Comment on “Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto

Correspondence/Rebuttal pubs.acs.org/est

Comment on “Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach”

W

e have read this work1 with great interest and would like to raise a few scientific questions related to a possible modification of surface reactivity of maghemite due to the presence of quartz. More precisely, we add a data on the dissolution of the quartz sample (Min-U-Sil 5) used by the authors,1 which proofs possible implications on the uptake properties of the iron oxide with respect to the adsorbing ions. We aim at a more comprehensive understanding of the sorption mechanisms involved in such complex mineral assemblages. The comparison of the intrinsic reactivity of coating phases in Fe−Si mineral assemblages vs pure individual phases should take into account the solubility of SiO2 particles and the concomitant release of silicate in solution. Such silicate release (from the dissolution of the quartz) and subsequent adsorption on Fe-phases in coated-sand systems can affect the fate and transport of organic and inorganic ligands.2−4 The dissolution of quartz in aqueous solution is of great complexity.5−7 In addition to classical parameters like temperature and solution conditions (pH, presence of cations, etc.), quartz dissolution kinetics can be affected by the particle size, because of the increasing surface area with diminishing grain size.5−12 Min-U-Sil quartz samples provided by U.S. Silica have different particle sizes (5, 10, 15, 30, and 40 μm) and have been the subject of several studies.4,13−16 Although the release of silicate from Min-U-Sil 5 (quartz sample used in this work1) has been previously evaluated,4 it was measured here for 2, 10, and 20 g/L of quartz. Prior to use, Min-U-Sil 5 was heated and acidwashed as in ref 1, to remove aluminum and iron oxides, amorphous silica and organic material.14,15 According to the data in Figure 1a, the aqueous silicate concentrations strongly depend on solid loading and also on pH, unlike the calculated quartz equilibrium concentrations, which are insensitive to pH below 9 (∼pKa of silicic acid). In contrast to what the name may suggest, the Min-U-Sil 5 contains smaller particles that are sometimes attached to the “5 μm particles” (as shown in Figure 1b). An increase in Min-U-Sil 5 solid concentration will increase the absolute amount of the finest quartz particles, and consequently the aqueous silicate concentrations in suspensions for nonequilibrium situations. Additionally, the quartz dissolution is time-dependent,5−7 but this time dependence does not seem to be crucial between 2 and 5 days in Min-U-Sil 5 suspensions, except at pH > 8 where [Si]aq (120h) > [Si]aq (48h) (Figure 1a). Overall, we content that the amounts of dissolved silicate in Min-U-Sil 5 suspensions would crucially depend on certain operational conditions adopted in sorption studies, for example, contact time and solid loading. For the conditions of the experiments chosen by Komárek et al.1 significant amounts of dissolved silica would have been present. For determining adsorption edges,1 the coated sand was left to equilibrate for 24 h for each pH value, and subsequently in a stirred aqueous solution for more than 1 week. Furthermore, an incipient contamination of maghemite by dissolved silicate is likely to have occurred during the coating procedures. Indeed, the maghemite-coated sand1 © 2016 American Chemical Society

Figure 1. (a) Aqueous Si concentrations measured in Min-U-Sil 5 suspensions. (a) At three different solid loadings (2, 10, and 20 g/L) and at two contact times for 2 g/L. 100 mM NaNO3 was used as background electrolyte. Lines are calculated equilibrium solubilities for quartz (solid line) and amorphous silica (dashed line); (b) SEM image of Min-U-Sil 5 particles showing the presence of various sizes of smaller particles (fines) attached to larger particles.

was prepared by mixing quartz sand and maghemite particles for a given time, according to Schwertmann and Cornell (or the method of Scheidegger et al.16). The presence of the dissolved silica may have various consequences for maghemite adsorption behavior. It has been reported that the adsorption of silicate onto iron oxides including maghemite takes place over a wide pH range.17−19 In these studies, the adsorption envelope of silica species bound to Fe-oxides by inner-sphere surface complexation typically showed maximum adsorption at pH ∼ 9 (pKa of silicic acid). Based on available sorption data for silicate onto pure maghemite,18 silicate surface coverage on maghemite can be estimated at around 1 μmol/m2 at circum-neutral pH ([≡FeOH]/[Si(OH)4] = 1:1). This may correspond to as much as 1 μmol or 2 μmol of surface sites per m2 (depending Published: January 21, 2016 1632

DOI: 10.1021/acs.est.5b05939 Environ. Sci. Technol. 2016, 50, 1632−1633

Environmental Science & Technology

Correspondence/Rebuttal

(8) Bennett, P. C. Quartz dissolution in organic-rich aqueous systems. Geochim. Geochim. Cosmochim. Acta 1991, 55, 1781−1797. (9) Tester, J. W.; Worley, W. G.; Robinson, B. A.; Grigsby, C. O.; Feerer, J. L. Correlating quartz dissolution kinetics in pure water from 25 to 625°C. Geochim. Cosmochim. Acta 1994, 58, 2407−2420. (10) Broekmans, M. A. T. M. Structural properties of quartz and their potential role for ASR. Mater. Charact. 2004, 53, 129−140. (11) Xu, B.; Wingate, C.; Smith, P. The effect of surface area on the modelling of quartz dissolution under conditions relevant to the Bayer process. Hydrometallurgy 2009, 98, 108−115. (12) Sergent, A.-S.; Jorand, F.; Hanna, K. Effects of Si-bearing minerals on the nature of secondary iron mineral products from lepidocrocite bioreduction. Chem. Geol. 2011, 289, 86−97. (13) Michael, H. L.; Williams, D. J. A. Electrochemical properties of quartz. J. Electroanal. Chem. Interfacial Electrochem. 1984, 179 (1), 131−139. (14) Coston, J. A.; Fuller, C. C.; Davis, J. A. Pb+2 and Zn+2 adsorption by a natural aluminum- and iron-bearing surface coating on an aquifer sand. Geochim. Cosmochim. Acta 1995, 59, 3535−3547. (15) Kohler, M.; Curtis, G.; Kent, D.; Davis, J. Experimental investigation and modeling of uranium (VI) transport under variable chemical conditions. Water Resour. Res. 1996, 32 (12), 3539−3551. (16) Scheidegger, A.; Borkovec, M.; Sticher, H. Coating of Silica Sand with Goethite: Preparation and Analytical Identification. Geoderma 1993, 58, 43−65. (17) Jordan, N.; Marmier, N.; Lomenech, C.; Giffaut, E.; Ehrardt, J.-J. Sorption of silicates on goethite, hematite, and magnetite: experiments and modelling. J. Colloid Interface Sci. 2007, 312 (2), 224−229. (18) Jolsterå, R.; Gunneriusson, L.; Forsling, W. Adsorption and surface complex modeling of silicates on maghemite in aqueous suspensions. J. Colloid Interface Sci. 2010, 342 (2), 493−498. (19) Yang, X.; Roonasi, P.; Jolsterå, R.; Holmgren, A. Kinetics of silicate sorption on magnetite and maghemite: An in situ ATR-FTIR study. Colloids and Surfaces A. Colloids Surf., A 2009, 343 (1−3), 24− 29. (20) Hiemstra, T.; Barnett, M. O.; Van Riemsdijk, W. H. Interaction of silicic acid with goethite. J. Colloid Interface Sci. 2007, 310, 8−17. (21) Meng, X.; Letterman, R. D. Effect of component oxide interaction on the adsorption properties of mixed oxides. Environ. Sci. Technol. 1993, 27 (5), 970−975. (22) Lützenkirchen, J.; Behra, P. A new approach for modelling potential effects in cation adsorption onto binary (hydr) oxides. J. Contam. Hydrol. 1997, 26 (1−4), 257−268.

on whether silicate is bound to goethite through a 1:1 mononuclear complex or a 2:1 binuclear complex, respectively).20 On the other hand, the presence of adsorbing silicate on surfaces of iron oxides may affect their respective values of PZC/IEP (point of zero charge/isoelectric point). Hiemstra et al.20 reported that adsorption of 1 mM of silicic acid on the surface of goethite decreased the IEP from 9 to 7. All these surface modifications occurring upon silicate adsorption are likely to affect the adsorption behavior and surface complexation mechanisms of Cd(II), Cr(VI), and Pb(II) onto maghemite.1 For the metal cations it could result in enhanced uptake via electrostatic interactions while for the anion it could cause competition. While such effects could be handled by a combined surface complexation model in principle, it has been shown that mixed oxide systems as those discussed here may be more complex and cannot be simply modeled by an additivity approach.21,22 To conclude, we suggest that in order to accurately assess interfacial mechanisms occurring at Fe-oxide/water interface in the context of Si complex mineral assemblages or Fe-coated sand systems, more attention should be paid to the possible release of silicate from Si-bearing minerals and its subsequent adsorption on reactive phases.

M. Kamagaté† J. Lützenkirchen‡ F. Huber‡ K. Hanna*,† †

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Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11, Allée de Beaulieu CS 50837, 35708 Rennes Cedex 7, France ‡ Institut fur Nukleare Entsorgung (INE), Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Komárek, M.; Koretsky, C. M.; Stephen, K. J.; Alessi, D. S.; Chrastný, V. Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach. Environ. Sci. Technol. 2015, 49 (21), 12851−12859. (2) Rusch, B.; Hanna, K.; Humbert, B. Coating of quartz silica with iron oxides: characterization and surface reactivity. Colloids Surf., A 2010, A (353), 172−180. (3) Rusch, B.; Hanna, K.; Humbert, B. Sorption and transport of salicylate in a porous heterogeneous medium of silica quartz and goethite. Environ. Sci. Technol. 2010, 44, 2447−2453. (4) Huber, F.; Luetzenkirchen, J. Uranyl Retention on QuartzNew Experimental Data and Blind Prediction Using an Existing Surface Complexation Model. Aquat. Geochem. 2009, 15, 443−456. (5) Van Lier, J. A.; De Bruyn, P. L.; Overbeek, J. T. G. The solubility of quartz. J. Phys. Chem. 1960, 64 (11), 1675−1682. (6) Dove, P. M. The dissolution kinetics of quartz in aqueous mixed cation solutions. Geochim. Cosmochim. Acta 1999, 63 (22), 3715− 3727. (7) Berger, G.; Cadore, E.; Schott, J.; Dove, P. M. Dissolution rate of quartz in lead and sodium electrolyte solutions between 25 and 300°C: Effect of the nature of surface complexes and reaction affinity. Geochim. Cosmochim. Acta 1994, 58, 541−551. 1633

DOI: 10.1021/acs.est.5b05939 Environ. Sci. Technol. 2016, 50, 1632−1633