Effect of rare earth elements on rice plant growth

The goals of this study were, (1) to quantify the effects of rare earth elements (REE) on rice plant growth and (2) to determine whether the presence of iron (III) oxides on the plant root surface (i.e. the iron oxide plaques) played a role in impeding any toxic effects caused by the presence of the REE. Hydroponic experiments were designed to grow rice plants in a greenhouse under controlled conditions, exposed to all rare earth elements simultaneously, and to iron (II) sulfate or iron (II) chloride. The results showed a significant decrease in root and plant height and biomass at rare earth element concentrations of 0.5 mg/L and 1 mg/L. Negative growth effects were observed for plant roots and shoots upon addition of 100 μmol/L Fe(II) chloride or Fe(II) sulfate. Even when the root biomass was enhanced upon addition of Fe (II) chloride at a 1 mg/L rare earth concentration, however, statistically significant decreases in root length and plant height were recorded. In the presence of Fe(II) sulfate, a negative growth effect was present for all REE concentrations, being more pronounced at the highest REE levels. For the Fe(II) chloride experiments, speciation modeling showed that the rare earth elements would remain "free" as hydrated ions (Ln3+) or would be complexed by Fe(III) oxyhydroxides. With Fe(II) chloride, the light rare earths (La, Ce, Pr, Nd; LREE) remained mostly soluble, whereas the middle (Sm, Eu, Gd; MREE) and heavy (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; HREE) elements were for the most part bound by Fe(III) solids. As negative growth effects were observed with Fe(II) chloride, the most soluble LREE could be concluded to play a role in rice plant growth inhibition. Furthermore, upon addition of Fe(II) sulfate, the MREE and HREE were significantly associated with SO42− and a regained toxic effect for rice plants was observed at the highest REE concentration, suggesting also an effect of the dissolved MREE-SO4+ and HREE-SO4+ species on rice growth. This observation, coupled to the knowledge that SO42− is an essential nutrient for plants, suggests absorption of the REE-SO4+ species by the rice plant. The recorded negative growth effects for both the Fe(II) chloride or Fe(II) sulfate conditions, strongly suggest that all REE are detrimental to the development of rice. For both the Fe(II) chloride and Fe(II) sulfate conditions the negative growth effects may have been attenuated as a consequence of REE sorption to Fe(III) oxyhydroxides (i.e. iron oxide plaques) identified on the root surface, as suggested by surface complexation modeling of the REE to iron (III) oxides, at the pH and ionic strength conditions in this study.