Kinetics and efficiency of H₂O₂ activation by iron-containing minerals and aquifer materials

Abstract

To gain insight into factors that control H₂O₂ persistence and OH yield in H₂O₂-based in situ chemical oxidation systems, the decomposition of H₂O₂ and transformation of phenol were investigated in the presence of iron-containing minerals and aquifer materials. Under conditions expected during remediation of soil and groundwater, the stoichiometric efficiency, defined as the amount of phenol transformed per mole of H₂O₂ decomposed, varied from 0.005 to 0.28%. Among the iron-containing minerals, iron oxides were 2–10 times less efficient in transforming phenol than iron-containing clays and synthetic iron-containing catalysts. In both iron-containing mineral and aquifer materials systems, the stoichiometric efficiency was inversely correlated with the rate of H₂O₂ decomposition. In aquifer materials systems, the stoichiometric efficiency was also inversely correlated with the Mn content, consistent with the fact that the decomposition of H₂O₂ on manganese oxides does not produce OH. Removal of iron and manganese oxide coatings from the surface of aquifer materials by extraction with citrate–bicarbonate–dithionite slowed the rate of H₂O₂ decomposition on aquifer materials and increased the stoichiometric efficiency. In addition, the presence of 2 mM of dissolved SiO₂ slowed the rate of H₂O₂ decomposition on aquifer materials by over 80% without affecting the stoichiometric efficiency.

Introduction

Over the past two decades, hydrogen peroxide-based in situ Chemical Oxidation (ISCO) has become increasingly popular as a means of remediating contaminated soil and groundwater (Huling and Pivetz, 2006; Krembs et al., 2010). In this practice, concentrated solutions of H₂O₂ are injected into groundwater or added to soils. Upon contacting iron-containing minerals, some of the H₂O₂ is converted into OH (reactions (1) and (2) (e.g., Lin and Gurol, 1998; Petigara et al., 2002)), which subsequently oxidizes contaminants. The technology is effective against many of the most recalcitrant organic contaminants typically encountered at contaminated sites (e.g., benzene, phenol, trichloroethylene, polycyclic aromatic hydrocarbons). The use of H₂O₂ is also attractive because it is relatively inexpensive and its byproducts, namely O₂ and H₂O, are benign.

$$\equiv \text{Fe}\left(\text{III}\right)+{\text{H}}_2{\text{O}}_2\to \equiv \text{Fe}\left(\text{II}\right)+{\text{O}}_2^{-}+2{\text{H}}^{+}$$

$$\equiv \text{Fe}\left(\text{II}\right)+{\text{H}}_2{\text{O}}_2\to \equiv \text{Fe}\left(\text{III}\right)+{}^{}\text{OH}+{\text{OH}}^{-}$$

$$2{\text{H}}_2{\text{O}}_2\stackrel{\text{Fe}-\text{and}\text{Mn}-\text{oxides},\text{enzymes}}{\to }2{\text{H}}_2\text{O}+{\text{O}}_2$$

The cost and efficiency of H₂O₂-based ISCO systems depend largely upon the distance that H₂O₂ travels in the subsurface, as well as the fraction of the H₂O₂ that is converted into OH. It is generally desirable that H₂O₂ persists in the subsurface for a significant amount of time so that it can penetrate deep into aquifers and react with contaminants distant from injection wells. Thus, conditions that maximize H₂O₂ persistence in the subsurface are usually desirable, because they minimize both the amount of H₂O₂ and the number of injection wells needed to decontaminate a given site. As H₂O₂ also can be decomposed by non-radical pathways (i.e., the pathways that do not produce OH, which are represented collectively by reaction (3)) (Petigara et al., 2002; Pham et al., 2009), the amount of H₂O₂ required also depends upon the yield of OH (i.e., the relative amount of H₂O₂ decomposed by reactions (1) and (2) to the total amount of H₂O₂ decomposed by reactions (1), (2), (3)).

To gain insight into factors that influence H₂O₂ persistence and OH yield, previous investigators have studied H₂O₂ decomposition and contaminant transformation in systems consisting of pure iron oxides (e.g., ferrihydrite, hematite and goethite (Valentine and Wang, 1998; Huang et al., 2001; Kwan and Voelker, 2002)), iron minerals (e.g., pyrite (Matta et al., 2007), iron-containing aluminosilicates (e.g., Lou et al., 2009)), and aquifer materials and soils (Ravikumar and Gurol, 1994; Miller and Valentine, 1999; Petigara et al., 2002; Bissey et al., 2006; Watts et al., 2007; Xu and Thomson, 2010). Although these studies help explain the trends in the rates of removal of contaminants in groundwater and sediments upon addition of H₂O₂, it is still difficult to predict the rates of these processes under conditions encountered at hazardous waste sites.

Studies conducted with pure minerals have suggested that the crystallinity of the oxide, the coordination of Fe, and the mineral surface area affect H₂O₂ decomposition rates and OH yields (Valentine and Wang, 1998; Huang et al., 2001). Nevertheless, these studies are of limited utility because iron in aquifer materials usually exists as a mixture of different phases and/or is associated with other oxides (e.g., silica, alumina or manganese oxides). The reactivity of oxides in mixed oxides and silicates is different from that of the constituent end-member minerals (Lim et al., 2006; Pham et al., 2009; Taujale and Zhang, 2012). Moreover, in addition to iron minerals, other components in soils, such as manganese oxides, organic matter, and enzymes (e.g., catalase or peroxidase), also serve as H₂O₂ sinks (Petigara et al., 2002; Xu and Thomson, 2010). Thus, results obtained with iron oxides and iron minerals do not capture the heterogeneity and complexity of sediment systems, and could overestimate the production OH in remediation systems. Although studies conducted with aquifer materials and soils have the potential to capture some of this heterogeneity, most previous studies have not related rates of H₂O₂ decomposition and OH yields to the surface properties of the sediments. For example, Xu and Thomson (2010) attempted to correlate the rate of H₂O₂ decomposition with aquifer materials' properties. Their results suggested that the H₂O₂ decomposition rate is correlated with the Fe and Mn content of the aquifer materials. However, they did not quantify OH yields. Thus, it is currently difficult to predict the performance of H₂O₂-based ISCO treatment systems without conducting extensive site-specific scoping studies. Hence, more research is needed to predict and optimize contaminant removal in H₂O₂-based ISCO treatment system.

To reconcile prior observations of the role of different types of transition metal catalysts with data on stoichiometric efficiency and H₂O₂ activation kinetics, the reactivity of various iron-containing minerals and a diverse set of aquifer materials have been investigated. By studying H₂O₂ activation with materials from ten different aquifers under similar conditions (i.e., well-buffered solution pH and identical initial concentration of H₂O₂ and target contaminant) and correlating results with data on materials' physico-chemical properties, new insight has been gained about the factors affecting the H₂O₂ decomposition rates and OH yields. To assess the role of free iron and manganese oxides (i.e., pure oxides that exist as discrete particles or as surface coatings), aquifer materials were leached with citrate–bicarbonate–dithionite (CBD) solution to remove both types of metal-oxide coatings prior to H₂O₂ addition. In addition, the effect of dissolved SiO₂, a solute that is ubiquitous in groundwater, on the reactivity of aquifer materials was also studied.