Biochar-loaded nano zero-valent iron from agricultural waste enhances copper immobilization during composting: Implications for safe soil amendment and crop production

This study presents a novel approach for the valorization of agricultural waste by converting waste tea residues into a functional composite, biochar-loaded nano zero-valent iron (nZVI@BC), and applying it to produce a value-added and safe organic fertilizer via swine manure composting. The addition of nZVI@BC, particularly at 50 g/kg (T5), dramatically enhanced the immobilization of copper (Cu), reducing its bioavailable fraction by 43.14 % and effectively transforming it into stable forms. We identified that the release and consumption of available phosphorus (AP), mediated by the compost microbial community (e.g., Clostridia), was the dominant mechanism (89.3 % explanation), leading to the precipitation of stable copper-phosphate minerals. The resulting compost product not only exhibited significantly reduced environmental risks but also possessed optimal physicochemical properties (e.g., CEC, pH, EC) for agricultural application. This work demonstrates a sustainable strategy for the synergistic valorization of multiple agricultural wastes—tea residue and swine manure—into a high-value, safe soil amendment, contributing to the circular bio-economy in the agro-industrial sector. • Valorization of waste tea residues into functional nZVI@BC for composting. • nZVI@BC drastically reduced (by 43.14 %) bioavailable Cu in the value-added compost. • The microbial-mediated release and consumption of available phosphorus (AP) was the key mechanism. • A green strategy for synergistic recycling of agricultural wastes into a safe, marketable soil amendment.