Synthesis of struvite by ion exchange isothermal supersaturation technique

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Recovery and recycling of phosphorus (P) from effluents and waste waters of different types is among the most important tasks of modern environmental chemistry. The importance of this task is determined by the rapid depletion of phosphate ore deposits, which are intensively exploited. The most pessimistic forecasts predict almost complete disappearance of high quality rock phosphates by the beginning of the next century. Phosphates are removed and recovered from the water under treatment as struvite (MgNH 4PO 4 · 6H 2O) or other low solubility compounds. The advantage of P recovery in the form of struvite lies in the possibility of using it as valuable mineral fertiliser. Ion exchange can be used both to recover phosphates from waste waters and to synthesize struvite of sufficiently high purity. This paper describes the results obtained by studying the conditions of struvite synthesis by using the ion exchange isothermal supersaturation (IXISS) technique. The IXISS phenomenon is observed in a number of ion exchange systems when using certain ion exchange separation techniques enabling the target substance to be concentrated beyond its solubility level at a given temperature. Moreover, this supersaturated solution remains stable for a long period, and after leaving the column it crystallizes spontaneously. Stable supersaturated struvite solutions were obtained by stripping magnesium from Lewatite CNP80 cation exchange resin in Mg-form by using ammonium phosphate and dihydrophosphate solutions under batch and dynamic conditions. The pH of struvite formation is around 8.0 at all eluate concentrations and temperatures studied. The results of X-ray diffraction analysis of the precipitates obtained confirm the formation of struvite crystals.