Environ. Sci. Technol. 2007, 41, 6315
Comment on “Reaction of Polycyclic Aromatic Hydrocarbons Adsorbed on Silica in Aqueous Chlorine” Nakamura et al. (1) investigated the reaction of polycyclic aromatic hydrocarbons (PAHs) with chlorine in aqueous medium. The authors claim “that the PAHs adsorbed on silica reacted more rapidly than the PAHs themselves in water.” Based on this finding the authors conclude that “the electrophilicity of which (the halogenating agents) may be raised by the catalytic effect of the silanol group of the silica surface.” For the following reasons we think that neither of these two statements is justified by the experiments described in this paper. The chlorination of PAHs in aqueous media can proceed as a homogeneous or a heterogeneous reaction. In the latter case, the interface area between the participating phases is of course an important parameter. According to the experimental conditions described by the authors, 25 µmol PAH was deposited on 0.5 g of silica gel. Unfortunately, the applied silica gel was not characterized (particle size, BET surface area, etc.). Further, the PAH-loaded solid was suspended in 10 mL of water and allowed to react with hypochlorite at a concentration of “1 ppm of available chlorine” at pH 5 and 9. One ppm of chlorine in 10 mL water corresponds to about 0.3 µmol, i.e., about 1.2 mol % of the applied PAH. Reference chlorination experiments in the absence of silica gel were carried out to reveal the effect of sorbents. Unfortunately, nothing is said in the experimental part about the details of these reference experiments. If the reader assumes that a procedure similar to that used with silica gel is carried out (i.e., identical amounts of PAHs are applied) then the question arises: what is the speciation of the PAHs in the pure aqueous system? If we consider pyrene as one of the investigated PAHs, its aqueous solubility is about 5 µM compared with 2500 µM as applied in the chlorination experiments. Hence, the great majority of the pyrene is either deposited at the glass walls of the vials used (not described in the paper) or suspended as crystals in the water phase. At the end of the reaction period the entire content of the vials was extracted with ethyl acetate and analyzed by means of GC-FID or GC-MS. From these analyses, degrees of PAH conversion were calculated and discussed. The authors state “it is obvious that silica-adsorbed PAHs react more rapidly than those in water.” First, it is not obvious how 0.3 µmol active chlorine can effect a 100% conversion of 25 µmol PAH, as stated in Table 1 of the paper. Second, the authors compare degrees of chlorination of PAHs which are in different speciation states: (I) adsorbed on a silica surface and (II) possibly suspended as crystals. The supplied information is insufficient for a comparison of rate constants of PAHs in water (i.e., in the dissolved state) and in the silica-adsorbed state. Although 23% conversion of pyrene in pure water at pH ) 5 is less than 100% conversion in the presence of silica gel, the available pyrene concentrations are 5 µM dissolved in water vs 2500 µM adsorbed as a thin layer, possibly as a monolayer. Nothing is known about the speciation and availability of the remaining 2495 µM pyrene in the pure water system.
10.1021/es070965j CCC: $37.00 Published on Web 07/31/2007
2007 American Chemical Society
Therefore, the higher degrees of PAH conversion in the presence of silica gel can be explained simply by a better availability to dissolved hypochlorite or chlorine. There is no reason to speculate about catalytic effects. Moreover, from the experimental point of view it is not useful to generate conditions for a 100% conversion of an educt when comparing reaction rates. Nevertheless, based on the conversion data given by the authors in Table 1 and the tabulated aqueous solubilities of the PAHs one can try to estimate relative rate constants for the homogeneous and the heterogeneous chlorination reaction. Based on this estimation it cannot be excluded that the homogeneous reaction has an even higher rate constant than the heterogeneous reaction. Regarding the mechanisms of chlorination, it is surprising that the authors do not consider Cl2 as a possible electrophilic species at pH 5. Although HOCl is by far the dominating chlorine species under the applied conditions, Cl2 may be competitive or even more reactive. Furthermore, the proposed catalytic mechanism of the heterogeneous reaction appears implausible to us. Silica materials (including natural quartz, gels, and sols) have an isoelectric point at pH ≈ 2 (2) and are negatively charged in the neutral and alkaline pH range. Therefore, the proposed hydrogen bond donor function of the silica surface is unlikely at pH ) 9. Regarding the environmental relevance of the model system chosen by Nakamura et al., we think that hydrophobic sorbents such as particulate and colloidal natural organic matter (NOM, DOM) are more likely to interact with the highly hydrophobic PAHs in natural aquatic environments than silica would be. The effect of DOM on the chlorination of PAHs is the topic of a forthcoming paper from the commenting authors (3). However, regarding our rigorous criticism with respect to some important aspects of the present paper, it is possible that we may have misunderstood details of the experimental part. If this is the case then we request the authors to make these points more clear for the reader. Otherwise the conclusions from the study of Nakamura et al. would have to be reduced to their introductory statement: “This paper describes that pyrene, fluorene, and fluoranthene ... adsorbed on silica easily react with aqueous chlorine in water ...”.
Literature Cited (1) Nakamura, H.; Tomonaga, Y.; Miyata, K.; Uchida, M.; Terao, Y. Reaction of Polycyclic Aromatic Hydrocarbons Adsorbed on Silica in Aqueous Chlorine. Environ. Sci. Technol. 2007, 41, 2190-2195. (2) Parks, G. A. The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chem. Rev. 1965, 65, 177-198. (3) Georgi, A.; Reichl, A.; Trommler, U.; Kopinke, F.-D. Influence of sorption to dissolved humic substances on transformation reactions of hydrophobic organic compounds in water. I. Chlorination of PAHs. Environ. Sci. Technol., submitted.
Frank-Dieter Kopinke* and Anett Georgi Department of Environmental Technology UFZ - Helmholtz-Centre for Environmental Research Ltd. Permoserstrasse 15 D-04318 Leipzig, Germany ES070965J
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