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Reply to “Comments on the Removal Mechanism of Hexavalent Chromium by Biomaterials or Biomaterial-Based Activated Carbons” (Comment on “Synthesis, Characterization, and Application as a Chromium(VI) Adsorbent of Amine-Modified Polyacrylamide Grafted Coconut Coir Pith”)† Maya R. Unnithan, V. P. Vinod, and T. S. Anirudhan* Department of Chemistry, UniVersity of Kerala, KariaVattom, TriVandrum 695 581, India Sir: We appreciate the interesting comments that Park et al.1 have made on the removal mechanism of hexavalent chromium by biomaterials or biomaterial-based activated carbons. In their comments, they discuss our recently published research article, on the removal mechanism of Cr(VI).2 Their remarks are mainly related to the claim that, when the Cr(VI) comes into contact with organic substances or reducing agents, especially in acidic medium, the Cr(VI) species is easily or spontaneously reduced to the Cr(III) species, because Cr(VI) has high redox potential value (above +1.3 V under standard conditions). They illustrated the importance of checking the reduction of Cr(VI) by organic materials. In our paper, “Synthesis, Characterization, and Application as a Chromium(VI) Adsorbent of Amine-Modified Polyacrylamide Grafted Coconut Coir Pith”,2 we reported the effectiveness of a new adsorbent prepared from coconut coir pith (CP) in removing Cr(VI) from aqueous solutions. We are interested in the application of biomaterials as the polymer support for constructing novel adsorbents. The adsorbent, PGCP-NH3+Cl, which has the -NH3+Cl functional group at the end of its chain, was synthesized via graft polymerization of acrylamide onto CP in the presence of N,NI-methylene bisacrylamide (cross linking agent) and peroxydisulfate (initiator), followed by transamidation and functionalization. The PGCPNH3+Cl is not a biomaterial or biomaterial-based activated carbon to consider the mechanism of Cr(VI) removal as biosorption or adsorption-coupled reduction, as suggested by Park et al.1 We disagree with the comments made by Park et al.,1 in regard to Cr(VI) removal mechanism in our system. Park et al.1 proposed two mechanisms for the reduction of Cr(VI) to Cr(III) and designated them as mechanism I (direct reduction) and mechanism II (indirect reduction). They are concerned that Cr(VI) is directly reduced to Cr(III) in the aqueous phase by contact with the electron-donor groups of the biomass, i.e., groups that have lower reduction potential values than 1.3 eV. The PGCP-NH3+Cl is not a non-living biomass to follow the mechanism of direct reduction (mechanism I) proposed by Park et al.1 for the Cr(VI) removal process. As already discussed in our paper, the mechanism of Cr(VI) removal by PGCP-NH3+Cl is an ion-exchange process, in which Cl- from the peripheral -NH3+Cl group of the adsorbent is exchanged with the Cr(VI) species from the solution. We have also considered the polynuclear species of Cr(VI) formed at different pH levels to explain the observed variations in adsorption capacity in the pH range of 2.0-9.0. The adsorption characteristics of Cr(VI) on the graft polymerized lignocellulosic residues were determined to be different from those observed on biomass or * To whom correspondence should be addressed. Tel: 0471-418782. E-mail:
[email protected],
[email protected]. † The original paper to which the comment refers was “Synthesis, Characterization, and Application as a Chromium(VI) Adsorbent of Amine-Modified Polyacrylamide Grafted Coconut Coir Pith”.
biomass-based activated carbons. The different adsorption behaviors were caused by the different surface properties of the materials. This idea is also supported by studies from other laboratories on similar types of materials.3,4 As suggested by Park et al.,1 mechanism II (indirect reduction), consists of three steps: (i) the binding of the anionic Cr(VI) ion species to the positively charged groups present on the biomass surface (PGCP-NH3+Cl is not a biomass to provide such positively charged groups); (ii) the reduction of Cr(VI) by adjacent electron-donor groups (PGCP-NH3+Cl does not contains electro-donor groups to reduce Cr(VI) to Cr(III)); and (iii) the release of the Cr(III) ions into the aqueous phase, because of electronic repulsion between the positively charged groups and the Cr(III) ions. In PGCP-NH3+Cl, such electronic repulsion is not possible, because Cr(VI) forms an inner sphere coordination complex with the reaction site, and, hence, Cr(III) is not formed on the adsorbent surface. We have studied the oxidation state of chromium bound on the PGCP-NH3+Cl, and we have not observed any Cr(III) species on the PGCPNH3+Cl surface, as well as in aqueous solution, even at very low pH values. In our earlier papers, we have reported the “adsorptioncoupled reduction” mechanism for the removal of Cr(VI) by biomaterial-based activated carbons at very low pH levels, because these carbons contain different electron donor groups. The reference has been cited in our paper.2 For example, in one of our papers,5 we reported that the amount of reduction of Cr(VI) to Cr(III) (5.1%-12.5%) was calculated when activated carbon equilibrated over a concentration range of 50-150 mg/L Cr(VI). To confirm the adsorption-coupled reduction mechanism for Cr(VI) removal by activated carbon, additional batch adsorption experiments using Cr(III) as adsorbate were also conducted and the results were explained in our earlier paper.5 As solution contains two adsorbate species, Cr(VI) and Cr(III), we have described kinetic data using a second-order kinetics equation derived by Fox et al.6 for systems involving simultaneous removal of two different species from aqueous solutions. The reduction of Cr(VI) to Cr(III) occurs on activated carbons only if it contains both acidic and basic functional groups. The adsorbent materials, such as hydrotalcites, organoclays, and surface-modified biomaterials, have not reported Cr(VI) reduction to Cr(III) during chromate removal process.4,7-10 Park et al.1 note that, when the Cr(VI) comes in contact with “organic substances” and biomasses, especially in acidic medium, the Cr(VI) is reduced to Cr(III). To our knowledge, the organic substances (such as lignin, tannin, pectin, cellulose, starch, terpenes, and alkaloids) have not been reported the reduction of Cr(VI) to Cr(III). The PGCP-NH3+Cl, even though it is prepared from coir pith, which is a biomass, the reaction sites for Cr(VI) are -NH3+Cl groups at the chain end. The adsorbents, prepared by the graft polymerization reaction,
10.1021/ie051346r CCC: $33.50 © 2006 American Chemical Society Published on Web 02/28/2006
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followed by functionalization should not be treated as biomaterials or organic substances to follow a biosorption process. Earlier workers3,4,11 have also been drawn similar conclusions while studying adsorption of Cr(VI) onto surface-modified soybean hull, coconut coir, and saw dust, which have different functionality. We thank D. Park, Y. S. Yun, and J. M. Park for their interest and comments on our paper. Literature Cited (1) Park, D.; Yun, Y. S.; Park, J. M. Comments on the Removal Mechanism of Hexavalent Chromium by Biomaterials or Biomaterial-Based Activated Carbons. Ind. Eng. Chem. Res. 2006, 45, 2405-2407. (2) Unnithan, M. R.; Vinod, V. P.; Anirudhan, T. S. Synthesis, Characterization and Application as a Chromium(VI) Adsorbent of Aminemodified Polyacrylamide Grafted Coconut Coir Pith. Ind. Eng. Chem. Res. 2004, 43, 2247. (3) Wartelle, L. H.; Marshall, W. E. Chromate Ion Adsorption by agricultural byproducts Modified with Dimethyloldihydroxyethylene Urea and Choline Chloride. Water Res. 2005, 39, 2869. (4) Baes, A. U.; Okuda, T.; Nishijima, W.; Shoto, E. Okada, M. Adsorption and Ion-Exchange of some Groundwater Anion Contaminants in an Amine Modified Coconut Coir. Water Sci. Technol. 1997, 35, 89.
(5) Raji, C.; Anirudhan, T. S. Chromium(VI) Adsorption by Sawdust Carbon: Kinetics and Equilibrium. Indian J. Chem. Technol. 1997, 4, 228. (6) Fox, I.; Malati, M. A.; Perry, R. The Adsorption and Release of Phosphate from Sediments of a River Receiving Sewage Effluents. Water Res. 1989, 23, 725. (7) Manju, G. N.; Gigi, M.C.; Anirudhan, T. S. Hydrotalcite as Adsorbent for the Removal of Chromium(VI) from Aqueous Media: Equilibrium Studies. Indian J. Chem. Technol. 1999, 6, 228. (8) Martinez-Gallegos, S.; Martinez, V.; Bulbulian, S. Chromium(VI) Separation from Tannery Wastes by Utilizing Hydrotalcites. Sep. Sci. Technol. 2004, 39, 667. (9) Das, N. N.; Konar, J.; Mohanta, M. K.; Srivastava, S. C. Adsorption of Cr(VI) and Se(IV) from Their Aqueous Solutions onto Zr4+-substituted ZnAl/MgAl-Layered Double Hydroxides: Effect of Zr4+ Substitution in the Layer. J. Colloid Interface Sci. 2004, 270, 1. (10) Krishna, B. S.; Murthy, D. S. R.; Jai Prakash, B. S. SurfactantModified Clay as Adsorbent for Chromate. Appl. Clay Sci. 2001, 20, 65. (11) Garg, V. K.; Gupta, R.; Kumar, R.; Gupta R. K. Adsorption of Chromium from Aqueous Solution on Treated Sawdust. Bioresour. Technol. 2004, 92, 79.
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