Molecular modeling of iron and arsenic interactions with carboxy groups in natural biomass

Environmental context. Arsenic has been considered one of the most important global environmental pollutants. Its occurrence in water systems is a result of natural processes and anthropogenic activities. In view of their high toxicity and the consequent health problems associated with human exposure to contaminated waters and food, there is an increasing interest in the study of the specific interactions of arsenic species with organic matter. Here, specific interactions among arsenic, iron and a vegetable biomass are investigated with a view to demonstrate how these interactions can affect arsenic mobility in the environment. Abstract. The interaction of iron and arsenic with dried lettuce leaves was investigated using a combination of spectroscopic techniques. Iron binding to carboxy groups is indicated by a decrease of 84% in iron loading after esterification. According to extended X-ray absorption fine structure (EXAFS) analysis, FeIII is coordinated by six oxygen atoms (Fe–O distance of 1.98 Å), two carbon atoms (Fe–C distance of 2.85 Å) in a bidentate mononuclear form, and 0.5 or 1 arsenic atoms (Fe–As distance of 2.93–2.94 Å). Arsenic is sorbed only when the biomass has been previously loaded with iron. AsV is coordinated by four oxygen atoms (As–O distance of 1.71 Å) and one iron atom in a bidentate mononuclear form or two iron atoms (As–Fe distance of 2.93–2.94 Å) in a bidentate binuclear form. In conclusion, the results demonstrate that carboxylic acid groups can affect AsV mobility in the environment so long as iron is available for bridging.