Decrease In Redox Potential Research Articles

Shelf Life of a Mixture of Pumpkin Puree (<em>Cucurbita moschata</em>) During Storage at 40xB0C

Pumpkin is used commonly as an ingredient for many food products because of its nutritional and health benefits. Current studies have been reported about pumpkin puree, but there is no information related to the mixture of flours and other ingredients. The development of a new formulation introduces into the market a different way of commercialization and consumption of pumpkin, favoring its production. The aim of this investigation was to determine the shelf life of the mixture by analyzing the physical-chemical and microbiological parameters of the product using a factorial experimental design of two factors, batches and time, with 3 and 13 levels respectively. The three levels of batches had 26 trays each one and processed with pumpkin puree, wheat flour, sugar, pasteurized egg, cinnamon, salt and butter. Potassium sorbate was added as a preservative agent, gum as a stabilizer and ascorbic acid as an acidulant agent. The mixture was packaged in polypropylene-polyethylene trays and stored at 4°C during 13 weeks. Two samples were taken weekly of each batch for physicochemical and microbial population analysis. The maximum charge of microorganisms reached was 4.07 Log CFU/g for aerobic mesophilic bacteria, 2.92 Log CFU/g for yeasts after day 28 and 1.48 Log CFU/g for molds after day 49. Coliforms were not found. It was observed a decrease in pH, water activity and redox potential. The results demonstrated that the shelf life of the product could be guaranteed up to day 35 when stored at 4°C.

Structural, electrical and electrochemical studies on doubly doped LiMn2−x(GdNi)xO4 cathode materials for Li-ion batteries

Doubly doped LiMn2−x(GdNi)xO4 (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) materials with spinel structure have been synthesized by sol-gel method. Powder X-ray diffraction results have confirmed the formation of cubic spinel structure. FE-SEM studies showed that cubic spinel structure remained unchanged with notably modified particle size in the range of 200–400 nm. Cyclic Voltammetry results showed enhancement in Li+ intercalation and de-intercalation through decrease in redox potentials as a function of doping. Electrochemical Impedance Spectroscopy (EIS) results show decreased charge transfer resistance with addition of Gd and Ni. Charge-discharge studies for LiMn1.98(GdNi)0.01O4 show 67.1 mAh/g discharge capacity over other doped derivatives and pristine LiMn2O4 (60.6 mAh/g) in aqueous Li2SO4 electrolyte.

Impacts of borehole heat exchangers (BHEs) on groundwater quality: the role of heat-carrier fluid and borehole grouting

This study investigated the role of two technical features of borehole heat exchangers (BHEs) in causing impacts on groundwater quality: the use of propylene glycol (PG) and other additives in heat-carrier fluids and the BHE grouting. The PG is the most diffused antifreeze compound used in heat-carrier fluids circulating into the ground loops. Laboratory-scale experiments were performed to simulate the release of PG into soil–water microcosms. PG itself is not toxic, but it is a potential organic nutrient able to stimulate the micro-organisms’ metabolism, thus resulting in O2 consumption and subsequent decrease in redox potential of the system. Indeed, a leaching of the main heavy metals was observed: Fe, Mn and Ni ions reached 102–104 μg l−1 concentrations after 60 days. Secondly, the role of BHE grouting in changing the hydraulic conditions of aquifers was investigated because defects and voids in the grout may increase the hydraulic conductivity along the boreholes, leading to inter-aquifer flux. A field-scale experiment was performed by grouting a 7-m-long pilot BHE, and many defects and void spaces were detected along the BHE. A simplified analytical model was thus applied to detect the magnitude of the inter-aquifer flux. Basing the calculations on typical hydraulic parameters of the alluvial aquifer of Piedmont Po Plain (NW Italy), the downward flux is up to 2.65 cubic metres per day in the worst scenarios with high head difference and thin separation layer between two superposed aquifers.

Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems.

Artificial photosynthetic constructs can in principle operate more efficiently than natural photosynthesis because they can be rationally designed to optimize solar energy conversion for meeting human demands rather than the multiple needs of an organism competing for growth and reproduction in a complex ecosystem. The artificial photosynthetic constructs described in this Account consist primarily of covalently linked synthetic chromophores, electron donors and acceptors, and proton donors and acceptors that carry out the light absorption, electron transfer, and proton-coupled electron transfer (PCET) processes characteristic of photosynthetic cells. PCET is the movement of an electron from one site to another accompanied by proton transfer. PCET and the transport of protons over tens of angstroms are important in all living cells because they are a fundamental link between redox processes and the establishment of transmembrane gradients of proton electrochemical potential, known as proton-motive force (PMF), which is the unifying concept in bioenergetics. We have chosen a benzimidazole phenol (BIP) system as a platform for the study of PCET because with appropriate substitutions it is possible to design assemblies in which one or multiple proton transfers can accompany oxidation of the phenol. In BIP, oxidation of the phenol increases its acidity by more than ten pKa units; thus, electrochemical oxidation of the phenol is associated with a proton transfer to the imidazole. This is an example of a PCET process involving transfer of one electron and one proton, known as electron-proton transfer (EPT). When the benzimidazole moiety of BIP is substituted at the 4-position with good proton acceptor groups such as aliphatic amines, experimental and theoretical results indicate that two proton transfers occur upon one-electron oxidation of the phenol. This phenomenon is described as a one-electron-two-proton transfer (E2PT) process and results in translocation of protons over ∼7 Å via a Grotthuss-type mechanism, where the protons traverse a network of internally H-bonded sites. In the case of the E2TP process involving BIP analogues with amino group substituents, the thermodynamic price paid in redox potential to move a proton to the final proton acceptor is ∼300 mV. In this example, the decrease in redox potential limits the oxidizing power of the resulting phenoxyl radical. Thus, unlike the biological counterpart, the artificial construct is thermodynamically incapable of effectively advancing the redox state of a water oxidation catalyst. The design of systems where multiple proton transfer events are coupled to an oxidation reaction while a relatively high redox potential is maintained remains an outstanding challenge. The ability to control proton transfer and activity at defined distances and times is key to achieving proton management in the vicinity of catalysts operating at low overpotential in myriad biochemically important processes. Artificial photosynthetic constructs with well-defined structures, such as the ones described in this Account, can provide the means for discovering design principles upon which efficient redox catalysts for electrolysis and fuel cells can be based.

Catalytic effect of pyrite on the leaching of arsenopyrite in sulfuric acid and acid culture medium

In this study, the influence of pyrite on the leaching of arsenopyrite in sulfuric acid and iron-free 9K medium was investigated by leaching experiments and electrochemical analysis. During acid leaching, the addition of pyrite resulted in the further oxidation of arsenopyrite, for the reason that the galvanic interaction made the pH increase and redox potential decrease, and then the formation of arsenate precipitates with high crystallinity was depressed. While leaching in iron-free 9K medium, the buffer action keeping the pH stable promoted arsenopyrite dissolution as compared with acid leaching. And the addition of pyrite facilitated the arsenic extraction, but increasing the amount of pyrite added had no distinct impact on the further leaching of arsenopyrite. Electrochemical studies revealed that the presence of pyrite increased the conductivity of electrodes and electrolytes, verifying the catalytic effect of galvanic interaction on arsenopyrite leaching. Additionally, in the sulfuric acid electrolyte, the addition of pyrite also decreased the thickness of passivation layer on the arsenopyrite surface, confirming that the formation of ferric arsenate precipitates with high crystallinity was inhibited in the presence of pyrite. Therefore, the pyrite-promoted arsenopyrite oxidation in sulfuric acid was due to the galvanic effect and the control of pH and redox potential, whereas that in culture medium was only associated with the galvanic effect. Moreover, the electrochemical measurements also showed that the oxidation of elemental sulfur was the rate controlling step for arsenopyrite dissolution.

Changes in soil redox potential in response to flood irrigation with waste water in central Mexico

SummaryIrrigation with untreated sewage water adds fresh organic matter to the soil. When it is applied by flooding, as in the Mezquital Valley of Mexico, many of the pores in the soil become temporarily waterlogged and depleted of oxygen, and reduction generates nitrous oxide (N2O) and methane (CH4). We monitored the redox potential, Eh, in the soil at two sites in the Mezquital Valley to discover whether the short‐term gaseous emissions matched the changes in Eh. One site is irrigated periodically by flooding with waste water and has alfalfa, rye grass and maize grown in succession; the other site grows maize with water from summer rain only. Each electrode buried in the soil for the purpose produced sequences of measurements that were correlated in time. We modelled the correlation and took it into account to analyse the effects of the irrigation by residual maximum likelihood (reml). After each flooding, the redox potential under alfalfa and rye grass decreased by 150–200 mV from a norm of about 450 mV for 2 days, after which it returned to its norm. The short‐term response to flooding under maize was similar, but the redox potential did not recover completely; instead there was a decrease from one irrigation event to another, in particular as a result of heavy rain in September that saturated the soil. The soil under rain‐fed maize was slower to respond, partly, we believe, because infiltration into the less aggregated clay soil and drainage from it were also slow. Irrigation with untreated sewage water caused a sharp decrease in Eh lasting 1–2 days. It seems to have depleted the soil of oxygen, causing reduction in microhabitats rich in carbon and nitrogen and generation of N2O, which was captured in the closed chambers that we had installed.Highlights Land in the Mezquital Valley of Mexico emits much greenhouse gas when irrigated with sewage water. The soil's redox potential was monitored in situ as crops were irrigated or depended solely on rain. Each irrigation caused a decrease in the soil's redox potential of 150–200 mV for 2 days. The decrease in redox potential corresponded in time with emission of nitrous oxide.

Experimental modeling of Au and Pt coupled transport by chloride hydrothermal fluids at 350–450°C and 500–1000 bar

The coupled solubility of Au(cr) and Pt(cr) has been measured in acidic chloride solutions at 350–450°С and 0.5 and 1 kb using the autoclave technique with determination of dissolved metal contents after quenching. The constants of the reaction combining the dominant species of Au and Pt in high-temperature hydrothermal fluids (K(Au–Pt)) have been determined: 2 Au(cr) + PtCl42- = Pt(cr) + 2AuCl2-; log K(Au–Pt) =–1.02 ± 0.25 (450°С, 1 kb), 0.09 ± 0.15 (450°С, 0.5 kb), and –1.31 ± 0.20 (350°С, 1 kb). It has been established that the factors affecting the Au/Pt concentration ratio in hydrothermal fluids and precipitated ores are temperature, pressure, redox potential, and sulfur fugacity. An increase in temperature results in an increase in the Au/Pt concentration ratio (up to ~550°С at P = 1 kb). A decrease in pressure and redox potential leads to enrichment of fluid in Au. An increase in sulfur fugacity in the stability field of Pt sulfides results in increase in the Au/Pt concentration ratio. Native platinum is replaced by sulfide mineral in low-temperature systems enriched in Pt (relative to Au).

Bioleaching of arsenopyrite by mixed cultures of iron-oxidizing and sulfur-oxidizing microorganisms

Arsenic is a critical environmental pollutant associated with acid mine drainage. Arsenopyrite is one of the major arsenic sulfide minerals whose weathering lead to the contamination of arsenic. In this study, the leaching behaviors of arsenopyrite by two mixed cultures of iron-oxidizing and sulfur-oxidizing microorganisms (Ferroplasma thermophilum and Acidithiobacillus caldus, Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus) were investigated, accompanying with community structure analysis of free microorganisms. The ratio of F. thermophilum to A. caldus of 1/1 showed a more favorable effect on the arsenic leaching than other ratios, and F. thermophilum played a dominant role in the solution all the leaching time. While adding A. caldus in the S. thermosulfidooxidans bioleaching system, the dissolution of arsenopyrite was suppressed. Notably, when the ratio of S. thermosulfidooxidans to A. caldus was 2/1, the arsenic extraction was accelerated at the early stage, but later it slowed down. The reason was because A. caldus was the predominant species at the later stage which made the redox potential decrease faster. XRD demonstrated that the proper addition of A. caldus could eliminate the sulfur passivation and promote the leaching in a degree. These studies are helpful to evaluate the environmental impact of arsenic.

Decreasing redox voltage of terephthalate-based electrode material for Li-ion battery using substituent effect

The preparation and assessment versus lithium of a functionalized terephthalate-based as a potential new negative electrode material for Li-ion battery is presented. Inspired from molecular modelling, a decrease in redox potential is achieved through the symmetrical adjunction of electron-donating fragments (–CH3) on the aromatic ring. While the electrochemical activity of this organic material was maximized when used as nanocomposite and without any binder, the potential is furthermore lowered by 110 mV upon functionalization, consistently with predicted value gained from DFT calculations.

Fate and Transformation of CuO Nanoparticles in the Soil-Rice System during the Life Cycle of Rice Plants.

Agricultural soil is gradually becoming a primary sink for metal-based nanoparticles (MNPs). The uptake and accumulation of MNPs by crops may contaminate food chain and pose unexpected risks for human health. Here, we investigated the fate and transformation of CuO nanoparticles (NPs) in the soil-rice system during the rice lifecycle. The results show that at the maturation stage, 1000 mg/kg CuO NPs significantly decreased redox potential by 202.75 mV but enhanced electrical conductivity by 497.07 mS/cm compared to controls. Moreover, the bioavailability of highest CuO NPs in the soil was reduced by 69.84% along with the plant growth but then was significantly increased by 165% after drying-wetting cycles. Meanwhile, CuO and Cu combined with humic acid were transformed to Cu2S and Cu associated with goethite by X-ray absorption near edge structure analysis. Additionally, CuO NPs had an acute negative effect on the plant growth than bulk particles, which dramatically reduced the fresh weight of grains to 6.51% of controls. Notably, CuO NPs were found to be translocated from soil to plant especially to the chaff and promoted the Cu accumulation in the aleurone layer of rice using micro X-ray fluorescence technique, but could not reach the polished rice.

Isotopic variation of dissolved and colloidal iron and copper in a carbonatic floodplain soil after experimental flooding

Many floodplain soils worldwide are contaminated by present and past industrial and mining activities. During flooding redox potential decreases, triggering the release of dissolved and colloidal metals. We used an anaerobic microcosm incubation to simulate flooding of a carbonate-rich floodplain soil for 40days. Soil solution samples were extracted to determine the release of dissolved (<0.02μm) and colloidal fractions (0.02–10μm). We analyzed stable isotope ratios of colloidal and dissolved Fe and Cu representing two groups of metals with different release behavior; release of Fe was steadily increasing, while that of Cu peaked sharply after flooding and decreased afterwards. The temporal trend of δ56Fe values of total Fe in solution indicated dissimilatory iron reduction. The apparent isotopic fractionation between dissolved and colloidal Fe (Δ56Fedissolved-colloidal=δ56Fedissolved−δ56Fecolloidal) varied from 0.31±0.04‰ to −1.86±0.26‰. Low δ56Fecolloidal (−1.16±0.04‰) values on day 4 of the experiment suggested colloid formation by precipitation of dissolved Fe, while the strong temporal variation in Δ56Fedissolved-colloidal indicated subsequent changes in colloid mineralogy, sorption to soil components and/or electron transfer-atom exchange. The variations in δ65Cu values (Δ65Cudissolved-colloidal from 0.81±0.03‰ to 1.58±0.09‰) are probably linked to the changing oxidation state of colloidal Cu. While at the beginning of the experiment colloidal Cu and solid soil Cu exchange, these systems decouple after the onset of sulfate reduction in the second half of the experiment. The experimental results fit well to findings from redoximorphic soils described in the literature and highlight the importance of colloids for metal release and the isotopic pattern in carbonatic soils.

Physico-Chemical and Microbiological Assessment of Organic Pollution in Plain Salty Lakes from Protected Regions

One of the most important goals of EU nature and water framework directives is to ensure healthy aquatic ecosystems by the protection of the most valuable species and habitats form the Natura 2000 network, while at the same time ensuring a balance between water/nature protection and the sustainable use of nature’s natural resources. The purpose of this study was to evaluate the physico-chemical and microbiological characteristics of four Romanian salty plain lakes included in Natura 2000 Network, in order to assess the degree of organic pollution and to generate the knowledge required for the design and implementation of appropriate measures for maintaining the balance between the water protection and the sustainable use of these protected ecosystems. The water and sediment sampling was performed in two consecutive years (2015 and 2016), in September and the following standard parameters have been determined: pH, chemical oxygen consumption (COC), the degree of trophicity and salinity of the environment, metals content, microbiological indicators and microbial physiological groups involved in nutrient cycling. The pH ranged from 7.56 to 8.93, close or above the upper normal limit of 8.5, being correlated with a high salinity characteristic of chlorinated, sulphated, high sodium and magnesium content waters. Despite the similar values recorded for the physico-chemical parameters in the two consecutive years suggesting a certain degree of stability of the investigated aquatic ecosystems, the COC values indicate a high degree of hypertrophy, which could be attributed to the reduced surface area, ecological pisciculture and agriculture input. The microbiological parameters revealed the existence of both recent and chronic fecal pollution source. The high hypertrophy degree could represent a positive premise for the high productivity of the investigated ecosystems, but also an alarm signal for excessive organic pollution, with the risk of redox potential decrease which can affect the fish and other life forms. Consequently, it is necessary to identify the sources of pollution and implement the appropriate measures to minimize the negative impact of organic contamination on the status of the respective ecosystems (water quality, biotic components) in order to maintain the health of both ecosystems and the surrounding human communities, allowing at the same time a sustainable use of the local resources.

Quality changes in refrigerated stored minced pork wrapped with plastic cling film and the effect of glucose supplementation

Meat spoilage greatly depends on meat composition and storage conditions. Microbial and biochemical changes in minced pork (100-g portions) wrapped with a polyvinyl chloride film during a 4-day refrigerated storage were studied. As glucose is the first substrate used by spoilage bacteria and when it is depleted bacteria could generate undesirable volatiles, the effect of the addition of glucose to minced meat was also studied. Three treatments were used: control (C), without added glucose, and low and high glucose concentration (L and H), 150mg and 750mg of glucose in 100g of meat, respectively. Spoilage bacteria, pH, redox potential, colour, basic volatile nitrogen, glucose, organic acids, and volatiles were analyzed in both recently prepared and stored pork samples. Storage resulted in increased levels of lactic acid bacteria and glucose-derived short chain alkyl volatiles, and a decrease in redox potential and volatile aldehyde levels. The addition of glucose to meat did not affect the biochemical characteristics of stored minced pork.

Enhancement of Biogas Production by Co-Digestion of Maize Silage with Common Goldenrod Rich in Biologically Active Compounds

This paper analyses the suitability of common goldenrod plants as mono- and co-substrates for biogas production. Furthermore, the role of bioactive compounds included in the biomass of this plant species was investigated. The results showed that the common goldenrod species produced lower biogas and methane yields than maize silage. However, the methane fermentation of their mixture resulted in approximately 9.5% higher biogas yield and 16.6% higher methane yield compared to the theoretical yields estimated based on two mono-digestions. A statistically significant increase in biogas production efficiency resulted from more favorable C/N ratio and the influence of bioactive compounds contained in common goldenrod. The addition of goldenrod crude extract caused an approximately 30% increase in the biogas yield of maize silage. This effect may be associated with a positive impact of biologically active substances on microorganisms or with a decrease in redox potential of the fermenting mass.

Poly(3-octylthiophene) nanoparticles for turn-on fluorescent sensor

A novel concept of optical fluorescent sensing using conducting polymer nanostructures poly(3-octylthiophene) as an optical receptor is proposed. A conducting polymer was encapsulated within polymeric nanospheres obtained from poly(maleic anhydride-alt-1-octadecene) in a simple procedure, resulting in formation of core-shell nanostructures. In the presence of carboxyl groups at the outer shell of nanospheres the conducting polymer undergoes spontaneous oxidation (e.g. by oxygen dissolved in the solution), leading to decrease of native fluorescence of poly(3-octylthiophene). Thus obtained core-shell nanospheres are sensitive to changes of the solution redox potential, due to dependence of poly(3-octylthiophene) emission spectra on its oxidation state. The decrease in redox potential is accompanied with increase of emission intensity of the conducting polymer core, which allows application of nanospheres as turn-on probe for reducing agents. The sensitivity and selectivity of obtained nanoprobes can be fine-tuned taking the advantage of the presence of charged groups in the shell layer of the nanostructures obtained. As a model analyte, ascorbic acid was chosen. A linear response was recorded, with increase of emission intensity for increasing ascorbic acid concentration, within the range from 10−6 to 0.1M.

Biogeochemical Importance of the Bacterial Community in Uranium Waste Deposited at Key Lake, Northern Saskatchewan

ABSTRACTThe long-term stability of immobilized elements of concern in uranium tailings deposited in the Deilmann Tailings Management Facility (DTMF), northern Saskatchewan, is dependent upon maintenance of highly oxic conditions within the tailings mass. The main objective of this study was to investigate the effect of stimulating microbial activity on the redox potential and state of ferrihydrite, which are considered to be the primary controlling condition and mineral phase, respectively, within the tailings. To determine the potential for biologically mediated decreases in redox potential and ferrihydrite reduction, a series of microcosm assays were performed. Non-sterile material from the tailings–water interface of the DTMF site was inoculated with indigenous flora previously isolated from the tailings material and enriched with a carbon source (50 ppm trypticase soy broth) and incubated under continuous-flow or intermittent-flow conditions, and compared with an uninoculated, no-carbon control that received continuous flow. Highly reducing conditions with redox potentials of less than −300 mV were detected after 2 days of incubation within the carbon-enriched tailings of microcosms receiving continuous flow, and less than −280 mV after 11 days of incubation within carbon-enriched tailings in microcosms receiving intermittent flow. The lowest recorded Eh value (−545 mV) was recorded after 14 days in a carbon-enriched microcosm receiving intermittent flow. In contrast, the redox conditions in the control microcosm never dropped below −93 mV; thus, it was clear that microbial activity and available carbon drove the Eh conditions to become highly reducing. The occurrence of low redox conditions was concomitant with the bulk chemical detection of Fe (II) in the effluent of treated microcosms. Sites of microbial ferrihydrite reduction were also detected using scanning transmission X-ray microscopy where Fe (II) species were observed in close proximity with bacterial cells. Analysis of the microbial diversity present within the microcosms confirmed that microbes indigenous to the DTMF system have the potential to generate conditions suitable for the proliferation of sulfate and iron reducing bacteria, such as Desulfosporosinus, which was detected by high-throughput 16S rRNA gene sequencing.

Remediation of antimony-rich mine waters: Assessment of antimony removal and shifts in the microbial community of an onsite field-scale bioreactor

An on-site field-scale bioreactor for passive treatment of antimony (Sb) contamination was installed downstream of an active Sb mine in Southwest China, and operated for one year (including a six month monitoring period). This bioreactor consisted of five treatment units, including one pre-aerobic cell, two aerobic cells, and two microaerobic cells. With the aerobic cells inoculated with indigenous mine water microflora, the bioreactor removed more than 90% of total soluble Sb and 80% of soluble antimonite (Sb(III)). An increase in pH and decrease of oxidation-reduction potential (Eh) was also observed along the flow direction. High-throughput sequencing of the small subunit ribosomal RNA (SSU rRNA) gene variable (V4) region revealed that taxonomically diverse microbial communities developed in the bioreactor. Metal (loid)-oxidizing bacteria including Ferrovum, Thiomonas, Gallionella, and Leptospirillum, were highly enriched in the bioreactor cells where the highest total Sb and Sb(III) removal occurred. Canonical correspondence analysis (CCA) indicated that a suite of in situ physicochemical parameters including pH and Eh were substantially correlated with the overall microbial communities. Based on an UPGMA (Unweighted Pair Group Method with Arithmetic Mean) tree and PCoA (Principal Coordinates Analysis), the microbial composition of each cell was distinct, indicating these in situ physicochemical parameters had an effect in shaping the indigenous microbial communities. Overall, this study was the first to employ a field-scale bioreactor to treat Sb-rich mine water onsite and, moreover, the findings suggest the feasibility of the bioreactor in removing elevated Sb from mine waters.

Near shore groundwater acidification during and after a hydrological drought in the Lower Lakes, South Australia

An extreme hydrological drought in the Lower Lakes of the Murray–Darling Basin (Ramsar listed site) resulted in exposure of large areas of lake bed (25% of pre-drought lake area), containing the reduced iron (Fe) sulfide mineral pyrite. The pyrite oxidised and the resulting acidification (pH<4) posed risks of acid and metals entering shallow groundwater and potentially discharging to the remaining lake water body. Piezometer transects were installed at four locations and monitoring of the groundwater levels and quality was undertaken for six years from 2009 (drought) to 2014 (4years post-reinundation). Acidic (pH3–5) groundwater was recorded at three of the four piezometer locations and included sites close to the lake water. The acidic groundwater (0.5–2m below lake bed) at these sites is likely to have originated from the transport of acid from the upper oxidised sediment layer formed during the drought. High soluble metal (Fe, Al, Mn) levels were also recorded at acidic locations. Acidic shallow groundwater has persisted at many sites for over 4years following reinundation post-drought, and is likely due to slow diffusion and limited sulfate reduction. Increases in dissolved Fe and Mn with decreases in redox potential suggest that reductive dissolution of Fe and Mn hydrous oxides and Fe oxy-hydroxysulfate minerals (e.g. jarosite) occurred post-drought. Groundwater hydraulic head gradients were low, indicating there was limited potential for groundwater to discharge to the lake. The hydraulic gradients at all locations were dynamic with complex relationships along the near-shore environment. The results highlight the long lasting and severe effects on groundwater that can occur following hydrological drought in aquatic environments with sulfidic sediments.

Electrochemical and spectroscopic studies of pyrite–cyanide interactions in relation to the depression of pyrite flotation

Cyanide is the most commonly used depressant to reject pyrite as a gangue mineral at alkaline pH in differential flotation. However, the depression mechanism of cyanide still remains uncertain. The depression effect has been proposed as the preferential adsorption of cyano species on pyrite, which has, however, not been confirmed experimentally. It is also believed that cyanide inhibits the chemisorption of thiol collectors due to the decrease of surface redox potential. In order to clarify the controversial depression mechanisms, the interactions between pyrite and cyanide at pH 10 were investigated in this study. Preferential adsorption of cyanide on pyrite rather than the decrease of surface redox potential was determined to be the depression contributor. Surface cyanoferric species were identified, both electrochemically and spectroscopically, on pyrite surface in the presence of cyanide, inhibiting the oxidation of pyrite and the oxidation/chemisorption of xanthate. This study provides new insights into the depression of pyrite flotation by cyanide.

Transcriptome analysis of Lactococcus lactis subsp. lactis during milk acidification as affected by dissolved oxygen and the redox potential

Performance of Lactococcus lactis as a starter culture in dairy fermentations depends on the levels of dissolved oxygen and the redox state of milk. In this study the microarray analysis was used to investigate the global gene expression of L. lactis subsp. lactis DSM20481T during milk acidification as affected by oxygen depletion and the decrease of redox potential. Fermentations were carried out at different initial levels of dissolved oxygen (dO2) obtained by milk sparging with oxygen (high dO2, 63%) or nitrogen (low dO2, 6%). Bacterial exposure to high initial oxygen resulted in overexpression of genes involved in detoxification of reactive oxygen species (ROS), oxidation–reduction processes, biosynthesis of trehalose and down-regulation of genes involved in purine nucleotide biosynthesis, indicating that several factors, among them trehalose and GTP, were implicated in bacterial adaptation to oxidative stress. Generally, transcriptional changes were more pronounced during fermentation of oxygen sparged milk. Genes up-regulated in response to oxygen depletion were implicated in biosynthesis and transport of pyrimidine nucleotides, branched chain amino acids and in arginine catabolic pathways; whereas genes involved in salvage of nucleotides and cysteine pathways were repressed. Expression pattern of genes involved in pyruvate metabolism indicated shifts towards mixed acid fermentation after oxygen depletion with production of specific end-products, depending on milk treatment. Differential expression of genes, involved in amino acid and pyruvate pathways, suggested that initial oxygen might influence the release of flavor compounds and, thereby, flavor development in dairy fermentations. The knowledge of molecular responses involved in adaptation of L. lactis to the shifts of redox state and pH during milk fermentations is important for the dairy industry to ensure better control of cheese production.