Valorisation of corn straw biomass power plant ash into magnetic bio-based composites for tetracycline removal: Performance, mechanisms and environmental risk

Biomass fly ash (BFA) derived from industrial crop-based bioenergy systems is an abundant by-product, but its direct utilization remains limited by poor functional properties and low reusability. As a result, upgrading its properties and functionality is essential for expanding its value-added applications. In this study, BFA generated from corn straw ( Zea mays L.) bioenergy production was valorized through iron modification using nano-Fe₃O₄ blending (BFA+Fe 3 O 4 ) and chemical co-precipitation (BFA+Fe 2 + /Fe 3+ ). The resulting materials were characterised and comparatively evaluated using tetracycline (TC) removal as a model application. The effects of contact time, initial concentration and pH were investigated to assess their practical applicability. The results showed that iron modification significantly improved the surface and physicochemical properties of BFA. Among the two modification materials, the BFA + Fe 2+ /Fe 3+ exhibited superior performance, owing to its larger specific surface area (137.32 m 2 /g) and higher abundance of surface functional groups. It achieved a TC removal efficiency above 95% over a wide pH range of 3–7 and at initial TC concentrations of 50–400 mg L⁻¹ , approximately twice that of raw BFA and BFA + Fe 3 O 4 . Mechanistic analysis indicated that TC transformation involved both adsorption and redox-related degradation processes, in which electron transfer mediated by dissolved oxygen and iron species played a key role. The BFA + Fe 2+ /Fe 3+ also demonstrated favourable stability and low environmental risk, maintaining 80.24% TC removal efficiency after five regeneration cycles. Overall, this study provides a feasible route for the valorisation of biomass ash derived from industrial crop bioenergy systems into functional magnetic bio-based materials, supporting the sustainable utilization of industrial crop residues within a circular bioeconomy framework.