The role of Al-Goethites on arsenate mobility

摘要

The geochemical fates of Fe and As are so closely correlated that methods of As removal from contaminated water are in general based on the high affinity of this metalloid for Fe (hydr) oxides. Dissimilatory Fe reducing bacteria, however, play a fundamental role in catalysing the redox transformations that ultimately control the mobility of As in anoxic environments. The potential of Al-goethites in adsorbing As(V) compared with hematite, goethite, ferrihy-drite, and gibbsite, and the stability of As retained by the Fe compounds under anoxic conditions were investigated in this study. The (hydr)oxides were synthesised, and adsorption isotherms and As(V) adsorption maxima at different pH were measured. Arsenic loaded samples were anaerobically incubated in the presence of Shewanella putrefuciens, and periodically sampled to evaluate the contents of soluble As and Fe. The As(V) adsorption maxima decreased in the following order: Fh > AlGt_(13) > AlGt_2o > AlGt_(23) > Gb > Hm > Gt. In terms of surface area, Gb, Gt, and Hm showed higher As(V) loading capacity than Fh, suggesting available reactive sites not fully occupied by arsenate on Fh. The same hypothesis can be considered for Al-goethites, as they showed even lower arsenate loading capacity per surface area. The presence of structural Al in the goethites enhanced considerably the As uptake capacity and stability under reducing conditions. Therefore, the Al-goethites showed good potential as adsorbents to remove As from water. S. putrefaciens cells were able to utilise both noncrystalline and crystalline Fe (hydr)oxides as electron acceptors, releasing As into solution. Al-goethites showed a decrease in Fe and As mobilisation as structural Al increased.