Evaluation of heat-treated struvite as a non-conventional sorbent for ammonia gas using STA-PTA-FTIR

Abstract

Struvite (MgNH4PO4·6H2O, S-25), a mineral recovered from nutrient-rich wastewaters, was heat-treated at 55–300 °C, and resultant substrates (HTS-55 to HTS-300) evaluated as sustainable, low-cost sorbents for ammonia gas (NH3(g)). Experiments were conducted using simultaneous thermal analysis (STA), coupled with pulse thermal analysis (PTA) for gas injection, and Fourier transform infrared spectroscopy (FTIR) for evolved gas analysis (EGA). The STA-PTA-FTIR technique was used to investigate in-situ sorption and desorption of NH3(g) by HTS substrates, and the associated energetics. The HTS-150 (dittmarite, MgNH4PO4·H2O) sorbent had the highest NH3(g) uptake, at 23.5 mg g−1. By comparison, commercial biochars exhibited higher NH3(g) sorption at 42.1–52.6 mg g−1. Sorption was partly reversible, with HTS-150 retaining 37.9% sorbed NH3(g), which was comparable to or better than biochar retention (12.0–40.8%). Sorption-desorption behavior and associated enthalpies indicate that HTS substrates sorb NH3(g) by both physisorption, via van der Waals and hydrogen bonding; and chemisorption, via hydroxyl and phosphate groups. Though biochars sorb more NH3(g), HTS-150 is produced at lower temperatures, requires less energy for pretreatment, and retains an equivalent fraction of NH3(g) following desorption. Additionally, the HTS starting material is readily recovered from wastewater, promoting valorization. Therefore, HTS substrates are promising non-traditional sorbents for NH3(g) sequestration.