Ammonium adsorption from anaerobic digestate on sugarcane hydrochar: performance, competitive ion mechanisms, and...

Hydrothermal carbonization (HTC) converts sugarcane residues into functional hydrochar, but its performance for ammonium (NH₄⁺) recovery from real wastewaters remains unexplored. It was hypothesized that divalent Mg²⁺ would cause disproportionately greater inhibition of NH₄⁺ adsorption than monovalent K⁺ due to charge density effects. Hydrochars produced at 200-240 °C were characterized, with sugarcane leaf hydrochar at 220 °C (SLH/220) exhibiting optimal uptake. Adsorption capacity reached 20 mg g^-1 in synthetic solutions, but decreased by approximately 50% to 10.25 mg g^-1 in liquid fraction of digestate. Despite its 11-fold lower concentration, Mg²⁺ caused stronger inhibition (32%) than K⁺ (24%), demonstrating that charge density, not concentration, governs cation competition. Adsorption followed the Langmuir isotherm (R² = 0.96) and pseudo-second-order kinetics (R² = 0.97), indicating monolayer chemisorption. Per ton of feedstock, the integrated HTC-adsorption-AD pathway yielded 41 m³ CH₄ and recovered 5.1 kg N as slow-release fertilizer, supporting decentralized circular economy systems. These findings establish that wastewater cation composition is the primary determinant of hydrochar performance, with direct implications for biorefinery design.