Thermal activation can induce persulfate (S2O8 2−) degradation to form sulfate radicals (SO4 −•) that can undergo radical interconversion to form hydroxyl radicals (HO•) under alkaline conditions. ...The radicals SO4 −•/HO• can be present either individually or simultaneously in the persulfate oxidation system. To identify the active radical species, a chemical probe method was developed. An excess of probe compounds was added to the system, and differences between the reactivity of the probes and the potential radical species were observed. The usage of various probes, including tert-butyl alcohol, phenol, and nitrobenzene (NB), for simultaneously identifying SO4 −•/HO• was investigated. NB can only react with radicals: it cannot react with persulfate. The reaction rate of NB with HO• is 3000−3900 times greater than that of NB with SO4 −•, which is a good candidate for use as a probe for differentiating between SO4 −•/HO• reactivity. Furthermore, the effects of pH on the formation of SO4 −•/HO• were demonstrated by the degradation kinetics of NB at varying pH values. The results indicated that SO4 −• is the predominant radical at pH <7; both SO4 −• and HO• are present at pH 9; HO• is the predominant radical at a more basic pH (i.e., pH 12). The use of chemical probe NB is a suitable compound for differentiating HO•/SO4 −• reactivity.
► Complete phenol degradations can be achieved by UV/SPS oxidation processes in the pH range of 3–11. ► During the phenol degradation the color change observed at 455 nm or in the range of 400–600 nm ...is a feasible indication either of a reduction of intermediate compounds or of TOC removal. ► A higher degree of phenol mineralization at pH 11 can be expected due to the fact that sulfate radicals undergo reactions with hydroxide ions to hydroxyl radicals.
Using ultraviolet photolytic persulfate activation to produce two sulfate radicals (
SO
4
-
) exhibits a potential for destroying organic contaminants in wastewater treatment applications. This study investigated both the feasibility of using a UV/SPS (sodium persulfate) process to treat phenol in aqueous phase and the effect of pH on degradation efficiency and TOC removal. The results revealed that a high initial persulfate concentration (i.e., 84
mM) and a lower initial phenol concentration (i.e., 0.5
mM) resulted in rapid and complete phenol degradation within 20
min. For all three pHs evaluated (i.e., 3, 7 and 11), complete phenol degradation was also achieved after 30
min of treatment by UV/SPS oxidation processes (i.e., under an SPS/phenol molar ratio of 84/0.5) with pseudo-first-order rate constants (
k
obs, phenol) of 0.14–0.16
min
−1 (average half-life (
t
1/2) = 4.5
min). UV–Vis spectrum scanning of the aqueous solution during treatment identified the development of brown color in the wavelength range of 400–460
nm. The colored intermediate compounds that formed were suspiciously similar to those observed during Fenton treatment. However, a more aggressive oxidation at pH 11 showed a rapid and more complete removal of TOC in aqueous phase. Therefore, it is recommended that UV photolytic persulfate activation under basic pH be a preferred condition for treatment of phenol.
Due to a gradual increase in the use of persulfate as an in situ chemical oxidation (ISCO) oxidant, a simple measurement of persulfate concentration is desirable to analyze persulfate distribution at ...designated time intervals on/off a site. Such a distribution helps evaluate efficacy of ISCO treatment at a site. This work proposes a spectrophotometric determination of persulfate based on modification of the iodometric titration method. The analysis of absorption spectra of a yellow color solution resulting from the reaction of persulfate and iodide in the presence of sodium bicarbonate reveals an absorbance at 352
nm, without significant interferences from the reagent matrix. The calibration graph was linear in the range of persulfate solution concentration of 0–70
mM at 352
nm. The proposed method is validated by the iodometric titration method. The solution pH was at near neutral and the presence of iron activator does not interfere with the absorption measurement. Also, analysis of persulfate in a groundwater sample using the proposed method indicates a good agreement with measurements by the titration method. This proposed spectrophotometric quantification of persulfate provides a simple and rapid method for evaluation of ISCO effectiveness at a remediation site.
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 synthesis of new materials, GAC–ZVI composites, which combine the advantages of a sorption material (granular activated carbon) and a reduction material (zerovalent iron) is developed. ► The ...kinetics of the sorption/dechlorination of TCE in short- and long-term experiments is studied. ► Mass balance during treatment is performed. ► Characterization before and after reaction using BET, XRD, SEM/EDS, and TEM is conducted.
The coupling adsorption and degradation of trichloroethylene (TCE) through dechlorination using synthetic granular activated carbon and zerovalent iron (GAC–ZVI) composites was studied. The GAC–ZVI composites were prepared from aqueous Fe
2+ solutions by impregnation with and without the use of a PEG dispersant and then heated at 105
°C or 700
°C under a stream of N
2. Pseudo-first-order rate constant data on the removal of TCE demonstrates that the adsorption kinetics of GAC is similar to those of GAC–ZVI composites. However, the usage of GAC–ZVI composites liberated a greater amount of Cl than when ZVI was used alone. The highest degree of reductive dechlorination of TCE was achieved using a GAC–ZVI700P composite (synthesized using PEG under 700
°C). A modified Langmuir–Hinshelwood rate law was employed to depict the behavior of Cl liberation. As a result, a zero-order Cl liberation reaction was observed and the desorption limited TCE degradation rate constant decreased as the composite dosage was increased. The GAC–ZVI composites can be employed as a reactive GAC that is not subject to the limitations of using GAC and ZVI separately.
Ascorbic acid (AA) mediated electron transfer may induce reductive dechlorination of carbon tetrachloride (CCl4). This study investigated the role of AA in conjunction with the presence of iron ...minerals over a wide pH range for the reduction of CCl4 in aqueous systems. The results indicate that CCl4 was reduced by AA at a pH of 13 (>pK a2, AA of 11.79) and chloroform (CHCl3) was a transformation byproduct of CCl4. When CCl4 levels were reduced to near complete disappearance, the decrease of CHCl3 was then observed. The degradation rate of CCl4 and also the formation rate of CHCl3 increased with increased AA concentrations. Analysis of reaction kinetics between CCl4 and AA revealed an overall second-order reaction with a rate constant of 0.253 ± 0.018 M–1 s–1. Furthermore, the reduction rate of CCl4 by AA at pH of 13 could be enhanced with the presence of iron minerals (Fe3O4, Fe2O3, FeOOH, and FeS2). In the absence or presence of iron minerals, the fraction of CCl4 transformed to CHCl3 was less than 1, indicating simultaneous one- and two-electron transfer processes. The end-products of AA at a pH of 13 included threonic acid and oxalic acid. This study highlights the potential of an alkaline AA solution for remediating chlorinated solvents.
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.
•Evaluation of the environmental risk of exposure to persulfate is studied.•Persulfate is considerably more toxic than its decomposition product sulfate.•Very acidic or basic pH conditions may cause ...toxic effects.•Water conductivity reveals minor influence on acute toxicity.•The bioassay results serve as reference toxicity values following ISCO application.
Persulfate anion (S2O82-), a widely used in situ chemical oxidation agent, is increasingly applied for environmental remediation. However, limited information on environmental and toxicological effects is available for the evaluation of the environmental risk of exposure to S2O82-, particularly after its application. In this study, the acute toxic effects on the common carp (Cyprinus carpio) were employed as a model to investigate S2O82-, sulfate ion (decomposition product of S2O82-), hydrogen/hydroxide ions and also the mixtures of these ion species. Acute toxicity test results showed 96h median lethal concentrations (LC50) of 540±23mgL−1 for S2O82- and 4100±110mgL−1 for SO42-. S2O82- was considerably more toxic than its decomposition product SO42-. Additionally, solution pH was also an important factor influencing toxicity, and S2O82- posed reduced acute toxicity when pH was in the range of 6–10. Water conductivity up to approximately 8000μScm−1 did not appear to significantly increase fish mortality. In the mixture toxicity test (i.e., S2O82-/OH−), LC50 values of 130±10mgL−1 for S2O82- and 23±2mgL−1 for OH− were lower than those obtained from the individual toxicity tests and therefore exhibited higher toxicity to fish. However, upon complete decomposition of S2O82- in the mixture, a reduction in acute toxicity may be expected. The results of this study revealed that it may be necessary and/or desirable to control the residual S2O82-and pH after S2O82- addition when potential exposure to an aquatic system is a concern.
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•Guava leaves are rich in polyphenols and serve as natural electron suppliers.•Guava leaf extract can effectively degrade carbon tetrachloride (CT) in solution.•Alkaline pH can ...accelerate the release of electrons to degrade CT.•CT degradation with GLE is enhanced in the presence of iron salt/iron mineral/soil.•CT degradation is through reductive dechlorination and mineralized to Cl.
Guava leaves contain polyphenols, which exhibit antioxidant activities and act as electron suppliers. It was postulated that guava leaf extract (GLE) may have the potential to be used as a natural reducing agent for in situ chemical reduction of chlorinated solvents, such as carbon tetrachloride (CT). Extraction of GLE was conducted by boiling guava leaves, and filtering the liquid to produce GLE-Filtrate. GLE-Powder was produced by drying the GLE-Filtrate at 50 °C. Characterization analysis showed a yield of 16.6 g GLE-Powder/100 g of leaves, with a total polyphenolic content of 300 mg g−1, and with gallocatechin as a major constituent. Both GLE-Powder and GLE-Filtrate exhibited the capability to degrade CT in aqueous phase. The CT degradation rate increased with increasing alkaline conditions, at pH higher than the pKa of polyphenols. GLE-Filtrate reductive degradation of CT can be enhanced with the presence of iron salts, and various minerals, or soils containing iron, at pH 10. The specific surface area rate constant (kSA) of CT degradation for the GLE-Filtrate/Fe2O3 aqueous system was as high as 5.52 × 10−3 L d−1 m−2. The analysis of byproducts formation found trichloromethane to be a major intermediate, which was further mineralized to generate Cl−.
For remediation of trichloroethylene (TCE) contaminated groundwater, activated carbon (AC) has been used to adsorb and reduce the TCE concentration and to manage contaminant migration. Additionally, ...AC may also act as an activator of the electron-transfer mediator in activating persulfate anion (S2O8 2−) to generate sulfate radical (SO4 −·) for contaminant destruction. The objective of the present research was to examine the combined use of AC and persulfate to treat TCE. The degradation of persulfate in the presence of AC follows a first-order kinetic behavior, and the faster persulfate degradation is observed when elevated AC dosage is used. Higher initial persulfate concentration results in a decrease of the persulfate degradation rate. Upon persulfate oxidation, the AC surface properties are altered including: an increase in acidity of surface concentration, a decrease in pHzpc, and a slight decrease in the surface area. The results of a study of adsorption kinetics and isotherms suggest that the adsorption behavior for the original AC fits the pseudo-second-order kinetic model while the pseudo-first-order kinetic model is suitable for predicting oxidized AC performance. During the persulfate oxidation of TCE with AC as an activator, studies show that TCE removal can be a net result of adsorption and oxidation, in which a partial mineralization of TCE to release chloride occurs.
