Structure-dependent removal of aromatic organophosphate flame retardants by combined adsorption and electrochemical oxidation.

The removal of aromatic organophosphate flame retardants (OPFRs) from water remains a significant challenge due to their persistence, hydrophobicity, and resistance to conventional treatment processes. In this study, a combined adsorption-electrochemical oxidation system using graphite intercalation compound (GIC) was evaluated for the removal of five representative aromatic OPFRs: triphenyl phosphate (TPP), 2-ethylhexyl diphenyl phosphate (EHDPP), cresyl diphenyl phosphate (CDPP), tricresyl phosphate (TCP), and tris(4-tert-butylphenyl) phosphate (TTBP). Batch adsorption experiments showed rapid initial removal governed by pseudo-second-order kinetics, indicating that surface interactions dominate the adsorption process. Adsorption capacity and rate varied among compounds, with more hydrophobic OPFRs exhibiting faster removal and lower residual concentrations. Cyclic adsorption-regeneration experiments demonstrated that overall performance is strongly dependent on electrochemical conditions. At low current (0.5 A), regeneration was incomplete and partially governed by desorption, whereas increasing the current to 3 A resulted in higher Apparent Oxidative Removal (AOR) values, lower residual concentrations and more stable treatment performance over successive cycles. Under optimal conditions (3 A, 20 min regeneration), several OPFRs were reduced to concentrations below the GC-MS detection limits, and all compounds were reduced to concentrations below 5 µg L-1. Mechanistic analysis revealed that adsorption dominates initial removal, while electrochemical oxidation is essential for regenerating adsorption sites and preventing contaminant release. The combined GIC-electrochemical system offers a synergistic and effective approach for the treatment of aromatic OPFRs, with performance governed by compound structure, hydrophobicity, and operating conditions. These findings provide new insights into adsorption-electrochemical processes for the removal of persistent organic contaminants.