Effect Of Ferrihydrite And Organic Matter On Adsorption And Transport Of Arsenic In Soil And Water

Arsenic（As）is one of the common toxic metal elements.The excessive of As in soil and groundwater in China is universal.As has a serious impact on human health,crop yield and quality.Many studies have shown that the use of iron oxides can reduce arsenic content in soil and water.Among iron oxides,ferrihydrite has the advantages of small particles,large specific surface area,and high reaction activity.Arsenic can be immobilized through adsorption and co-precipitation on ferrihydrite.Organic matter in soils can directly complex and chelate with arsenic,or react with iron（aluminum）oxides,affecting the adsorption and fixation of arsenic by iron oxides,thereby affecting the migration and transformation of arsenic in soil.In this study,2-Line and 6-Line ferrihydrite were synthesized in the laboratory,and we compared the sorption and immobilization of As（V）in solution.At the same time,three typical organic substances were selected,humus（HS）,fulvic acid（FA）and DOM from chickenmanure,to explore their effects on the adsorption and migration behavior of As（V）in soil.Firstly,adsorption kinetics,thermos-dynamics,and desorption of As（V）in water were carried out in the study.Then,the kinetic experiments were followed on the effects of different types and concentration of organic matter on the adsorption of As（V）by ferrihydrite.Finally,the effects of different types of organic matter on the migration of As（V）in the soil were discussed through soil adsorption experiments and simulated soil column experiments.The results of this study will provide a certain theoretical basis for the chemical behavior and pollution control of As（V）in soil and water.The main experimental results are as follows:1.2-Line ferrihydrite and 6-Line ferrihydrite synthesized in the laboratory were relatively small in particle size,and both were amorphous substances.The weak diffraction peak could be detected by XRD.The composition of 2-Line ferrihydrite and 6-Line ferrihydrite was similar,mainly composed of iron and oxygen elements,and the percentage of the content of the two elements Fe:O was about 1:1.6.The crystallinity of 6-Line ferrihydrite was better than 2-Line ferrihydrite.The main difference between 2-Line ferrihydrite and 6-Line ferrihydrite was that the size of the coherent scattering domain was inconsistent,and 6-Line ferrihydrite was not just higher crystalline form of 2-Line ferrihydrite.2.The adsorption effect of 6-Line ferrihydrite on As（V）was weaker than that of 2-Line ferrihydrite.The adsorption kinetic curve of As（V）on the surface of ferrihydrite could be described by second-order kinetics,when equilibrium was reached,the adsorption capacity of 2-Line and 6-Line ferrihydrite to arsenic was 49.40 mg·g^-1and 28.68 mg·g^-1,respectively.The adsorption of As（V）on ferrihydrite increased as the concentration increases,the Langmuir isotherm adsorption model had a good fitting effect.The maximum adsorption capacity decreased with higherthe temperature,indicatingan exothermic reaction.Water and Na OH were used as desorbents to desorb As（V）adsorbed on the surface of ferrihydrite.The amount of desorption increased with the increase of the initial concentration.The resolution rate of As（V）for 6-Line ferrihydrite was greater than that of 2-Line with water.The desorption rate of Na OH was much higher than that of water.3.After adding organic matter to ferrihydrite-arsenic reaction system,it was found that the type and concentration of organic matter significantly affect the adsorption effect of As（V）on ferrihydrite,and the amount of adsorption varied to varying degrees.DOM,humus and fulvic acid all reduced the amount of As（V）adsorbed by ferrihydrite.Among them,DOM extracted from chicken manure had the most significant reduction,followed by humus,and fulvic acid.As the concentration of organic matter increased,the reduction in As（V）adsorption capacity also increased when the reaction reached equilibrium.Three organic substanced in the study inhibited the adsorption of As（V）on 2-Line ferrihydrite to a greater degree than that of 6-Line ferrihydrite.4.The results of soil adsorption experiments showed that the addition of humus had little effect on the adsorption of As（V）in the soil.Fulvic acid and DOM would reduce the adsorption of As（V）in the soil.The results of the breakthrough curve and the vertical distribution of As（V）in the soil showed that the addition of organic matter had a certain effect on the migration of As（V）.As（V）transports in the soil with varying degrees of delayed tailing.The order of the migration performance in the soil was HS-soil≈CK<DOM-soil<FA-soil,the retardation coefficient R_dvalue were 19.53,17.82,12.20,10.13,respectively.