Crop root exudate-responsive fertilizers fabricated by structural conversion of wastewater activated sludges into phenolic polymer-incorporated mineral-like particles

Herein, scalable methods are proposed for synthesizing mineral-like particles from wastewater-derived activated sludge through a combination of heat and acid treatment, followed by alkaline precipitation. During the heat/acid co-treatment, solubilized elements (e.g., P, K, Ca, N, and Fe) and fragmented organic compounds are enriched from the sludge. Subsequently, mineral-like, low-crystalline particles containing high-molecular-weight phenolics are obtained through alkaline precipitation. Compared to raw sludge, these particles exhibit rapid nutrient release in response to citric acid present in crop root exudates. Notably, the nutrient release kinetics driven by the organic acid surpass those induced by the corresponding HCl-driven pH changes, suggesting that the complexation of phenolics and metal ions in the particles is directly disrupted by organic acids, leading to accelerated particle dissolution. Furthermore, lettuce plants capable of secreting organic acids from their roots are shown to grow more rapidly with the synthesized particles than with raw activated sludge or even commercial chemical fertilizer. These findings confirm that enhanced responsiveness to crop root exudates can be achieved by converting activated sludge into materials enriched with inorganic compounds and phenolic polymers, offering significant benefits for agronomic applications.