Persulfate activation by naturally occurring trace minerals

Abstract

The potential for 13 naturally occurring minerals to mediate the decomposition of persulfate and generate a range of reactive oxygen species was investigated to provide fundamental information on activation mechanisms when persulfate is used for in situ chemical oxidation (ISCO). Only four of the minerals (cobaltite, ilmenite, pyrite, and siderite) promoted the decomposition of persulfate more rapidly than persulfate–deionized water control systems. The other nine minerals decomposed persulfate at the same rate or more slowly than the control systems. Mineral-mediated persulfate activation was conducted with the addition of one of three probe compounds to detect the generation of reactive oxygen species: anisole (sulfate + hydroxyl radical), nitrobenzene (hydroxyl radical), and hexachloroethane (reductants and nucleophiles). The reduced mineral pyrite promoted rapid generation of sulfate + hydroxyl radical. However, the remainder of the minerals provided minimal potential for the generation of reactive oxygen species. The results of this research demonstrate that the majority of naturally occurring trace minerals do not activate persulfate to generate reactive oxygen species, and other mechanisms of activation are necessary to promote contaminant destruction in the subsurface during persulfate ISCO.

Introduction

In situ chemical oxidation (ISCO) has become a dominant technology for the remediation of contaminated soils and groundwater. Of the three common ISCO processes, permanganate, catalyzed H₂O₂ propagations (CHP), and activated persulfate, activated persulfate is the least mature and least understood technology. Nonetheless, persulfate appears to have many advantages as an ISCO reagent. Although the persulfate anion (S₂O₈²⁻) is a strong oxidant (E⁰ = +2.01 V), it is usually activated by heat, transition metals, or base to generate sulfate radical (SO₄⁻), a stronger oxidant (E⁰ = +2.6 V) [1], [2]. Sulfate radical can then react with water to form another oxidant, hydroxyl radical (OH) [3].

Numerous investigators have focused on the activation of persulfate. Thermal activation of persulfate has been studied for treating methyl tert-butyl ether (MTBE) [4] and trichloroethylene (TCE) [5]. Persulfate activation by iron has been approached from several perspectives. Liang et al. [6] found that iron (II), but not iron (III), activates persulfate, and proposed using thiosulfate to regenerate iron (II) after it was oxidized by persulfate activation. Anipsitakis and Dionysiou [7] demonstrated that out of nine transition metals tested, only three activated persulfate to promote degradation of 2,4-dichlorophenol: silver (I), iron (II), and iron (III), with silver (I) providing the most activation and iron (III) the least. Killian et al. [8] and Rastogi et al. [9] subsequently demonstrated that iron chelates are more effective than iron (II) in activating persulfate. Furman et al. [10] recently elucidated the mechanism of base activation of persulfate, which involves the base-catalyzed hydrolysis of persulfate to hydroperoxide anion. The hydroperoxide anion then reduces another persulfate molecule, generating sulfate radical and sulfate anion. High ratios of base to persulfate are often required for effective persulfate activation [11].

Although a modicum of fundamental information is known about persulfate activation in relatively simple aqueous systems, its chemistry in the subsurface has received little attention. Subsurface chemistry has been shown to be a dominant factor in the effectiveness of other ISCO technologies [12]. For example, minerals decompose hydrogen peroxide leading to the formation of reactive oxygen species and resulting in rapid decomposition of hydrogen peroxide in the subsurface [13], [14]. Subsurface minerals decompose H₂O₂ effectively enough that the addition of soluble iron is usually not required for CHP ISCO [15].

In contrast to research on CHP, minimal attention has been given to the possible activation of persulfate by naturally occurring minerals found in surface and subsurface soils. Liang et al. [16] studied TCE destruction in columns packed with a sandy soil; however, the authors did not investigate the potential for the minerals in the soil to activate persulfate. Costanza et al. [17] studied perchloroethylene (PCE) destruction by thermally activated persulfate in batch reactors containing several solids and soils, but they did not investigate the effect of soil minerals on persulfate activation. Similarly, Johnson et al. [18] studied the effect of soil natural oxidant demand, but not soil minerals, on persulfate decomposition. Ahmad et al. [19] investigated the potential for major soil minerals (e.g., goethite, birnessite) found in surface and subsurface soils to activate persulfate. They found that most of the dominant soil minerals, such as goethite and ferrihydrite, did not activate persulfate. Birnessite activated persulfate, but only at concentrations significantly greater than typically found in soils. Many trace minerals are characterized by surface charge couples significantly different from the dominant iron and manganese oxides found in soils. However, the potential for trace minerals, such as ilmenite, cuprite, etc., to activate persulfate has not been investigated to date. The objective of this research was to investigate the potential for soil trace minerals to promote the decomposition of persulfate and activate it to generate reactive oxygen species.