Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3.

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

The overlooked role of UV185 induced high-energy excited states in the dephosphorization of organophosphorus pesticide by VUV/persulfate.

Vacuum ultraviolet (VUV) based advanced oxidation processes (AOPs) recently attracted widespread interests. However, the role of UV185 in VUV is only considered to be generating a series of active species, while the effect of photoexcitation has long been overlooked. In this work, the role of UV185 induced high-energy excited state for the dephosphorization of organophosphorus pesticides was studied using malathion as a model. Results showed malathion degradation was highly related to radical yield, while its dephosphorization was not. It was UV185 rather than UV254 or radical yield that was responsible for malathion dephosphorization by VUV/persulfate. DFT calculation results demonstrated that the polarity of P-S bond was further increased during UV185 excitation, favoring dephosphorization while UV254 did not. The conclusion was further supported by degradation path identification. Moreover, despite the fact that anions (Cl-, SO42- and NO3-) considerably affected radical yield, only Cl- and NO3- with high molar extinction coefficient at 185 nm significantly affected dephosphorization. This study shed light on the crucial role of excited states in VUV based AOPs and provided a new idea for the development of mineralization technology of organophosphorus pesticides.

The degradation of municipal solid waste incineration leachate by UV/persulfate and UV/H2O2 processes: The different selectivity of SO4•- and •OH.

In this work, the different selectivity of SO4•- and •OH towards municipal solid waste incineration leachates (MSWILs) was studied by a comparative study of UV/persulfate (PS) and UV/H2O2. Results showed SO4•- preferentially mineralized carbon atoms of higher average oxidation state, while •OH showed a two-stage mechanism of partial oxidation and mineralization successively. Electron spin resonance (ESR) analysis showed SO4•- had superior selectivity towards MSWILs than •OH, and Fe(II) would significantly affect the selectivity via forming Fe-MSWILs complex. As the consequence, Fe(II) showed slightly negative effect on UV/PS, but greatly enhanced the performance of UV/H2O2/Fe(II). High concentration of Cl- affected the degradation of non-fluorescent substances by UV/PS, while SO42- and NO3- showed no effect. In contrast, anions showed no effect on UV/H2O2. In addition, •OH preferentially attacked large molecules, but SO4•- showed no selectivity. This study further revealed the selectivity of SO4•- and •OH in the treatment of hypersaline wastewater, and provided theoretical support for the development of targeted technology.

Comparative evaluation of MSW incineration leachate treatment by heterogeneous catalytic O3 and UV/O3: The unexpected contribution of high salinity and overlooked role of excited state.

The efficiency and mechanism of heterogeneous catalytic O3 and UV/O3 for municipal solid waste (MSW) incineration leachate advanced treatment was systematically compared. Prior to comparison, catalyst used in heterogenous catalytic O3 and operation parameters for each technology were optimized. The COD removal of CuO@Al2O3/O3 under its optimal parameters was 57.2%, which failed to meet the standard (≥75%). In contrast, the COD removal by UV/O3 could be 82.3%. The superior efficiency of UV/O3 over CuO@Al2O3/O3 could be summarized into three aspects: (I) Cu bounded ·OH (≡Cu-O·) preferentially attacked hydrophilic groups, while free hydroxyl radical (·OH) was non-selective, thus UV/O3 exhibited a unique three-stage mechanism; (II) The oxidation potential of ≡Cu-O· was higher than that of ·OH, therefore was more vulnerable to the negative effect of radical self-quenching; (III) The existence of UV-induced excited states made organics in UV/O3 more active than in CuO@Al2O3/O3 system, thus high concentration of anions enhanced COD removal in UV/O3 but affected that in CuO@Al2O3/O3. The study further revealed the characteristics of heterogeneous catalytic O3 and UV/O3, and UV induced excited state should be considered in UV-based advanced oxidation processes (AOPs).

Insight into the role of Fe in the synergetic effect of persulfate/sulfite and Fe2O3@g-C3N4 for carbamazepine degradation.

In this work, the role of Fe in the synergetic effect of persulfate/sulfite and Fe2O3@g-C3N4 (FCN) for carbamazepine (CBZ) degradation was studied. Unexpectedly, Fe2O3 in FCN plays very different roles for sulfite [S(IV)] and persulfate (PS) activation. Specifically, since photo-generated holes (h+) can transform S(IV) into SO4-, and photo-generated electrons (e-) can accelerate Fe(III) reduction which promotes transition metal based S(IV) activation, a synergetic effect of photocatalysis and Fe is observed in FCN/S(IV)/vis system. In contrast, in FCN/PS/vis system, both Fe(III)/Fe(II) cycle and PS activation compete for e-. Since PS is a stronger electron acceptor, Fe(III) reduction by e- is limited. Therefore, the contribution of Fe2O3 in FCN/S(IV)/vis system is 3 times higher than that in FCN/PS/vis system. Initial pH affects CBZ removal by changing surface charge of catalysts and oxidants species, while the effect varies for different catalysts and oxidants. This study provides new insight into the synergetic effect of photocatalysis and transition metal for SO4- generation, which contributes to catalyst design for environmental application.

Oil contamination drives the transformation of soil microbial communities: Co-occurrence pattern, metabolic enzymes and culturable hydrocarbon-degrading bacteria.

The land-based oil extraction activity has led to serious pollution of the soil. While microbes may play an important role in the remediation of contaminated soils, ecological effects of oil pollution on soil microbial relationships remain poorly understood. Here, typical contaminated soils and undisturbed soils from seven oilfields of China were investigated in terms of their physicochemical characteristics, indigenous microbial assemblages, bacterial co-occurrence patterns, and metabolic enzymes. Network visualization based on k-core decomposition illustrated that oil pollution reduced correlations between co-existing bacteria. The core genera were altered to those related with oil metabolism (Pseudarthrobacter, Alcanivorax, Sphingomonas, Chromohalobacter and Nocardioides). Under oil pollution pressure, the indigenous bacteria Gammaproteobacteria was domesticated as biomarker and the enzyme expression associated with the metabolism of toxic benzene, toluene, ethylbenzene, xylene and polycyclic aromatic hydrocarbons was enhanced. Functional pathways of xenobiotics biodegradation were also stimulated under oil contamination. Finally, twelve culturable hydrocarbon-degrading microbes were isolated from these polluted soils and classified into Stenotrophomonas, Delftia, Pseudomonas and Bacillus. These results show that the soil microbial communities are transformed under oil pollution stress, and also provide useful information for future bioremediation processes.

Different degradation mechanisms of carbamazepine and diclofenac by single-atom Barium embedded g-C3N4: the role of photosensitation-like mechanism.

This study reports the different degradation mechanisms of carbamazepine (CBZ) and diclofenac (DCF) by single-atom Barium (Ba) embedded g-C3N4. Single-atom Ba is anchored onto g-C3N4 by forming ionic bond with triazine ring, thus greatly enhances the photocatalytic activity with an atom ratio of 1.78%. CBZ undergoes a typical photocatalysis mechanism, while DCF is degraded via a photosensitization-like process, which does not need band gap excitation of photocatalyst. By means of Density Functional Theory (DFT) calculation, the selectivity is found to be related with the different valence excitation modes of CBZ and DCF. Specifically, CBZ undergoes a local excitation, which does not obviously affect molecular configuration. In contrast, DCF undergoes a charge transfer excitation, which significantly changes the reactive sites distribution and facilitates photosensitization-like degradation. Due to the different degradation mechanism, the effects of pH, co-existed anions, and water matrix are also different. Since photosensitization-like mechanism does not rely on photo-generated holes mediated oxidation, the degradation efficient of DCF shows higher anti-interference capacity in real water.

Insight into the synergetic effect of photocatalysis and transition metal on sulfite activation: Different mechanisms for carbamazepine and diclofenac degradation.

Sulfite [S(IV)] is a promising alternative for sulfate radical-based advanced oxidation processes (SR-AOPs). Transition metal and photocatalysis are generally considered to have a synergetic effect for S(IV) activation. However, the study shows that the synergetic effect is target specific. Herein, an ultra-small Fe2O3 clusters deposited graphitic carbon nitride is synthesized and used for S(IV) activation. For carbamazepine (CBZ) degradation, photogenerated holes can transform S(IV) into sulfate radical and photogenerated electrons can accelerate Fe(II)/Fe(III) cycle, which account for the synergetic effect. In contrast, the degradation of diclofenac (DCF) depends on the excitation of DCF rather than photocatalyst. Instead of radical precursor, S(IV) acts as the electron transfer bridge between excited DCF and photocatalyst. Thus, the deposition of Fe2O3 negatively affects DCF degradation. Density Functional Theory calculation shows that the first excited state rather than the ground state of diclofenac is more suitable for reactive site prediction, which confirms the photosensitization-like degradation mechanism. Moreover, the effects of pH and coexisted anions varies for CBZ and DCF. The study shed light on the synergetic effect of transition metal and photocatalysis for S(IV) activation, and also open an avenue for the study of target specific mechanisms for AOPs.

Single-atom silver induced amorphization of hollow tubular g-C3N4 for enhanced visible light-driven photocatalytic degradation of naproxen.

In this work, a novel strategy for building single-atom silver-induced amorphous graphitic carbon nitride (g-C3N4) with a hollow tubular morphology is developed. By forming a tubular supramolecular gel, silver is successfully isolated by the nitrogen atoms in both melamine and nitrate anions, impeding agglomeration in the subsequent thermal polymerization. The high density of single-atom-dispersed silver (atomic ratio up to 11.6%) selectively breaks the hydrogen bonds in layered g-C3N4, leading to a fully amorphous structure. Silver-induced full amorphization not only enhances the visible light absorption of g-C3N4 but also accelerates charge transfer, endowing the as-prepared photocatalyst having the optimal silver content with 52 times higher surface area specific naproxen (NPX) removal activity than pure g-C3N4. Both density functional theory (DFT) calculations and steric effects are applied to explain the degradation pathway of NPX. The toxicity of NPX is reduced by sufficient irradiation. This work provides useful insights into the design and morphology control of single metal ion-dispersed g-C3N4 for environmental applications.

Comprehensive reutilization of iron in iron ore tailings: preparation and characterization of magnetic flocculants.

A large number of iron ore tailings (IOTs) are produced in steel industry, posing threat to the environment during its storage and disposal. To effectively reutilize Fe in IOTs, we propose a comprehensive utilization scheme: (1) most Fe in IOTs is extracted by concentrated hydrochloric acid to form FeCl3 flocculants; (2) after separation from the FeCl3 flocculants, a small amount of Fe is absorbed on the residue solids, which is further washed out to synthesize micron Fe3O4 as magnetic seeds. Results show that the as-synthetic FeCl3 flocculants meet the product standard for FeCl3 flocculants in China (GB/T 4482-2018) after a series of treatments including rotary evaporation, neutralization, and dilution and have comparable performance with commercial polyaluminum chloride (PAC) and polyaluminum ferric chloride (PAFC). Moreover, the addition of synthetic superparamagnetic Fe3O4 (as magnetic seeds) doubled the flocculation rate compared with as-synthetic FeCl3 flocculants alone. Finally, the reutilization of Fe in IOTs can create a direct economic value of ¥ 1.27/kg IOTs, and produce 745 g high-silicon residues for further reutilization, which indicates that our comprehensive utilization scheme is of great application potential.

Remediation of hexavalent chromium in contaminated soil using amorphous iron pyrite: Effect on leachability, bioaccessibility, phytotoxicity and long-term stability.

A large amounts of arable land is facing a high risk of hexavalent chromium (Cr(VI)) pollution, which requires remediation using a low toxic agent. In this study, the remediation effect of amorphous iron pyrite (FeS2(am)) on Cr(VI) in Cr(VI)-contaminated soil was evaluated by systematically analyzing the variation of the leachability, bioaccessibility, phytotoxicity, and long-term stability of the remediated soil. The effectiveness of FeS2(am) on the leachability was assessed by alkaline digestion and the toxicity characteristic leaching procedure (TCLP); the effect on the bioaccessibility was evaluated via the physiologically based extraction test (PBET) and the Tessier sequential extraction; the effect on the phytotoxicity was assessed via phytotoxicity bioassay (seed germination experiments) based on rape (Brassica napus L.) and cucumber (Cucumis Sativus L.), and the long-term stability of the Cr(VI)-remediated soil was appraised using column tests with groundwater and acid rain as the influents. The results show that FeS2(am), with a stoichiometry of 4× exhibited a high efficiency in the remediation of Cr(VI) and decreased its leachability and bioaccessibility during the 30-day remediation period. In addition, seed germination rate, accumulation and translocation of Cr, and root and shoot elongation of rape and cucumber of remediated soil are not significantly different from those of clean soil, illustrating that FeS2(am) is suitable for remediating Cr(VI) contaminated arable soil. The stabilization of Cr(VI) in contaminated soil using FeS2(am) was maintained for 1575 days. The long-term effectiveness was further confirmed by the increasing amount of free Fe and Mn in the effluent and the decreasing redox potential. In summary, FeS2(am) has an excellent efficiency for the remediation of Cr(VI), demonstrating it is a very promising alternative for use in the contaminated arable soil.

Enhanced nitrogen removal from low C/N wastewater using biodegradable and inert carriers: Performance and microbial shift.

This work aims at investigating the enhancing effect of biodegradable and inert carriers on nitrogen removal from low C/N wastewater and revealing temporal changes in community succession. Natural corncobs and commercial fibers were used as biodegradable and inert carriers, respectively. Results showed that the TN removal efficiency was enhanced by 24% and 8.98% using biodegradable and inert carriers, respectively. For corncob carriers, denitrifiers achieved an obvious enrichment and reached a peak on the 30th day. On contrast, inert carriers were more favorable for the enrichment of nitirifiers. Additionally, the dominant denitrifying genus in the corncob system had changed to Dechloromonas, while it remained as Thauera at inert carriers. Finally, the potential coupling pattern of corncob conversion with denitrification in the corncob system was proposed based on the relevant functional enzymes. This work promotes a comprehensive understanding about carrier-enhanced nitrogen removal systems.

Carbon quantum dots modified tubular g-C3N4 with enhanced photocatalytic activity for carbamazepine elimination: Mechanisms, degradation pathway and DFT calculation.

A novel tubular graphitic carbon nitride (g-C3N4) modified with carbon quantum dots (CQDs) was fabricated and employed for the elimination of carbamazepine (CBZ) under visible light irradiation. The as-fabricated metal-free catalysts exhibited tubular morphologies due to the preforming of tubular protonated melamine with CQDs surface adsorption as the polymerization precursors. The surface bonded CQDs did not alter the band gap structure of g-C3N4, but greatly inhibited the charge recombination. Therefore, the CBZ degradation kinetics of tubular g-C3N4 were increased by over 5 times by the incorporation of CQDs. The main active species for CBZ degradation were found to be superoxide radical (O2-) and photo-generated holes (h+), which were further confirmed by electron spin resonance (ESR) analysis. In addition, the degradation pathways of CBZ were clarified via intermediates identification and quantum chemical computation using density functional theory (DFT) and wave function analysis. The olefinic double bond with the highest condensed Fukui index (f0 = 0.108) in CBZ molecule was found to be the most preferable sites for radical attack. Moreover, good stability of the as-prepared photocatalysts was observed in the consecutive recycling cycles, while the slight decline of photocatalytic activity was attributed to the minimal surface oxidation.

[Movement and transformation of heavy metals in water-sediment in water-level-fluctuating zone of Three Gorges Reservoir Area].

According to the off-season regulation model for Three Gorges Reservoir, the characteristic of movement and transformation of Cu, Pb, Cd and Cr in sediment of water-level-fluctuating zone of Three Gorge Reservoir Area was investigated. The results indicated that the chemical fraction composition characteristics of heavy metals are different. Cu mainly existed in organic/sulphidic fraction and residual fraction; Pb mainly existed in carbonates, Fe-Mn oxides and residual fraction; Cd mainly existed in carbonates and Fe-Mn oxides fraction; Cr mainly existed in residual fraction. In flood season, the exposure of sediment from river bottom to atmosphere caused the decrease of TOC, AVS and pH and increase of ORP, and lowered the amount of extractable heavy metals. Heavy metals have the tendency to release to water body. The migration rates of Cu, Pb, Cd and Cr were 30.50%, 26.10%, 33.50%, and 11.77% respectively, the migration ability of heavy metals followed: Cd > Cu > Pb > Cr. The contributions of extractable fractions of Cu, Pb, Cd and Cr to the migrated heavy metals were 77.15%, 86.09%, 94.86%, and 32.34% respectively, extractable fraction was the mainly part of the migrated heavy metals. The off-season regulation model for Three Gorges Reservoir lowered the ecological harm of heavy metals, the concentrations of Cu, Pb, Cr, Cd in the study areas were low.

[Preliminary study on bacterial culture through near-natural pure culture technique].

Based on the ecological features of mutual benefits of different microbes in natural environment, near-natural pure culture technique which was an improvement of conventional pure culture method had been put forward. The key point of this new method was to make hole petri dishes covered with millipore membranes. When these covered dishes being used to culture microorganisms, it could provide incubated microbes active materials from native environment, maintain free communicating with outside microbes, and improve their recovery and culturability of some bacteria. To some degree, this technique enhanced their strengths and overcome their weaknesses of mixed culture method and conventional pure culture method so as to increase microbial culturability and even possibly obtain some nonculturable microorganisms.

[Ecological interpretation and related strategies for low culturability of microorganisms].

Pure culture technique has been a fundamental method since its invention in microbiology, but its isolated and monotonous environment contradicts microbial diversity and cooperation relationship in natural environment. And this contradiction directly results in the uncultivability of some microbes in media. From ecological viewpoint, the recovery barriers of natural microbes mainly include the crash of cooperation in natural environment, the great change of nutrient in new environment and the loss of native niche. The main methods and strategies on low curturability of microorganisms are culture-independent methods, enrichment culture, mixed culture, dilution culture, simulating nature culture and synthetic method, which can to some degree compensate for the traditional approach and improve the low culturability of some bacteria.