Project Abstract

Evaluation of PCB Binding Energy in Soil and Sediments

Principal Investigators

Dharmaraj Raghavan and James H. Johnson Jr.
Howard University
E-mail: ramraj@enh.nist.gov

Introduction

Cleanup of contaminated sites by existing technologies is extremely costly, especially PCBs that are found at many Department of Defense installations and U.S. harbor areas. Of the many novel technologies being developed, bioremediation is the most attractive and promising because it is substantially less expensive to advocate and is efficient in converting contaminants into environmentally benign materials. For the successful development of the technology and implementation of the bioremediation process in the field, there is a need to understand the factors controlling bioavailability of PCBs and other hydrophobic contaminants in subsurface systems. Howard University researchers have studied one of the factors--i.e., how strongly the PCB contaminant is bound to the organic matrix and how to quantify the PCB binding energy. This important information will serve as predictors in designing strategies for promoting availability of PCB for microbial degradation.

Summary of the Problem

It has long been recognized that contaminant desorption is the rate controlling step in the availability of the contaminant in the aqueous phase and microbial remediation of the contaminant. The adsorbed hydrophobic contaminant resides in the mineral and the organic region. The condensed organic matrix region in soil is of primary importance because it plays an important role in limiting the availability of contaminant for biological degradation. Evaluating the PCB desorption kinetics for the condensed organic matrix fraction of soil is therefore one of our research objectives. The other objective is to use the desorption data at multiple temperatures to compute the binding energy via Arrhenius equation. Recent work in our laboratory has shown that the desorption of PCB from soil and sediment is strongly dependent on temperature. The desorption kinetics was found to observe Arrhenius like temperature dependance. While several studies have been conducted to correlate the availability kinetics of contaminant adsorbed to the type of soil and sediment, little is known about the process of desorption from natural organic matrix such as coal or humic acid.

Design and Implementation

To gather the necessary data, researchers carefully chose extensively characterized standard reference material (coal) and a single PCB congener. The study focussed on a Beulah-Zap Argonne Premium Coal sample and 2,3 dichorobiphenyl supplied by Argonne National Laboratories and Ultra Scientific. The coal-PCB mixture was equilibrated for 24 to 48 hours. A methodology was developed for the extraction of PCB from spiked coal using a Ruska Laboratory Thermal Extraction-Gas Chromatography (FID) HP 5890 system. The operating principle of thermal extraction is to volatilize the semi-volatile contaminant without oxidizing or degrading the components of the organic matrix. Three major aspects were examined:

effect of extraction temperature,
duration of isothermal extraction,
concentration of contaminant on the recovery of PCB from coal.

The type of information sought in the study is the kinetic (rate constant, desorption rate, order of the reaction, etc.) and thermodynamic data (activation or binding energy) of contaminant desorption from coal matrix.

Findings

The work with thermal extraction on PCB adsorbed coal showed an equilibration time of 24 to 30 hours. The PCB equilibrated coal was subject to isothermal heating at 170, 180, 190, 200 oC for 0, 10, 20 and 30 minutes. At temperatures above 215 degrees C, the matrix (i.e., coal and PCB) was found to undergo deterioration. At these working temperatures, there was a steady increase in the amount of PCB recovered as a function of duration of isothermal heating and the temperature of isothermal heating. For example, the lowest PCB extractability (~ 10%) was at 170 degrees C for 1 minute, while the highest extractability (~78%) was at 200 degrees C for 30 minutes. It was found that the PCB recovery reached a maximum upon extended duration of isothermal heating.

Studies are underway to examine the dependance of PCB extractability on the amount of PCB spiked on coal.

Future Research

The research work provides an opportunity for obtaining kinetic and thermodynamic data of PCB desorption from condensed organic matter. By constructing a plot of the logarithmic of the rate of PCB recovered as a function of the logarithmic of the PCB concentration, the kinetic data will be generated. Thermodynamic data, e.g. binding energy commonly referred as activation energy, will be obtained by conducting isothermal kinetic experiments at different temperatures. Knowing the binding energy, it should be possible to develop a system to improve the availability of contaminants. One way to improve the availability of contaminant is by adding surfactants that preferentially solubilize the contaminant from coal matrix. Future studies will also include the effect of surfactant on PCB binding energy.

Field engineers can use binding energy data to predict the accessibility of contaminant for microbial degradation. The information can be critical in estimating the efficacy of bioremediation technology at a given site. For example, the impact of surfactant aided bioavailability could be quantified as changes in binding energy as a function of surfactant concentration.