Equivalent Background Compound Research Articles

Competitive and Hindering Effects of Natural Organic Matter on the Adsorption of MTBE onto Activated Carbons and Zeolites

Equilibrium and kinetic adsorption of methyl tert-butyl ether (MTBE) onto three coal-based activated carbons, one coconut-based activated carbon, and two zeolites are elucidated in this study. Natural organic matter (NOM) and MTBE competed for the adsorption of activated carbons to different extents. The ideal adsorbed solution theory (IAST) combined with the equivalent background compound (EBC) model can adequately describe the NOM competition and predict the isotherms of MTBE onto the activated carbons. No competitive adsorption was observed for one of the zeolites, mordenite, due to the molecular effect. Besides, the aperture size, and the SiO2/Al2O3 ratio of the zeolite may also play an important role in the adsorption of MTBE from the aqueous phase. The surface diffusion model accurately simulated the transport of MTBE within the adsorbents employed in different water matrices. For all the activated carbons tested, the surface diffusivity of MTBE in natural water was nearly equal to that in deionized water, indicating that no apparently hindering effect occurs. A much slower adsorption kinetic of mordenite in natural water was observed since the opening apertures on mordenite may be appreciably hindered and blocked by NOM.

Simultaneous adsorption of MIB and NOM onto activated carbon: II. Competitive effects

The adsorption of an odour compound common in drinking water, 2-methylisoborneol (MIB), was studied on six activated carbons in the presence of six well-characterised natural organic matter (NOM) solutions. It was found that, although the carbons and the NOM solutions had a wide range of characteristics, the major competitive mechanism was the same in all cases. The low-molecular-weight NOM compounds were the most competitive, participating in direct competition with MIB for adsorption sites. Equivalent background compound calculations indicated a relatively low concentration of directly competing compounds in the NOM. Some evidence of pore blockage and/or restriction was also seen, with microporous carbons being the most affected by low-molecular-weight NOM and mesoporous carbons impacted by the higher-molecular-weight compounds.

Modeling equilibrium adsorption of 2-methylisoborneol and geosmin in natural waters

The impact of natural organic matter (NOM) on the adsorption of the taste and odor causing compounds 2-methylisoborneol (MIB) and geosmin by powdered activated carbon was investigated. Equilibrium experiments showed that adsorption of MIB and geosmin in natural waters was an order of magnitude lower than single-solute adsorption. It was found that when the equilibrium concentration was expressed as a percentage of the initial concentration, the removal of MIB and geosmin in natural waters was independent of initial concentration. This relationship was also mathematically derived from the ideal adsorbed solution theorem when the concentration of the target compound was assumed to be much lower than the concentration of the competing NOM. A single equivalent background compound (EBC) was used to simulate the competitive effects of the background NOM and describe the adsorption of MIB and geosmin. The same EBC was found to be applicable to both MIB and geosmin and, at different initial concentrations, for all four waters. Three of the waters had similar ratios of EBC initial concentration to total organic carbon concentration.

14C‐MIB adsorption on PAC in natural water

A new way of measuring very low concentrations of MIB allows calculation of the PAC dose needed to mitigate odors caused by MIB.Kinetic and equilibrium studies using five powdered activated carbons (PACs) and 14C‐labeled 2‐methylisoborneol (MIB) in Lake Michigan water produced guidelines for determining the dose of PAC needed to remove MIB. The equilibrium data, supported by predictions from the equivalent background compound competitive adsorption model, indicate that the percent MIB remaining at equilibrium is a function of the carbon dosage, independent of the initial MIB concentration. Kinetic experiments, analyzed in terms of the homogeneous surface diffusion model, demonstrated that the surface diffusion coefficient, Ds, is independent of both carbon dosage and initial MIB concentration. Together these equilibrium and kinetic studies show that for any given PAC dosage in natural water and for any fixed time of contact, the percent MIB remaining in an ideal reactor is independent of the initial MIB concentration. Thus the PAC dosage required to mitigate any MIB episode for any contact time of interest may be quickly determined from a Ct/C0 × 100 percent versus Cc plot. These studies have also exhibited a new application of 14C‐MIB allowing for the rapid collection of reproducible and accurate adsorption data.

An evaluation of multicomponent absorption equilibria models

An investigation into the modelling of competitive adsorption equilibria data using the ideal adsorbed solution theory (IAST), the modified simplified competitive adsorption models (MSCAM), and the equivalent background compound (EBC) method is presented. The models were evaluated with the only competitive isotherms available which quantified both natural organic matter (NOM) and synthetic organic chemical (SOC) adsorption. The data used a mixture of 1,1,2-trichloroethane (TCEA) and NOM from river water measured as dissolved organic carbon (DOC). The IAST method was applied by representing the NOM in the river water as pseudocomponents. It was observed that the laborious and time-consuming regressions of only the TCEA isotherm data yield different sets of parameters, many of which do not describe the NOM data well. Simultaneous regression of TCEA and NOM isotherm data, like TCEA isotherm data regression, is highly dependent on the initial guesses of the parameters. MSCAM simulated the NOM adsorption very well but TCEA less so. TCEA simulation with the EBC method provided good results, but unrealistic EBC characteristics led to poor predictions. Model predictions of the experiment showed that the MSCAM model described the data better than the IAST method, but not perfectly. DOC-based MSCAM simulation does not offer any significant improvements over molar-based simulation.

Prediction and Verification of Atrazine Adsorption by PAC

A procedure was developed to predict the removal of trace organic compounds from natural water by powdered activated carbon (PAC) adsorption systems, which function as a batch reactor or a continuous stirred tank reactor (CSTR). The procedure uses the equivalent background compound method coupled with the ideal absorbed solution theory to quantify the competition between trace organics and background organic matter in water, and uses the pseudo single-solute homogeneous surface diffusion model to describe the adsorption kinetics of the target compound under the influence of the background organic matter. The parameters required by the model as input data can be independently determined from adsorption isotherms and a set of batch kinetic test data. Good agreement between predicted aual performance was found for adsorption of atrazine from Central Illinois ground water at different initial concentrations and different carbon doses using a batch reactor and two CSTRs, one of which was a PAC/ultrafiltration system.

Effect of Initial Concentration of a SOC in Natural Water on Its Adsorption by Activated Carbon

One of the main factors affecting the equilibrium capacity of activated carbon for synthetic organic chemicals (SOCs) in natural water is the presence of background organic matter (BOM). However, the initial SOC concentration also plays an important role in determining the capacity of the carbon. The objective of this study was to develop a means of quantifying the effect of initial SOC concentration in different types of waters on its adsorption isotherm. The method used approximates the complex mixture of BOM with a single equivalent background compound (EBC). The initial concentration and single‐solute isotherm constants of the EBC were calculated based on the BOM's competitive effect on the adsorption of the SOC on activated carbon. The EBC method was then successfully used to predict the equilibrium capacity of powdered activated carbon for 2,4,6‐trichlorophenol in several natural and finished waters when present at different initial concentrations.