Role of Activated Carbon in Water Treatment

Abstract

Heavy metals, such as lead, mercury, zinc, aluminum, arsenic, nickel, chromium, and cobalt, are the common pollutants present within the environment from various natural and Industrial sources. Synthetic dyes are commonly used for dyeing and printing in a variety of industries. The traditional methods for the removal of heavy metals and dyes from wastewater are chemical precipitation, ion exchange, adsorption, membrane processes, and evaporation which require high capital investment and running costs. Activated carbon prepared from agricultural wastes and its by-products are good alternative sources for adsorption because they are low-cost, renewable sources with high carbon, volatile contents, low ash, and reasonable hardness. The preparation means of activated carbon are physical and chemical methods. The important advantages of chemical activation over physical activation are the process that can be accomplished even at lower temperatures and the yield obtained in chemical activation tends to be greater since burn-off char can be avoided. In this chapter, the removal of heavy metals and dyes, using activated carbon, which was prepared by using agricultural waste, biomass was presented. This helps the researchers to accumulate knowledge.

1. Introduction

Disposal of dye effluents from various industries containing heavy metals to water bodies causes water pollution. Due to the scarcity of water recycling, wastewater has become a worldwide concern for the past few decades. It is well known that heavy metals in water are harmful and cause toxic effects to human beings when it is consumed and affects the environment. Dyes are the major cause of water pollutants arising from dye manufacturing and textile industries. The waste chemicals and dye-house effluents liberated from industries must be treated properly to minimize the effects on the environment. Many traditional methods of separation, such as physical and chemical treatment, including coagulation, adsorption, filtration, precipitation, electrodialysis, oxidation, and membrane separation, have been used for the treatment of dye-containing effluents. The adsorption process is one of the best effective and cheaper methods of removing pollutants from wastewater. Green adsorbents used nowadays are high-cost and rare. Therefore, the adsorption process requires an up-gradation in its limitations. The adsorbent employed in the process should be inexpensive and readily available. Activated carbon prepared from agricultural biomass is an adaptable adsorbent because of its eminent properties, such as large surface area, pore volume, diverse pore structure, extensive adsorption capacity, and a high degree of surface reactivity. Due to large surface area and pore volume of the activated carbon, it can be employed in the removal of color, odor, and taste from water and wastewater. It can also be applicable for the recovery of natural gas and air purification in inhabited spaces, such as chemical industries, and it can act as catalyst and catalyst support material [1, 2]. Activated carbon can be prepared from various carbonous source materials, such as agricultural waste and textile waste. The adsorptive, chemical, structural, and catalytic properties of activated carbon were not only determined by the fundamental nature of the source, but also depends on the method of preparation and conditions used during the process. The preparation of activated carbon from carbonaceous raw material involves a series of processes that has to be done with almost care.

2. Carbonization and activation

Carbonization and activation are the most crucial steps for activated carbon production because these two processes determine the main surface properties and porous structure of the adsorbent. During carbonization, non-carbon and volatile carbon species are removed. An elementary pore structure with a fixed carbon mass is produced. The carbonized material obtained will be an elementary graphitic crystallite with a disordered and poorly developed porous structure. The process is usually achieved at temperatures below 800°C in a gaseous environment without any existing oxidants. The parameters which determine the quality and yield of the carbonized product are rate of heating, final temperature, processing time at the final temperature, and the nature like physical state of the carbonaceous precursor. The activation process increases the pore volume of the material, also enlarges the width of the pores formed during carbonization, and develops new pores of carbonized materials. Due to this, the property of the adsorbent will be increased after the activation process.

2.1 Physical and chemical activation

Physical and chemical activation are traditional processes to improve the properties of the adsorbents. The physical activation is usually carried out at temperatures between 800 and 1000°C. It takes place in the presence of oxidizing gases like steam, carbon dioxide (CO₂), air, or a mixture of these gases [3, 4, 5, 6]. Commercially prepared activated carbon uses steam activation due to its cost-efficiency. However, CO₂ activation develops a narrow micropore in the early stage of activation, whereas steam activation widened the initial microporosity from the beginning. After the process, activated carbon obtained will have lower micropore volume and larger meso and macropore volumes.

During chemical activation, carbonization and activation are carried out in a single-step process. The raw materials infused with chemical agents during chemical activation are thermally breakdown in between 300 and 800°C. The most commonly used reagents for chemical activation are zinc chloride (ZnCl₂), phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄), and alkaline salts, such as potassium hydroxide (KOH) and sodium hydroxide (NaOH). These reagents serve as oxidants and dehydrating agents so that carbonization and activation can take place simultaneously. Activated carbons prepared using KOH (aq) and NaOH (aq) activation obtained a surface area of 2000 m² g⁻¹ have been [7, 8, 9]. Chemical activation using H₃PO₄, ZnCl₂, or KOH and physical activation using CO₂ can also develop activated carbon with very high surface area and pore volume [9, 10, 11, 12]. Activated carbon prepared from corncob waste biomass has a pore volume of 1.533 cm³ g⁻¹ and a surface area of 2844 m² g⁻¹. It was obtained due to chemical activation with KOH at a KOH/char ratio of 4, followed by 30 min CO₂ gasification. An important merit of using chemical activation is that it can proceed at a lower temperature and takes less time when compared with physical activation. However, the demerit of chemical activation is further treatment or process required for reusing the leftover chemical reagent.

3. Structure of activated carbon

3.1 Porous structure activated carbon

The higher adsorption capability of activated carbon mainly depends on porous characteristics such as surface area, pore size distribution, and pore volume. Up to 15% of ash content is present in activated carbon in the form of mineral matter. The porous structure of activated carbon forms during the carbonization process and it further develops during the activation process. All activated carbons have different porous structures. The pore system of activated carbon differs from one another, and individual pores may vary in shape and size. Activated carbons possess pores from less than a nanometer to thousand nanometers. Pores are classified according to their average width. The distance between the walls of a slit-shaped pore or the radius of a cylindrical pore is an average width. Conventional classification of pore and width is proposed, and it is officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) [13]. The pore type and its width are shown in Table 1.

Type of pore	Width
Micro	<2 nm
Meso	2−50 nm
Macro	>50 nm

Table 1.

Classification of pore.

3.2 Crystalline structure of activated carbon

The microcrystalline structure of activated carbon develops during carbonization. Activated carbon structure is entirely different when compared to graphite. The interlayer spacing is different in graphite than in activated carbon. The interlayer spacing of graphite is 0.335 nm, whereas in activated carbon the interlayer spacing is 0.34 to 0.35 nm. Based on the graphitizing ability of activated carbons, they are classified into two types: graphitizing and non-graphitizing carbons. The graphene layers are oriented parallel to each other in graphitizing carbon. The carbon obtained was delicate due to the weak cross-linking between the neighbor micro crystallites and had a less developed porous structure. The non-graphitizing carbons are hard in nature. Strong cross-linking between crystallites in non-graphitizing carbons shows well-developed micropores structure. The formations of non-graphitizing structures with strong crosslink’s are promoted by the presence of associated oxygen or by the insufficiency of hydrogen in the original raw material. The structural differences between graphitizing and non-graphitizing carbons are shown in Figure 1.

Figure 1.

The structural difference between graphitizing (a) and non-graphitizing (b) carbons [14].

3.3 Chemical structure of activated carbon

Activated carbon has a porous and crystalline structure. With this, it also has a chemical structure. The adsorption capacity of activated carbon is determined by its porous structure. But it is strongly influenced by a relatively small amount of chemically bonded heteroatom, mainly oxygen and hydrogen [15]. The variation in the arrangement of electron clouds in the carbon skeleton results in the creation of unpaired electrons and incompletely saturated valences which influence the adsorption properties of active carbons, mainly for polar compounds.

4. Synthesis of activated carbon

Up to date, commercial activated carbon (AC) used in wastewater treatment is produced from coals, woods, coconut shells, and lignite [16, 17]. Activated carbons possess several desirable properties that enable their use in adsorption. Properties, such as large surface area and porosity, together with surface chemistry react with molecules with specific functional groups. The wastewater treatment process is less profitable compared to other industrial sectors; it is always preferable to reduce the cost involved in its treatment process. The potential of low-cost adsorbent prepared from bio-waste has been identified in the last decade, and a great number of studies have been conducted to determine the characteristics and efficiencies of activated carbon produced from different bio-waste in the removal of different pollutants from wastewater. Synthesis of activated carbon from biomass generally starts with pretreatment of the sample, including crushing, drying at ~100°C, and sieving to obtain small particles within a specific size range. After these processes, the sample is carbonized in a dry inert atmosphere at 300−500°C, which promotes the elimination of volatile matters and tars and leads to the formation of biochar. Nowadays, the use of hydrothermal carbonization is attaining popularity in activated carbon production. In the hydrothermal process, the biomass is mixed along with water or reagent solution before carbonization [18].

The product obtained from hydrothermal carbonization is termed hydro char. Due to the different synthesis methods followed in the preparation of biochar, it is claimed that hydrothermal carbonization is more advantageous than traditional carbonization because the drying step carried out in the preliminary stage is not required. In such a process, a lower temperature of 180−250°C is used. The pressure released from the steam due to its closed system acts as an extra driving force to convert the biomass into hydro char. The formation of subcritical water under such conditions degrades cellulose, hemicelluloses, and lignin in the biomass [19]. The acidic gases, such as carbon dioxide (CO₂), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂). eliminated during the heating will react with water to form an acidic solution. Therefore, the need to treat such gaseous pollutants is not required. The presence of several functional groups, especially oxygenated ones, on the hydro char was also found, which results in a higher adsorption capacity of contaminants [20]. The presence of functional groups improves the adsorption of heavy metals on the hydro char despite lower surface area compared to activated carbon [21]. After the carbonization of biomass, physical or chemical activations are required to activate the carbonized material. Physical activation is normally performed by passing inert gases, such as carbon dioxide (CO₂), nitrogen (N₂), or steam [22]. The gases are passed into the carbonized material at a high temperature of 700−900°C. Under these conditions, the conversion of carbonized material into CO₂ gas through oxidation is limited. Hence the yield of activated carbon is increased when compared to activation using air. Chemical activation proceeds only in the addition of activating reagents usually acid or base to the carbonous material. Former heating at 300−500°C is done, followed by washing the activated carbon to neutralize its pH. Potassium hydroxide (KOH) is one of the common basic reagents used in chemical activation. It inhibits tar formation in carbonized biomass [23]. In addition, KOH reacts with carbon in the precursor to form potassium carbonate (K₂CO₃), which then reacts further with carbon to form potassium (K), potassium oxide (K₂O), carbon monoxide (CO), and carbon dioxide (CO₂) [24]. These processes generate porosities in the adsorbents with large micropore volumes and narrower size distribution.

4.1 Preparation of activated carbon from agricultural waste biomass

Activated carbon is prepared from hemp stem [25]. The hemp stem was carbonized at 500°C in nitrogen (N₂) atmosphere for 1 hour. The carbonized material was then ground and mixed with potassium Hydroxide (KOH) solution for 24 hours, then it was dried and activated at 800°C in nitrogen (N₂) atmosphere. When zinc chloride (ZnCl₂) is added, it reacts with the char and governs the pore distribution during the heat treatment [26]. Acidic activating reagents are commonly used. Addition of phosphoric acid (H₃PO₄) to the carbonous material causes hydrolysis of glycosidic linkage in polysaccharides of hemicellulose and cellulose [27]. When phosphoric acid is used during chemical activation, it is possible to control the reaction of the acid with the carbonized biomass by utilizing the gases used. The resulting activated carbon prepared from hemp biomass will get an application of adsorbent for removing dyestuffs and also in wastewater treatment process.

Activated carbon prepared from spent tea leaves is mixed with phosphoric acid (H₃PO₄) and heated at 450°C in oxygen and air atmosphere [28]. Phosphoric acid (H₃PO₄) reacts with oxygen and air atmosphere to form phosphorus oxides, then phosphorus oxides react with oxygen to form cerium oxide (CeO₂) electron pair bonds. Due to the extension of polyaromatic cross-linking, the higher porosity and high surface area could be observed in the formation of activated carbon. Phosphoric acid reacts in the air atmosphere to form phosphorus pentoxide (P₂O₅), and it sublimes from the activated carbon to increase its porosity development to a further extent. Activated carbon prepared by using steam activation in the atmosphere increases the deposition of carbon and decreases the porosity. When physical activation is compared to chemical activation, it requires a higher cost due to the chemical activating reagent and acid/base utilized for neutralizing pH in activated carbon. Consumption of energy will be lowered due to the lower temperature requirement during chemical activation [29]. Activated carbon prepared from chemical activation will have a higher surface area and well-developed porosity compared to the physical activation process [30]. Therefore, chemical activation is more widely followed in the preparation of activated carbon.

The merits of chemical activation over physical activation can be explained by the microstructure model [31]. This model states that every activated carbon material contains numerous micro domains in spherical shape, where the micropores develop. Mesopores, on the other hand, formed in inter micro domain space. Using physical (steam) and chemical activation (KOH), the phenol resin-based spherical carbon was converted to activated carbon via chemical activation. Activated carbon prepared by potassium hydroxide (chemical activation) exhibits a larger surface area, micropore volume, and average pore width. Whereas activated carbon prepared by physical activation by means of steam activation has less surface area, pore width, and micropore volume even at the same activation temperature. Surface area of the activated carbon material using a chemical process is 2878 m²/g, at an activation temperature of 900°C, while activated by steam (physical activation) possesses a surface area of 2213 m²/g. A lower yield of activated carbon occurs in chemical activation when a loss of carbon mass obtains during homogeneous pore development in the intra-micro domain regions. During physical activation using steam, homogeneous activation was observed, which results in lowering the efficiency of micropore development. Reduction of sizes of particles may also occur. Steam activation produces a lower yield of activated carbon with limited porosity, while chemical activation produces a uniform pore development.

Modifications are attempted in synthesizing activated carbon. The biomass is mixed with activating agents for chemical activation before pyrolysis. The pyrolysis process is carried out at high temperatures of 550−900°C. This method of preparing activated carbon is termed a one-step process. The preparation of activated carbon at a low temperature below 550°C after that activating agent is added for chemical activation. This process is termed as two-step pyrolysis process.

Activated carbon is prepared from corn stalk. One- and two-step processes of activation methods are followed for adsorbing cadmium. After the preparation and treatment of the contaminants, two-step pyrolysis processes increased the microporosity and surface area of the activated carbon. Therefore, the activated carbon prepared using two-step pyrolysis process shows higher adsorption capacity. There is no difference in its electrochemical properties. Activated carbon prepared from corn stalk shows its ability in removing cadmium from wastewater.

Activated carbon is prepared from Cassia fistula commonly called golden shower tree. A three-step process was done to synthesize activated carbon. The C. fistula was cleaned, dried, and crushed. The crushed sample was carbonized. The hydrochar obtained was further pyrolyzed to form biochar. Finally, it was activated by using potassium carbonate (K₂CO₃). After the application to adsorption process, the prepared activated carbon shows high performance in cationic dyes. It is evident that three-step preparation processes are more advantageous than using one-step and two-step processes. Morphological studies show that the adsorbent’s character strongly depends on its preparation method. Activation process helps the adsorbent to increase its pore volume. Increase in pore volume enhances the adsorbent property to absorb more amounts of dyes. Preparation of Activated carbon using three-step processes reveals that it is a more effective method than other processes. Activated carbon prepared from C. fistula is a promising material for removing dyes from wastewater.

Activated carbon is prepared from Tamarindus indica fruit shell. It was washed with distilled water to remove its dirt and dried under sunlight to remove its water content. It is chemically activated by using ammonium chloride (NH₄Cl). The activated material is filtered and carbonized at 500°C for 2 hours [32]. The Langmuir model shows the formation of the adsorbent’s monolayer coverage is at the adsorbent’s outer space. While Freundlich model isotherm analysis confirms the monolayer adsorption capacity was high. The maximum dye removal percentage was 24.3 g l − ¹. From this, it is evident that the T. indica fruit shell biomass is a promising adsorbent for removing textile dyes and also it can be employed in wastewater treatment process. Table 2 lists some of the biomass prepared from agricultural waste which can be employed in the water treatment process.

S. No	Adsorbent	Activating agent	Temperature	Adsorption studies	Kinetic studies	Characterization
1.	Sugar beet pulp	H3PO4	Impregnated by 110°C for 12 hrs activated by (350, 400, 450, 500, and 550°C) time (0.5, 1, 1.5, 2, and 2.5 hrs)	—	Pseudo-second order	BET, FTIR
2.	Betel nut husk (BNH)	NaOH	BNH was mixed with powdered NaOH at three (1:1,1:2, and 1:3) ratios for 24 hrs at room temperature and activated by 500°C for 1 hr	Langmuir; Freundlich	Pseudo-second order	SEM, FTIR
3.	Tamarind Shell (Tamarindus indica fruit shell)	NH4Cl	Carbonized at 500°C for 2 hrs and activated for 24 hrs	Langmuir,; Freundlich,	Pseudo-second order	SEM,; FTIR,TGA/DTA
4.	Mango seed	ZnCl2	Impregnated with 100°C for 30 mins, activated by 500°C for 1 hr.	Phenol adsorption	—	—
5.	Papaya seed	—	RPS washed with distilled water to remove the dust particles. It was left at room temperature for 1 day. Dried in oven at 105°C for 24 hrs	Langmuir	Pseudo-second order	—

Table 2.

Preparation of activated carbon from various agricultural waste biomasses, activating agent and its studies [32].

5. Applications of activated carbon

Activated carbons are proven to be effective in the removal of various pollutants from aqueous solutions, including dyes, pharmaceutical personal care products (PPCPs), heavy metals, and organic pollutants.

5.1 Removal of dyes from water resources

Dyes are one of the heavy pollutants which affect water bodies, due to the usage of dyes in paints, clothing, paper products, and plastics. In the textile industry alone, there are more than 3600 types of dyes used (Pure Earth and Green Cross Switzerland, 2017). Around 2−20% of dyes used for coloring in the textile industry are eliminating effluent. Therefore, the textile industry is the root cause of water pollution [33]. Due to the complex structure of dye molecules, the dyes do not degrade in water. Dyes mixed in water bodies, such as ponds, lake, and river, reduce the amount of sunlight reaching water sources, and due to that, photosynthesis gets affected for aquatic plants as well as animals [34]. Polluted water intake by humans also causes mutagenic and carcinogenic effects [35, 36]. Therefore, advanced studies are carried out on removing toxic dyes. For example, methylene blue (MB), a dye used in the textile industry causes complications in eyes, affects brain functions, and also causes skin diseases [37]. Table 3 lists some of the biomass prepared from agricultural waste involved in removing dyes from water resources.

Source of biomass	Pollutants targeted to remove	Removal efficiency (%)
Bamboo cane powder (H3PO4/600°C, H2O + N2)	Lanasyn orange	2600
Cashew nutshell (ZnCl2/400°C, N2)	Methylene blue	476
Acorn shell (ZnCl2/700°C)	Methylene blue	330
Lemon citrus peel (H3PO4/500°C)	Rhodamine B	254

Table 3.

Dyes adsorption on adsorbents derived from biomass for water treatment [36, 37].

5.2 Removal of heavy metals from water

Heavy metals and anions present in drinking water become a challenging problem among the public due to their causes. The sources of these heavy metals and ions are paint industry, chemical plants, textile dying, dumping of waste in landfills, etc. Most of the ions liberate from industries are toxic and carcinogenic to the environment as well as to humans. Consumption of water, which contains metal ions and anions, usually causes chronic effects instead of acute effects. Long-term disease will be affected by humans due to the usage of this contaminated water. Up to date the effects of heavy metals present in drinking water are identified in human body [38]. Despite the efficiency of adsorption in the removal of contaminants from wastewater, the use of commercially activated carbon is undesirable due to the low affinity towards heavy metals [39]. Therefore, it is vital to develop other adsorbents, especially from biomass waste, to minimize the heavy metal ions from entering the water bodies. Table 4 lists out some of the biomass prepared from agricultural waste involved in removing heavy metal ions.

Source of biomass	Pollutants targeted to remove	Removal efficiency (mg/g)
Loblolly pine chips (300°C, 93% N2 + 7%O2/NaOH/ 800°C, N2)	Cd^2+ (Cadmium)	167.3
Rice Husk; (600°C/Na2CO3)	Pb^2+ (Lead)	0.6
Banana peel (500°C, air/KOH/500°C, air)	Cu^2+ (Copper)	13.24
Australian pinecones; (800°C /NaOH)	Cu^2+ (Copper)	12.82

Table 4.

Activated carbon from agricultural biomass in active removal of heavy metals from water [38, 39].

5.3 Removal of organic pollutants from water bodies

Palm oil is one of the main ingredients in cooking [40]. Usage of these oils globally, the economic growth raised in some countries like Malaysia and Indonesia, Around 39% of palm oil production is from Malaysia. Despite these huge benefits and economic credits, the removal of oil effluent is a major challenge. Palm oil mill effluent (POME) is a byproduct obtained after processing palm seeds. This effluent contains a high chemical oxygen demand (COD) and biological oxygen demand (BOD). Elimination of palm oil effluent in water sources affects aquatic lives due to the formation of harmful compounds in water. Palm oil mill effluent appears in black or brownish-colored slurry with a foul smell. The traditional method of treating Palm oil mill effluent was dumped in a large pit to degrade. It needs a large land area and long time to degrade [41]. This conventional method of treatment is not effective so the effluent will remain toxic. Therefore, adsorption is one of the best suitable techniques that can be applicable in removing oil effluent. Table 5 represents some of the adsorbents prepared from agricultural waste biomass that effectively remove toxic compounds from oil effluents.

Source of biomass	Pollutants targeted to remove	Removal efficiency (%)
Sugarcane bagasse; (700°C/KOH/600°C)	NH3-N; (Ammoniacal Nitrogen)	94.74% (color)
Sugarcane bagasse (700°C, N2/KOH/; microwave, N2)	NH3-N	97.83% (color)
Cow dung ash (CH3COOH)	NH3-N	79%
Bio sorbent from oil palm mesocarp fiber (600°C /steam/600°C)	palm oil mill effluent (POME)	80%

Table 5.

Activated carbon from agricultural biomass in active removal of organic pollutants from water bodies [40, 41].

5.4 Pharmaceutical and personal care products (PPCPs) - pollutant removal

Due to the increase in diseases all over the world people are practicing or in- taking various kinds of drugs. The commonly used drugs are analgesics, antibiotics, anti-inflammatories, as well as painkillers [42]. Due to the large demand for drugs, production is getting increased, and also pharmaceutical waste is getting increased. Pharmaceutical and personal care products (PPCPs) are also the major cause of pollution, due to the disposal of waste in water sources. The toxic compounds liberated in water sources are consumed by wild animals and also human beings and cause various health issues. Biological activity of humans gets affected due to the consumption of drugs even in low concentrations [43]. Carbamazepine, naproxen, diclofenac, and ibuprofen are some of the drugs that are commonly practiced and cause biological effects in humans [44]. The contamination sources of pharmaceutical and personal care products (PPCPs) include hospital effluents, medical waste from factories during production of drugs, and waste due to consuming medicine discharged from the body and disposal of medicinal waste in landfills. Table 6 represents the activated carbon synthesized from biomass to remove the medicinal waste in water.

Biomass source	Target pollutant	Adsorption capacity mg/g
Waste tea residue (H3PO4/450°C/steam)	Oxytetracycline	273.7
Sawdust (ZnCl2/microwave)	Bisphenol A	334.28
Peach stone; (H3PO4/400°C, air)	Carbamazepine (CBZ); Ciprofloxacin (CPX)	170.3 (Activated carbon-rice husk, CBZ); 113.0 (Activated carbon –Peach stone, CBZ)
Palm kernel shell; (900°C, N2 + CO2)	Acenolol	0.69 mmol/g
Peach stone; (600°C, air/300°C, air)	p-nitrophenol	234.3

Table 6.

Activated carbon from agricultural biomass in active removal of pollutants in pharmaceutical and personal care products (PPCPs) [43, 44].

5.5 Other applications of activated carbon

5.5.1 Removing contaminants in drinking water that add color, odor, and flavor

The surface of activated carbon helps in effective removal or adsorption of organic compounds. Change in the surface morphology of activated carbon removes flavor, odor, and color from drinking water. The activated carbon is calcinated at high temperatures to increase its surface area. After the carbon preparation, it is activated by steam. Due to this process, the surface area of the activated carbon increases. Adsorption property gets improved after the activation so that the odor and taste of the drinking water will be easily changed. The odor produced by the organic compounds and undesirable substance gets into the pores of the activated carbon. Therefore, the unwanted materials can be removed easily [45].

5.5.2 Decaffeination of coffee

Decaffeination is done to remove the extra content of caffeine present in coffee. Activated carbon is employed in removal of caffeine from coffee. Activated carbon is mixed with a solution of ethyl cellulose to adsorb ethyl cellulose. After this process, the activated carbon-containing ethyl cellulose is allowed to dry. Then, the aqueous coffee extract is added to the activated carbon-containing ethyl cellulose. This mixture will extract the caffeine present in the coffee. Activated carbon employed in decaffeination can be reused in the same application. This improves the cost-effective removal method and also cheaper adsorbent [45].

5.5.3 Refining sugar, honey, and candies

Refining or bleaching of cane sugar and honey is done in food industries. The bleaching process is made in liquid with a certain temperature to reduce its viscosity. Chemically activated carbons prepared from softwoods are best suited for treating darker syrups. The pH of the activated carbon is adjusted to neutral one to do this bleaching process. This refining method is cheaper with high efficiency [45].

5.5.4 Discoloration of liquors, juices, and vinegars

Activated carbon prepared from powdered wood is commonly used. The synthesis of activated carbon from powdered wood increases the size of the pores. An increase in porosity adsorbs the color molecules faster. Powdered wood-activated carbon can be directly added to liquors, juices, and vinegars. On constant stirring, the color will be adsorbed to the activated carbon. The contact time will depend upon the color of the compound. This discoloration method is effective cheaper and easy to process [45].

5.5.5 Water treatment in industries

Activated carbon is commonly used in water treatment industries. By using activated carbon-heavy metals, organic contaminants can be easily removed. Chemical activation will increase the surface and porosity of the activated carbon. Due to this, the adsorption process will occur faster and more effectively. Activated carbon from waste materials like agricultural biomass is used nowadays. This made the treatment process an economic one [45].

5.5.6 Tertiary wastewater treatment

Activated carbon with the mineral origin is more suitable for the treatment of wastewater. Due to the range of pore formation in activated carbon during the activation process, the adsorption behavior of the material increases. The contaminants present in the wastewater will be completely eliminated during the treatment process. The odor and color will also be removed [45].

5.5.7 Purification of air and industrial gases

Toxic gases emitted from industries can be treated by using activated carbon. It adsorbs the toxic material and organic pollutants present in the air. Coconut shell-activated carbon is commonly used because it is a micropore material. Greater granulometry than the coconut shell activated carbon is employed in water treatment process to avoid pressure drop. Chemically activated carbons are used to adsorb organic compounds. Standard activated carbon cannot retain organic compounds and acid gases like aldehydes, ammonia, or mercury vapors [45].

5.5.8 Compressed air purification (diving tanks and hospitals)

To fill oxygen or compressed air in a tank pump is essential. But the pumping equipment will release oils and impurities during pumping. The adsorption filters made using activated carbon are fixed in tanks and pumps to ensure that the air technically remains oil free. This effectively reduces petroleum-derived vapors. The coalescing filters and adsorption filters fixed in the pump will provide compressed air with the highest quality [45].

5.5.9 Recovery of gold, silver, and other precious metals

An activated carbon with a micropore can retain gold, silver, and precious metals. The right size of the adsorbent can give adsorption kinetics in accordance with hydraulics of the process which works better than other processes. The activated carbon should have a certain hardness to withstand acid elution, washing processes at various temperatures, and thermal reactivation [45].

6. Conclusion

Getting clean drinking water and also consuming becomes a major challenge nowadays. Therefore, recycling wastewater effectively fulfills these major problems. The most common problems faced in water treatment process are adsorbents. Adsorbents used in water treatment process are costly and also not available. Due to this, the entire process becomes expensive and non-profitable. Activated carbon prepared from agricultural biowaste can be used as an adsorbent. It has a high potential to replace commercial activated carbon in wastewater treatment processes due to its low cost and high performance. Advancement in adsorption process using biomass is a solution to the challenges of water scarcity. It will certainly lead to large-scale applications of activated carbons from renewable sources in wastewater treatment industry.

Acknowledgments

Dr. M. Jaya Rajan (Associate Professor, Department of Chemistry & Research) and Anish C.I (Research Scholar, Register No: 19113012031017) acknowledge the research center Annai Velankanni College, Tholayavattam-629157, Affiliated to Manonmaniam Sundaranar University, Tirunelveli- 627012, Tamil Nadu, India. providing support for this chapter.