Anoxic Groundwater Research Articles

Does submarine groundwater discharge contribute to summer hypoxia in the Changjiang (Yangtze) River Estuary?

The Changjiang (Yangtze) River Estuary (CJE) is one of the largest and most intense seasonal hypoxic zones in the world. Here we examine the possibility that submarine groundwater discharge (SGD) may contribute to the summer hypoxia. Spatial distributions of bottom water 222Rn suggest a hotspot discharge area in the northern section of the CJE. SGD fluxes were estimated based on a 222Rn mass balance model and were found to range from 0.002 ± 0.004 to 0.022 ± 0.011 m3/m2/day. Higher SGD fluxes were observed during summer hypoxia period. The well-developed overlap of the distribution patterns for SGD flux and dissolved oxygen (DO) implies that SGD could be an important contributor to summer hypoxia in the region off the CJE. We suggest that SGD contributes to the seasonal hypoxia either: (1) directly via discharge of anoxic groundwaters together with reducing substances; and/or (2) indirectly by delivering excess nutrients that stimulate primary productivity with consequent consumption of DO during organic matter decomposition.

Comprehensive Biogeochemical Analysis of Nitrogen Transformation Parameters

The potential for nitrogen attenuation in anoxic groundwater by examining the concentration and distribution patterns of electron acceptors, electron donors and microbiological parameters was assessed. Research was carried out for 25 monitoring bores in two anoxic alluvial aquifers Kovin-Dubovac and Knicanin-Centa during the four years monitoring period (2010–2013). Nitrate and nitrite concentrations were mostly undetectable while ammonium, ferrous and total organic carbon concentrations were increased with sporadic sulfide appearance. Piper diagrams qualified these groundwaters as predominantly Ca2+(Mg2+)–$${\text{HCO}}_{3}^{ - }$$ type. By Alekin classification waters belong to $${\text{C}}_{{\text{I}}}^{{{\text{Ca}}}}$$ type. The multiple electron donors (organic carbon, iron minerals and sulfide) and spatial and temporal variability of alternative electron acceptors proved to be determining factors of dominant nitrogen reduction path way. Concentrations trends of chemical and microbiological parameters revealed the conditions suitable for nitrogen conservation and they are presented via developed maps and BART test results. Statistical analysis was evaluated by Principal Component Analysis and Cluster Dendograms.

The Potential for CH<sub>4</sub> Production by Syntrophic Microbial Communities in Diverse Deep Aquifers Associated with an Accretionary Prism and its Overlying Sedimentary Layers

Accretionary prisms are thick masses of sedimentary material scraped from the oceanic crust and piled up at convergent plate boundaries found across large regions of the world. Large amounts of anoxic groundwater and natural gas, mainly methane (CH4), are contained in deep aquifers associated with these accretionary prisms. To identify the subsurface environments and potential for CH4 production by the microbial communities in deep aquifers, we performed chemical and microbiological assays on groundwater and natural gas derived from deep aquifers associated with an accretionary prism and its overlying sedimentary layers. Physicochemical analyses of groundwater and natural gas suggested wide variations in the features of the six deep aquifers tested. On the other hand, a stable carbon isotope analysis of dissolved inorganic carbon in the groundwater and CH4 in the natural gas showed that the deep aquifers contained CH4 of biogenic or mixed biogenic and thermogenic origins. Live/dead staining of microbial cells contained in the groundwater revealed that the cell density of live microbial cells was in the order of 104 to 106‍ ‍cells‍ ‍mL–1, and cell viability ranged between 7.5 and 38.9%. A DNA analysis and anoxic culture of microorganisms in the groundwater suggested a high potential for CH4 production by a syntrophic consortium of hydrogen (H2)-producing fermentative bacteria and H2-utilizing methanogenic archaea. These results suggest that the biodegradation of organic matter in ancient sediments contributes to CH4 production in the deep aquifers associated with this accretionary prism as well as its overlying sedimentary layers.

Sulphate Reducing Bacteria and Methanogenic Archaea Driving Corrosion of Steel in Deep Anoxic Ground Water

During the operation, maintenance and decommissioning of nuclear power plant radioactive contaminated waste is produced. This waste is stored in an underground repository 60-100 meters below the surface. The metallic portion of this waste comprises mostly carbon and stainless steel. A long-term field exposure showed high corrosion rates, general corrosion up to 29 μm a-1 and localized corrosion even higher. High corrosion rate is possible if microbes produce corrosive products, or alter the local microenvironment to favor corrosion. The bacterial and archaeal composition of biofilm formed on the surface of carbon steel was studied using 16S rRNA gene targeting sequencing, followed by phylogenetic analyses of the microbial community. The functional potential of the microbial communities in biofilm was studied by functional gene targeting quantitative PCR. The corrosion rate was calculated from weight loss measurements and the deposits on the surfaces were analyzed with SEM/EDS and XRD. Our results demonstrate that microbial diversity on the surface of carbon steel and their functionality is vast. Our results suggest that in these nutrient poor conditions the role of methanogenic archaea in corrosive biofilm, in addition to sulphate reducing bacteria, could be greater than previously suspected.

Influence of oxic and anoxic groundwater conditions on occurrence of selected agrochemicals

Abstract The occurrence and mitigation of selected pesticides and nitrates in surface and groundwater samples from two alluvions in the Republic of Serbia are investigated in this study. The influence of aquifer conditions on the degradation processes of pesticides and nitrates is considered as a potential removal mechanism of compound residues in alluvial groundwater. Nitrate concentration was analyzed in 144 groundwater samples in the Kovin-Dubovac drainage system and 63 samples at the Ključ groundwater source. The occurrence of 15 pesticides was monitored in groundwater in a total of 34 samples in the Kovin-Dubovac area and 14 samples at the Ključ groundwater source. Concentrations of selected pesticides and nitrates were monitored in eight samples from the Danube in the Kovin-Dubovac area and 15 samples from Velika Morava at the Ključ groundwater source. Both selected locations are agricultural areas. Results show that concentrations of nitrates (NO3−) are much higher in oxic groundwater conditions compared with concentrations in typical anoxic conditions. The opposite is the case for the concentration of pesticides that occurred in surface and groundwater samples. These results are very important for a better understanding of the self-purification potential of alluvial aquifers and assessment of aquifer condition influence, especially oxic or anoxic conditions, on the occurrence and mitigation of selected agrochemical residues in groundwaters.

Hydrogeochemical controls on arsenic contamination potential and health threat in an intensive agricultural area, northern China

The contamination of ground water with arsenic is a great public health concern. This paper discusses the possible formation mechanism of high As groundwater; identify the main influences of natural and anthropogenic factors on As occurrence in groundwater; and finally estimates As-induced potential health hazards in an intensive agricultural region, Datong Basin (Northern China). Our findings indicate that the predominant controlling factors of As in groundwater can be divided into natural factors and anthropogenic activities. Natural factors can be classified as natural potential source of As, environmental geological characteristics and hydrochemical conditions; anthropogenic activities are manifested in industrial coal mining, domestic coal burning, agricultural irrigation return flow and excessive application of fertilizers, and groundwater exploitation. Microbial and/or chemical reduction desorption of arsenate from Fe-oxide/hydroxide and/or clay minerals, As-bearing Fe-oxide/hydroxide reduction coupled with sulfate reduction, and competition with phosphorus are postulated to be the major process dominating As enrichment in the alkaline and anoxic groundwater. In addition, age-dependent human health risk assessment (HHRS) was performed, and high risk values reveal a high toxic and carcinogenic risk of As contaminate for population who is subject to the continuous and chronic exposure to elevated As.

Performance Evaluation of Metakaolin as Low Cost Adsorbent for Manganese Removal in Anoxic Groundwater

The recent climate condition and pollution problem related to surface water have led to water scarcity in Malaysia. Huge amount of groundwater has been identified as viable source for drinking water. This paper was aimed to investigate groundwater’s quality at specific location and metakaolin’s potential in the groundwater treatment in the removal of manganese. Groundwater purging was determined to be sufficient at 120 minutes where all three parameters (pH, dissolved oxygen and conductivity) were stabilized. The groundwater studied is classified as both anoxic and reductive due the low dissolved oxygen value. It also can be categorized as brackish due to high value of conductivity and total dissolved solid. Manganese content in groundwater was determined as higher than of that permissible limit for raw water and drinking water which makes it unsuitable for them not suitable for consumption and cleaning purpose. Average manganese concentration in samples was 444.0 ppb where the concentrations of manganese ranged from 229.4 ppb to 760.3 ppb. Manganese developed is not that a strong positive correlation against iron concentration, total dissolved solids and conductivity; whereas has a moderate negative correlation against dissolved oxygen. The capability adsorption of manganese by metakaolin was assessed via batch method which indicated optimum dosage and contact time was 14g that removed average 30.2% and contact time optimum at 120 minutes which removed 33.2% manganese from the sample.

Corrosion and biofouling tendency of carbon steel in anoxic groundwater containing sulphate reducing bacteria and methanogenic archaea

The corrosion of carbon steel in groundwater with or without of sulphate reducing bacteria (SRB) and/or methanogenic archaea (MA) was examined in a 12-month experiment simulating a geological underground repository for nuclear waste. In the abiotic environment, cumulative corrosion rate was lowest among the studied systems, 1 μm/a. In biotic environments, the cumulative corrosion varied between 5 μm/a and 10 μm/a, being highest in SRB enriched system and lowest in SRB + MA system. However, localised corrosion was more intensive in all biotic systems. Microbial community differed between environments from betaproteobacterial taxa dominated (SRB) to Desulfovibrionales and Methanobacteriales rich in SRB + MA/MA.

Microbially Mediated Release of As from Mekong Delta Peat Sediments.

Peat layers within alluvial sediments are considered effective arsenic (As) sinks under reducing conditions due to the binding of As(III) to thiol groups in natural organic matter (NOM) and the formation of As-bearing sulfide phases. However, their possible role as sources of As for anoxic groundwaters remains unexplored. Here, we perform laboratory experiments to provide evidence for the role of a sediment peat layer in releasing As. Our results show that the peat layer, deposited about 8,000 years ago in a paleomangrove environment in the nascent Mekong Delta, could be a source of As to porewater under reducing conditions. X-ray absorption spectroscopy (XAS) analysis of the peat confirmed that As was bound to NOM thiol groups and incorporated into pyrite. Nitrate was detected in peat layer porewater, and flow-through and batch experiments evidenced the release of As from NOM and pyrite in the presence of nitrate. Based on poisoning experiments, we propose that the microbially mediated oxidation of arsenic-rich pyrite and organic matter coupled to nitrate reduction releases arsenic from this peat. Although peat layers have been proposed as As sinks in earlier studies, we show here their potential to release depositional- and/or diagenetically-accumulated As.

Redox conditions and pH control trace element concentrations in a meandering stream and shallow groundwater of a semiarid mountain watershed, Red Canyon, Wyoming, USA

The interface between oxic surface water and anoxic groundwater plays an important role in trace element cycling in mountains streams. In this investigation, stream water and shallow groundwater were sampled in a semiarid mountain catchment at Red Canyon, Wyoming, USA to evaluate variability in redox conditions and trace element concentrations. Samples were collected in July 2013 during summer baseflow conditions and in May 2014 during snowmelt runoff. Field measurements showed dissolved oxygen-saturated stream water and anoxic groundwater, with relatively higher pH in surface water. Groundwater and surface water chemistry were remarkably similar during both the summer and spring sampling events, indicating overall stability of the oxic/anoxic transition and trace element concentrations. Similar stable water isotope values indicate a close connection between the surface water and groundwater (δ18O and δD ranged from − 18.1 to − 18.9‰ and from − 140 to − 147‰, respectively). Concentrations of most trace and major elements were a factor of two higher in groundwater relative to surface water. Notably, Mn, Fe, and Ce concentrations were 7–90-fold higher in groundwater relative to surface water. In contrast, As, Se, and V concentrations were 3–30-fold lower in groundwater relative to surface water, likely due to pH- and redox-driven changes in speciation and sorption. This study is important for characterizing trace element cycling in mountainous areas, where measurements are rare, and has implications for furthering understanding of the key biogeochemical processes that occur during groundwater–surface water interactions.

Highly Diverse Aquatic Microbial Communities Separated by Permafrost in Greenland Show Distinct Features According to Environmental Niches.

The Greenland Analog Project (GAP) study area in the vicinity of Kangarlussuaq, Western Greenland, was sampled for surface water and deep groundwater in order to determine the composition and estimate the metabolic features of the microbial communities in water bodies separated by permafrost. The sampling sites comprised a freshwater pond, talik lake, deep anoxic groundwater, glacier ice and supraglacial river, meltwater river and melting permafrost active layer. The microbial communities were characterized by amplicon sequencing of the bacterial and archaeal 16S rRNA genes and fungal ITS1 spacer. In addition, bacterial, archaeal and fungal numbers were determined by qPCR and plate counts, and the utilization pattern of carbon and nitrogen substrates was determined with Biolog AN plates and metabolic functions were predicted with FAPROTAX. Different sample types were clearly distinguishable from each other based on community composition, microbial numbers, and substrate utilization patterns, forming four groups, (1) pond/lake, (2) deep groundwater, (3) glacial ice, and (4) meltwater. Bacteria were the most abundant microbial domain, ranging from 0.2–1.4 × 107 16S rRNA gene copies mL-1 in pond/lake and meltwater, 0.1-7.8 × 106 copies mL-1 in groundwater and less than 104 copies mL-1 in ice. The number of archaeal 16S and fungal 5.8S rRNA genes was generally less than 6.0 × 103 and 1.5 × 103, respectively. N2-fixing and methane-oxidizing Actinomycetes, Bacteroidetes and Verrucomicrobia were the dominant microorganisms in the pond/lake samples, whereas iron reducing Desulfosporosinus sp. dominated the deep anaerobic groundwater. The glacial ice was inhabited by Cyanobacteria, which were mostly Chloroplast-like. The meltwater contained methano- and methylotrophic Proteobacteria, but had also high relative abundances of the nano-sized Parcubacteria. The archaea composed approximately 1% of the 16S rRNA gene pool in the pond/lake samples with nano-sized Woesearchaeota as the dominating taxon, while in the other sample types archaea were almost negligent. Fungi were also most common in the pond/lake communities, were zoospore-forming Chytridiomycetes dominated. Our results show highly diverse microbial communities inhabiting the different cold Greenlandic aqueous environments and show clear segregation of the microbial communities according to habitat, with distinctive dominating metabolic features specifically inhabiting defined environmental niches and a high relative abundance of putatively parasitic or symbiotic nano-sized taxa.

Model-Based Interpretation of Groundwater Arsenic Mobility during in Situ Reductive Transformation of Ferrihydrite.

Arsenic (As) release and mobility in groundwater is coupled to the iron (Fe) cycling and the associated transformation of Fe-oxides present in sediments. Recent in situ experiments have provided observations on arsenic mobilization and co-occurring reductive mineral transformation when placing As-loaded ferrihydrite-coated sand for 80 days in wells of an As-contaminated aquifer of Northern China. However, the complex temporal change in solid-associated arsenic and the multiple geochemical processes occurring when the flowing groundwater contacts the As-loaded ferrihydrite-coated sand hamper a detailed evaluation of the experimental data set. In this study, we develop a modeling approach that allows a quantitative interpretation of arsenic release and ferrihydrite transformation observed during the in situ experiments. The model accounts for the interplay of abiotic and biotic geochemical processes (i.e., surface complexation, reductive dissolution, formation of secondary iron minerals, and arsenic sequestration into the newly formed minerals) involved in the transformation of Fe-oxides and controlling arsenic mobility. The results show the capability of the proposed approach to reproduce the temporal trends of solid arsenic and ferrihydrite concentrations, as well as the spatial variability of mineral transformation, observed in different wells using a common set of surface complexation parameters and kinetic rate constants. The simulation outcomes allowed us to disentangle the specific contribution of the different mechanisms controlling the release of arsenic. It was possible to identify an initial rapid but minor release of As (13-23% of the initial surface concentration) due to desorption from ferrihydrite, as well as the reduction of adsorbed As(V) to As(III) upon contact with the flowing anoxic groundwater. Successively, reductive dissolution of ferrihydrite caused the decrease of the amount of the Fe mineral phase and led to a major depletion of solid-associated arsenic. The produced Fe(II) catalyzed the ferrihydrite conversion into more crystalline Fe(III) oxides (i.e., lepidocrocite and goethite) through Ostwald ripening, and resulted in the formation of siderite and mackinawite upon reaction with carbonates and sulfides naturally present in the groundwater. The model results also showed that, whereas the decrease in surface sites during reductive dissolution of ferrihydrite promoted arsenic mobilization, the mineral transformation limited As release through its sequestration into the newly formed secondary mineral phases.

Kinetic Study on Clogging of a Geothermal Pumping Well Triggered by Mixing-Induced Biogeochemical Reactions.

The sustainability of ground-source geothermal systems can be severely impacted by microbially mediated clogging processes. Biofouling of water wells by hydrous ferric oxide is a widespread problem. Although the mechanisms and critical environmental factors associated with clogging development are widely recognized, effects of mixing processes within the wells and time scales for clogging processes are not well characterized. Here we report insights from a joint hydrological, geochemical, and metagenomics characterization of a geothermal doublet in which hydrous ferric oxide and hydrous manganese oxide deposits had formed as a consequence of mixing shallow groundwater containing dissolved oxygen and nitrate with deeper, anoxic groundwater containing dissolved iron (FeII) and manganese (MnII). Metagenomics identify distinct bacteria consortia in the pumping well oxic and anoxic zones, including autotrophic iron-oxidizing bacteria. Batch mixing experiments and geochemical kinetics modeling of the associated reactions indicate that FeII and MnII oxidation are slow compared to the residence time of water in the pumping well; however, adsorption of FeII and MnII by accumulated hydrous ferric oxide and hydrous manganese oxide in the well bore and pump riser provides "infinite" time for surface-catalyzed oxidation and a convenient source of energy for iron-oxidizing bacteria, which colonize the surfaces and also catalyze oxidation. Thus, rapid clogging is caused by mixing-induced redox reactions and is exacerbated by microbial activity on accumulated hydrous oxide surfaces.

The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia

Black opal (opal-AG) owes its dark coloration to a fine-grained pigment commonly inferred to be mainly carbon, yet chemical compositions for black opals suggest there may be additional components. Here we search for such components in pigment concentrates prepared by dissolving black opal nodules (nobbies) from Lightning Ridge (NSW) in hydrofluoric acid, using electron microscopy (scanning electron microscopy, transmission electron microscopy), X-ray diffraction and laser-ablation ICP-MS. The results demonstrate the presence of sulfides—predominantly pyrite and chalcopyrite, with minor galena and Ti-oxide phases, as additional components of the pigment. ATR-FTIR analysis indicates the presence of C=O and C–H groups, consistent with an organic origin. Transmission electron microscopy images of pigment show variously deformed, originally spherical ∼100 nm particles rich in sulfide and carbon, which are interpreted as thin coatings of pigment on now dissolved opaline silica spheres. Laser-ablation ICP-MS analysis identifies remnant silica in pigment concentrates, which may be interpreted as opaline silica surviving HF treatment protected as inclusions in sulfides. When examined within the context of petrographic observations from more than 1000 opal nodules (nobbies) at Lightning Ridge, these new results suggest that pigment carbon and sulfides in the nodules formed microbially under initially anoxic groundwater conditions, within pre-existing cavities concurrently being filled with silica sol ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment. Intensely black pigment layers observed at the floor of many nodules indicate settling of dark, high-density (sulfide–Ti-oxide-rich) pigment within cavities, with the implication that sulfate-reducing bacterial (SRB) activity commences early during the silica sol-gel ripening process. Microbial activity may persist until after the cavity has completely filled with the silica sol, as illustrated by abundant black opals with uniformly distributed pigment. Pigment formed at this stage may no longer be able to settle out within the ripening and increasingly viscous silica gel, thus forming pigmentation throughout the opal cavity. The existence of ‘amber’, pigment-poor opal with intensely black basal pigment layers is interpreted as signalling a lack of sulfate to sustain further SRB activity, or a change to more oxidising conditions, possibly related to interaction with surface waters within a downward-penetrating weathering front. A change in redox conditions would shut off activity of SRB and thus sulfide pigment production and allow development of aerobic microbial activity as described by others.

Structural transformation of sulfidized zerovalent iron and its impact on long-term reactivity

This study elucidates the structural changes of sulfidized zerovalent iron (S-nZVI) in anoxic groundwater, presents a compelling evidence on the structural robustness of the material and explains how S-nZVI long-term reactivity is achieved.

Assessing and Reducing Risks from Mining of Technology Critical Elements

In recent years, there has been a rapid increase in the global demand for many elements used in green energy and high-tech applications, including antimony (Sb), cobalt (Co), indium (In), lithium (Li), niobium (Nb) and the rare earth elements (REE). Canada has abundant resources of these critical elements, and many companies are working to bring new mines into production. However, we know very little about the potential environmental impacts of mining critical elements compared to mining other commodities such as gold or copper. Processing resources such as REEs has led to environmental degradation in some parts of the world, but there are few published studies of these environmental impacts and related risks to human health. This talk will summarize the key geoenvironmental characteristics of critical element deposits, including results from recent studies of the St. Lawrence Columbium Mine in Oka, Quebec. This mine operated from 1961 to 1976 and at the time was one of the largest Nb producers in the world. Samples of waste rock, tailings, slag, surface water, and groundwater were collected from 2015 to 2018. Mineralogical analyses show that in waste rock and tailings most uranium (U), thorium (Th) and REEs are hosted by low-solubility oxide and silicate minerals. However, some REEs and fluorine (F) are contained in minerals such as calcite and fluorapatite, which are relatively soluble under weathering conditions. Smelter slag at the mine contains concentrations of U, Th and radioactive isotopes that exceed Canadian guidelines for the disposal of radioactive waste. Measurements of U, Th, F, Nb, REEs, and radionuclides in tailings seepage and pit lake waters indicate that these elements are relatively immobile in oxic surface waters, but may be transported in deeper, anoxic groundwaters. The results of this research should help to improve environmental predictions for future Nb- and REE-mining projects and support the development of new environmental guidelines.

Corrosion of Copper in Anoxic Ground Water in the Presence of SRB

Corrosion of Copper in Anoxic Ground Water in the Presence of SRB

Community dynamics and functional characteristics of naphthalene-degrading populations in contaminated surface sediments and hypoxic/anoxic groundwater.

Earlier research on the biogeochemical factors affecting natural attenuation in coal-tar contaminated groundwater, at South Glens Falls, NY, revealed the importance of anaerobic metabolism and trophic interactions between degrader and bacterivore populations. Field-based characterizations of both phenomena have proven challenging, but advances in stable isotope probing (SIP), single-cell imaging and shotgun metagenomics now provide cultivation-independent tools for their study. We tracked carbon from 13 C-labelled naphthalene through microbial populations in contaminated surface sediments over 6 days using respiration assays, secondary ion mass spectrometry imaging and shotgun metagenomics to disentangle the contaminant-based trophic web. Contaminant-exposed communities in hypoxic/anoxic groundwater were contrasted with those from oxic surface sediments to identify putative features of anaerobic catabolism of naphthalene. In total, six bacteria were responsible for naphthalene degradation. Cupriavidus, Ralstonia and Sphingomonas predominated at the earliest stages of SIP incubations and were succeeded in later stages by Stenotrophomonas and Rhodococcus. Metagenome-assembled genomes provided evidence for the ecological and functional characteristics underlying these temporal shifts. Identical species of Stenotrophomonas and Rhodococcus were abundant in the most contaminated, anoxic groundwater. Apparent increases in bacterivorous protozoa were observed following exposure to naphthalene, though insignificant amounts of carbon were transferred between bacterial degraders and populations of secondary feeders.

Biogeochemical phosphorus cycling in groundwater ecosystems – Insights from South and Southeast Asian floodplain and delta aquifers

The biogeochemical cycling of phosphorus (P) in South and Southeast Asian floodplain and delta aquifers has received insufficient attention in research studies, even though dissolved orthophosphate (PO43−) in this region is closely linked with the widespread contamination of groundwater with toxic arsenic (As). The overarching aim of this study was to characterize the enrichment of P in anoxic groundwater and to provide insight into the biogeochemical mechanisms underlying its mobilization, subsurface transport, and microbial cycling. Detailed groundwater analyses and in situ experiments were conducted that focused on three representative field sites located in the Red River Delta (RRD) of Vietnam and the Bengal Delta Plain (BDP) in West Bengal, India. The results showed that the total concentrations of dissolved P (TDP) ranged from 0.03 to 1.50 mg L−1 in groundwater, with PO43− being the dominant P species. The highest concentrations occurred in anoxic sandy Holocene aquifers where PO43− was released into groundwater through the microbial degradation of organic carbon and the concomitant reductive dissolution of Fe(III)-(hydr)oxides. The mobilization of PO43− may still constitute an active process within shallow Holocene sediments. Furthermore, a sudden supply of organic carbon may rapidly decrease the redox potential, which causes an increase in TDP concentrations in groundwater, as demonstrated by a field experiment. Considering the subsurface transport of PO43−, Pleistocene aquifer sediments represented effective sinks; however, the enduring contact between oxic Pleistocene sediments and anoxic groundwater also changed the sediments PO43−-sorption capacity over time. A stable isotope analysis of PO43−-bound oxygen indicated the influences of intracellular microbial cycling as well as a specific PO43− source with a distinct isotopically heavy signal. Consequently, porous aquifers in Asian floodplain and delta regions proved to be ideal natural laboratories to study the biogeochemical cycling of P and its behavior in groundwater environments.

A full-wave rectified alternating current wireless electrocoagulation strategy for the oxidative remediation of As(III) in simulated anoxic groundwater

The application of conventional DC electrocoagulation for remedying the anoxic As(III)-contaminated groundwater normally suffers from the external introduction of O2 and the vulnerability of the ohmic contact between iron electrode and conductive wire. To address these limitations, a novel full-wave rectified alternating current wireless electrocoagulation (FWAC-WEC) technology for the oxidative immobilization of As(III) from anoxic groundwater was developed in this study. The FWAC-WEC process produced Fe(II) and O2 simultaneously when utilizing iron bipolar electrodes (Fe-BPEs) and two Ti plate coated mixed IrO2/Ta2O5 (MMO) driving electrodes. The increase in Fe-BPEs number (1–3) and outage time (0–6 s) and the decrease in the angle for Fe-BPEs to electric field line separately benefited As(tot) removal. Besides, As(tot) removal was enhanced with increasing pH value but deteriorated by the presence of HCO3− and PO43−. In the FWAC-WEC process, As(III) was initially oxidized to ionic As(V) by intermediate Fe(IV) species. Thus, As(tot) removal was significantly promoted by adsorption and/or co-precipitation of As(V) with the fresh Fe(III) (oxyhydr)oxides, which appreciably obeyed the Langmuir isotherm and the second-order kinetics. Notably, the FWAC-WEC technology can effectively eliminate the passivation of Fe-BPEs and thereby increase the utilization of Fe-BPEs due to the periodic exchange of driving electrodes polarity. This strategy could remarkably reduce the energy consumption to 0.101 kW·h/m3 for As(III) removal, which was much less than 0.167 KW·h/m3 in the DC counterpart. Generally, the FWAC-WEC technology is energy-efficient and has the potential of being a sustainable treatment option to improve access to safe groundwater for millions of people.