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Periodic groundwater samples over ∼25 d werecollected from the drive point at an extraction rate of ∼0.4L/min after purging the drive point and tubing by >3 timestheir volume. Once the sample was collected the extractionsystemwas turned off and not activated again until the nextsampling episode. The sampleswere analyzed for persulfateusing the procedure outlined above, and Li using IonChromatography (DIONEX,DX300)with aMDL of 0.1mg/L.Results and DiscussionBatch Tests.Normalized persulfate and pHtemporal profilesfrom the batch reactor experiments conducted at the lowand high persulfate concentration are shown in Figure 1.Each data point represents the average fromtriplicate reactors(SD <7%).Data fromthe control reactors indicated that persulfateat the low concentration were stable for the entire ∼80 dexperimental period, while persulfate at the high con-centration was stable for the first 80 d and then decreasedsignificantly (2 data points) until the experiment wasterminated at ∼300 d. Uncatalyzed degradation of per-sulfate can be initiated through homolytic cleavage ofpersulfate to produce hydrogen sulfate and oxygen (20–22)as given byAt the experimental temperature conditions used here(∼20 °C) this is a very slow reaction with an estimated first-order rate coefficient of 3.7 × 10 4d 1(21) and thereforeleads to little (<5%) persulfate degradation in the controlsover the first 80 d. The ratio of the observed pH in thepersulfate controls to the theoretical pH (eq 2) was 1.04indicating that the persulfate degradation as described bythe hydrolysis reaction accounts for the observed Hproduction. In the high persulfate concentration controlsthe solution pH reduced to ∼3 by Day 40 as a result of theacidity generated (eq 2). When the pH decreases to <3persulfate degradation also occurs through the acid-catalyzedreaction (20, 21, 23, 24) to yield Caro’s acid represented byThis acid-catalyzed reaction was assumed to be respon-sible for much of the enhanced persulfate degradationobserved after Day 80 in the high persulfate concentrationcontrols. Loss of persulfate to acid-catalyzed degradationhas been observed to be detrimental to persulfate treatmentefficiency of some organic contaminants (25).Consistent with the notation used in ref 21, the overallrate lawfor the decomposition of persulfate in acid to neutralconditions in an aqueous solution (no solids present) is givenaswhere k1 is the first-order reaction rate coefficient foruncatalyzed degradation of persulfate (eq 2), and k2 is thesecond-order reaction rate coefficient for acid catalyzeddegradation of persulfate (eq 3) estimated as 1.1 × 10 1Lmol1d 1at 20 °C(21).In general, the solution pH in all the aquifer materialsystems decreased from the initial pH value due to thegeneration of H during persulfate degradation (eq 2)although the pH profiles were different for the two concen-trations employed (Figure 1). As expected the decrease inpH was more pronounced in the high persulfate concentra-tion systems since the degradation of a higher persulfateconcentration generates a larger H concentration. Thesignificant pH decrease observed in the controls due topersulfate degradation was not, in general, observed in thepresence of aquifer materials due to the buffering capacityof the solids. The total organic carbon (TOC) content is onlyS2O8 a small fraction of the total carbon (TC) content for 5 of the7 aquifermaterials used in this study (Table 1), and this highTC content is primarily due to the presence of calcite(carbonate) (7). Since the buffering capacity of carbonateminerals is well-known (26) it is expected that as persulfateis degraded, the high buffering capacity solids (i.e., Borden,LC34-LSU, LC34-USU, NIROP) wouldmaintain the solutionpH at near initial levels while the solution pH associatedwith the lower buffering capacity solids (i.e., DNTS, LAAP,MAAP) would decrease continuously. In fact, the aquifermaterial with the lowest TC content (i.e., MAAP) shows thelargest reduction in solution pH for both persulfate con-centrations. As the rate of H production decreases at latertimes the solution pH associated with the strongly bufferedsystems show a slight rebound. By Day 40, a solution pH ∼3was observed in the MAAP aquifer material systems at thehigh persulfate concentration and further decreased to ∼1by Day 290. It was assumed that acid-catalyzed degradationwas responsible for the enhanced persulfate degradationobserved beyondDay 80 for the high persulfate concentrationMAAP aquifer material systems.For all aquifer materials the decrease in persulfateconcentration followed a first-order kineticmass action lawwhich is consistent with the decomposition of persulfate inthe presence of catalysts and other reductants in aqueoussystems and in the presence of soil (20–22, 27, 28).