Thermal decomposition of struvite and its phase transition
Introduction
The problems associated with the accumulation of magnesium ammonium phosphate hexahydrate, struvite (MgNH4PO4 · 6H2O) on equipment surfaces of anaerobic digestion and post-digestion processes in the wastewater treatment industry (especially biological nutrient removal processes) affects the industry commercially through major downtime, loss of hydraulic capacity, and increased pumping and maintenance costs (Doyle and Simon, 2002). Although struvite can be a problem in wastewater treatment plants, the conditions for its formation found within the environment of wastewater treatment works can be exploited for extraction of struvite, as a commercial product.
Struvite can be used as slow release fertilizer at high application rates, without the danger of damaging plant roots (Bridger et al., 1962). Suggested uses for struvite are diverse and include ornamentals, forest outplantings, turf, orchards and potted plants. Research has also been undertaken with briquettes of magnesium ammonium phosphate, to investigate its slow release property in a small stream in British Columbia (BC), Canada because of its high P content (29% as P2O5) and a nutrient replacement source for salmon carcasses (Sterling, 1997). Compared to other fertilizers, the benefits of using struvite are low leaching rates and prolonged release of nutrients throughout the growing season of plants, with the possibility of only one single application (Gaterell et al., 2000).
Granular forms of struvite are one of the best, slow-release P fertilizers (Bridger et al., 1962, Gaterell et al., 2000). In the context of competition in the marketplace as a fertilizer with diammonium phosphate (DAP) and triple superphosphate, a modification of struvite has been proposed, whereby struvite should be treated with phosphoric acid (Gaterell et al., 2000). This modified form is termed as enhanced struvite, containing two parts of slowly-soluble, mono hydrogen magnesium phosphate (MgHPO4), to one part of highly soluble DAP ((NH4)2HPO4). This product may prove to be suitable where an initial high dose of P is required, followed by a sustainable slow release of P.
It is suggested that, the availability of P2O5 to the soil is higher for hexahydrate, struvite than that of the monohydrate, dittmarite (MgNH4PO4 · H2O) because of greater dilution with water of crystallization. When dittmarite contacts soil moisture at ambient temperatures, it gradually hydrates to the hexahydrate (Bridger et al., 1962).
Thermophilic digestion of sludge is normally carried out in a temperature range of 50–60 °C. The use of an optional pasteurization system, to treat the sludge at 70 °C for over 30 min prior to mesophilic (36–38 °C) digestion, is also used in Europe (Oleszkiewicz and Mavinic, 2001). Thus, a study on the thermal stability, phase transition and decomposition of the products of the struvite system would provide a better understanding of the fate of struvite in anaerobic digestion and post-digestion processes, as well as off-site, agricultural use.
The apparently fragile equilibrium of struvite in solution leads to the presence of other crystal phases as well (Andrade and Schuiling, 2001). The formation of magnesium phosphates such as MgHPO4 · 3H2O (newberyite), Mg3(PO4)2 · 8H2O (bobierrite) and Mg3(PO4)2 · 22H2O (cattiite), during struvite crystallization or dissolution process, is reported in the literature (Johnson, 1959, Taylor et al., 1963b, Michalowski and Pietrzyk, 2006). When the pH is increased from slightly acidic to slightly basic values, there is a change in predominant solid species from newberyite to struvite to cattiite (Taylor et al., 1963b, Dempsy, 1997). It is suggested that, for a pH between 6.4 and 7.7, both newberrite and struvite are thermodynamically stable (Dempsy, 1997), while both struvite and bobierrite are stable phases for alkaline pH (Taylor et al., 1963b). It was also reported that at room temperature, cattiite is stable in air but unstable in water, in which it reverts to bobierrite. The difference in the solubility products of these two hydrates of trimagnesium phosphates (see Table 1) indicates that they have distinctly different solubilities and that the reversion must involve dissolution of the higher hydrate and crystallization of the lower hydrate from the solution (Taylor et al., 1963b). Table 1 shows the solubility product values (Ksp) available in the literature for the precipitates in Mg2+ – – – H+ systems.
In this research project, a study was carried out in the laboratory to investigate the phase relationships of various reaction products of both synthetic and real magnesium ammonium phosphate systems, at different temperatures and different heating rates. In this paper, the decomposition behaviour of struvite at various temperatures, and the mechanism of its transformation to various other forms, including bobierrite and dittmarite, are reported.
Section snippets
Formation of struvite
Synthetic struvite was prepared by mixing equal volumes of equimolar quantities of magnesium chloride and diammonium phosphate. The solution was then made mildly alkaline (pH 7.4) by slow addition, with stirring, of filtered ammonium hydroxide (Johnson, 1959, Ohlinger, 1999, Babic-Ivancic et al., 2002). After initial mixing of reactant solutions, the solution was sealed in a tightly closed container, leaving a minimum space above the solution. The solution was then maintained at 25 °C, without
Identification of struvite
The powdered XRD pattern of synthetic and struvite pellets matched very well with that of the published pattern for struvite (Fig. 2).The slight difference between the XRD patterns of the two kinds of struvite may be due to the tress amount of impurity present in the struvite pellets. IR spectra for both synthetic struvite and struvite pellets were also consistent with the published IR spectrum of struvite in the wave number range of 400–4000 cm−1, with 100% recovery of spectra according to the
Conclusions
Phosphorus recovery through struvite crystallization and the possible reuse of struvite as a fertilizer is widely reported, as part of sustainable development. An understanding of the thermal stability of struvite, and its possible transition to other phases, would help to ensure the purity of struvite during crystallization. When applied as a fertilizer, its ability to supply the plant with sufficient nutrients without building up a concentration susceptible to leaching or fixation, largely
Acknowledgements
This research was primarily supported by an NSERC (Natural Science and Engineering Research Council of Canada) Grant, awarded to the second author. The authors are also grateful to the excellent technical assistance supplied by personnel in the Environmental Engineering Laboratory, University of British Columbia.
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