In situ chemical oxidation (ISCO) is a technology used for groundwater remediation. This laboratory study investigated the use of the oxidant sodium persulfate for the chemical oxidation of ...trichloroethylene (TCE) at near ambient temperatures (10, 20 and 30
°C) to determine the influence of pH (pH
=
4, 7 and 9) on the reaction rate (i.e., pseudo-first-order rate constants) over the range of temperatures utilized. TCE solutions (60
mg
l
−1; 0.46
mM) were prepared in phosphate buffered RO water and a fixed persulfate/TCE molar ratio of 50/1 was employed in all tests. Half-lives of TCE degradation at 10, 20 and 30
°C (pH 7) were 115.5, 35.0 and 5.5
h, respectively. Maximum TCE degradation occurred at pH 7. Lowering system pH resulted in a greater decrease in TCE degradation rates than increasing system pH. Radical scavenging tests used to identify predominant radical species suggested that the sulfate radical
(
SO
4
-
)
predominates under acidic conditions and the hydroxyl radical (
OH) predominates under basic conditions. In a side by side comparison of TCE degradation in a groundwater vs. unbuffered RO water it was demonstrated that when the system pH is buffered to near neutral pH conditions due to the presence of natural occurring groundwater constituents that the TCE degradation rate is higher than in unbuffered RO water where the system pH dropped from 5.9 to 2.8. The results of this study suggest that in a field application of ISCO, pH should be monitored and adjusted to near neutral if necessary.
The kinetics of dissolved trichloroethylene (TCE) oxidation by thermally activated sodium persulfate (Na2S2O8) was investigated in a phosphate-buffered solution at pH 7 and an ionic strength of 0.1 ...at a temperature of 40 °C. Thermal activation results in the production of a highly reactive sulfate free radical (SO4 •-). The method of half-lives was used to determine the reaction order with respect to TCE. This was accomplished by conducting a series of kinetic experiments and keeping in great excess (and therefore essentially constant) the sodium persulfate concentration while varying the TCE initial concentration. Resulting plots of normalized TCE concentration (C/C o) vs time were analyzed by polynomial regression analysis. Half-lives were calculated based on the resulting polynomial equations. To determine the reaction order with respect to persulfate, experiments were designed to examine a range of initial persulfate concentrations while maintaining the same initial TCE concentration. These results were also analyzed using polynomial regression analysis. The results of these studies indicate that the degradation rate of TCE by sodium persulfate could be described by the kinetic rate equation −dTCE/dt = (5.59 × 10-4 mM0.2 min-1)TCE0S2O8 2-0.8 within the limits of the experimental conditions utilized here (i.e., 40 °C, ionic strength 0.1, pH 7, TCE 0.14−0.52 mM, and S2O8 2- 13.5−43.7 mM). This equation indicates that the thermally activated persulfate oxidation of TCE appears to be pseudo zero order with respect to TCE (and therefore independent of the initial TCE concentration) and fractional order with respect to persulfate. This result should serve as a starting point for modeling of the TCE/persulfate system.
The objective of the laboratory study is to examine the conditions under which transition metal ions (e.g., ferrous ion, Fe
2+) could activate the persulfate anion (S
2O
8
2−) to produce a powerful ...oxidant known as the sulfate free radical (SO
4
−
) with a standard redox potential of 2.6 V. The SO
4
−
is capable of destroying groundwater contaminants in situ such as trichloroethylene (TCE). Experiments using Fe
2+ as an activator under various molar ratios of S
2O
8
2−/Fe
2+/TCE in an aqueous system indicated that partial TCE degradation occurred almost instantaneously and then the reaction stalled. Either destruction of SO
4
−
in the presence of excess Fe
2+ or the rapid conversion of all Fe
2+ to Fe
3+ limited the ultimate oxidizing capability of the system. Sequential addition of Fe
2+ in small increments resulted in an increased TCE removal efficiency. Therefore, it appeared that Fe
2+ played an important role in generating SO
4
−
. An observation of oxidation–reduction potential (ORP) variations revealed that the addition of sodium thiosulfate (Na
2S
2O
3) to the ferrous ion activated persulfate system could significantly decrease the strong oxidizing conditions. It was hypothesized that the thiosulfate induced reducing conditions might convert Fe
3+ to a lower valence state of Fe
2+, making the Fe
2+ available to activate persulfate decomposition. The sequential addition of thiosulfate (S
2O
3
2−), after the initial stalling of ferrous ion activated persulfate oxidation of TCE, resulted in an improvement in TCE removal. The ferrous ion activated persulfate–thiosulfate redox couple resulted in fairly complete TCE degradation in aqueous systems in a short time frame. In soil slurry systems, TCE degradation was slower in comparison to aqueous systems.
In situ chemical oxidation (ISCO) is a technique used to remediate contaminated soil and groundwater systems. It has been postulated that sodium persulfate (Na
2S
2O
8) can be activated by transition ...metal ions such as ferrous ion (Fe
2+) to produce a powerful oxidant known as the sulfate free radical (SO
4
−
) with a redox potential of 2.6 V, which can potentially destroy organic contaminants. In this laboratory study persulfate oxidation of dissolved trichloroethylene (TCE) was investigated in aqueous and soil slurry systems under a variety of experimental conditions. A chelating agent (i.e., citric acid) was used in attempt to manipulate the quantity of ferrous ion in solution by providing an appropriate chelate/Fe
2+ molar ratio. In an aqueous system a chelate/Fe
2+ molar ratio of 1/5 (e.g., S
2O
8
2−/chelate/Fe
2+/TCE ratio of 20/2/10/1) was found to be the lowest acceptable ratio to maintain sufficient quantities of Fe
2+ activator in solution resulting in nearly complete TCE destruction after only 20 min. The availability of Fe
2+ appeared to be controlled by adjusting the molar ratio of chelate/Fe
2+. In general, high levels of chelated ferrous ion concentrations resulted in faster TCE degradation and more persulfate decomposition. However, if initial ferrous ion contents are relatively low, sufficient quantities of chelate must be provided to ensure the chelation of a greater percentage of the limited ferrous ion present. Citric acid chelated ferrous ion appeared effective for TCE degradation within soil slurries but required longer reaction times. Additionally, the use of citric acid without the addition of supplemental Fe
2+ in soil slurries, where the citric acid apparently extracted native metals from the soil, appeared to be somewhat effective at enhancing persulfate oxidation of TCE over extended reaction times. A comparison of different chelating agents revealed that citric acid was the most effective.
Under thermally activated conditions (i.e., temperature of 40 ~ 99oC), there is considerable evidence that the persulfate anion (S(2)O(8)(2–)) can be converted to a powerful oxidant known as the ...sulfate free radical (SO(4) –•), which could be used in situ to destroy groundwater contaminants. In this laboratory study only limited trichloroethylene (TCE) degradation and no 1,1,1– trichloroethane (TCA) degradation was observed at 20oC. However, TCE and TCA were readily oxidized at 40o, 50o, and 60oC as a result of thermally activated persulfate oxidation. Experiments revealed that the pseudo-first-order reaction rate constants describing contaminant degradation increased with temperature. In aqueous systems activation energies for the TCE and TCA oxidation at an oxidant/contaminant molar ratio of 10/1 were determined to be 97.74 ± 3.04 KJ/mole and 163.86 ± 1.38 KJ/mole at pH 6 and an ionic strength of 0.1, respectively. A significant degradation of TCE and TCA occurs at 40o and 50oC, respectively, within less than 6 h. Aqueous system experiments revealed that oxidation reactions proceed more rapidly at increased persulfate/contaminant molar ratios. Soil slurry tests were conducted using medium to fine sands containing a range of fraction of organic carbon (foc) levels. Soil slurries were prepared at a 1/5 soil/water (mass ratio). In soil slurries the foc exhibited significant competition for sulfate free radicals produced. Based on these results, it was anticipated that higher temperatures, longer treatment times, and higher dosages of persulfate are required for the effective treatment of target contaminants in soil systems vs. aqueous systems.
The persulfate anion (S
2
O
8
2−
) is a strong oxidant with a redox potential of 2.01 V. However, when mixed with iron (II), it is capable of forming the sulfate radical (SO
4
−
.) that has an even ...higher redox potential (E
o
= 2.6 V). In these studies the sulfate radical was investigated to determine if it was a feasible oxidant for the destruction of BTEX and PAH compounds found in MGP contaminated soil. The sulfate radical was generated by either the sequential addition of iron (II) solutions or by a single addition of a citric acid chelated iron (II) solution. The sequentially added iron destroyed 86% of the total BTEX concentration and 56% of the total PAH concentration in the soil. The citric acid chelated iron destroyed 95% of the total BTEX concentration and 85% of the total PAH concentration. A second dose of persulfate and citric acid chelated iron (II) resulted in the destruction of 99% of the total BTEX concentration and 92% of the total PAH concentration. In both the sequential and chelated iron studies the lower molecular weight BTEX compounds were oxidized to a greater extent than the higher molecular weight BTEX compounds, whereas the oxidation of PAH compounds showed no preference to molecular weight.
Sodium persulfate is a common oxidant used for in situ chemical oxidation (ISCO) remediation of ground water contaminants (e.g., trichloroethylene (TCE)). Following ISCO unused persulfate and ...oxidation by-products such as sulfates remain within the groundwater at elevated levels. In this laboratory study the performance of three membranes: one nanofiltration (NF) membrane (designated DK) and two reverse osmosis (RO) membranes (designated AE and AG), manufactured by GE Osmonics, Inc., were evaluated for the removal of total dissolved solids (TDS), mainly persulfate and sulfates, and other associated by-products from a simulated groundwater. Experiments consisted of short-term sheet membrane evaluations, long-term spiral wound membrane evaluations and membrane compatibility tests. The general order of membrane performance was AE >AG>DK. SEM analysis of membrane surfaces clearly showed that both the DK and AG membranes experienced degradation and cracking after exposure to elevated sodium persulfate solutions for 30 days while the AE membrane appeared unchanged. This finding is in agreement with the membrane performance test results, again confirming that the AE membrane was the best membrane of this group.
Natural solutions containing clay extracts and synthetic solutions with varying concentrations of Al3+, Ca2+, and Na+ were used as anodic flushing solutions, and investigated for their enhanced ...in-situ removal of Pb from clay by electrokinetics (E-K). The experiments were conducted in a horizontal soil column apparatus with kaolin clay at an initial concentration of 340-410 g Pb g clay (dry) and 80-83% moisture content. Natural flushing solutions were prepared by mixing water and clay in ratios varying from 2:1 to 40:1 by weight, and the supernatant was used as an anodic flushing solution. The 7:1 natural solution was the most effective in Pb removal. Synthetic solutions were prepared from the salts of Al3+, Ca2+, Na+, Mg2+, and K+ ions. Pb removal was highest at a solution ionic strength of ∼0.001 M for each element group, and with trivalent Al3+ and divalent Ca2+ ions at concentrations of ∼6.4 × 10−5 M and 3.1 × 10−4 M, respectively. The 5.5 × Ca synthetic solution had the highest overall Pb removal efficiency. Energy expenditure unit volume of soil for all experiments was 8-31 kWs˙h m3.
Fenton's reaction is comprised of hydrogen peroxide (H
2
O
2
) catalyzed by iron, producing the hydroxyl radical (·OH), a strong oxidant. ·OH in turn may react with H
2
O
2
and iron and is capable of ...destroying a wide range of organic contaminants. In this laboratory study, Fenton's reaction was observed in aqueous and soil slurry systems using trichloroethylene (TCE) as the target contaminant, with the goal of maximizing TCE degradation while minimizing H
2
O
2
degradation. Fenton's reaction triggers a complex matrix of reactions involving ·OH, H
2
O
2
, iron, TCE, and soil organics. In soil slurries with a high fraction of organic carbon (f
OC
), iron tends to sorb to soil organics and/or particles. In aqueous systems the optimal ratio of H
2
O
2
:Fe
2+
:TCE to degrade TCE in a timely fashion, minimize costs, and minimize H
2
O
2
degradation is 300 mg/L: 25 mg/L: 60 mg/L (19:1:1 molar ratio), while soil slurries with a f
OC
up to approximately 1% and a soil:water ratio of 1:5 (weight ratio) require about ten times the amount of H
2
O
2
, the optimal ratio being 3000 mg/L: 5 mg/L: 60 mg/L (190:0.2:1 molar ratio). TCE degradation rates were observed to decrease in soil slurries with higher f
OC
because of competition by soil organic matter, which appears to act as a sink for ·OH. H
2
O
2
degradation rates tended to increase in soil slurries with higher f
OC
, most likely due to increased demand for ·OH by soil organics, increased available iron and other oxidation processes.
