  Industrial and agricultural waste streams, which contain high concentrations
of NH4+, PO43- and transition metals are environmentally harmful and toxic
pollutants. At the same time phosphorous and transition metals constitute
highly valuable resources. Typically, separate pathways have been considered to
extract hazardous transition metals or phosphate, independently from each
other. Investigations on the simultaneous removal of multiple components have
been studied only to a limited extent. Here, we report the synthesis routes for
Co- and Ni-struvites (NH4MPO4.6H2O, M = Ni2+, Co2+ ), which allow for P,
ammonia and metal co precipitation. By evaluating different reaction
parameters, the phase and stability of transition metal struvites, as well as
their crystal morphologies, and sizes could be optimized. Ni-struvite is stable
in a wide reactant concentration range and at different metal/phosphorus (M/P)
ratios, whereas Co-struvite only forms at low M/P ratios. Detailed
investigations of the precipitation process using ex situ and in situ
techniques provided insights into the crystallization mechanisms/crystal
engineering of these materials. M-struvites crystallize via intermediate
colloidal nanophases, which subsequently aggregate and condense to final
crystals after extended reaction times. However, the exact reaction kinetics of
the formation of a final crystalline product varies significantly depending on
the metal cation involved in the precipitation process: several seconds (Mg) to
minutes (Ni) to hours (Co). The achieved level of control over the morphology
and size, makes precipitation of metal struvites a promising method for direct
metal recovery and binding them in the form of valuable phosphate raw
materials. Under this paradigm, the crystals can be potentially upcycled as
precursor powders for electrochemical applications, which require transition
metal phosphates (TMPs).
