The Interaction Between Nano Zero-valent Iron And Soil Main Components And Its Environmental Implication

Nanoparticulate zero-valent iron?nZVI?,possessing excellent reductive activity and adsorption performance,has been and will be widely applied in the remediation of contaminated soil and groundwater.Clay mineral particle?CMP?and dissolved organic matter?DOM?as important and ubiquitous components of soils,sediments,and waters,affect soil properties and its ecological functions.The wide distributions of CMP and DOM in soils suggest their inevitable interaction with the increase of nZVI application in the environments.The migration and restoration of nZVI in the environment can be potentially affected by the interaction with CMP and DOM.In turn,the interaction can also likely alter the chemical composition and properties of CMP and DOM.Therefore,the interactions of three different nZVIs(two bare nZVIs of different sizes or one carboxymethyl cellulose?CMC?coated nZVI(nZVI_cmc))with two types of CMPs?kaolinite or montmorillonite?and two different sources of DOM?humic acid or fulvic acid?were systematically investigated.The heteroaggregation mechanism of nZVIs with CMPs,the effects of nZVI interaction with DOM on the nZVI transformation and DOM chemical stability,and the effect of heteroaggregation between nZVI and CMP on carbon sequestration were disscussed.In addition,the morphological transformation of nZVIs in 12 soils and its effect on soil physicochemical properties were simulatedly studied.The main results are as follows:?1?The mechanism and evolution of nZVI heteroaggregating with CMPs at different p H and mass ratios were studied.Under neutral or alkaline conditions,Lewis acid-base interaction contributed to the attachment between nZVIs and CMPs,while electrostatic attraction was involved in the nZVIs-CMPs attachment at the acidic condition.Compared with heteroaggregates formed by nZVIs attaching to CMP edges and faces under neutral and acidic conditions,the heteroaggregates were smaller with nZVIs mainly connecting to CMP edges under alkaline condition.Under neutral condition,small nZVI homoaggregates were bound to CMP edges at low nZVI concentrations;with the increase of nZVI concentration,the small nZVI homoaggregates further adsorbed more nZVIs to form larger aggregates and gradually extended to the CMP surface.?2?The interaction between nZVI and DOM and the effects on their chemical stability were studied.Aliphatic hydrocarbons especially rich in carboxyl and hydroxyl functional groups with large molecular weights?MW,>3 k Da?had a relatively high affinity to bare nZVI surfaces by ligand exchange,while quinone-like and protein-like fluorophores with low MW preferentially remained in aqueous phase after the nZVI adsorption;CMC coating on nZVI limited the adsorption of DOM.The interaction with DOM accelerated the oxidation of nZVI due to the iron ion chelation of DOM and the potential electron transfer mediator action of the adsorbed quinone-like fractions.Compared with the pristine DOM,the DOM fraction coating on nZVI was more stable,while the residual DOM in aqueous phase after nZVI adsorption was in lower chemical stabilities,more susceptible to chemical oxidation by KMn O₄ and photochemical degradation.?3?The heteroaggregation and adsorption of nZVI and CMP in DOM solutions were studied.Bare nZVIs easily formed heteroaggregates with montmorillonite and the coated nZVI tended to attach on kaolinite surface in DOM solutions.Compared with the individual CMPs,the formed bare-nZVI-CMP heteroaggregates promoted the sequestration of organic carbon,while the interaction of nZVI_cmcwith CMPs decreased the adsorption of organic carbon.The heteroaggregation of bare nZVIs with CMPs promoted the aromatic carbon adsorption and large MW aromatic carbon were prone to adsorb on the nZVI-montmorillonite complexes in DOM solutions.The heteroaggregation and competition between nZVIs and CMPs interfered with the adsorption of organic carbon on their surfaces;the ultimate immobilization of organic carbon determined by the formed nZVI-CMP complexes was irrelevant with the addition sequence of nZVIs and CMPs.?4?The morphological transformation of nZVI during ageing 60 days in 12 soils and its effect on soil physicochemical properties were studied.The application of nZVIs in soils was conducive to the immobilization of aromatic carbon and the decrement of the redox potential.While,the changes in other physicochemical properties?such as p H,electrical conductivity,and?-potential of colloids?of the soils with nZVI in the incubation were related to soil type and soil moisture content.Compared with the flooded conditions,the oxidation of core Fe⁰ in nZVI can be accelerated at 70%soil moisture content but limited at 30%soil moisture content.The spherical nZVI in the incubation gradually dissolved and collapsed and adhered to the clay minerals,with crystalline maghemite??-Fe₂O₃?and magnetite?Fe₃O₄?as the primary iron oxides.Under the flooded conditions,the oxidation contents of core Fe⁰ in different types of soil were significant negative correlation with the initial soil p H?p<0.05?.After nZVI aging in the soils,negligible fraction of Fe present in bioavailable metal fractions?ion-exchange and carbonate fractions?and vast fraction of Fe were combined with Fe-Mn oxides and organic matter during the oxidation of nZVI.