Correspondence/Rebuttal pubs.acs.org/est
Comment on “Polymerization of Silicate on Hematite Surfaces and Its Influence on Arsenic Sorption” uthors of the paper, “Polymerization of Silicate on Hematite Surfaces and Its Influence on Arsenic Sorption” 1 present a long-term investigation pertaining to polymerization of silicate (or hydrated silica) onto hematite surface and its influence on arsenic sorption. Authors emphasize how naturally present iron oxide or hematite will be affected in the presence of dissolved silica in attenuating arsenic(V) and arsenic(III) species even in the absence of human interventions. Experimental conditions and the observations of the study relate to the natural environment. This correspondence aims to expand and correlate some of the findings of the study to engineered systems for arsenic removal. We also want to raise a fundamental question: Will the sorption of other environmentally significant ligands, be it fluoride or phosphate, onto other metal oxide surfaces, be it aluminum or zirconium oxide or even amorphous iron oxide, encounter similar effects in the presence of hydrated silica that inevitably exists in every surface or groundwater? The authors observed “higher competitiveness of silica in suspension pre-equilibrated with silica for 48 h, compared to suspension to which arsenic and silica were simultaneously added.” Also, such competing effect is significantly greater at pH of 8 than at pH less than 7. Frequently, packed- or fixed-bed sorption processes are employed to remove trace concentrations of arsenic in water using hydrated Fe(III) oxide particles as the selective sorbent.2 In such engineered processes, in accordance with the principles of chromatographic separation, arsenic is sorbed at the
A
top of the column in preference to silica and other competing anions. The bulk of the silica, which is present at nearly 2 orders of magnitude greater than arsenic in contaminated groundwater, binds to iron oxide surfaces near the bottom of the column as illustrated in Figure 1A. This situation is exactly similar to the batch system investigated in the paper where iron oxide is presaturated with silica in the absence of arsenic. Such an environment promotes sorption of hydrated silicate anion in close proximity to each other followed by slow polymerization reaction. Once polymerized, desorption of di- or trimers of silicate species is more difficult than nonpolymerized single silicate species. Furthermore, the zero point of charge (pHzpc) of this silica layer is significantly lower than that of iron oxide, that is, they are negatively charged even at neutral pH. Access of anionic arsenate onto iron oxide surface is thus impaired due to the Donnan exclusion effect, thereby diminishing arsenic sorption. Following exhaustion with arsenic, Iron oxide- based sorbents in the packed bed are routinely regenerated using 1−3% NaOH solution. However, once polymerized, silicates are not amenable to complete regeneration under the regenerating conditions, thus lowering arsenic removal capacity in the subsequent sorption cycles. Figure 1B shows arsenic and silica recovery during regeneration of an anion exchanger supported iron oxide nanoparticles. Note that while arsenic desorption is 95% complete, the same for silica is significantly lower and only 45%, confirming progressive silica accumulation or fouling on iron oxide surfaces.
Figure 1. A. Illustration of arsenic and silica stratification in the column leading to enhanced silica polymerization; B. Experimental validation of poor desorption/recovery of silica from the column compared to arsenate during NaOH regeneration. Published: April 18, 2013 © 2013 American Chemical Society
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Environmental Science & Technology
Correspondence/Rebuttal
From a generic viewpoint, such silica fouling has also been observed for activated alumina 3 during fluoride removal process and is likely to occur for zirconium and titanium oxide surfaces. Additional experimental investigations by authors on silica polymerization or fouling onto other metal oxide surfaces through attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy may provide a unifying scientific mechanism that may offer sound operating guidelines for engineered processes.
Surapol Padungthon Arup SenGupta*
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Civil & Environmental Engineering, Lehigh University, 13 E. Packer Avenue, Bethlehem, Pennsylvania 18015, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: 610-758-3534; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Christl, I.; Brechbühl, Y.; Graf, M.; Kretzschmar, R. Polymerization of silicate on hematite surfaces and its influence on arsenic sorption. Environ. Sci. Technol. 2012, 46 (24), 13235−13243. (2) Sarkar, S.; Blaney, L. M.; Gupta, A.; Ghosh, D.; SenGupta, A. K. Arsenic removal from groundwater and its safe containment in a rural environment: Validation of a sustainable approach. Environ. Sci. Technol. 2008, 42 (12), 4268−4273. (3) Clifford, D. A. Ion exchange and inorganic adsorption. In Water Quality and Treatment: A Handbook of Community Water Supplies; American Water Works Association; McGraw-Hill Professional: New York, 1999; pp 9.1−9.91.
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