CO2-free Conditions Research Articles

Natural and synthetic plagioclases: Surface charge characterization and sorption of trivalent lanthanides (Eu) and actinides (Am, Cm)

The environmental fate of radiotoxic actinides may be controlled by their interactions with feldspars. Here, the sorption of trivalent minor actinides (Am, Cm) and their rare earth analog Eu onto synthetic pure Ca-feldspar (anorthite) and natural plagioclases of different Ca contents is investigated, covering ranges of [M3+] (52 nM–10 μM), solid-liquid ratios (1–3 g/L), pH (3–9), and ionic strengths (0.01–0.1 M NaCl) under both ambient and CO2-free conditions. As a first step to understand the uptake behavior of the heavy metals, the hitherto unknown surface charge and (de-)protonation reaction of Ca-feldspars is characterized. The zeta potential shows an unusual increase and charge reversal between pH 4 and 7 , which becomes more pronounced with increasing amounts of Ca in the crystal lattice and is likely connected to adsorption and/or surface precipitation of dissolved Al3+. Streaming potential measurements yield (de)protonation constants for anorthite surface sites of log K- = −6.94 ± 0.38 and log K+ = +6.84 ± 0.38. Batch sorption data shows strong immobilization of M3+ by plagioclases at mildly acidic and basic pH. Time-resolved laser fluorescence spectroscopy using Cm indicates the formation of an inner-sphere complex and its two hydrolyzed forms. The complex reactivity of dissolved Al3+ at the plagioclase-water interface severely complicated the development of a surface complexation model, emphasizing the need for additional research in this area. Our study highlights the importance of molecular-level studies to understand surface reactions and uncover unknown processes, which may have significant impact on the transport of (radiotoxic) contaminants in the geosphere.

A Methodology to Estimate Net Proton: Phosphorus Co-Adsorption Ratios for Acidic Soils

Despite extensive research, the behaviour of the key nutrient element, phosphorus (P), in soil is not yet fully understood. This study focussed on the outstanding issue of the co-adsorption of protons (H+) and P by soils. We developed a congruent set of measures to determine the net H+:P co-adsorption ratio and tested it on goethite, for which a ratio of 1.6:1 had been estimated under CO2-free conditions for additions of NaH2PO4. Under our conditions, and using additions of KH2PO4, the net H+:P co-adsorption ratio was estimated to be 1.44:1, i.e., in passable agreement with the published value. Application of the protocol to acidic soils resulted in a net H+:P co-adsorption ratio of 1.92:1, and substitution of H3PO4 for KH2PO4 gave a ratio of 1.96:1. These ratios for soils differ significantly from that for goethite. The soils for which we estimated net H+:P co-adsorption ratios had a wide range of properties and two had received previous applications of P fertiliser (Ca(H2PO4)2), which does not appear to have affected the net H+:P co-adsorption ratios. The H+:P co-adsorption ratio method could benefit from refinement, and further study is required to explore how these findings may apply to commercial P fertilisers under field conditions.

Spectrophotometric Determination of the First Hydrolysis Constant of Praseodymium (III)

The behavior of the trivalent ion praseodymium in 2M of NaCl at 303 K and in CO2 free conditions, was studied. Spectrophotometric titrations of the soluble species were used, in order to obtain the value of the first hydrolysis constant of Pr(III). The data obtained were treated with both the program SQUAD and by a graphic method, respectively. The result obtained using SQUAD was log*ß1 = –8.94 ± 0.03, while it was log*ß1 = –8.77 ± 0.03, when calculated graphically. These results are similar to the value obtained previously with the potentiometric method.
 

Reassimilation of Leaf Internal CO2 Contributes to Isoprene Emission in the Neotropical Species Inga edulis Mart.

Isoprene (C5H8) is a hydrocarbon gas emitted by many tree species and has been shown to protect photosynthesis under abiotic stress. Under optimal conditions for photosynthesis, ~70%–90% of carbon used for isoprene biosynthesis is produced from recently assimilated atmospheric CO2. While the contribution of alternative carbon sources that increase with leaf temperature and other stresses have been demonstrated, uncertainties remain regarding the biochemical source(s) of isoprene carbon. In this study, we investigated leaf isoprene emissions (Is) from neotropical species Inga edulis Mart. as a function of light and temperature under ambient (450 µmol m−2 s−1) and CO2-free (0 µmol m−2 s−1) atmosphere. Is under CO2-free atmosphere showed light-dependent emission patterns similar to those observed under ambient CO2, but with lower light saturation point. Leaves treated with the photosynthesis inhibitor DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to produce detectable Is in normal light under a CO2-free atmosphere. While strong temperature-dependent Is were observed under CO2-free atmosphere in the light, dark conditions failed to produce detectable Is even at the highest temperatures studied (40 °C). Treatment of leaves with 13C-labeled sodium bicarbonate under CO2-free atmosphere resulted in Is with over 50% containing at least one 13C atom. Is under CO2-free atmosphere and standard conditions of light and leaf temperature represented 19% ± 7% of emissions under ambient CO2. The results show that the reassimilation of leaf internal CO2 contributes to Is in the neotropical species I. edulis. Through the consumption of excess photosynthetic energy, our results support a role of isoprene biosynthesis, together with photorespiration, as a key tolerance mechanism against high temperature and high light in the tropics.

Acceleration of hydrogen production during water-olivine-CO2 reactions via high-temperature-facilitated Fe(II) release

The effects of temperature on hydrogen (H2) production in the alteration of olivine [(Mg,Fe)2SiO4] have been investigated under both CO2-rich (0.5 mol/L NaHCO3) and CO2-free conditions at 225–300 °C. Under CO2-rich conditions, H2 production rates showed strong temperature dependence, where the rate drastically increased for ≥275 °C, while the rate in CO2-free conditions was less temperature-dependent. At 300 °C under CO2-rich conditions, the H2 generation rate was 3 times faster than that under CO2-free conditions, and a considerable amount of CO2 was simultaneously stored as magnesite [(Mg,Fe)CO3]. Increasing the temperature under CO2-rich conditions promoted olivine consumption, while suppressing both brucite [(Mg,Fe)(OH)2] formation and incorporation of Fe(II) into secondary minerals, thus facilitating Fe(II) release and oxidation to produce H2. It has been revealed for the first time that H2 production during water-olivine-CO2 reactions is markedly accelerated via high-temperature-facilitated Fe(II) release.

NaHCO3-promoted olivine weathering with H2 generation and CO2 sequestration in alkaline hydrothermal system

Hydration of Fe(II) bearing minerals, such as olivine ((Mg,Fe)SiO4), potentially generate hydrogen (H2). However, because of the low Fe(II) dissolution rate, the H2 production rate is usually low. We have recently proposed a novel system to promote H2 production and simultaneous CO2 storage in hydrothermal conditions via NaHCO3-enhanced olivine weathering. The present study reports the role of NaHCO3 on both H2 production from olivine hydration and on minerals generations on laboratory experiments in CO2-rich (0.5 mol/L NaHCO3) and CO2-free hydrothermal conditions at pH range of 8-11. The highest H2, HCOOH yields and carbonation rate reached at the CO2-rich experiment with lower alkaline pH at 8.25. The addition of high concentration NaHCO3 decreased pH from 10.92 to 8.25, whereas olivine and brucite dissolutions were accelerated. Thus, more Fe(II) was released from olivine and brucite, and H2 production was accelerated. At higher pH range, olivine dissolution was promoted, but brucite dissolution was suppressed in both CO2-rich and CO2-free conditions. This study suggests H2 production was promoted with the presence of HCO3−, but not the pH variation. The consumption of Fe(II)-bearing brucite was important in enhancing H2 production during olivine hydration process.

The specific sorption of Np(V) on the corundum (α-Al2O3) surface in the presence of trivalent lanthanides Eu(III) and Gd(III): A batch sorption and XAS study

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as a competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl(V) ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10−5M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was applied to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopy (XAS) measurements were carried out on the samples containing only Np(V) and Np(V)+Gd(III). The results reveal the presence of a bidentate Np(V) edge-sharing complex on the corundum surface in the absence of Gd(III), while the coordination environment of Np(V) on the corundum surface could be changed when Gd(III) is added to the sample before the sorption of Np(V).

Effect of carbon dioxide and water on the performances of an iron-promoted copper/ceria catalyst for CO preferential oxidation in H2-rich streams

The catalytic behavior of an iron-promoted copper/ceria catalyst towards CO-PROX reaction under feed mixtures containing variable concentrations of CO2 (0–15 vol.%) and H2O (0–10 vol.%) was compared to that of an unpromoted sample with the same copper loading. At low temperature both species inhibit CO conversion on Fe-promoted catalyst more than on the reference Cu/CeO2 catalyst, although the performance is restored more rapidly to that corresponding to CO2-free conditions by increasing the temperature. CO2 temperature programmed desorption tests were carried out and experimental curves modeled to evaluate quantitatively the active sites for CO and H2 oxidation. Iron addition significantly reduced the amount of H2 oxidation sites, likely promoting copper dispersion, leading to the observed better selectivity under CO2-free conditions. The inhibiting effect of CO2 was ascribed to the formation of quite stable CO2 adsorbed species blocking copper active sites. The different dependence on the temperature of CO2 desorption results in a higher inhibition at low temperature for the Fe-promoted catalyst, but also in an easier CO2 desorption at higher temperature restoring the performance under CO2-free conditions for this sample.

Hydrolysis of Trivalent Holmium in Aqueous Solutions of 2 M Ionic Strength by Spectrophotometric and Potentiometric Methods

The first hydrolysis constants of trivalent holmium in 2 M NaClO4 and 2 M NaCl at 303 K and in CO2 free conditions were determined. The pCH borderlines of precipitation and first hydrolysis were determined by means of a spectrophotometric method and last one with the program SQUAD. Independently, the stability constant for the first hydrolytic species was determined, by means of potentiometric pH titrations whose data were treated with the program SUPERQUAD. The hydrolysis constants obtained were: and . These values attained by both methods are the same. The log10β1,Cl constants for the species HoCl2+ was also calculated for 2 M ionic strength and 303 K from the hydrolysis constant obtained in both perchlorate and chloride media. This value was logβ1,Cl = -0.56.

Requirement of carbon dioxide for initial growth of facultative methylotroph, Acidomonas methanolica MB58

The facultative methylotrophic bacterium Acidomonas methanolica MB58 can utilize C1 compounds via the ribulose monophosphate pathway. A large gene cluster comprising three components related to C1 metabolism was found in the genome. From upstream, the first was an mxa cluster encoding proteins for oxidation of methanol to formaldehyde; the second was the rmp cluster encoding enzymes for formaldehyde fixation; and the third was the cbb gene cluster encoding proteins for carbon dioxide (CO2) fixation. Examination of CO2 requirements for growth of A.methanolica MB58 cells demonstrated that it did not grow on any carbon source under CO2-free conditions. Measurement of ribulose-1,5-bisphosphate carboxylase activity and RT-PCR analysis demonstrated enzymatic activity was detected in A.methanolica MB58 at growth phase, regardless of carbon sources. However, methanol dehydrogenase and 3-hexlose-6-phosphate synthase expression was regulated by methanol or formaldehyde; it were detected during growth and apparently differed from ribulose-1,5-bisphosphate carboxylase expression. These results suggested that A.methanolica MB58 may be initially dependent on autotrophic growth and that carbon assimilation was subsequently coupled with the ribulose monophosphate pathway at early- to mid-log phases during methylotrophic growth.

Chlorite dissolution rates under CO2 saturated conditions from 50 to 120 °C and 120 to 200 bar CO2

Chlorite dissolution rates were measured in a series of batch reactor experiments testing the effect of pCO2, pH, chloride and bicarbonate concentrations and temperature. Chlorite is an important diagenetic mineral in sedimentary basins, often found cementing mineral grains and filling pore space in formations that may serve as reservoirs for storing carbon dioxide. Conflicting reports of whether chlorite acts as a barrier to reservoir rock reactivity or leads to enhanced porosity due to dissolution, after the injection of supercritical CO2 into a reservoir, makes studying the reactivity of chlorite in contact with CO2 saturated waters pertinent.Measured dissolution rates were initially rapid and decreased over time as the saturation state of solution relative to chlorite increased. Temperature had the strongest effect on dissolution rate, with an apparent activation energy of 16±0.5kJmol−1 and rate constant of logk0=−9.56±0.07molm−2s−1 assuming a rate law of the form: rate=k0exp(−EA/RT). The apparent activation energy is lower than previously accepted values, but is consistent with a study of chlorite dissolution using flow through techniques (Smith et al., 2013). Mineral dissolution rates are typically proton enhanced, but the lack of a significant pH effect or pCO2 effect on chlorite dissolution rate in this study suggests that the use of NaHCO3 to buffer the pH of CO2 saturated solutions led to an inhibition of mineral dissolution in competition with the expected pH effect. This is supported by the observed dissolution rate increasing dramatically (half a log unit) with the use of an organic acid buffer (KHpthalate) under CO2 free conditions. The effect of chloride (NaCl ∼5 to 50g/L) was found not to affect the dissolution rate of chlorite. Various empirical rate laws are proposed and fit to the data and lead to the development of a surface complex model describing proton promoted dissolution and bicarbonate inhibition of chlorite dissolution rates. The model can be applied to predict the rate of chlorite dissolution under elevated pCO2 conditions relevant to the storage of CO2 in reservoirs from 50 to 275°C in contact with fluids ranging in pH from 3.4 to 5.4.

In situ AFM and XPS Investigation of U6+ Reduction by Fe2+ on Hematite and Pyrite

ABSTRACTUranyl adsorption/reduction by Fe2+ on hematite and pyrite has been studied at neutral pH under anoxic and CO2-free conditions. XPS results confirm that more U3O8 precipitates on hematite than on pyrite reacted for 24 h in 160 μM uranyl nitrate and 160 μM Fe2+ solution at initial pH 7.3. These results are explained in terms of co-adsorption energy and U atom Mulliken charge transfer by quantum mechanical calculations. Moreover, in situ fluid tapping-mode AFM experiments on hematite indicate a deceleration of the U reduction rate within 24 h due to the passivation of the surface caused by the formation of orthorhombic U3O8 crystals. In addition, crystals observed using AFM show morphologies of orthorhombic schoepite appearing on hematite after 5 h.

In-situ X-ray diffraction study of carbonate formation and decomposition in perovskite-type BCFZ

The effect of CO 2 on the two phase perovskite with average composition of BaCo 0.4Fe 0.4Zr 0.2O 3− δ is investigated by in-situ X-ray diffraction (XRD) as well as transmission electron microscopy (TEM). Partial decomposition of the BCFZ into high-temperature rhombohedral BaCO 3 polymorph was observed during annealing in an atmosphere, which contained 50 vol% CO 2/50 vol% N 2 at 1173 K. This carbonate structure is not quenchable and cannot be detected by ex-situ methods. Additionally, the reversible phase transition of BaCO 3 from orthorhombic to rhombohedral to cubic at different temperatures accompanied by formation of CoO was shown by in-situ XRD and TEM. Furthermore, complete regeneration of perovskite phase was obtained after high-temperature treatment under CO 2-free conditions.

Chlorine partitioning between a basaltic melt and H 2O–CO 2 fluids at Mount Etna

Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H 2O or H 2O–CO 2 were performed at 1200–1260 °C, at pressures between 1 and 200 MPa, either near the nickel–nickel oxide (NNO) buffer or at two log units above it (NNO + 2), and with different chloride concentrations. Most of the experiments were done at chloride-brine-undersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients ( D Cl fluid/melt) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H 2O–Cl experiments in which the Cl concentration in the melt was ≤ 0.4 wt.%, a negative dependence between D Cl f/m and pressure is observed. D Cl fluid/melt in H 2O + Cl-bearing experiments ranges between 11–14 at 1 and 25 MPa to 6 at 200 MPa at NNO; and between 4 at 50 MPa and 13 at 100 MPa at ΔNNO ≥ 2. Addition of CO 2 at NNO yields lower partition coefficients than in CO 2-free conditions over the pressure range investigated. The negative pressure dependence observed for H 2O–Cl experiments disappears when CO 2 is present in the system. Overall, once CO 2 is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of D Cl f/m. Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid.

The composition of near-solidus melts of peridotite in the presence of CO 2 and H 2O between 40 and 60 kbar

Partial melting experiments at 40, 50 and 60 kbar pressure on three peridotite compositions with 0.5–0.63 wt.% H 2O and 2.0–3.2 wt.% CO 2 added indicate melting temperatures only marginally above continental geotherms. Most experiments were performed on a composition with 1.5 wt.% K 2O added, which causes a further decrease of about 40 °C in melting temperature. Melts progress gradually from carbonate-rich to carbonated silicate in composition: near-solidus melts have Ca/(Ca + Mg) of 0.46–0.53, which fall to < 0.40 more than 50 °C above the solidus. With increasing temperature for the K-enriched peridotite HPK at 50 kbar, melts are characterised by strong increases in SiO 2 (< 3 to > 30 wt.%) and Al 2O 3 (< 1 to > 9 wt.%) and concomitant decrease in CaO (> 20 to < 3 wt.%), with little change in MgO. K 2O and TiO 2 exhibit maxima at intermediate temperatures, reflecting the stability of phlogopite and ilmenite above the solidus, indicating the presence of up to 13 wt.% K 2O and 2.6 wt.% TiO 2 in carbonate-rich melts with only 13 wt.% SiO 2. Of 30 trace elements, only Cr, Mn, Ni, and Zn are compatible in the residue, whereas U, Rb, Ba and the LREE are most enriched in the melts. Low melt fractions exhibit troughs in trace element patterns for Hf, Nb, Ta and Cs that are diminished or eliminated at higher degrees of melting. Apart from these features and larger LREE/HREE ratios, trace element compositions are similar to silicate melts of peridotite in CO 2-free conditions. Partial melts of peridotite with CO 2 and H 2O are too low in MgO to resemble kimberlites, but will act as effective metasomatic agents enriching the lithosphere in K and carbonates, of relevance for ultramafic lamprophyres and kamafugites. Higher-degree melts (15–35%) have > 20 wt.% SiO 2, are only mildly enriched in trace elements, and will not cause large time-integrated isotopic in-growth except over long geological time intervals. The temperature interval over which melts are carbonatitic with SiO 2 contents < 10 wt.% narrows with increasing pressure from 30– 60 kbar, meaning that the metasomatic effects of carbonatitic melts may be greatest at 20–30 kbar.

Solubility and hydrolysis of lutetium at different [Lu3+]initial

Solubility product (Lu(OH)3(s)⇆Lu3++3OH−) and first hydrolysis (Lu3++H2O⇆Lu(OH)2++H+) constants were determined for an initial lutetium concentration range from 3.72·10−5 mol·dm−3 to 2.09·10−3 mol·dm−3. Measurements were made in 2 mol·dm−3 NaClO4 ionic strength, under CO2-free conditions and temperature was controlled at 303 K. Solubility diagrams (pLuaq vs. pCH) were determined by means of a radiochemical method using 177Lu. The pCH for the beginning of precipitation and solubility product constant were determined from these diagrams and both the first hydrolysis and solubility product constants were calculated by fitting the diagrams to the solubility equation. The pCH values of precipitation increases inversely to [Lu3+]initial and the values for the first hydrolysis and solubility product constants were log10β*Lu,H = −7.92±0.07 and log10K*sp,Lu(OH)3 = −23.37±0.14. Individual solubility values for pCH range between the beginning of precipitation and 8.5 were SLu3+ = 3.5·10−7 mol·dm−3, SLu(OH)2+ = 6.2·10−7 mol·dm−3, and then total solubility was 9.7·10−7 mol·dm−3.

Phase Equilibria of BaO-R2O3-CuO z Systems (R = Y and Lanthanides) under CO2-free Conditions

For applications ranging from phase equilibria to the processing of second-generation high Tc superconductor-coated-conductors, phase diagrams constructed under carbonate-free conditions are needed. Subsolidus phase equilibria of BaO-R2O3-CuOz (R = Ho) have been investigated at \( {\hbox{p}}_{{{\rm{O}}_{{\rm{2}}} }} \hbox{\,=\,} 100\;{\hbox{Pa}} \) (810°C), 21 kPa (875°C) and 0.1 MPa (850 and 930°C) by applying controlled atmosphere methods to minimize the presence of carbonate and CO2 and H2O contamination. Under carbonate-free conditions, most of these phase diagrams are different from those reported in the literature. In this paper, we also review and compare the phase diagrams of ten BaO-R2O3-CuOz systems (R = Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm and Yb) that were previously determined in this laboratory under \( {\hbox{p}}_{{{\rm{O}}_{{\rm{2}}} }} \hbox{\,=\,} 100\;{\hbox{Pa}}{\hbox{.}} \) Among these diagrams, a distinct trend of phase formation and tie-line relationships is observed.

Boron removal by hydrotalcite-like, carbonate-free Mg–Al–NO 3-LDH and a rationale on the mechanism

Removal of boron from boric acid and tetraborate solutions by hydrotalcite (HT)-like, carbonate-free, Mg–Al–NO 3-layered double hydroxide (LDH) was studied. Data from batch experiments were computed as a function of the adsorbent dose and contact time. Synthesis in CO 2-free conditions provided a material with high anion exchange capacity and more than 95% adsorption could be achieved. The physical and structural properties of the material before and after boron adsorption were investigated to describe the uptake mechanism. Interpretation of the results of d-spacing, surface area and zeta potential measurements indicated an anion exchange mechanism rather than surface adsorption. Calcination at 400 °C did not make any significant change in the removal process.

Application of the Specific Ion Interaction Theory → the Solubility Product and First Hydrolysis Constant of Europium

The specific ion interaction theory (SIT) was applied to the first hydrolysis constants of Eu(III) and solubility product of Eu(OH)3 in aqueous 2, 3 and 4 mol⋅dm−3 NaClO4 at 303.0 K, under CO2-free conditions. Diagrams of pEuaqversus pCH were constructed from solubilities obtained by a radiometric method, the solubility product log10Ksp, Eu(OH)3I {Eu(OH)3(s)\(\rightleftharpoons\)Euaq3++ 3OHaq− } values were calculated from these diagrams and the results obtained are log10Ksp,Eu(OH)3I = − 22.65 ± 0.29, −23.32 ± 0.33 and −23.70 ± 0.35 for ionic strengths of 2, 3 and 4 mol⋅dm−3 NaClO4, respectively. First hydrolysis constants {Euaq3++H2O\(\rightleftharpoons\)Eu(OH)(aq)2++H+ } were also determined in these media by pH titration and the values found are log10βEu,HI = − 8.19 ± 0.15, −7.90 ± 0.7 and −7.61 ± 0.01 for ionic strengths of 2, 3, and 4 mol⋅dm−3 NaClO4, respectively. Total solubilities were estimated taking into account the formation of both Eu3+ and Eu(OH)2+ (7.7 < pCH < 9) and the values found are: 1.4 × 10−6 mol⋅dm−3, 1.2 × 10−6 mol⋅dm−3 and 1.3 × 10−6 mol⋅dm−3, for ionic strengths of 2, 3 and 4 mol⋅dm−3 NaClO4, respectively. The limiting values at zero ionic strength were extrapolated by means of the SIT from the experimental results of the present research together with some other published values. The results obtained are log10Ksp, Eu(OH)3o = − 23.94 ± 0.51 (1.96 SD) and log10βEu,H0 = − 7.49 ± 0.15 (1.96 SD).

Reduced contribution from Na+/H+ exchange to acid extrusion during anoxia in adult rat hippocampal CA1 neurons.

The effect of anoxia on Na+/H+ exchange activity was examined in acutely isolated adult rat hippocampal CA1 neurons loaded with the H+-sensitive fluorophore, BCECF. Five-minute anoxia imposed under nominally HCO3-/CO2-free conditions induced a fall in pHi, the magnitude of which was smaller following prolonged exposure to medium in which N-methyl-D-glucamine (NMDG+) was employed as an extracellular Na+ (Na(+)(o)) substitute. Also consistent with the possibility that Na+/H+ exchange becomes inhibited soon after the induction of anoxia, rates of Na(+)(o)-dependent pHi recovery from internal acid loads imposed during anoxia were slowed, compared to rates of Na(+)(o)-dependent pHi recovery observed prior to anoxia. At the time at which rates of pHi recovery were reduced during anoxia, cellular adenosine triphosphate (ATP) levels had fallen to 35% of preanoxic levels, suggesting that ATP depletion might contribute to the observed inhibition of Na+/H+ exchange. In support, incubation of neurons with 2-deoxyglucose and antimycin A under normoxic conditions induced a fall in cellular ATP levels that was also associated with reduced Na(+)(o)-dependent rates of pHi recovery from imposed acid loads; conversely, pre-treatment with 10 mm creatine attenuated the effects of anoxia to reduce both ATP levels and Na(+)(o)-dependent rates of pHi recovery from internal acid loads. Taken together, the results are consistent with the possibility that functional Na+/H+ exchange activity in adult rat CA1 neurons declines soon after the onset of anoxia, possibly as a result of anoxia-induced falls in intracellular ATP.