Circumneutral pH Values Research Articles

PH and thiosulfate dependent microbial sulfur oxidation strategies across diverse environments.

Sulfur oxidizing bacteria (SOB) play a key role in sulfur cycling in mine tailings impoundment (TI) waters, where sulfur concentrations are typically high. However, our understanding of SOB sulfur cycling via potential S oxidation pathways (sox, rdsr, and S4I) in these globally ubiquitous contexts, remains limited. Here, we identified TI water column SOB community composition, metagenomics derived metabolic repertoires, physicochemistry, and aqueous sulfur concentration and speciation in four Canadian base metal mine, circumneutral-alkaline TIs over four years (2016 - 2019). Identification and examination of genomes from nine SOB genera occurring in these TI waters revealed two pH partitioned, metabolically distinct groups, which differentially influenced acid generation and sulfur speciation. Complete sox (csox) dominant SOB (e.g., Halothiobacillus spp., Thiomonas spp.) drove acidity generation and S2O3 2- consumption via the csox pathway at lower pH (pH ~5 to ~6.5). At circumneutral pH conditions (pH ~6.5 to ~8.5), the presence of non-csox dominant SOB (hosting the incomplete sox, rdsr, and/or other S oxidation reactions; e.g. Thiobacillus spp., Sulfuriferula spp.) were associated with higher [S2O3 2-] and limited acidity generation. The S4I pathway part 1 (tsdA; S2O3 2- to S4O6 2-), was not constrained by pH, while S4I pathway part 2 (S4O6 2- disproportionation via tetH) was limited to Thiobacillus spp. and thus circumneutral pH values. Comparative analysis of low, natural (e.g., hydrothermal vents and sulfur hot springs) and high (e.g., Zn, Cu, Pb/Zn, and Ni tailings) sulfur systems literature data with these TI results, reveals a distinct TI SOB mining microbiome, characterized by elevated abundances of csox dominant SOB, likely sustained by continuous replenishment of sulfur species through tailings or mining impacted water additions. Our results indicate that under the primarily oxic conditions in these systems, S2O3 2- availability plays a key role in determining the dominant sulfur oxidation pathways and associated geochemical and physicochemical outcomes, highlighting the potential for biological management of mining impacted waters via pH and [S2O3 2-] manipulation.

Surface reactivity of the iron and manganese-oxidizing bacterium Leptothrix cholodnii SP-6

Surfaces of prokaryotic cells play a significant role in the adsorption of metals from aqueous solution and the formation of authigenic minerals (Konhauser 2006). Although most studies focus on the cell wall, it is known that many bacteria synthesise an extracellular layer of polysaccharides and proteins, including what are known as sheaths. It has been shown that the cyanobacterium Calothrix sp. produces as sheath which is neutrally charged at circumneutral pH values, and it was hypothesized that such a sheath might allow the cyanobacterium to survive in geothermal settings with high silicification rates (Phoenix et al. 2002). Specifically, the dominance of hydroxyl sites on Calothrix’s sheath surface facilitates hydrogen bonding with aqueous silica species, inducing the precipitation of amorphous silica on the sheath and thus protecting the underlying cell (Phoenix et al. 2002). Leptothrix cholodnii is a sheathed, iron and manganese-oxidizing bacterium that frequently inhabits minerals seeps, where Fe2+ and Mn2+ discharge into oxygenated surface waters (Spring et al. 1996). As a result, the sheath becomes encrusted with Fe(III) and Mn(IV) oxyhydroxides while the underlying cells are protected from mineralization (Emerson and Ghiorse 1992, Emerson et al. 2010). However, unlike Calothrix, Leptothrix’s sheath composition suggests that it might behave differently at circumneutral pH (Emerson and Ghiorse 1993). To investigate the surface reactivity of Leptothrix's sheath and cell wall we analyzed isolated sheaths, sheathless cells, and intact filaments of L. cholodnii SP-6. We studied these components using potentiometric titration, zeta-potential, Cd-adsorption, and Fourier transform infrared (FTIR) spectroscopy to elucidate changes in surface charge between the cell wall and sheath. For the isolated sheaths and intact filaments, titration data were fit using a two-site protonation model, resulting in the following pKa values: 6.05 (±0.29) and 9.34 (±0.11); and 7.77 (±0.17) and 10.50 (±0.20), respectively. For the sheathless cells, the best fit was obtained by using a three-site protonation model, resulting in the following pKa values: 5.40 (±0.59), 8.11 (±1.64) and 10.73 (±0.45). Total proton-active site concentrations were lower in isolated sheaths compared to intact filaments. Additionally, at circumneutral pH, net negative charge was lower for sheathless cells compared to intact filaments and isolated sheaths (Fig. 1). This information agrees with the Cd adsorption behaviour found for the three materials (Fig. 2). Thus, our preliminary results suggest that Leptothrix’s sheath is less reactive than the intact filaments at circumneutral pH, leading us to hypothesize that the outermost layer would sequester relatively lower amounts of cations, including Mn2+, from solution and potentially would protect the underlaying cell from deleterious mineralization. In addition to that, the less reactive sheath’s surface would also contribute to cell attachment, which is important for a species commonly found in streams (Phoenix et al. 2002, Emerson et al. 2010).

Targeting oxidation sites on boreal acid sulfate soil macropore surfaces mitigates acid and metal release to recipient water streams

When reduced sulfidic parent sediments are oxidized, they become acid sulfate soils and discharge metal laden acidic solutions that can damage the environment, infrastructure, and human health. Consequently, methods to mitigate the effect of acid sulfate soils are a priority in affected areas. In this study, acid sulfate soil core samples, consisting of a natural network of preferential-flow soil macropores with defined macropore surfaces and inner cores of denser clay, were characterized and subjected to treatments with calcium carbonate and peat suspensions, or combinations thereof. The effects on the geochemistry and microbial communities were examined on both macropore surfaces and in inner cores. Although transport of treatment substances into the inner cores was demonstrated, no substantial effects were found on the geochemistry and microbial community that consisted of bacterial taxa commonly identified in acid mine drainage. In contrast, positive treatment effects were clearly detected on macropore surfaces and the most promising mitigation effects were detected for treatments combining calcium carbonate and peat suspensions. These treatments increased the pH of the macropore surfaces, added an electron donor in the form of peat, and significantly decreased the relative abundance of acidophilic bacterial populations while shifting the microbial community towards species typically growing at circumneutral pH values. These new environmental conditions were favorable for iron reduction that resulted in a positive effect on permeate quality. The study presents novel data regarding the important differences between acid sulfate soil macropore surfaces and inner cores, as well as their diverse biogeochemical characteristics. It further establishes that the major oxidation-reduction processes occur at the macropore surfaces, and that the combination treatment was the most effective at mitigating the negative environmental effects.

Contamination assessment and availability of potential toxic elements from the Sidi Driss tailing pile (NW Tunisia) based on geochemical and geophysical methods

Mine wastes are of particular relevance, either because they represent a source of contamination by potential toxic elements (PTEs), or even because the high concentrations of some critical raw materials with economic interest can make their mining viable in the future.This work presents mineralogical, geochemical, and geophysical data aimed at a detailed characterization of an old tailing pile of the Sidi Driss mine (northern Tunisia) that exploited Pb and Zn concentrates. The results obtained from four drilling boreholes, reaching a maximum depth of 11 m, show high concentrations of metal(loid)s in the tailings, namely As, Ba, Cd, Mn, Pb and Zn. Apart from barium, the higher concentrations of these PTEs were found between 3 and 8 m depth. The availability of such metal(loid)s is higher in deeper sections of the tailing pond than on the surface, which is explained by the reduction and oxidation environments prevailing at these depths, respectively. Thus, at surface sulphides are oxidised and several metal(loid)s are released and leached. This oxidation process generates acid mine drainage (AMD), causing the dissolution of carbonates, such as cerussite, from which Pb is released and leached underground. In deeper sections of the tailing pond, reduction conditions prevent sulphide oxidation and the prevalence of circumneutral pH values. In such environments, carbonates are not dissolved and Pb leached from the surface precipitates (e.g., as anglesite, an insoluble sulphate), as demonstrated by sequential selective chemical extraction.Electrical resistivity tomography (ERT) showed low resistivity areas from surface to 8 m depth. This is a consequence of waterlogging on the surface (due to rainy events) and, on the subsurface, of high metal(loid) contents in tailings. High electrical resistivity was measured below this level (bedrock), but a connection between heavily contaminated horizons (tailings) and a subsequent layer of low resistivities (associated with groundwater) is observed, which is of particular concern.

Evolution of Alkaline Mine Drainage and Unique Biodiversity on Soil-Free Mine Substrates, Southern New Zealand

Relatively impermeable soil-free substrates (clay-rich outcrops and sediments) exposed at eight abandoned placer gold mines generate alkaline mine drainage through evaporation and minor interaction between water and rock in a semi-arid rain shadow climate. Original mine sluicing a century ago created drainage channels over the land surface that still control the flow of ephemeral surface waters, with localised construction of erosional outwash pans. Ephemeral surface waters and associated evaporitic salts are dominated by halite derived from marine aerosols in rain, yielding circumneutral pH and electrical conductivity (EC) values locally exceeding 50 mS/cm. Weakly altered schist basement rocks and Miocene mudstone exposed at the surface contain abundant calcite, and surface waters are supersaturated with respect to Ca-carbonate minerals with a pH of ≈ 8 and an EC of ≈ 1 mS/cm. Water interaction with albite increases the dissolved Na/Cl molar ratio to > 1, and evaporative formation of Na-carbonate precipitates can raise the pH to > 10. Pyrite oxidation does not offset the alkaline pH in any of these processes, although dissolved sulfate can rise to high levels (> 2000 mg/L; EC ≈ 5 mS/cm), forming evaporative sulfate minerals. Ephemeral waters drain down outcrops and the relatively impermeable erosional pans, leaving salt encrustations with variable mineralogy and associated waters with pH and EC values that are partially controlled by rates of dissolution and reprecipitation of the minerals in the salts. The saline alkaline chemistry of the pans excludes most vegetation and has allowed development of salt-tolerant ecosystems with rare endemic halophytic plants. Maintenance of soil-free halophytic surfaces has potential applications in dryland mine sites around the world, especially as climate change enhances and extends aridity.

Catalytic Wet Peroxide Oxidation of Anionic Pollutants over Fluorinated Fe3O4 Microspheres at Circumneutral pH Values

Fluorinated Fe3O4 microspheres with 7.1 ± 1.4 wt% of fluoride (F-Fe3O4-1) were prepared via glycothermal synthesis. Fluorination significantly enhanced the activity of F-Fe3O4-1 in catalytic wet peroxide oxidation of anionic dyes (including orange G (OG) and congo red) at pH ~7. However, the promotional effect of fluorination became less obvious for amphoteric rhodamine B and was not observed for cationic methylene blue. After reacting with H2O2 (40 mM) for 2 h at pH 6.5 and 40 °C, the decolorization rates of OG (0.1 mM) and the pseudo-first-order rate constant were 96.8% and 0.0284 min−1 over F-Fe3O4-1 versus 17.6% and 0.0011 min−1 over unmodified Fe3O4. The effects of reaction parameters (initial H2O2 concentration and pH value and reaction temperature) on OG decolorization with H2O2 over F-Fe3O4-1 were investigated. The reusability of F-Fe3O4-1 was demonstrated by OG decolorization in eight consecutive runs. Fluorination increased the isoelectric point of F-Fe3O4-1 to 8.7 and facilitated the adsorption and degradation of anionic dyes on the surface of F-Fe3O4-1 at pH ~7. Scavenging tests and EPR spectra supported that hydroxyl radicals were the main reactive species for the OG decolorization over F-Fe3O4-1.

Occurrence of ammonium in the acidic-circumneutral coastal groundwater of Beihai, Southern China: δ15N, δ13C, and hydrochemical constraints

High levels of ammonium (NH4+) in groundwater can threaten the health of surface ecosystems and the safety of water supplies. To ascertain the occurrence of NH4+ in the coastal acidic groundwater of Beihai, Southern China, a combined hydrochemical and isotopic (δ15N in nitrate (NO3−) and NH4+; δ13C in dissolved inorganic carbon (DIC)) study was performed. In addition to a chloride concentration-based fresh water-seawater mixing model, δ15NNO3 vs δ15NNH4, Δδ13CDIC,react (changes in δ13CDIC values caused by reaction), and ΔCSO4,react (changes in sulfate concentrations caused by reaction) were applied to interpret the genesis of NH4+. The results show groundwater NH4+ loadings range from < 0.04 to 6.2 mg/L, with typically higher loadings along the coastline than in the plains. Elevated NH4+ loadings (>0.2 mg/L) are commonly accompanied by enriched δ15NNO3 and depleted δ15NNH4 values in relation to the anthropogenic endmember, depleted or enriched Δδ13CDIC,react values, and ΔCSO4,react > 0.00 or < 0.00 mmol/L. Such isotopic fingerprints and hydrochemical features indicate dissimilatory nitrate reduction to ammonium (DNRA), glucose fermentation, methanogenesis (acetate fermentation), and sulfide oxidation or sulfate reduction. The findings suggest acidic rainwater, sulfide oxidation, and NO3− reduction by ferrous iron (Fe2+) could jointly account for the formation of acidic groundwater (pH as low as 4.0) that is of meteoric origin. Fermentative and sulfide-driven DNRA with anthropogenic NO3− is notably responsible for the enrichment of groundwater NH4+ concentrations, with a minor contribution from anthropogenic NH4+ infiltration and algae decomposition. As a result of silicate and carbonate weathering, seawater inputs, and fermentation, the addition of bicarbonate (HCO3−) results in circumneutral pH values. These results indicate DNRA plays a critical role in the enrichment of NH4+ and, hence, improve understanding of its genesis in the groundwater of Beihai and other similar areas.

LED visible light assisted photochemical oxidation of HCHs in aqueous phases polluted with DNAPL

This work focuses on removing hexachlorocyclohexanes (HCHs) found in groundwater polluted with dense non-aqueous phase liquids (DNAPLs) by photo-oxidation with hydrogen peroxide or persulfate using LED visible light and ferrioxalate as the catalyst. Single oxidation tests were also performed to evaluate the contribution of LED-vis light on HCHs removal. Results show that it is possible to attain the degradation of HCHs up to 85% in 420 min with persulfate, whereas percentages lower than 40% are obtained when using hydrogen peroxide. Using both oxidants in the presence of ferrioxalate and LED visible light promotes the generation of hydroxyl and sulfate radicals under circumneutral pH values, which are the main responsible species for HCHs removal. Specifically, an oxidant conversion higher than 50% was achieved during the photochemical treatment with both oxidants, whereas conversions below 20% were obtained in the absence of LED visible light irradiation. On the other hand, DNAPL produced as liquid residuum of lindane production contains other chlorinated organic compounds (COCs), which are susceptible to being oxidized by hydroxyl and sulfate radicals, generating competitive oxidation reactions. The final conversion of chlorbenzenes reaches values close to 100% and HCHs are only effectively removed when persulfate is used as the oxidant. This better performance indicates that the photo-oxidation of DNAPL polluted groundwater with LED-vis light should be carried out with persulfate to ensure the removal of more dangerous COCs. This confirms the excellent ability of sulfate radicals for C-Cl bond breakdown.

Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships

Nanoparticles often exhibit size-dependent redox reactivities, with smaller particles being more reactive in some cases, while less reactive in others. Predicting trends between redox reaction rates and particle sizes is often complicated because a particle's dimensions can simultaneously influence its aggregation state, reactive surface area, and thermodynamic properties. Here, we tested the hypothesis that interfacial redox reaction rates for nanoparticles with different sizes can be described with a single linear free-energy relationship (LFER) if size-dependent reactive surface areas and thermodynamic properties are properly considered. We tested this hypothesis using a well-known interfacial redox reaction: the reduction of nitrobenzene to aniline by iron-oxide-bound Fe2+. We measured the reduction potential (EH) values of nano-particulate hematite suspensions containing aqueous Fe2+ using mediated potentiometry and characterized the size and aggregation states of hematite samples at circumneutral pH values. We used the measured EH values to calculate surface energies and reactive surface areas using thermodynamic relationships. Nitrobenzene reduction rates were lower for smaller particles, despite their larger surface areas, due to their higher surface energies. When differences in surface areas and thermodynamic properties were considered, nitrobenzene reduction kinetics for all particle sizes was described with a LFER. Our results demonstrate that when Fe2+ serves as a reductant, an antagonistic effect exists, with smaller particles having larger reactive surface areas but also more positive reduction potentials. When Fe3+ serves as an oxidant, however, these two effects work in concert, which likely explains past discrepancies regarding how iron oxide particle sizes influence redox reaction rates.

Polyamidoxime-based membranes for the rapid screening of uranium isotopes in water

Traditional radiochemistry approaches for the detection of trace-level alpha-emitting radioisotopes in water require lengthy offsite sample preparations and do not lend themselves to rapid quantification. Therefore, a novel platform is needed that combines onsite purification, concentration, and isotopic screening with a fieldable detection system. This contribution describes the synthesis and characterization of polyamidoxime membranes for isolation and concentration of uranium from aqueous matrices, including high-salinity seawater. The aim was to develop a field portable screening method for the rapid quantification of isotopic distribution by alpha spectroscopy. Membranes with varying degree of modification were prepared by chemical conversion of nitrile groups to amidoxime groups on the surface of polyacrylonitrile ultrafiltration (UFPAN) membranes. Attenuated total reflectance Fourier-transform infrared spectroscopy was used to analyze changes in surface chemistry. Flow through filtration experiments conducted using deionized (DI) water and simulated seawater solutions indicated that the modified membrane was effective in capturing more than 95% of the uranium in the solution prior to breakthrough even in the presence of salt ions. Batch uptake experiments were conducted and compared with the flow through experimental data to elucidate likely binding mechanisms. Alpha spectra of uranium loaded membranes were analyzed, and the effects of solution matrix and degree of modification on peak energy resolution were studied. Peak energy resolutions of 24 ± 2 keV and 32 ± 6 keV full width at half maximum (FWHM) were obtained by loading uranium from DI and seawater solutions onto modified membranes. Full width at 10% maximum of the same spectra were calculated to be 63 ± 9 keV and 160 ± 34 keV to quantify differences seen in peak tailing. Calculations performed based on the results show that it would take less than 3 h of analysis time to screen a sample provided enough volumes of solution are available. This work offers a facile method to prepare polyamidoxime-based membranes for uranium separation and concentration at circumneutral pH values, enabling the rapid, onsite screening of unknown samples.

Co-composting of biochar and nitrogen-poor organic residues: Nitrogen losses and fate of polycyclic aromatic hydrocarbons

Composting recycles nutrients and biodegrades organic pollutants, but often results in N leaching. Biochar can enhance the composting process and reduce N losses. Research, however, has focused on composting N-rich residues; also, information on the fate of biochar polycyclic aromatic hydrocarbons (PAHs) during composting is scarce. We explored the composting of biochar with N-poor organic residues as a strategy to reduce N losses and biochar PAHs. A small-scale composting experiment was performed with three treatments: 100% yard residues and two mixtures of 85% yard residues and 15% gasification- or pyrolysis-derived biochar. Temperatures were recorded daily during composting and Nlosses and changes in PAHs loads were calculated. Results across all treatments showed overall low N losses, likely caused by low temperatures and N contents, circumneutral pH values, and absence of leachate, and simultaneous immobilization and mineralization of PAH contents. Treatments with biochar showed a slower release of inorganic N (NO3--N and NH4+-N), although they also had overall lower inorganic N contents. This slower release of inorganic N may relate to biochar’s high surface area. We conclude that biochar provides valuable benefits for N-poor composting, and that composting should be further explored as a promising strategy to reduce the contents of PAHs in biochar.

Kinetics of diatrizoate degradation by ozone and the formation of disinfection by-products in the sequential chlorination

Abstract In this study, we studied the degradation kinetics of a common iodine contrast agent, diatrizoate, by ozone and the formation of disinfection by-products (DBPs) in the sequential chlorination. Effects of ozone concentration, solution pH, and bromide concentration on diatrizoate degradation were evaluated. The results indicate that diatrizoate can be effectively degraded (over 80% within 1 h) by ozone, and the degradation kinetics can be well described using the pseudo-first-order kinetic model. The pseudo-first-order rate constant (kobs) of diatrizoate degradation significantly increased with increasing ozone concentration and decreasing bromide concentration. The kobs kept increasing with the increase of pH value and reached a maximum of 6.5 (±0.05) × 10−2 min−1 at pH 9. As the ozone concentration gradually increased from 0.342 to 1.316 mg/L, the corresponding kobs of diatrizoate degradation increased from 1.76 (±0.20) × 10−3 to 4.22 (±0.3) × 10−2 min−1. The bromide concentration exhibited a strong inhibitory effect on diatrizoate degradation because of the competition for ozone with diatrizoate. Trichloromethane was the only detected DBP in the subsequent chlorination in the absence of bromide. However, in the presence of bromide, six other DBPs were detected, and bromochloroiodomethane and tribromomethane became the major products with concentrations 1–2 orders higher than those of the other DBPs. In order to provide safe drinking water to the public, water should be maintained at circumneutral pH values and low bromine concentrations (&amp;lt;5 μM) before reaching the chlorine disinfection process to effectively control the formation of DBPs.

Significant role of iron on the fate and photodegradation of enrofloxacin

Enrofloxacin (ENR) belongs to the fluoroquinolone (FQ) antibiotics family, which are contaminants of emerging concern frequently found in effluents. Although many works studying photo-Fenton process for FQ degradation have been reported, there are no reports analysing in deep the effect of iron complexation, as well as other metals, towards FQs’ photolysis, which, evidently, also contributes in the overall degradation of the pollutant. Therefore, in this work, we report a comparative study between the photochemical fate of ENR and its complex with Fe(III) under simulated sunlight irradiation. In addition, the effect of dissolved oxygen, self-sensitization process, and H2O2 addition on the studied photochemical systems are also investigated. Results indicate that, for free and iron-complexed ENR, singlet oxygen (1O2) is generated from the interaction of its triplet state with ground state oxygen. Half-life time (t1/2) of ENR under sun simulated conditions is estimated to be around 22 min, while complexation with iron enhances its photostability, leading to a t1/2 of 2.1 h. Such finding indicates that at least the presence of iron, might notably increase the residence time of these pollutants in the environment. Eventually, only with the addition of H2O2, the FQ-iron complex is efficiently degraded due to photo-Fenton process even at circumneutral pH values due to the high stability of the formed complex. Finally, after LC/FT-ICR MS analysis, 39 photoproducts are detected, of which the 14 most abundant ones are identified. Results indicate that photoproducts formation is pH and iron dependent.

Phosphate removal using surface enriched hematite and tetra-n-butylammonium bromide incorporated polyacrylonitrile composite nanofibers

The nanosized iron oxides-based adsorbent has been widely used to alleviate water eutrophication. However, it is challenging to industrialize the application of nanosized iron oxides-based adsorbent due to their poor stability, difficult separation and recovery.Herein, hematite and tetra-n-butylammonium bromide incorporated polyacrylonitrile (PAN/Fe2O3/TBAB) composite nanofibers with a controlled diameter (i.e., 66 to 305 nm) and composition were systematically synthesized as an adsorbent for phosphate removal from water using surfactant-mediated electrospinning. During the electrospinning process, polar TBAB surfactant enhanced the migration of Fe2O3 nanoparticles toward the surface of nanofibers resulting in Fe2O3 nanoparticles/TBAB surface enriched nanofibers. The synthesized nanofiber membranes were used for phosphate removal, and their adsorption kinetics, adsorption mechanism, and reusability were investigated. Data showed that adsorption kinetic followed the pseudo-second-order model whereas the adsorption mechanism follows the Langmuir model. The phosphate removal was mainly derived from the chemisorption of surface-enriched α-Fe2O3 nanoparticles at acidic and circumneutral pH values, with a small contribution from anion exchange at TBAB sites. The maximum phosphate removal capacity was approx. 8.76 mg/g (i.e., 23.1 mg/g, P/active materials) at pH 3. Additionally, the synthesized nanofiber membrane also shows excellent reusability.

Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.

New soft chemistry route to titanomagnetite magnetic nanoparticles with enhanced peroxidase-like activity

Titanomagnetite is widely distributed in nature, and has potentials in environmental remediation. Nanometric titanomagnetite particles were usually prepared via soft chemistry routes in aqueous solutions. Herein a novel soft chemistry route carried out in a deep eutectic solvent (DES) comprising choline chloride and urea in a molar ratio of 1:2 was successfully developed to make titanomagnetite nanoparticles (Fe2.5Ti0.5O4-DES, ~30 nm). The chemical composition analyzed by different methods revealed that Fe2.5Ti0.5O4-DES nanoparticles possess the Ti-rich core and Fe-rich shell. The Fe2.5Ti0.5O4-DES nanoparticles exhibited enhanced activities in catalytic decomposition of H2O2 and oxidative degradation of organic contaminants at circumneutral pH values and room temperature, which was mainly attributed to the increased content of ferrous (Fe2+) ions upon substitution of Ti4+ for ferric (Fe3+) ions in spinel structure of magnetite.

Uranium and neptunium retention mechanisms in Gallionella ferruginea/ferrihydrite systems for remediation purposes

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg−1 for uranium and 3.07 × 105 L kg−1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.

Bio‐electro‐Fenton systems for sustainable wastewater treatment: mechanisms, novel configurations, recent advances, LCA and challenges. An updated review

AbstractBio‐electro‐Fenton processes use biological electrons produced from bioelectrochemical systems to treat wastewater. The most significant advantages of bio‐electro‐Fenton systems are high effectiveness, low toxicity, gentle operation conditions, environmentally friendly treatment without sludge accumulation and energy conservation. Though promising, bio‐electro‐Fenton systems still face several challenges, such as high power density, H2O2 concentration, cathode materials, Fe2+ concentration and pH. This review comprehensively discusses the mechanisms of bio‐electro‐Fenton systems. Then, structural configurations are critically reviewed, including microbial fuel cells coupled with electro‐Fenton systems, microbial electrolysis cells coupled with electro‐Fenton systems and other bioelectrochemical systems coupled with electro‐Fenton systems. Furthermore, recent advances in bio‐electro‐Fenton systems for wastewater treatment are introduced, including dye solution, pharmaceuticals and personal care products, oily wastewater, landfill leachate and other pollutants. In addition, the current challenges and specific future prospects of bio‐electro‐Fenton, such as possible mechanisms for improving the power output, electrode materials that are potentially useful, self‐designed electrodes and methods of maintaining circumneutral pH values, are also explored. Heretofore, great progress in bio‐electro‐Fenton has been made, but further improvements are still needed in order to make this system more economical and practical. © 2020 Society of Chemical Industry

Study on the Species Distributions of Dissolved U(VI) and Adsorbed U(VI) on Silica Surface

Dissolved hexavalent uranium can exist in the form of several different chemical species. Furthermore, species distributions depend on the pH value of the aqueous solution. Representatively, UO2 2+, UO2OH+, (UO2)2(OH)2 2+, and (UO2)3(OH)5 + species coexist in solutions at acidic and circumneutral pH values. When amorphous silica particles are suspended in an aqueous solution, the dissolved chemical species are easily adsorbed onto silica surfaces. In this study, it was examined whether the species distribution of the adsorbed U(VI) on a silica surface followed that of the dissolved U(VI) in an aqueous solution. Time-resolved luminescence spectra of three different dissolved species (UO2 2+, UO2OH+, and (UO2)3(OH)5 +) and two different adsorbed species (≡SiO2UO2, ≡SiO2(UO2)OH–, or ≡SiO2(UO2)3(OH)5–) were measured in the pH range 3.5−7.5. The spectral shapes of these chemical species were compared by changing the pH value; consequently, it was confirmed that the species distribution of the adsorbed U(VI) species was different from that of the dissolved U(VI) species.

Kinetics of iohexol degradation by ozonation and formation of DBPs during post-chlorination

Degradation kinetics of the widely used iodinated X-ray contrast medium, iohexol, was investigated in this study by ozone advanced oxidation process (AOP). Factors influencing iohexol degradation, pH, ozone dosage, and the presence of different background ions were also evaluated. Results show that iohexol degradation during ozonation followed the pseudo-first-order kinetics well; the degradation rate increased with the increase of pH values and reached a maximum of 1.29(±0.05)×10−1 min−1 at pH 9. As the ozone concentration gradually increased from 0.342 to 1.13 mg/L, the corresponding pseudo-first-order rate constant of iohexol degradation increased from 7.25 (±0.30)×10−3 to 5.77(±0.26)×10−2 min−1. At pH 7, the presence of background ions have different inhibitory effects on iohexol during ozonation degradation. Br­ exhibited the strongest inhibitory effect followed by HCO3− and CO32-, while Cl­ possessed negligible influence. Less disinfection by-products (DBPs) were formed at circumneutral pH values and low bromine concentrations (< 10 μM), which can be beneficial for the operation of drinking water treatment plants to supply safe drinking water to the public.