Diffusion Of H2S Research Articles

Gas-phase removal of hydrogen sulfide using iron oxyhydroxide at low temperature: Measurement of breakthrough curve and modeling of sulfidation mechanism

Abstract With the aim of removing hydrogen sulfide (H2S) from a gaseous phase around room temperature, we studied the performance of an iron oxyhydroxide (ferrihydrite) adsorbent by measuring a packed bed breakthrough curve (BTC) in a constant gas flow. We determined the amount of H2S adsorbed on the adsorbent by integrating BTCs measured under different experimental conditions. These conditions included the initial concentration of H2S, the superficial gas velocity and the temperature of the packed bed. Under the experimental conditions, it was found that the amount of H2S adsorbed was almost unchanged by the superficial gas velocity, whereas the adsorbed amount depended on the initial concentration of H2S and temperature. To solve the mass transfer equations in the packed bed, we developed a model considering such parameters as the kinetic constant of the sulfidation reaction, the intragranular diffusivity of H2S, and the number density of active sites on the adsorbent. By fitting the calculated BTC to the measured BTC under one of the experimental conditions, the set of parameters could be estimated. Using the same set of estimated parameters, we calculated the BTCs under the other experimental conditions. As a result, we confirmed that the calculated BTC coincided with the BTC measured under a different initial H2S concentration and gas velocity. We also found that the number density of the active sites increased with increases in temperature.

A Liquid Crystal-Based Passive Badge for Personal Monitoring of Exposure to Hydrogen Sulfide

A new liquid crystal (LC)-based passive dosimeter badge for personal monitoring of exposure to hydrogen sulfide (H2S) gas is reported. When a thin film of LC supported on a surface functionalized with lead perchlorate Pb(ClO4)2 (the LC sensor) is exposed to H2S, the orientation of LC molecules in the film changes from perpendicular to parallel. This reorientation induces a change in the appearance of the LC film when viewed between crossed polarizers. A H2S dosimeter was fabricated by pairing a LC sensor with a glass substrate forming a headspace between the two surfaces, to control diffusion of H2S across the LC film. When the dosimeter is exposed to H2S, a bright front appears as a function of exposure time. An algorithm has been developed to correlate this response length and exposure dose. The dosimeters are functionally stable when subjected to extreme temperature and humidity fluctuations, and are immune to a number of potentially interfering chemicals, except mercaptans. These dosimeters detect H2S at 0.2 ppm TWA (8 hr) with ±20% overall accuracy. The dosimeters were used to monitor the personal exposure of personnel working in an oil refinery. The TWA concentrations measured by the LC-based dosimeters correlate strongly with the NIOSH 1063 method that uses a sorbent tube and a pump followed by laboratory analysis. Thus, the LC-based dosimeters can provide a sensitive tool for on-site assessment of personal exposure to H2S in different environments.

Diffusion of N2, O2, H2S and SO2 in MFI and 4A zeolites by molecular dynamics simulations

This paper presents diffusion data of N2, O2, H2S and SO2 in MFI and 4A zeolites obtained by molecular dynamics simulations, especially its dependence on temperature and loading. At high loadings and temperatures, the order of self-diffusivity of guests in two zeolites is O2 > N2 > H2S>SO2. The diffusion behaviour is different in different zeolites at lower loadings, reflecting different influences from straight channels (MFI) and α-cages (4A). Furthermore, with increasing loading, the self-diffusivity of guest molecules decreases in MFI but generally increases in 4A. The centre of mass (COM) probability densities and diffusion trajectories of guests give insight into molecular-level diffusion process. The simulation results reveal that with increasing loading, the diffusion mechanism would change from the inter-pore to intra-pore diffusion in MFI. However, in 4A, the intra-pore diffusion is predominant at low and high loadings, but inter-pore diffusion is more important at moderate loadings.

Natural near field sinks of hydrogen sulfide from two geothermal power plants in Iceland

Hydrogen sulfide (H2S) emissions have been growing with the increasing utilization of geothermal resources. Atmospheric H2S concentration has been measured and studied but less is known about the natural sinks of the chemical. This study investigates the atmospheric depletion of H2S within a 35 km distance from two Icelandic power plants. The results showed that atmospheric oxidation by the OH radical was the largest sink in the area. The second largest sink was H2S uptake in surface water, in a neighboring lake, but it was, however, small compared to the reported difference of sulfur amount in the lake in- and outflow. Sulfur was found to accumulate in moss close to the power plants at a maximum rate of about 1500 mg S/kg moss per year and decreased exponentially from the source, being negligible at a distance of a few kilometers. Soil uptake was limited by diffusion of H2S into the porous media and was thus much smaller than the estimated soil uptake potential. Washout with precipitation was estimated to be the smallest sink due to the low H2S reactivity in the precipitation (pH = 5.6), compared to the surface water (pH ∼8). Depletion of H2S from the atmosphere in the study area was estimated to be about 1.2% of the 2012 power plants emissions of over 20,000 tons. Although the uncertainties in the depletion estimates were considerable, most of the H2S emitted from the power plants was strongly indicated to be transported out of the study area as H2S.

H2S absorption at high pressure using hollow fibre membrane contactors

In this article, the removal of H2S from natural gas at high pressure using hollow fibre membrane contactors and water as the absorbent solvent was described and validated by a 2D comprehensive mathematical model. The modelling predictions were in a good agreement with the experimental data at low pressure of 1bar under non-wetting conditions. However, the experimental behaviour at high pressure in the range of 10–50bar was always lower than the modelling predictions. And thus, a sensitivity analysis was carried out to study the influence of gas, liquid, membrane diffusion and solubility coefficients of H2S at high pressure upon the modelling behaviour compared with the experimental trend. The modelling results confirmed that these parameters were still insensitive as the model predictions changed slightly with altering these coefficients. Conversely, membrane wetting was confirmed to be an important factor even for small ratios, i.e. pseudo-wetting (1–3%). The model was able to predict the absorption of H2S at higher pressures under 3% pseudo-wetting. Although the type of membranes used in this study was highly hydrophobic (ePTFE) and thus membrane wetting was not expected, pseudo-wetting phenomenon was confirmed to be an important factor at high pressure.

Synthesis and characterizations of flower-like ZnS and ZnS:Cu2+ nanostructures

Flower-like ZnS or ZnS:Cu2+ nanostructures were prepared in aqueous solution via H2S diffusion procedure. The morphology of products can be controlled by adjusting the growth time or concentration of Cu2+. A bit of Wurtzite phase attributed to the high surface energy has been found with X-ray diffraction. The strong photoluminescence emission centered at 368nm. XRD, HR-TEM, EDS map and FTIR all confirm that Cu2+ was doped in ZnS lattice.

Decoupling reduction–sulfurization synthesis of inorganic fullerene-like WS2 nanoparticles in a particulately fluidized bed

A novel decoupling reduction–sulfurization process for the synthesis of inorganic fullerene-like (IF) WS2 nanoparticles has been developed in this work. In this strategy, the raw WO3 nanoparticles are firstly reduced by hydrogen in a fluidized bed, followed by sulfurization with H2S in a reducing atmosphere. The diffusion and reaction are the two kinetic factors playing important roles in controlling the structure of WS2. Under diffusion limitation, the porous β-W phase and inordinate WS2 structure are formed, while the α-W phase and IF-WS2 structure are usually obtained in the reaction-limited condition. The particulate fluidization of the reduced WO3 nanoparticles in the agglomerates forms is realized in this decoupling reduction–sulfurization process. The porous β-W nanoparticles and their particulate fluidization enhance the diffusion of H2S, resulting in the completion of sulfurization reaction in short time. This newly developed approach is efficient and cost-effective, and might be extended to the large-scale synthesis of other inorganic fullerene-like metal sulfides.

Synthesis of highly ordered nanorod arrays 9Mn1CeMOs and character of high sulfur capacity for hot coal gas desulfurization

Highly ordered nanorod arrays 9Mn1Ce metal oxides (MOs) were prepared by negative replication of SBA‐15 hard template and were used directly for hot coal gas desulfurization at 600°C. The results indicated that 9Mn1CeMOs sorbent presented a superior desulfurization activity and small resistance of H2S diffusion compared with 9Mn1Ce mixed oxides. The breakthrough sulfur capacity (19.03 g‐S/100 sorbent) was significantly higher than that (7.94 g‐S/100 sorbent) over LaxFeyOz/MCM‐41 due to highly ordered nano‐rod array structure. The five successive sulfidation/regeneration cycles verified that 9Mn1CeMOs exhibited highly durable regeneration ability. The morphology and structure of the 9Mn1CeMOs were characterized by scanning electron microscopy, high‐resolution transmission electron microscope, X‐ray diffraction, and N2 adsorption, it revealed that parallel nanorod arrays formed with a diameter of 6–7 nm and the nanobridges between the nanorods played a crucial role for the stability of 9Mn1CeMOs. © 2014 American Institute of Chemical Engineers Environ Prog, 33: 1266–1273, 2014

Effect of the structure of the support and the aminosilane type on the adsorption of H2S from model gas

In the present work, three different porous supports, i.e. MCM-41, SBA-15 and UVM-7 were synthesized and functionalized with three different aminosilanes (C9H23NO3Si, C8H22N2O3Si and C10H27N3O3Si). All the synthesized samples were characterized using XRD, nitrogen physisorption, elemental analysis and acid–base titration, SEM and TEM. The obtained results revealed that all the samples have a well-ordered structure and amine groups were successfully grafted on their pore surfaces. Then, the ability of amine-functionalized samples to adsorb H2S was investigated. In the case of MCM-41, changing the aminosilane functional group did not change tbp (the time at which the outlet concentration of H2S reaches its maximum admissible value in the effluent). SBA-15 functionalized with C8H22N2O3Si showed enhanced tbp in comparison with C9H23NO3Si, but C10H27N3O3Si showed very low tbp (even lower than C9H23NO3Si) which might be attributed to pore blocking originating from the very large size of C10H27N3O3Si. UVM-7 had the highest tbp for all three of the aminosilane functional groups. This might be attributed to the very large pore size of UVM-7 with respect to those of SBA-15 and MCM-41. In addition, nanosize particles of UVM-7 facilitate the adsorption of H2S for this type of adsorbent, which should be due to the unconstrained diffusion of H2S within the mesopores of UVM-7. The efficiency of an aminosilane functional group strongly depends on the pore size of the support rather than its surface area.

Modeling of H2S migration through landfill cover materials

The emission of H2S from landfills in the United States is an emergent problem because measured concentrations within the waste mass and in ambient air have been observed at potentially unsafe levels for on-site workers and at levels that can cause a nuisance and potentially deleterious health impacts to surrounding communities. Though recent research has provided data on H2S concentrations that may be observed at landfills, facility operators and landfill engineers have limited predictive tools to anticipate and plan for potentially harmful H2S emissions. A one-dimensional gas migration model was developed to assist engineers and practitioners better evaluate and predict potential emission levels of H2S based on four factors: concentration of H2S below the landfill surface (C0), advection velocity (v), H2S effective diffusion coefficient (D), and H2S adsorption coefficient of landfill cover soil (μ). Model simulations indicated that H2S migration into the atmosphere can be mitigated by reducing H2S diffusion and advection or using alternative cover soils with a high H2S adsorption coefficient. Laboratory column experiments were conducted to investigate the effects of the four parameters on H2S migration in cover soils and to calculate the adsorption coefficient of different cover materials. The model was validated by comparing results with laboratory column experiments. Based on the results, the laboratory column provides an effective way to estimate the H2S adsorption coefficient, which can then be incorporated into the developed model to predict the depth of cover soil required to reduce emitted H2S concentrations below a desired level.

Performance of activated sludge diffusion for biological treatment of hydrogen sulphide gas emissions

Odours from wastewater treatment plants are comprised of a mixture of various gases with hydrogen sulphide (H(2)S) often being the dominant constituent. Activated sludge diffusion (ASD) as a biotreatment system for odour abatement has been conducted for over 30 years but has limited broad application due to disagreement in the literature regarding the effect that ASD may have on wastewater treatment performance. The effects of continuous H(2)S diffusion at 25 ppmv, with weekly peaks of approximately 100 ppmv, on H(2)S removal efficiency and wastewater treatment performance was evaluated over a 2-month period using an activated sludge pilot plant. H(2)S removal averaged 100% during diffusion at 25 ppmv, and 98.9% during the 100 ppmv peak periods. A significant increase in mixed liquor volatile suspended solids concentration (P < 0.01) was observed during H(2)S diffusion, which may be due to an increase in H(2)S-degrading microorganisms. There was no adverse effect of H(2)S on nitrification throughout the ASD trials. Ammonia (NH(3)) removal was slightly better in the test receiving H(2)S diffusion (87.6%) than in the control (85.4%). H(2)S diffusion appeared to improve robustness of the AS biomass to operational upsets.

Reactivity and Dynamics of H2S, NO, and O2 Interacting with Hemoglobins from Lucina pectinata

Hemoglobin HbI from the clam Lucina pectinata is involved in H2S transport, whereas homologous heme protein HbII/III is involved in O2 metabolism. Despite similar tertiary structures, HbI and HbII/III exhibit very different reactivity toward heme ligands H2S, O2, and NO. To investigate this reactivity at the heme level, we measured the dynamics of ligand interaction by time-resolved absorption spectroscopy in the picosecond to nanosecond time range. We demonstrated that H2S can be photodissociated from both ferric and ferrous HbI. H2S geminately rebinds to ferric and ferrous out-of-plane iron with time constants (τgem) of 12 and 165 ps, respectively, with very different proportions of photodissociated H2S exiting the protein (24% in ferric and 80% in ferrous HbI). The Gln(E7)His mutation considerably changes H2S dynamics in ferric HbI, indicating the role of Gln(E7) in controling H2S reactivity. In ferric HbI, the rate of diffusion of H2S from the solvent into the heme pocket (kentry) is 0.30 μM(-1) s(-1). For the HbII/III-O2 complex, we observed mainly a six-coordinate vibrationally excited heme-O2 complex with O2 still bound to the iron. This explains the low yield of O2 photodissociation and low koff from HbII/III, compared with those of HbI and Mb. Both isoforms behave very differently with regard to NO and O2 dynamics. Whereas the amplitude of geminate rebinding of O2 to HbI (38.5%) is similar to that of myoglobin (34.5%) in spite of different distal heme sites, it appears to be much larger for HbII/III (77%). The distal Tyr(B10) side chain present in HbII/III increases the energy barrier for ligand escape and participates in the stabilization of bound O2 and NO.

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H2S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H2S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H2S concentration at a temperature of 90°C. When the temperature and H2S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe2+ ions and the inward diffusion of H2S and HS− species.

Transport of H2S and HS(-) across the human red blood cell membrane: rapid H2S diffusion and AE1-mediated Cl(-)/HS(-) exchange.

The rates of H2S and HS(-) transport across the human erythrocyte membrane were estimated by measuring rates of dissipation of pH gradients in media containing 250 μM H2S/HS(-). Net acid efflux is caused by H2S/HS(-) acting analogously to CO2/HCO3(-) in the Jacobs-Stewart cycle. The steps are as follows: 1) H2S efflux through the lipid bilayer and/or a gas channel, 2) extracellular H2S deprotonation, 3) HS(-) influx in exchange for Cl(-), catalyzed by the anion exchange protein AE1, and 4) intracellular HS(-) protonation. Net acid transport by the Cl(-)/HS(-)/H2S cycle is more efficient than by the Cl(-)/HCO3(-)/CO2 cycle because of the rapid H2S-HS(-) interconversion in cells and medium. The rates of acid transport were analyzed by solving the mass flow equations for the cycle to produce estimates of the HS(-) and H2S transport rates. The data indicate that HS(-) is a very good substrate for AE1; the Cl(-)/HS(-) exchange rate is about one-third as rapid as Cl(-)/HCO3(-) exchange. The H2S permeability coefficient must also be high (>10(-2) cm/s, half time <0.003 s) to account for the pH equilibration data. The results imply that H2S and HS(-) enter erythrocytes very rapidly in the microcirculation of H2S-producing tissues, thereby acting as a sink for H2S and lowering the local extracellular concentration, and the fact that HS(-) is a substrate for a Cl(-)/HCO3(-) exchanger indicates that some effects of exogenous H2S/HS(-) may not result from a regulatory role of H2S but, rather, from net acid flux by H2S and HS(-) transport in a Jacobs-Stewart cycle.

Biomimetic fabrication of α-Fe2O3 with hierarchical structures as H2S Sensor

A facile sol–gel method is developed for the fabrication of α-Fe2O3 with quasi-honeycomb like structures inherited from Papilio paris butterfly wings. The exquisite hierarchical architecture is faithfully maintained in α-Fe2O3 from the skeleton of butterfly wings at the levels from macro to nano-scales. When used as a chemical sensor, the obtained α-Fe2O3 replica (P-α-Fe2O3) showed a much higher performance than that of the compared α-Fe2O3 nanoparticles synthesized under the same condition without biotemplate (S-α-Fe2O3). The P-α-Fe2O3-based sensor has a sensitivity of 19.2–50 ppm H2S, which is four times more than that of S-α-Fe2O3, accompanied by a rapid response/recovery time within 1/10 s even at a relatively low working temperature of 180 °C. Compare to the S-α-Fe2O3, surface area of which cannot be detectable, the high sensing feature of P-α-Fe2O3 would be attributed to the relatively high-specific surface area 24.12 m2/g thus fabricated together with the unique 3D-network structures, which provide channel for the diffusion of H2S. This strategy is expected to be used in fabrication of other kinds of metal oxide with unique structures for the potential application in gas sensor.

P27 Interaction of H2S with hemeproteins modulates the concentration of free sulfide compounds in solution and mediates H2S signaling

Metalloproteins, particularly hemeproteins have the potential to mediate H2S signaling in cells. In fact, using the hemoglobin I (HbI) from the clam L. pectinata as a model we have previously suggested that H2S binds tightly to the ferric iron of hemeproteins and that its release is controlled by two competing reactions involving simple sulfide dissociation and heme reduction [1] , [2] . At high H2S concentrations, reduction of the heme iron is stimulated by a second H2S molecule with the concomitant formation of ferrous heme and free polysulfides. To define the molecular details implicated in the electron transfer process and the role of distal residues in this process, the structural aspects associated with the heme-H2S moiety were further evaluated using several HbI mutants and X-ray absorption techniques (XAS). The dynamics of H2Swithin the HbI heme pocket were also evaluated by time-resolved absorption spectroscopy in the picosecond–nanosecond time range. XAS data shows that upon H2S binding to the HbI samples their absorption edges were red shifted by ∼1 eV with respect to the unliganded counterparts, indicating an increase of electron density on the ferric iron. Overall, the results suggest the formation of an “FeII–H2S” like derivative that coexists with the FeIII–H2S complex, upon H2S binding to rHbI and the HbI mutants. For HbI-H2S and the GlnE7His-H2S mutant, an H-bonding with the bound ligand can further modulate the FeII–H2S moiety of the intermediate (i.e. FeII–SH−), which can in principle facilitate interaction with a second H2S molecule, stimulating formation of the final deoxy and H2S2 species. Photo-excitation of the HbI FeIII–H2S complex resulted in photo-dissociation of the H2S from the ferric HbI, followed by fast H2S geminate rebinding with time constant τgem = 12 ± 1 ps. The fast rebinding reflects the high affinity of ferric HbI for H2S and supports the notion release of H2S is facilitated by another H2S molecule, which promotes heme reduction and polysulfide fromation. Since only one geminate rebinding is observed, we infer that there is no docking site for H2S within the heme and that only one energy barrier exists between solvent and heme pocket for sulfide binding. Thus, the rate-limmiting step for H2S releaseform ferric HbI is the diffusion of an aditional sulfide from the solvent to the heme pocket, which occurs at longer times scales. The fact that sulfide release in controlled by the rate of H2S diffusion from the solution to the vicinity of the heme, strongly suggest that hemeproteins modulate the concentration of free sulfide in the external environment, mediating in turn H2S signaling in cells. Disclosure: Nothing to disclose.

Analysis of Pit Stability in Type 316L Stainless Steel Exposed to H2S-Saturated Dilute Chloride Solutions Above the Critical Pitting Temperature

Abstract This study examines the effect of hydrogen sulfide (H2S) on the transition from metastable to stable propagating pits using Type 316L (UNS S31603) steel in brines with Cl− concentrations of 10−3 M to 2 × 10−2 M at 75°C and 150°C. The potentiostatic tests were performed at discrete intervals of 50 mV from the recorded open-circuit potential (OCP) to 300 mV above the OCP. The results based on analysis of the chrono-amperometric transients suggests that the transition to stable pit growth is hindered by H2S saturation at both temperatures. The comparison of the experimental results with similar tests in an acidified control brine (without H2S saturation) indicates that the effect of H2S in the promotion of pit metastability is associated with the diffusion of H2S to the bottom of the cavity. This increases the net anodic current by irreversibly forming metallic sulfides, which, in turn, affect the pit's anolyte by reducing the metal salt concentration. The consequence of this effect is that small pi...

Hydrogen sulfide intrusion in seagrasses from Shark Bay, Western Australia

Sulfides in sediments and hydrogen sulfide (H2S) intrusion in plant tissues were investigated for six species of seagrass in Shark Bay, Western Australia, at two sites with elevated salinities of 42 and 45 psu. H2S intrusion ranged from &lt;20% to 100% in roots and rhizomes, indicating a high degree of sulfide intrusion in some cases, although this did not vary consistently between larger, long-lived species and smaller, less persistent species. There were significant differences in accumulation of total sulfur (TS) among species. Anatomy of rhizomes and roots showed species-specific differences in aerenchyma, the air channels that allow oxygen to diffuse down to the roots and sediments, and tissues with thickened cell walls that could present a barrier to diffusion of H2S, suggesting that morphology may influence sulfide intrusion and sulfur accumulation. Sulfide concentrations in seagrass sediments were far lower in Shark Bay than in Florida Bay, a subtropical embayment where sulfide toxicity has been implicated in seagrass dieback. Despite significant H2S intrusion into tissues of some Shark Bay seagrasses, there was no evidence of any deleterious effects in the current conditions.

Hydrogen Sulfide: An Endogenous Mediator of Resolution of Inflammation and Injury

Hydrogen sulfide is emerging as an important mediator of many aspects of inflammation, and perhaps most importantly as a factor promoting the resolution of inflammation and repair of injury. In the gastrointestinal tract, H(2)S has been shown to promote healing of ulcers and the resolution of mucosal inflammation. On the other hand, suppression of endogenous H(2)S synthesis impairs mucosal defense and leads to increased granulocyte infiltration. H(2)S has been exploited in the design of more effective and safe anti-inflammatory drugs. Enteric bacteria can be a significant source of H(2)S, which could affect mucosal integrity; indeed, luminal H(2)S can serve as an alternative to oxygen as a metabolic substrate for mitochondrial respiration in epithelial cells. Enterocytes and colonocytes thereby represent a "metabolic barrier" to the diffusion of bacteria-derived H(2)S into the subepithelial space. A compromise of this barrier could result in modulation of mucosal function and integrity by bacterial H(2)S. Improvements in methods for measurement of H(2)S and development of more selective inhibitors are crucial for gaining a better understanding of the pathophysiological importance of this mediator. Results from animal studies suggest that H(2)S-releasing agents are promising therapeutic agents for many indications, but these compounds need to be assessed in a clinical setting.

No facilitator required for membrane transport of hydrogen sulfide

Hydrogen sulfide (H(2)S) has emerged as a new and important member in the group of gaseous signaling molecules. However, the molecular transport mechanism has not yet been identified. Because of structural similarities with H(2)O, it was hypothesized that aquaporins may facilitate H(2)S transport across cell membranes. We tested this hypothesis by reconstituting the archeal aquaporin AfAQP from sulfide reducing bacteria Archaeoglobus fulgidus into planar membranes and by monitoring the resulting facilitation of osmotic water flow and H(2)S flux. To measure H(2)O and H(2)S fluxes, respectively, sodium ion dilution and buffer acidification by proton release (H(2)S left arrow over right arrow H(+) + HS(-)) were recorded in the immediate membrane vicinity. Both sodium ion concentration and pH were measured by scanning ion-selective microelectrodes. A lower limit of lipid bilayer permeability to H(2)S, P(M,H(2)S) >or = 0.5 +/- 0.4 cm/s was calculated by numerically solving the complete system of differential reaction diffusion equations and fitting the theoretical pH distribution to experimental pH profiles. Even though reconstitution of AfAQP significantly increased water permeability through planar lipid bilayers, P(M,H(2)S) remained unchanged. These results indicate that lipid membranes may well act as a barrier to water transport although they do not oppose a significant resistance to H(2)S diffusion. The fact that cholesterol and sphingomyelin reconstitution did not turn these membranes into an H(2)S barrier indicates that H(2)S transport through epithelial barriers, endothelial barriers, and membrane rafts also occurs by simple diffusion and does not require facilitation by membrane channels.