Design of Functionalized Biochars for Dual Wastewater Treatment and Fertilizer Production

Wastewaters from the food industry and domestic sources contain large amounts of ammonium, a major contributor to eutrophication. Recovering this nutrient for fertilizer use offers both environmental and agricultural benefits. Poplar chop-derived biochars were prepared under different pyrolysis temperatures (300–500 °C) and chemical modifications (acidic and alkaline) to optimize ammonium (NH4+) adsorption and fertilizer reuse. The biochars were characterized by zeta potential, SEM–EDX, FTIR, and specific surface area measurements. Batch adsorption tests revealed that the alkaline-modified biochar produced at 300 °C achieved the highest capacity (4.63 mg NH4+/g biochar) and 62% removal efficiency. Adsorption kinetics followed a pseudo-second-order model (R2 = 0.97) but showed only marginal differences among models without independent mechanistic evidence. The Temkin isotherm described the equilibrium data the best (R2 > 0.99). Ammonium-enriched biochars enhanced seed germination by up to 54% compared to the control and increased plant biomass up to 12-fold in pot experiments. These results demonstrate that optimized biochars can effectively recover ammonium from wastewater; moreover, the observed plant growth improvement suggests potential slow-release behavior, promoting nutrient recycling and sustainable agriculture.