Valorized agricultural waste amendments for potentially toxic element immobilization in mine-contaminated paddy soil

The offsite impact of post-mining activities has resulted in the deposition of potentially toxic elements (PTEs) in soils devoted to farming in the catchment area and floodplains of the Taft River basin. This study evaluated the effects of raw and valorized carbonized rice hull and powdered mussel shell on soil properties and PTE availability. Carbonized rice hull (CRH) and powdered mussel shells (MS), as well as their acetic-acid valorized forms, were used as amendments in paddy soil where rice was cultivated in a pot experiment. The amendments were characterized and applied at 10% (w/w) singly or combined, comprising seven treatments, including the control. Soil properties, including pH, available nutrients, and total PTEs and speciation, were determined after rice cultivation. Results showed that the amendments enhanced soil pH, organic matter, and carbon content, influencing PTE speciation and availability. Notably, VCRH and VMS reduced total Cr by 29.6% and 24.4%, while MS reduced Zn by 7.9%, and VCRH and VMS by 14.1% and 10.1%, respectively. Amendments lowered exchangeable PTE fractions; VCRH and VMS reduced As levels by 8.8x to 7.1x, respectively. Specifically, VMS decreased Cr by over 27x and Cu by 10.3x, while VCRH+VMS led to the highest reductions in Ni (6x), Pb (4.1x), and Zn (2.5x) based on the control. Overall, the agricultural waste soil amendments were found to reduce PTE contents and availability in the soil, most notably VCRH, VMS, and VCRH+VMS at 10% w/w. Enhanced immobilization of the PTEs in valorized amendments is attributed to increased surface functional groups.