Granular Activated Carbon

PAC and GAC remove organic contaminants through adsorption, primarily a physical process in which dissolved contaminants adhere to the porous surface of the carbon particles.

From: Handbook of Water Purity and Quality (Second Edition), 2021

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New Developments and Application in Chemical Reaction Engineering

C.K. Ahn, ... S.H. Woo, in Studies in Surface Science and Catalysis, 2006

2 EXPERIMENTAL

Charcoal-based GACs (Darco 20~40, 12~20, and 4~12 mesh) were used as adsorbents for phenanthrene and Triton X-100. The pore size distribution and specific surface area of GACs were obtained from N2 gas adsorption at 77.3 K by using surface area analyzer (ASAP 2010, Micromeritics). Surface functional groups of GACs were determined by using X-ray photoelectron spectroscopy (EscaLab 220-IXL). In isotherm experiments, 0.1 g of each GAC was added to a 250-ml Erlenmeyer flask and then filled with 100 ml at various concentrations of compounds at 25 °C. In selective adsorption tests, the flasks containing surfactant solutions with phenanthrene (0~100 mg l−1) were shaken at 100 rpm at 25 °C for 48 h to reach equilibrium state. Phenanthrene and Triton X-100 were analyzed by using HPLC (Dionex) with an UV detector at 250 nm and 230 nm for phenanthrene and Triton X-100, respectively. The analytical column was a reversed-phase Supelcosil LC-PAH column (150 mm × 4.6 mm).

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Ozonation in the gaseous phase

Tatyana I. Poznyak, ... Alexander S. Poznyak, in Ozonation and Biodegradation in Environmental Engineering, 2019

10.6.2 Analytical methods

The intermediates and final products, obtained in ozonation in the gaseous phase, were adsorbed on the GAC. The samples of the GAC were analyzed using the Soxlhet method (Wang et al., 2012): the adsorbed compounds were extracted in methanol (30 mL per 0.5 g of the GAC). The extracts were analyzed by the high performance liquid chromatography (HPCL). The HPLC analysis were performed by a liquid chromatograph Perkin-Elmer series 40 with the UV-Vis detector and a chromatographic column Nova Pack C-18, 250×4.6mm, with the mobile phase of water: acetonitrile (30.0:70.0) with a flow of 0.5 mL min−1. The injected sample volume was 30 μL. The used wavelengths were 264 nm for benzene, 250 nm for toluene, 262 nm for ethylbenzene, and 249 nm for xylene. For the simple organic acids, the selected wavelength was 211 nm. The mobile phase, in the last case, was not modified.

The results are presented in two parts: the first one describes the stripping of the BTEX and their adsorption onto the GAC; the second one the evaluation of the BTEX decomposition degree in ozonation in the gaseous phase at different operating conditions.

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Green Synthesis of Iron Nanomaterials for Oxidative Catalysis of Organic Environmental Pollutants

Homer C. Genuino, ... George E. Hoag, in New and Future Developments in Catalysis, 2013

3.3.2.4 Other Chlorinated Compounds

The degradation of polychlorinated biphenyl (PCB) has been investigated by various researchers using various iron and iron bimetallic nanomaterials. When Fe/Pd bimetallic nanomaterials were impregnated onto a mesoporous granular activated carbon, PCBs were adsorbed on the surface of carbon and subsequently dechlorinated by the bimetallic nanoparticles [93]. The system was able to dechlorinate 2-chlorobiphenyl by 90% after 2 d. The remaining 10% was adsorbed on the activated carbon together with the biphenyl dechlorination product. Similarly, Fe–Pd bimetallic nanoparticles stabilized by a water-soluble starch were able to remove over 80% of PCB in less than 100 h, whereas the unstarched particles removed only 24% of PCB [24]. Using nZVI immobilized on a cation-exchange resin, the dechlorination of decachlorobiphenyl was successfully achieved after 10 d (Li et al., 2007). However, when particles were suspended in solution for the same duration, 21% of the parent compound remained.

nZVI was used to reduce eight chlorinated ethanes [102]. Most ethane molecules were dechlorinated, except 1,2-dichloroethane. The reactivity of the nZVI was dependent on the degree of chlorination and the location of chlorine atoms on the molecule. A higher rate of dechlorination was observed with highly chlorinated ethanes and when the chlorine atoms were localized on a single carbon atom. Dechlorination of hexachloroethane was most efficient and followed a reductive β-elimination (two chlorine atoms from adjacent carbons were removed followed by double bond formation) pathway forming PCE (tetrachloroethylene) as the major product.

Comparing biodegradation of monochloroethane with nZVI-assisted dechlorination of the compound, there was a significant difference in the degradation of the products formed [94]. The products of abiotic dichloro-elimination degraded 10% more than the products of biodegradation. This suggests that the degradation of chlorinated ethane by nZVI is superior to biodegradation since the products of the former degraded faster than those from the latter reaction.

Removal of 1,1,1,-trichloroethane using a series of bimetallic iron nanomaterials (Au, Cu, Ni, Pd, and Pt) did not show any periodic trend [95]. However, the first order rate constant for the reactions correlated with the solubility of atomic hydrogen on the second metal. This suggested that the enhanced activity of bimetallic iron nanomaterials was due to absorbed atomic hydrogen and not by galvanic corrosion processes.

Freshly prepared Pd/Fe nanoparticles were used to dechlorinate mono-, di-, and 1,2,4-trichlorobenzene [96]. Successful dechlorination to form benzene as the final product was achieved. The reaction followed a pseudo first-order kinetics with more chlorinated products having higher reaction rates than the less chlorinated compounds. Pd/Fe nanomaterial was active since the unpalladized iron showed very minimal reactivity. The aged bimetallic particles had less activity presumably due to the dislodgment of Pd and Pd encapsulation by the iron oxides formed during aging.

Further investigation on the dechlorination of trichlorobenzene by Pd/Fe particles revealed that the presence of surfactants influenced the reaction rate [97]. When the concentrations of surfactants were below the critical micelle concentration (CMC) values of cationic (cetyltrimethylammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (nonylphenol ethoxylate and octylphenol polyethylene glycol ether) surfactants, the rate constants increased as compared to pure water. However, above the CMC values, the rate decreased. The presence of natural organic matter (NOM) also reduced the catalytic dechlorination of 1,2,4-trichloro benzene by the particles due to competition with NOM as a H2 acceptor.

Other aromatic chlorinated compounds investigated are chlorophenols. Hydrodechlorination of 2,4-dichlorophenol by nano Pd/Fe bimetallic particles was investigated [98]. Phenol was found to be the major product; however, trace levels of 2-chlorophenol and 4-chlorophenol were also formed. Optimum conditions such as high Pd loading, higher reaction temperatures, and weak acid conditions favored the catalytic dechlorination of these compound.

Dror et al. deposited nZVI and cyanocobalamine on a diatomite matrix and used the composite to study the degradation of PCE and tribromoneopentyl alcohol [99]. The resulting material was superior than nZVI material since it prevents agglomeration making the surface area larger. Laboratory experiments showed rapid first-order decay of the contaminants containing the composite material, whereas the concentration in the control remained the same. In addition, various chlorinated compounds in well water were passed through a column containing the composite for 30 min. The concentrations of contaminants were reduced considerably but the degradation rate was slower for the less chlorinated compounds. Field experiments were also conducted and well water was made to pass through a 50 kg of the composite material in a column. Inlet contained significant amounts of TCE, PCE, cis-DCE, carbon tetrachloride, and chloroform. These contaminants were reduced substantially in the outlet. Specifically, the concentrations of TCE and PCE were reduced to a third of inlet values. Other compounds degraded include nitrate ions and pesticides such as bromacyl and prometryn.

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Nonbiological Degradation of Triazine Herbicides: Photolysis and Hydrolysis

Allan J. Cessna, in The Triazine Herbicides, 2008

Removal of Triazines from Water by Photolysis

Strategies to effect the elimination or removal of triazine herbicides from water have included adsorption, conversion to less toxic products, and complete mineralization. Thus, there have been investigations of adsorption by granular activated carbon (Battaglia, 1989), microbial biodegradation (Cook, 1987), oxidation with chlorine dioxide (Miltner et al., 1989), ozonation (Legube et al., 1987; Adams et al., 1990), ozonation followed by biodegradation (Kearney et al., 1988), ozonation combined with hydrogen peroxide (Trancart, 1990), and ferric ion in the presence of hydrogen peroxide (Sun and Pignatello, 1993). The triazine herbicides absorb light in the UV spectral range, and the photolytic methods studied have included the use of high-intensity UV light (Peterson et al., 1988), UV light in the presence of ozone (Kearney et al., 1987; Benitez et al., 1994; Lai et al., 1995; Zwiener et al., 1995) or hydrogen peroxide (Peterson et al., 1988; Chan et al., 1992; Beltrán et al., 1993, 1996; Hessler et al., 1993; Bourgine et al., 1995; DeLaat et al., 1997), vacuum-ultraviolet photolysis (Gonzalez et al., 1994), photocatalytic oxidation using TiO2 semiconductor particles (Pelizzetti et al., 1990b; Hustert et al., 1991; Pugh et al., 1995; Bellobono et al., 1998; Héquet et al., 2001), UV light in the presence of ferric ion or ferric ion plus hydrogen peroxide (Larson et al., 1991; Sun and Pignatello, 1993) and photocatalysis using polyoxometalates (Texier et al., 1999a, b; Hiskia et al., 2001).

Oxidative degradation of dissolved chemicals in water through catalytic or photochemical methods are generally referred to as Advanced Oxidation Procedures. These procedures are mainly light-induced oxidation processes in which highly reactive intermediates, such as hydroxyl radicals, are generated to oxidize dissolved organic compounds. Photo-induced methods that effect complete mineralization are preferred. However, the degree of mineralization of the substances versus formation of other products varies according to the method employed.

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Introduction

J.Y. Chen, in Activated Carbon Fiber and Textiles, 2017

1.4.1 Current market

It is reported that today's global active carbon market has $3 billion of revenue with these shares: powdery products 48%; granular products 35%; and other products 17%. Powdery and granular activated carbon products are still two major segments to generate market values. Dividing the world consumption of activated carbon via regions, the Asia–Pacific region is the largest market share, using about 45%, mainly by China, India, and Japan. The US market is the fastest growing market for activated carbon consumption, with a market size approximately the same as the Chinese market (~ 24%). Most activated carbon products used in the United States are imported from Sri Lanka, Canada, India, China, and Japan. China is the biggest producer of activated carbons in the world market. Its current manufacturing capacity is estimated at about 0.74 million metric tons per year. Chinese manufacturers are mainly located in two areas: the East and Southeast area (Jiangsu, Zhejiang, Jiangxi, Fujian) where woody biomass is produced from forest and agricultural production, and the North and Northwest area (Inner Mongolia, Shanxi, and Ningxia) where the coal mining industry is well established. Leading companies in cellulose-based active carbon production include Shanghai Xingchang Activated Carbon, Hangzhou Hengxing Activated Carbon, Zhixing Activated Carbon, Fujian Yuanli Carbon, and Xinsen Carbon. Major producers in coal-based activated carbon manufacturing include Huahui Carbon, Datong Carbon Corporation Ltd., Xinhua Activated Carbon, Xingtai Coal Chemical, Huaqing Activated Carbon, and Shenhua Ningxia Coal Industry Group (Research in China, 2014).

In contrast to the GAC/PAC market, the ACF market is relatively smaller. For one reason, ACF products do not enter the end-use market where activated carbons are dominating because of higher production cost. For another reason, market demands for ACF products are not as huge as for GAC/PAC commodities. Rather, the ACF market is a niche market that requires the manufacturers to have special equipment and expertise. Currently, estimated worldwide capacity of ACF production is about 14,000 metric tons. There are only a few companies continuously competing in this global ACF marketplace. Among those manufacturers are: Toyobo, Kuraray, Unitika, GCI, Etese, TCT, Anhui Jialiqi ACF Co., Ltd., Sutong Carbon Fiber, Jiangsu Tongkang, Nantong Yongtong, Jiangsu Kejing Carbon Fiber Co., Ltd., and Qinhuangdao Zichuan. Table 1.4 lists these major ACF manufacturers together with their websites, in order for readers to find more information about these companies.

Table 1.4. Current primary ACF producers

Company Website
TOYOBO http://www.toyobo-global.com/
Anhui Jialiqi http://www.ahsztxw.com/en/
Beyond Ocean http://www.cqfeiyang.cn/
Evertech Envisafe Ecology http://www.etese.com/
Gun Ei Chemical Industry http://www.gunei-chemical.co.jp/eng/
Jiangsu Tongkang http://www.jstk.com.cn/en/about.asp
Kejing Carbon Fiber http://kjacf.com/web/en/index.asp
Kuraray Chemical http://www.kuraraychemical.com/
Nantong Beierge http://beierge.jus668.com/
Nantong Senyou http://www.senyou.com/
Nature Technology http://www.actechintl.com/index.html
Sinocarb Carbon Fibers http://www.sinocarb.com/en/
Sutong Carbon Technology http://www.jsiec.cn/Item/list.asp?id=1505&cid=133
Taiwan Carbon Technology http://www.taicarbon.com.tw/
Unitika https://www.unitika.co.jp/e/
Yongtong Environmental Technology http://ntythb.de.b2b168.com/
Zichuan Carbon Fiber http://acf.sell.everychina.com/
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Analysis, Removal, Effects and Risk of Pharmaceuticals in the Water Cycle

Sung Kyu Maeng, ... Saroj K. Sharma, in Comprehensive Analytical Chemistry, 2013

4 Hybridization of Bank Filtration and Artificial Recharge and Recovery in Multibarrier Treatment

The occurrence of organic micropollutants, such as pharmaceuticals, in water resources including wastewater effluent is a major constraint to either indirect potable reuse or direct potable reuse purposes. Many individual advanced water treatment systems have been studied to enhance the removal of organic micropollutants, and there is a great interest in a multibarrier approach by providing synergies in which two or more systems can function as a hybrid system. Each treatment system in a hybrid system is based on different removal mechanisms (e.g., granular activated carbon (GAC), sorption; membranes, size exclusion and electrostatic interactions; and ozone/advanced oxidation process (AOP), oxidation). In particular, the hybrids of BF or ARR with advanced treatments such as AOP/ozone, nanofiltration (NF), or GAC gains much of interest since ARR and BF are more sustainable treatment. BF and ARR can be as a pretreatment process to NF (BF/ARR-NF), GAC (BF/ARR-GAC), AOP (BF/ARR-AOP), ozone (BF/ARR-ozone), and UV (BF/ARR-UV). BF and ARR reduce target compounds and bulk organic matter that reduces the performance of membranes, GAC, ozone, and AOP processes. There are a number of plants operating ARR or BF as a pretreatment to advanced treatment systems. ARR can be also used as a posttreatment to oxidation treatment systems (AOP-ARR and ozone-ARR). The problem associated with AOP or ozone is that metabolites are produced during oxidation; therefore, biodegradation during soil passage is a great option to reduce the metabolites.

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Membrane Applications

Sanghyun Jeong, ... Saravanamuthu Vigneswaran, in Comprehensive Membrane Science and Engineering (Second Edition), 2017

4.3.4.6 Deep Bed Biofilter (DBF)

When relatively slow flow rates are used, DBF functions as a biofiltration (Fig. 4). Here, biofilm is developed on the medium and it helps in decomposing biodegradable organic material.160 Following the adsorption of organic matters onto the filter media, the initial degradation is accomplished by extracellular enzymatic hydrolysis of macromolecules to smaller substrates, which can then be transported into the biofilm. Further degradation takes place by microbial biofilm community developed in the filter media.161

Fig. 4. Dual media filter (DMF) biofiltration tests with biofilm enrichment reactor (BER).

In this context, Naidu et al.162 evaluated the effectiveness of GAC biofilter based on detailed microbial activities for a 20-day period. It was observed that the high accumulation of active biomass (measured based on ATP) was present on the top layer of the media and the number of attached biomass decreased as filter media depth reduced. Correspondingly, a high bacteria cell number (1.0×108 CFU/g media) was detected on the top layer of GAC media. Further, the biomass concentration increased over time with a low value of 0.9±0.5 µg ATP/g media at the initial stage (0–5 days), which increased significantly to 51.0±11.8 µg ATP/g media after 15–20 days. The trends of DOC removal efficiency and total biomass in GAC media were observed, by which upon reaching a steady condition (15–20 days) and high quality of effluent (0.51±0.12 mg/L of DOC) was produced by GAC biofilter. The high active biomass accumulation on GAC media was attributed to the high organic removal. This study suggested that GAC media was able to hold high amount of biomass because microorganisms formed a thicker layer on porous GAC media in comparison to less porous media such as sand and anthracite.163

In this regard, Jeong et al.164 carried out a detailed microbial study on biofilter with GAC and anthracite media operated for 75 days. The microbial communities in the media were evaluated using terminal restriction fragment length polymorphism (T-RFLP) combined with principal component analysis (PCA), clustering of samples, and sequencing based on the 16S rRNA gene. The GAC biofilter consisted of diverse heterotrophs while the anthracite biofilter was mainly composed of sulfur-oxidizing and reducing bacteria, and alkalitrophic heterotrophs due to the presence of sulfur as an impurity in anthracite. The relatively low abundance of heterotrophic bacteria in the anthracite was attributed to its lower organic removal from seawater compared to GAC.

GAC biofilter also shows superior AOC reduction from seawater, which was related to its microbial contents.162 They observed that in the initial filter stages, the AOC concentration of effluent through the GAC biofilter was around 18.0±1.4 µg-C glucose/L. The high AOC concentration at the initial stage was related to the high-molecular-weight organic compounds transforming to LMW organic compounds during the microbial biodegradation process. Upon reaching mature stage of the filter operation (15–20 days), the AOC value reduced to 0.6±0.2 µg-C glucose/L. This was attributed to specific microbial community developed on the GAC media that assimilated the LMW organic compounds in the GAC biofilter. The AOC removal pattern was similar to the reduction trend of LMW organic compounds. The results of this study indicated the efficiency of GAC biofilter as the seawater pretreatment to control of biofouling potential.

In spite of the promising biofouling control offered by biofilter, this pretreatment still faces some challenges. For example, high concentrations of microorganisms and nutrients can pass through the granular filter during the period between the backwashes and before it gets the ideal level of filter compactness by which microorganisms may have the opportunity to pass through and colonize membrane surface and build a biofilm.42,99 Further, the filter media type, flow rate, and backwashing frequencies must be closely monitored for it to effectively perform as a biofilter.

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Ozonation as main method for organic contaminants degradation in three different phases: liquid, solid, and gaseous

Tatyana I. Poznyak, ... Alexander S. Poznyak, in Ozonation and Biodegradation in Environmental Engineering, 2019

1.3 Ozonation of volatile organic contaminants in the gaseous phase

The ozonation procedure is usually conducted in three sequential stages.

In the first stage, oxygen is passed through the aqueous solution (250 mL), containing the dissolved volatile organic compounds (VOCs). This solution is enclosed in a reactor (500 mL), which has a diffuser plate at its bottom for the feeding of oxygen. This procedure provokes the stripping of benzene, toluene, ethylbenzene, and xylene (BTEX) from the liquid phase to the tubular reactor. The liberation of BTEX is performed using an initial concentration of 200 mg L−1 for benzene, toluene, and xylene, while ethylbenzene was 150 mg L−1. The BTEX liberation from the water was performed with gas flows of 0.5 L min−1 and 0.2 L min−1. The liberated BTEX are adsorbed in granular activated carbon (GAC) (1.0g). The activated carbon is then settled in a bed. This bed was prepared with 1 g of GAC in a small tube with 1 cm diameter. To determine the stripping dynamics, the water samples are analyzed at different times (2,5,10,15,20,30 until 60 min).

On the second stage, the stripped BTEX pass through a tubular reactor, where the BTEX ozonation is carried out in gaseous phase. The ozonation in the tubular reactor (4) with fixed length (1.5 m length and diameter of 2.5 cm) is realized with the ozone concentration (30 and 15 mg L−1) and the variation of gas flow (0.2 L min−1 and 0.5 L min−1). Then the residual ozone, byproducts, and final products of ozonation are adsorbed in GAC (5) located at the reactor output.

In the third stage, all experiments are carried out at the ambient temperature. The measurements of ozone in gaseous phase at the reactor input was realized with an ozone sensor (6), connected to a PC (using an acquisition data board NI-6024(8)). Ozonation in the gaseous phase is carried out in the special tubular reactor, where ozone interacts with Volatile Organic Compounds (VOCs). Fig. 1.2 presents the schematic diagram of the laboratory scale for the ozonation in the gaseous phase. In this particular case, the synthetic solution of VOCs in water stripped from the reactor (3) by the oxygen flow to the tubular reactor (4) is mixed with the ozone–oxygen flow to react with ozone (Chairez et al., 2010).

Figure 1.2. Scheme of the reactor for ozonation in the gaseous phase (O3 = ozone, O2 = oxygen, c = contaminant, By-p = ozonation byproducts).

The schematic diagrams at the laboratory level, presented above, and the experimental conditions of the ozonation process in three different phases, are described in general format. In each particular cases, for the resolution of the particular treatment problem may be amended using modifications described below.

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Activated carbon filters for filtration–adsorption

S. Giraudet, P. Le Cloirec, in Activated Carbon Fiber and Textiles, 2017

9.2.3.2 Comparison with usual forms of activated carbon

Another aspect of the elimination of VOCs for the purification of gases is the efficiency of ACF or ACFC compared to the performances of the most common forms of activated carbon (grains or powders). Most of the advantages of ACF are related to their form (lower pressure drops, continuity of the fibers for electrical-swing adsorption, compactness, and alternative conformations of the filters). However, regarding mass transfer, ACF and ACFC are also potentially interesting for adsorption since the external surface area of the fibers is higher than for grains or powders and the micropores are mostly available on the outer surface of the fiber (with few “transport” pores, ie, macro- or mesopores) (Singh et al., 2002). Thus, the rate of adsorption should be significantly larger, leading to narrower mass transfer zones in dynamic adsorption. Several authors have compared ACF and/or ACFC with granular activated carbon for gas-phase filtration (Boulinguiez and Le Cloirec, 2009a, 2010a; Ramirez et al., 2005; Carter et al., 2011; Ortega and Subrenat, 2009).

There are two criteria of comparison, namely the adsorption capacity (equilibrium) and the rate of adsorption (kinetics). First, it should be stressed that comparisons of adsorption capacities are difficult since many parameters vary from one sample to another (porosity, chemical composition, content of functional surface groups, operating conditions). For example, while the average performances of ACF for the adsorption of formaldehyde at trace concentrations were linked to its lower content of basic groups than in granular activated carbons, the adsorption at higher concentrations was greater due to a larger pore volume and specific surface area (Carter et al., 2011). However, the adsorption of benzene and acetone was greater for an ACFC with higher porosity in comparison to granular activated carbon (specific surface areas of 1604 and 965 m2 g 1, respectively). These latter studies focussed only on adsorption capacities at equilibrium. Boulinguiez and Le Cloirec (2010a) gave a more complete comparison for a set of five different VOCs. In this study, the adsorption capacities were in favor of granular activated carbons made from wood (with the highest porosity), especially at high concentrations. The difference became slighter when the gas-phase concentration was decreased to trace concentrations (in the mg m 3 range). Nevertheless, the kinetics of adsorption pointed out the greater advantage of ACFC, with adsorption rates always higher due to its morphology and larger external surface area (Boulinguiez and Le Cloirec, 2010a).

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Adsorption and its Applications in Industry and Environmental Protection

A. Świątkowski, in Studies in Surface Science and Catalysis, 1999

Liquid-phase adsorption

One of the most widespread uses of activated carbons for liquid-phase adsorption is in water treatment. Recent years have seen an increase in the level of synthetic organic chemicals (SOC) in public water supplies. Hundreds of SOCs, such as pesticides, herbicides, detergents, polycyclic aromatic hydrocarbons, nitrosamines, phenolic compounds, trihalomethanes and other pollutants, have been identified in drinking water supplies. On the other hand, natural organic material (NOM) is found in varying concentrations in all natural water sources. It is a complex mixture of compounds formed as a result of the breakdown of animal and plant material in the environment. Most NOM consists of a range of compounds, from small hydrophilic acids, proteins and amino-acids to larger humic and fulvic acids. The reactions between NOM and disinfectants such as chlorine can produce disinfectant by-products, e.g. the reaction of chlorine with humic acids in groundwaters can produce chlorophenols and halomethanes, and these are indeed the most common products of chlorination. Many of these organic chemicals are carcinogenic. Several methods have been used with varying degrees of success for the control of organic pollutants in water. However, the use of activated carbons is perhaps the best broad-spectrum technology available at the present moment. As a consequence, the use of activated carbons in water treatment has increased throughout the world. GAC adsorption is an effective treatment technology for the removal of organics from drinking water supplies and for improving taste and odour. It is also a practicable technique for the removal of trace (heavy) metals such as Cd, Cr, Hg, Cu, Fe, V, Zn, Ni. Activated carbons are now being used on a much larger scale than ever before. Important properties of GAC for water treatment are their adsorptive capacity and selectivity, ability to withstand thermal regeneration and resistance to attrition losses during transport and handling.

The use of GAC for the treatment of municipal and industrial wastewaters has developed rapidly in the last 25 years. Moving beds, downflow fixed beds and upflow expanded beds have all been used in industrial wastewater applications. In most wastewater applications, the cost of virgin carbon usually precludes its use on a throwaway basis, so the thermal reactivation of hard, coal-based carbons has proved to be both economical and practicable. Chemical regeneration is generally limited to applications where partial recovery of capacity is acceptable and regenerant disposal is not a problem.

A second important use of activated carbons is in the removal of colour from sugars and in the purification of various foods and beverages. At present the sugar industry uses powdered and/or granular activated carbons as decolourants. Sugar solutions contain different colouring matter, such as caramels, melanoidines and iron-containing polyphenolic complexes. In part, the colour originates from the raw material, and in part it is formed during the refining process. The most easily adsorbed components are usually the melanoidines (nitrogeneous brown polymers formed by reactions between amino compounds and sugars). Activated carbons also help to remove surface-active agents and colloidal substances by raising their surface tension and decreasing their viscosity. This leads to higher rates of sugar crystallisation. In sugar processing, a typical procedure is to treat the sugar liquor with PAC. The carbon is subsequently separated by filtration. GAC is also used in various configurations of the decolourizing system (e.g. in countercurrent pulsed-bed columns).

Another, similar, application of activated carbons is in the treatment of edible oils and fats to remove undesirable components. Here, they are used in conjunction with certain bleaching clays. Activated carbon are also used in treating wines and spirits to remove any traces of fusel oil. In the production of brandies, they are used to remove undesirable flavours and to reduce the amount of aldehydes in the raw distillate. In the case of beers, activated carbons are used to improve their colour, and to remove flavours attributed to phenol and colouring matter.

A relatively recent use of activated carbons is in the recovery of gold from its ores. The subject has been extensively reviewed by Bansal et al. [7]. The process usually involves the treatment of finely ground ore with a very dilute solution of NaCN and oxygen. The gold and other metallic impurities present in the ore are oxidized and form cyanide complexes. After this stage they are leached together. The gold is recovered from the cyanide pulp by adsorption on activated carbons. In the carbon-in-pulp (CIP) process developed in the early 1950s [36] the carbon granules were directly added to the cyanide pulp and moved countercurrent to it. The gold-loaded carbon is removed by screening and the gold is recovered from it by elution using either solutions of metal salts, such as K2CO3-KOH, or mixtures of organic solvents [7]. After regeneration the carbon is recycled.

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