Trace metal(loid)s(TMs; As, Cd, Cu, and Pb) in polluted soils can exert a significant pressure on environmental quality which calls for measures to decrease the negative impacts of the class of pollutants. Ferrihydrite(Fh) and biochar are commonly used as soil additives to remediate TMs polluted soils in in Hunan Province in China. Both Fh and biochar are able to bind TMs, and their addition to trace metal polluted soils leads to the immobilization of TMs and a reduction in their bioavailability. However, a good understanding is still lacking regarding the effects of Fh and biochar on the bioavailability of TMs in paddy soils. Lowland paddy soils are usually subjected to flooding and drainage conditions, which affects the redox potential and bioavailability of trace metals. As such, water management of paddy soils may affect the efficiency of Fh and biochar to immobilize TMs. Moreover, the fate of water management on the stability of these immobilization agents is unclear. The objective of this project is to obtain a quantitative understanding of bioavailability of TMs in paddy soil, as affected by the fluctuated redox state and applied Fh stabilized by biochar. Batch experiments will be carried out to assess the adsorption capacity of Fh and biochar. Column experiments will be conducted to investigate the effect of the water management on the stability of Fh and biochar and the bioavailability of HMs in paddy soils. Spectrum technologies such as FTIR, XRD and XPS will be utilized in intuitively describe and explain the binding mechanism of adsorbents and TMs. Advanced surface complexation models will be applied to predict the complex interactions between the different reactive surfaces in paddy soils once when Fh and biochar are applied and TMs under flooded and drained conditions. The knowledge provided by this project will give guidance in reducing the bioavailability of TMs by applying Fh and biochar in paddy soil in a sustainable manner.