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Alleviating Na+ effect on phosphate and potassium recovery from synthetic urine by K-struvite crystallization using different magnesium sources

Huang, Haiming, Zhang, Dingding, Wang, Wenjun, Li, Bing, Zhao, Ning, Li, Jing, Dai, Jiankun
The Science of the total environment 2019 v.655 pp. 211-219
crystallization, crystals, electrodes, humans, magnesium, magnesium chloride, magnesium hydroxide, nitrogen content, pH, phosphates, phosphorus, potassium, sodium, urine
Human urine is characterized by high concentrations of nitrogen (N), phosphorus (P) and potassium (K), of which the P and K can be recovered as K-struvite crystals. This study first investigated the formation of Na-struvite because of the high Na+ present in the urine. From the results, the optimal pH for the Na-struvite crystallization was observed to be 12, and the rise in the Na+ concentration distinctly favored the Na-struvite formation. As magnesium needed to be added to induce the K-struvite crystallization, several magnesium sources including MgCl2, Mg sacrificial electrode and Mg(OH)2 were applied to recover P and K from synthetic urine. The findings indicated that when MgCl2 was used as the magnesium source, the K removal could be slightly enhanced by prolonging the reaction time, which would correspondingly decrease the Na concentration in the precipitates; besides, the intermittent addition of MgCl2 could noticeably improve the removal efficiency of K by 6%, but simultaneously raise the Na content in the precipitates recovered. With respect to the use of the Mg sacrificial electrode, the recovery efficiencies of the P and K from synthetic urine were close to those with the use of MgCl2. However, when Mg(OH)2 was used as the magnesium source, the recovery efficiencies of P and K achieved only roughly 50%, which was much lower than those noted when MgCl2 and the Mg sacrificial electrode were employed. A comprehensive analysis revealed that the MgCl2 was the best magnesium source for the K-struvite crystallization, followed by the Mg sacrificial electrode and Mg(OH)2.