Mechanochemically synthesized gypsum and gypsum drywall waste cocrystals with urea for enhanced environmental sustainability fertilizers

Abstract

Abstract New, cost-effective and sustainable methods are needed to decrease the reactive N-losses into the environment from mineral fertilizers. For this purpose, we demonstrate a successful mechanochemical synthesis of urea cocrystals containing secondary nutrients calcium (Ca) and sulfur (S) using widely abundant drywall gypsum waste. Resulting CaSO4⋅4urea cocrystal possesses unique reactive properties. In particular, it possesses a higher urea decomposition temperature of 195 °C as it foregoes the melting of urea at 137 °C as inferred from thermogravimetric measurements. A combination of Dynamic Vapor Sorption (DVS) and in situ Raman spectroscopy measurements suggested a much lower CaSO4⋅4urea propensity to adsorb and react with water as relative humidity than urea alone. Ab initio thermodynamics framework was devised and showed a large apparent thermodynamic barrier of crystalline CaSO4⋅4urea to transform into urea and the resulting reactive gases, NH3 and CO2. Detailed N and S measurements suggested their enhanced availability when using cocrystals especially during the early stages of the experiments. The synthesis process described using waste drywall gypsum to form CaSO4·4urea cocrystal mechanochemically thus can provide a new value proposition for this abundant waste and contributing to sustainable use as it can decrease the use of various enzyme inhibiting synthetic organophosphorus compounds.