Influence of natural organic matter on the extraction efficiency of flame retardants from surface waters

The influence of natural organic matter (NOM) on the solid-phase extraction (SPE) efficiency was investigated for legacy and emerging flame retardants (FRs; n = 26) in surface water. Three different groups of FRs were analyzed: polybrominated diphenyl ethers (PBDEs), halogenated flame retardants (HFRs), and organophosphorus flame retardants (OPFRs). In addition, five sorbents (Amberlite XAD-2, Amberlite IRA-743, Oasis HLB, Chromabond HR-P, and Chromabond HR-X) were evaluated for the extraction of FRs (n = 33) in water, of which Oasis HLB eluted with dichloromethane and acetone:n-hexane (1:1, v/v) provided the highest overall recoveries. In subsequent NOM experiments, where FRs were extracted from water containing different NOM concentrations, both increased and decreased extraction efficiency with increasing NOM level were observed. Physicochemical and semi-empirical quantum chemistry properties were calculated for the FRs and used for analyzing relations between FRs. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that the FRs separated into four different groups based on their properties. The FRs within each group responded similarly to increasing NOM, while differences in behavior were observed between the groups. This suggests that the structural properties of micropollutants highly influence NOM-FR interaction mechanisms. For instance, at high NOM levels, recoveries decreased substantially for FRs containing a moiety that can form strong hydrogen bonds (such as the double-bonded oxygen in e.g., OPFRs). Many of the compounds showed maximum extraction efficiency at higher levels of NOM. This suggests that binding of NOM to the sorbent and subsequent interaction between sorbent-bound NOM and FRs is an important mechanism for extraction of micropollutants from surface waters.