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Adsorption and Desorption of Organic Contaminants to Predict Fate, Transport, and Propensity to Electrochemically Degrade

Motivation
Sorption properties of polychlorinated biphenyls (PCBs) play a significant role in their mobility, ultimate fate in the sediments, and availability for degradation. The data generated during bench-scale adsorption studies and molecular-level study of the mechanism of adsorption of PCBs on substrates, which constitute or are natural sites of PCB occurrence in the environment, will help in effective desorption and the destruction of the persistent PCBs by electrochemical degradation.
Hypothesis
Generic structure of PCBs
The literature suggests that PCBs preferentially adsorb onto organic matter over adsorbing onto clay. It is further hypothesized that this is chemical in nature. This study will prove or disprove this hypothesis.
Objectives
  1. To generate isotherms for the adsorption of PCB congeners on substrates (clays and natural uncontaminated sediments)
  2. To study the sorption mechanism of PCBs on clays, natural, contaminated, and uncontaminated sediments in detail using molecular-level techniques like X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), and other techniques as necessary.
Approach
All substrates used in the study and as mentioned above will be well characterized for total organic carbon, surface area, particle size distribution, metal content and pH.

  1. Batch adsorption studies: The experimental design is based on a batch technique that involves mixing the soil substrate and the PCB solution in de-ionized water in different ratios, and agitating the mixture until the two phases reach equilibrium with respect to the solute distribution. Partitioning of the solute in the solid and liquid phases at equilibrium provides the basis for construction of adsorption isotherms. The substrates chosen for the study are Montmorillonite, Kaolin (two commonly found clays), and Caesar creek sediment. Adsorption experiments are run at eight substrate solution mass ratios lying between 1:10 to 1:500. The soil solution mixtures are placed in 40 mL borosilicate vials sealed with aluminum faced caps and tumbled in a Tumbler till equilibrium is reached at 30 rpm at 25 ± 1 ºC [3]. Samples are taken periodically and analyzed for PCB concentration. Mass balance is used to determine partitioning of the solute between the solid and solution phase at equilibrium.
  2. Molecular-level studies of adsorption mechanism: The techniques used will include XRD, FTIR, and TGA. Substrates will be equilibrated with an aqueous solution of PCBs. Substrate samples will then be prepared and analyzed following standard procedures. The basic approach of using these techniques is guided by the following fundamental principle: the signature of the contaminated substrate sample will deviate from the original signature of the uncontaminated or unspiked substrate depending on the adsorption mechanism. Deviations will be interpreted and supported by additional information from complementary techniques to establish the mechanism of adsorption.

Contact: Souhail Al-Abed (EIMS#135925)

Office of Research & Development | National Risk Management Research Laboratory

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