Rock-phosphate-enriched hydrochars derived from organic wastes: a sustainable amendment for multi-metal contaminated...

Soil contamination with toxic metal(loid)s represents a severe risk to food security and ecosystem stability. Hydrochars produced from organic wastes are promising soil amendments, but their capacity for metal immobilisation is limited by low surface functionality. In this study, natural rock phosphate (RP) was incorporated (2 %, w/w) into five feedstocks vegetable waste, mango seeds, dodder, carrot waste and moring a branches to synthesise RP‑modified hydrochars (RP‑MHCs) via hydrothermal carbonization at 200 °C for 2 h. RP‑MHCs were applied (0.5 and 1 %) to multi‑metal untreated municipal effluent polluted soil cultivated with Amaranthus viridis, to assess their impact on soil chemical properties metal bioavailability, and plant uptake under pot experimental conditions. The 1 % RP‑MBHC, (moringa‑based) treatment immobilised up to 84.05% Cr, 84.92% Cu, and 85.34% Zn whereas RP‑CWHC (carrot‑based) was most effective for 63.44% As 79.44% Cd, and 69.89% Pb stabilisation. Soil pH, CaCO₃, CEC, and organic matter increased significantly, supporting > 70 % biomass enhancement, of A. viridis and reducing bioaccumulation and translocation factors below unity. The immobilization effects were mainly attributed to surface sorption, phospate-induced precipitation, complexation, and pH mediated reduction in metal mobility. This research introduces a novel, low‑temperature, rock‑phosphate-hydrochar approach combining metal stabilisation with fertility improvement, offering a sustainable and cost‑effective strategy for remediating multi‑metal‑contaminated alkaline soils while ensuring food safety. Such RP-MHCs can be readily produced using existing hydrothermal treatment infrastructure, supporting their practical implementation at larger scales.