Long-term sorption behaviors of phenanthrene (Phen) on the Borden sand from 1 min to 365 days and of Phen and 1,2- dichlorobenzene (DCB) on the isolated ...
Environ. Sci. Technol. 2006, 40, 2491-2492
Response to Comment on “Sorption Kinetics of Organic Contaminants by Sandy Aquifer and Its Kerogen Isolate” We thank Allen-King and Ball (1) for their interest in, and constructive comments on, our recent article (2). They expressed their viewpoints and concerns on the representativeness of the isolated natural organic matter (NOM), the kinetics modeling and upper bound, and the predicted sorption equilibrium times for phenanthrene (Phen) and tetrachlorobenzene (TeCB). The detailed response to their comments and concerns follows. As the total organic carbon (TOC 0.021 wt %) in the Borden aquifer is very low, it would be a hard task to quantitatively isolate all of NOM from the aquifer. In this study (2), the Borden aquifer (16.6 kg) was treated with 1 M HCl. Only fine particles (silt and clay fractions) were collected and treated with concentrated 1:1 HCl + HF acids at ∼60 °C. A great deal of NOM in the treated aquifer was not collected during the isolation processes. The two NOM concentrates (I and II), respectively, with TOC of 0.736 and 5.56 wt % together account for 18.7% of TOC in the Borden aquifer (3). However, several lines of evidence are present to support our assumption that the isolated NOM is representative. First of all, the very close or even same nonlinear Freundlich parameters n (n < 1) for each of the four solutes (1,2-DCB, 1,3,5-TCB, naphthalene-Naph, and Phen) on both the isolated NOM and the bulk aquifer (the two sorbents) imply similar condensation degree of NOM (3-4). Second, the similar and consistent sorption kinetics rates (no observed fast sorption rate) for Phen on the two sorbents (2), and the very significant Kd-foc correlations (R 2 > 0.99) for TeCB and PCE on the bulk aquifer and the various size fractions (2, 5) also suggest the quite similar NOM property. Moreover, although the low sorption capacity was associated with the small size fractions (5), the fine particles were adequately extracted and the NMR spectrum of the kerogen isolate II showed minimal oxygencontaining functional groups (3). Furthermore, often-cited, reliable solute physicochemical parameters were used in the studies (2-3). The very significant correlations between the log KOC values at three different Ce levels and the hydro-
phobicity (log KOW) for the four HOC solutes (R 2 > 0.987) on the two sorbents, respectively (3), indicate that the investigated two classes of organic chemicals exhibit identical sorption behaviors, in contrast to their different sorption behaviors for other young NOM solids. Therefore, the nearly same modified-Freundlich parameters for Phen on the kerogen isolate and for TeCB on the acid-treated size fraction were also used to support the above assumption that the isolate kerogen would be similar to the majority of bulk NOM. Based on the above discussion, we respectfully disagree with the assumption proposed by Allen-King and Ball (1) that the other 81.3% of TOC comprises an amorphous NOM material as a linear partitioning medium. They used the dual carbonaceous material model (DCM) for interpreting our DCB sorption data (1) in order to support their hypothesis. However, this model depends on the choice of a linear partition coefficient (Kd) and faces difficulty with the following modeling. Using the Kd values derived from the nonlinear sorption isotherms of the four organic solutes at 75% of the solubility on the bulk aquifer (2), it was found that either the composite nonlinear factors (ntotal) for DCB and Naph or sorption quantities (qe-total) for Phen, TeCB, and Naph are significantly different from that of the bulk aquifer (Figure 1). Finally and more importantly, considering that the bulk NOM may be blocked and retarded by the clay minerals (6), we used the adsorption volumes of DCB on the two sorbents respectively to estimate the Phen equilibrium Koc values (2). The small molecules DCB and Naph were found to have higher adsorption volumes than the large molecules TCB and Phen (4, 7). In fact, whether the kerogen isolate is representative of the bulk NOM does not affect our kinetics modeling prediction for Phen as we have not estimated the equilibrium Koc value from the sorption isotherm on the kerogen isolate. So, Allen-King and Ball may have misunderstood our upper boundary conditions and kinetics modeling assumption (1). The slow change in sorption data was easily masked by the analytic errors when the sorption time increased because of power law sorption kinetics. We observed that some of the sorption data had relatively large scatters at longer time and at the lowest or two highest initial Phen concentrations (Figure 3 and Figure S1 in the Supporting Information) (2). But, the sorption data quality was higher at the other initial
FIGURE 1. Measured sorption isotherms of Phen, TCB, Naph, and DCB (bulk) on the bulk aquifer (3) and the composite total sorption isotherms (total) derived from the DCM model presented by Allen-King (1). The Koc values are, respectively, 14.2, 4.64, 0.8, and 0.59 L/g for Phen, TCB, Naph, and DCB derived from the nonlinear sorption isotherms of the four solutes on the bulk aquifer (3). 10.1021/es0680008 CCC: $33.50 Published on Web 02/18/2006
Phen concentrations in Figure S1. From Figure S1, the relatively consistent Koc values from 90 to 365 days for the bulk aquifer and at sampling times greater than 42 days for the isolate in Figure 3 (2) had not indicated an asymptote, but were brought about by experimental errors. Moreover, the fractional power equation, an approximate solution of the Fickian diffusion equation, can well describe both the original and the calculated time-dependent sorption data of Phen on the two sorbents (Figures 3 and S1 (2)). The similar rate parameter b for qe(t) vs t on the two sorbents, ranging from 0.077 to 0.099 and from 0.068 to 0.081, respectively, suggests the similar sorption kinetics rate (2). The estimated equilibrium time is longer than one decade for Phen on each of the two sorbents (2). Furthermore, as the equilibrium Kd value for TeCB on the 0.3-mm size fraction was underestimated by a factor of 1.76 (2, 5), the equilibrium time for TeCB with a molecular size similar to Phen is estimated to be much longer than that obtained by the prior investigation (over 10 years vs 2.5 years) (2, 8). Finally, the estimated time scales for Phen are also consistent with the theoretical calculation (2) using the reported diffusion coefficients of Phen and anthracence (1.3-5.8 × 10-17 cm2/s) by Ghosh et al. (9). In summary, the sorption equilibrium times for Phen on the two sorbents are much longer than previously understood. The bulk NOM could consist of the isolated kerogen, which is a highly complicated geopolymer or geomacromolecule, very different from young NOM in soils and sediments, and contains considerable nanometer pores (e.g., 2, 7). However, some questions remain to be answered, such as effect of kerogen-clay mineral interaction on sorption of HOCs, direct determination of HOC diffusion coefficient in kerogen matrix, etc. We also agree with Allen-King and Ball (1) that mechanistic kinetics modeling needs developed to advance our understanding on the sorption theory of HOCs in the supergene earth environment. This response to the comments by Allen-King and Ball (1) shows again the importance of kerogen for sorption of HOCs.
Literature Cited (1) Allen-King, R. M.; Ball, W. P. Comment on “Sorption kinetics of organic contaminants by sandy aquifer and its kerogen isolate.” Environ. Sci. Technol. 2006, 40, 2489-2490.
(2) Ran, Y.; Xing, B.; Rao, P. S. C.; Sheng, G.; Fu, J. Sorption kinetics of organic contaminants by sandy aquifer and its kerogen isolate. Environ. Sci. Technol. 2005, 39, 1649-1657. (3) Ran, Y.; Xiao, B. H.; Huang, W. L.; Peng, P. A.; Liu, D. H.; Fu, J. M.; Sheng, G. Y. Kerogen in aquifer material and its strong sorption for nonionic organic pollutants. J. Environ. Qual. 2003, 32, 1701-1709. (4) Ran, Y.; Huang, W. L.; Rao, P. S. C.; Liu, D. H.; Sheng, G. Y.; Fu, J. M. The role of condensed organic matter in the nonlinear sorption of hydrophobic organic contaminants by a peat and sediments. J. Environ. Qual. 2002, 31, 1953-1962. (5) Ball, W. P.; Roberts, P. V. Long-term sorption of halogenated organic chemicals by aquifer material. 1. Equilibrium. Environ. Sci. Technol. 1991, 25, 1223-1237. (6) Ran, Y.; Xiao, B. H.; Fu, J. M.; Sheng, G. Y. Sorption and desorption hysteresis of organic contaminants by kerogen in a sandy aquifer material. Chemosphere 2003, 50, 1365-1376. (7) Ran, Y.; Xing, B. S.; Rao, P. S. C.; Fu, J. M. Importance of adsorption (hole-filling) mechanism for hydrophobic organic contaminants on an aquifer kerogen isolate. Environ. Sci. Technol. 2004, 38, 4340-4348. (8) Ball, W. P.; Roberts, P. V. Long-term sorption of halogenated organic chemicals by aquifer materials. 2. Intraparticle diffusion. Environ. Sci. Technol. 1991, 25, 1237-1249. (9) Ghosh, U.; Talley, J. W.; Luthy, R. G. Particle-scale investigation of PAH desorption kinetics and thermodynamics from sediment. Environ. Sci. Technol. 2001, 35, 3468-3475.
Yong Ran, Guoying Sheng, and Jiamo Fu State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Wushan, Guangzhou 510640, PR China
Baoshan Xing Department of Plant, Soil and Insect Sciences University of Massachusetts Amherst, Massachusetts 01003-0960
P. Suresh C. Rao School of Civil Engineering Purdue University West Lafayette, Indiana 47907-1284 ES0680008