A kinetic study on slow natural degradation of polycyclic aromatic hydrocarbons in Bolgoda Lake waters and a technique for rapid destruction of them in the aquatic environment

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are unique environmental contaminants that exhibit strong toxic, carcinogenic and mutagenic properties. They are generated through incomplete combustion of organic matter such as fossil fuels (petrol, diesel, and natural gasses, etc.), wood and many other organic compounds used in variety of industrial and domestic processes. Once formed, they are adsorbed on to particulate matter and deposited on ground and in water bodies through continuous atmospheric fallouts. During rainy seasons, PAHs deposited on ground surfaces are washed into water bodies through surface runoff. A recent study revealed that water and sediment in Bolgoda lake to contain PAHs above safe levels. Further, the levels observed during dry seasons were found to increase after heavy rains, followed by gradual degradation during dry periods. As a part of the present investigation, the kinetics of degradations of four selected PAHs; naphthalene, phenanthrene, pyrene and chrysene (2, 3, 4 and 5 member ring compounds) found in Bolgoda lake were carried out. Four different environmental matrices given below (a) to (d): (a) Bolgoda lake water exposed continuously to incandescent radiation of a 60 W tungsten lamp at room temperature. (b) Bolgoda lake water kept in dark at room temperature. (c) Deionized water with pH and electrical conductivity (EC) adjusted to those of Bolgoda lake water, exposed continuously to incandescent radiation of 60 W tungsten lamp at room temperature (d). Deionized water with pH and EC adjusted to those of Bolgoda lake water, kept in dark at room temperature. These four different media could represent unique environmental components found in Bolgoda lake that could be responsible for degradations of PAHS in the Bolgoda lake. The results indicate that pseudo first order rate constants corresponding to degradations of all four PAHs in the four different media at room temperature decrease in the order a > b > c > d. Also, in all four media the order of the pseudo first order rate constants for degradations of the four PAHs found to vary as chrysene > naphthalene > pyrene > phenanthrene. As the other part of the investigation, the suitability of a modified Fenton based oxidation process to enhance the rate of degradation of PAHs in aquatic environment was investigated. The effect of initial concentration of H2 O2 & Fe2+ ions, pH of the solution and the presence of triton X-100, a surfactant which could bring non polar PAHs and polar Fenton reagents in close proximity to facilitate degradation reaction were examined for 3,4 and 5 member ring compounds: phenanthrene, pyrene and benzo(a)pyrene found in local aquatic environments. Pseudo first order rate constants for degradations of all three PAHs found to increase with increasing initial H2 O2 concentrations and initial Fe2+ concentration in aqueous media. High degradation rates were observed for concentration ratios, [H2 O2 ]:[Fe2+] in the range of ~50 to ~100. Decrease of pH of solutions, also increased the rates of degradation of all three PAHs. Presence of Triton X-100 at low levels, (1% v/v in the solution) found to increase the degradation rates of all three PAHs. Under all conditions studied, the pseudo first order rate constants for degradation of Benzo(a)pyrene were found to be the largest among the three PAHs .