Atypical temperature-dependent ammonium adsorption on co-pyrolyzed biochars: Low-temperature enhancement via cation exchange

High pyrolysis temperatures generally improve the adsorption performance of biomass-derived biochars. However, sludge-derived biochars frequently deviate from this trend due to their rich mineral composition and distinct inorganic transformations during thermal processing. This study investigated the ammonium removal performance of biochars produced via the co-pyrolysis of two distinct sewage sludges (TSS and KSS) with water chestnut shells (WCS) at 300 and 500 °C. Results demonstrated that co-pyrolyzed high-ash KBC produced at a lower pyrolysis temperature (300 °C) exhibited superior ammonium adsorption (2.33 mg NH₄⁺–N/g) compared to that produced at a higher temperature (500 °C). This atypical behavior was primarily governed by cation exchange capacity (CEC), which was influenced by inherent mineral phases and exchangeable cations (Mg²⁺, Na⁺, and K⁺) originating from the sludge matrix. Despite the presence of negatively charged surfaces and mineral-associated interactions in biochars produced at 500 °C, the loss of acidic functional groups and inorganic phase transformation reduced the overall adsorption capacity. These findings highlighted that for mineral-rich sewage sludge, the interplay between pyrolysis temperature and inorganic composition was critical in determining ammonium removal performance.