Is K-Struvite Precipitation a Plausible Nutrient Recovery Method from Potassium-Containing Wastes?—A Review

Abstract

The definition of technologies capable of removing and recovering nutrients from polluting effluents is a key environmental challenge. Through these technologies, it would be possible to protect aquatic systems and prevent the consumption of natural resources for the production of commercial fertilizers. In this regard, the application of the precipitation processes of struvite-type compounds is an attractive approach. Indeed, these processes are potentially able to remove nutrients from many effluents and produce a precipitate reusable as a slow-release fertilizer. The scientific community has largely focused on the precipitation of magnesium ammonium phosphate (MgNH4PO4·6H2O, MAP), while the recovery of the analogous magnesium potassium phosphate (MgKPO4·6H2O, MPP) has received extensive attention in the last decade. Research on this topic is continuously progressing to improve the precipitation process in different aspects (working conditions, reaction units, interference elimination, etc.). Until now, there has been no paper that comprehensively reviewed the applicability of MPP precipitation for the removal and recovery of nutrients from aqueous waste. To fill this gap, the present paper aimed to provide an exhaustive analysis of the literature reports on MPP processes to help researchers understand the theoretical and applicative aspects, the main problems, and the need for further research. In this regard, the applications in the treatment of various aqueous wastes were considered. The theoretical concepts, the main process parameters, and the effects of inhibiting substances and impurities are presented. Moreover, the development of reactor configurations and their working conditions are evaluated. Finally, the potential use of MPP as a fertilizer and some economic evaluations are reported. On the basis of the conducted analysis, it emerged that the recovery of MPP was mainly affected by the pH, dose, and nature of reagents, as well as the presence of competitive ions. The optimal pH values were reported to be between 9 and 11. Reagent overdoses with respect to the theoretical values improved the process and the use of pure reagents guaranteed superior performance. The stirred-tank reactors and fluidized bed reactors were the most used units with high process yields. The applicability of MPP in agronomic practices appears to be a suitable option.

Understanding the role of zinc ions on struvite nucleation and growth in the context of infection urinary stones

“…Regarding struvite, K-struvite is known, where the K + ion is substituted for the NH 4 + ion. 52 Such ion exchange is possible because the ionic radii of these ions do not differ much. The question arises whether, in the case of the described experiment, the Zn 2+ ion, which is a divalent cation, can occupy a nodal position instead of the Mg 2+ ion, which is also a divalent cation.…”

Can Tundish Deskulling Waste Be Used as a Magnesium Oxide Source to Develop Magnesium Phosphate Cement?

“…10. The phosphate (PO 4 3-) peak in K-struvite appears between 1000 and 1200 cm − 1 [35][36][37]. Besides, the asymmetrical deformation of PO 4 3is located at 1000 cm − 1 and the symmetrical one is identified at 575 cm − 1 [38].…”

“…The peak located around 1700 cm − 1 corresponds to the H-O-H bending modes of vibrations [33,38]. The Mg-O bond peak is located around 600 cm − 1 and the vibrations of coordinated water are at 750 cm − 1 [36,37,39]. Table 7 shows the leaching results and the regulatory limits following the European Commission legislation (Directive 2018/850).…”

Closing the Nutrient Loop—The New Approaches to Recovering Biomass Minerals during the Biorefinery Processes

“…), have been investigated, as reviewed in [324]. K-struvite (KMgPO 4 ) can be used to simultaneously recover P and K [325].…”