Study On The Transformation Of Cd-containing Ferrihydrite To Crystalline Iron Oxide

Ferrihydrite is a kind of weakly crystalline iron hydroxide commonly found in soil,and commonly has large specific surface area and high surface reactivity.Interaction mechanisms of ferrihydrite with heavy metal pollutants in soil has become a hot spot in recent years.The heavy metal-containing ferrihydrite in the environment will be transformed to various crystalline iron(hydrogen)oxides such as goethite and hematite,depending on temperature,pH,redox conditions and coexisting anions and cations,during which the content and species of heavy metals are also subjected to change.Cdis a typical heavy metal contaminant.However it is rarely reported how Cdaffects ferrihydrite transformationand how the Cd content and species changes concurrently.In this study,the transformation processes and products of Cd-containing ferrihydrite under the conditions of different pH and different aging time,and the changes of behavior and forms of Cd ions during the transformation process were carefully investigated.The results deepen our understanding the effect ofheavy metal pollutants on the transformation of iron hydroxide and the fate and mobility of heavy metal pollutants during the transformation.The research results are as follows:1.At both pH5 and pH7,Cd-free and Cd-containing ferrihydrite are transformed into mixed phases of goethite and hematite,and the relative content of goethite and hematite are almost equal at pH5 while at pH7 the products contain relatively more hematite with ppor crystallinity.At pH9,the Cd-free ferrihydrite are transformed into hematite-based mixed phases of goethite and hematite,but Cd-containing ferrihydrite are transformed into pure hematite.At pH10,the Cd-free ferrihydrite are transformed into goethite-based mixed phases of goethite and hematite,but Cd-containing ferrihydrite are transformed into pure hematite.This indicates that Cd inhibits the transformation of ferrihydrite to goethite but promotes the transformation to hematite.2.Under pH5 and pH7,Cd hardly enters the mineral structure during mineral transformation;Under pH9 and pH10,Cd enters the structure of hematite,and with the increase of Cd content,the content of Cd in the produced hematite is increased;the content of Cd entering the hematite structure at pH 9 is lower than that at pH10.3.During the ferrihydrite aging processat pH 5 and pH 7,the mineral suspension pH value gradually decreases to ~ 2.5and ~ 4.5 respectively.During transformation at pH 9 and pH 10,the mineral suspension pH gradually rises to ~11.5 and ~12.The pH of Cd-containing system rises faster than that of Cd-free ferrihydrite solution.4.With the increase of aging time,the produced hematite unit cell parameters and cell volume are gradually increased under pH9.While that at pH10 are gradually reduced.The hematite unit cell parameters and cell volume transformed from ferrihydrite at pH10 are higher than that at pH9.With the increase of the Cd concentration,the hematite unit cell parameters and cell volume transformed from ferrihydrite at pH 9 and pH 10 gradually increase.And the hematite morphology is changed from rhombic to circular.5.Cd ions are stable in the process of dry heat transformation of goethite to hematite.As the burning time increases,the hematite unit parameters and unit cell volume transformed from goethite containing 5% of Cd are gradually increased,while that of hematite transformed from goethite containing 10% of Cd are gradually reduced.With the increase of the concentration of Cd,the produced hematite unit cell parameters and unit cell volume are gradually increased.