Rapid Adsorption of Ammonium on Coffee Husk and Chicken Manure-Derived Biochars: Mechanisms Unveiled by Chemical Speciation, Physical, and Spectroscopic Approaches

Biochars have emerged as a sustainable technology for converting waste into high-value, stable carbon products. Depending on its properties, biochar can retain various elements, including nitrogen (N) as ammonium (N-NH4+). This study aimed to evaluate the rapid retention of N-NH4+ in biochars produced from coffee husk (CH) and chicken manure (CM) at different pyrolysis temperatures (PTs) (300 °C, 400 °C, and 900 °C) and investigate the mechanisms involved. A rapid N-NH4+ adsorption experiment was conducted, in which an NH4Cl solution was passed through the biochars. The following analyses were performed: cation exchange capacity (CEC), surface area, pore volume and size, total N content, N retention, infrared analysis (ATR-FTIR), and leachate solution analysis, followed by chemical speciation using Visual MINTEQ software. The results indicated that different mechanisms were involved in rapid N-NH4+ retention. In CH-derived biochars produced at 300 °C, N-NH4+ retention occurred primarily through electrostatic interactions with negative charges (CEC), as confirmed by ATR-FTIR analysis. In CM-derived biochars produced at 400 °C, N-NH4+ retention was mainly through the formation of struvite (NH4MgPO4·6H2O), as confirmed by chemical speciation of leachate solution in Visual MINTEQ. In CH-derived biochars produced at 900 °C, N-NH4+ ions were trapped in the pores of the charred matrix due to the increased biochar surface area, pore volume, and decreased pore size. The biochars studied proved effective in retaining N-NH4+ through different mechanisms, suggesting that biochars can enhance rapid N retention and reduce N leaching, potentially serving as a source of N for crops.