Improving the Sweeping Efficiency of Permanganate into Low Permeable Zones To Treat TCE: Experimental Results and Model Development

The residual buildup and treatment of dissolved contaminants in low permeable zones (LPZs) is a particularly challenging issue for injection-based remedial treatments. Our objective was to improve the sweeping efficiency of permanganate into LPZs to treat dissolved-phase TCE. This was accomplished by conducting transport experiments that quantified the ability of xanthan-MnO₄^– solutions to penetrate and cover (i.e., sweep) an LPZ that was surrounded by transmissive sands. By incorporating the non-Newtonian fluid xanthan with MnO₄^–, penetration of MnO₄^– into the LPZ improved dramatically and sweeping efficiency reached 100% in fewer pore volumes. To quantify how xanthan improved TCE removal, we spiked the LPZ and surrounding sands with ¹⁴C-lableled TCE and used a multistep flooding procedure that quantified the mass of ¹⁴C-TCE oxidized and bypassed during treatment. Results showed that TCE mass removal was 1.4 times greater in experiments where xanthan was employed. Combining xanthan with MnO₄^– also reduced the mass of TCE in the LPZ that was potentially available for rebound. By coupling a multiple species reactive transport model with the Brinkman equation for non-Newtonian flow, the simulated amount of ¹⁴C-TCE oxidized during transport matched experimental results. These observations support the use of xanthan as a means of enhancing MnO₄^– delivery into LPZs for the treatment of dissolved-phase TCE

Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate

The chemical oxidant permanganate (MnO4−) has been shown to effectively transform hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) at both the laboratory and field scales. We treated RDX with MnO4− with the objective of quantifying the effects of pH and temperature on destruction kinetics and determining reaction rates. A nitrogen mass balance and the distribution of reaction products were used to provide insight into reaction mechanisms. Kinetic experiments (at pH ∼ 7, 25 °C) verified that RDX−MnO4− reaction was first-order with respect to MnO4− and initial RDX concentration (second-order rate: 4.2 × 10−5 M−1 s−1). Batch experiments showed that choice of quenching agents (MnSO4, MnCO3, and H2O2) influenced sample pH and product distribution. When MnCO3 was used as a quenching agent, the pH of the RDX−MnO4− solution was relatively unchanged and N2O and NO3− constituted 94% of the N-containing products after 80% of the RDX was transformed. On the basis of the preponderance of N2O produced under neutral pH (molar ratio N2O/NO3 ∼ 5:1), no strong pH effect on RDX−MnO4− reaction rates, a lower activation energy than the hydrolysis pathway, and previous literature on MnO4− oxidation of amines, we propose that RDX−MnO4− reaction involves direct oxidation of the methylene group (hydride abstraction), followed by hydrolysis of the resulting imides, and decarboxylation of the resulting carboxylic acids to form N2O, CO2, and H2O

Dual Activation of Peroxymonosulfate Using MnFe₂O₄/g‑C₃N₄ and Visible Light for the Efficient Degradation of Steroid Hormones: Performance, Mechanisms, and Environmental Impacts

Single activation of peroxymonosulfate (PMS) in a homogeneous system is sometimes insufficient for producing reactive oxygen species (ROS) for water treatment applications. In this work, manganese spinel ferrite and graphitic carbon nitride (MnFe2O4/g-C3N4; MnF) were successfully used as an activator for PMS under visible light irradiation to remove the four-most-detected-hormone-contaminated water under different environmental conditions. The incorporation of g-C3N4 in the nanocomposites led to material enhancements, including increased crystallinity, reduced particle agglomeration, amplified magnetism, improved recyclability, and increased active surface area, thereby facilitating the PMS activation and electron transfer processes. The dominant active radical species included singlet oxygen (1O2) and superoxide anions (O2•–), which were more susceptible to the estrogen molecular structure than testosterone due to the higher electron-rich moieties. The self-scavenging effect occurred at high PMS concentrations, whereas elevated constituent ion concentrations can be both inhibitors and promoters due to the generation of secondary radicals. The MnF/PMS/vis system degradation byproducts and possible pathways of 17β-estradiol and 17α-methyltestosterone were identified. The impact of hormone-treated water on Oryza sativa L. seed germination, shoot length, and root length was found to be lower than that of untreated water. However, the viability of both ELT3 and Sertoli TM4 cells was affected only at higher water compositions. Our results confirmed that MnF and visible light could be potential PMS activators due to their superior degradation performance and ability to produce safer treated water