Synchronous anodic oxidation-cathodic precipitation strategy for efficient phosphonate wastes mineralization and recovery of phosphorus in the form of hydroxyapatite

Abstract To make up the gap of wasted phosphonates being too much as pollutants and phosphorus being too little as a natural resource, electrochemical “oxidation-precipitation” system was rationally developed to recover phosphorus with simultaneous mineralization of ethylenediamine tetra(methylene phosphonic acid) (EDTMP), a typical phosphonate. In this process, BDD(•OH) was the primary oxidative species for EDTMP degradation, which led to the bond cleavage in the sequence of C-N, C-P and C-C. Increasing current density from 3 to 30 mA cm-2 improved the mineralization efficiency of EDTMP from 14% to 72% within 120 min, accompanying 65%-95% of EDTMP conversion to orth-P. With the presence of 50 mg L-1 Ca2+, 21%-83% of phosphorus was recovered as insoluble calcium phosphate at the cathode surface. High initial Ca2+ concentration favored the phosphorus recovery, whereas it was negatively influenced with the presence of Mg2+ and HCO3-. In addition, phosphorus recovery was negligibly affected by solution pH in the range of 3.0 to 12.0 because the locally alkaline condition was well maintained at the cathode surface. Phosphorus was electrochemically recovered mainly in the form of thermodynamically most stable hydroxyapatite, which proceeded via the formation of brushite, octacalcium phosphate, and amorphous calcium phosphate with Δ[Ca]/Δ[P] molar ratio increasing from 1.0 to 1.67. Based on the results of phosphorus recovery from the real wastewater, we envision that this synchronous anodic oxidation-cathodic precipitation process can be applied as an environmentally compatible and feasible strategy for phosphorus recovery from the phosphonate wastewaters.