Environmental Context. Selenium (Se) is one of the most serious problems confronted in agricultural soils derived from Se-containing rocks. This metalloid, an essential nutrient for animals and humans, may be toxic at relatively low concentrations. Se removal from soil and water using plants is a promising alternative to traditional chemical or electrochemical techniques. However, very few plant species are able to accumulate Se at high concentrations. Since Se and sulfur (S) have similar chemical properties, sulfate (SO₄^2-) in the plant growth media may interfere in the process of Se uptake by plants. Thus, plant species able to uptake more Se than S when both elements are present are desired for Se phytoremediation purposes.

Abstract. Hydroponic experiments were performed to study several aspects of Se uptake by C. arvensis plants. Ten day old seedlings were exposed for eight days to different combinations of selenate (SeO₄^2-), sulfate (SO₄^2-), and selenite (SeO₃^2-). The results showed that in C. arvensis, SO₄^2- had a negative effect (P < 0.05) on SeO₄^2- uptake. However, a positive interaction produced a significant increase in SO₄^2- uptake when SeO₄^2- was at high concentration in the media. X-ray absorption spectroscopy studies showed that C. arvensis plants converted more than 70% of the supplied SeO₃^2- into organoselenium compounds. However, only approximately 50% of the supplied SeO₄^2- was converted into organoselenium species while the residual 50% remained in the inorganic form. Analysis using LC-XANES fittings confirmed that the S metabolic pathway was affected by the presence of Se. The main Se compounds that resembled those Se species identified in C. arvensis were Se-cystine, Se-cysteine, SeO₃^2-, and SeO₄^2-, whereas for S the main compounds were cysteine, cystine, oxidized glutathione, reduced glutathione, and SO₄^2-. The results of these studies indicated that C. arvensis could be considered as a possible option for the restoration of soil moderately contaminated with selenium even in the presence of sulfate.