Sulfide drives hydroxyl radicals production in oxic ferric oxyhydroxides environments

Photo-enhanced oxidation of arsenite by biochar: The effect of pH, kinetics and mechanisms

The persistent and photo-induced free radicals of biochar play significant roles in the transformation or degradation of inorganic and organic pollutants. However, the redox capacity of biochar for arsenite (As(III)) photochemistry under different pH conditions remains unclear. In this study, we discovered that solar radiation primarily expedited the oxidation of As(III) by biochar by augmenting the production of reactive oxygen species (ROS). Biochar demonstrated a strong pH reliance on the photooxidation of As(III). Under acidic and neutral conditions, solar radiation amplified the generation of •OH (hydroxyl radicals) by BC-P (phenolic -OH of biochar) and semiquinone-type BC-PFRs (persistent free radicals of biochar) by 4.9 and 2.0 times, respectively, resulting in enhanced As(III) oxidation. Under alkaline conditions, BC-P and BC-Q (quinoid CO of biochar) facilitated the production of H2O2 (hydrogen peroxide) by 2.1 times through the spontaneous formation of semiquinone-type BC-PFRs via an anti-disproportionation reaction, promoting approximately 88.2% of As(III) photooxidation. Furthermore, solar radiation elevated around 11.8% As(III) oxidation driven by BC-Q and semiquinone-type BC-PFRs. This study provides a crucial theoretical foundation for using biochar to treat arsenic pollution in aquatic systems and understanding the migration and transformation of arsenic in different environments.

On-site generation of reactive oxidative radicals from dithionite treated oxic soil slurry

Whereas dithionite has been extensively used as a reducing agent in soil and sediment remediation, here, we demonstrate that it can be used as a potential source of oxidizing radical in oxic soils with potential application in organic pollutant remediation. Benzoic acid was used as a probe compound and the generation of its oxidative product para-hydroxybenzoic acid (p-HBA) was detected to quantify the production of oxidative radicals (ROS). By increasing the dithionite concentration from 2.5-10 Mm, the accumulated P-HBA concentration in 120 min increased from 15.0-27 µM. Whereas, above 10 mM, the p-HBA concentration decreased due to radical scavenging. Increasing soil dosage from 2.5-15 g/100 mL the accumulated p-HBA amount increased from 22.8-33.7 µM. Temperature 25-35 ^oC and pH 6.2-7.5 were favoured for p-HBA generation. Furthermore, we investigated the roles of different active intermediates in the reaction system and proposed the mechanism behind the ROS genearation. This study suggested that dithionite can be used as an active reagent for advanced oxidation remediation in oxic soil medium.

Sulfidation of ferric (hydr)oxides and its implication on contaminants transformation: a review

Rapid industrialization and urbanization have resulted in elevated concentrations of contaminants in the groundwaters and subsurface soils, posing a growing hazard to humans and ecosystems. The transformation of most contaminants is closely linked to the mineralogy of ferric (hydr)oxides. Sulfidation of ferric (hydr)oxides is one of the most significant biogeochemical reactions in the anoxic environments, causing reductive dissolution and recrystallization of ferric (hydr)oxides and further affecting the transformation of iron-associated contaminants. This paper provides a comprehensive review on the sulfidation process of ferric (hydr)oxides and the transformation of relevant contaminants. This review presents detailed reaction mechanisms between ferric (hydr)oxides and dissolved sulfide, and elucidates the factors (e.g. crystallinity of ferric (hydr)oxides, the ratio of sulfide concentration to the surface area concentration of ferric (hydr)oxides) that control the formation of surface associated Fe(II), iron sulfide minerals, as well as transformation of secondary minerals. Then, we summarized the transformation mechanisms of a variety of typical environmentally relevant contaminants existing in groundwater and subsurface soils, including heavy metals, metal(loid) oxyanions (arsenic, antimony, chromium), radionuclides (uranium, technetium), organic contaminants and phosphate/nitrate species. The general mechanisms of contaminant transformation involve a combination of release, reduction and re-adsorption/incorporation processes, the specific pathway of which is highly dependent on the properties of the contaminant itself and the extent of sulfidation. Moreover, the challenge of extending our knowledge towards in situ remediation, as well as further research needs are identified.

Remediation of arsenic-contaminated paddy soil by iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue under flooded condition

Flooded condition enhances arsenic (As) mobility in paddy soils, posing an imminent threat to food safety and human health. Hence, iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue (CG-FeOH and CG-FeOS) were synthesized for remediation of As-contaminated paddy soils under a flooded condition. Compared to the control, CG-FeOH and CG-FeOS application decreased the soil pH by 0.10-0.80 and 0.13-1.63 units, respectively. CG-FeOH and CG-FeOS application significantly (P < 0.05) decreased available As concentration by 13.46-43.44% and 21.31-54.37%, respectively. CG-FeOH and CG-FeOS significantly (P < 0.05) reduced the non-specifically adsorbed and specifically adsorbed As fractions and increased As(V) proportion by 22.61-26.53% and 29.10-36.51%, respectively. Our results showed that CG-FeOH and CG-FeOS could change As geochemical fraction and valence state, consequently reducing available As concentration in paddy soils. Moreover, the sulfate could enhance the oxidation and co-precipitation of As with CG-FeOH. Compared to CG-FeOH, CG-FeOS was more effective in decreasing available As concentration and oxidizing As(III) to As(V). This study revealed that CG-FeOS is a potential amendment for As immobilization in paddy soils.

Interfacial reaction between organic acids and iron-containing clay minerals: Hydroxyl radical generation and phenolic compounds degradation

Reactive oxygen species, especially hydroxyl radicals (OH), exert a distinguished role in the transformation of contaminants, and their in-situ generation attracts wide attentions in environmental and geochemical areas. The present work explored the potential formation of OH during the interactions between iron-containing clay minerals and environmentally prevalent organic acids in dark environments. The results demonstrated that the accumulative OH concentrations were related to the solution pH, the types of clay minerals, and the nature of organic acid species. At pH 5.5, 1.2- 15.2 times of OH were generated from the reduction of Na-nontronite-2 (Na-NAu-2) compared with other clay minerals in the presence of ascorbic acid (AA) at 144 h. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analyses indicated that Fe(III) was reduced to Fe(II) by AA during OH formation. Meanwhile, chemical probe tests coupled with quenching experiments confirmed the generation of H₂O₂ and superoxide radical (O₂^-), which participated in the formation of OH. The produced OH/O₂^- can transform 68.4%, 86.4%, and 50.1% of phenol, p-nitrophenol, and 2,4-dichlorophenol within 168 h in AA-Na-NAu-2 suspension, respectively. This work provides valuable insights into OH production in the mutual interaction between organic acids and iron-bearing clays, which is helpful for the development of a new method for removing organic pollutants from contaminated water and soil environments.

Pyrogenic Carbon Initiated the Generation of Hydroxyl Radicals from the Oxidation of Sulfide

Sulfide is one of the most abundant reductants in the subsurface environment, while pyrogenic carbon is a redox medium that widely exists in sulfide environment. Previous studies have found pyrogenic carbon can mediate the reductive degradation of organic pollutants under anoxic sulfide conditions; however, the scenario under oxic sulfide conditions has rarely been reported. In this study, we found that pyrogenic carbon can mediate hydroxyl radicals (^•OH) generation from sulfide oxidation under dark oxic conditions. The accumulated ^•OH ranged from 2.07 to 101.90 μM in the presence of 5 mM Na₂S and 100 mg L^-1 pyrogenic carbon at pH 7.0 within 240 min. The Raman spectra and electrochemical cell experiments revealed that the carbon defects were the possible chemisorption sites for oxygen, while the graphite crystallites were responsible for the electron transfer from sulfide to O₂ to generate H₂O₂ and ^•OH. Quenching experiments and degradation product identification showed that As(III) and sulfanilamide can be oxidized by the generated ^•OH. This research provides a new insight into the important role of pyrogenic carbon in redox reactions and dark ^•OH production.

UV-Vis quantification of hydroxyl radical concentration and dose using principal component analysis

Hydroxyl radicals (∙OH) are powerful oxidizing species formed naturally in the environment or artificially produced to destroy contaminants in water treatment facilities. Their short lifetime and high reactivity, however, present a significant challenge to quantifying their concentration in solution. Herein, we developed a novel method to accurately measure the steady-state ∙OH concentration and total ∙OH dose produced during the UV photolysis of hydrogen peroxide (H₂O₂) by monitoring the loss of salicylic acid (SA). This information can be acquired using only benchtop UV-Vis spectroscopy, thus expanding measurement capabilities of resource-limited laboratories by eliminating the need for sophisticated instrumentation. To improve the precision with which the rate of SA loss was measured compared to previous methods, we applied principal component analysis (PCA) to fit the UV-Vis spectra collected during SA exposure to ∙OH. For our experimental conditions consisting of 12 mL solutions composed of ≤ 100 mM H₂O₂ and 0.07 mM SA, the steady-state ∙OH concentration throughout the complete photolysis of H₂O₂ was 1.33 × 10^-11 M ± 1.14 × 10^-12 M. This represents more than a ten-fold improvement in reducing the uncertainty of the measurement, with respect to narrowing the 95 % confidence interval, compared to a previous method that employed matrix analysis to process the spectra. Furthermore, the variance of the measured ∙OH concentrations was reduced by a factor of 100 compared to previous methods. Using PCA, the limit-of-detection and limit-of-quantitation for ∙OH are 5.33 × 10^-13 M and 1.23 × 10^-12 M, respectively. By developing quantitative relationships among ∙OH concentration, H₂O₂ concentration, and UV exposure time, we also show how to calculate the equivalent exposure to ∙OH generated in natural aquatic environments by indirect photolysis. Finally, we use this methodology to demonstrate that the presence of suspended carbonaceous nanoparticles at concentrations as high as 300 ppm does not affect ∙OH concentration.

Production of a Surface-Localized Oxidant during Oxygenation of Mackinawite (FeS)

The oxygenation of mackinawite (FeS) frequently occurs at anoxic-oxic interfaces in both natural and engineered systems such as intertidal sediment, in activated sludge in water treatment processes, and during sulfidized zero-valent iron particle corrosion. During reoxygenation events, FeS may drive a Fenton-like process leading to the production of strong oxidants though the details of this process are poorly understood. In this study, benzoic acid (BA) has been used to probe both the magnitude and identity of these strong oxidants under circumneutral pH conditions. The major product of BA oxidation during FeS oxygenation was found to be 2,5-dihydroxybenzoic acid (2,5-DHBA) rather than monohydroxybenzoic acids identified to be the major products in a range of hydroxyl radical (HO^·)-dominated systems. Based upon relative reactivity with other competitive probes and nature of the hydroxybenzoate product distribution, it is hypothesized that the strong oxidant must be a surface-localized entity such as high-valent iron or surface-associated hydroxyl or sulfur-based radicals with reactivity differing from those formed in free solution. The importance of both the reactivity of the oxidant and adsorption of the substrate to the surface is demonstrated.