Surface Complexation Modeling Of Heavy Metals Binding On Soil Mineral-organic Composites

The heavy metal contamination is one of the most pressing concerns about food security in China and globally.It has been restricting the construction of our ecological civilization.The bioavailability of heavy metals in soil is controlled by adsorption onto different solid surfaces and their complexes.Surface complexation model?SCM?can not only be used to explore heavy metal adsorption mechanisms,but also to predict their speciations in soils and sediments.The SCM has been successfully built for heavy metal adsorption onto single mineral and organic matter fraction,and component additivity?CA?method was widely adopted for predicting heavy metal adsorption onto mineral-organo composites.The CA method ignores the interactions between mineral and organic matter,and ternary mineral-organo-heavy metal reactions,which reduce the accuracy for complex systems.The mechanistic SCMs for mineral-organo composites including soil mineral-organo interfacial reactions are still lack.In this study,typical soil secondary minerals?montmorillonite,goethite and ferrihydrite?,bacteria?Pseudomonas putida?and humic acid?HA?were chosen as soil representative component,and their composites were made through adsorption or coprecipitation.The batch adsorption,isothermal titration calorimetry?ITC?,synchron-based X-ray absorption spectroscopy?XAFS?were adopted to reveal the thermodynamic driving forces and coordination structures for Cu?II?,Cd?II?and Pb?II?onto mineral-organo associations.The potentiometric titration and Fourier Transform Infrared Fpectrometer?FTIR?were used to detect the type and density of functional sites.The molecular-level SCMs were developed based on these information for the mineral-organo composites.The main results and conclusions are showing below:?1?SCM was built for Cu?II?adsorption onto montmorillonite,P.putida and their composites.The model calculation found cation bridge was the main configuration for Cu?II?adsorption in phylisilicate-bacteria complex.In composite systems,the physical blockage between montmorillonite and P.putida resulted in reduced Cu?II?adsorption at pH<5.5,while the formation of Cu?II?bridging structures enhanced their binding at high pHs.These deviations in the CA method were corrected by adding reactions of>RCOOH…XNa and>RCOO-CuOH-XNa in the new“CA-site masking-bridging”model.?2?Molecular level SCM was built for Pb?II?adsorption onto montmorillonite-P.putida composites.Pb?II?ions mainly coordinated with phosphoryl and carboxyl groups on P.putida at low and high concentrations,respectively.Contrary enthalpy values were found for Pb?II?adsorption to permanent?-2.91 kJ/mol?and variable charge sites?6.93 kJ/mol?on montmorillonite.The ternary bridging model,EXAFS and ITC provided molecular and thermodynamic evidences for the formation of enthalpy driven?-4.74 kJ/mol?ternary complex?>AlO-Pb-PO₄?in montmorillonite-P.putida composites.The proportion for the bridging structures increased with bacterial mass ratios at pH>5.The ternary complex did not result in the enhanced adsorption for Pb?II?on the composites,but promoted the allocation of Pb onto the mineral fraction.?3?The site masking between goethite and bacteria resulted in the reduced adsorption of Cd?II?ions at high concentrations.The coordination structure for Cd?II?onto goethite and P.putida were>?FeO?₂HCd^0.5+and>RPO₄Cd⁺,respectively.Cd?II?adsorption on the 5:1 composite was consistent with the additive rule.However,the CA method over predicted Cd?II?adsorption by approximately 8%on the 1:1composite at high Cd concentration.The deviation was corrected by adding the site blockage reactions between phosphoryl and carboxyl on P.putida and>FeOH on goethite.Both CA and“CA-site masking”models for Cd?II?adsorption onto the composites were in line with the ITC data.?4?The effects of goethite-HA-bacteria interface reactions on Cd?II?retention were investigated.The increase in negative charge and steric advantage for HA on goethite resulted in an enhanced capacity for Cd?II?immobilization,meanwhile more adsorbed Cd?II?were located on HA fraction.The site masking in ternary goethite-HA-bacteria composites lead to significant reduction of the functional groups on bacteria,corresponding to a reduced adsorption and significantly migration of Cd?II?to HA fraction compared to the CA prediction.The concentration controls the applicability of additive rule.When the interfacial reaction leads to increase in the active sites,the adsorption conforms to additivity at high Cd?II?concentrations.When site masking exists in the complex,the CA method can predict the adsorption at low concentrations.?5?The mechanisms for Fe?III?and HA interactions during coprecipitation and their impact on Cd?II?immobilization were evaluated.The coprecipitation with Fe?III?was two times more efficient for Cd?II?immobilization than adsorption onto ferrihydrite at the same quantity.The bidentate complex?>?FeO?₂Cd?with Cd-Fe bond length 3.43?was identified in the coprecipitates.The interactions between Fe and HA resulted in the blockage of functional sites on HA,but increased the site density of ferrihydrite.At high HA ratio?Fe/C=1:3?,the ternary complex?>FeO-Cd-COO?dominated Cd?II?immobilization in the precipitates.The CA method could not precisely predict Cd?II?immobilization in Fe?III?-HA coprecipitation systems,and the accuracy of model was improved by adding new reactions and functional sites.