Effects of phosphorous precursors and speciation on reducing bioavailability of heavy metal in paddy soil by engineered biochars.

Ammonium phosphate (AP), phosphoric acid (PC), and potassium phosphate (TKP) were used for the modification of biochar for enhanced heavy metal passivation in soil. The effect of various phosphorus (P) precursors on adsorption-related properties, P speciation distribution pattern, and the passivation mechanism was investigated by BET, FTIR, XRD, XPS, and 31P NMR analysis. The mobility and bio-availability of cadmium (Cd) were studied by extraction experiments, and the P release kinetics was also determined. Results showed that the immobilization efficiency of Cd (II) by biochars followed the order: TKP-BC > PC-BC > AP-BC > BC, and TKP-BC reduced available Cd content by 81% treated with 2% addition. The P speciation shows a significant effect on the P-enriched biochars' passivation performance, especially orthophosphate, which is essential for the immobilization of Cd2+ by forming phosphate precipitation. Pyrophosphate and orthophosphate monoester in AP-BC and PC-BC can promote Cd2+ passivation via the formation of P-Cd complexes or organometallic chelates. It is also shown that PC-BC has the lowest P release rate while TKP-BC has the highest percentage of P (15.50%) remaining in the biochar. The results may contribute to the development of modified biochar for soil remediation based on P-related technologies.

Reactive transport modeling of pollutants in heterogeneous layered paddy soils: a) Cadmium migration and vertical distributions.

Cadmium (Cd) pollution in soil has attracted more attention recently for its high toxicity and easy accumulation in crops. This study aims to investigate the mechanisms governing the transport behavior of Cd, and to simulate and predict the long-term migration of Cd in different paddy soil layers. Therefore, a layer-by-layer (LBL) model based on the geochemical model PHREEQC was developed. A dual-porosity finite difference method was applied to model the two-region diffusion process. The solute transport parameters were obtained by field measurement, literature review, or inversely estimation using PHREEQC based on the experimental results. Modeling and experimental results both indicate that different mechanisms (cation exchange reaction, preferential flow, etc.) control the transport and vertical distribution of Cd. The prediction results show that only the surface soil (< 0.3 m) would pose the risk of Cd2+ pollution. The coupled LBL model could correctly simulate the migration of Cd under near-field conditions.