Increasing Sulfate Concentration Research Articles

Effects of sulfur poisoning on physicochemical properties and performance of MnO2/AlNi-PILC for toluene catalytic combustion

AlNi pillared clay (AlNi-PILC) was synthesized firstly, and then MnO2 was supported via wetness impregnation from nitrate precursors. Sulphation was performed by in-situ decomposing ammonium sulfate with different concentrations over MnO2/AlNi-PILC. Catalysts before and after sulfur poisoning were characterized by XRD, N2 adsorption/desorption, HRTEM, XPS, H2-TPR and NH3-TPD. MnO2/AlNi-PILC exhibited high catalytic activity, allowing the complete toluene combustion. Structure of the catalyst was obviously damaged after sulfur poisoning. (001) crystal plane strength of AlNi-PILC was decreased significantly. Meanwhile, the specific surface area and pore volume reduced with increase of sulfate concentration. Sulfur species were readily formed on the surface of poisoned catalyst and deposited in the pore structure of AlNi-PILC, which resulted in significant impacts on the structural stability, acidity and the number of active species. These changes were responsible for the decreased catalytic performance.

Discerning the biodegradation of binary dyes in microbial fuel cell: Interactive effects of dyes, electron transport behaviour, autocatalytic mechanism, and degradation pathways

This research presented the first attempt to investigate the effect of biodegradation of binary Acid Orange 7 (AO7) and Reactive Green 19 (RG19) on the performances of wastewater treatment and bioelectricity generation, using anti-gravity flow microbial fuel cell (AGF-MFC) system. The influences of initial dye concentration, substrate loading, sulphate concentration and application of quinones on system performances were comprehensively evaluated. The decolourization efficiencies of AO7 were higher than RG19 in binary solutions, at every tested concentrations. The addition of higher concentration of RG19 in binary solution was also found to have increased the overall performances of MFC, owing to electron mediating characteristics of its decolourized intermediates. However, the power density declined with the addition in dye concentration. Further increase of substrate loading by 3-folds (2.43 g/L) improved the decolourization efficiency approximately by 7%, but deteriorated power performance by 42%, to 63.40 ± 0.07 mW/m2. Increasing sulphate concentration from 20 to 400 mg/L had resulted in a high decolourization extent of binary dyes ascribed to sulphide-mediated dye degradation, whereas the power generation was reduced. The increase of sulphate to 800 mg/L led to decrease in decolourization and power density of the system. These outcomes deciphered the competitions of electrons between different electron acceptors in the anodic compartment. Moreover, the autocatalytic mechanism of RG19 decolourized intermediates, 1-amino-2-naphthol-3,6-disulphonate (1A2N36S) as electronophore was thoroughly unearthed. Detailed degradation pathways of dyes were proposed based on UV-Visible spectra and gas chromatograph-mass spectrometer (GC-MS) analyses.

Biogeochemical assessment of coastal urban soils and ecological safety

As a result of field and laboratory studies, ecological diagnostics of coastal soils was carried out in 11 points of the city of Irkutsk along the coastline of the Angara River in the recreation zone for 5 km. Revealed alkaline soil with pH up to 9.0. An increased sulfate content was found in the aqueous extract of soil substrates. This leads to soil toxicity, especially as a result of the subsequent transformations of sulfates into more toxic compounds -hydrogen sulfide, sulfuric acid and insoluble metal sulfates. Indicators of the biological activity of soils revealed a decrease in the activity of soil enzymes. More than 70% of soil samples were found to be toxic. The parameters of the biogeochemical state of soils indicate the inhibition of the enzymatic process. The inverse dependence of the biological activity of the soil on its pH has been established. The distribution of total forms of heavy metals Pb, Cu, Zn, Ni, Cd, Hg and As at observation points is obtained. The excess of the normative contents of elements was revealed. The studies carried out have shown an increased environmental hazard of the recreation area in the city center and the need for environmental protection measures.

New Insights into Surfactant Adsorption Estimation in Carbonates under Harsh Conditions Using Surface Complexation Modeling

Summary Several laboratory experiments demonstrated that the use of sodium hydroxide could increase the solution pH and reduce the adsorption of anionic surfactants. However, a better understanding of rock-oil-brine interactions and their effect on surfactant adsorption during alkaline-surfactantpolymer (ASP) flooding is needed for realistic and representative estimations of surfactant adsorption levels. Therefore, this study presents a novel approach to capture these interactions and better predict their effect on surfactant adsorption as well as effluent concentrations of surfactant and various aqueous species using the Phreeqc simulator. Currently, surface complexation models (SCMs) consider rock-brine, oil-brine, and surfactant-brine reactions. In this work, four new surface complexation reactions with intrinsic stability constants that honor oil-surfactant interactions have been proposed for the first time and then validated against experimental data reported in the literature. In addition, we analyzed the effect of various parameters on surface adsorption under harsh conditions of high temperature and high salinity using the proposed SCM. The results showed that the developed surfactant-based SCM is capable of estimating surfactant adsorption and its concentration in the effluent during chemical floods. The model was validated against two sets of ASP corefloods from the literature including single-phase and two-phase dynamic surfactant adsorption studies. The findings highlighted that oil-surfactant surface complexation reactions are important and should be captured for a more representative and accurate estimation of surfactant adsorption during chemical flooding. Moreover, the detailed and comprehensive analysis showed that surfactant adsorption increased and its concentration in the effluent decreased with the increase in temperature of the chemical flood. The latter shows that the adsorption process is endothermic and it is more of chemisorption as opposed to physio-adsorption. It was also showed that a decrease in water total salinity decreases the surfactant adsorption on the rock surface, which is related to the increase in the repulsive forces between the rock surface and adsorbed species. Additionally, with the increase in surfactant concentration in the chemical flood, surfactant concentration in the effluent increases, with a slight increase in surfactant adsorption. This slight increase in adsorption can be neglected compared with the injected and produced masses of the surfactant that are proportional. Moreover, the effect of sulfate spiking is significant where the increase in sulfate concentration reduces the surfactant adsorption. Furthermore, it is worth highlighting that the lowest surfactant adsorption levels were achieved through injected water dilution; less than 0.1 mg/g of rock. This is the first study to test a novel formulation of SCM considering the oil-surfactant effect on surfactant adsorption properties. The proposed framework to estimate surfactant adsorption is conducted for high-temperature and high-salinity reservoir conditions. Thus, it could be used in numerical reservoir simulators to estimate oil recovery due to wettability alteration by chemical flooding in carbonates, which will be investigated in our future work. The surfactant adsorption mechanisms during chemical flooding are very case-dependent and hence, the findings of this study cannot be generalized.

Mixed salt precipitation and water evaporation during smart water alternative CO2 injection in carbonate reservoirs

The main objective in this study is to experimentally elucidate the challenges associated with smart water alternating CO2 (smart WAG-CO2) with regard to permeability impairment that occurs due to interaction between injection water-formation water and injection gas-water. For this purpose, static tests including compatibility and zeta potential measurement, and dynamic tests including water, dry and wet CO2 coreflooding experiments are conducted. The findings illustrate that formation scales due to mixing formation water and injection water are mixed-salt with SrSO4 as the dominant scale type. Despite the fact that zeta potential outcomes point up that an increase in sulfate concentration in the injection water advantageously leads to proper potential oil recovery, the coreflooding tests reveal that sulfate ion concentration is the main factor in controlling k/ki ratio; the more the sulfate concentration, the more the level of permeability impairment. During gas injection, evaporation phenomenon was observed, with the evaporation rate being function of core water saturation. In addition, as salinity increases, while evaporation rate is reduced, more salt precipitation occurs in the sample with higher salinity. The results exhibited that initial permeability is more effective than water salinity in changing gas effective permeability. The results of this study can aid the engineers working in upstream sections of petroleum industry to reduce the scale removal expenditures and enhance the smart WAG-CO2 technique.

The insignificant effect of increased sulfate concentration on nitrogen dynamics in eutrophic lakes: the neglected role of iron ions

Context Sulfate (SO42−) concentrations in eutrophic lakes are continuously increasing. Aims This study aimed to explore the effects on nitrogen and their limiting factors in eutrophic lakes. Methods We constructed a series of microcosms with different initial SO42− concentrations (0–180 mg L−1) to simulate the SO42− concentration in eutrophic lakes. Key results The NO3−-N concentration rapidly decreased in all treatments. The respective 2.21 and 2.77 mg L−1 concentrations of NO3−-N recorded on Day 5 in the initial 150 and 180 mg L−1 SO42− concentration treatments were higher than in other treatments. The highest N2O concentrations in all treatments showed insignificant differences. The existence of free Fe2+ that was released from iron reduction resulted in an insignificant effect of increased SO42− concentration on nitrogen dynamics. With a decrease in Fe2+, the acid volatile sulfate (AVS) increased, and the highest AVS concentrations were positively correlated with the initial SO42− concentration (7.20–12.88 mg kg−1). Conclusion This effect could be ascribed to the fact that the rapid combination of sulfide (∑S2−) and Fe2+ prevents ∑S2− from producing a toxic effect on the denitrification. Implications Our study highlighted the role of iron in eutrophic lakes, especially in the coupling of sulfate reduction and nitrogen dynamics.

Arsenate decreases production of methylmercury across increasing sulfate concentration amendments in freshwater lake sediments.

Arsenic (As) and sulfate pollution are often found co-occurring as a result of smelting metal ores. Previous studies showed that sulfate reducing microbes (SRMs) can use As(V) as a terminal electron acceptor, while others reported that SRMs are the main mercury (Hg) methylators in freshwater systems. However, we have yet to fully explore how As(V) can affect methylmercury (MeHg) production. In this study, we examined whether additions of As(V) and sulfate in freshwater sediments collected near a major gold mine with a history of S and As emissions affect Hg methylation. First, we show that Hg methylation in lake sediments was primarily limited by carbon substrate availability rather than by that of sulfate as terminal electron acceptors. Then, under conditions where carbon is not limiting, sulfate addition to the system significantly increased Hg methylation rate constants. Finally, we show that MeHg production rates in sediments significantly decreased with increasing As(V) concentrations, regardless of the sulfate concentration amended to sediments. This work underscores the apparent antagonistic effects of As(V) on the one hand, and carbon and sulfate on the other hand on the kinetics of Hg methylation. Arsenic controls on Hg methylation are complex and a combination of direct impact on the methylators' fitness, the formation of As-bearing mineral phases affecting Hg bioavailability, or changes in the microbial community structures over increasing As concentrations should be the focus of additional investigations.

Rapidly Increasing Sulfate Concentration: A Hidden Promoter of Eutrophication In Shallow Lakes

Rapidly Increasing Sulfate Concentration: A Hidden Promoter of Eutrophication In Shallow Lakes

Elevated particle acidity enhanced the sulfate formation during the COVID-19 pandemic in Zhengzhou, China

The significant reduction in PM2.5 mass concentration after the outbreak of COVID-19 provided a unique opportunity further to study the formation mechanism of secondary inorganic aerosols. Hourly data of chemical components in PM2.5, gaseous pollutants, and meteorological data were obtained from January 1 to 23, 2020 (pre-lockdown) and January 24 to February 17, 2020 (COVID-lockdown) in Zhengzhou, China. Sulfate, nitrate, and ammonium were the main components of PM2.5 during both the pre-lockdown and COVID-lockdown periods. Compared with the pre-lockdown period, even though the concentration and proportion of nitrate decreased, nitrate was the dominant component in PM2.5 during the COVID-lockdown period. Moreover, nitrate production was enhanced by the elevated O3 concentration, which was favorable for the homogeneous and hydrolysis nitrate formation despite the drastic decrease of NO2. The proportion of sulfate during the COVID-lockdown period was higher than that before. Aqueous-phase reactions of H2O2 and transition metal (TMI) catalyzed oxidations were the major pathways for sulfate formation. During the COVID-lockdown period, TMI-catalyzed oxidation became the dominant pathway for aqueous-phase sulfate formation because the elevated acidity favored the dissolution of TMI. Therefore, the enhanced TMI-catalyzed oxidation affected by the elevated particle acidity dominated the sulfate formation, resulting in the slight increase of sulfate concentration during the COVID-lockdown period in Zhengzhou.

How Might Changing Climate Limit Cyanobacteria Growth in Shallow Prairie Lakes? An Empirical Space-For-Time Evaluation of the Potential Role of Increasing Sulfate

Cyanobacteria blooms alter aquatic ecosystems and occur frequently in shallow prairie lakes, which are predicted to increase in salinity as the regional climate becomes hotter and drier. However, flat landscapes that experience depression bottom salinity with high concentrations of sulfate in addition to sodium and chloride, may mitigate nutrient increases or even inhibit cyanobacteria growth. Cyanobacteria can dominate shallow lakes with low N:P ratios because many cyanobacteria species fix dissolved N2, whether due to in-lake denitrification or exchange with the atmosphere, a process that requires molybdenum as an enzyme cofactor. Sulfate can compete with molybdate at cellular uptake sites, potentially limiting the competitive advantage of cyanobacteria. We studied 25 lakes located in a relatively limited geographic region of southern Alberta (Canada) and used a space-for-time analysis to model scenarios of increased sulfate concentrations under changing climate. Monthly, we measured nitrogen, phosphorus, sulfate, molybdenum, and cyanobacterial pigments and used mixed effects models to identify empirical relationships. Sulfate drives conductivity in the region and we found that most saline lakes we sampled are turbid lakes with high nutrients and high cyanobacteria biomass. In addition to phosphorus, molybdenum predicted cyanobacterial pigments in the top two models, showing a positive relationship with cyanobacterial biomass. We also found a negative relationship between sulfate concentration and cyanobacteria pigments, which suggests that as lakes get saltier, even with increased nutrients, there may not be an incremental increase in cyanobacteria biomass. Our results therefore suggest that competition between sulfate and molybdate may limit future cyanobacteria growth in shallow lakes and that with a warmer and drier climate it may not be inevitable that shallow lakes will continue to be dominated by cyanobacterial blooms, a hypothesis that could be tested directly via experimentation.

Effect of soluble gypsum on swell behaviour of lime-treated expansive soil – a micro-level investigation

ABSTRACT Lime treatment of soil is a widely adopted technique to improve its properties. However, the durability of lime-treated soil under varying environmental conditions such as contamination with chemical agents is still a matter of concern. An attempt has been made to examine the swelling behaviour of untreated and lime-treated expansive soil subjected to sulphate contamination (i.e. soluble gypsum). Series of the free swell index and one-dimensional oedometer swell tests have been performed in untreated and lime-treated soil inundating with soluble gypsum of varying sulphate concentration (i.e. low to high; 0 to 30000 ppm). The results of one-dimensional oedometer swell tests revealed that the sulphate contamination leads to the reduction in the maximum swelling percentage of untreated soil. On contrary, a drastic increase in the swelling percentage has been observed in the lime-treated soil subjected to contamination up to 16000 ppm, which diminishes thereafter with an increase in sulphate concentration. It is revealed that the sulphate contamination has observed promising effect on untreated soil; whereas a susceptible behaviour is pronounced in the lime-treated soil up to 16000 ppm sulphate concentration. Further, experimental outcomes are elucidated based on the detailed microanalyses (XRD, SEM, EDAX and FTIR) of samples.

Increasing sulfate concentration and sedimentary decaying cyanobacteria co-affect organic carbon mineralization in eutrophic lake sediments

Sulfate (SO42−) concentrations in eutrophic lakes are continuously increasing; however, the effect of increasing SO42− concentrations on organic carbon mineralization, especially the greenhouse gas emissions of sediments, remains unclear. Here, we constructed a series of microcosms with initial SO42− concentrations of 0, 30, 60, 90, 120, 150, and 180 mg/L to study the effects of increased SO42− concentrations, coupled with cyanobacterial blooms, on organic carbon mineralization in Lake Taihu. Cyanobacterial blooms promoted sulfate reduction and released a large amount of inorganic carbon. The SO42− concentrations in cyanobacteria treatments significantly decreased and eventually reached close to 0. As the initial SO42− concentration increased, the sulfate reduction rates significantly increased, with maximum values of 9.39, 9.44, 28.02, 30.89, 39.68, and 54.28 mg/L∙d for 30, 60, 90, 120, 150, and 180 mg/L SO42−, respectively. The total organic carbon content in sediments (51.16–52.70 g/kg) decreased with the initial SO42− concentration (R2 = 0.97), and the total inorganic carbon content in overlying water (159.97–182.73 mg/L) showed the opposite pattern (R2 = 0.91). The initial SO42− concentration was positively correlated with carbon dioxide (CO2) emissions (R2 = 0.68) and negatively correlated with methane (CH4) emissions (R2 = 0.96). The highest CO2 concentration and lowest CH4 concentration in the 180 mg/L SO42− treatment were 1688.78 and 1903 μmol/L, respectively. These biogeochemical processes were related to competition for organic carbon sources between sulfate reduction bacteria (SRB) and methane production archaea (MPA) in sediments. The abundance of SRB was positively correlated with the initial SO42− concentration and ranged from 6.65 × 107 to 2.98 × 108 copies/g; the abundance of MPA showed the opposite pattern and ranged from 1.99 × 108 to 3.35 × 108copies/g. These findings enhance our understanding of the effect of increasing SO42− concentrations on organic carbon mineralization and could enhance the accuracy of assessments of greenhouse gas emissions in eutrophic lakes.

Reliability assessment of concrete under external sulfate attack

The variability in characteristics of concrete material, such as pore size and chemical composition, has significant impact on the long-term performance of concrete structures under sulfate attack. This paper presents a reliability-based numerical model for assessing the long-term corrosion behaviour of concrete subject to sulfate attack. Using a reliability approach, the model can describe the reactive transport behaviour of sulfate ions and concrete degradation process. Specially, the variables in pore characteristics in interfacial transition zone (ITZ) and concrete chemical composition were quantified using a large amount of experimental data to determine the time-dependent reliability index (β) under long-term sulfate attack. The results show that the filling of ettringite and monosulfate after sulfate attack could modify the physical pore structure of ITZ at early stage. The time for concrete to reach the reliability index limit state shorten remarkably with the increase of sulfate concentration. For example, a four-fold increase of sulfate concentration (e.g. from 57 mol/m3 to 14.25 mol/m3) could shorten the time to reach the reliability index limit (i.e. β = 4.7) by 70 %. In addition, aggregate surface treatment especially with silica fume could improve the reliability of deteriorating concrete significantly by prolonging the time to reach the reliability index limit by 40 % in this study.

In Situ X-ray Diffraction at High Temperatures: Formation of Ca2SiO4 and Ternesite in Recycled Autoclaved Aerated Concrete

This study provides an insight into possible recycling processes for autoclaved aerated concrete (AAC) at low temperatures (<1000 °C). Belite binders were synthesized from wastes of AAC by the addition of CaCO3 and adaption of the molar CaO/SiO2 (C/S ratio) in the range of 2 to 2.5. An in situ XRD study performed during heating up to 1000 °C and subsequent quenching to room temperature evidenced the formation of ternesite besides C2S in sulfate-containing systems. Several factors influencing the reaction kinetics and the evolution of the phase composition were investigated thoroughly. Increased sulfate content and dwelling time during heating increase the ternesite content and promote the formation of highly crystalline α’H-C2S. The C/S-ratio of the starting materials has to be adjusted to the sulfate content in order to prevent the formation of ternesite at the expense of C2S. Ternesite remains stable during quenching to room temperature or even increases in amount, except in cases of very low cooling rates or high residual quartz contents (C/S-ratio ≤ 2). Temperature and range of α’H-C2S to β-C2S phase transition on quenching strongly depend on the cooling rate. The onset temperature for β-C2S formation varies between 540 °C (slow quench) and 450 °C (fast quench). Thermal expansion coefficients of ternesite are calculated showing similarity with C2S. The incorporation of CaSO4 modules in the structure switches the direction of maximum compression.

Investigating the importance of sub-grid particle formation in point source plumes over eastern China using IAP-AACM v1.0 with a sub-grid parameterization

Abstract. The influence of sub-grid particle formation (SGPF) in point source plumes on aerosol particles over eastern China was firstly illustrated by implementing an SGPF scheme into a global–regional nested chemical transport model with an aerosol microphysics module. The key parameter in the scheme was optimized based on the observations in eastern China. With the parameterization of SGPF, the spatial heterogeneity and diurnal variation in particle formation processes in sub-grid scale were well resolved. The SGPF scheme can significantly improve the model performance in simulating aerosol components and new particle formation processes at typical sites influenced by point sources. The comparison with observations at Beijing, Wuhan and Nanjing showed that the normal mean bias (NMB) of sulfate and ammonium could be reduced by 23 %–27 % and 12 %–14 %, respectively. When wind fields are well reproduced, the correlation of sulfate between simulation and observation can be increased by 0.13 in Nanjing. Considering the diurnal cycle of new particle formation, the SGPF scheme can greatly reduce the overestimation of particle number concentration in nucleation and Aitken mode at night caused by fixed-fraction parameterization of SGPF. On the regional scale, downwind areas of point source experienced an increase in sulfate concentration of 25 %–50 %. The results of this study indicate the significant effects of SGPF on aerosol particles over areas with the point source and the necessity of a reasonable representation of SGPF processes in chemical transport models.

Effectiveness of Oxygen-Saturated Seawater Injections and Air Sparging Technologies in Remediation of Coastal Marine Sediments from Sludge.

The occurrence of hypoxic muddy sediments on shallow beaches and other sheltered areas is a well-known environmental problem, which negatively affects coastal areas, tourism potential, the public use of beaches and sediment biodiversity. The usual solution is limited to dredging and removal of sludge to a landfill site. In this study, a laboratory-scale experiment was performed to determine the effectiveness of two technologies: a modification of air sparging and a new approach based on injecting oxygen-saturated seawater in hypoxic muddy sediments (oxygen-saturated seawater injections method), for remediating sludge in coastal sediments, minimizing environmental impact respect to dredging. Our results showed that both technologies significantly increased dissolved oxygen content in pore water, facilitating the oxidation of more than 90% of the organic matter, and other reduced inorganic compounds such as sulphide, with the consequent increase in sulphate concentration from 0.3 to 3.0 g·L-1. Moreover, a rise of redox potential from - 258 mV to above 200 mV, and a dramatic drop in chemical oxygen demand were also indicators that oxic conditions had been restored. After 65 days, soft, black, muddy and hypoxic sediment with high organic matter content and a characteristic foul odour was transformed into well-oxygenated sediment, which had a low organic matter content and had lost its initial shiny black colour and odour. The main difference between both technologies was the depth influenced by sediment remediation; oxygen-saturated seawater injections affected deeper areas than clean pressurized air injections.

Selenium uptake, volatilization, and transformation by the cyanobacterium Microcystis aeruginosa and post-treatment of Se-laden biomass

With a narrow margin between beneficial and toxic effects, selenium (Se) is of great concern due to its increasing level in aquatic environments. The accumulation and transformation of Se by the cyanobacterium Microcystis aeruginosa and effects of nutrients, particularly sulfate, were investigated. The nutrient-deprived cyanobacterium removed water-borne selenate (82.2 ± 0.93%) faster than selenite (58.9 ± 1.77%), with 86.0 ± 1.41% and 77.2 ± 1.00%, respectively, of the Se accumulated in the biomass and the rest volatilized. When supplied with excess nutrients, the Se accumulation and volatilization rates were significantly inhibited, with the removal efficiency dropping to 50.2 ± 2.59% and 7.37 ± 0.93% for selenite and selenate, respectively. When M. aeruginosa was tested with inadequate, appropriate, and adequate levels of sulfate, Se uptake decreased with increasing sulfate concentrations, particularly for selenate (from 34.1 to 4.81%). Using X-ray absorption near-edge structure to speciate biomass Se, selenite and selenate were transformed to organo-Se (87.3–100%), with or without nutrients present, suggesting M. aeruginosa could efficiently reduce Se oxyanions to more bioavailable forms. With increasing sulfate levels (5.0 and 10.0 mg S/L), percentages of SeMet converted from selenite decreased by 28.2–33.0%, with 19.1–33.2% as elemental Se, while organo-Se remained dominant (93.6–95.1%) in selenate-treated M. aeruginosa. Transmission electron microscopy shows structural damage in the cell wall at exposure to selenite (1600 μg Se/L), with the intracellular structure intact. To prevent Se biomagnification along aquatic food chains, the Se-laden biomass was combusted as a post-treatment, leading to a significant reduction in Se content (∼99.2%) and Se bioavailability, with inorganic Se (45.0–70.5%) predominant in the residue.

Carbon and sulfur isotope variations through the Upper Ordovician and Lower Silurian of South China linked to volcanism

Pronounced excursions of carbon and sulfur isotopes (δ13C and δ34S) spanning the Ordovician–Silurian (O–S) transition have been recognized globally, yet the causes of these isotope events and their causal relationships to environmental changes, especially during Early Silurian time, remain the topics of debate. In this study, we present organic carbon and pyrite sulfur isotope (δ13Corg and δ34Spyr) records and elemental concentrations from a drill core spanning from the Upper Ordovician (Katian) to Lower Silurian (Aeronian) succession of the Yangtze Platform, China. The newly presented δ13Corg profile records previously recognized positive excursions, including the Hirnantian isotopic carbon excursion (HICE), the middle Rhuddanian excursion, and the early Aeronian positive carbon isotope excursion (EACIE). In addition, two negative δ13Corg excursions are recognized during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages. The δ34Spyr profile exhibits a great amount of variation (−17.8‰ to +19.6‰) but mimics the δ13Corg record. Coupled positive δ13Corg and δ34Spyr excursions during the middle Hirnantian and late Rhuddanian to Aeronian are associated with decreased total organic carbon (TOC) content and less reducing water column conditions, whereas the negative δ13Corg and δ34Spyr excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian are associated with enhanced TOC values and the establishment of more strongly anoxic to euxinic conditions. The δ13C and δ34S excursions during the O–S transition, then, cannot be explained by changes in redox conditions or the burial of organic carbon and pyrite. The heavier and less variable middle Hirnantian and late Rhuddanian to Aeronian δ34Spyr values probably reflect low marine sulfate concentration. In contrast, negative Katian to early Hirnantian and late Hirnantian to middle Rhuddanian δ34Spyr excursions may reflect increased sulfate concentration. The two negative δ13Corg and δ34Spyr excursions associated with the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian correspond with episodes of volcanism. We therefore propose that volcanism was the main driver of coupled negative δ13Corg and δ34Spyr excursions. As volcanism diminished during the middle Hirnantian and late Rhuddanian to Aeronian, both δ13Corg and δ34Spyr returned to more positive values. Our results indicate that volcanism played a crucial role in triggering carbon and sulfur isotope perturbations and environmental changes during the Late Ordovician and Early Silurian.

Benzene degradation in contaminated aquifers: Enhancing natural attenuation by injecting nitrate

Natural attenuation processes depend on the availability of suitable electron acceptors. At the megasite Zeitz, concentrations of the main contaminant benzene were observed to increase constantly in the lower aquifer to levels of more than 2.5 mM. This was accompanied by decreasing concentrations of sulphate (SO42−), which has been previously shown to be the main electron acceptor for benzene oxidation at this site, resulting in an electron acceptor-limited, sulphidic benzene plume. Therefore, a field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO3−) into the sulphidic benzene plume aiming (i) to recycle sulphate by nitrate-dependent sulphide oxidation, and (ii) to serve as direct electron acceptor for benzene oxidation. Within 60 days, 6.74 tons sodium nitrate (NaNO3) were injected into the lower aquifer, and the resulting biogeochemical effects within the benzene plume were monitored for more than one year by chemical and microbiological analyses of groundwater samples taken from various depths of ten monitoring wells located in three observation lines downstream of nitrate injection. Nitrate was microbiologically consumed, as shown by changes in δ15N-NO3− and δ18O-NO3− values, partial nitrite accumulation, and changing ratios of Na+/NO3−. Main electron donors for nitrate reduction were reduced sulphur compounds, verified by changing δ34S-SO42− and δ18O-SO42− values, partially increasing sulphate concentrations, and strongly increasing abundances of typical sulphur-oxidizing, nitrate-reducing bacterial taxa within the nitrate plume. The general absent hydrogen isotope fractionation of benzene, also in the sulphidic, nitrate-free part of the plume, indicates that benzene was not biodegraded by sulphate-reducing consortia. However, detected small carbon isotope fractionation of benzene points to in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulphate in the sulphidic, sulphate-depleted benzene plume due to microbial oxidation of reduced sulphur species, leading to presumably favored conditions for in situ benzene biodegradation.

Pitting Corrosion Probability of Copper in Solutions Containing Chloride and Sulfate Using Coupled Multielectrode Arrays

The preferred disposal method for used nuclear fuel is to seal it in containers emplaced in a deep geologic repository (DGR) in a suitable rock formation. In Canada, the container is a robust steel vessel coated with 3 mm of Cu as a corrosion barrier. Eventually, the DGR environment will become anoxic, and in the absence of oxygen copper is thermodynamically stable in water. However, since exposure conditions will initially be oxidizing due to the presence of trapped O2 upon sealing the DGR, there is a possibility of localized corrosion to occur. Mass balance calculations show that uniform corrosion during this period will be within the designed corrosion allowance, however, since the Cu coating is relatively thin and the required lifetime is long, the risk of pitting corrosion during this period must be carefully evaluated.Pitting corrosion susceptibility is determined by the corrosion potential of the material (Ecorr, the potential at which the rates of all anodic and cathodic reactions are equal), and the breakdown potential (Eb, the potential at which the copper oxidation rate on the surface increases abruptly due to the breakdown of the protective film), both of which are distributed parameters. This is due to the stochastic nature of passive film rupture and any uncontrollable variations in the structure and local environment at the metal surface. It is important to note that the analysis is based on the concept that pitting is only possible if the Ecorr is equal to, or more positive than the Eb.Our previous studies showed a passive behavior of copper surface when exposed to high pH solutions. In this work, a multielectrode array was used to obtain a statistically meaningful set of potential measurements through the simultaneous monitoring of 30 electrodes to determine their corrosion and breakdown potentials. From the extensive database that acquired, the Ecorr and Eb distributions were defined for the copper electrodes exposed to solutions containing various chloride concentrations and a more thorough evaluation of the relative values of Ecorr and Eb has undertaken to determine the susceptibility to pitting.Experiments were conducted using a multielectrode array in chloride and sulfate containing solutions at 25 °C and pH 8 and 11. In alkaline solutions, the copper oxide is more stable and can potentially form a passive film, whereas chloride and sulfate promote the breakdown of the passive film. As the concentration of aggressive anions increased, the values of Ecorr and Eb decreased, suggesting that the solubility of copper increased with in the presence of both chloride and sulfate. However, there is a critical sulfate concentration (~5 × 10-3 M) that above which the pitting probability of copper decreased.In general, the possibility of pitting increased with growing overlap between the distribution curves of Ecorr and Eb. To evaluate the possibility of pitting, histograms and distribution curves of Ecorr and Eb in the solutions that contain different chloride and sulfate concentrations were compared. In lower chloride concentrations, there was a very small overlap between distributions of Ecorr and Eb values; therefore, the possibility of pitting under these conditions is very low. On the other hand, a high chloride concentration at high pH indicated a higher probability of pitting, as the overlap between Ecorr and Eb distributions increased. Also, the extent of the passive zone decreased with increasing chloride concentration. In solutions containing sulfate content, the overlap between Ecorr and Eb increased with increasing sulfate concentration up to a certain content (~5 × 10-3 M), after this point the overlap decreased as the sulfate concentration increased. The results from potentiodynamic experiments indicated that increasing the chloride concentration resulted in decreased the extent of the passive zone; however, the sulfate did not seem to play a significant role in changing the extent of the passive zone. Figure 1