Electrochemical Activation of Natural Magnesite for Phosphorus Recovery from Wastewater via In Situ Struvite Crystallization

Phosphorus recovery from wastewater can reduce dependence on phosphate rock and help control eutrophication. Struvite crystallization is a promising recovery route, but conventional processes often require soluble magnesium salts and alkaline reagents. This study used electrochemical activation of natural magnesite to supply Mg2+ in situ for magnesium ammonium phosphate (MAP) formation. Magnesite dissolution provided Mg2+, while cathodic water reduction generated a local alkaline zone that promoted phosphate precipitation. The effects of current density, N:P molar ratio, initial phosphorus concentration and initial pH were examined. XRD, SEM–EDS, and FTIR supported the formation of struvite-containing precipitates. Under the optimized conditions, namely an initial pH of 7.75, a nominal anodic current density of 28.21 A/m2, an initial phosphorus concentration of 6.41 mM, and an N:P molar ratio of 1.42:1, PO43−–P recovery, NH4+––N apparent removal and apparent MAP purity reached 57.56%, 52.26% and 62.56%, respectively. The apparent MAP purity represents an NH4+––N-based operational estimate rather than an absolute quantitative phase fraction. The decrease in aqueous NH4+––N should be regarded as apparent removal because NH3 volatilization or stripping may also occur near the alkaline cathode. This work provides a reagent-saving approach for phosphorus recovery by coupling Mg2+ release, cathodic alkalization and struvite crystallization.