Elucidating the effects of TiO nanoparticles on the toxicity and accumulation of Cu in soybean plants (Glycine max L.).

Copper (Cu) pollution is common in the soil. Due to the widespread application of TiO NPs, there is a high propensity for the co-occurrence of TiO nanoparticles (NPs) and Cu in agricultural soils. It is therefore imperative to evaluate the joint effects of TiO NPs and Cu on crops. In this study, the mutual effects of TiO NPs and Cu on their toxicity and accumulation in soybean seedlings and on their fates in a hydroponic system were determined. When Cu was at levels of 1 and 2 mg/L, the co-occurring TiO NPs at a non-toxic concentration (10 mg/L) significantly enhanced the toxicity and accumulation of Cu and Ti in soybeans, and inhibited the translocation of Cu from soybean roots to shoots. However, when the Cu concentration for co-exposure was ≥ 5 mg/L, such mutual effects disappeared. The amount of Cu ions adsorbed onto TiO NPs after 48 h of co-exposure gradually increased from 31 to 118 mg/g when the Cu concentration was increased from 1 to 20 mg/L. The aggregation and sedimentation of TiO NPs were significantly increased after 48 h of co-exposure with the Cu at a concentration higher than 5 mg/L, as compared to the single TiO NPs exposure. The increasing aggregation and sedimentation might reduce the bioavailability of TiO NPs associated with the adsorbed Cu to soybeans, and consequently alleviate or even neutralize the enhanced toxicity and accumulation of Cu in soybeans exerted by the co-existing TiO NPs. Our results thus suggest that consideration of the impact of TiO NPs on the phytotoxicity of heavy metals, and specifically Cu, needs to be interpreted with care, and highlight the importance of integrating the interaction and fates of TiO NPs and metals into their risk assessment.