In-situ generation of reactive oxygen species using combination of electrochemical oxidation and metal sulfide

In-situ groundwater remediation of contaminant mixture of As(III), Cr(VI), and sulfanilamide via electrochemical degradation/transformation using pyrite

Electrochemical advanced oxidation processes (EAOPs) are effective in removing persistent contaminants from groundwater. However, their practical applicability depends significantly on various site-specific characteristics. Therefore, the primary objective of this investigation was to study the feasibility of EAOPs and pyrite, which is a sulfide mineral, to effectively remove the mixture of arsenic (As (III)), chromium (Cr (VI)), and sulfanilamide in groundwater. We conducted a comparison of three systems: (1) EAOP alone, (2) pyrite alone, and (3) a combined EAOP and pyrite system. In EAOP alone, sulfanilamide was effectively oxidized (80%), while the electrochemical transformation of As(III)/Cr(VI) into As(V)/Cr(III) was limited. In just the pyrite system, As(III), Cr(VI), and sulfanilamide were adsorbed onto the surface of pyrite (60%, 20%, and 18%). Neither the EAOP nor the pyrite system alone could effectively treat the contaminants mixture. Nonetheless, the combined system completely removed As(III), Cr(VI), and sulfanilamide by the synergistic reaction. This could be attributed to the formation of green rust, a natural adsorbent mineral produced as a reaction of dissolved iron, generated via electrochemical pyrite oxidation, with the groundwater electrolyte (e.g., CO3 or SO4). This system harmonized the combined approach of EAOP and pyrite to effectively eliminate both organic and inorganic contaminants. ENVIRONMENTAL IMPLICATION: A paper proposed electrochemical oxidation (EO) with pyrite to remove both organic and inorganic contaminants from groundwater. The removal performance of the combined system was evaluated, and the synergistic mechanism was revealed. The combination of EO and pyrite with synergistic removal effectively removed the mixture of both contaminants. This could be attributed by the formation of green-rust by electrochemical activation for pyrite. Compared to the single system of EO and pyrite alone, the combined system with EO and pyrite improved removal performance. Results suggested that the combined system could be used for green groundwater remediation.

Electrochemical oxidation and mechanism of sulfanilamide from groundwater in a flow-through system using carbon fiber (CF) anode

Metal-based anodes have been used for a long time in the electrochemical oxidation processes to remediate groundwater. However, the high cost of this technique as well as the release of potentially toxic metals (ex, lead), are major barriers being fully implemented. As an alternative of metal-based anodes, in recent years, carbon-based anodes have been paid attention due to their eco-friendliness and cost-effectiveness. This study evaluated the oxidation performance of carbon fiber (CF) anode in a flow-through system. The CF anode degraded 45-87% of the target pollutant (sulfanilamide), depending on the current intensity applied. However, no further degradation of sulfanilamide was observed after the cathode, indicating that sulfanilamide degradation occurred mainly at the anode. This study also determined the effect of electrolytes on electrochemical oxidation using chloride (Cl-), sulfate (SO42-), bicarbonate (CO3-), and synthetic groundwater. Cl- and SO42- electrolytes were converted electrochemically into active species, thereby enhancing sulfanilamide degradation, while the bicarbonate and groundwater electrolytes inhibited oxidation performance by scavenging hydroxyl radicals. A series of scavenger tests and characterization showed that the direct oxidation and hydroxyl radicals involved the sulfanilamide degradation. Especially, the production of hydroxyl radicals is more favorable in high currents than in low currents. That is, CF anode contributed to the degradation by direct oxidation of carbon-based electrodes and generation of hydroxyl radicals. In summary, this study highlights how a CF anode is capable of effectively degrading organic pollutants via anodic oxidation.

Adsorption of antibiotics onto low-grade charcoal in the presence of organic matter: Batch and column tests

Antibiotics contaminate diverse ecosystems and threaten human health. In ecosystems including water, sediment, and soil, the amount of antibiotics present is tiny compared to the amount of natural organic matter. However, most studies have ignored the co-presence of natural organic matter in the adsorption of target antibiotics. In this study, we quantitatively evaluated the effect of co-presenting natural organic matter on the adsorption of sulfamethazine (SMZ) through batch and column experiments using low-grade charcoal, an industrial by-product. SMZ was used as a model antibiotic compound and humic acid (HA) was used to represent natural organic matter. The co-presence of 2000 mg/L HA (400 times the concentration of SMZ) lowered the adsorption rate of SMZ from 0.023 g/mg·min to 0.007 g/mg·min, and the maximum adsorption capacity from 39.8 mg/g to 15.6 mg/g. HA blocked the charcoal's pores and covered its surface adsorption sites, which dramatically lowered its capacity to adsorb SMZ. Similar results were obtained in the flow-through column experiments, where the co-presence of natural organic matter shortened the lifetime of the charcoal. As a result, the co-presence of a relatively high concentration of natural organic matter can inhibit the adsorption of SMZ and likely other antibiotic compounds, and thus the presence of natural organic matter should be accounted for in the design of adsorption processes to treat antibiotics in water.

Factors hindering the degradation of pharmaceuticals from human urine in an iron-activated persulfate system

This study investigated the degradation of clofibric acid (CFA), bezafibrate (BZF), and sulfamethoxazole (SMX) in synthetic human urine using a novel mesoporous iron powder-activated persulfate system (mFe-PS system), and identified the factors limiting their degradation in synthetic human urine. A kinetic model was established to expose the radical production in various reaction conditions, and experiments were conducted to verify the modeling results. In the phosphate-containing mFe-PS system, the 120 min removal efficiency of CFA decreased from 95.1% to 76.6% as the phosphate concentration increased from 0.32 to 6.45 mmol/L, but recovered to 90.5% when phosphate concentration increased to 16.10 mmol/L. Meanwhile, the increased concentration of phosphate from 0.32 to 16.10 mmol/L reduced the BZF degradation efficacy from 91.5% to 79.0%, whereas SMX removal improved from 37.3% to 62.9%. The mFe-PS system containing (bi)carbonate, from 4.20 to 166.70 mmol/L, reduced CFA and BZF removal efficiencies from 100% to 76.8% and 80.4%, respectively, and SMX from 83.5% to 56.7% within a 120-min reaction time. In addition, alkaline conditions (pH ≥ 8.0) inhibited CFA and BZF degradations, while nonacidic pH (pH ≥ 7.0) remarkably inhibited SMX degradation. Results of the kinetic model indicated the formation of phosphate (H2PO4·/HPO4·-) and/or carbonate radicals (CO3·-) could limit pharmaceutical removal. The transformation products (TPs) of the pharmaceuticals revealed more incompletely oxidized TPs occurred in the phosphate- and (bi)carbonate-containing mFe-PS systems, and indicated that H2PO4·/HPO4·- mainly degraded pharmaceuticals via a benzene ring-opening reaction while CO3·- preferentially oxidized pharmaceuticals via a hydroxylation reaction.

In-situ hydrogen peroxide formation and persulfate activation over banana peel-derived biochar cathode for electrochemical water treatment in a flow reactor

Electrochemical advanced oxidation processes (EAOPs) are effective for the removal of organic contaminants from groundwater. The choice of an affordable cathode material that can generate reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) and hydroxyl radicals (•OH) will increase practicality and cost effectiveness of EAOPs. Carbon enriched biochar (BC), which is derived from pyrolysis of biomass, has emerged as an inexpensive and environmentally-friendly electrocatalyst for removing contaminants from groundwater. In this study, a banana peel-derived biochar (BP-BC) cathode packed in a stainless steel (SS) mesh was used in a continuous flow reactor to degrade the ibuprofen (IBP), as a model contaminant. The BP-BC cathodes generate H₂O₂ via a 2-electron oxygen reduction reaction, initiate the H₂O₂ decomposition to generate •OH, adsorb IBP from contaminated water, and oxidize IBP by formed •OH. Various reaction parameters such as pyrolysis temperature and time, BP mass, current, and flow rate, were optimized to maximize IBP removal. Initial experiments showed that H₂O₂ generation was limited (∼3.4 mg mL^-1), resulting in only ∼ 40% IBP degradation, due to insufficient surface functionalities on the BP-BC surface. The addition of persulfate (PS) into the continuous flow system significantly improves the IBP removal efficiency via PS activation. The in-situ H₂O₂ formation and PS activation over BP-BC cathode results in concurrent generation of •OH and sulfate anion radicals (SO₄^•-, a reactive oxidant), respectively, which collectively achieve ∼ 100% IBP degradation. Further experiments with methanol and tertiary butanol as potential scavengers for •OH and SO₄^•- confirm their combined role in complete IBP degradation.

Synergistic effect between Ce-doped SnO2 and bio-carbon for electrocatalytic degradation of tetracycline: Experiment, CFD, and DFT

Carbon-based sandwich-like electrocatalyst with a hierarchical structure, carbon sheet (CS)-loaded Ce-doped SnO₂ nanoparticles, were successfully prepared using a simple method, which presented a high-efficiency electrocatalytic performance for tetracycline decomposition. Among them, Sn_0.75Ce_0.25O_y/CS exhibits superior catalytic activity, such as more than 95% of tetracycline was removed (120 min), and over 90% of total organic carbon was mineralized (480 min). It is found from morphology observation and computational fluid dynamics simulation that the layered structure is conducive to improving the mass transfer efficiency. Through X-Ray powder diffraction, X-ray photoelectron spectroscopy, Raman spectrum, and density functional theory calculation analyze that the structural defect in Sn_0.75Ce_0.25O_y caused by Ce doping is considered to play the key role. Moreover, electrochemical measurements and degradation experiments further prove that the outstanding catalytic performance is attributable to the initiated synergistic effect established between CS and Sn_0.75Ce_0.25O_y. These results explain the effectiveness of Sn_0.75Ce_0.25O_y/CS for the remediation of tetracycline-contaminated water and mitigating the potential risks and imply that the Sn_0.75Ce_0.25O_y/CS composite has a deeply practical value in tetracycline wastewater degradation and a promise for further application.

Mechanistic investigation into flow-through electrochemical oxidation of sulfanilamide for groundwater using a graphite anode

The technical effectiveness/merit of electrochemical oxidation (EO) has been recognized. Nonetheless, its practical application to groundwater remediation has not been fully implemented due to several technical challenges. To overcome the technical incompleteness, this study adopted a graphite anode in the flow-through system and studied the mechanistic roles of a graphite anode. To this end, groundwater contaminated with sulfanilamide was remediated by means of EO, and sulfanilamide oxidation was quantitatively determined in this study. Approximately 60% of sulfanilamide was degraded at the anode zone, and such observation offered that the removal of sulfanilamide was not closely related with current variations (10-100 mA). However, this study delineated that sulfanilamide removal is contingent on the flow speed. For example, the removal of sulfanilamide was lowered from 59 to 25% owing to a short contact time when the flow velocity was increased from 0.14 to 0.55 cm/min. This study also delineated that a shorter anode-cathode distance could offer a favorable chance to enhance the removal of sulfanilamide even under an identical current. A shorter distance could offer a chance to save energy due to the lower voltage operation. This study also offered that chloride (Cl^-) and sulfate (SO₄^2-) electrolytes served a crucial role in the generation of active species. In contrast, bicarbonate (HCO₃^-) and synthetic groundwater electrolytes impeded the oxidation rate because HCO₃^- scavenged the other active species. In an effort to seek the oxidation mechanisms of a graphite anode, scavenger, cyclic voltammetry test, and electron https://en.wikipedia.org/wiki/Electron_paramagnetic_resonanceparamagnetic resonance (EPR) analysis were done. From a series of the tests, it was inferred that a graphite anode did not directly affect the generation of the active species. Thus, the prevalence of the oxygenated functional groups on an anode surface could be the main mechanism in sulfanilamide removal due to the enhanced electron transfer.

CaO2-based electro-Fenton-oxidation of 1,2-dichloroethane in groundwater

It has been well recognized that the Fenton reaction requires a rigorous pH control and suffers from the fast self-degradation of H₂O₂. In an effort to resolve the technical demerits of the conventional Fenton reaction, particular concern on the use of CaO₂-based Fenton reaction was paid in this study. To realize the practical use of CaO₂ in the Fenton reaction for groundwater remediation, it could be of great importance to control its reaction rate in the subsurface. As such, this study laid great emphasis on the combined process of electrochemical oxidation and CaO₂-based Fenton oxidation, using 1,2-dichloroethane (1,2-DCA) as a model compound. It was hypothesized that the reaction rate is also highly contingent on the formation of Fe(II) (stemmed from iron anode oxidation). Eighty percent of 1,2-DCA were degraded by the CaO₂-based Fenton reaction. The final pH was neutral, inferring that the reaction could be a viable option for the subsurface environment. Moreover, the supply of electric current in an iron anode expedited 1,2-DCA degradation efficiency from 35 % to 62 % via electrically generated Fe(II), which donated electrons to H₂O₂, producing more hydroxyl radicals. An anode-cathode configuration from the single-well system enhanced the degradation of 1,2-DCA, with less amount of energy consumption than the double-well system. Based on results, CaO₂-based electro-Fenton oxidation can remove well 1,2-DCA in groundwater and can be a strategic measure for groundwater remediation.

Enhanced-oxidation of sulfanilamide in groundwater using combination of calcium peroxide and pyrite

Fenton reaction using hydrogen peroxide (H₂O₂) has been widely applied to achieve the in-situ chemical oxidation of contaminated soil and groundwater. However, injecting and transporting H₂O₂ to a contaminated zone consumes the chemical through reactions with other substances and self-decomposition. Additionally, Fe(II), an activator for the Fenton reaction, scavenges hydroxyl radicals, greatly reducing its activity. Therefore, this study proposes a novel oxidation system combining calcium peroxide (CaO₂) and pyrite for the degradation of oxidizable contaminants in groundwater. CaO₂ is an oxygen releasing compound, and pyrite is a natural mineral that provides Fe(II). The individual applications of CaO₂ and pyrite cannot generate OH radicals and oxidize the target pollutant, sulfanilamide. However, the combination of pyrite and CaO₂ oxidized well sulfanilamide even in mild pH and 1.0 wt% of pyrite. Moreover, H₂O₂ and OH radicals are the dominant oxidants in the reaction. A speciation analysis shows the oxidation of pyrite in this combined system. Furthermore, this system oxidized 80% of 0.1 mM sulfanilamide, whereas only 30% was oxidized by conventional Fenton reaction, indicating that this combined system is effective and applicable to remediate groundwater. This study provides an alternative oxidation process to achieve in-situ chemical oxidation.