Molecular Mechanisms Of Cr(Ⅲ) Immobilization By Organo-iron(Oxy) Hydroxides Coprecipitates

To achieve safe production and pollution control in the farmland with the concentrations of heavy metals（HMs）exceeding China’s soil environmental quality limits,it is necessary to effectively assess and predict the bioavailability of HMs in soils.Interactions between HMs and soil reactive phases significantly determine the molecular speciation and bioavailability of HMs in soils.Therefore,it is critical to investigate the molecular immobilization mechanisms of HMs by soil reactive phases.Iron（Fe）（oxy）hydroxides and soil organic matter are both reactive sorbents for HMs,and usually coprecipitate with each other to form organo-Fe（oxy）hydroxides coprecipitates（OFC）in soils.Heavy metals in soils can be immobilized by the OFC via sorption and coprecipitation.However,due to the complex structures of the OFC with multiple components,the molecular immobilization mechanisms of HMs by the OFC remain largely unknown.In addition,soil environmental conditions（e.g.,p H,ionic strength）are dynamic,but systematic investigation on the immobilization mechanisms of HMs by the OFC under varied conditions are lacking.The development of synchrotron-based techniques provides a unique platform to probe the interaction mechanisms of HMs at the interfaces of multi-components in the OFC at the molecular level and the nano/submicron scales.Furthermore,substantial crop straws are produced and returned into farmlands in China,which could release massive of dissolved organic matter（DOM）and subsequently affected the formation of OFC and its interaction mechanisms with HMs.Therefore,in this study,given the wide straw returning into farmlands in China,the DOM,extracted from rice and rape straws,were used to synthetize the OFC,and Cr（Ⅲ）was selected for study.The effects of Fe/C molar ratio,p H,ionic strength,and Fe（Ⅱ）on Cr（Ⅲ）immobilization by OFC via sorption and coprecipitation were investigated through batch experiments.The molecular speciation and nano/submicron-scale distribution of Cr,Fe,and C were characterized using synchrotron-based techniques including X-ray absorption spectroscopy（XAS）and scanning transmission X-ray microscopy（STXM）,which provides a systematic and deep understanding of interaction mechanisms between OFC and Cr（Ⅲ）in the aforementioned processes at the molecular level.The main results are listed as following:（1）The significant role of iron（oxy）hydroxides and organic matter in Cr（Ⅲ）immobilization by OFC via sorption and coprecipitation was found.Regardless of DOM sources and reaction types,OFC could immobilize more than 88%of totally added Cr（Ⅲ）when the Fe/C molar ratio was more than 0.3.However,the immobilized Cr（Ⅲ）was significantly reduced at the Fe/C molar ratio of 0.1,which exhibited a stronger decreasing trend in the coprecipitation than sorption reaction.Cr K-edge X-ray absorption near-edge structure（XANES）analysis suggested that Cr（Ⅲ）was dominantly immobilized by iron（oxy）hydroxides,but the proportion of organically bound Cr（Ⅲ）increased with the decrease of Fe/C molar ratio.STXM results indicated that C,Cr,Fe distributions were unevenly at the submicron-scale,especially for the samples from coprecipitation.Additionally,STXM-XANES analysis suggested that Cr（Ⅲ）was bounded by the carboxylic groups associated with organic fraction of the OFC,which provided the first direct evidence of the complexation between Cr（Ⅲ）and carboxylic groups on the OFC.（2）The impacts of C loading amount,p H,and ionic strength on Cr（Ⅲ）sorption by OFC and their underlying molecular sorption mechanisms were revealed.Cr（Ⅲ）sorption by the OFC was inhibited with the increasing C loading,implying a critical role of organic matter in blocking sorption sites on the ferrihydrite surfaces.The Cr（Ⅲ）sorption increased with the increase of p H.Inhibition of Cr（Ⅲ）sorption to OFC with high C loading occurred when ionic strength increased,suggesting the presence of outer-sphere complexed Cr（Ⅲ）.Extended X-ray absorption fine structure（EXAFS）analysis revealed that more Cr（Ⅲ）were bound to ferrihydrite fraction of the OFC at a relatively high p H,and organically bound Cr（Ⅲ）enhanced when increasing C loading and decreasing ionic strength.STXM analysis directly provided the spectroscopic evidence of the blockage of binding sites on the ferrihydrite by organic matter,which overwhelmed Cr（Ⅲ）retention by the direct binding of Cr（Ⅲ）to carboxylic groups of the organic matter fractions in the OFC.（3）The impacts of Fe/C molar ratio,p H,and ionic strength on Cr（Ⅲ）coprecipitated with OFC was studied,and multiple roles of DOM in Cr（Ⅲ）retention in the coprecipitation were clarified.The batch results showed that Cr（Ⅲ）retention was enhanced in the presence of DOM,a maximum of which was achieved at a Fe/C molar ratio of 0.5.The increase of p H and ionic strength could also promote Cr（Ⅲ）immobilization.Cr K-edge XANES results indicated that Fe（oxy）hydroxide fractions,instead of organic matter,provided the predominant binding sites for Cr（Ⅲ）,which was directly confirmed by high spatial resolution STXM-ptychography analysis at the nanoscale.Moreover,DOM could indirectly facilitate Cr immobilization by improving the aggregation and deposition of coprecipitate particles through DOM bridging or electrostatic interactions.Additionally,C K-edge XANES analysis further indicated that the carboxylic groups of DOM were complexed with Fe（oxy）hydroxides,which advances our understanding on the mechanism that DOM bridging-induced the coprecipitation of OFC,which indirectly promoted Cr（Ⅲ）immobilization by the OFC at the molecular level.（4）Fe（Ⅱ）-catalyzed transformation of OFC with immobilized Cr（Ⅲ）via sorption and coprecipitation and its impact on Cr（Ⅲ）stability were studied.Results indicated that the immobilized Cr（Ⅲ）and organic matter fraction in the OFC could inhibited the transformation of ferrihydrite to goethite,and organic matter fraction also induced the information of lepidocrocite.Although there was undetectable soluble Cr（Ⅲ）released from the OFC during the transformation,diluted HNO₃ extraction and acid dissolution results suggested that the transformation of ferrihydrite or OFC could result in the Cr（Ⅲ）enhancement on the surface of formed secondary iron（oxy）hydroxides.Meanwhile,Fourier-transform infrared spectroscopy and Cr K-edge XANES results indicated that the immobilized Cr（Ⅲ）by ferrihydrite could enter into the interior of goethite via isomorphous substitution during the aforementioned transformation,but Cr（Ⅲ）in the OFC was predominantly associated with ferrihydrite fraction throughout the transformation.These results suggested that organic matter fraction in the OFC could suppress the Cr（Ⅲ）isomorphous substitution,thus decreasing extent of Cr（Ⅲ）stabilization on the OFC during the transformation.