Activated carbon has a random and imperfect structure with high porosity and visible cracks and crevices. Synthesis of activated carbon involves pretreatment of carbon source material, carbonization (calcination under inert atmosphere), and activation.⁴⁷ Medhat et al.⁴⁸ have used corn cob for the preparation of activated carbon powder. The dried corn powder was calcined at 700 °C for 2 h in a muffle furnace under an N₂ atmosphere. The carbon powder was activated using KOH or ammonium sulfate. Suhas et al.⁴⁹ have synthesized activated carbon with relatively high surface area (569 m² g⁻¹) by hydrothermal method. The carbon source used in this research was Phyllanthus emblica fruit stone. Generally, the as-synthesized activated carbon contains surface groups such as hydroxyl (OH), carboxyl (COOH), carbonyl (CO), phenol (C₆H₅OH), lactones ((CO)O) etc., which are responsible for adsorption of aquatic contaminants. Importantly, carbon sources have a significant impact on the properties of activated carbon. Commonly, activated carbon is prepared from high carbon content with a low concentration of inorganic substances like coal, biowastes (leaves, rice husks, coconut shells, waste paper, etc.) and non-degradable wastes (plastics, tires, etc.).⁵⁰

(i)

Pure activated carbon

Owing to availability and excellent adsorption capacity, pure activated carbon was much studied as an adsorbent in wastewater treatment. For example, Shokry et al.⁵¹ have prepared activated carbon nanoparticles (average diameter = 38 nm) with a pore volume of 0.183 cm³ g⁻¹ using raw Maghara coal (volatile matter = 50.6% and ash content = 4.12%), an eco-friendly, abundant natural material and an aqueous solution of NaOH (50%) as an activating agent for removal of methylene blue from water. NaOH activates the carbon via multiple chemical reactions (Eqs. 1–4).⁵¹

From the batch adsorption experimental results, it was found that a monolayer methylene blue adsorption (28.09 mg g⁻¹) on activated carbon occurred. The enhanced adsorption capability of Maghara coal-derived activated carbon was due to surface hydroxyl (OH) groups that form ionic interactions with charged methylene blue (MB⁺) molecules (Eq. 5). Moreover, spent activated carbon could be effectively regenerated using HCl aqueous solution.

Studies have also been performed to explore the utilization of wastes for the generation of activated carbons. For example, the tannery sludge biomass was converted to mesoporous activated carbons via carbonization (500 °C/3 h, inert atmosphere) followed by a physical activation route.⁵² The surface area and average pore diameter of the synthesized activated carbon are 187.21 m² g⁻¹ and 3.24 nm, respectively. This activated carbon was effectively utilized for the adsorption of malachite green (231.34 mg g⁻¹), 2,4-dichlorophenoxyacetic acid (20.09 mg g⁻¹), and Cr(VI) ions (Cr₂O₇^2 −, 86.26 mg g⁻¹) from their respective aqueous solutions. The palm shell (agricultural waste) derived activated carbon (surface area = 506.84 m² g⁻¹) with the aid of oleic acid activation followed by 50 kHz ultrasonic irradiation was used for acenaphthene (poly aromatic hydrocarbons) adsorption from wastewater.⁵³ Under the optimized condition, this porous activated carbon showed a higher adsorption capacity of 52.75 mg g⁻¹, while normal palm shell-derived activated carbon (without activation) showed only 15 mg g⁻¹ toward acenaphthene adsorption. Thermal treatment was successfully applied to regenerate spent activated carbon. Chaudhary et al.⁵⁴ have investigated the role of functional groups of a few selected anthraquinone acid dyes on the adsorption ability of activated carbons. These researchers have found that acid blue 129 dye molecules containing methyl group show enhanced adsorption than other dyes (acid blue 25 and acid blue 40). They also found that tautomerism is responsible for the adsorption of acid blue 40 on the synthesized activated carbons.

(ii)

Functionalized activated carbon

The surface functionalization of activated carbon has remarkably improved the adsorption capacities. The incorporation of oxygen, nitrogen, sulfur, and phosphorus containing functional groups maximizes active sites in activated carbon.⁵⁵ In this regard, activated carbon functionalized with ethylenediaminetriacetic acid was reported for the adsorption of Nd(III) ions from wastewater.⁵⁶ Based on the Langmuir adsorption model, the highest adsorption ability was observed for Nd(III) ions with a good recovery rate. In another study, activated carbon was functionalized by 8-hydroxyquinoline.⁵⁷ This adsorbent was used for the extraction of Cd(II), Ni(II), Mn(II), Zn(II), and Pb(II) ions from contaminated groundwater by the solid phase extraction method. The 8-hydroxyquinoline functionalized sample showed more than 50% enhanced adsorption capacities than the pure adsorbent. The order of adsorption capacity (mmol g⁻¹) is Mn(II) = 0.393 >  Ni(II) = 0.345 >  Zn(II) = 0.314 >  Cd(II) = 0.170 >  Pb(II) = 0.092.

(iii)

Activated carbon-based composites