LCD Model-based Quantification Analysis Of Multiple Factor Influence On Arsenic Adsorption Onto Soil Particles

The mobility and bioavalability of Arsenic?As?in soils are mainly controlled by the reaction of adsorption-desorption,oxidation-reduction and biotransformation of As at soil solid-water interface.These reactions are generally influenced by environmental factors,including pH,organic matters?types and contents?,co-occurrence anions and cations,and redox potential.In this study,pH dependency of arsenite?As?III??and arsenate?As?V??adsorption to goethite under the influence of calcium(Ca²⁺)?as a major cation?and phosphate(PO₄^3-)?as a major anion?was studied,effects of different types of natural organic matter?NOM?on the adsorption of As at goethite-water interface were quantitatively studied,and the inflence mechnisms of multiple factors on the adsorption of As to goethite were illuminated.On the basis of the above study,the adsorption of As onto natural soil samples were conducted,and the experimental results were compared and analysed with the model results,which were calculated by two advaced surface complexation models,i.e.Ligand and Charge Distribution?LCD?and Natural Organic Matter-Charge Distribution?NOM-CD?model,in order to reveal the possible influence mechnisms.The main results are:?i?In the goethite system,the presence of Ca²⁺significantly promoted the adsorption of As?V?to goethite,because the electrostatic interactions between adsorbed Ca²⁺and As?V?are stronger than that between adsorbed Ca²⁺and As?III?;the presence of PO₄^3-decreased As adsorption strongly which is mainly caused by site competition and electrostatic repulsion;when As?III?and As?V?were present simultaneously,both As?III?and As?V?competed for surface sites on goethite,and As?V?showed a stronger competition than As?III?;when Ca²⁺,PO₄^3-,As?III?and As?V?were all existed together,the changes of dissolved As in solution became more complicated at pH 6?10,because of competitive and synergetic effects.?ii?The interactions between of PO₄^3-or As?V?and different NOM at goethite-water interface are well described with both the LCD and NOM-CD model.According to these two models:site density of carboxylic groups,protonation constant of carboxylic groups,and particle size of NOM,and the spatial distribution of the NOM adsorbed on goethite surfaces are major properties of NOM determining its effect on oxyanion adsorption to oxides.Differences were observed between different HA in their competition with PO₄^3-and As?V?,whereas the effects of different FA on the adsorption of oxyanions were similar.The competition effect was more significant between FA and PO₄^3-or As?V?than HA,which is mainly determined by the higher density of carboxylic groups on FA and consequently a stronger electrostatic repulsion between the FA and oxyanions adsorbed.Additionally,adsorbed FA can get closer to the goethite surface because of the small particle size of FA,leading to the stronger decrease in adsorption of PO₄^3-and As?V?.The effect of NOM on the adsorption of As?III?was not significant,since the negative charge of adsorbed As?III?attributed to goethite surface is lower than that of As?V?and PO₄^3-,which results in the weaker electrstatic repulsion between adsorbed NOM and As?III?.?iii?The speciation of As in paddy soils was notablely affted by soil redox conditions and pH,and the influence mechnisms of PO₄^3-on the distribution of As in paddy soils varied with soil redox conditions.Under suboxic conditions,PO₄^3-inhibited Fe and As reduction probably due to stabilization of Fe-?hydr?oxides,but increased soluble As?V?concentration as a result of competitive adsorption between As?V?and PO₄^3-.In anoxic soils,PO₄^3-stimulated Fe and As reduction and caused increases of As?III?in soil solution.?iv?In the NOM-goethite and soil-water systems,both the NOM-CD and LCD model can describe the distribution of As at solid-water interface.The NOM-CD model is relatively simple to use,whereas the LCD model can directvely reveal the physical and chemical influence mechnisms of NOM on the adsorption of ions on mineral surfaces by considering the NOM propertities,including site density of carboxylic and phenolic groups on NOM and the spatial distribution of adsorbed NOM on oxide surface.In summary,this study revealed the influence of mutlible factors on As adsorption onto mineral surfaces,elucidated the influence of pH,Ca and PO₄^3-on the micro-interface process of As on natural soils,and tested the applicability of NOM-CD and LCD model.The understanding obtained in current study can provide a theroy fundation for predicting As mobility and bioavalability in the environment.