Effect of inorganic anions and biomolecules on adsorption/desorption of arsenate and arsenite on/from clay minerals

A. VIOLANTE; M. PIGNA; M. FERNANDEZ; R. M. TORRES S¨¢NCHEZ

Rio de Janeiro

Conferencia; XV International Clay Conference; 2013

Arsenic is a toxic element for humans, animals and plants and can be present in all natural environments. Sorption/desorption of arsenate As(V) and arsenite (As(III) on/from different soil components may have a dominant role in regulating arsenic mobility and toxicity in soils and polluted waters. To have information on the factors which may influence the mobility and potential toxicity of arsenic in soil environments, we studied the sorption/desorption of As(V)and As(III) on/from phyllosilicates, various metal oxides and synthetic materials (Fe-montmorillonite; Fe-MMT and its thermal and mechanical products) as affected by i) pH (4.0-8.0); ii) the presence and concentration of phosphate, selenate, selenite, sulphate and low molecular mass organic ligands (LMMOLs; e.g., malate, oxalate, citrate and tartrate), iii) the sequence of anions additions and iv) the residence time. We found that minerals, particularly those rich in Fe (Fe-MMT) were more effective in fixing As(V) than phosphate after 24-48 hours of reaction. Arsenite also strongly competed with phosphate and other ligands on minerals containing iron. Sulfate showed a very poor influence in preventing As (V) adsorption or in removing As(V) from soil minerals and soils, particularly at pH ¡Ý 6.0, and even at an initial sulfate/arsenate molar ratio >> 1.0. We found that at an initial sulfate/arsenate molar ratio < 10, sulfate did not prevent but only retarded arsenate adsorption on ferrihydrite. Vice versa, sulfate and selenate inhibited the sorption of arsenite on minerals containing aluminum (gibbsite, noncrystalline Al oxide, kaolinite and so on), but poorly prevented arsenite adsorption on minerals containing iron. The capability of organic and inorganic ligands in preventing As sorption follows the sequence: selenate ¡Ö sulfate < oxalate < malate ¡Ö tartrate < citrate < selenite < phosphate. The efficiency of most of the competing ligands in preventing arsenite and arsenate sorption increased by ecreasing pH, but phosphate whose efficiency increased by increasing pH. In acidic systems all the competing ligands inhibited the sorption of arsenite more than arsenate.