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Batch and stop-flow column experiments were performed toestimate persulfate decomposition kinetic parameters inthe presence of seven well-characterized aquifer materials.Push pull tests were conducted in a sandy aquifer to representpersulfate decomposition under in situ conditions. Thedecomposition of persulfate followed a first-order rate law forall aquifer materials investigated. Reaction rate coefficients(kobs) increased by an order of magnitude when persulfateconcentration was reduced from 20 g/L to 1 g/L, due to ionicstrength effects. 29294
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The column experiments yielded higher kobs thanbatch experiments due to the lower oxidant to solids massratio. The kinetic model developed from the batch test data wasable to reproduce the observed persulfate temporal profilesfrom the push pull tests. The estimated kobs indicate thatunactivated persulfate is a persistent oxidant for the range ofaquifer materials explored with half-lives ranging from 2 to600 d.IntroductionThe treatment of contaminated soil and groundwater by insitu chemical oxidation (ISCO) relies on the oxidationpotential of chemical reagents to destroy harmful organiccompounds. The interaction of these oxidants with targetand nontarget compounds in the subsurface plays animportant role in oxidant choice and overall loading re-quirements. These interactions in conjunction with the sitespecific oxidant delivery technique tomaximize contactwithtarget compounds determine the effectiveness and efficiencyof an ISCO treatment system. Over the last two decadesstudies conducted at the bench- and pilot-scale employingpermanganate or peroxide have highlighted critical issuesrelated to the ability of these oxidants to destroy environ-mentally relevant organic contaminants and how theyinteractwith aquifermedia (1, 2). Permanganate has a limitedability to oxidize aromatic hydrocarbons and other fuelrelated contaminants relative to catalyzed hydrogen peroxide,and the interaction of peroxide and permanganate withaquifer solids is very different (3).
The latest oxidant to gainwidespread use is persulfate which has a high oxidationpotential on activation to the sulfate radical (Eo 2.6 V),exhibits widespread reactivity toward organic compounds, is relatively stable in near-neutral aqueous solutions, andhas aminimal impact onsoilmicroorganisms (4–6). Persulfatealone can be an effective oxidant or it can be activated byaquifer material constituents. The study of unactivatedpersulfate stability also forms the basis for further evaluationof activated persulfate persistence. The peer-reviewed lit-erature that focuses on persulfate interactionwith potentiallyreductive and catalytic constituents in aquifermaterials andits stability in groundwater systems is limited (5).In the presence of aquifer materials peroxide undergoesenhanced decompositionwithout significantly changing thetotal reductive capacity (TRC) of the aquifer solids such thatrepeated addition of peroxide leads to continual peroxidedecomposition (7). This enhanced decomposition or deg-radation is so rapid that it limits the treatment radius ofinfluence in the subsurface to a fewmeters around injectionlocations (5). In contrast to peroxide, permanganate isconsumed by the natural organic matter (NOM) and inor-ganic reductants associated with aquifermaterials (8). Thusaquifer materials exposed to permanganate exhibit a finiteor an ultimate natural oxidant demand (NOD) (8–10). Oncethe ultimate NOD of the solids has been satisfied, perman-ganate is relatively stable in the subsurface for extendedperiods of time (3, 11).The TRC of an aquifer material can be theoreticallydetermined by estimating the sum of specific reductivespecies (e.g., natural organic matter (NOM), Fe(II), andMn(II)) or experimentally quantified by the chemical oxidantdemand (COD) test (10, 12). For some aquifer materials thereductivemineralsmay be the largest oxidant sink, while forothers it may be the NOM (10). Recent efforts by Xu andThomson (7, 10, 11) using a variety of aquifer solids haveshown that permanganatemass consumption and peroxideenhanced decomposition are highly correlatedwith the totalorganic carbon (TOC) content and/or the amorphous iron(FeAm) content.The limited data on persulfate interaction with aquifermaterials are conflicting. The data presented by Dahmani etal. (13) support the view that persulfate is stable once thepersulfate demand of the aquifer solids has been satisfied,similar to the behavior of permanganate. In contrast, Brownand Robinson (1) indicate that persulfate interaction withaquifer materials was fundamentally different than perman-ganatewith persulfatemass loss increasingwith time similarto the decomposition behavior of peroxide. In some shallowsoils persulfate only mildly oxidized the amorphous andrelatively young NOM (14, 15), while others (16) observed a93%decrease in the TOCcontent of loamy and sandy surfacesoils.Most of these studieswere performed on single aquifermaterial or soil samples from surficial horizons, and hencethe findings, while illustrative, have limited applicability todifferent aquifer materials.This paper identifies relevant aquifermaterial propertiesthat control the stability of persulfate in the presence ofmultiple aquifer solids and defines kinetic parameters thatcan be used in predictive tools for the design of a persulfate-based ISCO system.