CHAPTER 13 ADSORPTION OF ORGANIC COMPOUNDS Vernon L. Snoeyink, Ph.D. Ivan Racheff Professor of Environmental Engineering Department of Civil and Environmental Engineering University of Illinois at Urbana-Champaign Urbana, Illinois R. Scott Summers, Ph.D. Professor of Environmental Engineering Civil, Environmental, and Architectural Engineering University of Colorado Boulder, Colorado Adsorption of a substance involves its accumulation at the interface between two phases, such as a liquid and a solid or a gas and a solid. The molecule that accumu- lates, or adsorbs, at the interface is called an adsorbate, and the solid on which adsorption occurs is the adsorbent. Adsorbents of interest in water treatment include activated carbon; ion exchange resins; adsorbent resins; metal oxides, hydroxides, and carbonates; activated alumina; clays; and other solids that are sus- pended in or in contact with water. Adsorption plays an important role in the improvement of water quality. Acti- vated carbon, for example, can be used to adsorb specific organic molecules that cause taste and odor, mutagenicity, and toxicity, as well as natural organic matter (NOM) that causes color and that can react with chlorine to form disinfection by- products (DBPs). NOM is a complex mixture of compounds such as fulvic and humic acids, hydrophilic acids, and carbohydrates.The aluminum hydroxide and fer- ric hydroxide solids that form during coagulation will also adsorb NOM.Adsorption of NOM on anion exchange resins may reduce their capacity for anions (see Chap- ter 9),but ion exchange resins and adsorbent resins are available that can be used for efficient removal of selected organic compounds. Calcium carbonate and magne- sium hydroxide solids formed in the lime softening process have some adsorption capacity, and pesticides adsorbed on clay particles can be removed by coagulation and filtration (Chapters 6 and 8). The removal of organic compounds by adsorption on activated carbon is very important in water purification and therefore is the primary focus of this chapter.A 13.1 study conducted by two committees of the AWWA showed that approximately 25 percent of 645 United States utilities, including the 500 largest, used powdered acti- vated carbon (PAC) in 1977 (American Water Works Association, 1977). In 1986, 29 percent of the 600 largest utilities reported using PAC (American Water Works Association, 1986), predominantly for odor control. More attention is being given now to granular activated carbon (GAC) as an alternative to PAC. GAC is used in columns or beds that permit higher adsorptive capacities to be achieved and easier process control than is possible with PAC.The higher cost for GAC can often be off- set by better efficiency, especially when organic matter must be removed on a con- tinuous basis. GAC should be seriously considered for water supplies when odorous compounds or synthetic organic chemicals of health concern are frequently present, when a barrier is needed to prevent organic compounds from spills from entering finished water, or in some situations that require DBP control. GAC has excellent adsorption capacity for many undesirable substances and it can be removed from the columns for reactivation when necessary. The number of drinking water plants using GAC,principally for odor control, increased from 65 in 1977 (American Water Works Association, 1977) to 135 in 1986 (Fisher, 1986); in 1996, there were approxi- mately 300 plants treating surface water and several hundred more treating contam- inated groundwater. The promulgated as well as proposed DBP regulations will drive many utilities to consider GAC for removal of organic compounds in the next 10 years. GAC is also used as a support medium for bacteria in processes to biologi- cally stabilize drinking water before distribution. This chapter also covers the use of ion exchange and adsorbent resins for the removal of organic compounds. Removal of inorganic ions by ion exchange resins and activated alumina is discussed in Chapter 9. ADSORPTION THEORY Adsorption Equilibrium Adsorption of molecules can be represented as a chemical reaction: A + B ⇔ A⋅B where A is the adsorbate, B is the adsorbent, and A⋅B is the adsorbed compound. Adsorbates are held on the surface by various types of chemical forces such as hydrogen bonds, dipole-dipole interactions, and van der Waals forces. If the reac- tion is reversible, as it is for many compounds adsorbed to activated carbon, molecules continue to accumulate on the surface until the rate of the forward reac- tion (adsorption) equals the rate of the reverse reaction (desorption). When this condition exists, equilibrium has been reached and no further accumulation will occur. Isotherm Equations. One of the most important characteristics of an adsorbent is the quantity of adsorbate it can accumulate. The constant-temperature equilibrium relationship between the quantity of adsorbate per unit of adsorbent q e and its equi- librium solution concentration C e is called the adsorption isotherm. Several equa- tions or models are available that describe this function (Sontheimer, Crittenden, and Summers, 1988), but only the more common equations for single-solute adsorp- tion, the Freundlich and the Langmuir equations, are presented here. 13.2 CHAPTER THIRTEEN The Freundlich equation is an empirical equation that is very useful because it accurately describes much adsorption data.This equation has the form q e = KC e 1/n (13.1) and can be linearized as follows: log q e = log K + log C e (13.2) The parameters q e (with units of mass adsorbate/mass adsorbent, or mole adsor- bate/mass adsorbent) and C e (with units of mass/volume, or moles/volume) are the equilibrium surface and solution concentrations, respectively. The terms K and 1/n are constants for a given system; 1/n is unitless, and the units of K are determined by the units of q e and C e . Although the Freundlich equation was developed to empiri- cally fit adsorption data, a theory of adsorption that leads to the Freundlich equation was later developed by Halsey and Taylor (1947). The parameter K in the Freundlich equation is related primarily to the capacity of the adsorbent for the adsorbate, and 1/n is a function of the strength of adsorp- tion. For fixed values of C e and 1/n, the larger the value of K, the larger the capacity q e . For fixed values of K and C e , the smaller the value of 1/n, the stronger is the adsorption bond.As 1/n becomes very small, the capacity tends to be independent of C e and the isotherm plot approaches the horizontal level; the value of q e then is essentially constant, and the isotherm is termed irreversible. If the value of 1/n is large, the adsorption bond is weak, and the value of q e changes markedly with small changes in C e . The Freundlich equation cannot apply to all values of C e , however. As C e increases, for example, q e increases (in accordance with Equation 13.1) only until the adsorbent approaches saturation.At saturation, q e is a constant, independent of fur- ther increases in C e , and the Freundlich equation no longer applies. Also, no assur- ance exists that adsorption data will conform to the Freundlich equation over all concentrations less than saturation, so care must be exercised in extending the equa- tion to concentration ranges that have not been tested. The Langmuir equation, q e = (13.3) where b and q max are constants and q e and C e are as defined earlier, has a firm theo- retical basis (Langmuir, 1918).The constant q max corresponds to the surface concen- tration at monolayer coverage and represents the maximum value of q e that can be achieved as C e is increased.The constant b is related to the energy of adsorption and increases as the strength of the adsorption bond increases. The Langmuir equation often does not describe adsorption data as accurately as the Freundlich equation. The experimentally determined values of q max and b often are not constant over the concentration range of interest, possibly because of the heterogeneous nature of the adsorbent surface (a homogeneous surface was assumed in the model develop- ment), lateral interactions between adsorbed molecules (all interaction was neglected in the model development), and other factors. Factors Affecting Adsorption Equilibria. Important adsorbent characteristics that affect isotherms include surface area, pore size distribution, and surface chem- q max bC e ᎏ 1 + bC e 1 ᎏ n ADSORPTION OF ORGANIC COMPOUNDS 13.3 13.4 CHAPTER THIRTEEN FIGURE 13.1 Pore size distributions for different acti- vated carbons. (Source: Lee, Snoeyink, and Crittenden, 1981.) istry.The maximum amount of adsorption is proportional to the amount of surface area within pores that is accessible to the adsorbate. Surface areas range from a few hundred to more than 1500 m 2 /g, but not all of the area is accessible to aqueous adsorbates.The range of pore size distributions in an arbitrary selection of GACs is shown in Figure 13.1.A relatively large volume of micropores (pores less than 2 nm diameter d) (Sontheimer, Crittenden,and Summers, 1988) generally corresponds to a large surface area and a large adsorption capacity for small molecules, whereas a large volume of mesopores (2 < d < 50 nm) and macropores (d > 50 nm) is usually directly correlated to capacity for large molecules.The fulvic acid isotherms in Fig- ure 13.2 are for the same activated carbons whose pore size distributions are shown in Figure 13.1. Note that the activated carbons that have a relatively small volume of macropores also have a relatively low capacity for the large fulvic acid molecule. Lee et al. (1981) showed that the quantity of humic substances of a given size that was adsorbed was correlated with pore volume within pores of a given size.The rel- ative positions of the isotherms for the activated carbons in Figure 13.1 might be entirely different than those in Figure 13.2 if the adsorbate were a small molecule, such as a phenol, which can enter pores much smaller than those accessible to ful- vic acid. Summers and Roberts (1988b) showed that if the amount adsorbed was normalized for the available surface area, the differences in adsorption capacity of different carbons for a humic acid could be attributed to the surface chemistry of the carbon. The surface chemistry of activated carbon and adsorbate properties also can affect adsorption (Coughlin and Ezra, 1968; Gasser and Kipling, 1959; Kipling and Shooter, 1966;Snoeyink and Weber,1972;Snoeyink et al., 1974). Several researchers (Coughlin and Ezra, 1968; Gasser and Kipling, 1959; Kipling and Shooter, 1966) demonstrated that extensive oxidation of carbon surfaces led to large decreases in the amounts of phenol, nitrobenzene, benzene, and benzenesulfonate that could be adsorbed. Oxidation of the activated carbon surface with aqueous chlorine was also found to increase the number of oxygen surface functional groups and correspond- ingly to decrease the adsorption capacity for phenol (Snoeyink et al., 1974). Thus, oxygenating a carbon surface decreases its affinity for simple aromatic compounds. The tendency of a molecule to adsorb is a function of its affinity for water as com- pared to its affinity for the adsorbent. Adsorption onto GAC from water, for exam- ple, generally increases as the adsorbate’s solubility decreases (Weber, 1972). As a molecule becomes larger through the addition of hydrophobic groups such as ᎏ CH 2 ᎏ , its solubility decreases and its extent of adsorption increases as long as the molecule can gain entrance to the pores. When an increase in size causes the molecule to be excluded from some pores, however, adsorption capacity may decrease as solubility decreases. As molecular size increases, the rate of diffusion within the activated carbon particle decreases, especially as molecular size approaches the particle’s pore diameter. The affinity of weak organic acids or bases for activated carbon is an important function of pH. When pH is in a range at which the molecule is in the neutral form, adsorption capacity is relatively high. When pH is in a range at which the species is ionized, however, the affinity for water increases and activated carbon capacity accordingly decreases. Phenol that has been adsorbed on activated carbon at pH below 8, where phenol is neutral, can be desorbed if the pH is increased to 10 or above, where the molecule is anionic (Fox, Keller, and Pinamont, 1973). If adsorp- tion occurs on resins by means of the ion exchange mechanism, the specific affinity of the ionic adsorbate for charged functional groups may also cause good removal. The inorganic composition of water also can have an important effect on the extent of NOM adsorption, as shown in Figure 13.3 for fulvic acids (Randtke and Jepsen, 1982). After 70 days, a small GAC column was nearly saturated with fulvic acid. Addition of CaCl 2 at this point resulted in a large increase in adsorbability of fulvic acid, as reflected in the reduced column effluent concentration. After 140 days, elimination of the CaCl 2 resulted in desorption of much of the fulvic acid. Cal- cium ion apparently associates (complexes) with the fulvic acid anion to make ful- vic acid more adsorbable (Randtke and Jepsen, 1982; Weber, Voice, and Jodellah, 1983). Presumably many other divalent ions can act in similar fashion, but calcium is of special interest because of its relatively high concentration in many natural waters. Similar effects are expected for other anionic adsorbates, but salts are not expected to have much effect on the adsorption of neutral adsorbates (Snoeyink, Weber, and Mark, 1969). ADSORPTION OF ORGANIC COMPOUNDS 13.5 FIGURE 13.2 Adsorption isotherms for peat fulvic acid. (Source: Lee, Snoeyink, and Crittenden, 1981.) Inorganic substances such as iron, manganese, and calcium salts or precipitates may interfere with adsorption if they deposit on the adsorbent. Pretreatment to remove these substances, or to eliminate the supersaturation, may be necessary if they are present in large amounts. Adsorption isotherms may be determined for heterogeneous mixtures of com- pounds using group parameters such as total organic carbon (TOC), dissolved organic carbon (DOC), chemical oxygen demand (COD),dissolved organic halogen (DOX),UV absorbance, and fluorescence as a measure of the total concentration of substances present. Because the compounds within a mixture can vary widely in their affinity for an adsorbent, the shape of the isotherm will depend on the relative amounts of compounds in the mixture. For example,isotherms with the shape shown in Figure 13.4 are expected if some of the compounds are nonadsorbable and some are more strongly adsorbable than the rest (Randtke and Snoeyink, 1983). The strongly adsorbable compounds can be removed with small doses of adsorbent and yield large values of q e . In contrast, the weakly adsorbable compounds can only be removed with large doses of adsorbent that yield relatively low values of q e .The nonadsorbable compounds produce a vertical isotherm at low C e values. In contrast to single-solute isotherms, the isotherm for a heterogeneous mixture of compounds will be a function of initial concentration and the fraction of the mixture that is adsorbed. The relative adsorbabilities of compounds within a mixture have an important effect on the performance of adsorption columns. The nonadsorbable fraction cannot be removed regardless of the column design, whereas the strongly adsorbable fraction may cause the effluent concentration to slowly approach the influent concentration. Competitive Adsorption in Bisolute Systems. Competitive adsorption is impor- tant in drinking water treatment because most compounds to be adsorbed exist in solution with other adsorbable compounds. The quantity of activated carbon or other adsorbent required to remove a certain amount of a compound of interest 13.6 CHAPTER THIRTEEN FIGURE 13.3 Effects of calcium chloride addition and withdrawal on column per- formance (pH = 8.3; TOC = 5.37 mg/L, peat fulvic acid buffer = 1.0 mM NaHCO 3 ). (Source: Randtke and Jepsen, 1982.) from a mixture of adsorbable compounds is greater than if adsorption occurs with- out competition, because part of the adsorbent’s surface is utilized by the competing substances. The extent of competition on activated carbon depends upon the strength of adsorption of the competing molecules, the concentrations of these molecules, and the type of activated carbon. Some examples illustrate the possible mag- nitude of the competitive effect. Jain and Snoeyink (1973) showed that as p-bromophenol (PBP) equilibrium con- centration increased from 10 −4 to 10 −3 M (17 to 173 mg/L), the amount of p-nitrophenol (PNP) adsorbed at an equilibrium concentration of 3.5 × 10 −5 M (∼ 5 mg/L) decreased by about 30 percent. Displacement of previously adsorbed compounds by competition can result in a column effluent concentration of a com- pound that is greater than the influent concentration, as shown in Figure 13.5. A dimethylphenol (DMP) concentration about 50 percent greater than the influent resulted when dichlorophenol (DCP) was introduced to the influent of a column ADSORPTION OF ORGANIC COMPOUNDS 13.7 FIGURE 13.4 Nonlinear isotherm for a het- erogeneous mixture of organic compounds. (Source: Randtke and Snoeyink, 1983.) FIGURE 13.5 Breakthrough curves for sequential feed of DMP and DCP to a GAC adsorber (C 0 = 0.990 mmol/L, C 02 = 1.02 mmol/L, EBCT = 25.4 s). (Source: W. E. Thacker, J. C. Crittenden, and V. L. Snoeyink, 1984. “Modeling of Adsorber Performance: Variable Influent Concentration and Comparison of Adsorbents,” Journal Water Pollution Control Federation 56: 243. Copyright © Water Environment Federation, reprinted with permission.) saturated with DMP (Thacker, Crittenden,and Snoeyink, 1984).Similar occurrences have been observed in full-scale GAC systems. Effluent concentrations in excess of influent concentrations can be prevented through careful operation. Crittenden et al. (1980) showed that the magnitude of the displacement decreased when the value of C eff /C inf was lowered at the time the second compound was introduced. Thus, a reasonable strategy to prevent the occurrence of an undesirable compound at a con- centration greater than the influent is (1) to monitor the column for that compound and (2) to replace the activated carbon before complete saturation at the influent concentration occurs (i.e., before C eff = C inf ). A number of isotherm models have been used to describe competitive adsorp- tion.A common model for describing adsorption equilibrium in multiadsorbate sys- tems is the Langmuir model for competitive adsorption, which was first developed by Butler and Ockrent (1930) and which is presented in the fourth edition of this book (Snoeyink, 1990). This model is based on the same assumptions as the Lang- muir model for single adsorbates. Jain and Snoeyink (1973) modified this model to account for a fraction of the adsorption taking place without competition. This can happen if the adsorbates have different sizes and only the smaller adsorbate can enter the smaller pores (Pelekani and Snoeyink, 1999), or if some of the surface functional groups adsorb one compound but not the other.Other models that can be used to describe and predict competitive effects are the Freundlich-type isotherm of Sheindorf, Rebhun, and Sheintuck (1981) and the ideal adsorbed solution theory of Radke and Prausnitz (1972) described in the next section. The latter has proven to be applicable to a large number of situations. Competitive Adsorption in Natural Waters. Adsorption of organic compounds at trace concentrations from natural waters is an important problem in water purifica- tion. Essentially all synthetic organic chemicals that must be removed in water treat- ment by adsorption must compete with natural or background organic matter for adsorption sites. The heterogeneous mixture of compounds in natural waters adsorbs on activated carbon and reduces the number of sites available for the trace compounds, either by direct competition for adsorption sites or by pore blockage (Pelekani and Snoeyink, 1999). The amount of competition and the capacity for the trace compound depend on the nature of the background organic matter and its con- centration, as well as the characteristics of the activated carbon. Also important is the concentration of the trace compound, because this concentration affects how much of this compound can adsorb on the carbon. For example, Figure 13.6 shows that the adsorption capacity of 2-methylisoborneol (MIB), an important earthy/ 13.8 CHAPTER THIRTEEN 100 10 1 1 10 100 C e (ng/L) q e (ng/mg) K = 9.56 (ng/mg)(L/ng) I/n K = 9.56 (ng/mg)(L/ng) I/n l/n = 0.492 1,000 10,000 C 0 = 1245 ng/L C 0 = 150 ng/L Single-Solute Isotherm FIGURE 13.6 Effect of initial concentration on MIB capacity in Lake Michigan water. (Source: Gillogly et al., 1998b.) musty odor compound, is lower in natural water than in distilled water, and that this capacity is further reduced as initial concentration decreases (Gillogly et al., 1998b). It is important to have a procedure to predict capacity as a function of initial con- centration, because the capacity of activated carbon depends in such an important way on initial concentration and because the concentrations of trace organic com- pounds vary widely in natural waters. The ideal adsorbed solution theory (IAST) can be used for this purpose. The following two equations, based on the IAST as developed by Radke and Prausnitz (1972) and modified by Crittenden et al. (1985) to include the Freundlich equilibrium expression, describe equilibrium in a two- solute system, C 1,0 − q 1 C c − n 1 = 0 (13.4) C 2,0 − q 2 C c − n 2 = 0 (13.5) where q 1 and q 2 = equilibrium solid phase concentrations of compounds 1 and 2 q 1 = (C 1,0 − C 1,e )/C c and q 2 = (C 2,0 − C 2,e )/C c C 1,0 and C 2,0 = initial liquid phase concentrations of compounds 1 and 2 C 1,e and C 2,e = equilibrium concentrations of compounds 1 and 2 K 1 and K 2 = single-solute Freundlich parameters for compounds 1 and 2 1/n 1 and 1/n 2 = single-solute Freundlich exponents for compounds 1 and 2 C c = carbon dose These equations show the relationship between the initial concentration of each adsorbate, the amount of adsorbed compound per unit weight of carbon, and the carbon dose. The Freundlich parameters are derived from single-solute tests in organic-free water. In natural waters, the organic matter present is a complex mixture of many dif- ferent compounds; representing each of these compounds, even if they could be identified, would be computationally prohibitive. Several researchers have modeled NOM adsorption by defining several fictive components that represent groups of compounds with similar adsorption characteristics, as expressed by Freundlich K and n values (Sontheimer, Crittenden, and Summers, 1988). Extending Equations 13.4 and 13.5 to N components yields C i,0 − C c q i − ΄ ΅ n i = 0 (13.6) where N = number of components in the solution C i,0 = initial liquid-phase concentration of compound i C c = carbon dose q i = equilibrium solid-phase concentration of compound i n i and K i = single-solute Freundlich parameters for compound i These equations can be solved simultaneously to determine the concentrations for each component assumed to be in solution. Crittenden et al. (1985) used this fictive component approach to describe the adsorption of a target compound in the presence of NOM. With a single-solute isotherm of the target compound and experimental results from isotherms measured Α N i = 1 n j q j ᎏ n i K i q i ᎏ Α N j = 1 q j n 1 q 1 + n 2 q 2 ᎏᎏ n 2 K 2 q 2 ᎏ q 1 + q 2 n 1 q 1 + n 2 q 2 ᎏᎏ n 1 K 1 q 1 ᎏ q 1 + q 2 ADSORPTION OF ORGANIC COMPOUNDS 13.9 13.10 CHAPTER THIRTEEN FIGURE 13.7 EBC model results for atrazine isotherms in Illinois ground- water. (Source: Reprinted with permission from D. R. U. Knappe et al. 1998. “Predicting the capacity of powdered activated carbon for trace organic com- pounds in natural waters.” Environmental Science & Technology, 32:1694–1698. Copyright 1998 American Chemical Society.) using the natural water, parameters for each of the fictive components were found through a best-fit search procedure. These results were then applied to describe the adsorption of other compounds in that water. The IAST was applied to the problem of trace organic adsorption in natural waters by Najm, Snoeyink, and Richard (1991) using a procedure that was subse- quently modified by Qi et al. (1994) and Knappe et al. (1998). These researchers assumed that the background organic matter that competed with the trace com- pound could be represented as a single compound, called the equivalent background compound (EBC). This approach involved the determination of the single-solute isotherm for the trace compound, and an isotherm in natural water for the trace compound at two different initial concentrations. A search routine was used to find the Freundlich parameters K and 1/n and the initial concentration C 0 for the EBC that gave the observed amount of competition. For example, Figure 13.7 shows isotherms determined for atrazine in organic-free water and in Illinois groundwater at initial concentrations of 176 and 36 µg/L (Knappe et al., 1998). These data were used to determine the following EBC characteristics: K EBC > 1.0 × 10 6 (µmole/g)(L/µmole) 1/n ,1/n EBC = 0.648, C 0,EBC = 0.870 µmole/L The K value for the EBC was arbitrary above 1.0 × 10 6 (µmole/g)(L/µmole) 1/n . These EBC parameters are specific for the type of carbon, the type and concentra- tion of background organic matter, and the type of synthetic organic chemical (SOC). They can be used in Equations 13.4 and 13.5 together with the initial con- centration of the trace compound and its single-solute Freundlich parameters to cal- culate the surface coverage of trace compound as a function of carbon dose C e . Given the surface coverage q, the initial concentration C 0 , and the carbon dose C c , the equilibrium concentration of the trace compound can be calculated from the [...]... 13.11 Adsorption column breakthrough curve mass of GAC in column mass CUR ᎏ = ᎏᎏᎏᎏ volume volume treated to breakthrough VB (13.9) Breakthrough curves are strongly affected by the presence of nonadsorbable compounds, the biodegradation of compounds in a biologically active column, slow adsorption of a fraction of the molecules present, and the critical depth of the column relative to the length of the... volume of interparticle voids ADSORPTION OF ORGANIC COMPOUNDS 13.19 enter pores that are unavailable to larger adsorbates As a result, all of the BET surface area may not be available for adsorbates in drinking water Tabulations of single-solute isotherm constants are very useful when only rough estimates of adsorption capacity are needed to determine whether a more intensive analysis of the adsorption. .. THMs and total organic halide (TOX), and assimilable organic carbon (AOC), a measure of the biodegradable fraction of NOM, by biofiltration of ozonated and settled Ohio River water All filters had a total depth ADSORPTION OF ORGANIC COMPOUNDS 13.29 TABLE 13.2 Process Parameters for Activated Carbon Following Ozone Parameter Value Ozone dosage Biological degradation Oxygen demand for DOC oxidation EBCT... Within the MTZ, the degree of saturation with adsorbate varies from 100 percent (q = [qe]0) to zero The length of the MTZ, LMTZ, depends upon the rate of adsorption and the solution flow rate Anything that causes a higher rate of adsorption, such as a smaller carbon particle size, higher temperature, a larger diffusion coefficient of adsorbate, and/or greater strength of adsorption of adsorbate (i.e., a... concentration increased as (1) the diffusion coefficient of the ADSORPTION OF ORGANIC COMPOUNDS 13.13 adsorbate increased, (2) the amount of compound adsorbed increased, (3) the strength of adsorption decreased (e.g., as the Langmuir b value decreased or the Freundlich l/n value increased), and (4) the activated carbon particle size decreased Volatile organic compounds are especially susceptible to displacement... carbonization, or pyrolysis, is usually performed in the absence of air at temperatures less than 700°C, while activation is carried out with oxidizing gases such as ADSORPTION OF ORGANIC COMPOUNDS 13.17 FIGURE 13.12 Effect of biodegradation and the presence of nonadsorbable compounds on the shapes of breakthrough curves steam and CO2 at temperatures of 800 to 900°C Chemical activation combines carbonization... the desorption diffusivity is lower than that during adsorption Adsorption Kinetics Transport Mechanisms Removal of organic compounds by physical adsorption on porous adsorbents involves a number of steps, each of which can affect the rate of removal: 1 Bulk solution transport Adsorbates must be transported from bulk solution to the boundary layer of water surrounding the adsorbent particle The transport... important effects on adsorption that have been discussed in earlier sections Additional factors that must be considered in the design of full-scale systems are presented here ADSORPTION OF ORGANIC COMPOUNDS 13.25 GAC Particle Size The effect of particle size on the rate of approach to equilibrium in isotherm determination was discussed previously It has a similar effect on the rate of adsorption in columns... (1987) found a small amount of biological oxidation if the water was not preozonated, but application of 1.1 mg O3 /mg DOC resulted in removal of 35 to 40 percent of the influent DOC by biological oxidation Biodegradable compounds may be removed by microbes, without prior 13.28 CHAPTER THIRTEEN FIGURE 13.15 DOC removal by adsorption and biodegradation during GAC filtration of an ozonated humic acid solution... solution (Source: Sontheimer and Hubele, 1987.) adsorption to the GAC, if a biofilm capable of degrading such compounds is developed before they are applied Adsorbable biodegradable compounds may be adsorbed first if the biofilm is not developed when the compounds enter the column, and then desorbed and degraded as the biofilm develops The use of GAC in biofilters has recently been reviewed by Servais . to the influent of a column ADSORPTION OF ORGANIC COMPOUNDS 13.7 FIGURE 13.4 Nonlinear isotherm for a het- erogeneous mixture of organic compounds. (Source:. distribution of lengths, however. ADSORPTION OF ORGANIC COMPOUNDS 13.17 FIGURE 13.12 Effect of biodegradation and the presence of nonadsorbable compounds on