Cr(VI) Adsorption Onto Biomass Waste Material-Derived Activated Carbon

Abstract

Water pollution by chromium is due to both natural and anthropogenic sources like leather, textile, steel producing, magnetic tapes, wood protection, paper, chemical manufacturing, metal plating, battery, and pigment industries. According to the EPA (2000), chromium is the second common metal which exists in contaminated fields. It exists in different oxidation states ranging from –2 to +6. However, the most frequently encountered chromium species are trivalent and hexavalent forms since the +2, +4, and +5 forms are unstable and are easily converted to trivalent form. Cr(III), which is the most thermodynamically stable chromium form, is relatively innocuous due to the poor biological cell permeability and is an essential trace element for living organisms. According to Eh–pH diagram, chromium hydroxyl species exist as CrOH2+, Cr(OH)2+, Cr(OH)3, and Cr(OH)4– at pH > 3.5 while Cr(III) predominates at pH < 3.0. The second stable chromium form, Cr(VI), is generally produced by anthropogenic activities. Cr(VI) is a strong oxidant and reacts readily with any oxidizable organic material, hence converts to trivalent form. In the absence of organic matter, especially under aerobic condi‐ tions, it can be stable for long time. It exists primarily as salts of chromic acid (H2CrO4), hydrogen chromate ion (HCrO4–), and chromate ion (CrO42–), depending on pH (Figure 2). Predominant Cr(VI) species in the solution is HCrO4– at pH between 1.0 and 6.5 while CrO42– ions are dominant above pH 6.5. Dichromate anions (Cr2O72–) can be found only at high concentrations about more than 1 g/L. Chromium is considered the fourteenth most noxious heavy metal. Cr(VI) compounds are more toxic and carcinogenic even at low concentrations and over a wide range of pH values. High concentrations of Cr(VI) cause water quality problem in natural water sources, and health problem included lung cancer as well as kidney, liver, and gastric damage for human. Contact with chromium can also cause skin irritation. Because of its toxicity and carcino‐ genic nature, Cr(VI) compounds must be removed before discharging to the environment. The permissible limit of Cr(VI) for discharge into inland surface water is 0.1 mg/L . However, American Environmental Protection Agency (EPA) limits Cr(VI) concentration in potable water as 0.05 mg/L]. The most common methods of Cr(VI) removal from aqueous system are chemical precipitation, ion exchange, membrane processes, electrodialysis, and adsorptio. Among them, chemical precipitation is the most commonly used method. However, chemical precipitation yields large quantities of sludge, which is difficult to dispose directly or treat further. More‐ over, chemical precipitation method is not applicable for low concentrations. However, adsorption by activated carbon is considered to be an effective process for chromium removal due to high specific surface area, pore volume, and specific surface functionalities of activated carbon; application in this field is not common. The limited selectivity and high cost are the main obstacles for using commercial activated carbon in order to remove chromium species from aqueous media. Many researchers have studied on chromium adsorption onto activated carbon during the past ten years. A significant amount of them have used several biomass waste materials as activated carbon source. Activated carbons derived from biomass waste material can be favorable compared to conventional activated carbons with respect to their adsorptive properties, low cost, and renewable sources; in addition, evaluation of this biomass waste materials as activated carbon precursor prevents solid waste pollution problem.