Effects of surfactants on the removal of nitrobenzene by Fe(II) sorbed on goethite.

Surfactants are easily accumulated in groundwater, sediment, and aquifers, due to their excessive use in household, industrial, and agricultural processes. These residual surfactants are expected to influence the transformation and fate of organic contaminants by Fe(II) sorbed on iron oxides in anaerobic environments. Here, we investigated the effects of various surfactants including nonionic TX-100, anionic SDBS and bio-surfactant saponin on the removal of nitrobenzene (NB) by Fe(II) sorbed on goethite (goethite/Fe(II)) through batch experiments. We also elucidated the mechanism behind the effects by XPS, XRD, and determination of the amounts of sorbed Fe(II) on goethite. The results showed that the presence of TX-100 improved NB removal from 77.2% in the absence of surfactant, to 93.8% within 6 h, and improved the removal rate by about 1.3 times. In contrast, the presence of SDBS decreased the removal efficiency to 45.5%, and the presence of saponin nearly inhibited the removal of NB completely. The removed NB was finally nearly reduced to aniline in the absence or presence of surfactants, except in the case of saponin. The amounts of sorbed Fe(II) listed in the sequence as goethite/Fe(II)+TX-100 > goethite/Fe(II) > goethite/Fe(II)+SDBS > goethite/Fe(II)+saponin, and this order negatively correlated with that in the redox potential of these systems. These results confirmed that the presence of surfactants influenced the sorption of Fe(II) on goethite and changed the reactivity of goethite/Fe(II) for NB removal. This finding will promote a clear understanding of the impact of coexisting surfactants on the transformation and fate of organic contaminants in real anaerobic environments.