Fractionating magnesium ion from seawater for struvite recovery using electrodialysis with monovalent selective...

As the consumption of global phosphorus reserves accelerates, recovering phosphorus as struvite (MgNH4PO4·6H2O) from wastewater is an important option for phosphorus recycling. However, magnesium source is one of the major limiting factors for struvite recovery. In this work, different from previous studies where seawater was used directly as magnesium source in struvite precipitation, an electrodialysis stack equipped with monovalent selective cation-exchange membranes was designed to fractionate Mg2+ from seawater for struvite recovery. Results revealed that Mg2+ fractionation was achieved effectively. The comparison on applying the driving force for ionic transport showed that constant voltage was more preferable than constant current due to its higher Mg2+ separation efficiency, current efficiency and lower energy consumption. Increasing voltage from 7 V to 13 V would improve Mg2+ permeation ratio from 72.9% to 80.5% into the product stream but simultaneously increased the energy consumption from 5.40 (kWh/kg MgCl2) to 11.69 (kWh/kg MgCl2). In addition, the investigation on the influence of Ca2+ co-existence and further struvite recovery experiments revealed that the variation of Ca2+ concentrations in seawater did not influence Mg2+ fractionation significantly, nevertheless it might reduce struvite recovery efficiency through forming calcium phosphate.