Modeling trichloroethylene adsorption by activated carbon preloaded with natural dissolved organic matter using a modified IAST approach.

A model was developed, using an approach based on the Ideal Adsorbed Solution Theory (IAST), to predict trichloroethylene (TCE) adsorption by granular activated carbon (GAC) preloaded with natural dissolved organic matter (DOM) isolated from three surface water sources. The IAST model was formulated for a bi-solute system in which TCE and DOM single-solute uptakes were described by the Langmuir-Freundlich and Freundlich isotherms, respectively. The effect of DOM molecular size and polarity (as measured by XAD 8 resin fractionation) on TCE uptake by preloaded GAC was assessed to identify a reactive fraction of natural water DOM for the purpose of modeling competitive adsorption. Consistent with previous work that identified low molecular weight species as the most reactive with regard to preloading effects (i.e., reducing target compound uptake), the low molecular weight components of the polar (hydrophilic) and nonpolar (hydrophobic) DOM fractions, isolated using ultrafiltration (1 kDa molecular weight cutoff membrane), exhibited significant competitive effects. Furthermore, the effects of these fractions on TCE uptake were similar; therefore, theywere considered together to represent a single "reactive fraction" of DOM. On the basis of this finding, isotherms for the <1 kDa low molecular weight DOM fraction of the whole water were measured, and molar concentrations were computed based on an average molecular weight determined using size-exclusion chromatography. The IAST model was modified to incorporate surface area reduction due to pore blockage by DOM and to reflectthe hypothesis thatTCE molecules can access adsorption sites which humic molecules cannot, thus preventing competition on these sites. The model was calibrated with data for TCE uptake by carbon preloaded with the <1 kDa low molecular weight DOM fraction and was verified by predicting TCE uptake by carbon preloaded with whole natural waters for both constant GAC dose (hence constant DOM loading) and variable GAC dose (hence variable DOM loading) TCE isotherms. Preloading by DOM reduced volume in GAC pores having widths smaller than 1.25 nm (likely accessible only to TCE) to a greater extent than total pore volume, suggesting preferential blockage of micropores. Such preferential pore blockage may explain, in part, why increased DOM loading decreases the fraction of the total surface area on which no competition between TCE and DOM occurs.

Effects of reverse osmosis isolation on reactivity of naturally occurring dissolved organic matter in physicochemical processes.

A field reverse osmosis system was used to isolate dissolved organic matter (DOM) from two lacustrine and two riverine surface water sources. The rejection of DOM was on the order of 99% and did not vary significantly with pressure. A simple mass balance model using a single measured value of rejection predicted the concentration within the closed-loop isolation system. The effect of operating pressure and solution flux on mass recovery of DOM was evaluated in laboratory and field trials. Under controlled laboratory conditions, >99% of a lacustrine DOM could be accounted for. A fraction of the isolated DOM was not recoverable using hydrodynamic cleaning; however, this fraction was recovered by using a pH 10 NaOH wash solution. The mass recovered in the NaOH solution increased from <1% to >6% with increasing transmembrane pressures from 414 kPa (60 psi) to1000 kPa (145 psi), respectively. This is consistent with fouling that results from an increase in solution flux, and a decrease in tangential crossflow velocity. Under field conditions, mass balances were generally >95% and mass recovery was >90% in all cases. The effects of temperature on solution flux were consistent with changes in fluid viscosity; effects of temperature on membrane diffusivity or morphological properties were small. RO isolation under low pressure conditions designed to maximize DOM recovery had little effect on DOM reactivity evaluated in terms of nanofiltration membrane fouling, XAD-8 resin adsorption, activated carbon adsorption, competition with trichloroethylene for adsorption sites on activated carbon, and molecular weight distribution measured using size exclusion chromatography.

Probing reactivity of dissolved organic matter for disinfection by-product formation using XAD-8 resin adsorption and ultrafiltration fractionation.

The disinfection by-product (DBP) reactivity (yield and speciation upon reaction with chlorine) of dissolved organic matter (DOM) isolated from two surface waters was investigated. The source waters, each having significantly different specific ultraviolet absorbance (SUVA254), molecular weight (MW) distribution and polarity, were fractionated using XAD-8 resin adsorption and ultrafiltration (UF), with good DOM mass balance closures (based on dissolved organic carbon). It was found that such fractionation preserved both the SUVA and the reactivity of the source waters, as demonstrated by statistically similar DBP formation and speciation from chlorinated source water and source waters reconstituted from XAD-8 or UF fractions. In addition, there was no evidence of synergistic effects among DOM components when reacting with chlorine. Consistent trends between DBP yields and MW were not found. Hydrophobic fractions of DOM (isolated by XAD-8) were the most reactive DOM components; however, hydrophilic components also showed appreciable DBP yields, contributing up to 50% of total DBP formation. In contrast, strong and unique correlations were observed between the SUVA of individual fractions and their trihalomethane (THM) and haloacetic acid (HAA9) yields, confirming that the aromaticity of DOM components is more directly related to reactivity than other physicochemical properties. The finding of a single correlation independent of the fractionation process employed is notable because XAD-8 adsorption and UF fractionate DOM by significantly different mechanisms. These results confirm that SUVA is a distributed parameter that reflects DOM heterogeneity. Therefore, the SUVA distribution within natural water represents an important property that can be used as a reliable predictor of DBP formation. Finally, bromine appears to be more effectively incorporated into low UV-absorbing (i.e., low SUVA), low MW and hydrophilic DOM fractions.