Nitrogen (N) and phosphorus (P) nutrient recovery in a solid form from wastewater streams is of utmost importance while managing their global cycles. Here, we show that water insoluble MgO precursor can be utilized to synthesize MgNH 4 PO 4 •6H 2 O (struvite). Hence, water-soluble magnesium precursors, such as MgCl 2 , that require expensive synthesis routes can be substituted with more sustainable, naturally occurring but less soluble magnesium precursors, such as MgO. Time-resolved analysis of solid and liquid products showed two MgO concentration regimes during struvite formation, e.g. a kinetically controlled one at [Mg:NH 4 + :PO 4 3− ] ratio of 4.80:1:1 and an equilibrium limited one at [Mg 2+ ]: [NH 4 + ]:[PO 43− ] = 1.44:1:1. In both cases, >70% of NH 4 + and PO 4 3− species were removed from solution with the pseudo-second-order dependence of the reaction rate on the starting MgO concentration. The dual adsorption/reaction behavior of both ions needed in equimolar amounts to form struvite resulted in heterogeneous product distribution with magnesium phosphate as the dominant component at equilibrium. Spatially resolved Raman and pXRD analyses showed that crystalline struvite (MgNH 4 PO 4 •6H 2 O), magnesium phosphate (Mg 3 PO 4 •22H 2 O) and complex amorphous species are present. The Raman spectral footprint of the intermediate component was assigned to dypingite, Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O. This suggests that parallel hydration reaction takes place, under ambient conditions, to form magnesium hydroxycarbonates when excess MgO is available. In turn, this implies that various carbonates of magnesium can potentially be utilized with improved conversion during struvite precipitation from aqueous N and P precursor solutions. Lastly, sustainability and economic analysis of struvite synthesis from MgO clearly showed that employing MgO instead of MgCl 2 can significantly decrease overall energy requirements, and the resulting carbon footprint by a factor of ∼3.

Only 47% of the total fertilizer nitrogen applied to the environment is taken up by the plants whereas approximately 40% of the total fertilizer nitrogen lost to the environment reverts back into unreactive atmospheric dinitrogen that greatly affects the global nitrogen cycle including increased energy consumption for NH3 synthesis, as well as accumulation of nitrates in drinking water. In this letter, we provide a mechanochemical method of inorganic magnesium and calcium salt–urea ionic cocrystal synthesis to obtain enhanced stability nitrogen fertilizers. The solvent-free mechanochemical synthesis presented can result in a greater manufacturing process sustainability by reducing or eliminating the need for solution handling and evaporation. NH3 emission testing suggests that urea ionic cocrystals are capable of decreasing NH3 emissions to the environment when compared to pure urea, thus providing implications for a sustainable global solution to the management of the nitrogen cycle.

A smart ionic co-crystal of urea with KCl and ZnCl2 has been obtained in two polymorphic modifications via mechanochemical and solution methods and proven to be a very efficient urease inhibitor while, simultaneously, able to provide soil nutrients to complement N supply.

The mechanochemical reaction of urea and catechol affords the quantitative formation of a 1:1 urea·catechol (URCAT) cocrystal that can act simultaneously as a urease inhibitor and as a soil fertilizer. The novel compound has been characterized using solid-state methods, and its environmental activity has been assessed using the inhibition of Canavalia ensiformis urease and water vapor sorption experiments at room temperature. The urea molecules within the cocrystal were organized in hydrogen-bonded dimers bridged by two catechol molecules, with the OH groups interacting via hydrogen bonds with the urea carbonyl groups. The inhibition of jack bean urease enzyme by URCAT led to the complete loss of urease activity after a 20 min incubation period. A large difference of water vapor adsorption was observed between urea and URCAT, with the latter adsorbing 3.5 times less water than urea. Our results suggested that cocrystal engineering strategies can be successfully applied to tackle sustainability problems at the food–energy–water nexus.

While population growth necessitates a significant increase in crop production, stringent environmental regulations require that it be done using sustainable nutrient sources. Nutrients in the form of NH4 + and PO4 3– are recovered from wastewater streams via precipitation, using water-soluble magnesium ions to form sustainable, slow-release fertilizer, struvite (MgNH4PO4·6H2O). However, the magnesium needed is mainly incorporated in the crystal lattices of very low-solubility minerals. This work utilizes a combination of powder X-ray diffraction (pXRD) and ex situ Raman and energy-dispersive X-ray spectroscopies combined with ion chromatography to characterize transformation products of low-solubility MgCO3 particles in NH4 +- and PO4 3–- containing aqueous solutions with and without Ca2+ ions present. Although pXRD showed struvite as the predominant solid product for the molar ratio [Mg2+/NH4 +/PO4 3–] of [0.2:1:1] and higher, ex situ Raman spectra evidenced formation of a dypingite-like phase along with struvite. Single-crystal Raman spectroscopy in combination with scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy showed Ca2+ incorporation into the structure of struvite crystals as submicron crystallites at the Ca2+/Mg2+ ratio of 0.2, from both CaCO3 and CaCl2 and at the Ca2+/Mg2+ ratio of 1, in the case of CaCO3. Moreover, distinct solid product speciation was observed when Ca2+ was present in aqueous solutions when using CaCl2; for example, hydroxyapatite was observed for Ca2+/Mg2+ = 1 when CaCl2 was used. The results reported here unravel the effect of the physicochemical solution parameters, such as concentration of MgCO3, pH, Ca2+ concentration, and solubility of Ca-containing precursors, on the formation of struvite crystals. This shows that recovery of nutrients containing N and P from wastewater streams is possible in the form of a slow-release fertilizer (struvite) using low-solubility, abundant magnesium-containing minerals.