Transformation of 4-nonylphenol isomers during biosolids composting Original Research Article

4-Nonylphenol, a degradation intermediate of commercial surfactant and known endocrine disruptor, has been frequently detected at levels up to several thousand μg l−1 in surface waters and up to several hundred mg kg−1 (dry weight) in soil and sediment samples. Large quantities of 4-NP can be quickly sorbed by the organic rich solid phase during wastewater treatment and are concentrated in biosolids, a possible major source for 4-NP in the environment. Microbial transformation in culture studies followed different mechanisms for different 4-NP isomers, which have different estrogenic activity. Composting is a process of solid matrix transformation where biological activity is enhanced by process control. This approach has been used successfully in remediation of contaminated soils and sludges. In this study, the transformation kinetics of 4-NP and its isomers were characterized during biosolids composting. Five distinctive 4-NP isomer groups with structures relative to α- and β-carbons of the alkyl chain were identified in biosolids. Composting biosolids mixed with wood shaving at a dry weight percentage ratio of 43:57 (C:N ratio of 65:1) removed 80% of the total 4-NP within two weeks. At this biosolids/wood shaving ratio (B:WS), the transformation of total 4-NP and its isomers followed second-order kinetic. Higher B:WS ratios yielded significantly slower 4-NP transformation which followed first-order kinetic. Isomers with α-methyl-α-propyl structure transformed significantly slower than those with less branched tertiary α-carbon and those with secondary α-carbon, suggesting isomer-specific degradation of 4-NP during biosolids composting.