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.