Enhanced strategies for phosphate recovery from urine by magnesium galvanic process

Abstract

Magnesium galvanic process (MGP) can be applied to recover phosphate from source-separated urine. However, information on how the urine matrix affects MGP performance is limited. Therefore, this study investigated the mechanism of phosphate recovery by MGP in synthetic and real urine matrixes. Our results showed that the major components in urine (i.e., NH4+, Cl−, and HCO3−) all exhibited acceleration effects on corrosion of Mg plate. However, the underlying action mechanism of each component was distinct. Ammonium facilitated the conversion from MgO to Mg(OH)2, chloride complexed with Mg2+ ions, and bicarbonate led to complexation as well as formation of MgCO3. Furthermore, our results revealed an interesting aspect where although bicarbonate alone accelerated the corrosion of Mg plate, its coexistence with other ions inhibited overall performance due to the blocking effect of formed MgCO3 on chloride penetration and reduction in free magnesium ion concentration. After elucidating the interaction of NH4+, Cl−, and HCO3− on the passive layer of the Mg plate, we proposed to pretreat urine with HCl, which resulted in a significant enhancement in current production and phosphate recovery. This improved MGP was further tested in a continuous flow reactor, which recovered over 95% of phosphate in real urine for more than 1 h. The phosphate precipitates were confirmed as high purity struvite. Generally, the improved MGP, which simultaneously produced Mg2+, dihydrogen, and electricity with no energy input, is a promising sustainable and green alternative for phosphate recovery from source-separated urine.