Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Iodine-129 and Iodine-127 Speciation in Groundwater at Hanford Site, U.S.: Iodate Incorporation into Calcite” n his comment on our paper “Iodine-129 and Iodine-127 Speciation in Groundwater at Hanford Site, U.S.: Iodate Incorporation into Calcite”, Lu specified three concerns for Zhang et al’s study,1 including (1) precipitation mechanism (degassing vs freezing), (2) analytical methods, and (3) mass balance control. In response, comparative and comprehensive discussions on the precipitation mechanisms and iodine incorporation can be found in the paper, as well as below. This includes additional experiments of iodine distribution and speciation in calcite precipitates. In addition, the measurements of total iodine in soils/sediment were clarified below as well. The calculations on mass balance in this comment were clarified by using correct data sets. Lu proposed that freezing samples might have caused calcite to precipitate from the original groundwater. The processes described in the references2,3 provided by Lu do not, however, apply to our study. Naturally occurring cryogenic calcites are formed from rapid freezing of bicarbonate groundwater followed by CO2 degassing due to evaporation.2 In laboratory production of cryogenic calcites, CO2 degassing occurs when ice is sublimated under vacuum and well-controlled laboratory conditions.2 In other words, CO2 degassing is a necessary step for calcite precipitation from bicarbonate groundwater. In our study, in contrast, the groundwater was simply stored in capped Nalgene bottles and frozen at about −12 °C. Defrosting without a vacuum in the capped bottles is not robust enough to cause sufficient degassing of CO2. To prove this, a mixture of calcium nitrate and sodium bicarbonate, which contained 100 mg/L Ca2+ and 164 μg/L HCO3−, close to the highest Ca2+ and HCO3− concentrations we found in the studied Hanford Site groundwater, was prepared in a Nalgene bottle and frozen for 3 days. No significant precipitation was observed after it was defrosted. In addition, except for the groundwater from the Hanford Site that was sent to us without filtration, all other groundwater samples that we have studied in our lab over the last 15 years (e.g., Savannah River Site groundwater, cave water, seawater samples) were frozen after in situ filtration and did not precipitate from freezing. Therefore, for the groundwater from the Hanford Site, the only possibility for CO2 degassing is due to the dramatic decrease in barometric pressure during groundwater sampling, that is, the precipitation must have occurred before freezing. In addition, a new ion exchange experiment followed by an acid treatment was applied to the suspended particles from CO2 degassing and to those from CaCO3 precipitation by CaCl2 and Na2CO3 to investigate whether the incorporated iodine is absorbed on surface or inside precipitate structure. Briefly, 5 mg of precipitate was extracted by 3 mL of 0.1 M KCl twice on a Vortex for 60 min and then rinsed with 3 mL 0.1 M KCl. The 9 mL of extraction and rinse solutions were combined and subjected for iodide and iodate analysis. The KCl-extracted iodine was considered as absorbed iodine on the surface of the precipitates.4,5 The residual precipitates were resuspended in 4.5 mL Milli-Q water and then 0.5 mL of 1 M HCl was added.
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© 2013 American Chemical Society
The mixtures were capped and shaken on a Vortex for 1 h, allowing the precipitate to dissolve in the acid solution. Iodine in the acid solution was considered as incorporated iodine inside the precipitate/calcite structure. The results of iodine distribution and speciation experiment demonstrate that 11− 21% of iodine was absorbed on the surface of the CO2degassing precipitate, among which, iodide accounted only 80%) incorporated inside the calcite structure, with iodate accounting for 77−86%. A similar incorporation pattern was also observed for the precipitates from CaCO3 precipitation by CaCl2 and Na2CO3. More than 90% of incorporated iodine was not ion exchangeable and exclusively existed as iodate. Organo-iodine concentrations in the acid solution were below the detection limit, further confirming our discussion in Zhang et al’s study,1 that organics play a limited influence on iodine incorporation into the precipitates. Regarding Lu’s second concern about the need for additional information about analytical methodology, the total iodine concentrations in precipitates in this study were measured following the same procedure used to measure total iodine in soils, as described in Zhang et al 2010.6 Briefly, precipitates were combusted at 850 °C under an O2 stream. Total iodine (I−, IO3− and OI) of the sample was all converted to inorganic iodine and then collected in distilled water, and the resulting solutions were further analyzed according to the procedure for iodate measurement with GCMS. ICPMS was not used for any iodine measurements in this study. Finally, in response to Lu’s third comment, it appears we may have not clearly provided information in the manuscript, leading Lu to use the wrong values (the lower part of the Table 11) to independently calculate mass balance. The gain/loss ratios calculated by Lu were incorrect. Here we recalculated them using upper part of the Table 1 and Table 5.1 The gain/ loss ratios are 1.40, 1.55, and 1.60, respectively. The relatively high gain/loss ratios might have been caused by deviations in measuring total iodine. The empirical standard deviation for total iodine is around 10%7 which is good for iodine speciation determination, but could contribute >0.4 to a gain/loss ratio. In conclusion, CO2 degassing induced by a dramatic decrease in barometric pressure during groundwater sampling was the primary cause for the formation of precipitates, which is mainly consist of calcium carbonate. Iodate is the main species of iodine to coprecipitate with calcite by two processes, surface sorption and iodate substitution for CO32‑, with the latter being the dominant process, accounting for ∼80% iodine coprecipitation.
Saijin Zhang*,† Chen Xu† Danielle Creeley† Yi-Fang Ho†
Published: November 4, 2013 13205
dx.doi.org/10.1021/es4046132 | Environ. Sci. Technol. 2013, 47, 13205−13206
Environmental Science & Technology
Correspondence/Rebuttal
Hsiu-Ping Li† Russell Grandbois† Kathleen A. Schwehr† Daniel I. Kaplan‡ Chris M. Yeager§ Dawn Wellman∥ Peter H. Santschi† †
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Department of Marine Science, Texas A&M University at Galveston, Texas 77554, United States ‡ Savannah River National Laboratory (SRNL), Aiken, South Carolina 29802 , United States § Los Alamos National Laboratory (LANL), Los Alamos, New Mexico 87545, United States ∥ Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States
AUTHOR INFORMATION
Corresponding Author
*Corresponding author at: 200 Seawolf Pkwy, Bldg. 3029, Galveston TX, USA 77554. Tel +1 409 354 4530; fax: + 1 409 740 4787 E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Zhang, S.; Xu, C.; Creeley, D.; Ho, Y.-F.; Li, H.-P.; Grandbois, R.; Schwehr, K. A.; Kaplan, D. I.; Yeager, C. M.; Wellman, D.; Santschi, P. H. Iodine-129 and iodine-127 speciation in groundwater at Hanford Site, U.S.: Iodate incorporation into calcite. Environ. Sci. Technol. 2013, 47, 9635−9642. (2) Clark, I. D.; Lauriol, B. Kinetic enrichment of stable isotopes in cryogenic calcites. Chem. Geol. 1992, 102 (1−4), 217−228. (3) Bein, A.; Arad, A. Formation of saline groundwaters in the baltic region through freezing of seawater during glacial periods. J. Hydrol. 1992, 140 (1−4), 75−87. (4) Yamaguchi, N.; Nakano, M.; Takamatsu, R.; Tanida, H. Inorganic iodine incorporation into soil organic matter: Evidence from iodine Kedge X-ray absorption near-edge structure. J. Environ. Rad. 2010, 101, 451−457. (5) Xu, C.; Miller, E. J.; Zhang, S.; Li, H. −P.; Ho, Y. −F.; Schwehr, K. A.; Kaplan, D.; Otosaka, S.; Roberts, K. A.; Brinkmeyer, R.; Yeager, C. M.; Sanschi, P. H. Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River site. Environ. Sci. Technol. 2011, 45, 9975− 9983. (6) Zhang, S.; Schwehr, K. A.; Ho, Y.-F.; Xu, C.; Roberts, K. A.; Kaplan, D. I; D., I.; Brinkmeyer, R.; Yeager, C. M; Santschi, P. H. A novel approach for the simultaneous determination of iodide, iodate, and organo-iodide for 127I and 129I in environmental samples using gas chromatography mass spectrometry. Environ. Sci. Technol. 2010, 44, 9042−9048. (7) Zhang, S.; Du, J.; Xu, C.; Schwehr, K. A.; Ho, Y. -F.; Li, H. -P.; Roberts, K. A.; Kaplan, D. I.; Brinkmeyer, R.; Yeager, C. M.; Santschi, P. H. Concentration dependent mobility, retardation and speciation of iodine in surface sediment from the Savannah River Site. Environ. Sci. Technol. 2011, 45, 5543−5549.
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