Characterization of recalcitrant intermediates from biotransformation of the branched alkyl side chain of nonylphenol ethoxylate surfactants

Nonylphenol polyethoxylates (A(9)PEs) are complex mixtures of isomers and oligomers widely used as surfactants. The weakly estrogenic effects of the A(9)PE breakdown products are impelling reformulation of many commercial products and have made "A(9)PE free" products an industry trend. Initial A(9)PE biotransformation is known to produce metabolites with shortened ethoxy chains, mainly A(9)PE(2), and carboxylated ethoxy chains, mainly A(9)PE(2)C. Very little is known on the ultimate fate of the branched alkyl side chain. We have used liquid chromatography-electrospray-mass spectrometry to elucidate structures of species generated from biotransformation of the branched alkyl chain. According to an OECD protocol, a laboratory A(9)PE biodegradation experiment was conducted. After about two weeks from the beginning of the experiment, relevant amounts of species having both side chains oxidized (CAPECs) and 3-8 carbons in the residual alkyl chain appeared in the test solution. These species were presumably generated from less extensively alkyl branched A(9)PEC isomers by various oxidative mechanisms. The unreacted most extensively alkyl branched A(9)PEC isomers disappeared from the test liquor after more than 3 months by an unknown mechanism. Less abundant, metabolites having only the alkyl chain carboxylated (CAPEs) were also formed. With time, these species were slowly transformed to CAPECs. Apart from a very slow conversion of CAPE(2)Cs to CAPE(1)Cs, this metabolite class was extremely recalcitrant to further biotransformation, as they persisted in the test liquor even more than 5 months after their generation. Analysis of a sewage treatment plant effluent showed CAPECs, as a total, were present at concentration of 58 mu g/L, accounting for 63% of the total AgPE metabolites leaving the plant.