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Reply to “Comment on the Removal Mechanism of Hexavalent Chromium by Biomaterials or Biomaterials-Based Activated Carbons” (Comment on “Diffusion Kinetic Study of Chromium(VI) Biosorption by Aeromonas caWiae”)† Maria X. Loukidou, Thodoris D. Karapantsios, Anastasios I. Zouboulis, and Kostas A. Matis* Chemical Technology DiVision, School of Chemistry, Aristotle UniVersity, GR-54124 Thessaloniki, Greece Sir: Park et al., in their long commentary (including 49 references!) of our paper, start with a paragraph that briefly describes our work. Yet, the description is not objective; for instance, there is no reference to our note (summarized clearly in the Conclusions section) that what we describe is a complex process. By putting emphasis on the removal mechanism itselfs which, however, was not the focus of our attemptsthese authors have tried to generalize it, based on findings from their recent work. This tacitly assumes that the same mechanism holds for all aqueous systems that use biomaterial-based activated carbons or even dead biomasses, which is, by far, not the case. It is well-known that a common scheme to control chromates in wastewater treatment is by reducing Cr(VI) to its trivalent form at acidic pH values (pH ∼2) in the presence of reductants, being followed by hydroxide precipitation of Cr(III) (see, for example, the referenced work of Zouboulis et al.1). On the other hand, the existence of parallel mechanisms (i.e., anion exchange and chromium reduction) was also reported,2 often overlapping with the aforementioned scheme under certain experimental conditions. The removal of hexavalent chromium oxyanions at low pH values may be attributed to their attraction/affinity toward positively charged surface groups of biomass or reduction of Cr(VI) to Cr(III), followed by its bonding to the negatively charged biosorbent. The characterization of our biomass, Aeromonas caViae, including zeta-potential measurements (which can be found elsewhere3), supports clearly the aforementioned arguments to which, however, Park et al. do not seem to have given the proper attention and significance. † This original paper to which the comment refers was “Diffusion Kinetic Study of Chromium(VI) Biosorption by Aeromonas caViae”. * To whom correspondence should be addressed. Tel.: +30 31 99 77 43. Fax: +30 31 99 77 59. E-mail:
[email protected].
A first indication that Cr(VI) reduction indeed does occur comes from optical observations of the biomass, which is expected to turn green during the process “that did not happen”. Meanwhile, the value of the solution pH during chromates biosorption should also change, because of the occurrence (in this case) of oxidation-reduction reactions.4 One definitive way to identify the above is certainly, as Park et al. argued, to conduct chemical analysis of total chromium in the system (i.e., by atomic absorption spectroscopy (AAS)) and compare it with results for only Cr(VI), obtained usually via a colorimetric ultraviolet-visible (UV-vis) spectroscopy method. If the measured concentrations coincide, then the conclusion is apparent. Indeed, this is what we did and all the above clearly lead to a possible mechanism of the observed fast and effective removal process (capacity of qmax ≈ 284 mg/g, according to the Langmuir isotherm, obtained with 0.5 g/L biomass load at pH 2.5), which compares favorably to other metal biosorbents available in the literature; note that, for the equilibrium runs, ample contact time (∼2 h) was allowed. The latter information (unpublished) is presented in the Ph.D. thesis of Loukidou.5 Literature Cited (1) Zouboulis, A. I.; Kydros, K. A.; Matis, K. A. Removal of hexavalent chromium anions from solutions by pyrite fines. Water Res. 1995, 29, 1755. (2) Kratochvil, D.; Pimentel, P.; Volesky, B. Removal of Trivalent and Hexavalent Chromium by Seaweed Biosorbent. EnViron. Sci. Technol. 1998, 32, 2693. (3) Loukidou, M. X.; Zouboulis, A. I.; Karapantisios, T. D.; Matis, K. A. Equilibrium and kinetic modeling of chromium(VI) biosorption by Aeromonas caViae. Colloid Surf., A 2004, 242, 93. (4) Kratochvil, D.; Volesky, B. Advances in the biosorption of heavy metals. Trends Biotechnol. 1998, 16, 291. (5) Loukidou, M. X. Ph.D. Thesis, Aristotle University, Thessaloniki, Greece, 2003, p 188.
IE050971T
10.1021/ie050971t CCC: $33.50 © 2006 American Chemical Society Published on Web 02/28/2006
