Effects of Activated Carbon Surface Chemistry and Pore Structure on the Adsorption of Trace Organic Contaminants from Aqueous Solution
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Date
2002-08-26
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Abstract
The principal objectives of this research were (1) to identify activated pore structure and surface chemistry characteristics that assure the effective removal of trace organic contaminants from aqueous solution, and (2) to develop a procedure to predict the adsorption capacity of activated carbons from fundamental adsorbent and adsorbate properties.
To systematically evaluate pore structure and surface chemistry effects on the adsorption of organic micropollutants from aqueous solution, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three commercially available granular activated carbons (GACs) were studied to verify whether correlations developed for the ACF matrix are valid for adsorbents that are typically used for water treatment. BET surface area, pore size distribution, elemental composition, point of zero charge and infrared spectroscopy data were obtained to characterize the adsorbents. The results showed that the ACF matrix prepared in this study permits a fairly independent evaluation of surface chemistry and pore structure effects on organic contaminant adsorption from aqueous solution. Methyl tertiary-butyl ether (MTBE), a relatively hydrophilic adsorbate, and trichloroethene (TCE), a relatively hydrophobic adsorbate, served as adsorbate probes. To evaluate the effects of natural organic matter (NOM) on MTBE and TCE adsorption capacities, isotherm experiments were conducted in ultrapure water and Sacramento-San Joaquin Delta water.
With respect to surface chemistry, both single-solute isotherms and isotherms in the presence of NOM indicated that hydrophobic adsorbents more effectively removed TCE and MTBE from aqueous solution than hydrophilic adsorbents. Enhanced water adsorption on polar surface sites explained the poorer performance of the hydrophilic adsorbents. Based on the elemental composition of the low-ash carbons evaluated in this study, activated carbons should have oxygen and nitrogen contents that sum to no more than 2 to 3 mmol/g to assure sufficient hydrophobicity.
With respect to pore structure, both single-solute isotherms and isotherms in the presence of NOM indicated that adsorbents should exhibit a large pore volume in micropores with widths that are about 1.5 times larger than the kinetic diameter of the target adsorbate. Furthermore, micropollutant isotherm data obtained in the presence of NOM showed that an effective adsorbent should possess a micropore size distribution that extends to widths that are approximately twice the kinetic diameter of the target adsorbate to prevent pore blockage or restriction as a result of NOM adsorption.
A procedure based on the Polanyi Potential Theory (PPT) was developed to predict the adsorption capacities of activated carbons from fundamental adsorbent and adsorbate properties. It was assumed that trace organic compound adsorption from aqueous solution is primarily controlled by non-specific dispersive forces while water molecules interact with the oxygen-containing functional groups on carbon surfaces. A correlation between the coalescing factor for water adsorption and adsorbent oxygen content was developed. Based on this correlation, the PPT yielded reasonable estimates of aqueous phase adsorption capacities for both relatively polar and non-polar adsorbates on both relatively hydrophobic and hydrophilic activated carbons. With the developed procedure, the adsorption capacities of organic compounds that are partially miscible in water can be predicted from (1) N2 and CO2 adsorption isotherms of a given adsorbent, (2) the adsorbent oxygen content, and (3) the molar volume and parachor of the target adsorbate.
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Keywords
trace organic contaminants, pore structure, surface chemistry, adsorbent
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Degree
PhD
Discipline
Civil Engineering
