also more reactive in the heterogeneous reduction of Cr（Ⅵ）than FeS and FeS₂,the performance of sulfureted Fe₂TiO₅ for Cr（Ⅵ）removal was remarkably improved through Mo loading.Since the highly dispersed Cr（OH）₃ formed on the surface of the sulfureted Mo/Fe₂TiO₅ after regeneration increased the positive charges on the surface of the sorbent,resulting in an improvement in the in the physical adsorption of Cr（Ⅵ）,the Cr（Ⅵ）removal efficiency over sulfureted Mo/Fe₂TiO₅ gradually increased from approximately 46%to 85%after five cycles.Thus,the sorbent usage for Cr（Ⅵ）removal could be gradually reduced from 3.5 g/L to 2.8 g/L during the five cycles.