Correspondence/Rebuttal pubs.acs.org/est
Comment on “Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments”
T
ong et al., 20161 reported a study affirming the production of abundant hydroxyl radicals (•OH) from oxygenation of subsurface sediments at circumneutral pH. Their study was prompted by several recent studies reporting the production of •OH in the dark such as Page et al., 20132 and Minella et al., 2015.3 Knowing that ferrous iron [Fe(II)] serves as a catalyst for the Fenton mechanism4 and that the subsurface sediments represent a more significant source of Fe (II) than the materials involved in the previous studies, i.e., arctic surface water and pore water contained in soils2 and anoxic lake water,3 Tong et al. hypothesized that more •OH may be produced from the oxygenation of the Fe(II)-rich minerals such as phyllosilicates in aquifer sediments. They reported that the hypothesis was corroborated by the results from their experiments conducted both in the laboratory and in the field showing orders of magnitude of more •OH produced than those reported by the previous studies. Tong et al. claimed that they have identified an abundant source of •OH in natural subsurface environment that was previously overlooked and their findings could open new areas for environmental remediation practices. Their findings, if true, are indeed of significant environmental importance. However, I think that Tong et al. may have overstretched the findings from the relevant previous studies.2,3 As pointed out by Tong et al., constant production of •OH in abundance is expected for aquifers with an oxic/anoxic interface where frequent interactions of O2 with the aquifer sediments occur, given that Fe (II) is a ubiquitous element in natural sediments. Most natural shallow or water table aquifers fall into this category, because they periodically receive percolated rainwater with many of them in frequent interactions with surface water and, in response to climatic changes, undergo temporal water table fluctuation causing alternating wetting and drying. We also know that with the exception of the presence of dense nonaqueous phase liquids (DNAPLs), contaminants are first found in the shallow aquifers where, according to the findings by Tong et al., •OH is available in abundance to initiate the advanced oxidation of contaminants through the Fenton mechanism.4 As such, spontaneous and sustaining contamination reduction in such aquifers would have been a common phenomenon. The fact is, however, aquifer remediation in general does not occur without human intervention. Otherwise, we would be today facing a much smaller environmental challenge caused by contaminated soils and groundwater.5 I want also to raise questions in Figure 4 reported by Tong et al., 2016.1 Figure 4 displays the results of a field injectionextraction test showing good agreement between the •OH concentrations and the Cl− concentrations measured in the extracted groundwater samples taken during the course of the extraction. For the field test, Tong et al. reported that a total of 360 L of oxygenated water containing Cl− at 100 mg/L as a tracer was injected into the aquifer through a 23 m deep well (screened between 20.6 and 23.0 m in depth) at a rate of 40 L/ min, followed by injecting 30 L deoxygenated and deionized © XXXX American Chemical Society
water to serve as a baseline. The injected water was allowed to settle in the aquifer overnight. The experiment resumed by extracting water from the same well at a rate of 0.72 to 0.9 L/ min over a course of about 13 h. A total of 663 L groundwater, less than doubling the injection amount, was extracted, sampled and analyzed at approximately 10 mL intervals. With respect to the Cl− data displayed in Figure 4, I found myself had a difficult time to comprehend the following observations: (1) the first sample showed near zero Cl− concentration thus representing deionized water, suggesting that at least part of the 30 L injected deionized water remained unmixed with the ambient groundwater after settling in the aquifer overnight, given Cl− as a major anion in groundwater; (2) the bell-shaped Cl− concentration curve with a maximum value equaling or slightly exceeding the injection concentration of 100 mg/L, added with the steeply sloped ascending and descending limbs, showing that the injected water was to a great extent unaffected by the ambient groundwater flowing in the aquifer; and (3) the bulk of the injected Cl− mass was recovered through extracting merely 663 L water after settling in the aquifer overnight. Given the small amount of the injected water relative to the vast volume of water contained in the aquifer, and the attenuation in the aquifer occurring in three dimensions dictated by the advection and hydrodynamic dispersion processes, a Cl− concentration time-series plot remarkably different from the type of curve displayed in Figure 4 is expected, irrespective of the specific hydrogeological conditions of the aquifer or the construction details of the injection well involved in the test. The shape of Cl− concentration curve displayed in Figure 4 actually reveals typical characteristics of restricted one-dimensional flow and is similar to, for example, a solute transport breakthrough curve generated from a column experiment with a pulse injection. To demonstrate the expected Cl− concertation behavior in a field test conducted in a three-dimensional aquifer, I constructed a numerical model using the commercially available finite-element modeling software FEFLOW6 to approximate the field injection-extraction conditions as reported by the authors. Because very limited information regarding the tested aquifer was provided, many assumptions had to be made. The modeling assumed a 7000 × 7000 × 30 m confined aquifer with a uniform groundwater flow with a hydraulic gradient of 0.001 m/m maintained by the constant head boundaries as detailed below. An overlying aquitard of 1 m thick serves as the confining unit. Model boundaries included constant head boundaries for the up- and the downstream boundaries with respective heads of 37.5 and 30.5 m, and two no-flow side boundaries. The testing well was placed at the center of the model domain. The model had four layers, with the top layer representing the aquitard and, the rest layers the aquifer with the middle layer matching the screened interval of the testing well. The model domain was discretized into 11,804 triangular
A
DOI: 10.1021/acs.est.6b00376 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Figure 1. Simulated piezometric heads and Cl− concentrations at the testing well.
Figure 2. Simulated Cl- distributions in the vicinity of the injection/extraction well at different times. Different concentration contour intervals and color schemes are used for the simulated plumes. Labeled concentrations are in mg/L.
elements with 5987 nodes per layer; the sizes of the model elements in the vicinity of the testing well approach the diameter of the well of 33 mm. Model parameters were chosen in a way to prevent the situation of simulating a heterogeneous aquifer with fast flowing groundwater where attenuation of the injected water by the ambient water is overly pronounced. The
lateral and vertical hydraulic conductivity for the aquifer layers were 3 m/day and 0.3 m/day, respectively. The aquitard layer had a homogeneous and isotropic hydraulic conductivity of 0.001 m/day. Uniform transport parameters were assumed for all four model layers, including a porosity of 0.3, a molecular diffusion coefficient of 1 × 10−9 m2/s, and longitudinal and B
DOI: 10.1021/acs.est.6b00376 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
(5) “Front Matter.″ National Research Council. Alternatives for Managing the Nation’s Complex Contaminated Groundwater Sites; The National Academies Press: Washington D.C., 2013. (6) Diersch, H. G. FEFLOW − Finite Element Modeling of Flow, Mass and Heat Transport in Porous and Fractured Media; Springer: Berlin Heidelberg, 2014.
transverse dispersivity of 5 and 0.5 m, respectively. An ambient Cl− concentration of 9.5 mg/L was estimated from the reported Cl− data shown in Figure 4. The model had a simulation period of 3,600 min. The field test was replicated in the model as follows: (1) the injection of Cl−-containing water was simulated using an injection well with a rate of 40 L/min operating from 1440 min to 1449 min; during this period, a constant Cl− concentration of 100 mg/L was prescribed for the model node representing the well; (2) from 1449 min to 1450 min, injection rate was changed to 30 L/min and the prescribed concentration was changed to 0 mg/L to simulate the subsequent injection of 30 L deionized water; (3) to simulate the overnight settling period after the injection, the well and the concentration boundaries were removed between 1,450 min and 2,160 min, assuming a 12 h settling period; (4) the 13 h extraction was simulated by adding an extraction well of 0.85 L/ min operated from 2160 min to 2940 min. Figure 1 shows the simulated Cl− concentrations at the testing well over the entire course of the simulated field test. Figure 2 shows the modeled Cl− plumes at four different times corresponding, respectively, to (1) end of the 9 min Cl−-containing water injection; (2) end of the 1 min clean water injection; (3) beginning of the 13 h extraction; and (4) end of the 13 h extraction. Although there are some minor numerical dispersions cause by the abrupt changes in aquifer stresses corresponding to the starting and the ending of both the injection and the extraction, the simulation results are deemed reliable. The resulting time-series plot for Cl− concentration at the testing well reveals the expected transport behavior of the injected water in a threedimensional aquifer. Admittedly, the numerical model is likely far from an exact replica of the injection-extraction test because the flow and transport parameters assumed in the model are not expected to represent the true aquifer conditions. Nevertheless, the modeling exercise serves to demonstrate that the Cl− concentrations reported by Tong et al. do not resemble a field test conducted in a natural aquifer.
Kerang Sun*
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CH2M HILL Inc., 6 Hutton Center Drive, Santa Ana, California 92707, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: 714-435-6324; e-mail:
[email protected]. Notes
The authors declares no competing financial interest.
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REFERENCES
(1) Tong, M.; Yuan, S.; Ma, S.; Jin, M.; Liu, D.; Cheng, D.; Liu, X.; Gan, Y.; Wang, Y. Production of abundant hydroxyl radicals from oxygenation of subsurface sediments. Environ. Sci. Technol. 2016, 50, 214−221. (2) Page, S. E.; Kling, G. W.; Sander, M.; Harrold, K. H.; Logan, J. R.; McNeill, K.; Cory, R. M. Dark formation of hydroxyl radicals in arctic soil and surface waters. Environ. Sci. Technol. 2013, 47 (22), 12860− 12867. (3) Minella, M.; Laurentiis, E. D.; Maurino, V.; Minero, C.; Vione, D. Dark production of hydroxyl radicals by aeration of anoxic lake water. Sci. Total Environ. 2015, 527, 322−327. (4) Pignatello, J.; Oliveros, E.; Mackay, A. Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Critical Rev. in Environ. Crit. Rev. Environ. Sci. Technol. 2006, 36 (1), 1−84. C
DOI: 10.1021/acs.est.6b00376 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
