Study on the removal of typical organophosphate flame retardants by iron-carbon micro-electrolysis constructed wetlands

To address the limited removal efficiency of conventional constructed wetlands for organophosphate flame retardants (OPFRs), this study developed intensified constructed wetland systems by incorporating iron-carbon micro-electrolysis (ICME) substrates. Three substrate combinations, biochar-sponge iron (BC+SI), activated carbon-iron filings (AC+Fe), and a commercial iron-carbon material (FeC), were evaluated for their performance in removing three typical OPFRs—triphenyl phosphate (TPHP), tris(2-chloroethyl) phosphate (TCEP), and tris(chloropropyl) phosphate (TCPP). The effects of different substrates, hydraulic retention times (HRT, 0.5-2 d), and influent OPFR concentrations (300-1500 μg/L) on removal efficiencies were systematically investigated. The results showed that the BC+SI combination achieved the highest performance, with average removal rates exceeding 95% for all three OPFRs. The system also demonstrated excellent removal of conventional pollutants, with maximum average removal rates of 85.5% for COD, 75.5% for TN, and 93.6% for TP, meeting the Grade 1 A standard of China's Discharge Standard for Pollutants from Municipal Wastewater Treatment Plants. Mechanism analysis revealed that the ICME substrates significantly altered the system's microenvironment, increasing the effluent pH to 8.0-8.5 and decreasing the dissolved oxygen in the intensified layer to 0.3-0.9 mg/L, thereby creating favorable conditions for pollutant degradation. Shortening the HRT from 2 d to 0.5 d or increasing the influent OPFR concentration to 1500 μg/L reduced OPFR removal rates by approximately 30% and 35%, respectively. This study concludes that the BC+SI combination, through the synergistic effects of adsorption and micro-electrolysis, provides an efficient and stable method for removing both typical OPFRs and conventional pollutants from water.