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Response to Comment on “Adsorption and Desorption of Oxytetracycline and Carbamazepine by Multiwalled Carbon Nanotubes” We thank Drs. Bui and Choi for their interest in our paper. In their comment on our paper (1) regarding oxytetracycline (OTC) and carbamazepine (CBZ) sorption and desorption by multiwalled carbon nanotubes (MWCNTs), Bui and Choi (2) made some interesting remarks. These remarks can be summarized in three main points (i) compounds properties, (ii) pKa values of carbamazepine and possible mechanism of CBZ interaction with MWCNTs, and (iii) effect of calcium ions on OTC sorption by MWCNTs. We appreciate the comments and suggestions given by Bui and Choi (2). We offer the following responses to the comments. In first case we misplaced the properties of two tested pharmaceuticals. The correct expression should be OTC (Sw ) 121 mg/L, log Kow ) -1.22, pKa ) 3.27, 7.32, 9.11; density ) 1.63 g/cm3) and CBZ (Sw ) 112 mg/L, log Kow ) 2.45, pKa ) 7; density )1.15 g/cm3). The calculated values of OTC and CBZ based on the Polanyi-Manes model as well the discussion in our paper (1) used the proper values of OTC and CBZ. Therefore our results and discussion were not affected. Moreover, we have submitted an “Addition/Correction” to correct this misunderstanding. Second, the value of CBZ pKa was cited from the paper (3) and this paper was cited 40 times up to April, 2010. Bui and Choi (2) excluded the electrostatic interactions between CBZ and MWCNT in their comments. However, we have suggested that the electrostatic interaction could be the one of the possible interaction mechanisms between CBZ and MWCNT, and we did not exclude other mechanisms such as π-π interactions and hydrogen bonding. In addition, we still do not have sufficient evidence to exclude the electrostatic interaction in the case of CBZ. This hypothesis could be further verified by studying the CBZ desorption from
10.1021/es101242j
2010 American Chemical Society
Published on Web 05/14/2010
MWCNTs in much wider range of pH than that in our study. We agree with Bui and Choi (2) that the change of MWCNT properties may result in different CBZ desorption. Changes of MWCNT properties depending on pH have been also suggested by us in the case of OTC. Third, the explanation given by Bui and Choi (2) regarding the possibility of calcium ions influence on OTC binding by MWCNTs is very interesting. However, their explanation was not able to explain the desorption of CBZ as affected by pH. The effect of background solution surely needs more detailed investigations including different compositions of background solution as Bui and Choi suggested. But this was not the focus of our paper. Some more specific experimental design is needed to address the problem. We have supposed that calcium ions may simultaneously bind the negatively charged OTC and make an interaction with MWCNTs nevertheless we did not want to speculate without additional work. The full characteristics of multiwalled carbon nanotubes were presented in a previous paper (4). We agree that ref 10 is more relevant than ref 9 during discussion of OH relevance in sorption and desorption processes of organic contaminants by MWCNTs.
Literature Cited (1) Oleszczuk, P.; Pan, B.; Xing, B. Adsorption and desorption of oxytetracycline and carbamazepine by multiwalled carbon nanotubes. Environ. Sci. Technol. 2009, 43, 9167–9173. (2) Bui, T.-X. Choi, H. Comment on “adsorption and desorption of oxytetracycline and carbamazepine by multiwalled carbon nanotubes Environ. Sci. Technol. 2010, doi: 10.1021es100684f. (3) Drillia, P.; Stamatelatou, K.; Lyberatos, G. Fate and mobility of pharmaceuticals in solid matrices. Chemosphere 2005, 60, 1034– 1044. (4) Lin, D.; Xing, B. Tannic acid adsorption and its role for stabilizing carbon nanotube suspensions. Environ. Sci. Technol. 2008, 42, 5917–5923.
Patryk Oleszczuk, Bo Pan, and Baoshan Xing ES101242J
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