Carbon, adsorption

Adsorption occurs when an organic molecule is brought to the activated carbon surface and held there by physical and/or chemical forces. The quantity of a compound 

From an isotherm test it can be determined whether a particular organic material can be removed effectively. It will also show the approximate capacity of the carbon for the application and provide a rough estimate of the carbon dosage required. Isotherm tests also afford a convenient means of studying the effects of pH and temperature on adsorption. Isotherms put a large amount of data into concise form for ready evaluation and interpretation. Isotherms obtained under identical conditions using the same contaminated groundwater for two or more carbons can be quickly and conveniently compared to determine the relative merits of the carbons. 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

FIGURE 8.2 Schematic showing fixed-bed and moving-bed adsorption system. (After Knox et al., 1986.) 

In a typical countercurrent moving-bed carbon column employing upflow of the water, two or more columns are usually provided and are operated in series. The influent contaminated groundwater enters the bottom of the first column by means of a manifold system that uniformly distributes the flow across the bottom. The groundwater flows upward through the column. The unit hydraulic flow rate is usually 2 to 10 gpm/ft2. The effluent is collected by a screen and manifold system at the top of the column and flows to the bottom manifold of the second column. The carbon flow is not continuous, but instead is pulsed. 

Adsorption is a process by which organics are retained on the surface of granulated solids. The solid adsorbent particles are highly porous and have very large surface-to-volume ratios. Gas molecules penetrate the pores of the adsorbent and contact the large surface area available for adsorption. Activated carbon is the most common adsorbent for organic removals. 

The amount of VOC retained on the carbon may be represented by adsorption isotherm, which relate the amount of VOC adsorbed to the equilibrium pressure (or VOC concentration) at a constant temperature. The adsorptive capacity of the carbon (expressed as VOClb/Clb) depends not only on properties on the carbon, but also on the properties of the organic. Generally, the adsorptive capacity increases with  

One important consideration of this system is the operating temperature of the process gas stream. Operating temperature must be less than 100°F. This is because the adsorption capacity decreases with the increase in temperature. The efficiency of carbon adsorption depends on both the concentration of VOC in the gas stream and its composition. Generally, efficiencies of over 95% can be achieved when the organic concentrations are greater than 1,000 ppmv. 

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Chlorine can be removed by either activated carbon adsorption or by reaction with olefins such as ethylene over-activated carbon at temperatures of 30—200°C (44). Addition of Hquid high boiling paraffins can reduce the chlorine content in the HCl gas to less than 0.01% (45). 

Activated carbon adsorption generally uneconomical for removal of >1000 ppm contaminant from large stream unless bed regenerated steaming often easiest regeneration method but creates new wastewater problem usually 3—5 kg steam requited per kg of carbon for regeneration. 

The air stream exiting a stripper may requite some type of emissions control, depending on local and regulatory requirements. Carbon adsorption is often used catalytic oxidation is another option. 

Carbon Adsorption. Carbon adsorption is a well estabflshed and widely used technology for the removal of organics from wastewaters and gaseous streams. Carbon adsorption is a proven technology for potable water treatment and capable of reducing organic concentrations to very low or nondetectable levels. 

In carbon adsorption, contaminants are physically attracted or adsorbed on the surface of the carbon. Adsorption capacities are high for carbon because its porous nature provides a large surface area relative to its volume. Activated carbon is prepared from lignite, bituminous coal, coke, wood, or other organic materials such as coconut shells. 

Common examples of compounds that are amenable to carbon adsorption are aromatics (benzene, toluene) and chlorinated organics (trichloroethylene, trichloroethane [71-55-6, 75 -(9(9-j5y, tetrachloroethylene, polychlorinated biphenyls (PCBs), DDT /T(9-77-77, pentachlorophenol [87-86-5J. Compounds that are not adsorbed effectively by carbon include ethanol [64-17-5], diethylene glycol [111-46-6], and numerous amines (butylamine [109-73-9, 13952-84-6, 75-64-9], triethanolamine [102-71-6], cyclohexylamine [108-91-8], hexamethylenediamine [108-91-8] (1). Wastewater concentrations that are suitable for carbon adsorption are generally less than 5000 mg/L. 

Most carbon adsorption units use granular activated carbon (GAC). The powdered form of activated carbon (PAC) typically is less than 100 microns in diameter and may be used to reduce dioxins in incinerator emissions (2) and in the treatment of drinking water and wastewater treatment (see the section on "Activated Sludge"). 

Fig. 2. Round robin operation of a tiiree-bed carbon adsorption unit.

Design criteria for carbon adsorption include type and concentration of contaminant, hydrauhc loading, bed depth, and contact time. Typical ranges are 1.4—6.8 L/s/m for hydrauhc loading, 1.5—9.1 m for bed depth, and 10—50 minutes for contact time (1). The adsorption capacity for a particular compound or mixed waste stream can be deterrnined as an adsorption isotherm and pilot tested. The adsorption isotherm relates the observed effluent concentration to the amount of material adsorbed per mass of carbon. 

Pretreatment of aqueous streams may be required prior to using ion exchange. Suspended soHds that can plug an ion-exchange unit should be reduced to the 10 p.m level. Organics that can foul resins can be removed by carbon adsorption. Iron [7439-89-6] and manganese [7439-96-5], commonly present in ground waters, should be removed because they precipitate on the resin. 

Thermal Desorption. Thermal desorption is an innovative treatment that has been appHed primarily to soils. Wastes are heated to temperatures of 200 to 600°C to increase the volatilization of organic contaminants. Volatilized organics in the gas stream are removed by a variety of methods including incineration, carbon adsorption, and chemical reduction. 

XAlD-4 for a group of chlorinated hydrocarb Dns is given in Table 8. The EPA may recommend a combination of air Stripping and carbon adsorption... 

R. A. Dobbs, R. J. Middendorf, and J. M. Cohen, Carbon Adsorption Isotherms for Toxic Organics, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio, EPA-600/8-80-023, 1980. 

Chemical precipitation Chemical oxidation/re duction Air and/or steam stripping Activated carbon adsorption Resin adsorption Ion exchange Ultrafiltra-tion and/or reverse osmosis Flo atation / ph ase separation... 

Hydrocarbon Recovery. Toluene is typically recovered from the oxidizer vent gases through the use of refrigeration followed by activated carbon adsorption. 

P. N. Cheremisinoff and E. EUerbusch, Carbon Adsorption Handbook, Ann Arbor Science PubHshers, Inc., Ann Arbor, Mich., 1978, pp. 539—626. An exceUent reference book on activated carbon, ranging from theoretical to appHed aspects. 

U.S. Enviionmental Piotection Agency, Process Design Manualfor Carbon Adsorption, SwiadeU-Diesslei Co., Pittsbuigh, Pa., 1971, pp. 3—68. 

Adsorption. Adsorption (qv) is an effective means of lowering the concentration of dissolved organics in effluent. Activated carbon is the most widely used and effective adsorbent for dyes (4) and, it has been extensively studied in the waste treatment of the different classes of dyes, ie, acid, direct, basic, reactive, disperse, etc (5—22). Commercial activated carbon can be prepared from lignite and bituminous coal, wood, pulp mill residue, coconut shell, and blood and have a surface area ranging from 500—1400 m /g (23). The feasibiUty of adsorption on carbon for the removal of dissolved organic pollutants has been demonstrated by adsorption isotherms (24) (see Carbon, activated carbon). Several pilot-plant and commercial-scale systems using activated carbon adsorption columns have been developed (25—27). 

For color removal, ozonization has achieved the greatest practical importance as seen by the plethora of articles and patents on this method (147—163). Ozonization in combination with treatments such as coagulation, flocculation, carbon adsorption, uv irradiation, gamma radiation, and biodegradation significantly and successfully remove dye wastes and reduce costs (156,164—170). 

A number of papers have appeared on the removal of heavy metals in the effluents of dyestuff and textile mill plants. The methods used were coagulation (320—324), polymeric adsorption (325), ultrafiltration (326,327), carbon adsorption (328,329), electrochemical (330), and incineration and landfiU (331). Of interest is the removal of these heavy metals, especiaUy copper by chelation using trimercaptotria2ine (332) and reactive dyed jute or sawdust (333). 

Cheremisinoff and Ellerbusch, Carbon Adsorption Handbook, Ann Arbor Science, Ann Arbor, 1978. 

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