A feasibility study of metal sulfide (FeS and MnS) on simultaneous denitrification and chromate reduction

Superior nitrate and chromium reduction synergistically driven by multiple electron donors: Performance and the related biochemical mechanism

Nitrate and Cr(VI) are the typical and prevalent co-contaminants in the groundwater, how to synchronously and effectively diminish them has received growing attention. The most problem that currently limits the nitrate and Cr(VI) reduction technology for groundwater remediation is with emphasis on exploring the optimal electron donors. This study investigated the feasibility of utilizing the synergistical effect of inorganic electron donors (pyrite, sulfur) and inherently limited organics to promote synchronous nitrate and Cr(VI) removal, which meets the requirement of naturally low-carbon and eco-friendly technologies. The NO3--N and Cr(VI) removal efficiencies in the pyrite and sulfur involved mixotrophic biofilter (PS-BF: approximately 90.8 ± 0.6% and 99.1 ± 2.1%) were substantially higher than that in a volcanic rock supported biofilter (V-BF: about 49.6% ± 2.8% and 50.0% ± 9.3%), which was consistent with the spatial variations of their concentrations. Abiotic and biotic batch tests directly confirmed the decisive role of pyrite and sulfur for NO3--N and Cr(VI) removal via chemical and microbial pathways. A server decline in sulfate production correlated with decreasing COD consumption revealed that there was sulfur disproportionation induced by limited organics. Metagenomic analysis suggested that chemoautotrophic microbes like Sulfuritalea and Thiobacillus were key players responsible for sulfur oxidation, nitrate and Cr(VI) reduction. The metabolic pathway analysis suggested that genes encoding functional enzymes related to complete denitrification, S oxidation, and dissimilatory sulfate reduction were upregulated, however, genes encoding Cr(VI) reduction enzymes (e.g. chrA, chrR, nemA, and azoR) were downregulated in PS-BF, which further explained the synergistical effect of multiple electron donors. These findings provide insights into their potential cooperative interaction of multiple electron donors on greatly promoting nitrate and Cr(VI) removal and have implications for the remediation technology of nitrate and Cr(VI) co-contaminated groundwater.

Enhanced Cr(VI) elimination from water by goethite-impregnated activated carbon coupled with weak electric field

A weak electric field (WEF, 2 mA cm-2) was employed to promote Fe(III)/Fe(II) cycle on goethite-impregnated activated carbon (FeOOH@AC) filled in a continuous-flow column for enhanced Cr(VI) elimination from water. Surficial analysis and Cr species distribution showed that α-FeOOH of 0.2-1 μm was successfully synthesized and evenly loaded onto AC. Electron transfer from WEF to α-FeOOH was facilitated with AC as electron shuttles, thereby boosting Fe(III) reduction in the α-FeOOH. The generated Fe(II) reduced Cr(VI) and the resultant Cr(III) subsequently precipitated with OH- and Fe(III) to form Cr(OH)3 and (CrχFe1-χ)(OH)3. Therefore, the WEF-FeOOH@AC column exhibited a much lower Cr(VI) migration rate of 0.0018 cm PV-1 in comparison with 0.0037 cm PV-1 of the FeOOH@AC column, equal to 104 % higher Cr(VI) elimination capacity and 90 % longer column service life-span. Additionally, under different Cr(VI) loadings by varying either seepage velocities or influent Cr(VI) concentrations, the WEF-FeOOH@AC column maintained 1.0-1.5 folds higher Cr(VI) elimination and 0.9-1.4 folds longer longevity than those of the FeOOH@AC column owing to the interaction between FeOOH@AC and WEF. Our research demonstrated that WEF-FeOOH@AC was a potential method to promote Cr(VI) elimination from water and offer an effective strategy to facilitate Fe(III)/Fe(II) cycle in iron oxides.

Sulfur-manganese carbonate composite autotrophic denitrification: nitrogen removal performance and biochemistry mechanism

A novel composite sulfur-manganese carbonate autotrophic denitrification (SMAD) system was developed to reduce sulfate production and provide pH buffer function while improving denitrification efficiency without external organics. The average removal efficiency of total nitrogen (TN) was 98.09% and 96.29%, and that of NO3--N was 99.53% and 97.77%, respectively, in the SMAD system with a hydraulic retention time (HRT) of 6 h and 3 h. They were significantly higher than that in the controls (quartz sand, manganese carbonate ore, and sulfur systems). The H+ produced by the sulfur autotrophic denitrification (SAD) process promoted the release of Mn2+ in the SMAD system. And this system had a stable pH with no accumulation of NO2--N. The decrease of sulfate and formation of Mn oxides through Mn2+ electron donation confirmed the presence of the manganese autotrophic denitrification (MAD) process in the SMAD system. Dominant functional bacteria in the SMAD system were Thiobacillus, Chlorobium, and Sulfurimonas, which were linked to nitrogen, sulfur, and manganese conversion and promoted denitrification. Meanwhile, Flavobacterium participating in Mn2+ oxidation was found only in the SMAD system. The SMAD system provided a new strategy for advanced tailwater treatment.

Simultaneous nitrate and chromium removal mechanism in a pyrite-involved mixotrophic biofilter

Microbially mediated NO3--N and Cr(VI) reduction is being recognized as an eco-friendly and cost-effective remediation strategy. Iron sulfide mineral, as a natural inorganic electron donor, has a strong influence on NO3--N and Cr(VI) transformation, respectively. However, little is known about the simultaneous nitrate and chromium removal performance and underlying mechanism in an iron sulfide mineral-involved mixotrophic biofilter. This study demonstrated that the NO3--N and Cr(VI) removal efficiencies were stable at 62 ± 8% and 56 ± 10%, and most of them were eliminated in the 0-100-mm region of the biofilter. Cr(VI) was reduced to insoluble Cr(III) via microbial and chemical pathways, which was confirmed by the SEM-EDS morphology and the XPS spectra of biofilm and pyrite particles. SO42- was as a main byproduct of pyrite oxidation; however, the bacterial SO42- reduction synchronously occurred, evidenced by the variations of TOC and SO42- concentrations. These results suggested that there were complicated and intertwined biochemical relations between NO3--N/Cr(VI)/SO42-/DO (electron acceptors) and pyrite/organics (electron donors). Further investigation indicated that both the maximal biomass and greatest denitrifiers' relative abundances in microbial sample S1 well explained why the pollutants were removed in the 0-100-mm region. A variety of denitrifiers such as Pseudoxanthomona, Acidovorax, and Simplicispira were enriched, which probably were responsible for both NO3--N and Cr(VI) removal. Our findings advance the understanding of simultaneous nitrate and chromium removal in pyrite-involved mixotrophic systems and facilitate the new strategy development for nitrate and chromium remediation.

High-rate iron sulfide and sulfur-coupled autotrophic denitrification system: Nutrients removal performance and microbial characterization

Iron sulfides-based autotrophic denitrification (IAD) is a promising technology for nitrate and phosphate removal from low C:N ratio wastewater due to its cost-effectiveness and low sludge production. However, the slow kinetics of IAD, compared to other sulfur-based autotrophic denitrification (SAD) processes, limits its engineering application. This study constructed a co-electron-donor (FeS and S⁰ with a volume ratio of 2:1) iron sulfur autotrophic denitrification (ISAD) biofilter and operated at as short as 1 hr hydraulic retention time (HRT). Long-term operation results showed that the superior total nitrogen and phosphate removals of the ISAD biofilter were 90-100% at 1-12 h HRT, with the highest denitrification rate up to 960 mg/L/d. Considering low sulfate production, HRT of 3 h could be the optimal condition. Such superior performance in the ISAD biofilter was achieved due to the interactions between FeS and S⁰, which accelerated the denitrification process and maintained the acidity-alkalinity balance. Metagenomic analysis found that the enriched nitrate-dependent iron-oxidizing (NDFO) bacteria (Acinetobacter and Acidovorax), sulfur-oxidizing bacteria (SOB), and dissimilatory nitrate reduction to ammonia (DNRA) bacteria likely supported stable nitrate reduction. The metabolic pathway analysis showed that completely denitrification and DNRA, coupled with sulfur oxidation, disproportionation, iron oxidation and phosphate precipitation with FeS and S⁰ as co-electron donors, were responsible for the high-rate nitrate and phosphate removal. This study provides the potential of ISAD as a highly efficient post-denitrification technology and sheds light on the balanced microbial S-N-Fe transformation.

A critical review of wastewater quality variation and in-sewer processes during conveyance in sewer systems

In-sewer physio-biochemical processes cause significant variations of wastewater quality during conveyance, which affects the influent to a wastewater treatment plant (WWTP) and arguably the microbial community of biological treatment units in a WWTP. In wet weather, contaminants stored in sewer deposits can be resuspended and migrate downstream or be released during combined sewer overflows to the urban water bodies, posing challenges to the treatment facilities or endangering urban water quality. Therefore, in-sewer transformation and migration of contaminants have been extensively studied. The compiled results from representative research in the past few decades showed that biochemical reactions are both cross-sectionally and longitudinally organized in the deposits and the sewage, following the redox potential as well as the sequence of macromolecule/contaminant degradation. The sewage organic contents and sewer biofilm microorganisms were found to covary but more systematic studies are required to examine the temporal stability of the feature. Besides, unique communities can be developed in the sewage phase. The enrichment of the major sewage-associated microorganisms can be explained by the availability of biodegradable organic contents in sewers. The sewer deposits, including biofilms, harbor both microorganisms and contaminants and usually can provide longer residence time for in-sewer transformation than wastewater. However, the interrelationships among contaminant transformation, microorganisms in the deposits/biofilms, and those in the sewage are largely unclear. Specifically, the formation and migration of FOG (fat, oil, and grease) deposits, generation and transport of contaminants in the sewer atmosphere (e.g., H₂S, CH₄, volatile organic compounds, bioaerosols), transport and transformation of nonconventional contaminants, such as pharmaceuticals and personal care products, and wastewater quality variation during the biofilm rehabilitation period after damages caused by rains/storms are some topics for future research. Moreover, systematic and standardized field analysis of real sewers under dynamic wastewater discharge conditions is necessary. We believe that an improved understanding of these processes would assist in sewer management and better prepare us for the challenges brought about by climate change and water shortage.

Role of iron(II) sulfide in autotrophic denitrification under tetracycline stress: Substrate and detoxification effect

Autotrophic denitrification using inorganic compounds as electron donors has gained increasing attention in the field of wastewater treatment due to its numerous advantages, such as no need for exogenous organic carbon, low energy input, and low sludge production. Tetracycline (TC), a refractory contaminant, is often found coexisting with nutrients (NO₃^- and PO₄^3-) in wastewater, which can negatively affect the biological nutrient removal process because of its biological toxicity. However, the performance of autotrophic denitrification under TC stress has rarely been reported. In this study, the effects of TC on autotrophic denitrification with thiosulfate (Na₂S₂O₃) and iron (II) sulfide (FeS) as the electron donors were investigated. With Na₂S₂O₃ as the electron donor, TC slowed down the nitrate removal rate, which decreased from 1.32 to 0.18 d^-1, when TC concentration increased from 0 mg/L to 50 mg/L. When TC concentration was higher than 2 mg/L, nitrite reduction was seriously inhibited, leading to nitrite accumulation. With FeS as the electron donor, nitrate removal was much more efficient under TC-stressed conditions, and no distinct nitrite accumulation was observed when the initial TC concentration was as high as 10 mg/L, indicating the effective detoxification of FeS. The detoxification effects in the FeS autotrophic denitrification system mainly resulted from the rapid adsorption of TC by FeS and effective degradation of TC, as proven by a relatively higher living biomass area. This study offers new insights into the response of sulfur-based autotrophic denitrifiers to TC stress and demonstrates that the FeS-based autotrophic denitrification process is a promising technology for the treatment of wastewater containing emerging contaminants and nutrients.

Arsenic and cadmium bioavailability to rice (Oryza sativa L.) plant in paddy soil: Influence of sulfate application

Arsenic (As) and cadmium (Cd) accumulate easily in rice grains that pose a non-negligible threat to human health worldwide. Sulfur fertilizer has been shown to affect the mobilization of As and Cd in paddy soil, but the effect of co-contamination by As and Cd has not been explored. This study selected three soils co-contaminated with As and Cd from Shangyu (SY), Tongling (TL) and Ma'anshan (MA). Incubation experiments and pot experiments were carried out to explore the effect of sulfate supply (100 mg kg^-1) on the bioavailability of As and Cd in soil and the rice growth. The results showed that the exogenous sulfate decreased As concentrations in porewater of SY and TL by 51.1% and 29.2% through forming arsenic-sulfide minerals. The exchangeable Cd in soil also declined by 25.6% and 18.6% and transformed into Fe and Mn oxides-bound Cd. The relative abundance of Desulfotomaculum, Desulfurispora and dsr gene increased remarkably indicated that sulfate addition stimulated the activity of sulfate-reducing bacteria. In MA soil, sulfate addition immobilized Cd but had little effect on As solubility, which was speculated to be related to the high sulfate background of the soil. Further pot experiments showed that sulfate application significantly increased rice tillers, biomass, chlorophyll content in shoots, and decreased electrolyte leakage in root. Finally, sulfate significantly reduced As and Cd in SY rice shoots by 60.2% and 40.8%, respectively, while As decreased by 39.6% in TL rice shoots and Cd decreased by 23.0% in MA rice shoots. These results indicate that the application of sulfate can reduce the bioavailability of As and Cd in the soil-rice system and promote rice growth, and it is possible to reduce the accumulation of As and Cd in rice plants simultaneously.

Removal of CrO42-, a Nonradioactive Surrogate of 99TcO4-, Using LDH-Mo3S13 Nanosheets

Removal of chromate (CrO₄^2-) and pertechnetate (TcO₄^-) from the Hanford Low Activity Waste (LAW) is beneficial as it impacts the cost, life cycle, operational complexity of the Waste Treatment and Immobilization Plant (WTP), and integrity of vitrified glass for nuclear waste disposal. Here, we report the application of [Mo^IV₃S₁₃]^2- intercalated layer double hydroxides (LDH-Mo₃S₁₃) for the removal of CrO₄^2- as a surrogate for TcO₄^-, from ppm to ppb levels from water and a simulated LAW off-gas condensate of Hanford's WTP. LDH-Mo₃S₁₃ removes CrO₄^2- from the LAW condensate stream, having a pH of 7.5, from ppm (∼9.086 × 10⁴ ppb of Cr⁶⁺) to below 1 ppb levels with distribution constant (K_d) values of up to ∼10⁷ mL/g. Analysis of postadsorbed solids indicates that CrO₄^2- removal mainly proceeds by reduction of Cr⁶⁺ to Cr³⁺. This study sets the first example of a metal sulfide intercalated LDH for the removal of CrO₄^2-, as relevant to TcO₄^-, from the simulated off-gas condensate streams of Hanford's LAW melter which contains highly concentrated competitive anions, namely F^-, Cl^-, CO₃^2-, NO₃^-, BO₃^3-, NO₂^-, SO₄^2-, and B₄O₇^2-. LDH-Mo₃S₁₃'s remarkable removal efficiency makes it a promising sorbent to remediate CrO₄^2-/TcO₄^- from surface water and an off-gas condensate of nuclear waste.