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Activated : Fundamentals and New Applications Activated carbon sorbents are important tools in and air-pollution control. This article provides information on the fundamentals of this diverse sorbent and on new applications for which it is being employed

Cabot Ken Koehlert lobal sustain- Cabot Corp. ability trends are creating in- Gcreased demand IN BRIEF for purification of air and ACTIVATED CARBON water, as well as more en- BASICS vironmentally friendly pro- cess alternatives. Activated RAW MATERIALS carbon (AC) sorbents play ACTIVATION PROCESSES important roles in applica- tions throughout the chemi- POST-PROCESSING cal process industries (CPI). These range from traditional 1,000 nm 100 nm FUNDAMENTALS applications, such as mu- FIGURE 1. These images from a - microscope show the pore structure of FACTORS AFFECTING nicipal water purification -based activated carbon PERFORMANCE and fluegas treatment, to cutting-edge applications, such as ad- ner. The degree of order varies based on APPLICATION TYPES sorbed storage and double- the starting raw material and thermal his- BIOGAS PURIFICATION layer capacitors. tory. Graphitic platelets in steam-activated SEDIMENT REMEDIATION In this article we review the basics of ac- are somewhat ordered, while more tivated carbon, as well as the link between amorphous aromatic structures are found in activated carbon properties and adsorption chemically activated . performance. In addition, the article provides Randomized bonding creates a highly po- an overview of two emerging applications for rous structure with numerous cracks, crev- activated carbon sorbents — biogas purifi- ices and voids between the carbon layers. cation and sediment remediation. Activated carbon’s molecular size and the resulting enormous internal surface Activated carbon basics area make this material extremely effective for Activated carbon is a highly porous, high- adsorbing a wide range of impurities from liq- surface-area adsorptive material with a uids and gases. To provide some perspective largely amorphous structure. It is composed for the internal surface area of activated car- primarily of aromatic configurations of car- bon, the annual activated carbon production bon atoms joined by random cross-linkages. of the author's employer has a surface area Activated carbon differs from another form nearly equal to the total land area on Earth of carbon — — in that activated (148 million km²). Figure 1 shows two micro- carbon has sheets or groups of atoms that graphs of the internal structure of steam-ac- are stacked unevenly in a disorganized man- tivated lignite.

32 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 Cabot Activated carbon sorbents are tailored reactions in the raw material at tempera- for specific applications mainly based on tures between 100 and 400°C. pore size and pore volume requirements. Charring. Higher-molecular-weight or- Porosity and other parameters are con- ganic materials are converted to a car- trolled by the following: 1) raw material bonaceous char residue at selection; 2) activation process condi- of 400–600°C. At this point, incipient tions; and 3) post-processing steps. porosity and increased internal surface Depending on the application, activated area begin to form. A B carbon may be in the form of powder Activation. Activation is generally con- (PAC), granule (GAC) or extrudate (EAC). ducted in a steam atmosphere at tem- All three forms are available in a range of peratures between 700 and 1,050°C, particle sizes. depending on the pore structure of the carbon being produced. The desired re- Raw materials action is shown in Equation (1). Almost any carbon-containing mate- rial can be used to produce activated C + H2O ➞ CO + H2 (1) C D carbon. In practice, economics and FIGURE 2. This series of diagrams il- target product properties are the deter- This reaction gasifies portions of the lustrates pore development during steam mining factors in the selection of raw solid carbon to create pore volume. activation materials. The raw material has a Typically, about half of the carbona- significant impact on the final product ceous char material entering the activa- properties, including pore size distribu- tion step will be reacted away to create tion and volume, hardness and purity. the desired internal pore structure. The Most commercial activated diagrams in Figure 2 depict the develop- are manufactured from the following ment of porosity during steam activation. raw materials: Chemical activation is used to pro- • Coal (anthracite, bituminous, sub- duce carbons with pore structures and bituminous, lignite) compositions that are somewhat differ- • shell ent from those in steam-activated car- • Wood bons. For example, chemically activated Some types of activated carbon are wood has higher mesoporosity and produced from less conventional raw higher oxygen content than a steam- materials, such as , olive stones, activated carbon. The chemical activa- fruit pits, petroleum , , syn- tion process consists of mixing raw bio- thetic , scrap tires and waste mass with a strong dehydrating agent cellulose materials. and heating to about 400 to 700°C. The Raw materials may undergo pre-pro- dehydrating agent (typically phosphoric cessing steps to control size, form and or ) extracts the mois- other properties. They may be crushed, ture from the raw material and fixes the milled, briquetted or mixed with binders volatile component of the biomass while and extruded prior to activation. the activation occurs. The degree of activation is determined by the ratio of Activation processes raw material to dehydrating agent and Activated carbons are manufactured via by the heating time and . one of two processes: steam activation After activation, the product is extracted at high temperature or chemical activa- to yield a highly porous activated carbon tion using a strong dehydrating agent. product and the the dehydrating agent Steam activation is the most com- is recovered. monly used method for activated car- Chemical activation with potassium bon production. It is performed in rotary hydroxide is of great recent interest be- kilns, shaft kilns, multi-hearth furnaces cause it can produce highly micropo- or fluidized beds, and proceeds through rous carbons with specific surface areas the following steps: at or beyond the theoretical value for Drying. Activated carbon raw materi- (about 2,600 m2/g). als in commercial use contain residual moisture that must be removed before Post-processing activation can take place. Following activation, activated carbon Devolatilization. Volatile organic com- sorbents may undergo a series of post- pounds (VOCs) are formed by processing steps, including the following:

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 33 Cabot

0.9 Bituminous range of applications. is commonly used to disinfect water. A more special- 0.8 Lignite ized example is impregnation with 0.7 Wood Peat salts and amines for military gasmasks. 0.6 On a larger scale, impregnation is used 0.5 to promote the oxidation of in 0.4 coal-fired utility fluegas applications.

Pore volume, mL/g volume, Pore 0.3 Products for enhanced sulfur removal are impregnated with potassium iodide 0.2 or hydroxide. Another example 0.1 of impregnation by end-users is the de- 0 position of precious on activated < 2 nm 2-50 nm > 50 nm Pore diameter carbon to form heterogeneous catalysts. Re-activation. Some granular and ex- FIGURE 3. The distribution of pore vol- Washing. Activated carbon may be truded products can be re-activated umes differs depending on the starting acid-washed to reduce ash content and and recycled after use to minimize cost material for the activated carbon remove soluble impurities, such as . and environmental impact. In this pro- Varying levels of increased purity are re- cess, spent carbon loaded with organic quired for some applications, including contaminants is thermally treated in a double-layer capacitors, pharmaceuti- process similar to steam activation. Re- cals and food and beverages. activation desorbs and destroys volatile Sizing. Granular products may be contaminants and restores much of the sieved to specific particle size ranges. original pore volume and adsorptive ca- Common mesh-size specifications pacity. Typical product loss during re- range from 20 x 40 to 4 x 8 mesh (ap- activation is 5–15%, requiring makeup proximately 0.4 mm x 0.8 mm to 2 mm with virgin material. x 5 mm). Extrudates are produced with diameters ranging from 0.8 to 5 mm and Adsorption fundamentals length-to-diameter ratios between about Activated carbon sorbents remove low 1:1 to 4:1. Granule and extrudate sizes concentrations of chemicals (adsorbates) are specified to balance mass transfer from a fluid (liquid or gas) by adsorption, and drop. Powder products the process of accumulation of materials are often ground in roller, hammer or onto a solid surface. Adsorption occurs jet mills to achieve a target particle-size within the activated carbon pore struc- distribution (PSD), often specified by the ture by two distinct mechanisms: physi- terms d5, d50 and d90, which are mea- cal and chemical adsorption. sures of particle diameters. D50 (median Physical adsorption. Molecules are particle size) can range from 5 to about attached to the carbon surface by van 20 micrometers (µm). Smaller particles der Waals attractive forces. These in- improve mass transport, but can termolecular forces are very weak and to filterability challenges. Tight control of diminish with increasing distance be- PSD can optimize the balance. tween the carbon surface and the ad- Shaping. Powdered activated car- sorbate molecule. Because the weak bons may be formulated with binders attractive forces are greatly dependent and shaped into cylinders, tubes, hon- on distance, physical adsorption occurs eycombs and other extruded forms to primarily within pores that have a radius optimize surface-to-volume ratio and only a few times greater than the molec- pressure drop for various applications. ular diameter of the adsorbed molecule. Granules and extrudates may also be Pores that are smaller than the size of formed into filter blocks and plates using the impurity molecule are inaccessible polymeric binders. and do not participate in the adsorp- Impregnation. Some contaminants, tion process. Pores that are significantly such as formaldehyde, elemental mer- larger than the adsorbate molecule are cury and sulfide are not ad- not as effective at adsorption because sorbed on activated carbon. Chemical the attractive force diminishes as the impregnation can add functionality to the distance between the pore surface and carbon sorbent to catalyze a reaction or the adsorbate is increased. promote . Activated car- Adsorption occurs after the impurity bon manufacturers add functionality for a molecule has diffused into the carbon

34 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 gas mixture Process & Industrial user ( w/C02) treatment H2S removal system Electricity generation Commercial user Moisture separator Activated Activated carbon carbon

Landfill well Boiler room Siloxane Processed gas Landfill Blower removal Additional processing

Biogas system Residential user

Gas collection Gas control and processing Gas utilization pore structure to an adsorption site. The tion isotherm. The Freundlich isotherm is FIGURE 4. This schematic diagram shows process is -rate-limited and the commonly used to empirically define this the collection, purification and use of bio- large pores play a role in transporting relationship. The Freundlich adsorption gas from a landfill site adsorbate to the adsorption sites. Phys- isotherm equation is shown here: ical adsorption is an equilibrium process 1/n and is very dependent on the concen- Ccarbon = kfCwater (2) tration of the adsorbate in the . The relationship between the amount Where: of adsorbate on the surface versus that in Ccarbon = concentration of adsorbate the solution is described by the adsorp- adsorbed on carbon

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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 35 Coconut AC Lignite AC Factors affecting performance Factors that affect the performance of activated carbons can come from the specifics of the application, or from the 100 activated carbon itself. Application-related factors. Several 75 factors can aid or hinder adsorption. The principal factor is the molecular size

50 of the compound being adsorbed. Ac- tivated carbon adsorption increases as AC capacity DI, % AC capacity DI, AC capacity NOM/ the size of the molecule being adsorbed 25 increases. Certain types of functional groups on the impurity molecule can 0 also affect its adsorbability. 0 5 10 15 Because the physical adsorption PCB-118 conc. (ng/L) process is an equilibrium reaction, the FIGURE 5. This diagram shows a com- Cwater = equilibrium concentration of concentration of the molecule to be ad- parison of lignite- and coconut-based ac- adsorbate in solution sorbed strongly affects the amount ad- tivated carbon performance with 50 mg/L natural organic material (NOM) Kf and n are constants for a given HOC sorbed, as described by the adsorption and carbon at room temperature isotherm. The of the adsorbed The partition coefficient (Ccarbon / compound is also important, with lower Cwater) can be calculated from Freun- solubility resulting in greater adsorption. dlich equation at a given equilibrium Adsorption rate increases with tem- concentration. If a single compound is perature. However, desorption rate also being adsorbed by activated carbon, all increases with temperature and it is not pores of a suitable size are available for possible to predict the net effect in liquid- adsorption and the amount of material phase applications. The principal reason adsorbed is the maximum achievable for adsorbing at elevated temperatures in by the particular carbon. the liquid phase is to lower viscosity and If several adsorbate molecules are increase diffusion rate. In gas-phase ap- present, there can be competition for plications, adsorption always decreases the adsorption sites. In that situation, with higher temperature. the larger impurity molecules will block Activated-carbon-related factors. In some of the smaller pores. This may re- adsorption applications, the most criti- duce the carbon’s ability to adsorb the cal performance parameter is the distri- smaller-sized adsorbates. bution of pore size and volume. Since Chemical adsorption. Chemical ad- adsorption occurs almost exclusively in sorption refers to the direct reaction of pores just a few times larger than the ad- the adsorbed molecule with an active sorbate molecule, it is the pore volume site on the carbon surface. As a result of within this size range that determines the reaction, a chemical bond is formed adsorption capacity. The highly porous between the adsorbate and the carbon nature of activated carbon gives rise to surface. The active sites on the carbon surface areas as large as 3,000 m²/g. surface involved in chemisorption are Pore sizes are classified as micro-, mainly functional groups that contain meso- or macropores, according to the oxygen and alter the electron balance conventions of the International Union of the carbon surface. If the molecule of Pure and Applied Chemisty (IUPAC; being adsorbed is chemically bound to iupac.org). Micropores with widths less the carbon surface (shared electrons), than 2 nm are useful for adsorbing small the process is termed chemisorption. molecules, especially in vapor applica- If the adsorbed molecule resides only tions. Mesopores, ranging in width from temporarily on the carbon surface, picks 2 to 50 nm, are in the right size range up an electron, and then leaves the car- to adsorb many contaminant materi- bon surface, the process is described as als. Macropores greater than 50 nm in catalytic conversion. This is the mecha- width have minimal adsorption capac- nism responsible for the dechlorination ity, but are critical in determining ad- reaction used by bottlers to convert free sorption kinetics within the particle. The in water to chlorides. larger pores provide transport paths for molecules to diffuse into the mesopores

36 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 and micropores, where adsorption High purity is critical in some appli- takes place. Figure 3 compares the cations. Food, beverage and potable pore distribution of four different ac- water applications require low levels tivated carbons from a variety of raw of extractables. Pharmaceutical acti- materials and activation processes. vated carbons must have ultra-high Surface area can be determined purity and full traceability. Emerging by adsorption using the uses in double-layer capacitors and BET (Brunauer, Emmet, Teller) other electrochemical applications method. Pore size distribution can depend on high purity for extended be quantified by examining the ad- cycle life. sorption and desorption of nitrogen, and other adsor- Application types bates. Mercury porosimetry is well Activated carbon sorbents are used suited to measure macropore and in two broad application classes: large mesopore volume. However, vapor and liquid purification. Within it is more common and convenient each class are examples of two to measure and rank activated car- types of fluid-sorbent contacting. bon performance using specific ad- These are PAC dosing and GAC/ sorbates that mimic the application. EAC packed columns. Beyond purifi- Examples include adsorption cation, activated carbons are used in to assess small pore capacity and a number of specialized applications. dye molecule adsorption (methylene In PAC dosing systems, activated blue, bromophenol blue, and so on) carbon particles are injected into the to assess medium-sized pores. In contaminated fluid, dispersed within some cases, the characterization the fluid for an appropriate contact test is directly related to the final ap- time, and then removed by sedimen- plication, as in the case of molasses tation or . PAC dosing may decolorizing efficiency as a predictor be used in batch or continuous in- of performance in applica- jection systems. PAC contact times tions, and butane working capacity range from 0.05 to 2 seconds in gas- as a performance metric for carbons phase systems and 1 to 60 minutes used to control automobile in liquid-phase batch applications. In vapor emissions. municipal , PAC may Particle size and distribution im- be added continuously as a slurry or pact performance in both PAC and powder and then can be removed GAC applications. For PAC, size and by , sedimentation and size distribution correlate to mass- filtration. In coal-fired utility mercury transfer resistance and filterability. removal, PAC is injected into the In batch applications, a fine particle fluegas through a distribution lance size provides rapid adsorption, but and removed in the electrostatic pre- also a high pressure drop and slow cipitator or fabric filter. filtration when removing the sorbent. In packed-bed systems, fluid flows The best balance of performance is through a static bed of GAC or EAC. often found by narrowing the size As the contaminant concentration distribution. In GAC and extrudate in the fluid decreases, the loading packed-bed applications, size is on the carbon increases, creating again related to mass-transfer resis- a concentration gradient along the tance and pressure drop. column. The mass-transfer zone is Durability is of particular concern defined by the gradient between for GAC and EAC forms. Particles the inlet concentration existing in must be able to resist damage and the fully loaded bed and the outlet fines formation during transport, col- concentration. Breakthrough occurs umn loading and use — especially if when the mass-transfer zone travels the application involves column back- to the exit of the column. Packed- washing. Activated carbons for gold bed systems are sized based on extraction have some of the most either calculated or experimentally stringent durability requirements. Du- determined isotherms, contact time rable particles are also required to based on estimated or measured ki- minimize losses during re-activation. netics and mass transfer, and pres-

Circle xx on p. 62 or go to adlinks.chemengonline.com/66430-xx CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 37 sure drop requirements. bic digestion using animal manure or Vapor-phase purification applications vegetation. The main constituents of cover a wide range of contaminant mol- biogas are methane and carbon diox- ecules from mercury to volatile organic ide, however, it also contains undesir- materials. Some specific examples in- able impurities like clude gasoline vapor in automobile evap- (H2S), siloxanes and VOCs. These un- orative-loss-control devices, mercury wanted impurities are present in low in coal-fired utility fluegas and dioxin in concentrations, but must be removed incinerator offgas. Another vapor-phase to reduce combustion equipment dam- application is pressure- or temperature- age and downtime, to ensure emission swing adsorption for solvent recovery targets are met, and to meet gas-purity and hydrogen purification. Impregnated specifications. After purification, the carbons are used in military gasmasks biogas can either be sent to an engine and industrial to protect per- to generate power, sold into the natu- sonnel from toxic gases. An emerging ral gas grid or sold as a transportation fuel in the form of biomethane. Figure 4 shows where activated carbon adsorp- One of the most effective technologies for high- tion is used in a typical biogas system. performance impurity removal in biogas is a two-step One of the most effective technolo- gies for high-performance impurity re- treatment with activated carbon moval in biogas is a two-step treatment with activated carbon. For optimal per- application in this area — removal of hy- formance, an operator will first direct a drogen sulfide and siloxanes from biogas humid gas through an activated carbon — is highlighted below. bed where H2S is reduced to elemental Activated carbon sorbents are used sulfur by a catalytic process. After H2S to purify a wide range of liquid systems, removal, the gas is dried and sent to a including potable water, wastewater, in- separate activated carbon bed, where dustrial process water, chemicals, phar- siloxane and VOC impurities are re- maceuticals, foods and beverages. Re- moved by physical adsorption. moval of taste and odor contaminants H2S purification. H2S is a hazardous and harmful from potable chemical compound present in biogas. water is one of the main purification ap- Biogas producers need to remove H2S plications. Activated carbon is used to because it is poisonous, corrosive, flam- remove precious metal catalysts after mable and malodorous. Traditionally, synthesis of pharmaceutical active in- biological scrubbers, iron-oxide-based gredients. Liquid sugar decolorization is media, or impregnated activated car- one of the earliest activated carbon ap- bons have been used for H2S removal in plications and remains important today. biogas. All technologies are capable of Activated carbon is used to remove un- removing H2S; however, none of these desirable taste compounds and color technologies has been optimized spe- precursors from processed fruit juices. cifically for biogas. Biological scrubbers Color removal from natural-gas liquids are very effective at removing high con- is a more recent application. Activated centrations of H2S, but can be difficult to carbon application in sediment remedia- operate and capital-intensive to install. tion is covered in more detail below. Iron oxide media are low-priced, but Non-purification applications of ac- generally demonstrate very low removal tivated carbon include gold recovery efficiency. Iron oxide media are also no- from leaching , stor- torious for bricking (bridging between age of gases, such as hydrogen and particles, forming lumps), which methane, and electrodes for double- to costly and time-intensive removal ef- layer capacitors and catalysts. forts. Chemically impregnated activated carbon has a higher loading efficiency, Biogas purification but can be quite expensive. Impreg- Biogas is an environmentally friendly nated carbons can also pose safety and sustainable fuel derived from the challenges, as they may cause bed fires breakdown of organic matter. It is typi- due to an exothermic reaction with H2S. cally produced at , wastewater Many operators today are focusing on treatment facilities, or through anaero- total cost of ownership and are choos-

38 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 ing a media or engineered solution that gas-limited source, they will likely prefer offers the lowest cost per mass of H2S using activated carbon only. TSA units removed. In making this calculation, the are also known to struggle with remov- operator considers capital investment ing some lower-molecular-weight si- for each technology, followed by analysis loxanes, which means that they often of long-term operating costs. For media- require activated carbon polishing ves- based solutions, they must consider the sels to remove all siloxane compounds. amount of media needed to fill the ves- High concentrations of siloxane in a bio- sel, the operating time until media satu- gas can lead to high operational costs ration, the cost and time associated with to replace a standard activated carbon media changeouts, and ultimately, how because much of the carbon pore vol- the downtime impacts the ability to pro- ume is spent removing different families cess gas. It is important to consider total of compounds from the gas. Specifi- lifecycle cost in developing activated cally designed surface modifications to carbons for biogas H2S removal. standard activated carbon can allow the Siloxanes and VOC purification. Si- preferential adsorbtion of siloxanes over loxanes are a group of synthetic com- VOCs. This results in significantly better pounds used in the manufacture of per- siloxane loading capacity versus stan- sonal hygiene, healthcare and industrial dard activated carbon products. Since products. Disposal of these products this type of modified product does not creates the risk of siloxane impuri- require a TSA unit to achieve high per- ties in biogas, posing significant risks formance, it is a way that processors depending on the final application. In could avoid the need to invest in an ex- combustion engines, boilers, turbines pensive system upfront, but still realize and fuel cells, siloxanes can form silica long-term savings. that causes damage, destruction and Biogas is one of the more challenging reduced operating efficiency. Siloxane gas-purification applications. Depend- damage leads to higher operational ing on the source, the amount and the costs and can seriously impact biogas speciation of impurities is constantly upgrading and even prevent sales of variable. When deciding which adsorp- contaminated biogas. tion technology to deploy, an operator VOCs are impurities often found in bio- must consider many factors to deter- gas derived from agriculture, landfills and mine the highest-performance and most wastewater-treatment facilities. Depend- cost-effective solution for their site. ing on the application, VOCs can be viewed either as beneficial or detrimental Sediment remediation to the quality of the gas. If the biogas is Sediments accumulated on the bot- sent for combustion, the VOCs are often tom of waterbodies are sinks for toxic, considered a beneficial high-Btu addi- bio-accumulative chemicals that can tive. When biogas is further upgraded be transferred to invertebrates and fish to biomethane, VOCs must be removed via food webs. Increasingly, remedia- to prevent significant damage to mem- tion practitioners are turning to in-situ branes and to limit emissions of sulfur treatment of contaminated sediments oxides and nitrogen oxides. Pipeline gas to reduce the ecological and human- has strict limits on VOC concentration. health risks posed by contaminants. Activated carbon has traditionally This is mainly because conventional been used for siloxane and VOC re- technologies (such as dredging and moval, as it is effective at physically conventional sand capping) have not removing both compounds. For dedi- always demonstrated risk reduction. A cated siloxane removal, users often new approach of in-situ sediment treat- opt for temperature-swing adsorption ment via contaminant sequestration in- (TSA) using an activated-carbon bed volves placing activated carbon amend- as a polishing step. The decision to use ments on contaminated sediments. activated carbon versus TSA typically In this application, activated carbon is comes down to operating conditions used to adsorb HOCs, such as poly- and costs. TSA systems have relatively cyclic aromatic (PAHs) high upfront capital costs and also use and polychlorinated biphenyl (PCBs) significant amounts of gas for regen- from sediment pore water. One chal- eration. If an operator is pulling from a lenge in this application is interference

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 39 from natural organic matter (NOM). which can be calculated from a Freun- Large organic molecules present in the dlich adsorption isotherm equation at a water matrix, such as humic and fulvic given equilibrium concentration of the , can cause blockage of the acti- contaminant in pore water. vated carbon pore structure, leading to The author’s employer has generated reduced adsorption capacity for target partition coefficients for a wide range contaminants. Highly microporous car- of PAH and PCB contaminants for ac- bons are particularly prone to blockage. tivated carbon products in this area. Activated carbon with a tailored blend These data, combined with knowledge of mesoporosity and microporosity is of site conditions, enables the design of required for resistance to NOM and re- reactive caps using the CAPSIM model. moval of target contaminants. Using the right activated carbon amend- ment and cap design can provide in-situ Increasingly, remediation practitioners are turning to sediment remediation solutions that pre- in-situ treatment of contaminated sediments to reduce vent breakthrough from contaminated sediments for 100 years or more. human health and ecological risks Summary remarks Lignite-derived, activated carbon Activated carbon is a sorbent with wide- grades now exist specifically for remov- ranging uses in the purification of vapor ing PCBs, PAHs, dioxins and furans in and liquid systems, as well as special- contaminated sediments. Activated ized uses in fuel storage, and carbons developed for this application electrochemistry. Activated carbon can have an optimal pore size distribution be tailored for specific applications by that is designed to be highly effective for a combination of raw material selec- the removal of these compounds in the tion, activation process conditions and presence of NOM concentrations typi- post-processing, including shaping and cal in sediment pore water. This broad chemical impregnation. Adsorption per- pore-size distribution is critical for high formance is driven by pore size and vol- performance, as NOM can partially or ume distribution, and other factors, such completely block the adsorptive capac- as durability and particle size, influence ity of more microporous carbons. Figure the choice of activated carbon for a spe- 5 compares microporous coconut AC cific application. Activated carbon can and lignite AC with tailored porosity in be used in liquid or vapor applications removal of PCB-118. The chart shows in either PAC or GAC/EAC form. Biogas the ratio of PCB removal in water with purification and sediment remediation 50 mg/L NOM to removal in distilled are two recent applications that highlight water and clearly demonstrates lignite’s the versatility of activated carbon sor- resistance to pore blockage. bents for purification applications. n Activated carbon amendments are Edited by Scott Jenkins applied to the sediment either by direct mixing, as a component in a geotextile Author mat, or by incorporating it into a cap that Ken Koehlert is the product line and creates a physical and adsorptive bar- technology director for Cabot Norit Acti- vated Carbon (157 Concord Road, Bill- rier between the contaminated sediment erica, MA 01821; Phone: 1-978-663- and water. The cap consists of sand or 3455; Email: ken.koehlert@cabotcorp. aggregate and carbon can be added as com). Koehlert has spent more than 30 years at Cabot, and has held positions GAC or as a PAC coating applied to the in process and product R&D and manu- surface of the aggregate. facturing across several of the compa- ny’s businesses. He received B.S.Ch.E. In order to design reactive caps using and M.S.Ch.E. degrees from the Massachussets Institute of activated carbon, engineering firms Technology. Koehlert’s team develops and supports a wide frequently use the CAPSIM model, de- range of products, including two unique activated carbons for veloped by professor Danny Reible of biogas purification. DARCO BG1 material optimizes the cost of H2S removed while limiting the concern for exothermic Texas Tech University (Lubbock, Tex.; reactions associated with impregnated activated carbons. www.ttu.edu). To assess and approve Cabot’s recently launched Norit Silpure product has a spe- cifically de­signed surface to preferentially adsorb siloxanes a specific activated carbon grade in a over VOCs. The team also works with Norit SedimentPure reactive cap, engineering firms require activated carbons with tailored porosity to remove target con- specific activated-carbon performance taminants in the presence of natural organic matter. data, namely the partition coefficient,

40 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2017 Circle xx on p. 62 or go to adlinks.chemengonline.com/66430-xx