Effects Of Interactions Between Organic Matter And Ferrihydrite On Reactivity Of Organic Matter With Heavy Metals

The mobility and bioavailability of heavy metals are dependent on their chemical speciation,which was affected by multiple environmental processes.Organic matter(OM)contains abundant functional groups and heterogeneous binding sites,ferrihydrite(Fh)has large specific surface areas and high reactivity,they can effectively bind metals and reduce their bioavailability.In natural environments,OM and ferric iron ions may precipitate together at the redox interfaces and form the OM-Fh co-precipitation composites,which has significant implications on the fate of OM,Fh,and metal contaminants.On the other hand,the continuous reactions between OM and Fh will result in the dynamic changes of OM composition and reactivity.The fractionation of OM molecules may result in distinct composition of OM at the redox interfaces in water or the mineral-water interfaces.A mechanistic and quantitative understanding on how the diverse molecular composition and molecular fractionation of OM on mineral surfaces affecting OM reactivity with metals will help to develop accurate models for predicting OM and metal behavior in the environment.Firstly,kinetics of nickel(Ni)dissociation from humic acids(HA)and fulvic acids(FA),two most important OM,was studied with a competing ligand exchange method(CLE)at varying Ni concentrations,reaction pH,and ionic strength.The kinetic data were analyzed using a mechanistic kinetics model developed based on the Windermere Humic Aqueous Model(WHAM 7).Experimental results showed that Ni dissociation rates were affected by both Ni concentrations and pH,and Ni dissociation from FA had faster rates than that from HA.The kinetics model can reproduce the experimental data well,with only two model fitting parameters for different reaction conditions.The optimized dissociation rate coefficients(kd,s-1)of OM AA-weak and AAB-weak binding site are 4.39×10-3 and 9.08×10-4 s-1 for HA,1.41×10-2 and 1.51×10-3 s-1 for FA,respectively.The modeling results showed that various HA and FA binding sites played different roles in controlling Ni dissociation rates,with bidentate binding sites and the weak tridentate sites being the most significant.At high pH values the Ni re-association reactions were significant for controlling the overall rates of Ni dissociation but had minimal impact at lower pH values.Secondly,two typical OM,HA and FA,were used to synthesize OM-Fh co-precipitation composites.We investigated the interactions between OM and ferrihydrite and their impact on kinetic reactions of Ni with OM-Fh composites using spherical aberration corrected scanning transmission electron microscopy(Cs-STEM),batch and kinetic experiments,and X-ray adsorption spectroscopy(XAS).Cs-STEM results,at both nano and sub-nano scales,showed three typical distributions of OM on ferrihydrite,adsorbed on ferrihydrite surfaces,accumulated as OM nanoparticles,and being blocked in tiny pores in composites.Batch experiments and modeling quantified the effects of OM and ferrihydrite interactions using the effective concentration coefficients(fOM and fFh),which was supported by XAS results.The values of fOM and fFh are 0.56 and 0.69 for HA-Fh composites,0.86 and 0.88 for FA-Fh composites,respectively,which demonstrated that the interactions between OM and ferrihydrite decreased the effective binding sites of both OM and ferrihydrite and reduced Ni adsorption.The kinetics model developed in this study integrated both WHAM 7 and CD-MUSIC models and successfully predicted the kinetics of Ni adsorption and desorption on OM-Fh composites under the impact of OM-Fh interactions.The model can quantitatively assess the dynamic changes of Ni speciation on different OM and Fh binding sites during the kinetic processesFinally,we investigated the effect of adsorptive fractionation of an FA on ferrihydrite on FA composition and reactivity with copper(Cu)using batch adsorption experiments,Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS),and Cu titration experiments.A suite of theoretical modeling methods,including WHAM,VSOMM,SPARC,and LFERs,were employed to construct the representative molecular models and quantify the proton and Cu binding characteristics of FA molecules.Results showed that FA molecules high in acidic functional groups had large Cu binding capacity and were preferentially adsorbed by ferrihydrite.Despite of the complex molecular composition of FA,FA molecules can be divided into three representative groups(group(+),group(0)and group(-))and each group of molecules had distinct chemical properties and different distributions of proton and Cu binding ability Interestingly,molecules within the same groups behaved similarly during the adsorptive fractionation.Using VSOMM,we constructed simple molecular models(model(+),model(0)and model(-))representing those three groups of molecules in FA.We further assessed the chemical reactivity in terms of the proton and metal binding ability for the three molecular models,and results showed that the molecules in group(+)and group(0)that contained a mixture of carboxylic and phenolic groups were capable of Cu binding under most environmental relevant pH(e.g.,5-7).The molecules in group(-)as represented by model(-),in comparison,mainly contained the phenolic groups,which were strong Cu binding sites but did not deprotonate sufficiently for Cu binding in most acidic to neutral pH conditionsOverall,we investigated the effects of interaction between OM and Fh on OM reaction properties at both mechanistic and quantitative levels.We illustrated the role of binding sites of OM,each component and binding site of OM-Fh,and variation of OM molecular composition in metal environmental activity.And we constructed molecular models to describe the OM reaction properties and developed kinetics model to describe metal behavior under complicated conditions.Our results,at both mechanistic and quantitative levels,have advanced our understanding on the OM-mineral-metal reaction system,help to develop comprehensive models for predicting OM and metal cycling in the environment,and guide the work of environmental remediation and risk assessment.