Ind. Eng. Chem. Res. 2006, 45, 2411-2412
2411
Reply to “Comment on the Removal Mechanism of Hexavalent Chromium by Biomaterials or Biomaterial-Based Activated Carbons” (Comment on “Removal of Hexavalent Chromium from Aqueous Solution Using Low-Cost Activated Carbons Derived from Agricultural Waste Materials and Activated Carbon Fabric Cloth”)† Dinesh Mohan, Kunwar P. Singh,* and Vinod K. Singh EnVironmental Chemistry DiVision, Industrial Toxicology Research Centre, Post Box No. 80, Mahatma Gandhi Marg, Lucknow 226 001, India Sir: We are thankful to Dr. Jong Moon Park for taking interest in our paper, and, of course, his valuable suggestions. Recently, we published a paper1 on the removal of hexavalent chromium (Cr(VI)) from aqueous solution using low-cost activated carbons derived from agricultural waste materials and activated carbon fabric cloth. In this paper, the agricultural waste materials were converted to low-cost activated carbons, viz., FAC (activated carbon derived from coconut fibers), SAC (activated carbon derived from coconut shells), ATFAC (activated carbon derived from acid-treated coconut fibers), and ATSAC (activated carbon derived from acid-treated coconut shells) successfully. Batch experiments showed that the removal efficiency of Cr(VI) follows the order FAC > ATSAC > ATFAC > SAC. However, a commercially available activated carbon fabric cloth (ACF) performed better than the tested adsorbents. In any event, various kinetic models and isotherm models were used to describe the adsorption kinetics and adsorption equilibrium of Cr(VI) removal by these adsorbents. In this paper, we have reported that the maximum adsorption of Cr(VI) species on the various adsorbents was at pH 2.0 and negligible at pH >8. Cr(VI) can exist in several stable forms, such as CrO42-, HCrO42-, Cr2O72-, and HCr2O7-, and the relative abundance of a particular complex is dependent on the concentration of the Cr ion and the pH of the solution. At low pH, the sorbent is positively charged, because of the protonation, whereas the sorbate (dichromate ions) exists mostly as an anion, leading to an electrostatic attraction between the sorbent and sorbate. This results in increased adsorption at low pH. As the pH of the solution increases, the sorbent undergoes deprotonation, and the adsorption capacity decreases. Therefore, all subsequent studies were performed at pH 2.0. Authors suggested to reconsider the removal mechanism of Cr(VI) adsorption on activated carbons. Park et al. noted that we analyzed only the total chromium in aqueous solution, using atomic absorption spectrophotometry (AAS). According to Park et al., it has been proven that, when Cr(VI) comes in contact with organic substances or reducing agents, especially in an acidic medium, the Cr(VI) is easily or spontaneously reduced to the Cr(III), because Cr(VI) has high redox potential value (above +1.3 V under standard conditions). Therefore, it is very important to check the reduction of Cr(VI) by organic materials. Park et al. noted that several researchers analyzed both the Cr(VI) content and the total chromium content in aqueous solution, and they reported that the removal of Cr* To whom correspondence should be addressed. Tel.: 0091-5222508916. Fax: 0091-522-2628227. E-mail:
[email protected]. † The original paper to which the comment refers was “Removal of Hexavalent Chromium from Aqueous Solution Using Low-Cost Activated Carbons Derived from Agricultural Waste Materials and Activated Carbon Fabric Cloth”.
(VI) occurred partially through reduction, as well as anionic adsorption, and the reduction could occur only under strongly acidic conditions (pH
