Thermochemical Analysis of Ammonia Gas Sorption by Struvite from Livestock Wastes and Comparison with Biochar and Metal–Organic Framework Sorbents

Struvite-bearing solids from swine (S) and dairy (D) wastewater, heat-treated to 150–300 °C, were evaluated as ammonia gas (NH3(g)) sorbents and compared to biochar (BC) and a metal–organic framework (MOF). Simultaneous thermal analysis-pulse thermal analysis-Fourier-transform infrared spectroscopy (STA-PTA-FTIR) was used to determine sorption capacity, reversibility, thermodynamics, and kinetics. For wastewater-derived sorbents, S solids heated to 150 °C (S-150) had the highest NH3(g) sorption capacity (47.2–49.9 mg g–1), comparable to BC (50.8 mg g–1). Enthalpies increased with sorption capacity, and the energy released per mole sorbed NH3(g) indicated stronger bonds formed with S sorbents than BC. After desorption, S-150 retained more NH3(g) (48–51%) than BC (39%). The MOF had the highest sorption capacity (289.7 mg g–1) and irreversibly bound NH3(g) (81%) but similar sorption activation energy (Ea) as S-150. The rates (k) of NH3(g) sorption and desorption were fastest for S-150. Overall, S-150 sorbents performed similarly to BC but were less effective than MOF for NH3(g) sequestration. However, advantages of S-150 for NH3(g) mitigation include wastewater valorization, minimal synthesis, low heat treatment, and potential use in agricultural applications. Evaluation of struvite-based wastewater-derived sorbents, comparison with commonly used sorbents, and the implementation of thermochemical analysis for this purpose are all novel aspects of this study.