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GAC Isotherm
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| Figure 1: Typical GAC isotherm plot. |
Granular activated carbon (GAC) is commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment. Adsorption is both the physical and chemical process of accumulating a substance at the interface between liquid and solids phases. The compound that adsorbs at the interface is called the adsorbate and the solid on which adsorption occurs is the adsorbent. If a reaction is reversible, as is the case with many compounds removed by GAC in drinking water treatment, compounds continue to accumulate on the activated carbon until equilibrium is reached (i.e. when the rate of adsorption equals the rate of desorption), at which time no further accumulation occurs.
GAC adsorption isotherms are frequently used by carbon manufacturers to characterize the ability of a particular GAC to remove a specific contaminant. An important characteristic of GAC is the quantity of adsorbate that it can accumulate. The adsorption isotherm describes the equilibrium relationship between the adsorbate, adsorbent, and the equilibrium concentration of the adsorbate in water. GAC isotherms are typically shown graphically on log-log plots. On such plots, more adsorbable compounds have higher and flatter lines than less adsorbable compounds.
The two most common mathematical expressions used to relate the adsorption isotherm are the Freundlich equation and the Langmuir equation. The Freundlich equation is an empirical equation that accurately describes much adsorption data. The Freundlich equation has the following form:
qe = KCe1/n
and can be linearized as follows:
log qe = log K +1/n *log Ce
where: qe = equilibrium loading on the GAC (mg chemical/g GAC)
Ce = equilibrium concentration in the water (mg chemical/L)
K = adsorption capacity at unit concentration (mg/g)(L/mg)1/n
1/n = strength of adsorption (dimensionless)
Figure 1 shows the log-log Freundlich relationship. For constant Ce and 1/n values, the capacity qe increases with increasing K values. For constant Ce and K values, the adsorption bond is stronger with decreasing 1/n values. As 1/n values become very small, qe remains essentially constant with increasing values of Ce. This condition is called an irreversible isotherm. As 1/n values become very large, the adsorption strength weakens, and the qe value changes significantly with small changes in Ce.
The Langmuir equation has the following form:
qe = (qmaxbCe)/(1+bCe)
and can be linearized as follows:
1/qe = 1/(qmaxbCe + 1/qmax
where: qmax = ultimate adsorption capacity (mg chemical/g GAC)
b = relative energy of adsorption (L/mg)
An isotherm is typically determined by running several batch reactors, typically bottles, in parallel. The bottles have the same chemical concentration and water volume. Each bottle is dosed with a different amount of carbon and gently mixed until equilibrium between phases is reached. Ce is measured and the GAC loading (qe) in each bottle is calculated assuming conservation of mass throughout the experiment. A plot of qe and Ce is made and isotherm parameters are determined using linear regression analysis.
Competitive adsorption can also occur in drinking water treatment when there are several adsorbable compounds present in the water. The amount of GAC required to remove a certain compound from a mixture is higher than if there are no other adsorbable compounds because some of the adsorbent surface is taken by competing substances. Total organic carbon (TOC) serves as a gross measure of competing organic compounds. Several models can be used to develop GAC isotherms for competitive adsorption.
There are several factors that must be considered and controlled during GAC isotherm development. If the adsorbing chemical is volatile, the bottles should be completely filled and head space free. pH control is important for compounds whose isotherms are a function of pH (i.e. weak organic acids or bases) since pH changes can leave the compound in ionized form, which will have a higher affinity for water and decrease GAC capacity. Salts in solution should be considered since they can impact the isotherm for ionic substances or for substances that compete with organic ions for adsorption sites. Biodegradation must be minimized if the adsorbing compound is biodegradable. The GAC isotherm should be determined within the concentration range of each particular application, since isotherm parameters sometimes vary with changing concentrations. Adsorption capacity increases with temperature, consequently, these tests are preformed isothermally.
Adsorption capacity is also affected by the type of carbon that was activated. Generally, coal-based materials have higher capacities than do wood-based or other materials.
The size of the compound significantly impacts the rate of adsorption. It takes longer to remove large molecular weight compounds than low molecular weight compounds. The GAC particle size (mesh) also affects the adsorption rate because it determines the time required to transport the compound within the pore to available adsorption sites. With GAC, equilibrium times can take several weeks to months. GAC is typically pulverized for isotherm measurement to shorten the equilibrium time to a range of days to a week. Pulverizing the GAC does not impact the total surface available for adsorption. Shortening the equilibration time also can be important with biodegradable chemicals since preventing biodegradation over time period is not always be feasible.
Since GAC isotherms provide information on the adsorption capacity, they can be used (1) to compare different GAC manufacturers, (2) as a quality control measure of purchased GAC, and (3) to predict performance of a GAC system. Isotherms can be used to obtain a rough estimate of the carbon usage rage (CUR) and bed life, which can be useful in determining the applicability of GAC. Isotherm parameters can also be used as input parameters for mathematical models to predict performance of a GAC adsorption process.