Correspondence/Rebuttal pubs.acs.org/est
Comment on “Thermally Released Arsenic in Porewater from Sediments in the Cold Lake Area of Alberta, Canada”
J
aved and Siddique1 presented laboratory experiments demonstrating that arsenic (As) is released from sediments when heated to 200 °C. This study agrees with earlier studies showing arsenic is released from sediments upon heating,2 and from other media containing iron-(hydr)oxides.3−7 The work applies a sequential extraction technique to show that thermally released arsenic is primarily (∼89−100%) the result of desorption from specifically bound and exchangeable fractions. Although the study provides new and interesting insights, the underlying mechanisms of arsenic desorption are only limitedly discussed. The understanding of the key biogeochemical processes is a prerequisite for predicting the degree of arsenic (im)mobilization and the spatial extent of elevated arsenic concentrations in thermally impacted aquifers. In this comment, we address these issues and make recommendations for further research.
over a 2 orders of magnitude concentration range from 0 to 200 °C (Figure 1). The increased desorption of arsenic was
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EXPLANATIONS GIVEN FOR ARSENIC RELEASE The authors describe, in brief, two potential mechanisms that can release arsenic upon heating. First, the authors propose that the desorption of arsenic “might be facilitated by sulfate and bicarbonate ions present in the synthetic aquifer water through ion-exchange processes which can be enhanced under intense heat”. This type of competitive desorption is described in the literature for isothermic conditions8 for situations where displacement and flushing occurs of the one with the other aqueous composition in porous media; but not in preequilibrated static batch experiments. Moreover, the proposed mechanism fails to explain the stronger observed desorption with deionized water. Second, “heat may have lowered the redox potential and pH that favors the dominance of arsenite (AsIII) which has a low adsorption potential compared to arsenate (AsV)”. This explanation is not clearly elaborated nor convincing because (i) it is unlikely that short-term heating (1 h) would result in major AsV reduction given the unfavorable kinetics of this process,8 and (ii) while a temperature increase may decrease pH due to increased self-ionization of water, experimental data by Dixit and Hering9 show that desorption of AsIII from a pH decrease cannot explain the near complete arsenic desorption, whereas AsV sorption increases.
Figure 1. Results of temperature-dependent surface complexation modeling of arsenic containing sediments in equilibrium with “aquifer” water and with deionized water using the TD-SCM.10 The ratio of arsenic (Δ) at given temperature and arsenic at 10 °C (AsIII = 10 μg/l, AsV = 0 μg/l) is plotted. The model is based on the PHREEQC code which includes the surface complexation database for ferrihydrite by Dzombak and Morel.11 This database was extended using reaction enthalpies calibrated to sediment heating experiments.10 Aquifer water composition is taken similar to these earlier experiments.10
attributed largely to the negative enthalpy of arsenic sorption rather than the decrease in pH. Sorption reactions of oxyanions like arsenite and arsenate are exothermic similar to the exothermic nature of the protonation reaction of oxyanions.10 These protonation reactions are also used in combination with linear free energy relationships to demonstrate the consistency between different surface complexation reactions in the Dzombak and Morel database.11
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RELATED HYDROCHEMICAL CHANGES AND REVERSIBILITY OF SORPTION Besides thermally induced (de)sorption, a suit of other hydrochemical processes are thermally stimulated with the potential to influence arsenic mobility, including precipitation of carbonates,12 dissolution of silicates,13 mineralization of sedimentary organic matter,14 and changing redox reactions and kinetics.15 A temperature rise has been observed to trigger and/or accelerate sulfate reduction,15,16 even by thermophilic microbial species up to 70 °C.15 Therefore, sulfide formation and its subsequent precipitation resulting in arsenic sequestration,17 could be relevant at the fringe of the thermal plume
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TEMPERATURE-DEPENDENT SURFACE COMPLEXATION MODELING TO UNDERSTAND ARSENIC DESORPTION Temperature-dependent surface complexation modeling (TDSCM) has been shown to improve the understanding of the driving mechanisms leading to arsenic desorption.10 We applied our TD-SCM, calibrated to sediment heating column experiments (7−80 °C),10 to predict aqueous arsenic concentrations upon heating of sediment in equilibrium with both “aquifer water” and deionized water. Note we assumed for simplicity that arsenic occurred as AsIII. Our model confirmed the observed arsenic increase in both deionized and aquifer water © 2016 American Chemical Society
Published: June 15, 2016 7263
DOI: 10.1021/acs.est.6b02106 Environ. Sci. Technol. 2016, 50, 7263−7264
Environmental Science & Technology
Correspondence/Rebuttal
(3) Negrea, A.; Lupa, L.; Ciopec, M.; Lazau, R.; Muntean, C.; Negrea, P. Adsorption of As(III) Ions onto Iron-containing Waste Sludge. Adsorpt. Sci. Technol. 2010, 28 (6), 467−484. (4) Maji, S. K.; Pal, A.; Pal, T.; Adak, A. Adsorption thermodynamics of arsenic on laterite soil. J. Surface. Sci. Technol. 2007, 22, 161−176. (5) Kanel, S. R.; Manning, B.; Charlet, L.; Choi, H. Removal of Arsenic(III) from Groundwater by Nanoscale Zero-Valent Iron. Environ. Sci. Technol. 2005, 39 (5), 1291−1298. (6) Kersten, M.; Vlasova, N. Arsenite adsorption on goethite at elevated temperatures. Appl. Geochem. 2009, 24 (1), 32−43. (7) Hong, H. J.; Yang, J. S.; Kim, B. K.; Yang, J. W. Arsenic Removal Behavior by Fe-Al Binary Oxide: Thermodynamic and Kinetic Study. Sep. Sci. Technol. 2011, 46 (16), 2531−2538. (8) Smedley, P. L.; Kinniburgh, D. G. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17 (5), 517−568. (9) Dixit, S.; Hering, J. G. Comparison of Arsenic(V) and Arsenic(III) Sorption onto Iron Oxide Minerals: Implications for Arsenic Mobility. Environ. Sci. Technol. 2003, 37 (18), 4182−4189. (10) Bonte, M.; Stuyfzand, P. J.; van Breukelen, B. M. Reactive Transport Modeling of Thermal Column Experiments to Investigate the Impacts of Aquifer Thermal Energy Storage on Groundwater Quality. Environ. Sci. Technol. 2014, 48 (20), 12099−12107. (11) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; John Wiley & Sons: New York, 1990; p 393. (12) Griffioen, J.; Appelo, C. A. J. Nature and extent of carbonate precipitation during aquifer thermal energy storage. Appl. Geochem. 1993, 8 (2), 161−176. (13) Holm, T. R.; Eisenreich, S. J.; Rosenberg, H. L.; Holm, N. P. Groundwater Geochemistry of Short-Term Aquifer Thermal Energy Storage Test Cycles. Water Resour. Res. 1987, 23 (6), 1005−1019. (14) Brons, H. J. Biogeochemical aspects of aquifer thermal energy storage. PhD Thesis, Wageningen University, Wageningen, 1992. (15) Bonte, M.; Röling, W. F. M.; Zaura, E.; van der Wielen, P. W. J. J.; Stuyfzand, P. J.; van Breukelen, B. M. Impacts of Shallow Geothermal Energy Production on Redox Processes and Microbial Communities. Environ. Sci. Technol. 2013, 47 (24), 14476−14484. (16) Jesußek, A.; Köber, R.; Grandel, S.; Dahmke, A. Aquifer heat storage: sulphate reduction with acetate at increased temperatures. Environ. Earth Sci. 2013, 69, 1−9. (17) Radloff, K. A.; Zheng, Y.; Stute, M.; Weinman, B.; Bostick, B.; Mihajlov, I.; Bounds, M.; Rahman, M. M.; Huq, M. R.; Ahmed, K. M.; Schlosser, P.; van Geen, A., Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh. Appl. Geochem., in press.201510.1016/j.apgeochem.2015.11.003 (18) Schultz, M. F.; Benjamin, M. M.; Ferguson, J. F. Adsorption and Desorption of Metals on Ferrihydrite: Reversibility of the Reaction and Sorption Properties of the Regenerated Solid. Environ. Sci. Technol. 1987, 21 (9), 863−869. (19) Saito, T.; Hamamoto, S.; Ueki, T.; Ohkubo, S.; Moldrup, P.; Kawamoto, K.; Komatsu, T. Temperature change affected groundwater quality in a confined marine aquifer during long-term heating and cooling. Water Res. 2016, 94, 120−127. (20) Ford, R. G.; Bertsch, P. M.; Farley, K. J. Changes in transition and heavy metal partitioning during hydrous iron oxide aging. Environ. Sci. Technol. 1997, 31 (7), 2028−2033. (21) Smith, K. S., Metal sorption on mineral surfaces: an overview with examples relating to mineral deposits. In Reviews in Economic Geology: Vol. 6A and 6B, Plumlee, G. S.; Logsdon, M .J., Filipek, L.F. Eds.; Society of Economic Geologists: 1999.
where temperatures have decreased to levels where biological activity can occur. A key unknown that warrants further investigation concerns the reversibility of sorption when elevated temperatures decline to ambient conditions. Sorption is generally a reversible process under limited temperature variations.18 Cyclic aquifer heating experiments by Saito et al.19 showed potassium and boron concentrations increasing and subsequently decreasing following cooling but this was at a ΔT of only 7 °C. Aging of amorphous reactive surfaces to thermodynamically more stable structures such as goethite or hematite impacts sorption capacity and could reduce sorption reversibility.20,21 A (sustained) temperature increase may accelerate the aging process and decrease sorption capacity,9,21 especially in the core of the thermal plume. The extent of arsenic sorption around the periphery of a thermal treatment zone or during cooling after thermal treatment is therefore uncertain.
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CONCLUDING REMARKS Complementing Javad’s and Siddique’s work with observations on other relevant hydrogeochemical processes and assessing thermal reversibility will provide a more thorough conceptual understanding of the fate of arsenic and determine maximum mobilization distances. The experimental data collected by Javad and Siddique and conceptual models should in our view be tested with reactive transport models that include TDSCM.10 This integrated approach will help improve the understanding of critical geochemical processes and risk management decision making related to subsurface heating and associated arsenic mobilization.
Boris M. Van Breukelen*,‡ Matthijs Bonte† ‡
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Department of Water Management, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands † Shell Global Solutions, Lange Kleiweg 20, Rijswijk, The Netherlands
AUTHOR INFORMATION
Corresponding Author
*Phone: +31 (0)15-278-5227; e-mail: b.m.vanBreukelen@ tudelft.nl. Author Contributions
Both authors contributed equally and have given approval to the final version of the manuscript. Notes
The authors declare the following competing financial interest(s): M.B. works for Shell, a company which applies in-situ thermal technologies to extract bitumen.
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ACKNOWLEDGMENTS We thank colleagues of M.B. and especially Matthew Lahvis for comments and textual improvements. REFERENCES
(1) Javed, M. B.; Siddique, T. Thermally Released Arsenic in Porewater from Sediments in the Cold Lake Area of Alberta, Canada. Environ. Sci. Technol. 2016, 50 (5), 2191−2199. (2) Bonte, M.; van Breukelen, B. M.; Stuyfzand, P. J. Temperatureinduced impacts on groundwater quality and arsenic mobility in anoxic aquifer sediments used for both drinking water and shallow geothermal energy production. Water Res. 2013, 47 (14), 5088−5100. 7264
DOI: 10.1021/acs.est.6b02106 Environ. Sci. Technol. 2016, 50, 7263−7264
