Sulfidation Of Zero-valent Manganese For Chromium (Ⅵ) Removal From Wastewater

Chromium（Cr）is a typical heavy metal element,widely used in metallurgy,electroplating,chemical industry,leather tanning,textile printing and dyeing processes.Chromium mainly exist in forms of trivalent（Cr（III））and hexavalent（Cr（Ⅵ））in wastewater,in which Cr（Ⅵ）has high toxicity to bacteria,animals,plants and human,even at trace concentrations.It is potentially carcinogenic and mutagenic and can cause a serious threat to living organisms.Therefore,it is necessary to treat and purify chromium in water.At present,among many treatment technologies of heavy metal wastewater,adsorption method based on nano zero-valent metal and its modified materials is one of the most effective methods.Owing to strong reducibility,sulfidized zero-valent manganese（SZVMn）should have efficient adsorption performance towards hexavalent chromium（Cr（Ⅵ））.To date,the feasibility and characteristics of removal of Cr（Ⅵ）from wastewater using SZVMn still remain unclear.This study aims at utilizing the methods for controllable synthesis of sulfidized zero-valent iron（SZVI）to prepare SZVMn,and to investigate the correlation between the influencing factors removal of Cr（Ⅵ）and the morphology/structure of SZVMn prepared with different preparation methods and sulfurizing agents,as well as provide a technological basis for their application in actual Cr（Ⅵ）-containing wastewater.Firstly,SZVMn was prepared by synthesis of nanoscale Zero-Valent Manganese（n ZVMn）via borohydrides reduction followed by sulfidation with in-situ formed elemental sulfur.The Cr（Ⅵ）removal performance and mechanism using SZVMn were investigated.The results show that SZVMn particles were composed of Mn⁰,Mn O_x and Mn S_x.The proportion of Mn S_xincreased with the increase in S/Mn molar ratio.The Cr（Ⅵ）removal efficiency reached 99.4%at an S/Mn molar ratio of 0.1,with a SZVMn dosage of 1 g/L,an initial Cr（Ⅵ）concentration of 30 mg/L,a reaction p H of 5,and a reaction time of 24 h.The Cr（Ⅵ）removal decreased when the S/Mn molar ratio was higher than 0.1,due to higher consumption of Mn⁰ with more S dosed during synthesis of SZVMn.The decrease in solution p H was favorable to the removal of Cr（Ⅵ）.Coexisting ions could hinder Cr（Ⅵ）removal,especially PO₄^2-.The kinetic fitting results show that the Cr（Ⅵ）removal process by SZVMn was fitted well with the pseudo-second-order kinetic model.The isotherm of Cr（Ⅵ）adsorption through the SZVMn followed the Freundlich model.The removal of Cr（Ⅵ）by SZVMn involves the following mechanisms:Cr（Ⅵ）was quickly adsorbed and complexed on the material surface by SZVMn;Mn⁰,Mn S_xreduced Cr（Ⅵ）to Cr（III）,and the dissolved Mn²⁺participated in the reduction of Cr（Ⅵ）;Cr（III）was precipitated in the forms of Cr（OH）₃ and Cr₂O₃,thus was removed from the water.Secondly,SZVMn material was prepared by sulfidation modification with Na₂S as sulfidizing agent,and the Cr（Ⅵ）removal characteristics and mechanism were studied.It is found that similar to the SZVMn modified by elemental sulfur,the particles were composed of Mn⁰,Mn O_x and Mn S_x.SEM shows that SZVMn had a multi-layer structure,and the results of HRTEM and SAED show that SZVMn was a low crystallinity material.The removal efficiency of Cr（Ⅵ）by SZVMn first increased and then decreased with the increase in S/Mn molar ratio,and the optimal S/Mn molar ratio was 0.3.The removal rate of Cr（Ⅵ）reached 99%at a molar ratio of S/Mn 0.3,the dosage of 0.5 g/L,an initial Cr（Ⅵ）concentration of of 30 mg/L,the initial p H value of 5,and the reaction time of 24 h,.The increase in solution p H was not conducive to the removal of Cr（Ⅵ）by SZVMn,and the optimal p H is 4-6 under the condition of fixed p H.Coexisting anions inhibited the removal of Cr（Ⅵ）,and PO₄^2-had the strongest inhibition.SZVMn was superior to n ZVMn in preserving and resisting coexisting ions.The kinetic fitting results show that the removal process of Cr（Ⅵ）by SZVMn conformed to the pseudo-second-order kinetic model.The adsorption isotherm of Cr（Ⅵ）removed by SZVMn can be well described by Freundlich model,and the maximum adsorption capacity was 187.19 mg/g.The the main mechanisms for Cr（Ⅵ）removal are as follow:the adsorption of Cr（Ⅵ）by electrostatic attraction of SZVMn,Cr（Ⅵ）complexation on the surface of SZVMn,and Mn⁰,Mn S_x and Mn²⁺reduction on Cr（Ⅵ）and Cr（III）precipitation.The above-mentioned two kinds of SZVMn were applied to the treatment of Cr（Ⅵ）containing industrial wastewater.The results showed that both of the SZVMn could adapt to the actual wastewater environment and remove Cr（Ⅵ）,and the effect of sulfidizing modified SZVMn with Na₂S was better.After alkaline pretreatment（p H=4）,the removal rate of SZVMn modified by elemental sulfur was close to 95%when the dosage of 10 g/L,while that of Na₂S modified SZVMn was close to 100%.The removal rate of SZVMn was close to 100%when 5g/L Na₂S were added directly without alkaline pretreatment（p H=1.5）.This study proposes a new method and material for removing Cr（Ⅵ）from wastewater,explores the interaction between Cr（Ⅵ）and SZVMn,and provides a new way for the treatment of Cr（Ⅵ）-containing wastewater.