Activated Carbon as Superadsorbent and Sustainable Material for Diverse Applications

Sharma, Gaurav; Sharma, Shweta; Kumar, Amit; Lai, Chin Wei; Naushad, Mu.; Shehnaz, undefined; Iqbal, Jibran; Stadler, Florian J.

doi:https://doi.org/10.1155/2022/4184809

Adsorption Science & Technology

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Advanced Nanocomposites for Environmental Remediation of Hazardous Pollutants

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Review Article | Open Access

Volume 2022 | Article ID 4184809 | https://doi.org/10.1155/2022/4184809

Activated Carbon as Superadsorbent and Sustainable Material for Diverse Applications

Gaurav Sharma,^1,2,3,4Shweta Sharma,²Amit Kumar,^1,2Chin Wei Lai,⁵Mu. Naushad,^6,7 Shehnaz,⁸Jibran Iqbal,⁹and Florian J. Stadler¹

Academic Editor: Chinenye Adaobi Igwegbe

Received04 Oct 2021

Accepted11 Jan 2022

Published24 Mar 2022

Abstract

Activated carbon is a carbonaceous material with highly porous structure. Different functionalities can be introduced to its surface by various physical and chemical treatments. Various precursors can be used for the synthesis of activated carbon such as fossil fuels, agricultural wastes, and lignocellulosic wastes, etc. Number of papers have been reported in literature devoted to the synthesis, characterization, and various applications of activated carbon. Herein, in this review, special attention has been paid to the basic properties of activated carbon and its surface chemistry originated due to physical and chemical treatment. In addition, a general introduction to adsorption process, various adsorption isotherms, and adsorption kinetics is also included. A brief description of mechanism of adsorption onto activated carbon is also presented. At last, most probable applications of activated carbon such as adsorption of pollutants (e.g., dyes, heavy metal ions, pesticides, pharmaceutical waste products, and volatile organic organic), as catalyst support, anduse in food and pharmaceutical industries is also presented.

1. Introduction

1.1. Activated Carbon (AC)

Activated carbon (AC) refers to the highly carbonaceous materials having high porosity and sorption ability produced from the wood, coconut shells, coal, and cones, etc. AC is one of the frequently used adsorbents utilized in various industries for the removal of numerous pollutants from water and air bodies. Since, AC synthesized from the agricultural and waste products, it has proved to be a great alternative to the traditionally used nonrenewable and expensive sources. For the preparation of AC, two basic processes, carbonization and activation, are used. In the first process, precursors are subjected to high temperatures, between 400 and 850°C, to expel out all the volatile components. High elevated temperature removes all the noncarbon components from the precursor such as hydrogen, oxygen, and nitrogen in the form of gases and tars. This process produces char having high-carbon content but low surface area and porosity. However, the second step involves the activation of previously synthesized char. Pore size enhancement during the activation process can be categorized into three: opening of previously inaccessible pores, new pore development by selective activation, and widening of existing pores.

Usually, two approaches, physical and chemical, are used for activation to get desired surface area and porosity. Physical activation involves the activation of carbonized char using oxidizing gases such as air, carbon dioxide, and steam at high temperatures (between 650 and 900°C). Carbon dioxide is usually preferred because of its pure nature, easy handling, and controllable activation process around 800°C. High pore uniformity can be obtained with carbon dioxide activation in comparison to steam. However, for physical activation, steam is much preferred as compared to carbon dioxide since AC with relatively high surface area can be produced. Due to the smaller molecule size of water, its diffusion within char’s structure occurs efficiently. Activation by steam has been found to be around two to three times higher than carbon dioxide with same degree of conversion.

However, chemical approach involves the mixing of precursor with activating agents (NaOH, KOH, and FeCl₃, etc.). These activating agents acts as oxidants as well as dehydrating agents. In this approach, carbonization and activation is carried out simultaneously at comparatively lower temperature 300-500°C as compared to the physical approach. As a result, it effects the pyrolytic decomposition and, then, results in expansion of improved porous structure and high carbon yield. Major benefits of chemical over physical approach are the low temperature requirement, high microporosity structures, large surface area, and minimized reaction completion time.

The superiority of chemical activation method can be explained on the basis of a model proposed by Kim and his coworkers [1] according to which various spherical microdomains responsible for the formation of micropores are found in the AC. On the other hand, mesopores are developed in the intermicrodomain regions. Experimentally, they formed activated carbon from phenol-based resin by chemical (using KOH) and physical (using steam) activation (Figure 1). Results showed that AC synthesized by KOH activation possessed high surface area of 2878 m²/g as compared to 2213 m²/g by steam activation. In addition, other factors such as pore size, surface area, micropore volume, and average pore width were all found to be better in KOH-activated conditions as compared to steam activated.

Depending upon the particle size and method of preparation, it can be categorized into three types: powered AC, granular AC, and bead AC. Powered AC is formed from fine granules having size 1 mm with average diameter range of 0.15-0.25 mm. Granular AC has comparatively larger size and less external surface area. Granular AC are used for various liquid phase and gaseous phase applications depending upon their dimension ratios. Third class: bead AC is generally synthesized from the petroleum pitch with diameter ranging from 0.35 to 0.8 mm. It is known for its high mechanical strength and low dust content. It is extensively utilized in fluidized bed applications such as water filtration due to its spherical structure.

1.1.1. Surface Chemistry of AC

Surface chemistry of AC is greatly influenced by the type of heteroatom present on its surface such as nitrogen, phosphorus, sulfur, and oxygen. The type of heteroatom present is determined from nature of activation process used. On the basis of type of group present, the surfaces of AC can be categorized into acidic and basic surfaces.

(1) Acidic AC. Groups present on the exterior surface determine the reactivity of AC. Presence of oxygen-containing compounds on the surface of AC contributes to its acidic character. Since the exterior sites constitutes the majority of adsorbent area, the concentration of such reactive groups influences the adsorption capability of AC to a great extent [2–5]. Oxygen-containing acidic groups include carboxylic, quinone, carbonyl, and pyrone, etc. Carboxylic groups can also exist in the form of carboxylic anhydride [6, 7]. Carboxylic acid, lactone, and phenolic hydroxyl groups are categorized as the sources of surface acidity. AC contains oxygen functionalities by the oxidation of gases and aqueous oxidants [7, 8]. For the gas phase treatment, oxygen, air, steam, and carbon dioxide can be used. During such processes, low temperature oxidations results into the development of strong acidic groups. However, high temperature oxidations create weak acid groups [9]. As compared to gas phase, liquid phase oxidation treatments can be used to introduce large amount of oxygen into the carbon moieties. Mixture of nitric acid and carboxylic acid exhibits strong oxidizing properties for introducing momentous oxygenated acidic functionalities such as carboxylic, phenolic hydroxyl, and lactone onto the AC surface [10]. Findings have validated that oxidation of AC in gas phase enhanced mainly due to the concentration of hydroxyl and carbonyl groups present on its surface; however, oxidation in liquid phase can integrate high amount of oxygen in the form of carboxylic and phenolic hydroxyl groups onto the AC surface at comparatively much lower temperatures in comparison to the gas phase oxidation.

(2) Basic AC. Basicity of activated carbon can be related to the resonating electrons of aromatic carbon rings and basic surface functionalities (e.g., nitrogen-containing groups) that are capable of binding with protons. In addition, some oxygen-containing functionalities such as ketone, pyrone, and chromene can also result into basic AC. In basic AC, present electrons act as Lewis bases. Increase in the quantification of nitrogen-containing groups increases the adsorbing ability of AC towards carbon dioxide. Nitrogen-containing functionalities can be hosted into AC either by reaction with nitrogen-containing reagents (such as NH₃, nitric acid, or amines) or activation with nitrogen-containing precursors. Basic AC interacts with acid molecules generally through dipole-dipole interactions, covalent bonding, and hydrogen bonding.

2. General Introduction to Adsorption Process

Adsorption is a surface phenomenon involving the adsorption of adsorbate molecules onto adsorbent. Basic principle involving the adsorption process is the variable surface energy. Urge to stabilize the surface energy results into interactions between the adsorbent surface and adsorbate molecules. Adsorption of pollutants onto adsorbent depends upon number of factors such as temperature, solution pH, adsorbate concentration, contact time, coexisting ions, temperature, and surface properties of adsorbent including maximum available surface area, pore size, and pore distribution. Adsorption can be classified into two categories: physical and chemical based on the type of interactions between adsorbate and adsorbent molecules. Chemical adsorption is categorized by the chemical interactions involving exchange of electrons between the two phases, thus forming permanent bonds and resulting in an irreversible process. However, physical adsorption involves the electrostatic, π-π interactions, Vander-Waal’s interactions, or H-bonding, resulting into a reversible process [11, 12]. Figure 2 presents the basic terminologies used in adsorption process.

The electrical characteristics of adsorbent surface can easily be determined using two methods; point of zero charge (PZC) and isoelectric point (IEP). PZC determines the solution pH at which the surface charge equals to zero. It can be determined using pH drift method or potentiometric titration, etc. However, IEP occurs when the electrokinetic parameter at shear planes equals to zero. Major difference between the two is that the IEP determines only the external surface charge of adsorbent but PZC determines its internal as well as external surface charge. Difference of the two determines the internal surface charge of adsorbent.

During the adsorption experiments, the efficient performance of adsorbent is determined by its adsorption capacity which can be calculated using equation: where is the initial concentration of adsorbate in mg/L; is the equilibrium concentration of adsorbate in mg/L; represents the mass of adsorbent in grams; and denotes the volume of total reaction solution in liters. is defined to be the solid to liquid ratio. In addition, percent adsorption is also used for denoting the total adsorption occurred during a process which can be calculated using the formula [14]: where represents the initial concentration of adsorbate in mg/L and is the concentration of adsorbate at time (minute) in mg/L. To some extent, percent adsorption causes error in the observation, so mostly, is preferred over it.

Number of techniques has been put forward for environmental remediation purposes such as osmosis, photocatalysis, membrane filtration, and adsorption; out of all, adsorption has been found to be quite effective due to the low cost production of adsorbents, high efficiency, simple designing, and insensitivity to toxic pollutants, etc. This method not even produce toxic side products at the end of reaction. Number of toxic pollutants such as pesticides, organic dyes, metal ions, gases, and PPCPs can be removed using this technique. Table 1 presents the different carbon precursors and their surface area being used for adsorption of various pollutants.

2.1. Adsorption Isotherm

Adsorption isotherms help in evaluating the mechanism of adsorption of adsorbate molecules onto the adsorbent. In addition, models aids in determining the maximum adsorption capacity of adsorbent. Adsorption can be mono-layered or multilayered depending upon the type of interactions between adsorbate and adsorbent that can be easily determined from the Langmuir and Freundlich isotherm models, respectively. In addition to these two, some other isotherm models commonly used are Temkin, Redlich-Peterson, and Dubinin-Radushkevich isotherm model. Table 2 shows the linear equations and significance of various adsorption isotherm and kinetic models.

2.1.1. Langmuir Isotherm Model

Langmuir isotherm model assumes the adsorption of adsorbate molecules onto energetically homogeneous sites. It also generalizes that the interaction occurs by monolayer adsorption onto the adsorbent surface [28, 29]. Equations explaining this model have the linear as well as nonlinear forms and these are: where is the adsorption capacity of adsorbent at equilibrium in mg/g; represents the maximum adsorption capacity in mg/g; is the Langmuir constant in L/mg; and is the equilibrium concentration of adsorbate in mg/L.

2.1.2. Freundlich Isotherm Model

Compared to Langmuir isotherm, Freundlich isotherm model assumes the adsorption of adsorbate molecules onto energetically heterogeneous sites. This model explains the multilayered adsorption of adsorbate. This model can be represented by the nonlinear equation as:

The linearized form of Freundlich isotherm model is where is the amount of metal ion adsorbed at equilibrium time (mg/g), is the equilibrium concentration of dye in solution (mg/L), is the capacity of the adsorbent, and is the intensity of adsorption constant for Freundlich. The plot of ln versus ln is employed to determine the and from intercept and slope, respectively.

2.2. Adsorption Kinetics

Number of governing mechanisms such as mass transfer coefficient, chemical reaction, or diffusion control generalize the adsorption kinetic models. Kinetic studies help in determining the best reaction conditions for undertaking the adsorption experiment for full-scale batch process. Adsorption kinetics demonstrates the rate of solute uptake and the mechanism of how this rate govern the residence time of adsorbate at the interface of solution. Number of models can be utilized for analyzing the kinetic data such as seudo-first-order, pseudo-second-order, intraparticle diffusion model, and Elovich model.

2.2.1. Pseudo-first-Order Kinetics

Pseudo-first-order kinetic model explains the adsorption at the solid-liquid interface. The linearized form of pseudo-first-order kinetic model can be formulated as [30]: where and represents the adsorption capacity of adsorbent at equilibrium and time in mg/g, respectively, is the pseudofirst-order rate constant, and is the contact time (minute). Linear plot of log (-) versus gives the value of and which can be calculated from its slope and intercept, respectively.

2.2.2. Pseudo-second-Order Kinetics

Pseudo- second-order kinetic model assumes that the rate-limiting step in adsorption is controlled by chemical interactions between adsorbate and adsorbent molecules. Differential form of this model can be explained by the following equation [31, 32]: where represents the pseudo-second-order rate constant (g (mg min)^-1). The linearized form of this model is [33]:

Linear plot of versus gives the value of and which can be calculated from its slope and intercept, respectively.

2.2.3. Intraparticle Diffusion Method

The intraparticle diffusion model specifies that rate-limiting step involves the transference of solute from the solution into the adsorbent pores through an intraparticle process. Weber and Morris formulated that for most of the adsorption processes, amount of adsorption varies proportional with inspite with the contact time, and this can be expressed as [34]. where is the adsorption capacity at time , is the half-life time in second, and (mg/g min^0.5) is the rate constant of intraparticle diffusion. To find out the rate constants, plot versus gives a linear relationship, and can be determined from the slope of the plot.

2.3. Thermodynamic Studies

Thermodynamic studies help in determining (a) type of adsorption; either physical or chemical (b) randomness; (c) exothermic or endothermic nature; and (d) spontaneity of the adsorption reaction taking place. All these factors can be determined from the following presented equations [33, 35]: where is the universal gas constant 8.3144 J/(mol × K) and is the operational temperature in kelvin. Value of (Gibbs free energy change) can be calculated directly from Equation (8); however, (Change in entropy) and (change in enthalpy) are determined from the intercept and slope of the thermodynamic graph plotted between ln and . In addition, is determined from the adsorption-isotherm constants. is expressed in J/mol and temperature in .

3. Mechanism of Adsorption onto AC

Mechanism of adsorption of various pollutants onto AC is very important to be elaborated. Mainly four processes are involved; (a) bulk transfer, (b) film diffusion, (c) pore diffusion, and (d) intraparticle diffusion. Bulk transfer involves the transport of adsorbate molecules in the solution phase and occurs instantaneously. In the film diffusion form, the adsorbate molecules are transferred to the external surface of adsorbent molecule by hydrodynamic boundary. Pore diffusion involve the transference of adsorbate molecules into the pores of adsorbent, thus decreasing the overall adsorption rate. Lastly, intraparticle diffusion involves the diffusion of adsorbate molecules from exterior of adsorbent into the pores of adsorbent and also along the pore-wall surfaces [36]. Figure 3 presents the probable adsorption mechanism.

Various interactions play an important role in their adsorption such as bonding, electrostatic, and π-π and n-π interactions [37]. π-π interactions, also known as π-π electron donor-acceptor interactions, exist between π-electrons of AC and adsorbate molecules. Presence of electron withdrawing groups at the edges and surface of AC results into appreciable drop in the π electron density. Removal of metal cations by AC is generally favored by Cπ-cation interactions that exists between the π-electron cloud of AC and metal cations. For example, in 2015, Tran et al. synthesized biochar having properties similar to that of AC from orange peels by pyrolysis. Adsorption of cadmium ions onto biochar surface followed the Cπ-cation interactions with maximum adsorption capacity of 114.69 mg/g [38]. The adsorption of metal cations has been found to be strongly dependent upon the solution pH as observed by Bui and Choi [39].

In addition to the above-reported interactions, hydrogen bonding also plays an important role in the adsorption of organic aromatic compounds onto AC. These interactions exist between the functional groups such as N-H in atrazine and O-H in paracetamol of the adsorbent and AC. Considering particular mechanism, O-H groups present on the surface of AC form hydrogen bond with the functional groups present in the organic adsorbates. Net intensity of the interactions regulates the magnitude of adsorption. In this mechanism, some complexes between solvent molecules and surface oxides are formed, which could block the migration of the solute molecules from the outside to the micropore structure of AC [40]. These interactions are highly temperature dependent as they vanish with the increase in temperature due to rise in kinetic energy of the adsorbed molecules.

4. Adsorption of Various Pollutants onto Activated Carbon-Based Materials

4.1. Heavy Metals

Metals having atomic weight between 63.5 and 200.6 g/mol are categorized into heavy metals [41]. Fertilizer, paper, pesticide, tannery, and battery industries are the major contributor for the addition of heavy metals into the environment directly or indirectly. Heavy metals are non-biodegradable in nature and accumulate in living organisms and most of heavy metal ions are known to be toxic or carcinogenic in nature such as mercury, cobalt, cadmium, and lead [42–44].

Zinc is a transition metal with atomic number 30 which is quite essential for human health. Intake of zinc in excess amount causes serious issues such as skin irritations, stomach cramps, nausea, vomiting, and anemia. Another one is mercury which is neurotoxin that causes damage to the central nervous system. High concentrations of mercury cause impairment of pulmonary and kidney function, chest pain, and dyspnea, etc. One most common disease caused by the presence of mercury is Minamata Bay [45]. In addition to mercury, lead can also cause central nervous system damage. It can also affect the kidney, liver, and reproductive system and malfunctions the brain activities. The toxic side effects are anemia, headache, dizziness, insomnia, weakness of muscles, and renal damages [46]. Chromium occurs in environment in mainly two states: Cr(III) and Cr(VI), out of which is Cr(VI) which is highly toxic [47]. It affects human physiology, gathers in the food chain, and causes many health issues ranging from simple skin irritation to lung carcinoma [48]. Table 3 summarizes the adsorption of heavy metal ions onto activated carbon derived from various sources with their maximum adsorption capacity and reaction conditions.

Metal ions interact with AC through Cπ-cation interactions and thus get adsorbed onto its surface and into the pores. A number of papers have been reported in the literature that mark the utilization of Cπ-cation interactions for the adsorption of heavy metal ions onto AC. Production of AC from rapeseed was used for the removal of lead ions from aqueous effluents. For providing an evidence for the involvement of Cπ-Pb(II) interactions, FTIR analysis was carried out. Disappearance of characteristic peak of C=O at 1710 cm^-1 after adsorption of Pb(II) ions showed the complexation with Cπ-cation interactions [49].

In another study, AC synthesized from coconut shell was utilized for the adsorption of Pb(II) ions from aqueous solution. Adsorption was reported to be strongly dependent upon solution pH, and maximum was observed at pH 4.5 with maximum adsorption capacity of 26.50 mg/g. Reaction was found to be endothermic as generalized by thermodynamic analysis [50]. AC synthesized from coirpith was chemically activated and utilized for the adsorption of Ni(II) ions from aqueous solution. The concentration ratio of carbon/nickel ions affected the adsorption rate to a great extent. pH studies revealed the maximum adsorption of Ni(II) ions in the basic medium and consequently showed desorption ability in acidic environment of HCl. Maximum adsorption capacity of coirpith-designed activated carbon was 62.5 mg/g [51].

Adsorption of Pb(II) and Cd(II) ions onto AC preactivated by physical (steam) and chemical treatment (H₃PO₄ and KOH) was studied by El-Hendawy. Out of the three activation treatment processes, H₃PO₄ chemical treatment activation was found to be the most effective with maximum adsorption capacity of 139 mg/g for Pb(II) and 129 mg/g for Cd(II). Pb(II) ions were removed or adsorbed in preference to Cd(II) ions due to their lower solubility and lower pH requirement for complexation [52]. Another group of researchers, Ameh synthesized activated carbon by chemical treatment with NaOH and utilized it for the adsorption of Cu(II) and Cd(II) ions. Maximum adsorption capacity calculated from Langmuir isotherm for Cu(II) and Cd(II) ions was 118.06 and 88.42 mg/g, respectively. Adsorption capability was found to be strongly dependent upon the adsorbent dosage [53].

4.2. Pesticides

Pesticides are defined as the group of chemical or biological compounds having antimicrobial and disinfectant properties applied intentionally against various microbes and pests comprising insects, weeds, birds, mammals, and nematodes, etc. It help in increasing the productivity, processing, storage, and transport of diverse variety of food [60]. Specifically, pesticide is characterized by its exceptional chemical structure established to mimic and substitute specific molecules to the targeted pests, which offer a lethal disruption to the anticipated biological reactions [61]. Pesticides actually helps in diminishing the effect of unwanted microbes or other agents and helps in fulfilling the ever increasing food demands due to increasing world population. These can be applied to the agricultural field in the form of herbicides, insecticides, ovicides, rodenticides, fungicides, avicides, nematicides, and bactericides, etc [62, 63]. Pesticides can be classified in accordance to their physicochemical properties and targeted implications as presented in Figure 4.

Recent decades have marked the advent of pesticide practice as an agricultural repellant, fumigant, defoliant, or sterilant and has inferred as the most challenging tasks, due to which gained high momentum and popularity across the world [64]. Diverse purification or treatment techniques such as precipitation, flotation, sedimentation, membrane processes, filtration, coagulation–flocculation, biological process, adsorption, electrochemical techniques, chemical reactions, and ion exchange with variable level of success have augmented a dramatic progress in the scientific community [65–68]. Among them, adsorption process is documented as the most effective and auspicious approach in the wastewater treatment processes. Table 4 shows various forms of AC used for removal of pesticides with their maximum adsorption capacity.

2,4-Dichlorophenoxyacetic acid (2,4-D) is an herbicide that is used for the careful control of broadleaf weeds. Hameed et al. studied its removal from aqueous solution by employing AC treated with KOH/CO₂ for physiochemical activation. Various adsorption isotherms were employed, and results showed that Langmuir data fitted best. Maximum adsorption capacity calculated was 238.10 mg/g. Herbicide initial concentration was of greater influence as compared to solution pH and other factors [69]. In 2006, Chingombe et al. [70] described the heat treatment via annealing the AC in a reactor that enhanced the micropore content, thus increasing the surface area from 790 to 960 m²/g due to the reduction in oxygen-containing functional groups. Such enhancement in surface area increased the uptake of 2,4-dichlorophenoxyacetic acid and benzolin onto its heat-treated AC. Adam et al. investigated the adsorption of a fungicide, propiconazole under two different activation treatments, one by basic treatment (NaOH) and other by acidic treatment (HNO₃ and H₂O₂). Results presented that acidic treatment of AC decreased its adsorption abilities. Actually, acidic groups increased the oxygen-containing functionalities onto the surface that showed negative effect on adsorption of propiconazole as a result of which the adsorption capacity decreased [71]. Yoo et al. stated that nitric acid-treated AC increased the concentration of carboxylic groups onto its surface for the treatment. However, this treatment did not mark any strong effect on the adsorption rate of oligosaccharides (carbohydrate compounds) from aqueous solutions [72].

4.3. Pharmaceutical and Personal Care Products (PPCPs)

Number of possible routes has been put forward for designating the sources responsible for the enhancement of pharmaceutical pollution which includes incomplete metabolized and excreted drugs in the urine and feces, which enter into the water system. Other feasible source is the spare, or expired drugs, that may be dumped into toilets. Few drugs are not easily degradable and thus persist in the water system. Adverse effects of PPCPs include aquatic toxicity, genotoxicity, and endocrine disruption. Additionally, presence of trace amount of PPCPs in drinking water is also a public health concern, as little is known about probable chronic health issues related to long-term ingestion of mixtures of such compounds through drinking water. Table 5 presents the adsorption of various pharmaceutical and personal care products onto AC.

Salman with his coworkers studied the removal of bentazon and carbofuran pesticides from aqueous solution. For this, they synthesized date seed-based AC with potassium hydroxide and carbon dioxide used as the activating agents [84]. Freundlich model for both pesticides fitted better to the adsorption equilibrium data as compared to Langmuir model. Maximum adsorption capacities as calculated from Langmuir isotherms were 86.26 and 137.04 mg/g for bentazon and carbofuran, respectively. Due to smaller size of carbofuran as compared to bentazon, it easily penetrated into the pores of AC thus offering high removal efficiency. Pesticide initial concentration was found to be the more influential factor as compared the solution pH.

4.4. Dyes

Dyes are the colorful compounds designed intentionally to give hue to fabrics and paper. These have the tendency of attaching to any responsive material. These have been utilized since the past many centuries. In the initial times, these were derived from the natural sources such as plants, insects, or microbes, but one major problem with these dyes was their short-term application part. Under the influence of sunlight and temperatures, these dyes faded away which led to the need for the emergence of synthetic dyes. Synthetic dyes are the laboratory-synthesized dyes first invented by WH Perkins in 1856 that solved the issue of persistency of dye color. Such synthetically produced dyes were structurally stable and difficult to degrade [94–96]. With their emergence, industries started utilizing them as their major product constituent and disposed them off into water system after use without any treatment process that raised serious environmental and health issues. Relative proportion of dye effluents added by various industries to the environment has been presented in Figure 5. Although it seems that resuming the usage of natural dyes are a better alternative, it was found to be more bad as employing synthetic dyes alone because in order to increase the bonding capability with the fabrics, a mordant is required. Mordants are basically binding agents that help in the attachment of natural dye to materials. These are much more toxic and dangerous than that of synthetic dyes.

Dyes can be classified into numerous classes based on various categories such as structure, color, application, and chemical composition [34]. Dyes classified on the basis of chemical structure are presented in Figure 6 [34]. In addition, these can also be categorized on the basis of particle charge acquired upon dissolution in aqueous medium [97–99], for example, cationic, anionic, and nonionic.

Dye-utilizing industries purge dye effluents into the water sources without any treatment process. Water pollution due to dyes is objectionable to environmentalists due to their toxic and hazardous nature [100, 101]. Addition of concentrated dye effluents to water bodies with acidic characteristics and high temperature disturbs the oxygen transfer mechanism and the self-purification ability of environmental water bodies. Mixing of water bodies with dye effluents also results into turbidity that forms a visible foam layer above the water surface due to which it blocks the penetration of sunlight into water system, thus limiting the requirement of sunlight by aquatic species for photosynthesis and respiration. Presence of dye effluents in the water bodies becomes a breeding ground for various microbes such as bacteria and fungi, and their presence in water makes it unsuitable for consumption. Recently, a law governing the level of presence of these effluents has been established by the International Dye Industry Wastewater Discharge Quality Standards according to which industries have to ensure the wastewater released into the water bodies [102–104]. Table 6 summarizes the removal of dyes by various AC-based absorbents.

Adsorption of dyes onto AC is mainly influenced by the surface properties. It is an amphoteric material having ability to exhibit cationic as well as acidic characters. Its interaction with the dye molecules is mainly influenced by the solution pH that makes the surface to behave differently in different solution pH. In 2007, activated carbon prepared from groundnut shell by chemical treatment with ZnCl₂ was utilized for the adsorptive removal of malachite green from aqueous solution [125]. High surface area of 1200 m²/g favored the adsorption of malachite green onto it. Maximum adsorption capacity of 114 mg/g was calculated. In another work, cotton fiber was used as the carbon precursor, and ZnCl₂ was employed as the activating agent. As designed cotton fiber-activated carbon (CFAC) was utilized for the removal of methylene blue from aqueous solution, BET surface analysis determined the high surface area, 2060 m²/g of CFAC with maximum adsorption capacity of 597 mg/g. Adsorption data fitted best to the Langmuir isotherms. Study showed that acidic pH favored the adsorption process. Chemical interactions favored the adsorption of methylene blue onto CFAC as generalized by pseudosecond kinetic model [126].

In addition to agricultural products, microorganisms such as E. prolifera were also used for the preparation of carbon. It was treated with NaAlO₂ for chemical activation to add functionalities onto its surface and make it reactive. Activation by NaAlO₂ endorsed it with high surface area of 1374 m²/g. It was successfully employed for the adsorptive elimination of brilliant scarlet from aqueous solution. High adsorption capacity of 1000 mg/g was obtained as calculated by Langmuir isotherm [127].

A number of activated carbon precursors have been reported in the literature designed particularly for the removal of methylene blue such as Albizia lebbeck [128], hazelnut husk [129], macadamia nut endocarp [130], oil palm wood [131], olive stone [132], acacia fumosa seed [133], vetiver roots [134], waste tea [135], and date stones [136].

4.5. Volatile Organic Compounds

Volatile organic compounds (VOCs) are the chemical compounds documented as the major cause of air pollution. VOCs such as phenol, benzene, formaldehyde, ketone, butane, xylene, toluene, and chlorinated compounds are quite dangerous since these are carcinogenic, mutagenic, and toxic. Increase in industrialization has led to the increase in VOC emission into the environment. Other sources of VOCs in environment include transportation, construction, and natural emissions. Usually, these are present in higher concentration near petrochemical plants as compared to the suburban areas. Presence of VOCs in the environment leads to global warming, ozone depletion, acid rain, and photochemical smog. In human beings, they cause skin irritation, liver damage, liver malfunctioning, and asthma. There has been a rapid increase in concentration of VOCs in the environment, such as, in China, the total amount of VOCs has increased from 1.15 in 1980 to 13.35 Tg in 2010 [137].

Number of techniques has been suggested in literature for VOC removal such as catalytic oxidation, photocatalytic oxidation, reverse osmosis, ion exchange condensation, thermal oxidation, biological treatments, and adsorption. Out of all, adsorption process is the simplest, efficient, and economical alternative to this since it offers high degree of operation stability, no toxic by-product formation, no additional treatment requirement, and cost effectiveness. A number of articles have been published in literature for VOC removal by activated carbon. AC prepared from avocado kernels and activated by carbon dioxide possesses surface area of 206 m²/g. It was utilized for the removal of phenol. Results presented that slightly acidic pH favored the adsorption, and maximum adsorption capacity obtained was 90 mg/g [138]. Chemical interactions favored the adsorption of phenol onto AC as generalized by kinetic studies. Durian shell-derived activated carbon was also reported in literature, activated by H₃PO₄ which possessed high surface area of 1404 m²/g. It was used for adsorptive removal of toluene with maximum adsorption capacity of 874 mg/g. Isotherm studies showed that Freundlich isotherm explained the adsorption process better in comparison to Langmuir isotherm [139]. Kenaf rapeseeds were also utilized as the carbon precursors. It was then activated by carbon dioxide which endorsed it high surface area of 1112 m²/g. It was used for the removal of phenol, and maximum adsorption capacity as recorded by Langmuir isotherm was 84.1 mg/g. Results showed that adsorption was strongly dependent upon solution pH, and maximum adsorption was observed at pH 10 [140].

5. Activated Carbon as a Catalyst Support

Overall activity and reactivity of any catalyst depends upon the chemical composition, surface area, and physical properties. Any active material or phase can be imprinted onto a suitable support to provide it high surface area and improve its mechanical strength. In addition, extremely dispersed active phase can be stabilized by depositing it into the micropores of support. A synergistic relation exists between the active phase and support that enhances its catalytic activity. It helps in preventing the agglomeration rate. Most important parameter during catalyst designing is the proper dispersion of active phase onto the support that is described as the ratio of number of surface atoms of active phase to the total number of atoms present.

Number of materials can act as a support, and one among them is the AC that is extensively utilized due to nontoxic, cheap availability and easy synthetic routes. It acts as a sink for the produced electron-hole pairs and thus helps in lowering the recombination of e^--h⁺ pairs. However, their activity gets highly influenced by the presence of impurities, also called catalyst poison such as copper, zinc, and iron. Major advantage of AC as a catalyst support is the fact that its pore structure and internal surface both can be widely varied. It consists of small area of disordered graphitic structure elements. Their catalytic activity is determined by electron transfer from the edges of carbon sheets. Therefore, the presence and distribution of these active sites are important. The catalytic activity of the graphite structure can be changed by the presence of heteroatoms such as sulphur and nitrogen in the lattice. Its activity also got influenced by the presence of acid or base surface oxide complexes. Key features of AC as the support material include (a) high resistance to acidic or basic mediums; (b) designed pore size distribution for particular reactions; (c) minimized cost as compared to silica and alumina supports; and (d) stability at high temperatures.

Activated carbon and its composites can be utilized as a catalyst or catalyst support for various chemical reactions such as hydrolysis, hydrogenation, esterification, and hydrogenation. The carbon catalyst having amorphous morphology and highly reactive groups such as -COOH, -OH, and -SO₃H acts as an efficient catalyst for the hydrolysis of pure cellulose to glucose which was not possible with conventionally used catalysts such as Amberlyst-15, Nafion, and H-mordenite. The carbon catalyst enhanced the hydrolysis rate due to its low activation energy requirement. High catalyst performance is accredited to its tendency to adsorb 1,4-glucan [141]. Figure 7 shows the conversion of cellulose to glucose by hydrolysis.

Hydrodeoxygenation reactions is another branch where carbon acts as a catalyst or as its support. Such reactions involve the conversion of carbohydrates to unsaturated compounds by catalyst favoring the cleavage of C-O in comparison to C-C with minimized amount of H₂ utilization. MXenes such as Mo₂C supported on activated carbon helps in the conversion of vegetable oil such as rapeseed, olive, and soya bean to diesel-range hydrocarbons [143].

Hydrogenation reactions involves the treatment of compounds with molecular hydrogen. Hydrogenation being carried out at industrial scale involves the reduction of nitro groups in nitroaromatic compounds to reduce the chances of production of other compounds such as aniline. Hydrogenation of nitrobenzene was studied by Pt catalyst supported on AC in 1977 [144]. Results presented show that catalytic activity of Pt got enhanced by its impregnation onto AC. Catalytic activity was found to be highly dependent upon the dispersion of Pt onto AC, pore structure of AC, chemical composition of AC surface, and temperature also. Linear relationship between activity and Pt dispersion (calculated at equivalent reduction temperature) has been found.

AC also acts as support material in case of oxidation of phenol. Shukla and his coworkers designed a catalyst synthesized by impregnation of Co(II) ions onto AC and utilized it for the activation of peroxymonosulphate that carried out the oxidation of phenol. Co(II) ions were uniformly distributed onto AC surface. Results indicated that Co/AC demonstrated high activity in oxidation of phenol with sulphate radicals and 100% decomposition and 80% TOC removal could be achieved in 60 minutes [145].

6. Activated Carbon in Food and Pharmaceutical Industries

Activated carbon and its various derivatives have found to be used in food and pharmaceutical industries. AC utilized as acidulants found a prominent use in food and pharmaceutical industries as a pH regulator, chelating agents, and antioxidant synergist. AC as acidulants helps in decolorizing maleic acid and adipic acid prior to their conversion and crystallization. Sodium benzoate and sodium sorbate are used as preservatives in food, beverage, and pharmaceutical industries. Granular AC helps in decolorizing and deodorizing them even in less than 1% dosage during manufacturing [146].

Due to high purity and performance, AC has also been found to be a proficient adsorbent during pharmaceutical processing applications that entail highly efficient filtration system. This safeguards soluble minerals with low acid content that lessens the adulteration of the final product with negligible impact on solution pH. Due to exclusive adsorptive properties and bio-origin endorse AC with antitoxin properties also, it quickly enables to lessen the effects of poisons or overconsumption of stimulants, through aid with digestion and sensitive alimentary conditions. In veterinary medicine field, AC is extensively employed as a feed additive. It functions as a curative agent the same way as in human medicine by adsorbing undesirable or potentially harmful substances from animal’s gastrointestinal tract, making it an effective treatment for poisoning and diarrhea.

7. Activated Carbon as Storage System for Gases

Increase in industrialization has led to the enhancement in the presence of gases (above their permissible limit) such as carbon dioxide (CO₂), sulphur dioxide (SO₂), hydrogen (H₂), and methane (CH₄) in the environment.

Carbon dioxide (CO₂) is the major reason for temperature increase of earth, and it also contributes to the ozone layer depletion. Major sources of CO₂ in the atmosphere are the industrial practices, fuel burning, and deforestation. Researchers have suggested the adsorption and sequestration of CO₂ from the point of its origin. A number of techniques have been put forward for CO₂ capturing such as membrane separation, cryogenic techniques, liquid solvent absorption, temperature adsorption, solid sorbents, and pressure adsorption [6, 147]. Adsorption is categorized as the most effective one due to its ease of operation, cost effectiveness, and economic processing. CO₂ adsorption onto AC requires the presence of basic functionalities such as the nitrogen-containing functional groups on its surface that favor the maximum CO₂ capturing [148]. When the carbon materials are treated with ammonia at ambient temperatures, ammonia decomposes to form free radicals (-NH₂, -NH, and atomic hydrogen and nitrogen). These generated free radicals attack the carbon surface to form nitrogen-containing functional groups [149]. CO₂ capturing can be successfully determined using various techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD).

Methane has established significant consideration as a future clean energy source due to its natural profusion, relative safety (compared to other fuels), cost effectiveness, and low carbon emission. For convenient storage solicitations in future transportation technologies, intended materials must offer extraordinary storage aptitude, high storage, and release kinetics and long cycle effectiveness under modest thermodynamic conditions [150]. It has been found that CH₄ storage using AC synthesized from polymers, hardwood, and coconut shell is equivalent to the compressed natural gas at 250 bar, with high methane uptake of approximately 200 at 27°C and pressure between 34 and 48 bar [151].

In addition to methane, hydrogen is also a supreme fuel for future generations. Hydrogen infrastructure play an essential role in the commercialization of fuel cells. The U.S. Department of Energy (DOE) has fixed the necessity of 5.5 wt% H₂ for gravimetric and 40 g H₂/L for volumetric systems [152]. For acquiring convenient hydrogen storage systems to function fuel cells using H₂, storage containers having high volumetric and gravimetric density are required. Now-a-days, conventional technologies for H₂ storage/transport are compression, cryogenic and cryo-compressed storage, metal hydrides, and activated carbon. Table 7 avails the summary of storage conditions of various gases onto AC.

8. Regeneration of AC

Regeneration is very important as it helps in increasing the productive life of the adsorbent and enhances its practical utility. Considering the regeneration of AC, it can be done by various methods, and some of them are explained below.

8.1. Thermal Regeneration

This method involves the temperature treatment which is accomplished using fixed bed furnaces, fluidized bed furnaces, and rotary kilns. Under this, adsorbed gases such as carbon dioxide, partially oxidized contaminants, and particulate matter can be removed from the adsorbent surface. It includes gasification of organism pollutants, drying at 105°C, and pyrolysis under inert atmosphere. However, due to the high-temperature conditions, the adsorption efficiency of adsorbent gets drastically affected. Although thermal regeneration is quite a simple regeneration method, it is an energy- and time-consuming way and also leads to significant amount of C loss. In addition, numerous hazardous by-products can also be produced during the implication of this method [160].

8.2. Chemical Regeneration

In this method, certain chemicals are used for the regeneration of adsorbent by desorbing the adsorbed molecules from the adsorbent surface. The proficiency of this method depends strongly on the type of contaminant present on or in the spent of AC. This method typically leads to zero carbon-attrition and helps in maintaining the actual adsorption capacities of AC. The affinity of adsorbed molecules with that of the acidic or alkaline reagents used determines the extent of regeneration. The efficiency of this type of regeneration therefore depends on the solubility and the reactivity of the adsorbents with the chemical reagents [161].

8.3. Electrochemical Regeneration

This technique is quite efficient as it involves the use of moderate temperature and requires no addition of chemicals. In this method, organic contaminants adsorbed on the surface of AC get cracked in situ while the structural integrity and characteristics of the carbon are not altered. The major mechanisms include electrooxidation, electroreduction, and electrosorption [162].

8.4. Bioregeneration

It makes the use of microbial agents for regenerating the adsorption capacity and surface activity of AC. Bioregeneration occurs when the organic contaminant species can be easily desorbed. The concentration gradient of the organic contaminants between the adsorbent and the bulk liquid aids the bioregeneration. This two-step process of desorption and biological removal of the adsorbed organics has been verified in a closed-loop recirculating batch system [163]. It has been found that biodegradation occurs due to the presence of concentration gradient of the pollutant as one moves from the carbon surface to the bulk solvent. There are many factors that crucially affects the biogeneration of AC such as carbon activation method used, porosity of AC, concentration gradient and carbon saturation, type of microorganism used, and concentration of biomass. Major target compounds for the adsorption bioregeneration are synthetic organic compounds, explosive-contaminated ground water, industrial wastewater, and residual disinfectants [164].

9. Conclusions

An overall review has been presented on the activated carbon. Chemical and physical approach treatments have been discussed in detail. These treatments influence the overall activity of activated carbon towards various pollutants. Data presented emphasize the need for the presence of basic functionalities on activated carbon surface for the adsorption and storage of carbon dioxide. Adsorption capacities of activated carbon depends upon the surface area, pore size and volume, surface functionalities, and mineral matter content. Different precursors can be used for the activated carbon synthesis such as fossil fuels, agricultural wastes, and lignocellulosic. Precursors of biobased origin offer economical and easy availability that serves as a potent alternative to activated carbon derived from fossil fuels. Due to its attractive properties, it serves as a potential applicant for the adsorption of heavy metals, dyes, pesticides, and pharmaceutical effluents. Activated carbon also helps in the storage of various gases such as carbon dioxide, methane, and hydrogen. Despite such high application fields, we think its applications can still further be explored in various other fields such as antimicrobial, photocatalysis, and hydrogen production.

Abbreviations

AC:	Activated carbon
PZC:	Point of zero charge
:	Adsorption amount at equilibrium
:	Initial concentration
:	Equilibrium concentration
%:	Percent
C:	Celsius
g:	Gram
m:	Meter
PPCPs:	Pharmaceutical and personal care products
VOCs:	Volatile organic compounds
TPD:	Temperature-programmed desorption
XPS:	X-ray photoelectron spectroscopy
FTIR:	Fourier-transform infrared
DSAC:	Activated carbon derived from date stones
2,4-D:	2,4-Dichlorophenoxyacetic acid
:	Volume
:	Maximum adsorption capacity
CFAC:	Cotton fiber-activated carbon
:	Langmuir constant
:	Freundlich constant
DOE:	Department of Energy
IEC:	Isoelectric point
:	Temperature
:	Enthalpy change
:	Gibbs free energy change
:	Entropy change.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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