1、Persulfate Persistence underThermal Activation ConditionsRICHARD L. JOHNSON,*PAUL G. TRATNYEK, ANDREID OBRIEN JOHNSONDepartment of Environmental and Biomolecular Systems,Oregon Health fax: 503-748-1464;e-mail: rjohnsonebs.ogi.edu.Environ. Sci. Technol. 2008, 42, 9350935693509ENVIRONMENTAL SCIENCE (i
2、i)once soil NOD is met, reaction rates return to first-orderthermolysis values; and (iii) for each individual respike,disappearance of persulfate appeared to be pseudo-firstorder. Thus, the model results indicate that the rate ofpersulfate consumption by NOD proceeds in a well-behaved manner until t
3、he NOD is met, and based on thisbehavior we believe that the persulfate/NOD reaction (eq18) can be effectively modeled in groundwater systems.Persulfate Diffusion into Contaminant Source Zones.In many groundwater remediation contexts, contactbetween injected chemical oxidant and contaminantrequires
4、diffusion of the oxidant into low-permeabilitysource zones. For thermally activated persulfate, access todiffusion limited source zones may be limited becausepersulfate decomposition at moderate to high tempera-tures has half-lives on the order of hours to days (Sup-porting Information Figure S4, Ta
5、ble 1). The impact ofthis can be examined using a simple one-dimensionaldiffusion model. Initial and boundary conditions for themodel were (i) the diffusion zone was assumed to be in-finitely thick; (ii) initially there was no persulfate in thediffusion zone; (iii) the persulfate concentration at th
6、einterface between the advection-dominated and diffusionzones was maintained at 100 mM (200 mM oxidizingequivalents) throughout the simulations; (iv) the soil wasassumed to have an initial oxidant demand of 0.019 mmolof reducing equivalents per gram (i.e., the value determinedfor the soil in the res
7、pike experiments above); and (v) thesoil bulk density and porosity were assumed to be 1.6 gcm-3and 0.4, respectively.Effective diffusion coefficients used in the model wereadjusted to vary linearly with the absolute temperature(i.e., to follow the Stokes-Einstein equation and Wilke-Changcorrelation)
8、. Temperature-dependent values of the rateconstants for persulfate decomposition by eqs 17 and 18,determined from the batch experiments discussed above,are listed in Table 1. Diffusion and reaction were simulatedusing an explicit finite difference solution to the followingequations:S2O82-t)DMolec2S2
9、O82-x2-k1S2O82- -k4S2O82-NOD(21)NODt)-k4S2O82-NOD (22)where Dmolecis the effective molecular diffusion coefficient(Table 1), the persulfate concentration is expressed in molesof oxidizing equivalents per liter of water (i.e., twice the molarconcentration), and the aqueous NOD concentration iscalcula
10、ted as the oxidant demand per gram of soil times thebulk density of the soil divided by the porosity.Results of the diffusion modeling are shown in Figure4. The curves in the main figure are for persulfatedegradation by thermolysis only. They show that the depthof persulfate penetration at elevated
11、temperatures islimited, even in the absence of soil NOD. If the persulfate/NOD reaction is included, the resulting steady-state profilesare essentially identical to the thermolysis-only cases.However, the time required to reach steady state includingNOD are significantly longer than the thermolysis-
12、onlycase (Table 1).As discussed above, the lifetime of persulfate at elevatedtemperatures is short when compared to transport times.FIGURE 4. Calculated steady-state diffusion profiles forpersulfate in saturated soil without soil oxidant demand. Inset:calculated persulfate concentration profiles vs
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