Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions

The adsorption of carbon dioxide on hydrotalcite-like compounds was investigated. Two different powdered hydrotalcites were used containing the cations nickel and iron. The powdered materials were screened for carbon dioxide adsorption using a thermogravimetric method and it was found that NiMgAl (Sample 1) hydrotalcite has the largest capacity for CO₂, adsorbing 1.58 mmol g⁻¹ at 20 °C, and highest rate of adsorption of up to 0.17 mmol g⁻¹ min⁻¹. This represented an increase of 53% in adsorption capacity, compared with NiMgAlFe (Sample 2). In order to improve the rheological behaviour of hydrotalcite paste for extrusion, hydrotalcite powders were combined with boehmite alumina (70:30 and 50:50 ratios of hydrotalcite:boehmite) before extrusion into pellets suitable for use in a fixed bed adsorber. These pellets were then re-crushed and further tested by thermogravimetric methods. The effects of temperature, composition and pre-treatment of the hydrotalcites on the adsorption of carbon dioxide and nitrogen are reported. At 20 °C, the amount of carbon dioxide adsorbed was between 2.0 and 2.5 mmol g⁻¹ for all the hydrotalcite/alumina samples in this study, although this decayed rapidly with increasing temperature. The results are compared with silica gel as a common sorbent reference, and with literature values. Hydrotalcite/alumina samples have thermal stability and a high adsorption capacity for carbon dioxide over a wide range of temperatures. The composition of the hydrotalcite/alumina pellets investigated in this study has less effect upon the adsorption behaviour compared with the non-calcined hydrotalcite powder, thus allowing a wide choice of pellet compositions to be used.     

Electrochemical catalytic modification of activated carbon fabrics by ruthenium chloride for supercapacitors

A novel method, electrochemical catalytic oxidation via a ruthenium redox couple in an aqueous RuCl₃ · xH₂O solution rather than the anodic deposition of Ru oxides, was developed to modify the microstructure and electrochemical properties of activated carbon fabrics (ACFs). The variation in microstructures (i.e., specific surface area and mean pore size) for the modified ACFs was examined by means of nitrogen gas adsorption isotherms. The distribution of oxygen-containing functional groups within the modified ACFs was identified by temperature programmed desorption (TPD) and X-ray photoelectron (XPS) spectra. Effects of the electrochemical catalytic modification on the electrochemical characteristics and reversibility of ACFs were investigated systematically by means of cyclic voltammetry (CV) and chronopotentiometry (CP) in 0.5 M H₂SO₄. The total specific capacitance of ACFs reached a maximum (ca. 180 F/g measured at 10 mA/cm²) when they were catalytically modified at 1.15 V with a passed charge density of 5 C/cm². These modified ACFs were demonstrated to be an excellent candidate for the supercapacitor application.     

Adsorption of methimazole on the mercury electrode

The electrosorption behaviour of methimazole (vetranal) at the mercury/1 M NaClO₄ interface was determined from a double layer differential capacity measurements extrapolated to zero frequency. Methimazole is the thiourea derivative, which is used as a drug in the overactivity of the thyroid gland. The values of zero charge potential were shifted to negative potentials ca. 230 mV with an increasing methimazole concentration from 10⁻⁴ M to 10⁻¹ M. The relative surface excesses Γ′ of methimazole were calculated from surface pressure at the density of surface charge σ = constant. The values of Γ′ increase with the increase of σ. Adsorption of methimazole was analyzed on the basis of the constants obtained from the Frumkin and virial isotherms. The values of the standard Gibbs energy of adsorption ΔG° increase with the increase of σ while the values of interaction parameters A and B decrease.     

Electrochemical quartz crystal nanobalance and chronocoulometry studies of phenylalanine adsorption on Au

The interfacial adsorption behaviour of the amino acid, phenylalanine (Phe), was studied at a polycrystalline Au surface in 0.05 M KClO₄ using cyclic voltammetry, chronocoulometry (CC) and electrochemical quartz crystal nanobalance (EQCN) frequency measurements. The frequency was observed to decrease with increasing concentration of Phe, indicating that the frequency measurements were following analyte adsorption directly. Both CC and EQCN frequency measurements showed a two-stage adsorption process, consistent with the molecule being adsorbed in the horizontal position at negative potentials, but rearranged to the more upright position at potentials more positive to the potential of zero charge. From the slopes at the onset of each of these two regions in plots of change in mass from the EQCN frequency measurements versus the surface charge density from CC measurements, the calculated molar mass corresponded to that of Phe displacing adsorbed water molecules for EQCN measurements made with small bulk concentrations of Phe (i.e., <1 × 10⁻⁴ mol L⁻¹).The adsorption process from CC measurements for Phe, described using the Henry adsorption isotherm, gave Gibbs energies of adsorption (ΔG_ADS) ranging from −18 to −35 kJ mol⁻¹ over the potential range of −0.6 to 0.6 V. The observed decrease in frequency of the EQCN measurements with additions of aliquots of amino acid and the substantial ΔG_ADS values suggests that Phe adsorbs onto the surface via chemisorption. Surface concentrations (1.2 × 10⁻¹⁰ mol cm⁻²) were in excellent agreement between the EQCN and CC measurements for small bulk concentrations of Phe (4.0 × 10⁻⁵ mol L⁻¹), in very good agreement with previously published results at the Au(1 1 1) surface. Thus, for small bulk concentrations of analyte, these electrochemical techniques complement one another to enhance our knowledge of the behaviour of thin organic films at electrode surfaces.     

Defluorination-enhanced hydrogen adsorptivity of activated carbon fibers

Fluorinated activated carbon fibers (F-ACFs) were prepared by direct thermal fluorination of pristine activated carbon fibers. By the pyrolysis of F-ACFs at 1073 K under nitrogen gas flow, fluorine was subsequently eliminated and the sp²-bonded ACF structures were recovered. The micropore widths were 1.1 and 0.8 nm, and the isosteric heats of adsorption of nitrogen were 11.3 and 12.8 kJ/mol for pristine and defluorinated ACFs, respectively. These results strongly suggest that changes occurred in the structural properties of micropores in defluorinated ACFs. The hydrogen adsorption isotherms showed that the defluorinated ACFs adsorbed more hydrogen gas than pristine ACFs at 77 K, suggesting that the potential for interaction between hydrogen molecules and the defluorinated slit nanospaces was increased due to the changes in the pore structural properties and/or to the induced polarization of the pore walls making up the modified π-electron systems.     

Influence of halide ions on the adsorption of diphenylamine on iron in 0.5 M H2SO4 solutions

Halide ions are found to enhance the inhibition performance of amines due to enhanced adsorption of amines by the adsorbed halide ions on the metal surface. In this work, the synergistic action of halide ions on the corrosion inhibition of iron in 0.5 M H₂SO₄ by diphenylamine has been found out by electrochemical impedance and polarization methods. Analysis of impedance data has been made with equivalent circuit with constant phase angle element for calculation of double layer capacitance values. Experiments have been carried out in the concentration range of 100-1000 ppm of diphenylamine in the presence of 0.5-1.0 × 10⁻³ M of halide ions. Diphenylamine is found to be a cathodic inhibitor and the inhibition efficiency of about 65% is obtained at 1000 ppm. The anodic and cathodic Tafel slopes in the presence of diphenylamine alone and with halide ions are 65 ± 5 and 105 ± 5 mV, respectively. Diphenylamine inhibits corrosion by adsorption and the surface coverage values are increased considerably in the presence of halide ions. In the presence of iodide ions, the inhibition efficiency of diphenylamine at 100 ppm is increased to more than 90%. In the case of other halide ions, the inhibition efficiency of diphenylamine in increased to 80% at 1000 ppm. The order of synergism of halide ions is I⁻ ? Br⁻ > Cl⁻. The highest synergistic effect of iodide ions is due to chemisorption with metal surface due to its larger size and ease of polarizability.     

Electrochemically enhanced adsorption of aniline on activated carbon fibers

For adsorptive separation processes, the adsorption rate and capacity are two important factors affecting the costs. This study describes the anodic polarization of activated carbon fibers (ACFs), which can enhance the adsorption rate and capacity of aniline. The electrosorption kinetics and the affecting factors (bias potential, electrolyte, and pH) of isotherms for aniline on ACFs were investigated. The adsorption/electrosorption of aniline on ACFs follow pseudo-first-order adsorption kinetics, and the adsorption rate improves with increasing bias potential. The electrosorption isotherms, which exhibit a variety of responses depending on bias potential, electrolyte and pH, follow the two classical models of Langmuir and Freundlich. With electrosorption of aniline from aqueous solution, a two-fold enhancement of adsorption capacity is achievable. The initial and saturated ACFs were characterized using scanning electron micrograph (SEM) and Fourier transform infrared spectroscopy (FT-IR). The SEM micrographs show that the surface of ACFs is not oxidized, which is also verified by cyclic voltammetry results. The FT-IR spectroscopy suggests that the interaction between aniline and ACFs is main weak physisorption instead of chemisorption. These experimental results suggest that the electrochemical polarization of ACFs can effectively improve the adsorption rate and capacity of aniline, which may be due to the enhanced affinity between aniline and ACFs instead of the oxidation on the surface of ACFs or in the solution.     

Meldola blue immobilized on a new SiO2/TiO2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO₂/TiO₂/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol-gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si-O-Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm⁻¹) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO₂/TiO₂/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l⁻¹ and limit of detection of 0.008 mmol l⁻¹. SiO₂/TiO₂/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.     

Adsorption of Cu from water using raw and modified coal fly ashes

In this study, we found that both raw and modified coal fly ashes effectively adsorb Cu²⁺ from wastewater. The adsorption capacities followed the order CFA> CFA-600> CFA-NaOH. The adsorption isotherms for Cu²⁺ on the raw and modified coal fly ashes fit the Langmuir, Freundlich, and DKR isotherms quite well. These adsorptions were endothermic in nature; the values of E (between 1.3 and 9.6 kJ mol⁻¹) were consistent with an ion-exchange adsorption mechanism. The adsorptions of Cu²⁺ onto CFA, CFA-600, and CFA-NaOH followed pseudo-second-order kinetics.     

Adsorption of thiourea on monocrystalline silver electrodes in neutral solution

Impedance spectroscopy and radiometric method have been used in the study of thiourea (TU) adsorption on monocrystalline silver electrodes of basal indices: (1 1 1), (1 0 0) and (1 1 0) in neutral solution. The dependence of the surface concentration of TU on the electrode potential and on the bulk concentration was determined for each studied surface. From radiometric measurements it follows that adsorption of TU on silver electrodes takes place in the entire range of applied potentials. The process of adsorption is practically reversible with respect to the electrode potential (in the range of the double layer) and the bulk concentration of TU. Differential capacity of silver electrodes in 0.01 M NaClO₄ solution containing TU of concentrations from 10⁻⁶ to 5 × 10⁻⁴ M has been measured. The isotherms of TU adsorption, determined from the capacitance and radiometric measurements have been compared and the Gibbs energy of adsorption was calculated. The values of limiting surface concentration of adsorbed TU as well as the Gibbs energy of adsorption depend on the plane of Ag electrode and follow the sequence: Ag(1 1 1) > Ag(1 0 0) > Ag(1 1 0) which is in agreement with the surface density of Ag atoms.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Adsorption of asparagine on the gold electrode and air/solution interface

The adsorption of asparagine (Asn) on a gold electrode from 0.1 M LiClO₄ aqueous solutions was investigated. The experimental data obtained from ac impedance measurements were analyzed to determine the dependence of adsorption parameters, i.e. the standard Gibbs energy of adsorption (ΔG⁰), maximal value of surface excess concentration (Γ_max) of Asn and parameter of interactions in the adsorbed layer (A) on the electrode potential. The relatively large value of Gibbs energy of adsorption (∼ −47 kJ mol⁻¹) gives the evidence of a very strong adsorption of Asn at the polycrystalline Au electrode. The comparison of the adsorption behavior of Asn at the air/solution and the Au/solution interfaces points out to the significant electronic interactions of adsorbate molecules with the Au electrode, since the adsorption of Asn on a free surface (from the same solutions) is very week. The analysis of the electrochemical data as well as the infrared reflection absorption spectroscopy (IRAS) results reveal that Asn molecules are anchored to the Au surface through oxygen atoms of the carboxylate group COO⁻ and through the amide carbonyl group.     

Correlation between retention and adsorption of phenolic compounds in nanofiltration membranes

Equilibrium adsorption experiments of phenol, 3-chlorophenol, 4-chlorophenol, and 3-nitrophenol aqueous solutions on NF90 membrane were conducted to obtain the corresponding adsorption isotherms at 25 ºC. Single-compound solutions with concentration ranging from 0.1 to 8 mmol L^− 1 were used. Freundlich and Langmuir models were compared to the experimental isotherms and their characteristic parameters were obtained from linear fits. In addition, the adsorptive behaviour of twelve aqueous phenolic compounds on the NF90 membrane was studied in order to investigate the relationship between adsorption and retention of selected solutes. An inverse correlation between the adsorbed amount, at the same equilibrium concentration (1 mmol L^− 1), and retention was found. The influence of the molecular hydrophobicity and dipole moment of phenolic compounds on membrane adsorption, solute retention and water flux decline was also investigated.     

Electrogravimetric investigation of formaldehyde oxidation at Pt electrodes in acidic media

The electrochemical and adsorptive behavior of formaldehyde at Pt electrodes in acidic media was investigated using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques. All chemical and electrochemical steps related to formaldehyde oxidation (e.g. bulk adsorption and oxidation, CO (sub)monolayer adsorption and oxidation and electrons per Pt site) were analyzed. All the mass and charge density data in this paper are referred to the real surface area. The charge density associated with formaldehyde oxidation was close to 420 μC cm⁻², which is related to the oxidation of approximately one CO monolayer with two electrons transferred. For CO adsorption the experimental mass value was 50 ng cm⁻². In the region of CO oxidation the analysis of mass and charge variations indicates simultaneous CO oxidation, anion and water adsorption and CO readsorption. The mechanism was confirmed by CO and CO₂ flux calculations. From the analysis of the mass-charge ratio and species flux it was concluded that CO, an intermediate produced during formaldehyde oxidation, is adsorbed at the Pt surface and the main contribution to the mass increase during formaldehyde oxidation is CO readsorption, and water adsorption.     

Immobilization of Nafion-ordered mesoporous carbon on a glassy carbon electrode: Application to the detection of epinephrine

A stable suspension of ordered mesoporous carbon (OMC) was obtained by dispersing OMC in a solution of Nafion. By coating the suspension onto glassy carbon (GC) electrode, cyclic voltammetry was used to evaluate the electrochemical behaviors of Nafion-OMC-modified GC (Nafion-OMC/GC) electrode in 0.1 mmol L⁻¹ hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)/0.1 mol L⁻¹ KCl solution, where Nafion-OMC/GC electrode shows a faster electron transfer rate as compared with OMC/GC, Nafion/GC and GC electrodes. Due to the unique properties of Nafion-OMC, an obvious decrease in the overvoltage of the epinephrine (EP) oxidation (ca. 100 mV at pH 4.1 and 115 mV at pH 7.0) as well as a dramatic increase in the peak current (12 times at pH 4.1 and 6 times at pH 7.0) was observed at Nafion-OMC/GC electrode compared to that seen at GC electrode. By combining the advantages of OMC with those of Nafion, the anodic peak of EP and that of ascorbic acid (AA) were separated successfully (by ca. 144-270 mV) in the pH range of 2.0-10.0, which may make Nafion-OMC/GC electrode potential for selective determination of EP in the presence of AA at a broad pH range. As an EP sensor, the EP amperometric response at Nafion-OMC/GC electrode in pH 7.0 PBS is extremely stable, with 99% of the initial activity remaining (compared to 32% at GC surface) after 120 min stirring of 0.20 mmol L⁻¹ EP. And Nafion-OMC/GC electrode can be used to readily detect the physiological concentration of EP at pH 7.0. These make Nafion-OMC/GC electrode potential candidates for stable and efficient electrochemical sensor for the detection of EP. The solubilization of OMC by Nafion may provide a route to more precise manipulation, and functionalization for the construction of OMC-based sensors, as well as allowing OMC to be introduced to biologically relevant systems.     

Effects of 1,4-naphthoquinone on aluminum corrosion in 0.50 M sodium chloride solutions

Effects of 1,4-naphthoquinone (NQ) have been investigated as a corrosion inhibitor for aluminum in aerated and de-aerated solutions of 0.50 M NaCl using potentiodynamic polarization, chronoamperometry (CA), open-circuit potential (OCP), electrochemical impedance spectroscopic (EIS), scanning electron microscopic (SEM), cyclic voltammetric, and quartz crystal analyzer (QCA) techniques. These measurements revealed that the presence of NQ shifted the corrosion and pitting potentials to more noble values and decreased the anodic currents in the passive region in both aerated and de-aerated chloride solutions, and the surface and polarization resistances are increased as the concentration of NQ is increased. The most effective concentration of NQ for corrosion inhibition was found to be 1.0 × 10⁻³ M in both aerated and de-aerated chloride solutions. The QCA data indicate that adsorption of NQ molecules plays an important role in protecting the pits on the aluminum surface. The SEM images show that the presence of NQ decreased the severity of the pitting corrosion of aluminum to a great extent at −675 mV versus Ag/AgCl.     

Oxygen and phosphorus enriched carbons from lignocellulosic material

Activated carbons were prepared by phosphoric acid activation of fruit stones in air at temperatures 400-1000 °C. The surface chemistry was investigated by elemental analysis, cation exchange capacity, infrared spectroscopy and potentiometric titration. The porous structure was analyzed from adsorption isotherms (N₂ at 77 K and CO₂ at 273 K). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (2.2 mmol g⁻¹) being attained at 700 °C, which coincides with the maximum contents of phosphorus and oxygen. The use of air instead of argon during thermal treatment increased the amount of cation exchangeable surface groups for carbons obtained at 400-700 °C. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK 1.8-3.1 (carboxylic and polyphosphates), 4.8-6.3 (second dissociation of carboxylic, weak acid in polyphosphates and enol structures) and 8.1-9.7 (phenols and enol structures). Carbons obtained in air at 400-600 °C show enhanced copper adsorption from 0.001 mol L⁻¹ Cu(NO₃)₂ in acidic solutions as compared to carbons obtained in argon. Carbons obtained in air show well-developed porous structure that is equivalent or higher as compared with carbons obtained in argon; the difference being progressively increased with increasing treatment temperature.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Probing the electrochemical behaviour of SWCNT-cobalt nanoparticles and their electrocatalytic activities towards the detection of nitrite at acidic and physiological pH conditions

The electrochemical decoration of edge plane pyrolytic graphite electrode (EPPGE) with cobalt and cobalt oxide nanoparticles integrated with and without single-walled carbon nanotubes (SWCNTs) is described. Successful modification of the electrodes was confirmed by field emission scanning electron microscopy (FESEM), AFM and EDX techniques. The electron transfer behaviour of the modified electrodes was investigated in [Fe(CN)6]^3−/4− redox probe using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and discussed. The study showed that cobalt nanoparticles modified electrodes exhibit faster electron transfer behaviour than their oxides. The catalytic rate constant (K) obtained at the EPPGE-SWCNT-Co for nitrite at pH 7.4 and 3.0 are approximately the same (∼3 × 10⁴ cm³ mol⁻¹ s⁻¹) while the limits of detection (LoD = 3.3δ/m) are in the μM order. From the adsorption stripping voltammetry, the electrochemical adsorption equilibrium constant β was estimated as (13.0 ± 0.1) × 10³ M⁻¹ at pH 7.4 and (56.7 ± 0.1) × 10³ M⁻¹ at pH 3.0 while the free energy change (ΔG°) due to the adsorption was estimated as −6.36 and −10.00 kJ mol⁻¹ for nitrite at pH 7.4 and 3.0, respectively.     

Preparation, characterization and Methylene Blue adsorption of phosphoric acid activated carbons from globe artichoke leaves

Activated carbons were prepared by the pyrolysis of artichoke leaves impregnated with phosphoric acid at 500 °C for different impregnation ratios: 100, 200, 300 wt.%. Materials were characterized for their surface chemistry by elemental analysis, “Boehm titrations”, point of zero charge measurements, infrared spectroscopy, as well as for their porous and morphological structure by Scanning Electron Microscopy and nitrogen adsorption at 77 K. The impregnation ratio was found to govern the porous structure of the prepared activated carbons. Low impregnation ratios (~ 100 wt.%) led to essentially microporous and acidic activated carbons whereas high impregnation ratios (> 100 wt.%) gave essentially microporous-mesoporous carbons with specific surface areas as high as 2038 m²·g^− 1, pore volume as large as 2.47 cm³·g^− 1, and a slightly acidic surface. The prepared activated carbons were studied for their adsorption isotherms of Methylene Blue at pH = 3 and pH = 9. The supermicroporous structure of the material produced at 200 wt.% H₃PO₄ ratio was found to be appropriate for an efficient adsorption of this dye controlled by dispersive and electrostatic interactions depending on the amount of oxygen at the surface.