Effects of iron oxide on antimony(V) adsorption in natural soils: transmission electron microscopy and X-ray photoelectron spectroscopy measurements

Antimony (Sb) contamination in the environment is a worldwide concern. To address such contamination issues, we studied the adsorption of Sb in four different types of soils. We investigated the main chemical and physical factors that influenced the adsorption of Sb, and distinguished between the different adsorption abilities of naturally occurring crystalline and amorphous iron (Fe) compounds in these soils. Adsorption of Sb in ferrosol, primosol, isohumosol, and sandy soil was studied using batch experiments. Transmission electron microscopy and X-ray photoelectron spectroscopy were used to examine the character and location of Sb adsorbed on individual particles in these soils without affecting its geochemical environment. In addition, the crystalline and amorphous Fe compounds in these soils were separated and analyzed using X-ray diffraction. The relationship between these Fe compounds and Sb adsorption was also explored. The sorption capacities of the four soils increased on addition of Sb in solution, reaching values of 10.8, 4.33, 5.45, and 1.19 g kg(-1) for ferrosol, primosol, isohumosol, and sandy soil, respectively. The adsorption of Sb in ferrosol was much higher than for other soils because of its higher Fe oxide content. In fact, the Sb content adsorbed on ferrosol showed a good exponential relationship with its Fe content. The X-ray photoelectron spectroscopy results indicated that the Fe-2p and O-1s binding energies decreased after the adsorption of Sb in the ferrosol. This suggests that an electron transfer occurred between Sb and Fe through an oxidation-reduction reaction, after Sb adsorption in the ferrosol. The adsorption abilities of Sb in the four soils were in the order of ferrosol > isohumosol > primosol > sandy soil. The amounts of Sb adsorbed by these soils were significantly positively correlated with their Fe contents (Sb = -3.78 + 2.88 x Fe, P < 0.01), but were negatively correlated with their sand contents (Sb = 12.30 -aEuro parts per thousand 0.12 x Sand, P < 0.01). The X-ray diffraction analysis results showed that crystalline Fe compounds have a higher capacity for Sb adsorption than amorphous Fe compounds.