Modified Biochar for Wastewater Nutrient Recovery and Slow‐Release Fertilization in Sandy Agroecosystems

ABSTRACT

Modified biochars are increasingly recognized as multifunctional materials for wastewater nutrient recovery and slow‐release fertilization in sandy agroecosystems. This review evaluates modified biochars, with particular emphasis on tyre‐derived biochar (TBC), for (i) recovery of nitrogen (N) and phosphorus (P) from wastewater and digestate and (ii) reuse of nutrient‐enriched biochar as a slow‐release fertilizer in sandy agroecosystems. Importantly, pristine biochar often shows limited affinity for anionic phosphorus species such as orthophosphate because its surface is commonly negatively charged, whereas chemical or mineral modification can markedly enhance P adsorption by introducing reactive sites and improving surface functionality. Across the literature, modified biochars generally outperform pristine materials in nutrient recovery due to increased surface area, porosity, and functional group density, with adsorption governed by ion exchange, electrostatic attraction, surface complexation, pore filling, and mineral precipitation. Following soil application, nutrient‐loaded biochars can improve nutrient retention, reduce leaching losses, and supply nutrients gradually, making them particularly relevant for sandy soils in arid and semi‐arid regions where low organic matter and rapid drainage constrain nutrient‐use efficiency and crop productivity. In this context, biomass‐derived biochars more directly support biogenic nutrient recycling, whereas TBC is more appropriately framed as a waste‐derived resource recovery material within a broader waste‐valorization pathway rather than a closed circular nutrient cycle. However, performance varies with feedstock, modification strategy, wastewater composition, nutrient form, and soil conditions, while contaminant risks including heavy metals, polycyclic aromatic hydrocarbons (PAHs), and phytotoxicity remain important limitations, especially for waste‐derived chars. Overall, modified biochars show strong potential to link wastewater treatment, nutrient recovery, and improved fertilization efficiency in sandy agroecosystems, but future work should prioritize long‐term field validation, contaminant safety, and site‐specific biochar design for arid and semi‐arid farming systems.