Transitioning weathered oil fields towards new energy: A review on utilizing hydrogenotrophic methanogens for petroleum hydrocarbons remediation

The weathering process can cause the volatilization of light components in crude oil, leading to the accumulation of total petroleum hydrocarbons (TPH) in weathered oil field soils. These TPH compounds are relatively resistant to biodegradation, posing a significant environmental hazard by contributing to soil degradation. TPH represents a complex mixture of petroleum-based hydrocarbons classified as persistent organic pollutants in soil and groundwater. The release of TPH pollutants into the environment poses serious threats to ecosystems and human health. Currently, various methods are available for TPH-contaminated soil remediation, with bioremediation technology recognized as an environmentally friendly and cost-effective approach. While converting TPH to CO2 is a common remediation method, the complex structures and diverse types of petroleum hydrocarbons (PHs) involved can result in excessive CO2 generation, potentially exacerbating the greenhouse effect. Alternatively, transforming TPH into energy forms like methane through bioremediation, followed by collection and reuse, can reduce greenhouse gas emissions and energy consumption. This process relies on the synergistic interaction between Methanogens archaea and syntrophic bacteria, forming a consortium known as the oil-degrading bacterial consortium. Methanogens produce methane through anaerobic digestion (AD), with hydrogenotrophic methanogens (HTMs) utilizing H2 as an electron donor, playing a crucial role in biomethane production. Candidatus Methanoliparia (Ca. Methanoliparia) was found in the petroleum archaeal community of weathered Oil field in northeast China. Ca. Methanoliparia has demonstrated its independent ability to decompose and produce new energy (biomethane) without symbiosis, contribute to transitioning weathered oil fields towards new energy. Therefore, this review focuses on the principles, mechanisms, and developmental pathways of HTMs during new energy production in the degradation of PHs. It also discusses strategies to enhance TPH degradation and recovery methods.

Bioremediation of trichloroethylene-contaminated groundwater using green carbon-releasing substrate with pH control capability

In this research, a sustainable substrate, termed green and long-lasting substrate (GLS), featuring a blend of emulsified substrate (ES) and modified rice husk ash (m-RHA) was devised. The primary objective was to facilitate the bioremediation of groundwater contaminated with trichloroethylene (TCE) using innovative GLS for slow carbon release and pH control. The GLS was concocted by homogenizing a mixture of soybean oil, surfactants (Simple Green™ and soya lecithin), and m-RHA, ensuring a gradual release of carbon sources. The hydrothermal synthesis was applied for the production of m-RHA production. The analyses demonstrate that m-RHA were uniform sphere-shape granules with diameters in micro-scale ranges. Results from the microcosm study show that approximately 83% of TCE could be removed (initial TCE concentration = 7.6 mg/L) with GLS supplement after 60 days of operation. Compared to other substrates without RHA addition, higher TCE removal efficiency was obtained, and higher Dehalococcoides sp. (DHC) population and hydA gene (hydrogen-producing gene) copy number were also detected in microcosms with GLS addition. Higher hydrogen concentrations enhanced the DHC growth, which corresponded to the increased DHC populations. The addition of the GLS could provide alkalinity at the initial stage to neutralize the acidified groundwater caused by the produced organic acids after substrate biodegradation, which was advantageous to DHC growth and TCE dechlorination. The addition of m-RHA reached an increased TCE removal efficiency, which was due to the fact that the m-RHA had the zeolite-like structure with a higher surface area and lower granular diameter, and thus, it resulted in a more effective initial adsorption effect. Therefore, a significant amount of TCE could be adsorbed onto the surface of m-RHA, which caused a rapid TCE removal through adsorption. The carbon substrates released from m-RHA could then enhance the subsequent dechlorination. The developed GLS is an environmentally-friendly and green substrate.

An innovative permeable reactive bio-barrier to remediate trichloroethene-contaminated groundwater: A field study

Permeable reactive bio-barrier (PRBB), an innovative technology, could treat many contaminants via the natural gradient flow of groundwater based on immobilization or transformation of pollutants into less toxic and harmful forms. In this field study, we developed an innovative PRBB system comprising immobilized Dehalococcoides mccartyi (Dhc) and Clostridium butyricum embedded into the silica gel for long-term treatment of trichloroethene (TCE) polluted groundwater. Four injection wells and two monitoring wells were installed at the downstream of the TCE plume. Without PRBB, results showed that the TCE (6.23 ± 0.43 μmole/L) was converted to cis-dichloroethene (0.52 ± 0.63 μmole/L), and ethene was not detected, whereas TCE was completely converted to ethene (3.31 μmole/L) with PRBB treatment, indicating that PRBB could promote complete dechlorination of TCE. Noticeably, PRBB showed the long-term capability to maintain a high dechlorinating efficiency for TCE removal during the 300-day operational period. Furthermore, with qPCR analysis, the PRBB application could stably maintain the populations of Dhc and functional genes (bvcA, tceA, and vcrA) at >108 copies/L within the remediation course and change the bacterial communities in the contaminated groundwater. We concluded that our PRBB was first set up for cleaning up TCE-contaminated groundwater in a field trial.

Development of innovative and green adsorbents for in situ cleanup of fluoride-polluted groundwater: Mechanisms and field-scale studies

In this study, the magnesium oxide (MgO)-based adsorbents [granulated MgO aggregates (GA-MgO) and surface-modified MgO powder (SM-MgO)] were developed to remediate a fluoride-contaminated groundwater site. Both GA-MgO and SM-MgO had porous, spherical, and crystalline structures. Diameters for GA-MgO and SM-MgO were 1-1.7 mm and 1-10 μm, respectively. The pseudo second-order dynamic adsorption and the Freundlich isotherm could be applied to express the chemical adsorption phenomena. The monolayer adsorption was the dominant mechanism at the initial adsorption period. During the latter part of fluoride adsorption, the multilayer adsorption became the dominant mechanism for fluoride removal from the water phase, which also resulted in the increased adsorption capacity. Higher hydroxide, phosphate, and carbonate concentrations caused a decreased fluoride removal efficiency due to the competition of sorption sites between fluoride and other anions with similar electronic properties. Fluoride removal mechanism using GA-MgO and SM-MgO as the adsorbents was mainly carried out by the chemical adsorption. Reaction paths contained two main processes: (1) formation of magnesium hydroxide after the reaction of MgO with water, and (2) the hydroxyl group of the magnesium hydroxide was replaced by fluoride ions to form magnesium fluoride precipitation. Results from column tests show that up to 61 and 73% of fluoride removal (initial fluoride concentration = 9.3 mg/L) could be obtained after 50 pore volumes of groundwater pumping with GA-MgO and SM-MgO injection, respectively. The GA-MgO system could be applied to contain and remediate fluoride-contaminated groundwater, and SM-MgO could be applied as an immediate fluoride removal alternative to achieve a rapid pollutant removal for emergency responses. Up to 71% of fluoride removal (fluoride concentration = 10.8 mg/L) could be obtained with GA-MgO injection after 30 days of operation. The developed GA-MgO system is a potential and green remediation alternative to contain the fluoride plume significantly.

Application of novel nanobubble-contained electrolyzed catalytic water to cleanup petroleum-hydrocarbon contaminated soils and groundwater: A pilot-scale and performance evaluation study

Soil and groundwater contamination caused by petroleum hydrocarbons is a severe environmental problem. In this study, a novel electrolyzed catalytic system (ECS) was developed to produce nanobubble-contained electrolyzed catalytic (NEC) water for the remediation of petroleum-hydrocarbon-contaminated soils and groundwater. The developed ECS applied high voltage (220 V) with direct current, and titanium electrodes coated with iridium dioxide were used in the system. The developed ECS prototype contained 21 electrode pairs (with a current density of 20 mA/cm2), which were connected in series to significantly enhance the hydroxyl radical production rate. Iron-copper hybrid oxide catalysts were laid between each electrode pair to improve the radical generation efficiency. The electron paramagnetic resonance (EPR) and Rhodamine B (RhB) methods were applied for the generated radical species and concentration determination. During the operation of the ECS, high concentrations of nanobubbles (nanobubble density = 3.7 × 109 particles/mL) were produced due to the occurrence of the cavitation mechanism. Because of the negative zeta potential and nano-scale characteristics of nanobubbles (mean diameter = 28 nm), the repelling force would prevent the occurrence of bubble aggregations and extend their lifetime in NEC water. The radicals produced after the bursting of the nanobubbles would be beneficial for the increase of the radical concentration and subsequent petroleum hydrocarbon oxidation. The highly oxidized NEC water (oxidation-reduction potential = 887 mV) could be produced with a radical concentration of 9.5 × 10-9 M. In the pilot-scale study, the prototype system was applied to clean up petroleum-hydrocarbon polluted soils at a diesel-oil spill site via an on-site slurry-phase soil washing process. The total petroleum hydrocarbon (TPH)-contaminated soils were excavated and treated with the NEC water in a slurry-phase reactor. Results show that up to 74.4% of TPH (initial concentration = 2846 mg/kg) could be removed from soils after four rounds of NEC water treatment (soil and NEC water ratio for each batch = 10 kg: 40 L and reaction time = 10 min). Within the petroleum-hydrocarbon plume, one remediation well (RW) and two monitor wells (located 1 m and 3 m downgradient of the RW) were installed along the groundwater flow direction. The produced NEC water was injected into the RW and the TPH concentrations in groundwater (initial concentrations = 12.3-15.2 mg/L) were assessed in these three wells. Compared to the control well, TPH concentrations in RW and MW1 dropped to below 0.4 and 2.1 mg/L after 6 m3 of NEC water injection in RW, respectively. Results from the pilot-scale study indicate that the NEC water could effectively remediate TPH-contaminated soils and groundwater without secondary pollution production. The main treatment mechanisms included (1) in situ chemical oxidation via produced radicals, (2) desorption of petroleum hydrocarbons from soil particles due to the dispersion of nanobubbles into soil pores, and (3) enhanced TPH oxidation due to produced radicals and energy after nanobubble bursting.

Cleanup of Cr(VI)-polluted groundwater using immobilized bacterial consortia via bioreduction mechanisms

Cr(VI) bioreduction has become a remedial alternative for Cr(VI)-polluted site cleanup. However, lack of appropriate Cr(VI)-bioreducing bacteria limit the field application of the in situ bioremediation process. In this study, two different immobilized Cr(VI)-bioreducing bacterial consortia using novel immobilization agents have been developed for Cr(VI)-polluted groundwater remediation: (1) granular activated carbon (GAC) + silica gel + Cr(VI)-bioreducing bacterial consortia (GSIB), and (2) GAC + sodium alginate (SA) + polyvinyl alcohol (PVA) + Cr(VI)-bioreducing bacterial consortia (GSPB). Moreover, two unique substrates [carbon-based agent (CBA) and emulsified polycolloid substrate (EPS)] were developed and used as the carbon sources for Cr(VI) bioreduction enhancement. The microbial diversity, dominant Cr-bioreducing bacteria, and changes of Cr(VI)-reducing genes (nsfA, yieF, and chrR) were analyzed to assess the effectiveness of Cr(VI) bioreduction. Approximately 99% of Cr(VI) could be bioreduced in microcosms with GSIB and CBA addition after 70 days of operation, which caused increased populations of total bacteria, nsfA, yieF, and chrR from 2.9 × 10⁸ to 2.1 × 10¹², 4.2 × 10⁴ to 6.3 × 10¹¹, 4.8 × 10⁴ to 2 × 10¹¹, and 6.9 × 10⁴ to 3.7 × 10⁷ gene copies/L. In microcosms with CBA and suspended bacteria addition (without bacterial immobilization), the Cr(VI) reduction efficiency dropped to 60.3%, indicating that immobilized Cr-bioreducing bacteria supplement could enhance Cr(VI) bioreduction. Supplement of GSPB led to a declined bacterial growth due to the cracking of the materials. The addition of GSIB and CBA could establish a reduced condition, which favored the growth of Cr(VI)-reducing bacteria. The Cr(VI) bioreduction efficiency could be significantly improved through adsorption and bioreduction mechanisms, and production of Cr(OH)₃ precipitates confirmed the occurrence of Cr(VI) reduction. The main Cr-bioreducing bacteria included Trichococcus, Escherichia-Shigella, and Lactobacillus. Results suggest that the developed GSIB bioremedial system could be applied to cleanup Cr(VI)-polluted groundwater effectively.

Toxicity Evaluation of a Heavy-metal Polluted River: Pollution Identification and Bacterial Community Assessment

The Salt River is an important urban river in Kaohsiung, Taiwan. In this study, the source identification and risk and toxicity assessment of the heavy-metal contaminated sediments in the Salt River were investigated. The geo-accumulation index (Igeo), enrichment factor (EF), sediment quality guidelines (SQGs), potential ecological risk index (RI), pollution load index (PLI), and toxic units (TU) were applied to determine effects of heavy metals on microbial diversities and ecosystems. Results from the ecological and environmental risk assessment show that high concentrations of Zn, Cr, and Ni were detected in the midstream area, the sum of toxic units (ΣTUs) in the midstream (7.2 - 32.0) is higher than in the downstream (14.0 - 19.7) and upstream (9.2 - 17.1). It could be because of the continuous inputs of heavy-meta-contained wastewaters from adjacent industrial parks. Results also inferred that the detected heavy metals in the upstream residential and commercial areas were possibly caused by nearby vehicle emissions, non-point source pollution, and domestic wastewater discharges. Results of metagenomic assays show that the sediments contained significant microbial diversities. Metal-tolerant bacterial phyla (Proteobacteria: 24.4% to 46.4%, Bacteroidetes: 1.3% to 14.8%, and Actinobacteria: 2.3% to 11.1%) and pathogenic bacterial phyla (Chlamydiae: 0.5% to 37.6%, and Chloroflexi: 5.8% to 7.2%) with relatively high abundance were detected. Metal-tolerant bacteria would adsorb metals and cause the increased metal concentrations in sediments. Results indicate that the bacterial composition in sediment environments was affected by anthropogenic pollution and human activities, and the heavy-metal-polluted ecosystem caused the variations in bacterial communities.

Permanganate activation by glucose-derived carbonaceous materials for highly efficient degradation of phenol and p-nitrophenol: Formation of hydroxyl radicals and multiple roles of carbonaceous materials

Owing to its inertness toward refractory organic pollutants and the release of Mn²⁺, the use of permanganate was limited in soil and groundwater remediation. The present study proposed an improvement strategy based on glucose-derived carbonaceous materials, which enhanced the potential of permanganate degrading organic pollutants. The glucose-derived carbonaceous material with 1000 °C charring temperature was named C1000, which was exploited in activating KMnO₄ for the elimination of refractory organic contaminants. The addition of C1000 in the KMnO₄ system triggered the degradation of refractory p-nitrophenol and quicken phenol degradation. Unlike the detection of Mn(III) species in a solo KMnO₄ system, the presence of C1000 facilitated the formation of ^•OH in the KMnO₄ system, which was confirmed by the use of quenchers such as methanol, benzoic acid, tertiary butanol, and carbonate. Additionally, the glucose-derived carbonaceous material played multiple roles in improving the performance of permanganate, including the enrichment of organic pollutants, donation of electrons to permanganate, and acting as an electron shuttle to facilitate the oxidation of organic pollutants by permanganate. The study's novel findings have the potential to expand the use of permanganate in the remediation of organic pollutants.

Bioremediation potential of cadmium by recombinant Escherichia coli surface expressing metallothionein MTT5 from Tetrahymenathermophila

Cadmium (Cd) is a common heavy metal contaminant in industrial wastewater that causes many diseases in humans. Metallothionein (MT), a cysteine-rich metal-binding protein, is well known in chelate-heavy metals. In this study, we expressed MTT5 of Tetrahymena thermophila fused with Lpp-OmpA in the outer membrane of Escherichia coli to determine its ability to accumulate and adsorb Cd. Our results revealed that our recombinant E. coli had a 4.9-fold greater Cd adsorption compared to wild E. coli. Adsorption isothermic analysis demonstrated that the adsorption behavior for Cd in our recombinant bacteria was better fitted into the Freundlich isotherm model than Langmuir isotherm model. Fourier-transform infrared spectroscopy indicated that phosphate and organic phosphate groups were involved in the interaction between Cd and the bacterial surface. Using quantitative reverse transcription polymerase chain reaction, we further showed that the expression of metal-resistance genes (dnaK and clpB) was downregulated due to surface MTT5 protected our recombinant bacteria from Cd²⁺ adsorption. Furthermore, we showed that our recombinant bacteria could adsorb Cd from the contaminated wastewater containing other metals and were suggested to be applied in the field study.

Toxicity determination, pollution source delineation, and microbial diversity evaluation of PAHs-contaminated sediments for an urban river

The Feng-Sang River is a metropolitan river in Kaohsiung City, Taiwan. In this study, Feng-Sang River sediments were analyzed to investigate the distributions and sources of polycyclic aromatic hydrocarbons (PAHs). The Sediment Quality Guidelines (SQGs), potentially carcinogenic PAHs (TEQcarc), and toxic equivalence quotient (TEQ) were applied to evaluate influences of PAHs on ecosystems and microbial diversities. Results indicate that PAHs concentrations varied between seasons and locations. The concentrations of ∑16PAHs ranged from 73.6 to 603.8 ng/kg in dry seasons and from 2.3 to 199.3 ng/kg in wet seasons. This could be because of the flushing effect during wet seasons, which caused the movement and dilution of the PAH-contaminated sediments. Diagnostic ratio analysis infers that high PAHs levels were generated by combustion processes and vehicle traffic, and results from multivariate descriptive statistical analysis also demonstrate that the vehicular traffic pollution could be the major emission source of PAHs contamination. Comparisons of PAHs with SQGs indicate that PAHs concentrations in sediment were below the effects range low (ERL) values, and thus, the immediate threat to organisms might not be significant. The diagnostic ratio analyses are effective methods for PAH source appointment. The metagenomic assay results imply that sediments contained essential microbial species with eminent diversity. The detected PAH-degrading bacteria (Desulfatiglans, Dechloromonas, Sphingomonas, Methylobacterium, Rhodobacter, Clostridium, and Exiguobacterium) played a key role in PAHs biotransformation, and Dechloromonas and Rhodobacter had a higher relative abundance. Results of microbial diversity analyses indicate that the contaminated environment induced the changes of governing microbial groups in sediments. PRACTITIONER POINTS: Diagnostic ratio analyses are effective methods for PAHs source appointment. Microbial composition in sediments are highly affected by anthropogenic pollution. Combustion and vehicle traffic contribute to urban river sediments pollution by PAHs. Dechloromonas and Rhodobacter are dominant PAHs-degrading bacteria in sediments.

Draft genome sequence of Pseudomonas sp. A46 isolated from mercury-contaminated wastewater

Pseudomonas sp. A46 was first isolated from mercury-contaminated groundwater in Taiwan. This study is the first to report the draft whole-genome sequence of Pseudomonas sp. A46. Its genome consists of 126 contigs, with a total length of 6,782,516 bp and a GC content of 64.7%. Phylogenetic analysis based on 16 S rRNA gene sequences revealed that Pseudomonas sp. A46 is closely related to Pseudomonas citronellolis. Assessment of the draft genome sequence revealed that Pseudomonas sp. A46 harbors sets of genes conferring resistance to heavy metals, such as mercury, zinc, lead, copper, cadmium, chromate, and arsenate. These identified genes enable this bacterium to tolerate heavy metal stress.

Long-term dechlorination of cis-DCE to ethene with co-immobilized Dehalococcoides mccartyi BAV1 and Clostridium butyricum in silica gel system

Chloroethenes are common groundwater pollutants, and have been classified as toxic and carcinogenic to humans. The metabolites of chloroethenes, cis-dichloroethene (cis-DCE) and vinyl chloride (VC) commonly accumulate in groundwater due to their recalcitrant reductive dechlorination under anaerobic conditions. Dehalococcoides mccartyi (Dhc) is the key anaerobic bacteria for complete dechlorination of chloroethene, and Clostridium butyricum (C. butyricum) can provide hydrogen for supporting the growth of Dhc. In this study, we co-immobilized Dhc strain BAV1 and C. butyricum in a silica gel to determine the ability of the complete dechlorination of cis-DCE. Our results showed that our immobilized system could protect BAV1 from a high concentration (8 mM) of cis-DCE to carry out complete dechlorination. After the long-term use of our immobilized system, the activity of complete dechlorination was maintained for more than 180 consecutive days. Furthermore, we applied the immobilized system to remediate contaminated groundwater and uncovered the complete dechlorination of cis-DCE into ethene, a non-toxic product, within 28 days. Therefore, this novel co-immobilized system could serve a solution for bioremediation at chloroethene-contaminated sites.

Bioremediation of hexavalent-chromium contaminated groundwater: Microcosm, column, and microbial diversity studies

Sulfate reducing bacteria (SRB) have the capability of bioreducing hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] under sulfate-reducing conditions for toxicity reduction. However, a high amount of sulfate addition would cause elevated sulfide production, which could inhibit the growth of SRB and result in reduced Cr(VI) bioreduction efficiency. A slow release reagent, viscous carbon and sulfate-releasing colloidal substrates (VCSRCS), was prepared for a long-lasting carbon and sulfate supplement. In the column study, VCSRCS was injected into the column system to form a VCSRCS biobarrier for Cr(VI) containment and bioreduction. A complete Cr(VI) removal was observed via the adsorption and bioreduction mechanisms in the column with VCSRCS addition. Results from X-ray diffractometer analyses indicate that Cr(OH)_3(s) and Cr₂O_3(s) were detected in precipitates, indicating the occurrence of Cr(VI) reduction followed by Cr(III) precipitation. Results from the Fourier-transform infrared spectroscopy analyses show that cell deposits carried functional groups, which could adsorb Cr. Addition of VCSRCS caused increased populations of total bacteria and dsrA, which also enhanced Cr(VI) reduction. Microbial diversity results indicate that VCSRCS addition resulted in the growth of Cr(VI)-reducing bacteria including Exiguobacterium, Citrobacter, Aerococcus, and SRB. Results of this study will be helpful in developing an effective and green VCSRCS biobarrier for the bioremediation of Cr(VI)-polluted groundwater.

Effects of treatment processes on AOC removal and changes of bacterial diversity in a water treatment plant

The effectiveness of different treatment processes on assimilable organic carbon (AOC) removal and bacterial diversity variations was evaluated in a water treatment plant. The van der Kooij technique was applied for AOC analysis and responses of bacterial communities were characterized by the metagenomics assay. Results show that the AOC concentrations were about 93, 148, 43, 51, 37, and 38 μg acetate-C/L in effluents of raw water basin, preozonation, rapid sand filtration (RSF), ozonation, biofiltration [biological activated carbon (BAC) filtration], and chlorination (clear water), respectively. Increased AOC concentrations were observed after preozonation, ozonation, and chlorination units due to the production of biodegradable organic matters after the oxidation processes. Results indicate that the oxidation processes were the main causes of AOC formation, which resulted in significant increases in AOC concentrations (18-59% increment). The AOC removal efficiencies were 47, 28, and 60% in the RSF, biofiltration, and the whole system, respectively. RSF and biofiltration were responsible for the AOC treatment and both processes played key roles in AOC removal. Thus, both RSF and biofiltration processes would contribute to AOC treatment after oxidation. Sediments from the raw water basin and filter samples from RSF and BAC units were collected and analyzed for bacterial communities. Results from scanning electron microscope analysis indicate that bacterial colonization was observed in filter materials. This indicates that the surfaces of the filter materials were beneficial to bacterial growth and AOC removal via the adsorption and biodegradation mechanisms. Next generation sequencing analyses demonstrate that water treatment processes resulted in the changes of bacterial diversity and community profiles in filters of RSF and BAC. According to the findings of bacterial composition and interactions, the dominant bacterial phyla were Proteobacteria (41% in RSF and 56% in BAC) followed by Planctomycetes and Acidobacteria in RSF and BAC systems, which might affect the AOC biodegradation efficiency. Results would be useful in developing AOC treatment and management processes in water treatment plants.

Cleanup chlorinated ethene-polluted groundwater using an innovative immobilized Clostridium butyricum column scheme: A pilot-scale study

In this study, the developed innovative immobilized Clostridium butyricum (ICB) (hydrogen-producing bacteria) column scheme was applied to cleanup chlorinated-ethene [mainly cis-1,2-dichloroethene (cis-DCE)] polluted groundwater in situ via the anaerobic reductive dechlorinating processes. The objectives were to assess the effectiveness of the field application of ICB scheme on the cleanup of cis-DCE polluted groundwater, and characterize changes of microbial communities after ICB application. Three remediation wells and two monitor wells were installed within the cis-DCE plume. In the remediation well, a 1.2-m PVC column (radius = 2.5 cm) (filled with ICB beads) and 20 L of slow polycolloid-releasing substrate (SPRS) were supplied for hydrogen production enhancement and primary carbon supply, respectively. Groundwater samples from remediation and monitor wells were analyzed periodically for cis-DCE and its degradation byproducts, microbial diversity, reductive dehalogenase, and geochemical indicators. Results reveal that cis-DCE was significantly decreased within the ICB and SPRS influence zone. In a remediation well with ICB injection, approximately 98.4% of cis-DCE removal (initial concentration = 1.46 mg/L) was observed with the production of ethene (end-product of cis-DCE dechlorination) after 56 days of system operation. Up to 0.72 mg/L of hydrogen was observed in remediation wells after 14 days of ICB and SPRS introduction, which corresponded with the increased population of Dehalococcoides spp. (Dhc) (increased from 3.76 × 103 to 5.08 × 105 gene copies/L). Results of metagenomics analyses show that the SPRS and ICB introduction caused significant impacts on the bacterial communities, and increased Bacteroides, Citrobacter, and Desulfovibrio populations were observed, which had significant contributions to the reductive dechlorination of cis-DCE. Application of ICB could effectively result in increased populations of Dhc and RDase genes, which corresponded with improved dechlorination of cis-DCE and vinyl chloride. Introduction of ICB and SPRS could be applied as a potential in situ remedial option to enhance anaerobic dechlorination efficiencies of chlorinated ethenes.

Bioremediation of trichloroethylene-polluted groundwater using emulsified castor oil for slow carbon release and acidification control

In this study, the emulsified castor oil (ECO) substrate was developed for a long-term supplement of biodegradable carbon with pH buffering capacity to anaerobically bioremediate trichloroethylene (TCE)-polluted groundwater. The ECO was produced by mixing castor oil, surfactants (sapindales and soya lecithin [SL]), vitamin complex, and a citrate/sodium phosphate dibasic buffer system together for slow carbon release. Results of the emulsification experiments and microcosm tests indicate that ECO emulsion had uniform small droplets (diameter = 539 nm) with stable oil-in-water characteristics. ECO had a long-lasting, dispersive, negative zeta potential (-13 mv), and biodegradable properties (viscosity = 357 cp). Approximately 97% of TCE could be removed with ECO supplement after a 95-day operational period without the accumulation of TCE dechlorination byproducts (dichloroethylene and vinyl chloride). The buffer system could neutralize acidified groundwater, and citrate could be served as a primary substrate. ECO addition caused an abrupt TCE adsorption at the initial stage and the subsequent removal of adsorbed TCE. Results from the next generation sequences and real-time polymerase chain reaction (PCR) indicate that the increased microbial communities and TCE-degrading bacterial consortia were observed after ECO addition. ECO could be used as a pH-control and carbon substrate to enhance anaerobic TCE biodegradation effectively. PRACTITIONER POINTS: Emulsified castor oil (ECO) contains castor oil, surfactants, and buffer for a slow carbon release and pH control. ECO can be a long-term carbon source for trichloroethylene (TCE) dechlorination without causing acidification. TCE removal after ECO addition is due to adsorption and reductive dechlorination mechanisms.

Application of zeolitic imidazolate framework for hexavalent chromium removal: A feasibility and mechanism study

The mechanisms and effectiveness of using zeolitic imidazolate framework (ZIF-8) [a sub-family of metal-organic framework (MOF)] particles on hexavalent chromium [Cr(VI)] removal were evaluated. The ultrasonic mixing method was applied for the preparation of ZIF-8, and chemicals used for ZIF-8 synthesis included ammonium hydroxide, zinc nitrate hexahydrate, and 2-methylimidazole. ZIF-8 particle had a clear rhombic dodecahedron morphology shape and a strong peak intensity with high crystallinity. The adsorption capacity (AC) of ZIF-8 was 30.3 mg of Cr(VI)/g of ZIF-8 [Cr(VI) = 50 mg/L]. The AC of Cr(VI) raised to 34.3 mg/g under acidic conditions (pH = 5), and the AC dropped to below 13.7 mg/g with a pH range from 7 to 11. It could be because of the competitive effects between CrO₄ ^2- and hydroxide ions for adsorption locations of ZIF-8. Cr(VI) removal relied on the amount of Cr(VI) adsorbed on the particles of ZIF-8, and the mechanisms of Cr(VI) adsorption by ZIF-8 included chemical/physical processes and the rate-limiting step was the chemical adsorption. A fraction of sorbed Cr(VI) was reduced to Cr(III), and thus, ZIF-8 could serve as a reducing agent during Cr(VI) reduction. Cr(VI) was removed effectively from the water phase by ZIF-8 via adsorption and reduction mechanisms. PRACTITIONER POINTS: ZIF-8 particles had an adsorption capacity of 30.33 mg of Cr(VI)/g of ZIF-8. Cr(VI) sorption by ZIF-8 has chemical (rate-limiting step) and physical processes. ZIF-8 can serve as a reducing agent for Cr(VI) reduction. Cr(VI) can be removed by ZIF-8 via the adsorption and reduction mechanisms.

Novel rapid room temperature synthesis of conjugated microporous polymer for metal-free photocatalytic degradation of fluoroquinolones

The existence of Fluoroquinolones (FQs), non-biodegradable pharmacophores, in the natural environment possesses a serious threat. We herein report a novel, rapid, room-temperature synthesis of semiconducting conjugated microporous polymer (CMP) for the decontamination of four second-generation FQs, Norfloxacin, Enrofloxacin, Ciprofloxacin, and Ofloxacin. The CMP demonstrated impressive gas uptake and FQ adsorption ability. Decreased HOMO-LUMO bandgap resulted in enhanced exciton pair generation on visible-light-illumination. Additionally, a high degree of photocurrent response and suitable redox potentials of the material conjointly endorsed its almost quantitative FQ-degradation efficiency. Ofloxacin showed the best removal efficiency with 0.061 and 0.207 min^-1 adsorption and degradation rate constants, respectively, one of the highest values reported. The CMP exhibited equipotent activity for other FQs as well. On increasing the concentration of the FQs or decreasing the incident photo-intensity, quantitative removal efficiencies are observed. Changing the pH of the medium from acidic to alkaline did not impart any change in catalytic activity as well. The reactive species involved viz. O₂^-, ¹O₂, etc. and their roles in the degradation process were determined through control and trapping experiments. A plausible in-depth mechanistic pathway was assessed from the FQ degradation intermediates, and the reactive catalytic species substantiating step-by-step break down of the antibiotic backbone.

Application of iron/aluminum bimetallic nanoparticle system for chromium-contaminated groundwater remediation

When the nanoscale zero valent iron (nZVI) is used for the reduction of hexavalent chromium (Cr⁶⁺) to trivalent chromium (Cr³⁺) in groundwater, the reduction efficiency is decreased due to the passivation of reactive sites by precipitation. The bimetallic nanoparticle (BNP) can be created with the addition of the second metal to achieve a higher activity and reduce the occurrence of the ferrous/ferric hydroxide precipitation. In this study, the iron-coated aluminum (Fe/Al) BNP and aluminum-coated iron (Al/Fe) BNP systems were designed for remediating Cr⁶⁺-contaminated groundwater. The chemical liquid-phase deposition and co-reduction method was applied to produce BNPs. Cr⁶⁺ removal rate by Fe/Al BNPs was directly proportional to the saturation concentration and reactive sites, which caused a higher Cr⁶⁺ removal rate. The pseudo-first-order kinetic model could be used to describe the Cr⁶⁺ adsorption mechanism by Fe/Al BNPs. Results show that Fe/Al BNPs and Al/Fe BNPs could reduce Cr⁶⁺ to Cr³⁺, and the removal efficiencies for Cr⁶⁺ were 1.47 g/g BNP and 0.07 g/g BNP, respectively. Detection of Cr³⁺ in the aqueous phase was observed during the Cr⁶⁺ removal process. Results from X-ray diffraction (XRD) analysis confirmed that Cr(OH)₃ was present on the surface of BNPs. Main mechanisms caused Cr⁶⁺ removal included reduction, precipitation, and adsorption. The reduction of Cr⁶⁺ produced OH^-, which created alkaline environment and facilitated the formation of chromium hydroxide precipitates [Cr(OH)₃]. Thus, the migration of Cr³⁺ was prevented and the environmental risk was reduced. BNP had a higher activity and stability, and it was applicable for Cr⁶⁺-contaminated site remediation.

Growth inhibition of sulfate-reducing bacteria for trichloroethylene dechlorination enhancement

Trichloroethylene (TCE) is a frequently found organic contaminant in polluted-groundwater. In this microcosm study, effects of hydrogen-producing bacteria [Clostridium butyricum (Clostridium sp.)] and inhibitor of sulfate-reducing bacteria (SRB) addition on the enhancement of TCE dechlorination were evaluated. Results indicate that Clostridium sp. supplement could effectively enhance TCE reductive dechlorination (97.4% of TCE removal) due to increased hydrogen concentration and Dehalococcoides (DHC) populations (increased to 1 × 10⁴ gene copies/L). However, addition of Clostridium sp. also caused the increase in dsrA (dissimilatory sulfide reductase subunit A) (increased to 2 × 10⁸ gene copies/L), and thus, part of the hydrogen was consumed by SRB, which would limit the effective application of hydrogen by DHC. Control of Clostridium sp. addition is a necessity to minimize the adverse impact of Clostridium sp. on DHC growth. Ferric citrate caused the slight raise of the oxidation-reduction state, which resulted in growth inhibition of SRB. Molybdate addition inhibited the growth of SRB, and thus, the dsrA concentrations (dropped from 4 × 10⁷ to 9 × 10⁵ gene copies/L) and sulfate reduction efficiency were decreased. Increased DHC populations (increased from 8 × 10³ to 1 × 10⁵ gene copies/L) were due to increased available hydrogen (increased from 0 to 2 mg/L), which enhanced TCE dechlorination (99.3% TCE removal). Metagenomic analyses show that a significant microbial diversity was detected in microcosms with different treatments. Clostridium sp., ferric citrate, and molybdate addition caused a decreased SRB communities and increased fatty acid production microbial communities (increased from 4.9% to 20.2%), which would be beneficial to the hydrogen production and TCE dechlorination processes.

Application of enhanced bioreduction for hexavalent chromium-polluted groundwater cleanup: Microcosm and microbial diversity studies

Hexavalent chromium (Cr⁶⁺) is a commonly found heavy metal at polluted groundwater sites. In this study, the effectiveness of Cr⁶⁺ bioreduction by the chromium-reducing bacteria was evaluated to remediate Cr⁶⁺-contaminated groundwater. Microcosms were constructed using indigenous microbial consortia from a Cr⁶⁺-contaminated aquifer as the inocula, and slow-releasing emulsified polycolloid-substrate (ES), cane molasses (CM), and nutrient broth (NB) as the primary substrates. The genes responsible for the bioreduction of Cr⁶⁺ and variations in bacterial diversity were evaluated using metagenomics assay. Complete Cr⁶⁺ reduction via the biological mechanism was observed within 80 days using CM as the carbon source under anaerobic processes with the increased trivalent chromium (Cr³⁺) concentrations. Cr⁶⁺ removal efficiencies were 83% and 59% in microcosms using ES and NB as the substrates, respectively. Increased bacterial communities associated with Cr⁶⁺ bioreduction was observed in microcosms treated with CM and ES. Decreased bacterial communities were observed in NB microcosms. Compared to ES, CM was more applicable by indigenous Cr⁶⁺ reduction bacteria and resulted in effective Cr⁶⁺ bioreduction, which was possibly due to the growth of Cr⁶⁺-reduction related bacteria including Sporolactobacillus, Clostridium, and Ensifer. While NB was applied for specific bacterial selection, it might not be appropriate for electron donor application. These results revealed that substrate addition had significant impact on microbial diversities, which affected Cr⁶⁺ bioreduction processes. Results are useful for designing a green and sustainable bioreduction system for Cr⁶⁺-polluted groundwater remediation.

Distribution, sources, and behavior of PAHs in estuarine water systems exemplified by Salt River, Taiwan

Water, suspended particulate matter (SPM), and sediment samples were collected from Salt River in Taiwan and analyzed the concentrations of 16 types of polycyclic aromatic hydrocarbons (PAHs). The analysis results were used to examine the distribution, source, partition behavior, and potential ecological risks of PAHs in the estuarine water systems. The mean concentration of total PAHs in water, SPM, and sediment samples was 0.485-10.2 μg/L, 26.7-169 mg/kg dw, and 0.343-29.4 mg/kg dw, respectively. The highest concentration was found at the river mouth and decreased toward the river and sea with the tide. The distribution of the diagnostic ratios of PAHs showed that the combustion of coal and petroleum products are the main sources of PAHs in Salt River. The in site organic carbon normalized partition coefficients for SPM-water (K'_oc(SPMW)) and sediment-water (K'_oc(SedW)) were 2.8-4.5 and 4.6-6.0 (log units), respectively, increasing with the number of rings in PAHs. The values log K'_oc(SedW) and log K'_oc(SPM-W) showed a significant linear correlation with their octanol-water partition coefficients (p < 0.01), and their slopes were 0.427 and 0.316, respectively. The fugacity fraction was used to evaluate the exchange of PAHs in water-SPM-sediment systems. Results showed that in SPM, 2-4-ring PAHs tend to be released into water, whereas 5-6-ring PAHs in water tend to be adsorbed onto SPM. The exchange of PAHs between water and sediment occurs in the direction of adsorption onto sediment from water. The assessment of the mean risk quotient, total toxicity equivalence, and mean effect range-median quotient of PAHs showed that the PAHs in the water and SPM of Salt River may have moderate to high ecological risk. In sediment, PAHs in the lower reaches and estuary may pose moderate to high ecological risk, whereas PAHs in the middle and upper reaches show low to moderate ecological risk.

Enhanced reductive dechlorination of trichloroethene with immobilized Clostridium butyricum in silica gel

Deteriorated environmental conditions during the bioremediation of trichloroethene (TCE)-polluted groundwater cause decreased treatment efficiencies. This study assessed the effect of applying immobilized Clostridium butyricum (a hydrogen-producing bacterium) in silica gel on enhancing the reductive dechlorination efficiency of TCE with the slow polycolloid-releasing substrate (SPRS) supplement in groundwater. The responses of microbial communities with the immobilized system (immobilized Clostridium butyricum and SPRS amendments) were also characterized by the metagenomics assay. A complete TCE removal in microcosms was obtained within 30 days with the application of this immobilized system via reductive dechlorination processes. An increase in the population of Dehalococcoides spp. was observed using the quantitative polymerase chain reaction (qPCR) analysis. Results of metagenomics assay reveal that the microbial communities in the immobilized system were distinct from those in systems with SPRS only. Bacterial communities associated with TCE biodegradation also increased in microcosms treated with the immobilized system. The immobilized system shows a great potential to promote the TCE dechlorination efficiency, and the metagenomics-based approach provides detailed insights into dechlorinating microbial community dynamics. The results would be helpful in designing an in situ immobilized system to enhance the bioremediation efficiency of TCE-contaminated groundwater.