Speciation and location of arsenic and antimony in rice samples around antimony mining area.

Arsenic (As) and antimony (Sb) are considered as priority environmental pollutants and their accumulation in crop plants particularly in rice has posed a great health risk. This study endeavored to investigate As and Sb contents in paired soil-rice samples obtained from Xikuangshan, the world largest active Sb mining region, situated in China, and to investigate As speciation and location in rice grains. The soil and rice samples were analyzed by coupling the wet chemistry, laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), synchrotron-based micro X-ray fluorescence mapping (µ-XRF) and micro X-ray absorption near-edge structure (µ-XANES) spectroscopy. The results of field survey indicated that the paddy soil in the region was co-polluted by Sb (5.91-322.35 mg kg-1) and As (0.01-57.21 mg kg-1). Despite the higher Sb concentration in the soil, rice accumulated more As than Sb indicating the higher phytoavailability of As. Dimethylarsinic acid (DMA) was the predominant species (>60% on average) in the rice grains while the percentage of inorganic As species was 19%-63%. The µ-XRF mapping of the grain section revealed that the most of As was distributed and concentrated in rice husk, bran and embryo. Sb was distributed similarly to As but was not in the endosperm of rice grain based on LA-ICP-MS. The present results deepened our understanding of the As/Sb co-pollution and their association with the agricultural-product safety in the vicinity of Sb mining area.

Effect of iron plaque on antimony uptake by rice (Oryza sativa L.).

Although iron (Fe) plaque has been shown to significantly affect the uptake of toxic antimony (Sb) by rice, knowledge about the influence of iron plaque on antimony (Sb) (amount, mechanisms, etc) is, however, limited. Here, the effect of Fe plaque on Sb(III) and Sb(V) (nominal oxidation states) uptake by rice (Oryza sativa L.) was investigated using hydroponic experiments and synchrotron-based techniques. The results showed that iron plaque immobilized Sb on the surface of rice roots. Although the binding capacity of iron plaque for Sb(III) was markedly greater than that for Sb(V), significantly more Sb(III) was taken up by roots and transported to shoots. In the presence of Fe plaque, Sb uptake into rice roots was significantly reduced, especially for Sb(III). However, this did not translate into decreasing Sb concentrations in rice shoots and even increased shoot Sb concentrations during high Fe-Sb(III) treatment.

Photo-induced oxidation of Sb(III) on goethite.

Goethite widely exists in soils and sediments, and plays a very important role in the environmental fate of toxic metal(loid)s. In the present study, photo-induced oxidation of antimonite [Sb(III)] on goethite was investigated with kinetic measurements and X-ray photoelectron spectroscopy (XPS) techniques. Effects of environmental factors including solution pH, the content of goethite as well as humic acid on the photo-induced oxidation of antimonite were tested. The results indicated that no oxidation of antimonite occurred in goethite suspension in the dark, but significant amounts of antimonite were transformed to antimonate when the suspension was exposed to light. Ferrous ions were found in the solution during the antimonite oxidation process, and its concentration decreased with increasing solution pH, which strongly affected the oxidation rate of antimonite. The initial solution pH has great impact on Sb oxidation. After 2h illumination, the highest oxidation rate was found at pH 3, while the initial oxidation rate was even higher at pH 9. In conclusion, the antimonite can be adsorbed and oxidized on goethite irradiated with light, which will greatly reduce its environmental risk.