Mechanism Of The Effect Of Iron Oxides On The Reductive Remediation Of Cr(?)-Contaminated Soil With Calcium Polysulfide

In recent years,the increasingly serious soil chromium pollution has aroused widespread concern in the world.Chromium contamination in soils mainly comes from the non-organised emission of chromium wastewater,gas and non-standard stacks of chromium slag from chromium productivity,metallurgy,electroplating and leather industry.Compared to other chemical forms,Cr(?)has higher potential hazards to ecosystems and human health due to its large environmental toxicity and strong mobility.This article focus on the Cr(?)-contaminated purple soil in Chongqing,and takes advantage of iron oxide to strength the remediation process of Cr(?)-contaminated soil with CPS for the first time,in order to slow down the excessive odor,H2 S pollution and other issues.Based on the literature,typical iron oxides were screened.BET analyzer,X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)were used to study the characterization of typical iron oxides.Batch tests were taken to investigate the effect of iron oxide on the reduction of Cr(?)in solution by CPS.Through studying the influence of pH and iron species on the facilitation role of iron oxide,the key factors to facilitation was discussed,and the solid products of the equilibrium system were analyzed.By above researches,the possible mechanism was deduce.Then the soil batch tests were taken to study the effect of iron oxide on the reductive remediation of Cr(?)-contaminated soil by CPS and on the physical and chemical properties of soil.Influence of iron oxide content and environmental acidity and alkalinity on this falicitation were also dicussed.Finally,the distribution of Cr in fraction of sequential extraction procedure was carried out before and after treatment,and the mobility of Cr was compared.The main conclusions are as follows:(1)Adsorbed Fe(?)was the main reason which facilitated the reduction of Cr(?)by CPS.The facilitation mechanism of iron oxide on the reduction of Cr(?)by CPS mainly consists of three processes: the production of adsorbed Fe(?)followed by the reductive dissolution of iron oxide by CPS;the oxidation of adsorbed Fe(?)to adsorbed Fe(III)while Cr(?)was reduced to Cr(III);reduction of Fe(III)to Fe(?)again by the excess of CPS.Therefore,Fe(?)/Fe(III)cycling electrons formed by Fe in the system play a role of electron transport between CPS and Cr(?).(2)The addition of goethite and hematite increased the Cr(?)removal rate and reduced the equilibrium time in CPS-Cr(?)system.pH is a key factor affecting the facilitation of iron oxide on the reduction of Cr(?)by CPS.With the increasing pH,the facilitation tended to weaken firstly and the strength,and the facilitation was the most obvious when the pH was slightly acidic,(3)The reduction of Cr(?)in the soil and the total chromium and Cr(?)in the leachate were significantly improved with the addition of goethite and hematite to the CPS-Cr(?)system.The equilibrium time of the reaction was shortened from 72 h to 36 h and 48 h,respectively.The removal rate of Cr(?)in soil was increased by 8.96% and 0.34%,respectively.The removal rate of total Cr in the leachate were increased by 5.77% and 1.11%,respectively.The removal rate of Cr(?)in the leachate were increased by 10.57% and 7.05%,respectively,and the facilitation of goethite was stronger than that of hematite.The effect of iron oxide addition on soil pH was not significant,but it could increase of soil ORP,and the time of soil in reductive conditions was increased;It could also change the exsited state of Cr,helping it transform to the more stable sate--iron and manganese-bound Cr,in order to reduce the mobility of chromium in the environment.(4)With the increase of iron oxide dosage,the content of Cr(?)and the leaching toxicity of chromium showed the trend of decreasing first and then rising.When the iron oxide dosage was 9 g/50 g soil,the promotion of Cr(?)and the decrease of soil OPR were the most obvious for the reduction of CPS.The different dosage of iron oxide addition had little effect on soil pH,and the pH was between 8.65-8.95.The addition of iron oxide has a certain effect on the reductive stabilization of acid and neutral Cr(?)-contaminated purple soil by CPS,and The facilitation was stronger in acid soil.Under the two acid-base conditions,iron oxide had little influence on the change of soil pH and the ORP.