Static die-away of a nonylphenol ethoxylate surfactant in estuarine water samples

The static die-away of the nonylphenol ethoxylate (NPE) surfactant used to stabilize the bitumen-water emulsion in the fuel Orimulsion was studied using estuarine water samples collected at four stations in Tampa Bay, FL. Incubations were in the dark at 28 degrees C for 183 days. Primary degradation was complete in 4-24 days with lag periods between 0 and 12 days. The concentration of five NPE intermediate degradation products and total surfactant were monitored at 4-8-day intervals for the first 89 days and at 30-day intervals thereafter. The intermediates detected included nonylphenol diethoxylate (NP2EO) and nonylphenoxy ethoxy acetic acid (NP2EC). Much smaller amounts of nonylphenoxy acetic acid (NP1EC) and nonylphenol monoethoxylate (NP1E0) were detected. The completely deethoxylated metabolite nonylphenol (NP) was not detected. On a molar basis, NP2EC accounted for 66.6-93.3% of degradation products and for 22.7-75.6% of the starting material at the termination of the experiment. The sequence of degradation and intermediate accumulation and decay indicated the initial formation of NP2EO from higher NPE homologues followed by oxidation to NP2EC. A second phase of the experiment was initiated on day 296. The remaining water in two sample incubation containers was combined with equal volumes of freshly collected Tampa Bay water. In three of five replicates from one sample, die-away of the residual NP2EC began at day 20 and declined to approximately 50% of the initial concentration on day 32. In other replicates of this sample and in a second sample, no change in NP2EC or NP1EC concentrations was observed. The data have confirmed relatively rapid primary degradation of the parent NPE and the formation in sequence of two degradation products, NP2EO and NP2EC. The later compound appears to be relatively resistant to further degradation However, data did indicate that microorganisms were present in the Bay that are capable of transformation and degradation of the compound.