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The observations reported here are ofpractical interest and provide the most comprehensivepicture of persulfate decomposition in the presence of typicalaquifer materials.Materials and MethodsA series of batch and column experiments was conductedusing seven well-characterized, uncontaminated, sandy tosilty-sand calcite-rich aquifermaterials collected fromacross North America. These aquifer materials exhibit a range ofphysicochemical properties and variations in composition(10) that arewidely applicable to subsurface conditionswherepersulfate ISCO treatmentmay be used (Table 1). The initialCOD or reductive capacity of these materials is largely dueto the presence of TOC and a minor amount of mineral-based Fe and Mn. To avoid handling difficulties, onlymaterials <2mmgrain sizewere air-dried to a constantweightat 80 °C and used in these bench-scale investigations. Tosupport the bench-scale findings a sequence of pilot-scaleexperiments using the well established push pull method-ology (17) were performed at Canadian Forces Base (CFB)Borden located near Aliston, Ontario,
Canada to quantifypersulfate stability under in situ conditions.Chemicals. Sodium persulfate (Na2S2O8, purity g98%,Aldrich Chem. Co., Milwaukee), ACS grade ferrous am-monium sulfate (Fe(NH4)2(SO4)2 6H2O) (EMD Chemicals,Gibbstown), ammonium thiocyanate (NH4SCN) (Baker,Phillipsbourg), and sulfuric acid (H2SO4) (EMD Chemicals,Gibbstown) solutions were prepared to required concentra-tions using Milli-Q water. Sodium persulfate (Na2S2O8)(Klozur, FMC) and lithium chloride (LiCl, purity 99 %, ACSgrade, Aldrich Chem. Co., Milwaukee) injection solutionsfor the pilot-scale testswere prepared using uncontaminatedambient groundwater collected on-site.Batch Tests. To observe persulfate degradation rates andto evaluate the impact of persulfate exposure on the reductivecapacity of the aquifer materials used in this study, well-mixed batch reactor tests were conducted at a low (1.0 g/L)and high (20 g/L) persulfate concentration. Each reactor (300mL) was filled with 100 g of aquifer solids and 100 mL ofMilli-Q water and mixed manually for 5 min. Following a24 h equilibration period, each reactor was spiked with therequisite persulfate mass (0.1 or 2.0 g) to establish an initialpersulfate concentration of ∼1.0 or 20 g/L (∼4or80mM)oran oxidant to solids mass ratio of 1.0 or 20 g/kg. Temporalsampling for persulfate concentration and pHwas performedfrequently (∼5 d at early time to >15 d at later time). Thereactors were shaken by hand ∼2 h prior to sampling andfrequently between sampling events. All reactorswere storedat ∼20 °C in the dark. Quantification of persulfate wasperformed in duplicate on a 0.1 mL aqueous aliquot usingthe procedure described in ref 18 with a slight variation tocorrect for aquifer solids color interference. An Orion pHmeter (Model 290A) was used to measure pH. Controlsconsisted of persulfate at the two concentrations in Milli-Qwater only. All experimentswere conducted in triplicate. Forall aquifermaterials, except LC34-LSU, themass of persulfatein the high concentration reactors exceeded the stoichio-metric requirement for persulfate to oxidize the TRC(captured as COD) based on the following two electron halfReaction See Supporting Information (SI) Section SI1 for details ofthe COD tests used here.Column Tests. Column tests were conducted to closelymimic in situ conditionswith respect to the oxidant to solidsmass ratio of ∼0.25 g/kg for a persulfate concentration of 1g/L. Based on the slow persulfate depletion observed in thebatch experiments a stop-flow column operation scheme asopposed to continuous flow was adopted. The inherentassumption in these stop-flow column experiments is thatthere is insignificant depletion of the aquifer materialreduction capacity on exposure to persulfate. This allowsrepeated sampling fromthe same location in the column torepresent prolonged persulfate/aquifer material contact.Plexiglas columns (length of 40 cm, internal diameter of5 cm) were packed with air-dried solids. CO2 was flushedthrough the columns for ∼5 min followed by Milli-Q waterfor 24 h in the up-flow mode. Following this initial watersaturation process, ∼2 pore volumes (PVs) of the lowpersulfate concentration solution were injected into thecolumn in up-flowmode at a flow rate of ∼8mL/min for ∼1h to establish a uniform initial persulfate concentrationthroughout the column. Once complete breakthrough wasachieved (effluent concentration ∼1 g/L), the injectionwas stopped, and the tubing at both ends of the column wasclamped.Asampling port located at the bottomof the column(0 cm) was used to repeatedly obtain ∼5 mL samples of thecolumn pore-water as required. Sampling was performedfrequently during the first 10 d and then less frequent inresponse to observed concentration changes. ForCODtests,see SI Section SI1.Push Pull Tests. Push pull tests were conducted in apresumably no-drift, hydraulically isolated section of a sheet-pile walled gate, similar to that described in ref 19. A steeldrive point (3 cm diameter, 20 cm long screened section)was driven to 2.75 m below ground surface such that thescreen was approximately in the middle of the saturatedaquifer thickness of ∼3 m. The water table remained stableduring the course of these experiments. The push pull testswere designed to capture persulfate degradationwith respectto the depletion of a conservative tracer (Li ) and wereconducted at both the low(1 g/L) and high (20 g/L) persulfateconcentration in duplicate. Similar to the column experi-ments, an assumption relating to an insignificant depletionin the aquifer material reduction capacity was made tofacilitate multiple tests at the same location and depth.Bromide was deliberately not used as a conservative tracersince it showed signs of depletion within ∼5 d of exposureto persulfate (data not shown).A 100 L solution of the desired persulfate concentrationand ∼220 mg/L of Li (as LiCl) was injected into the aquiferover a period of ∼4 h under a gravity controlled flow rate of∼0.5 L/min.