Engineering MgHPO4·3H2O/Mg(OH)2 composite for boosting NH3 adsorption properties

The emission of ammonia gas (NH3) in agricultural composting not only leads to a decrease in fertilizer efficiency, but also causes environmental pollution. Here, an alkaline, insoluble composites was prepared and its adsorption performance for NH3 was explored. The results indicate that the material synthesized using a one-pot method with magnesium hydroxide (Mg(OH)2) and phosphoric acid (H3PO4) consists of a mixture of MgHPO4·3H2O and Mg(OH)2 (M1). TEM images reveal the adhesion of the two materials together, with Raman spectroscopy indicating that MgHPO4·3H2O is located on the outer surface of Mg(OH)2. The microenvironment formed by the adhesive construction of MgHPO4·3H2O and Mg(OH)2 mixture not only provides sufficient magnesium salts and phosphate for the growth of struvite but also creates alkaline conditions conducive to struvite formation. The kinetic results show that the NH3 adsorption of M1 follows pseudo second-order adsorption kinetics and belongs to surface chemical reaction control. Compared with a simple mixture of MgHPO4·3H2O and Mg(OH)2, M1 has a 4.5 fold increase in NH3 adsorption rate and a 3.15 fold increase in equilibrium adsorption capacity. Preliminary on-site tests demonstrated the potential application of the M1 adsorbent for NH3 adsorption in composting.