Remediation of groundwater contaminated with trichloroethylene (TCE) using a long-lasting persulfate/biochar barrier

Groundwater contamination by chlorinated solvents causes potential threats to water resources and human health. Therefore, it is important to develop effective technologies to remediate contaminated groundwater. This study uses biodegradable hydrophilic polymers, hydroxypropyl methylcellulose (HPMC), hydroxyethyl cellulose (HEC) and polyvinyl pyrrolidone (PVP) as binders to manufacture persulfate (PS) tablets for the sustained release of persulfate to treat trichloroethylene (TCE) in groundwater. The release time for different tablets decreases in the order: HPMC (8-15 days) > HEC (7-8 days) > PVP (2-5 days). The efficiency with which persulfate is released is: HPMC (73-79%) > HEC (60-72%) > PVP (12-31%). HPMC is the optimal binder for the manufacture of persulfate tablets and persulfate is released from a tablet of HPMC/PS ratio (wt/wt) of 4/3 for 15 days at a release rate of 1127 mg/day. HPMC/PS/biochar (BC) ratios (wt/wt/wt) between 1/1/0.02 and 1/1/0.0333 are suitable for PS/BC tablets. PS/BC tablets release persulfate for 9-11 days at release rates of 1243 to 1073 mg/day. The addition of too much biochar weakens the structure of the tablets, which results in a rapid release of persulfate. TCE is oxidized by a PS tablet with an efficiency of 85% and a PS/BC tablet eliminates more TCE, with a removal efficiency of 100%, due to oxidation and adsorption during the 15 days of reaction. Oxidation is the predominant mechanism for TCE elimination by a PS/BC tablet. The adsorption of TCE by BC fits well with the pseudo-second-order kinetics and the pseudo-first-order kinetics, which describes the removal of TCE by PS and PS/BC tablets. The results of this study show that a PS/BC tablet can be used in a permeable reactive barrier for long-term passive remediation of groundwater.

Low-temperature persulfate activation by powdered activated carbon for simultaneous destruction of perfluorinated carboxylic acids and 1,4-dioxane

Carbonaceous materials have emerged as a method of persulfate activation for remediation. In this study, persulfate activation using powdered activated carbon (PAC) was demonstrated at temperatures relevant to groundwater (5-25 °C). At room temperature, increasing doses of PAC (1-20 g L^-1) led to increased persulfate activation (3.06 × 10^-6s^-1 to 2.10 × 10^-4 with 1 and 20 g L^-1 PAC). Activation slowed at lower temperatures (5 and 11 °C); however, substantial (>70 %) persulfate activation was achieved. PAC characterization showed that persulfate is activated at the surface of the PAC, as indicated by an increase in the PAC C:O ratio. Similarly, electron paramagnetic resonance (EPR) spectroscopy studies with a spin trapping agents (5,5-dimethyl-1-pyrroline N-oxide (DMPO)) and 2,2,6,6-tetramethylpiperidine (TEMP) revealed that singlet oxygen was not the main oxidizing species in the reaction. DMPO was oxidized to form 5,5-dimethylpyrrolidone-2(2)-oxyl-(1) (DMPOX), which forms in the presence of strong oxidizers, such as sulfate radicals. The persulfate/PAC system is demonstrated to simultaneously degrade both perfluorooctanoic acid (PFOA) and 1,4-dioxane at room temperature and 11 °C. With a 20 g L^-1 PAC and 75 mM persulfate, 80 % and 70 % of the PFOA and 1,4-dioxane, respectively, degraded within 6 h at room temperature. At 11 °C, the same PAC and persulfate doses led to 57% dioxane degradation and 54 % PFOA degradation within 6 h. Coupling PAC with persulfate offers an effective, low-cost treatment for simultaneous destruction of 1,4-dioxane and PFOA.

Insights into carbon isotope fractionation on trichloroethene degradation in base activated persulfate process: The role of multiple reactive oxygen species

Understanding the role of reactive oxygen species (ROS) is essential to elucidate the mechanism of contaminants degradation in in-situ chemical oxidation (ISCO). In this study, compound specific isotope analysis (CSIA) and radicals quenching methods were integrated to investigate the roles of hydroxyl radical (HO), sulfate radical (SO₄^-), and superoxide radical (O₂^-) on trichloroethene (TCE) degradation during persulfate (PS) activated with base. The carbon isotope fractionation of TCE was found to be dependent of the base:PS ratios, yielding carbon enrichment factors (ε values) from -9.8 ± 0.5‰ to -16.7 ± 1.0‰ at base:PS molar ratios between 0.5:1 and 10:1, which was attributed to multi-pathways degradation of TCE by multiple ROS. The expected ε value (-31.6 ± 1.6‰) for TCE degradation via O₂^- attacking pathway, was more negative than those values via SO₄^- or HO pathways. The relative contributions of HO, SO₄^- and O₂^- for TCE degradation during base activated PS were estimated with observed ε values. HO and O₂^- were the predominant ROS for TCE degradation (with the relative contribution of 55-69% and 22-45%, respectively) in base activated PS. This work highlights the prospect of CSIA application for identifying degradation pathways of contaminants with ROS in environment.

The role of soil organic matters and minerals on hydrogen peroxide decomposition in the soil

Application of H₂O₂ in in-situ chemical oxidation (ISCO) for soil remediation has been limited by its rapid decomposition. However, effect of main factors involving in this phenomenon are not well understood. In this contribution, H₂O₂ decomposition in the six types of natural soils was investigated by kinetic analyses and soil characterizations. The grassland soil (GS) and red soil (RS) have the highest H₂O₂ decomposition rates (respective 0.048 and 0.069 min^-1), while the paddy soil (PS) shows the lowest one (0.004 min^-1). The decomposition mainly takes place on the surface adsorption sites of soil particles. PS has the highest content of SOM, which can block the active adsorption sites for H₂O₂ decomposition. The effects of dissolved organic matter (DOM) and biological debris in the soil are minor. Iron and manganese containing minerals are significantly influential on H₂O₂ decomposition, and the soil with a higher content of clay can induce faster H₂O₂ decomposition. The immobilized goethite (GM) and birnessite (BM) on montmorillonite were synthesized to simulate soil minerals. Results show H₂O₂ decomposition rates in BM is even faster than GM when the former dosage is two orders of magnitude lower than that of the latter. This indicates the crucial role of manganese minerals although their contents are generally much lower than that of iron in the soils. This study advanced the understanding of H₂O₂ decomposition in the soil and bring insights for H₂O₂ based ISCO technology in soil remediation.