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1.
We used titration calorimetry to measure the bulk heats of proton and Cd adsorption onto a common Gram positive soil bacterium Bacillus subtilis at 25.0 °C. Using the 4-site non-electrostatic model of Fein et al. [Fein, J.B., Boily, J.-F., Yee, N., Gorman-Lewis, D., Turner, B.F., 2005. Potentiometric titrations of Bacillus subtilis cells to low pH and a comparison of modeling approaches. Geochim. Cosmochim. Acta69 (5), 1123-1132.] to describe the bacterial surface reactivity to protons, our bulk enthalpy measurements can be used to determine the following site-specific enthalpies of proton adsorption for Sites 1-4, respectively: −3.5 ± 0.2, −4.2 ± 0.2, −15.4 ± 0.9, and −35 ± 2 kJ/mol, and these values yield the following third law entropies of proton adsorption onto Sites 1-4, respectively: +51 ± 4, +78 ± 4, +79 ± 5, and +60 ± 20 J/mol K. An alternative data analysis using a 2-site Langmuir-Freundlich model to describe proton binding to the bacterial surface (Fein et al., 2005) resulted in the following site-specific enthalpies of proton adsorption for Sites 1 and 2, respectively: −3.6 ± 0.2 and −35.1 ± 0.3 kJ/mol. The thermodynamic values for Sites 1-3 for the non-electrostatic model and Site 1 of the Langmuir-Freundlich model of proton adsorption onto the bacterial surface are similar to those associated with multifunctional organic acid anions, such as citrate, suggesting that the protonation state of a bacterial surface site can influence the energetics of protonation of neighboring sites. Our bulk Cd enthalpy data, interpreted using the 2-site non-electrostatic Cd adsorption model of Borrok et al. [Borrok, D., Fein, J.B., Tischler, M., O’Loughlin, E., Meyer, H., Liss, M., Kemner, K.M., 2004b. The effect of acidic solutions and growth conditions on the adsorptive properties of bacterial surfaces. Chem. Geol.209 (1-2), 107-119.] to account for Cd adsorption onto B. subtilis, yield the following site-specific enthalpies of Cd adsorption onto bacterial surface Sites 2 and 3, respectively: −0.2 ± 0.4 and +14.4 ± 0.9 kJ/mol, and the following third law entropies of Cd adsorption onto Sites 2 and 3, respectively: +57 ± 4 and +128 ± 5 J/mol K. The calculated enthalpies of Cd adsorption are typical of those associated with Cd complexation with anionic oxygen ligands, and the entropies are indicative of inner sphere complexation by multiple ligands. The experimental approach described in this study not only yields constraints on the molecular-scale mechanisms involved in proton and Cd adsorption reactions, but also provides new thermodynamic data that enable quantitative estimates of the temperature dependence of proton and Cd adsorption reactions.  相似文献   

2.
The subsurface mobility of Np is difficult to predict in part due to uncertainties associated with its sorption behavior in geologic systems. In this study, we measured Np adsorption onto a common gram-positive soil bacterium, Bacillus subtilis. We performed batch adsorption experiments with Np(V) solutions as a function of pH, from 2.5 to 8, as a function of total Np concentration from 1.29 × 10−5 M to 2.57 × 10−4 M, and as a function of ionic strength from 0.001 to 0.5 M NaClO4. Under most pH conditions, Np adsorption is reversible and exhibits an inverse relationship with ionic strength, with adsorption increasing with increasing pH. At low pH in the 0.1 M ionic strength systems, we observed irreversible adsorption, which is consistent with reduction of Np(V) to Np(IV). We model the adsorption reaction using a nonelectrostatic surface complexation approach to yield ionic strength dependent NpO2+-bacterial surface stability constants. The data require two bacterial surface complexation reactions to account for the observed adsorption behavior: R-L1− + NpO2+ ↔ R-L1-NpO2° and R-L2− + NpO2+ ↔ R-L2-NpO2°, where R represents the bacterium to which each functional group is attached, and L1 and L2 represent the first and second of four discrete site types on the bacterial surface. Stability constants (log K values) for the L1 and L2 reactions in the 0.001 M system are 2.3 ± 0.3 and 2.3 ± 0.2, and in the 0.1 M system the values are 1.7 ± 0.2 and 1.6 ± 0.2, respectively. The calculated neptunyl-bacterial surface stability constants are not consistent with values predicted using the linear free energy correlation approach from Fein et al. (2001), suggesting that possible unfavorable steric interactions and the low charge of NpO2+ affects Np-bacterial adsorption.  相似文献   

3.
A thermodynamic model is proposed for calculation of liquidus relations in multicomponent systems of geologic interest. In this formulation of mineral-melt equilibria, reactions are written in terms of the liquid oxide components, and balanced on the stoichiometry of liquidus phases. In order to account for non-ideality in the liquid, a ‘Margules solution’ is derived in a generalized form which can be extended to systems of any number of components and for polynomials of any degree. Equations are presented for calculation of both the excess Gibbs free energy of a solution and the component activity coefficients.Application to the system CaO-Al2O3-SiO2 at one atmosphere pressure is achieved using linear programming. Thermodynamic properties of liquidus minerals and the melt are determined which are consistent with adopted error brackets for available calorimetric and phase equilibrium data. Constraints are derived from liquidus relations, the CaO-SiO2 binary liquid immiscibility gap, solid-solid P-T reactions, and measured standard state entropies, enthalpies, and volumes of minerals in this system.Binary and ternary liquidus diagrams are recalculated by computer programs which trace cotectic boundaries and isothermal sections while checking each point on a curve for metastability. The maximum differences between calculated and experimentally determined invariant points involving stoichiometric minerals are 17°C and 1.5 oxide weight per cent. Because no solid solution models have been incorporated, deviations are larger for invariant points which involve non-stoichiometric minerals.Calculated heats of fusion, silica activities in the melt, and heats of mixing of liquids compare favorably with experimental data, and suggest that this model can be used to supplement the limited amount of available data on melt properties.  相似文献   

4.
《Applied Geochemistry》1993,8(2):127-139
Ligand adsorption on δ-Al2O3 at pH 8 was examined for a series of organic ligands (aromatic acids, monochlorophenols and aliphatic acids) including both monodentate and bidentate ligands. Adsorption isotherms for the aromatic acids exhibited saturation at high dissolved ligand concentrations; saturation was not observed (over the concentration range examined) for the chlorophenols. Small, though measurable, amounts of heat were evolved on reaction of the aromatic acids, the monochlorophenols and propionate (but not of the longer chain fatty acids) with the oxide surface; overall ligand adsorption reactions wereexothermic (ΔHobs < O). For adsorption of (partially or fully) protonated ligands, the favorable ΔHobs was due largely to the exothermic proton transfer reaction between phenolic hydroxyl groups of the ligands and hydroxide ions displaced from the oxide surface. The enthalpy corresponding to the ligand-exchange reaction of surface hydroxyl groups for the various ligands (as fully deprotonated species), ΔHcorr, appeared to be related to the ligand structure. The surface ligand-exchange reaction was more exothermic for the dicarboxylic acid phthalate than for the monocarboxylic acids benzoate or propionate or for salicylate and was endothermic for the chlorophenols.  相似文献   

5.
We report on the thermochemistry of proton hydration by water in the gas phase both experimentally using high-pressure mass spectrometry (HPMS) and theoretically using multilevel G3, G3B3, CBS-Q, CBS-QB3, CBS/QCI-APNO as well as density functional theory (DFT) calculations. Gas phase hydration enthalpies and entropies for protonated water cluster equilibria with up to 7 waters (i.e., n ? 7H3O+·(H2O)n) were observed and exhibited non-monotonic behavior for successive hydration steps as well as enthalpy and entropy anomalies at higher cluster rank numbers. In particular, there is a significant jump in the stepwise enthalpies and entropies of cluster formation for n varying from 6 to 8. This behavior can be successfully interpreted using cluster geometries obtained from quantum chemical calculations by considering the number of additional hydrogen bonds formed at each hydration step and simultaneous weakening of ion-solvent interaction with increasing cluster size. The measured total hydration energy for the attachment of the first six water molecules around the hydronium ion was found to account for more than 60% of total bulk hydration free energy.  相似文献   

6.
The adsorptive removal of Cr(VI) was studied using activated carbon derived from Leucaena leucocephala (ACLL). The physico-chemical properties of ACLL were determined using proximate analysis and N2 BET surface area analysis. The N2 BET surface area of ACLL was determined to be 1131 m2 g?1. The point of zero charge (pHpzc) of 5.42 indicated that ACLL surface was positively charged for pH below the pHPZC, attracting anions. The effect of experimental operating parameters such as time of contact, ACLL dose, pH, initial concentration and temperature was investigated. The optimum values of parameters such as concentration of 100 mg L?1, 300 mg of ACLL dose, time of contact of 60 min, pH of 4 indicated the maximum Cr(VI) uptake of 13.85 mg g?1. The pseudo-second-order kinetic model best fitted with the Cr(VI) adsorption data. Adsorptive removal of Cr(VI) onto ACLL satisfactorily fitted in the order of Redlich–Peterson > Freundlich > Langmuir > Temkin adsorption isotherm model. The thermodynamic parameters showed the adsorption of Cr(VI) onto ACLL was an endothermic and spontaneously occurred process.  相似文献   

7.
The purpose of the present work is to extend our knowledge of metal–cyanobacteria interactions and to contribute to the database on adsorption parameters of aquatic microorganisms with respect to metal pollutants. To this end, the surface properties of the cyanobacteria (Gloeocapsa sp. f-6gl) were studied using potentiometric acid–base titration methods and ATR-FTIR (attenuated total reflection infrared) spectroscopy. The electrophoretic mobility of viable cells was measured as a function of pH and ionic strength (0.01 and 0.1 M). Surface titrations at 0.01–1.0 M NaCl were performed using limited residence time reactors (discontinuous titration) with analysis of Ca, Mg and dissolved organic C for each titration point in order to account for alkali-earth metal–proton exchange and cell degradation, respectively. Results demonstrate that the cell-wall bound Ca and Mg from the culture media contribute to the total proton uptake via surface ion-exchange reactions. This has been explicitly taken into account for net proton balance calculations. Adsorption of Zn, Cd, Pb and Cu was studied at 25 °C in 0.01 M NaNO3 as a function of pH and metal concentration. The proportion of adsorbed metal increases as a function of culture age with cells of 44 days old having the largest adsorption capacities. A competitive Langmuir sorption isotherm in conjunction with a linear programming method (LPM) was used to fit experimental data and assess the number of surface sites and adsorption reaction constants involved in the binding of metals to the cyanobacteria surface. These observations allowed the determination of the identity and concentration of the major surface functional groups (carboxylate, amine, phosphoryl/phosphodiester and hydroxyl) responsible for the amphoteric behavior of cyanobacterial cell surfaces in aqueous solutions and for metal adsorption. Results of this work should allow better optimizing of metal bioremediation/biosequestration processes as they help to define the most efficient range of pH, cell biomass and duration of exposure necessary for controlled metal adsorption on cyanobacteria cultures. It follows from comparison of adsorption model parameters between different bacteria that technological application of cyanobacteria in wastewater bioremediation can be as efficient as other biological sorbents.  相似文献   

8.
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9.
The adsorption of hydrogen sulfide (ΓH2S) and protons (ΓH+) on the surface of crystalline sulfur was investigated experimentally in H2S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm−3 and log CH+ values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of ΓH2S were very close to the highest values determined for ΓH+. This finding, combined with the lack of detectable proton adsorption in H2S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of ≡ S − H2S complexes in the presence of H2S.We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: ≡ S + H2S = ≡SSH20 β° The deprotonation of the ≡ SH° complex occurs via the reaction: ≡ SSH20 = ≡SSH− + H+ β− Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β− at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.  相似文献   

10.
The enthalpies of solution of synthetic Mg2SiO4-Fe2SiO4 olivine solid solutions have been measured in Pb2B2O5 melt at 970 K. The heat of solution of forsterite was found to be 15.62 ± 0.3 kcal mol?1 and that of fayalite 9.39 ± 0.14 kcal mol?1. Solid solutions between these end-members exhibit small positive deviations from mixing ideality, asymmetric towards the Fe end-member. In terms of the sub-regular solution model, excess enthalpies of intermediate olivine are adequately represented by the equation Hxs = 2(1000 + 1000XFe) XFeXMgThe enthalpies of solution at 970 K are consistent with high temperature phase equilibrium measurements of activity-composition relationships in the olivine series. Excess entropy terms are not needed to relate the phase equilibrium data to the calorimetric data presented here.The enthalpy of solution of FeSiO3 ferrosilite at 970 K was found to be 4.36 ± 0.10 kcal mol?1. This value, when taken together with calorimetric measurements on fayalite and quartz, is consistent with phase equilibrium investigations of the reaction: 2FeSiO3 = Fe2SiO4 + SiO2 Ferrosilite Fayalite QuartzThese provide a check on the internal consistency of the calorimetric data presented here.  相似文献   

11.
In the present study, adsorption of lead (II) ions from aqueous solution by alluvial soil of Bhagirathi River was investigated under batch mode. The influence of solution pH, sorbent dose, initial lead (II) concentration, contact time, stirring rate and temperature on the removal process were investigated. The lead adsorption was favored with maximum adsorption at pH 6.0. Sorption equilibrium time was observed in 60 min. The equilibrium adsorption data were analyzed by the Freundlich, Langmuir, Dubinin–Radushkevich and Temkin adsorption isotherm models. The kinetics of lead (II) ion was discussed by pseudo first-order, pseudo second-order, intra-particle diffusion, and surface mass transfer models. It was shown that the adsorption of lead ions could be described by the pseudo second-order kinetic model. The activation energy of the adsorption process (E a) was found to be ?38.33 kJ mol?1 using the Arrhenius equation, indicating exothermic nature of lead adsorption onto alluvial soil. Thermodynamic parameters, such as Gibbs free energy (?G 0), the enthalpy (?H 0), and the entropy change of sorption (?S 0) have also been evaluated and it has been found that the adsorption process was spontaneous, feasible, and exothermic in nature. The results indicated that alluvial soil of Bhagirathi River can be used as an effective and low cost adsorbent to remove lead ions from aqueous solutions.  相似文献   

12.
Macro- and molecular-scale knowledge of uranyl (U(VI)) partitioning reactions with soil/sediment mineral components is important in predicting U(VI) transport processes in the vadose zone and aquifers. In this study, U(VI) reactivity and surface speciation on a poorly crystalline aluminosilicate mineral, synthetic imogolite, were investigated using batch adsorption experiments, X-ray absorption spectroscopy (XAS), and surface complexation modeling. U(VI) uptake on imogolite surfaces was greatest at pH ∼7-8 (I = 0.1 M NaNO3 solution, suspension density = 0.4 g/L [U(VI)]i = 0.01-30 μM, equilibration with air). Uranyl uptake decreased with increasing sodium nitrate concentration in the range from 0.02 to 0.5 M. XAS analyses show that two U(VI) inner-sphere (bidentate mononuclear coordination on outer-wall aluminol groups) and one outer-sphere surface species are present on the imogolite surface, and the distribution of the surface species is pH dependent. At pH 8.8, bis-carbonato inner-sphere and tris-carbonato outer-sphere surface species are present. At pH 7, bis- and non-carbonato inner-sphere surface species co-exist, and the fraction of bis-carbonato species increases slightly with increasing I (0.1-0.5 M). At pH 5.3, U(VI) non-carbonato bidentate mononuclear surface species predominate (69%). A triple layer surface complexation model was developed with surface species that are consistent with the XAS analyses and macroscopic adsorption data. The proton stoichiometry of surface reactions was determined from both the pH dependence of U(VI) adsorption data in pH regions of surface species predominance and from bond-valence calculations. The bis-carbonato species required a distribution of surface charge between the surface and β charge planes in order to be consistent with both the spectroscopic and macroscopic adsorption data. This research indicates that U(VI)-carbonato ternary species on poorly crystalline aluminosilicate mineral surfaces may be important in controlling U(VI) mobility in low-temperature geochemical environments over a wide pH range (∼5-9), even at the partial pressure of carbon dioxide of ambient air (pCO2 = 10−3.45 atm).  相似文献   

13.
Numerous studies have utilized surface complexation theory to model proton adsorption behaviour onto mesophilic bacteria. However, few experiments, to date, have investigated the effects of pH and ionic strength on proton interactions with thermophilic bacteria. In this study, we characterize proton adsorption by the thermophile Anoxybacillus flavithermus by performing acid-base titrations and electrophoretic mobility measurements in NaNO3 (0.001-0.1 M). Equilibrium thermodynamics (Donnan model) were applied to describe the specific chemical reactions that occur at the water-bacteria interface. Acid-base titrations were used to determine deprotonation constants and site concentrations for the important cell wall functional groups, while electrophoretic mobility data were used to further constrain the model. We observe that with increasing pH and ionic strength, the buffering capacity increases and the electrophoretic mobility decreases. We develop a single surface complexation model to describe proton interactions with the cells, both as a function of pH and ionic strength. Based on the model, the acid-base properties of the cell wall of A. flavithermus can best be characterized by invoking three distinct types of cell wall functional groups, with pKa values of 4.94, 6.85, and 7.85, and site concentrations of 5.33, 1.79, and 1.42 × 10−4 moles per gram of dry bacteria, respectively. A. flavithermus imparts less buffering capacity than pure mesophilic bacteria studied to date because the thermophile possesses a lower total site density (8.54 × 10−4 moles per dry gram bacteria).  相似文献   

14.
In a recent study [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830.] the hydrolysis of Np(V) was investigated at 10-85 °C by absorption spectroscopy, potentiometry, and microcalorimetry along the titration of Np(V) solutions with tetramethylammonium hydroxide up to pH 10. However, there is strong evidence that the precautions to avoid competing reactions with carbonate were not sufficient and that the measured effects are not caused by the formation of Np(V) hydroxide complexes but primarily by the formation of Np(V) carbonate complexes. The reported equilibrium constants, enthalpies, entropies, and heat capacities for the complexes NpO2OH(aq) and are severely in error and must not be used for the geochemical modeling of neptunium. If the hydrolysis constants reported by Rao et al. [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830] are used to calculate neptunium solubilities in alkaline solutions relevant for nuclear waste repositories, the Np(V) concentrations are overestimated by orders of magnitude.  相似文献   

15.
Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO3 electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (65Cu/63Cu) and Zn (66Zn/64Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (αsoln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δsoln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).  相似文献   

16.
X-ray absorption fine structure (XAFS) measurements was used at the U L3-edge to directly determine the pH dependence of the cell wall functional groups responsible for the absorption of aqueous UO22+ to Bacillus subtilis from pH 1.67 to 4.80. Surface complexation modeling can be used to predict metal distributions in water-rock systems, and it has been used to quantify bacterial adsorption of metal cations. However, successful application of these models requires a detailed knowledge not only of the type of bacterial surface site involved in metal adsorption/desorption, but also of the binding geometry. Previous acid-base titrations of B. subtilis cells suggested that three surface functional group types are important on the cell wall; these groups have been postulated to correspond to carboxyl, phosphoryl, and hydroxyl sites. When the U(VI) adsorption to B. subtilis is measured, observed is a significant pH-independent absorption at low pH values (<3.0), ascribed to an interaction between the uranyl cation and a neutrally charged phosphoryl group on the cell wall. The present study provides independent quantitative constraints on the types of sites involved in uranyl binding to B. subtilis from pH 1.67 to 4.80. The XAFS results indicate that at extremely low pH (pH 1.67) UO22+ binds exclusively to phosphoryl functional groups on the cell wall, with an average distance between the U atom and the P atom of 3.64 ± 0.01 Å. This U-P distance indicates an inner-sphere complex with an oxygen atom shared between the UO22+ and the phosphoryl ligand. The P signal at extremely low pH value is consistent with the UO22+ binding to a protonated phosphoryl group, as previously ascribed. With increasing pH (3.22 and 4.80), UO22+ binds increasingly to bacterial surface carboxyl functional groups, with an average distance between the U atom and the C atom of 2.89 ± 0.02 Å. This U-C distance indicates an inner-sphere complex with two oxygen atoms shared between the UO22+ and the carboxyl ligand. The results of this XAFS study confirm the uranyl-bacterial surface speciation model.  相似文献   

17.
The internal energies and entropies of 21 well-known minerals were calculated using the density functional theory (DFT), viz. kyanite, sillimanite, andalusite, albite, microcline, forsterite, fayalite, diopside, jadeite, hedenbergite, pyrope, grossular, talc, pyrophyllite, phlogopite, annite, muscovite, brucite, portlandite, tremolite, and CaTiO3–perovskite. These thermodynamic quantities were then transformed into standard enthalpies of formation from the elements and standard entropies enabling a direct comparison with tabulated values. The deviations from reference enthalpy and entropy values are in the order of several kJ/mol and several J/mol/K, respectively, from which the former is more relevant. In the case of phase transitions, the DFT-computed thermodynamic data of involved phases turned out to be accurate and using them in phase diagram calculations yields reasonable results. This is shown for the Al2SiO5 polymorphs. The DFT-based phase boundaries are comparable to those derived from internally consistent thermodynamic data sets. They even suggest an improvement, because they agree with petrological observations concerning the coexistence of kyanite?+?quartz?+?corundum in high-grade metamorphic rocks, which are not reproduced correctly using internally consistent data sets. The DFT-derived thermodynamic data are also accurate enough for computing the P–T positions of reactions that are characterized by relatively large reaction enthalpies (>?100 kJ/mol), i.e., dehydration reactions. For reactions with small reaction enthalpies (a few kJ/mol), the DFT errors are too large. They, however, are still far better than enthalpy and entropy values obtained from estimation methods.  相似文献   

18.
Acacia nilotica was used for the adsorption of Reactive Black 5 (RB5) dye from an aqueous solution. Both the raw and activated (with H3PO4) carbon forms of Acacia nilotica (RAN and ANAC, respectively) were used for comparison. Various parameters (including dye concentration, contact time, temperature, and pH) were optimized to obtain the maximum adsorption capacity. RAN and ANAC were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The maximum experimental adsorption capacities for RAN and ANAC were 34.79 and 41.01 mg g?1, respectively, which agreed with the maximum adsorption capacities predicted by the Langmuir, Freundlich, and Dubinin–Radushkevich equilibrium isotherm models. The adsorption data of ANAC showed a good fit to the isotherm models based on the coefficient of determination (R 2): Langmuir type II (R 2 = 0.99) > Freundlich (R 2 = 0.9853) > Dubinin–Radushkevich (R 2 = 0.9659). This result suggested monolayer adsorption of RB5 dye. The adsorption of RB5 dye followed pseudo-second-order kinetics. The RAN adsorbent reflected an exothermic reaction (enthalpy change, ΔH = ?0.006 kJ mol?1) and increased randomness (standard entropy change, ΔS = 0.038 kJ mol?1) at the solid–solution interface. In contrast, ANAC reflected both exothermic [?0.011 kJ mol?1 (303–313 K)] and endothermic [0.003 kJ mol?1 (313–323 K)] reactions. However, the ΔS value of ANAC was lower when the RB5 adsorption increased from 313 to 323 K. The negative values for the Gibbs free energy change at all temperatures indicated that the adsorption of RB5 dye onto RAN and ANAC was spontaneous in the forward direction.  相似文献   

19.
In this study, we measure proton, Pb, and Cd adsorption onto the bacteria Deinococcus radiodurans, Thermus thermophilus, Acidiphlium angustum, Flavobacterium aquatile, and Flavobacterium hibernum, and we calculate the thermodynamic stability constants for the important surface complexes. These bacterial species represent a wide genetic diversity of bacteria, and they occupy a wide range of habitats. All of the species, except for A. angustum, exhibit similar proton and metal uptake. The only species tested that exhibits significantly different protonation behavior is A. angustum, an acidophile that grows at significantly lower pH than the other species of this study. We demonstrate that a single, metal-specific, surface complexation model can be used to reasonably account for the acid/base and metal adsorption behaviors of each species. We use a four discrete site non-electrostatic model to describe the protonation of the bacterial functional groups, with averaged pKa values of 3.1 ± 0.3, 4.8 ± 0.2, 6.7 ± 0.1, and 9.2 ± 0.3, and site concentrations of (1.0 ± 0.17) × 10−4, (9.0 ± 3.0) × 10−5, (4.6 ± 1.8) × 10−5, and (6.1 ± 2.3) × 10−5 mol of sites per gram wet mass of bacteria, respectively. Adsorption of Cd and Pb onto the bacteria can be accounted for by the formation of complexes with each of the bacterial surface sites. The average log stability constants for Cd complexes with Sites 1-4 are 2.4 ± 0.4, 3.2 ± 0.1, 4.4 ± 0.1, and 5.3 ± 0.1, respectively. The average log stability constants for Pb complexes with Sites 1-4 are 3.3 ± 0.2, 4.5 ± 0.3, 6.5 ± 0.1, and 7.9 ± 0.5, respectively. This study demonstrates that a wide range of bacteria exhibit similar proton and metal adsorption behaviors, and that a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes yields a reasonable model for the adsorption behavior of many of these species. The differences in adsorption behavior that we observed for A. angustum demonstrate that genetic differences do exist between the cell wall functional group chemistries of some bacterial species, and that significant exceptions to the typical bacterial adsorption behavior do exist.  相似文献   

20.
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