Effects Of Anions And Organic Matters On Arsenic（Ⅲ） Adsorption By δ-MnO₂and Goethite

Theδ-MnO₂and gothite were synthesized and used as adsorbents of arsenic （Ⅲ）.The effects of anions and organic matters on the adsorption of arsenic （Ⅲ） were studi-ed，and influence mechanism was discussed. The results were important for regulationof arsenic bioavailability and restoration and prevention of arsenic-contaminated soil.The main research contents were shown below:1. The effects of anions on the adsorption of arsenic （Ⅲ）The species and concentrations of anion in soil were used as a reference. Theeffects of PO43-、HPO₄^2-、H2PO4-、Cl-、SO42-、SiO32-and mixed anions on the a-dsorption of arsenic （Ⅲ） byδ-MnO₂and goethite were tested. The results indicatedthat when arsenic （Ⅲ） concentration was fixed to10mg/L，the effects of anions onarsenic （Ⅲ） byδ-MnO₂decreased in order of PO43-、SiO32->HPO₄^2-、H2PO4-> Cl-、SO42-, PO43-、SiO32-showed great influence on arsenic （Ⅲ） adsorption by goethite, theeffects of Cl-、HPO₄^2-、SO42-、H2PO4-were negligible. When arsenic （Ⅲ） concentrationwas changed in the range of0⁸5mg/L，to the blank，all the anions had impacts on theadsorption of arsenic （Ⅲ） byδ-MnO₂and goethite.The impact was stronger in high c-oncentration than in low concentration, and was stronger by goethite than byδ-MnO₂.SiO32-enhanced the effects of PO43-on the adsorption of arsenic （Ⅲ） byδ-MnO₂andgoethite. The effects of arsenic （Ⅲ） by goethite were more obvious than byδ-MnO₂.The effects of anions on the adsorption of arsenic （Ⅲ） byδ-MnO₂and goethite weredecided by the nature of anions and adsorbents. The influences of anions on the adso-rption included electric competition, ligand exchange.When the concentrations of ars-enic（Ⅲ） were high, the anions might hinder diffusion and precipitation of arsenic （Ⅲ）in the adsorbent surface. 2. The effects of organic matters on the adsorption of arsenic （Ⅲ）The species and concentrations of organic matter in soils were used as a refere-nce. The influences of citric acid, oxalic acid, salicylic acid, lactic acid, humic acid a-nd tannic acid on the adsorption of arsenic（Ⅲ） byδ-MnO₂and goethite were tested,the results were showed as follows:（1） The effects of low molecular weight organic acids: when arsenic（Ⅲ） concen-tration was fixed to10mg/L, the effects of citric acid, oxalic acid, salicylic acid, lacticacid on adsorption of arsenic（Ⅲ） byδ-MnO₂were not obvious; citric acid and oxalicacid had affected arsenic（Ⅲ） adsorption by goethite, the effects of lactic acid and sal-icylic acid on the adsorption of arsenic（Ⅲ） were not obvious. When arsenic（Ⅲ） con-centration was changed, to the blank, all tested low molecular weight organic acids h-ad impacts on the adsorption of arsenic（Ⅲ） by theδ-MnO₂and goethite, the impactswere greater in high concentrations than in low concentrations, and greater by goethitethan byδ-MnO₂. Oxalic acid and salicylic acid obviously enhanced the effects of citr-ic acid on the adsorption of arsenic（Ⅲ） byδ-MnO₂and goethite, salicylic acid show-ed synergistic action, oxalic acid enhanced the effect of citric acid by goethite. The ef-fects of low molecular weight organic acids on the adsorption of arsenic（Ⅲ） by goet-hite andδ-MnO₂were decided by the nature of low molecular weight organic acids a-nd adsorbents. In neutral solution, the existing forms of low molecular weight organicacids were radical negative ions, the negative ions could compete active site on the a-dsorbent surface by electrostatic forces; on the other hand, the negative ions could als-o form complexes with cationic on the adsorbent surface, which making the As（Ⅲ） a-dsorption reduced. Adsorption include electrostatic adsorption, ligand exchange withhydroxyl or hydrated on the surface ofδ-MnO₂and goethite, forming hydrogen bonds,complexation with free Mn ion、Fe ion on theδ-MnO₂and goethite surface, low mo-lecular weight organic acids could hinder arsenic（Ⅲ） diffusion and precipitation on t-he surface of absorbents when arsenic（Ⅲ） concentrations were high.（2）The effects of humic acid and tannic acid: when arsenic（Ⅲ） concentrationwas fixed to10mg/L, the humic acid was used as interfering substance, the adsorptio-n rate and capacity of arsenic（Ⅲ） decreased byδ-MnO₂. The adsorption rate and cap-acity of arsenic（Ⅲ） declined rapidly and then came to balance by goethite. The effec-ts of tannic acid on the adsorption of arsenic（Ⅲ） by goethite andδ-MnO₂were not o-bvious. When arsenic（Ⅲ） concentration was changed, to the blank, both humic acidand tannic acid had impacts on the adsorption of arsenic（Ⅲ） by theδ-MnO₂and goe- thite, the impacts were greater in high concentrations than in low concentrations. Itmay be due to that when the arsenic（Ⅲ） concentration was low, outside complex fo-rmed between humic acid and free Mn ion on theδ-MnO₂surface, internal limitingcomplex formed between humic acid and Fe ion on the goethite surface free, mean-while, the humic acid also could affect arsenic（Ⅲ） adsorption by absorbing arsenic（Ⅲ）. When arsenic（Ⅲ） concentration was low, the affinity between arsenic（Ⅲ） andadsorbants was stronger than tannic acid, so preferentially absorbed arsenic（Ⅲ）. Wh-en arsenic（Ⅲ） concentration was high, surface diffusion and precipitation also exis-ted excepting electro static adsorption and complexation on the surface ofδ-MnO₂a-nd goethite, the humic acid and tannic acid could hinder arsenic （Ⅲ） diffusion and pr-ecipitation on the surface of absorbents.