| Abstract
| - A model was developed, using an approach based on theIdeal Adsorbed Solution Theory (IAST), to predicttrichloroethylene (TCE) adsorption by granular activatedcarbon (GAC) preloaded with natural dissolved organic matter(DOM) isolated from three surface water sources. TheIAST model was formulated for a bi-solute system in whichTCE and DOM single-solute uptakes were described bythe Langmuir−Freundlich and Freundlich isotherms,respectively. The effect of DOM molecular size and polarity(as measured by XAD 8 resin fractionation) on TCEuptake by preloaded GAC was assessed to identify areactive fraction of natural water DOM for the purpose ofmodeling competitive adsorption. Consistent with previouswork that identified low molecular weight species as themost reactive with regard to preloading effects (i.e., reducingtarget compound uptake), the low molecular weightcomponents of the polar (hydrophilic) and nonpolar(hydrophobic) DOM fractions, isolated using ultrafiltration(1 kDa molecular weight cutoff membrane), exhibitedsignificant competitive effects. Furthermore, the effects ofthese fractions on TCE uptake were similar; therefore,they were considered together to represent a single “reactivefraction” of DOM. On the basis of this finding, isothermsfor the <1 kDa low molecular weight DOM fraction of thewhole water were measured, and molar concentrationswere computed based on an average molecular weightdetermined using size-exclusion chromatography. The IASTmodel was modified to incorporate surface area reductiondue to pore blockage by DOM and to reflect the hypothesisthat TCE molecules can access adsorption sites which humicmolecules cannot, thus preventing competition on thesesites. The model was calibrated with data for TCE uptakeby carbon preloaded with the <1 kDa low molecularweight DOM fraction and was verified by predicting TCEuptake by carbon preloaded with whole natural waters forboth constant GAC dose (hence constant DOM loading)and variable GAC dose (hence variable DOM loading) TCEisotherms. Preloading by DOM reduced volume in GACpores having widths smaller than 1.25 nm (likely accessibleonly to TCE) to a greater extent than total pore volume,suggesting preferential blockage of micropores. Suchpreferential pore blockage may explain, in part, why increasedDOM loading decreases the fraction of the total surfacearea on which no competition between TCE and DOM occurs.
|
