(731d) Conversion of Recycled Drywall into Nitrogen Efficient Fertilizers

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 ^oC as it foregoes the melting of urea at 137 ^oC 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.