Compound-Specific Sorption and Desorption of Organophosphate Flame Retardants in Surface Soil: Strong Aromatic Retention Versus High Chlorinated Secondary Release

Organophosphate flame retardants (OPFRs) are widely detected in terrestrial ecosystems. Evaluating their soil–water transfer remains challenging because initial sorption loading does not necessarily dictate long-term retention. Here, the compound-specific sorption and desorption behaviors of 10 structurally diverse OPFRs were systematically investigated in a surface soil using batch kinetic and isotherm experiments. Adsorption proceeded rapidly within the first 4 h and approached equilibrium by 24 h. Single-solute kinetic desorption revealed strong compound-specific divergence: aromatic OPFRs exhibited near-complete solid-phase immobilization (>99.5% operational retention), whereas chlorinated and ether-linked OPFRs displayed substantial secondary release, desorbing 57.3–79.5% of their pre-sorbed mass. Equilibrium isotherms indicated stronger soil affinity for aromatic OPFRs than for more hydrophilic analogues. This contrast highlights a competitive site-blocking and matrix occlusion mechanism, wherein hydrophobic aromatic species occupy high-energy sites and pore throats, physically restricting the outward diffusion of co-sorbed compounds. Freundlich hysteresis indices (0.830–0.905) reflected progressive occupation of high-energy sorption sites. The results indicate that initial solid-phase loading and short-term release potential should be evaluated separately when comparing OPFRs behavior in soil. These insights provide a crucial foundation for accurately modeling the environmental fate and ecological risks of emerging OPFRs in soil–water systems.