Chenju Liang

A rapid spectrophotometric determination of persulfate anion in ISCO

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.

Thermally Activated Persulfate Oxidation of Trichloroethylene (TCE) and 1,1,1-Trichloroethane (TCA) in Aqueous Systems and Soil Slurries

Under thermally activated conditions (i.e., temperature of 40∼99°C), there is considerable evidence that the persulfate anion () can be converted to a powerful oxidant known as the sulfate free radical (), 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 20°C. However, TCE and TCA were readily oxidized at 40°, 50°, and 60°C 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 40° and 50°C, 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.

Feasibility study of ultraviolet activated persulfate oxidation of phenol

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., 0.5mM) 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.

Potential for activated persulfate degradation of BTEX contamination.

The present study focused on evaluation of activated persulfate (PS) anion (S(2)O(8)(2-)) oxidative degradation of benzene, toluene, ethylbenzene, and xylene (constituents of gasoline and known collectively as BTEX) contamination. The results indicated that BTEX were effectively oxidized by PS in aqueous and soil slurry systems at 20 degrees C. PS can be activated thermally, or chemically activated with Fe(2+) to form the sulfate radical (SO(4)(-)) with a redox potential of 2.4V. The degradation rate constants of BTEX were found to increase with increased persulfate concentrations. For two PS/BTEX molar ratios of 20/1 and 100/1 experiments, the observed aqueous phase BTEX degradation half-lives ranged from 3.0 to 23.1 days and 1.5 to 20.3 days in aqueous and soil slurry systems, respectively. In the interest of accelerating contaminant degradation, Fe(2+) and chelated Fe(2+) activated persulfate oxidations were investigated. For all iron activation experiments, BTEX and persulfate degradations appear to occur almost instantaneously and result in partial BTEX removals. It is speculated that the incomplete degradation reaction may be due to the cannibalization of SO(4)(-) in the presence of excess Fe(2+). Furthermore, the effects of various chelating agents including, hydroxylpropyl-beta-cyclodextrin (HPCD), ethylenediaminetetraacetic acid (EDTA), and citric acid (CA) on maintaining available Fe(2+) and activating PS for the degradation of benzene were studied. The results indicated that HPCD and EDTA may be less susceptible to chelated Fe(2+). In contrast, CA is a more suitable chelating agent in the iron activated persulfate system and with a PS/CA/Fe(2+)/B molar ratio of 20/5/5/1 benzene can be completely degraded within a 70-min period.

pH dependence of persulfate activation by EDTA/Fe(III) for degradation of trichloroethylene.

The ability of free ferrous ion activated persulfate (S(2)O(8)(2-)) to generate sulfate radicals (SO(4)(-)) for the oxidation of trichloroethylene (TCE) is limited by the scavenging of SO(4)(-) with excess Fe(2+) and a quick conversion of Fe(2+) to Fe(3+). This study investigated the applicability of ethylene-diamine-tetra-acetic acid (EDTA) chelated Fe(3+) in activating persulfate for the destruction of TCE in aqueous phase under pH 3, 7 and 10. Fe(3+) and EDTA alone did not appreciably degrade persulfate. The presence of TCE in the EDTA/Fe(3+) activated persulfate system can induce faster persulfate and EDTA degradation due to iron recycling to activate persulfate under a higher pH condition. Increasing the pH leads to increases in pseudo-first-order-rate constants for TCE, S(2)O(8)(2-) and EDTA degradations, and Cl generation. Accordingly, the experiments at pH 10 with different EDTA/Fe(3+) molar ratios indicated that a 1/1 ratio resulted in a remarkably higher degradation rate at the early stage of reaction as compared to results by other ratios. Higher persulfate dosage under the EDTA/Fe(3+) molar ratio of 1/1 resulted in greater TCE degradation rates. However, increases in persulfate concentration may also lead to an increase in the rate of persulfate consumption.

Synthesis of granular activated carbon/zero valent iron composites for simultaneous adsorption/dechlorination of trichloroethylene

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.

Iron (II) Activated Persulfate Oxidation of MGP Contaminated Soil

The persulfate anion (S2O8 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 (SO4 −.) 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.

In situ iron activated persulfate oxidative fluid sparging treatment of TCE contamination — A proof of concept study

In situ chemical oxidation (ISCO) is considered a reliable technology to treat groundwater contaminated with high concentrations of organic contaminants. An ISCO oxidant, persulfate anion (S(2)O(8)(2-)) can be activated by ferrous ion (Fe(2+)) to generate sulfate radicals (E(o)=2.6 V), which are capable of destroying trichloroethylene (TCE). The property of polarity inhibits S(2)O(8)(2-) or sulfate radical (SO(4)(-)) from effectively oxidizing separate phase TCE, a dense non-aqueous phase liquid (DNAPL). Thus the oxidation primarily takes place in the aqueous phase where TCE is dissolved. A bench column study was conducted to demonstrate a conceptual remediation method by flushing either S(2)O(8)(2-) or Fe(2+) through a soil column, where the TCE DNAPL was present, and passing the dissolved mixture through either a Fe(2+) or S(2)O(8)(2-) fluid sparging curtain. Also, the effect of a solubility enhancing chemical, hydroxypropyl-beta-cyclodextrin (HPCD), was tested to evaluate its ability to increase the aqueous TCE concentration. Both flushing arrangements may result in similar TCE degradation efficiencies of 35% to 42% estimated by the ratio of TCE degraded/(TCE degraded+TCE remained in effluent) and degradation byproduct chloride generation rates of 4.9 to 7.6 mg Cl(-) per soil column pore volume. The addition of HPCD did greatly increase the aqueous TCE concentration. However, the TCE degradation efficiency decreased because the TCE degradation was a lower percentage of the relatively greater amount of dissolved TCE by HPCD. This conceptual treatment may serve as a reference for potential on-site application.

Evaluation of persulfate oxidative wet scrubber for removing BTEX gases

Soil vapor extraction (SVE) coupled with air sparging of groundwater is a method commonly used to remediate soil and groundwater contaminated with volatile organic petroleum contaminants such as gasoline. These hazardous contaminants are mainly attributable to the compounds-benzene, toluene, ethylbenzene, and xylenes (known collectively as BTEX). Exhaust gas from SVE may contain BTEX, and therefore must be treated before being discharged. This study evaluated the use of iron-activated persulfate chemical oxidation in conjunction with a wet scrubbing system, i.e., a persulfate oxidative scrubber (POS) system, to destroy BTEX gases. The persulfate anions can be activated by citric acid (CA) chelated Fe(2+) to generate sulfate radicals (SO(4)(*-), E degrees =2.4V), which may rapidly degrade BTEX in the aqueous phase and result in continuous destruction of the BTEX gases. The results show that persulfate activation occurred as a result of continuous addition of the citric acid chelated Fe(2+) activator, which readily oxidized the dissolved BTEX. Based on initial results from the aqueous phase, a suitable Fe(2+)/CA molar ratio of 5/3 was determined and used to initiate activation in the subsequent POS system tests. In the POS system, using persulfate as a scrubber solution and with activation by injecting Fe(2+)/CA activators under two testing conditions, varying iron concentrations and pumping rates, resulted in an approximate 50% removal of BTEX gases. During the course of the tests which in corporate activation, a complete destruction of BTEX was achieved in the aqueous phase. It is noted that no removal of BTEX occurred in the control tests which did not include activation. The results of this study would serve as a reference for future studies into the practical chemical oxidation of waste gas streams.

Persulfate regeneration of trichloroethylene spent activated carbon

The objective of this study was to demonstrate the regeneration of trichloroethylene (TCE) spent activated carbon using persulfate oxidation and iron activated persulfate (IAP) oxidation. Both processes resulted in decreases in the adsorbability of regenerated activated carbons. IAP was shown to rapidly degrade the aqueous TCE and causes a significant mineralization of the TCE. The release of chloride ions provided evidence of this. Persulfate oxidation mainly resulted in desorption of TCE from the activated carbon and only partial mineralization of the TCE through a carbon activated persulfate reaction mechanism. Concerning destruction of the TCE, in the regeneration test using persulfate, 30% of the original TCE was present in the solution and 9% remained on the activated carbon after the first regeneration cycle. In contrast, in the test that used IAP, it was observed that no TCE was present in the solution and only approximately 5% of the original TCE remained on the activated carbon after the first regeneration. Following the regeneration cycles, elemental analysis was carried out on the samples. BET surface area and EDS analysis showed some effects on the physico-chemical properties of the activated carbon such as a slight decrease in the surface area and the presence of iron precipitates on the carbon.