Tailings ponds are a main heavy metal pollution source in mining areas. In this study, the geo-accumulation index ($I_{geo}$) and the potential ecological risk index (RI) are used to evaluate the environmental impact of Hongtou Mountain (HTM) tailings pond on the surrounding area. Farmland soil, surface water, and sediment samples in the Hun River around the HTM tailings pond were collected. Heavy metal contents in the samples were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Results show that Cu, Zn, and Cd content in the farmland soil and sediment around the lower reaches of the Hun River (HTM tailings pond section) are obviously higher than the upper reaches. The $I_{geo}$ values show that the farmland soil near the outlet of the tailings pond is the most polluted area. Cu was classified as moderate–strongly pollution, Zn was moderately pollution, and Cd was strongly pollution. Cd is the major pollutant in farmland soil, the monomial ecological risk ($E_r^i$) for Cd is a very high potential ecological risk. The potential ecological risk of sediment in the dry season is more serious than in the raining season. In the dry season, the $I_{geo}$ index shows strong pollution for Cu and Cd at the confluence of the Hun River and the tributary from the HTM tailings pond, and a moderate–strongly pollution for Zn. Whereas, the $E_r^i$ index shows that the monomial ecological risk for Zn at H3 is low, and Cu is moderate. The potential ecological risk at H3 is high, and Cd is the main source of the ecological risk around the HTM tailings pond.

The impact of engineered nanoparticles (ENPs) on the migration and toxicity of coexisting pollutants is still unclear, especially in soil media. This study aims to evaluate the impact of titanium dioxide nanoparticles (TiO₂ NPs) on the phytotoxicity of cadmium (Cd) to Oryza sativa L., and the migration of cadmium (Cd) in the soil-rice system. Three different Cd stress groups (C1 group: 1.0 mg kg⁻¹, C2 group: 2.5 mg kg⁻¹ and C3 group: 5.0 mg kg⁻¹) were set in the pot experiment, and the target concentration of TiO₂ NPs in each group were 0 mg kg⁻¹ (T0), 50 mg kg⁻¹ (T1), 100 mg kg⁻¹ (T2) and 500 mg kg⁻¹ (T3). Plant height and biomass decreased with the increasing of Cd content in paddy soil. TiO₂ NPs could lower the phytotoxicity of Cd in terms of the changes in the morphological and biochemical characteristics, especially in the tillering and booting stage. In the tillering stage, TiO₂ NPs addition caused a significant increase in plant height, biomass and the total chlorophyll content in the leaves of Oryza saliva L. In the booting stage, TiO₂ NPs addition caused a 15% to 32% and 24% to 48% reduction of malondialdehyde (MDA) content for the C2 and C3 group, respectively, compared to that of the respective control treatment (T0). TiO₂-NPs addition reduced the activity of peroxidase (POD) in the leaves in the booting and heading stage, and the activity of catalase (CAT) in the tillering stage. In the C1 and C2 group, the grain Cd content in the 100 and 500 mg kg⁻¹ TiO₂ NPs treatments reached 0.47–0.84 mg kg⁻¹, obviously higher than that of the treatment without TiO₂ NPs (0.27–0.32 mg kg⁻¹), suggesting that TiO₂-NPs could promote Cd migration in the soil-rice system.

The effects of engineered nanoparticles (ENPs) on heavy metal fate and biotoxicity in farmland soil are mostly unknown. A flooding–drying simulation experiment was conducted to study the effects of three typical metal oxide nanoparticles (TiO₂-NPs, ZnO-NPs and CuO-NPs) on the chemical speciation of heavy metals and micronutrient bioavailability in paddy soil. The results showed that the addition of ZnO-NPs and CuO-NPs caused significant increases in soil pH, Eh and EC after a 90-d flooding–drying process. ZnO-NPs and CuO-NPs addition caused clearly increase in the Zn and Cu concentrations in the acid-soluble fraction, Fe/Mn oxides-bound fraction and organic-bound fraction, leading to higher bioavailability in the soil. DTPA-extractable Zn and Cu increased to 184.6 mg kg⁻¹ and 145.3 mg kg⁻¹ in the maximum ZnO-NPs and CuO-NPs concentration treatments (500 mg kg⁻¹). TiO₂-NPs promoted the transformation of Mn from a Fe/Mn oxides-bound fraction to an acid-soluble fraction. Soil Cd bioavailability obviously decreased in the TiO₂-NPs treatment but increased in the ZnO-NPs and CuO-NPs treatments.