Enzymatic Treatment of Phenolic Wastewater: Effects of Salinity and Biosurfactant Addition

Water contaminated with phenols is produced from several oil and gas related industries. Although there are a number of treatment methods, enzymatic wastewater treatment is more attractive due to its sustainability, environmental-friendliness, and mild nature. A key limitation of this process, however, is the enzymatic deactivation (whether complete or partial) during the treatment process. This limitation might be addressed to a certain extent through the addition of biosurfactants to the reaction medium. Thus, the key aim of this study is to utilize laccase (an oxidoreductase enzyme from Trametes versicolor) to remove bisphenol A (BPA) from wastewaters in the presence of rhamnolipid biosurfactant. Since most wastewaters contain inorganic salts, the efficacy of enzymatic treatment of high saline wastewaters has been evaluated. The beneficial effect of the biosurfactant addition during the enzymatic treatment of highly saline phenolic wastewater has been also assessed. Additionally, the effect of increasing the biocatalyst and the phenolic pollutant concentrations have been also probed. The results showed that the BPA degradation rate increases with increasing the enzyme concentration. The extent of BPA removal also increased with increasing the biocatalyst concentration, approaching almost a complete removal at an enzyme concentration of 400 ppm. The BPA degradation rate also increased almost linearly with increasing its initial concentration; however, its removal extent showed the opposite trend. The addition of as low as 1 ppm rhamnolipid biosurfactant to the reaction medium increased both the BPA degradation rate and the removal extent relative to the biosurfactant-free wastewater samples. The addition of the biosurfactant to the reaction medium boosted the BPA degradation rate and the removal extent by 1.1- to 1.23-fold. The highest BPA degradation rate and removal enhancement (about 23% higher than those in the absence of the biosurfactant) was obtained for BPA-rhamnolipid mass ratio of 50:1. The presence of salt severely reduced the BPA degradation rate and removal. The addition of 20 mM NaCl resulted in about 1.7-fold drop in the BPA degradation rate and removal. The drop in the BPA degradation rate and removal reached more than 3.6-fold at 500 mM NaCl. The addition of 1 ppm rhamnolipid partially compensated the negative effect of salinity, providing relatively higher BPA degradation rate and removal at all examined salinity levels. The findings reported herein reveal the positive effect of biosurfactant addition to the enzymatic reaction medium and the need for the salt removal prior to subjecting the saline wastewaters to enzymatic treatment.