Sorption of pentachlorophenol to humic acids and subsequent effects on biodegradation and solvent extraction

Abstract

The focus of this research was to acquire a better understanding of the sorption and desorption of pentachlorophenol to soil organic matter. In order to separate the reactions controlling the interactions with the soil organic matter from those associated with mineral surfaces, these experiments used only humic acids extracted from soil samples. The major focus of this study was to examine the effects of solution pH, humic acid concentration and contact time on the degree of sorption. The association reactions proceeded slowly. Even after 28 days, many solutions had not attained equilibrium. An increase in the solution pH led to a reduction in the amount of partitioning onto the humic material. At solution concentrations between 100 mg/L and 800 mg/L of total organic carbon (TOC), an increase in the humic acid concentration resulted in a lower partition coefficient. However, above a concentration of 800 mg/L TOC, further increases in the amount of humic material caused enhanced sorption. The particulate humic acids demonstrated a higher affinity for the pentachlorophenol than did the dissolved polymers. In the concentrated solutions, the majority of the humic acids were present in the particulate form.

Two experiments focused on the effect of sorption on the bioavailability and solvent extraction of pentachlorophenol. The bioavailability data Suggested that the sorbed contaminant was not readily accessible to the microorganisms. The humic acids prevented the extraction of the sorbate by methyl-tert-butyl ether and methylene chloride. Recovery of the pentachlorophenol sorbed to the dissolved humic acids ranged from zero to 42.9 percent, depending on the solution pH. The removal of pentachlorophenol from the particulate matter varied from 25 percent to 90 percent. Longer contact times diminished the transfer of PCP associated with the solid humic acids to the solvent phase.

The experimental results were not consistent with a simple, one mechanism model. The best explanation of the data was provided by a model which included liquid-liquid partitioning, surface sorption, absorption, and chemisorption. The dominant process depended on the contact time, solution pH, and concentration and nature of the humic acids.