Significant Promotion Of Transition Metal Oxides Loading On Sulfureted Fe-Ti Spinel For Cr(Ⅵ) Removal:Performance And Mechanism

Chromium pollution is one of the hot environmental issues in the world.Electroplating wastewater is the main source of anthropogenic chromium emission,and its maximum allowable emission concentration has been strictly limited.Chromium mainly exists in the form of trivalent Cr（Ⅲ）and hexavalent Cr（Ⅵ）in electroplating wastewater,among which Cr（Ⅵ）is the majority.Cr（Ⅲ）has low toxicity and is easily removed by precipitation.Cr（Ⅵ）is highly toxic,usually in the form of Cr O₄^2-or Cr₂O₇^2-and difficult to remove.Therefore,it is very important to control Cr（Ⅵ）discharge in electroplating wastewater to solve the problem of chromium pollution.Adsorption method is a promising method to control Cr（Ⅵ）discharge in electroplating wastewater.Its core is the development of high efficiency adsorbent.Previous studies have found that magnetic sulfureted Fe-Ti spinel can not only regenerate in multiple cycles without reducing Cr（Ⅵ）removal efficiency,but also that chromium species can accumulate continuously on its surface.However,the Cr（Ⅵ）removal performance of sulfureted Fe-Ti spinel is relatively ordinary,the amount of adsorbent is larger,and the operation cost is higher.In this study,transition metal oxides（WO₃ or CuO）were respectively loaded on the surface of Fe-Ti spinel before vulcanization to further improve the Cr（Ⅵ）removal performance of Fe-Ti spinel,so as to achieve efficient recovery of chromium resources in electroplating wastewater.The main research results of this paper are as follows:（1）WO₃ loading significantly improved the Cr（Ⅵ）removal performance of sulfureted Fe-Ti spinel in electroplating wastewater,and its three-hour Cr（Ⅵ）removal efficiency increased from 51%to 69%.Removal of Cr（Ⅵ）from sulfureted Fe-Ti spinel and sulfureted WO₃/Fe-Ti spinel mainly follows an“adsorption-reduction”mechanism:Cr（Ⅵ）in the liquid phase is first physically adsorbed on the surface of the adsorbent,and then reduced to Cr（Ⅲ）by reducing substances（Fe S or Fe S₂）on the surface of the adsorbent.It was found that the Cr（Ⅵ）removal efficiency of the adsorbent was positively correlated with the amount of reducing substance on the adsorbent surface and the amount of Cr（Ⅵ）and H⁺adsorbed on the surface.Although WO₃ loading did not increase the amount of reducing substances on the surface of sulfureted Fe-Ti spinel,it significantly increased the amount of positive charge on the surface,thus promoting the physical adsorption of Cr（Ⅵ）and H⁺.Therefore,WO₃ loading significantly improves the Cr（Ⅵ）removal performance of sulfureted Fe-Ti spinel.In addition,the used sulfureted WO₃/Fe-Ti spinel still maintains excellent spinel structure and can be recycled and reused through magnetic separation recovery.After regeneration,the Cr（Ⅵ）removal performance of sulfureted WO₃/Fe-Ti spinel does not decrease basically,and chromium can be continuously enriched on the surface of sulfureted WO₃/Fe-Ti spinel,and the chromium content（in terms of chromium element）reaches 14.5 wt.%after ten cycles.（2）CuO loading can further improve the removal performance of chromium in electroplating wastewater,and the removal efficiency of Cr（Ⅵ）in three hours increased from51%to 84%,which is obviously better than that of WO₃/Fe-Ti sulfide spinel.After CuO loading,new Cu₂S species are formed on the surface of sulfureted Fe-Ti spinel.On the one hand,the formation of Cu₂S increases the amount of reducing substances on the surface of sulfureted Fe-Ti spinel,on the other hand,increases the amount of positive charge on the surface of Fe-Ti sulfide spinel,and promotes the physical adsorption of Cr（Ⅵ）and H⁺.The Cu²⁺released by CuO/Fe-Ti spinel in the process of Cr（Ⅵ）removal can return to the surface of the adsorbent in the form of Cu（OH）₂ precipitation.After heat treatment,the CuO is decomposed into CuO,and Cu₂S are formed after vulcanization.Cu₂S play the role of“inducer”on the surface of the adsorbent,repeatedly inducing chromium enrichment on the surface of sulfureted CuO/Fe-Ti spinel during the cycle.After the enrichment of chromium reaches 30 wt.%after multiple cycles,it can be used as chromium ore resources for industrial applications and become raw materials for smelting and recycling chromium.Therefore,sulfureted CuO/Fe-Ti spinel is an environmentally friendly and low-cost adsorbent that can efficiently recover Cr（Ⅵ）from electroplating wastewater.