Potentially toxic elements in flooded-gold-mine fields: compost stabilises Cd, biochar and manure mobilize As and Pb

The study aimed to investigate how potentially-toxic-elements (PTEs) mobility in flooded-contaminated soils could be mitigated via the addition of soil amendments. We simulated an anaerobic environment via flooding in an incubation experiment. Slurry samples were collected from each pot with a syringe after the incubation period for the laboratory analysis of the concentrations of the PTEs. Further, we employed near-infrared-spectroscopy (NIRS) to qualitatively examine the differences in the treatments through the existence of C-H and N-H bonds. We also developed starter calibration and validation models to predict the presence of PTEs in the flood-induced mining sites. The results indicated the dominance of C-H and N-H bonds in the compost-amended soils contributing to mitigating Cd. Arsenic (As) increased sharply in all treatments, especially corn cob biochar (CCB) and rice husk biochar (RB), above 900 % points. Compost reduced Cd by 43 %, whilst Hg rose in corn cob biochar and manure. Lead (Pb) showed the highest increases, with over 1100 % rise in manure treatment and 600-700 % increase with biochar. Compost and CCB markedly altered the soil biogeochemistry and limited the availability of Cd via elevated Fe/Cd molar ratios and improved sorption. Compost exhibited enhanced efficacy by markedly reducing Cd concentrations, seemingly owing to its elevated Fe and Mn levels. Multiple regression analyses emphasised pH, redox potential and Mn in the mobilisation of As, Cd, Hg and Pb during flooded regimes at contaminated mining sites. The NIR via partial least square modelling achieved a robust model for the prediction of As (R²cv = 0.802 and RMSEcv = 2.943 = As mg L^-1). The study endorses compost as an effective ameliorant for stabilisation of Cd, while biochar and manure promoted As and Pb release.