engleski

The transformation of struvite to newberyite in aqueous solutions

Magnesium ammonium phosphate hexahydrate, MgNH4PO4&#8226; 6H2O, known as struvite, has often been reported to be found as a constituent of slightly soluble scale deposits in pipes and at equipment surfaces of wastewater treatment industry. In addition, the precipitation of struvite from wastewaters can be used as an alternative method for the removal of major nutrient compounds, phosphorous and nitrogen, causing eutrophication and its subsequent use as high quality fertilizers. The research reported in this paper was undertaken with the aim to get some answers concerning mostly the kinetics and mechanisms of struvite transformation into newberyite, MgHPO4&#8226; 3H2O, which was found to coexist with struvite in aqueous solutions under certain conditions. The transformation process was studied at 25 and 37 °C in the systems in which the initial pH was varied in the range 5.0 < pHi < 9.0. The propagation of the reaction was followed by measuring pH of the solution and by determining composition of the solid phase by means of FT-IR spectroscopy. Apart from the study of spontaneously precipitated systems, a series of experiments was performed using different well defined mixtures of struvite and newberyite with which the systems were initially seeded. Such an approach to the investigation enabled easier drawing conclusions about the kinetics and mechanism of the transformation process. The results obtained indicate that struvite transformation is solution mediated process, in which the dissolution of unstable phase (struvite) is followed by the growth of stable one (newberyite). The transformation process was found to be partially retarded because of aggregation of the stable phase crystals.

Transformation; Struvite; Newberyite; Spontaneous precipitation; Seeding

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