Effects of a rhamnolipid biosurfactant on the reductive dechlorination of polychlorinated biphenyls by St. Lawrence River (North America) microorganisms

The effect of a rhamnolipid biosurfactant on the reductive dechlorination of polychlorinated biphenyls was investigated with the use of clean sediments spiked with Aroclor 1248. The surfactant was added to the contaminated sediments at four different concentrations (5, 10, 25, and 50 microg/g sediment [ppm] on a sediment dry wt basis), and dechlorination was followed over a 40-week period. The rate of overall dechlorination was enhanced at the three highest concentrations. Dechlorination at the lowest concentration (5 ppm) was not different from that in surfactant-free sediments. On a congener basis, enhanced dechlorination was mostly found in the congeners that have high threshold concentrations for dechlorination. These congeners are characterized by an initial increase in concentration before dechlorination starts. At the three highest biosurfactant concentrations, this initial concentration increase was absent or dramatically reduced. Therefore, the enhancement in dechlorination appears to be caused by an increase in bioavailability at high surfactant concentrations. The biosurfactant also reduced the lag time before dechlorination began in these congeners. Among those congeners that have low threshold concentrations, dechlorination enhancement was found only in two peaks. For these two, there was no lag period, either with or without the rhamnolipids. The maximum level of dechlorination and the congener pattern of final dechlorination products were identical, regardless of biosurfactant concentration.