Response to Comment on “Inhibitory Effect of Dissolved Silica on H2O2 Decomposition by Iron(III) and Manganese(IV) Oxides: Implications for H2O2–Based In Situ Chemical Oxidation”
Year: 2012
Type: editorial
Abstract: 2 Dr. Hanna has raised questions about: (a) the mechanism through which dissolved silica (SiO 2 ) inhibits hydrogen peroxide (H 2 O 2 ) decomposition in our system; and, (b) the feasibility of using dissolved SiO 2 to stabilize H 2 O 2 in in situ chemical oxidation (ISCO) systems.Here we address these questions in the sequence presented in his comment.Mechanism Through Which Dissolved SiO 2 Stabilizes H 2 O 2 .Dr. Hanna suggested that the slower rate of H 2 O 2 decomposition observed in the presence of SiO 2 was due to interactions between dissolved SiO 2 and H 2 O 2 .In support of this assertion, Dr. Hanna cited refs 15-18 (numbers correspond to references in the comment), which involved the decomposition of H 2 O 2 by transition metals.In our opinion, the slower H 2 O 2 decomposition in the presence of dissolved SiO 2 observed in these studies was most likely due to SiO 2 decreasing the catalytic activity of the metals.This wellstudied phenomenon is a much more rational explanation than the speculative interaction between SiO 2 and H 2 O 2 invoked by Dr. Hanna.As was reported in our paper, the concentration of dissolved iron in our experiments was below the method detection limit (i.e., 5 μM).On the basis of our previous research, 3 H 2 O 2 decomposition under the conditions employed in our study was catalyzed mainly by iron mineral surfaces.For this reason, the dissolved Fe III experiment proposed by Dr. Hanna would not provide any useful information.Moreover, given the wellstudied effect of iron-complexing ligands on Fenton and Fenton-like reactions, 4,5 the slower H 2 O 2 decomposition observed in Dr. Hanna's experiment with dissolved Fe III and SiO 2 is not surprising.Additionally, while dissolved SiO 2 might stabilize H 2 O 2 , the proposed experiment would neither prove nor disprove Dr. Hanna's hypothesis.Dissolved Silica as a H 2 O 2 -Stabilizing Reagent in ISCO.Dr. Hanna questioned the merit of using dissolved SiO 2 as a H 2 O 2 -stabilizing reagent in ISCO, because "the groundwater contains a lot of organic and inorganic ligands that can strongly compete with silicate for sorption". 1This comment, in our opinion, is erroneous because it is contrary to the SiO 2 adsorption effect widely observed by researchers studying the activity of iron oxides in groundwater systems (e.g., refs 6 and 7).Dr. Hanna pointed out that dissolved SiO 2 was unable to stabilize H 2 O 2 in a previous study. 8To further evaluate the feasibility of employing dissolved SiO 2 in ISCO, we performed additional experiments with aquifer sediments.Our results (Figure 1) indicated that the presence of 2 mM of dissolved SiO 2 approximately doubled the half-life of H 2 O 2 in three different aquifer samples.These results, together with the data in ref 8, suggest that the inhibitory effect of dissolved SiO 2 could vary among aquifers.On the basis of the data presented in Figure 1 and the finding described in our paper, we believe that SiO 2 has potential merit as a stabilizer for H 2 O 2 in ISCO treatment systems.
DOI: 10.1021/es300559w
Institution: University of California, Berkeley
Language: English
Subfield: Water Science and Technology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2332618950