Cr Release Behavior During Aqueous Fe(?)-induced Recrystallization Of Cr-substituted Goethite

The Cr-substituted goethite is widespread in natural soil.Under certain conditions,Cr ions would be released from the structure of goethite,which increase the mobility and activity of Cr in soil environment,and harm to the environment and human health.In anaerobic soil environment,Fe???could sorb on the surfaces of iron?hydr-?oxides,and reacted directly with Fe???in the iron?hydr-?oxides,which drive the iron oxides with poor crystallinity converted to a higher degree of crystallinity mineral.More importantly,the reaction between Fe???_aq and Fe???_oxide is often coupled with the incorporation or release of heavy metals.In this study,we focus on the Cr???release from a series of Cr-substituted goethites during reaction with _aqueous Fe???,as well as the impact factors,such as pH and initial Fe???concentration,for this process.This is very helpful to quantify the environmental behaviors of Cr in soil,understand the chemical mechanisms of Cr and provide basic data for the efficient remediation of Cr in soil.The main contents and conclusions are as follows:?1?To study the effect of Cr substituted ratio on the reaction between Fe???_aq and Cr-goethite,five substituted ration Cr-goethite?0,0.02,0.05,0.16 and 0.32?were chose as research objects.The results of stable Fe isotopic composition and M?ssbauer characterization studies show that Fe atom exchange occurred between Fe???_aq and Fe???_oxide in Cr-substituted goethites and Cr-substituted goethites were recrystallized.The Fe atom exchange rates decreased in the Cr–goethite with higher Cr contents.The concentration of Cr ions released from the structure of Cr-goethite increased with the rate of Cr substitution in goethite during the Fe???catalyzed Cr-goethite recrystallization.The released and adsorbed Cr were reincorporated in the structure of goethite after incubation for a long time.?2?To understand the effect of pH on the reaction between Fe???_aqq and Cr-goethite,four pH value?3.5,5.5,7.5,and 8.5?were set up.The results showed that the rate of Fe atom exchange increased with higher pH value,when pH values were below 7.5;However,the rate of Fe atom exchange decreased with pH increased?above 7.5?.The fastest rate of absorption occurred at pH 7.5,due to point of zero charge for goethite(pH_ZPC=7.5).And the fastest rate of Fe atom exchange carried out at pH7.5 when the rate of adsorption on surface of goethite received highest.?3?To understand the effect of the initial Fe???concentration on the reaction between Fe???_aq and Cr-goethite,four concentrations?0.5,1.0,1.5 and 2.5 mM?were set up.The electrostatic surface potential could be changed by the rate of Fe???adsorbed on the surface of goethite.The more Fe???adsorbed on the goethite surface,the higher Fe atom exchange rate will received.However,when the Fe???concentration beyond the monolayer coverage,the exist of redundant Fe???would decreased the rate of Fe???atom exchange.The rate of Fe atom exchange with various initial Fe???concentration showed that the fastest rate of Fe atom exchange were carried out at 1 mM Fe???.?4?To further investigated the behavior of Cr release and reincorporation during the process of Fe???-induced recrystallization of Cr-substituted goethite,the concentration of Cr adsorbed on the surface and structure were studied.The results found that the release of Cr was positively correlated with Fe atom exchange rate.More Cr was released from the Cr-goethite with the higher rate of Fe atom exchange.More importantly,part of released Cr would be reincorporated into the goethite during the recrystallization of Cr-substituted goethite induced by Fe???_aq.