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1.
A study on energy changes and mechanisms of Cd sorbed on goethite was performed using the technique of microcalorimetry.The data of the amount of Cd sorpion(Aq) and concentration of Cd in equilibrium solution(Ce),and the data of Aq and the heat effect(AH) caused by Cd^2 sorption on goethite all fitted Langmuir isotherm.The amount of heat released from Cd sorption on goethite increased with the amount of Ce or Aq.The reaction process of Cd sorption on goethite may be divided into five stages and three plateaus,depending on the variation of enthalpy change(ΔaHm) of Cd sorption with Aq,which implied three mechanisms of interaction between Cd and goethite.The experimental results showed that the microcalorimetry may be useful for determination of microcalorie variation in soil. 相似文献
2.
胡敏酸对铵钾在粘土矿物上交互作用的影响 总被引：1，自引：0，他引：1
ZHANG Wen-Zhao CHEN Xiao-Qin ZHOU Jian-Min LIU Dai-Huan WANG Huo-Yan DU Chang-Wen 《土壤圈》2013,23(4):493-502
Interaction of ammonium (NH+4) and potassium (K+) is typical in field soils. However, the effects of organic matter on interaction of NH+4 and K+have not been thoroughly investigated. In this study, we examined the changes in major physicochemical properties of three clay minerals (kaolinite, illite, and montmorillonite) after humic acid (HA) coating and evaluated the influences of these changes on the interaction of NH+4 and K+on clay minerals using batch experiments. After HA coating, the cation exchange capacity (CEC) and specific surface area (SSA) of montmorillonite decreased significantly, while little decrease in CEC and SSA occurred in illite and only a slight increase in CEC was found in kaolinite. Humic acid coating significantly increased cation adsorption and preference for NH+4, and this effect was more obvious on clay minerals with a lower CEC. Results of Fourier transform infrared spectrometry analysis showed that HA coating promoted the formation of H-bonds between the adsorbed NH+4 and the organo-mineral complexes. HA coating increased cation fixation capacity on montmorillonite and kaolinite, but the opposite occurred on illite. In addition, HA coating increased the competitiveness of NH+4 on fixation sites. These results showed that HA coating affected both the nature of clay mineral surfaces and the reactions of NH+4 and K+with clay minerals, which might influence the availability of nutrient cations to plants in field soils amended with organic matter. 相似文献
3.
重金属污染对典型湿润富铁土上钾素行为的影响 总被引：2，自引：2，他引：2
Difference of montmorillonite(Mt),illite(It) and kaolinite(Kt) in lead sorption characteristics and the effects of amorphous Fe and Al oxide coatings on the characteristics were experimentally sutdied with logistic model.The sorption curves had sigmoid feature due to use of acetate-type buffer solution.With the model the sorption process could be divided into four stages and the sorption characteristics at the stages were discussed.The results showed that,after Mt,It and Kt were coated by amorphous Fe oxide,their maximum sorption capacity(MSC) and percentage of high-SSC concentration scope(HCS) of Pb^2 increased markedly,but the specific sorption capacity(SSC) decreased.with regard to effects of amorphous Al oxide coating,except for It Al,the SSC of other samples showed a downtrend,despite that their MSC remained unchanged.Eventually,the gray correlation degrees to Pb^2 sorption for different physicochemical characteristics of the clay minerals were indicated to be higher for hydronium,zero point of surface charge and hydroxy,but lower for specific surface area.density of surface charge and amount of surface charges. 相似文献
4.
The adsorption kinetics of Pb2+ on different soils and minerals with variable charges was studied by the two ion-selective electrode technique at different pH and concentrations. The results showed that more than 95% of adsorption on all the samples occurred during the first 5 minutes. All adsorption time-dependent data could fit the surface second-order equation very well. The values of Xm were goethite > kaolinite, and latosol > red soil at the same initial reaction concentration. The values of k were kaolinite > > goethite, and latosol > red soil at the same reaction pH and initial concentration. 相似文献
5.
Experiments were conducted to study the influences of synthetic bayerite, non-crystalline aluminum oxide (N-AlOH), goethite, non-crystalline iron oxide (N-FeOH) and kaolinite on the adsorption, activity, kinetics and thermal stability of invertase. Adsorption of invertase on iron, aluminum oxides fitted Langmuir equation. The amount of invertase held on the minerals followed the sequence kaolinite > goethite > N-AlOH > bayerite > N-FeOH. No correlation was found between enzyme adsorption and the specific surface area of minerals examined. The differences in the surface structure of minerals and the arrangement of enzymatic molecules on mineral surfaces led to the different capacities of minerals for enzyme adsorption. The adsorption of invertase on bayerite, N-AlOH, goethite, N-FeOH and kaolinite was differently affected by pH. The order for the activity of invertase adsorbed on minerals was N-FeOH > N-AlOH > bayerite > reak goethite > kaolinite. The inhibition effect of minerals on enzyme activity was kaolinite > crystalline oxides > non-crystalline oxides. The pH optimum of iron oxide- and aluminum oxide-invertase complexes was similar to that of free enzyme (pH 4.0), whereas the pH optimum of kaolinite-invertase complex was one pH unit higher than that of free enzyme. The affinity to substrate and the maximum reaction velocity as well as the thermal stability of combined invertase were lower than those of the free enzyme. 相似文献
6.
中国黑土上腐殖酸和腐殖物质的提取及其描述 总被引：7，自引：0，他引：7
Twenty-three progressive extractions were performed to study individual humic acids (HAs) and humin fractions from a typical black soil (Mollisol) in Heilongjiang Province, China using elemental analysis and spectroscopic techniques. After 23 HA extractions the residue was separated into high and low organic carbon humin fractions. HA yield was the highest for the first extraction and then gradually decreased with further extractions. Organic carbon (OC) of the humin fractions accounted for 58% of total OC even after 23 successive HA extractions. In addition, the atomic C/H ratio decreased during the course of extraction while C/O increased; the E4/E6 ratio from the UV analysis decreased with further extraction while E~/E3 increased; the band assigned to aliphatic carbon (2 930 cm-1) in the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra gradually increased with progressive extraction; the calculated ratio of the sum of aromatic carbon peak heights to that of aliphatic carbon peak heights from DRIFTS spectra declined with extractions; and nuclear magnetic resonance (NMR) data suggested that HA aliphatic carbons increased with extractions while aromatic carbons decreased. Thus, hydrophobicity and aliphaticity of HAs increased with extractions while polarity and aromaticity decreased. These data showed substantial chemical, structural, and molecular differences among the 23 HAs and two humin fractions. Therefore, these results may help explain why soil and sediment humin fractions have high sorption capacity for organic contaminants. 相似文献
7.
Chemical characteristics of humic substances in soils with different mineralogical characteristics and under different utilization paterns in Zhangpu,Fujian Province,together with two pairs of cultivated soils in North China Plain were studied by chemical analysis,visible and IR spectroscopy and ^13C NMR spectrometry.For soils in Zhanpu the HA/FA ratio and both the aromaticity and the degree of humification of HA were higher in soils with montmorillonite as the predominant clay mineral than in those with kaolinite as the predominant clay mineral,provided these soils were under the same utilization pattern.While for each pair of soils with similar mineralogical characteristics the HA/FA ratio was higher and the C/H ratio and the contnet of carboxyl group of HA were lower in paddy soil than in upland soil.Among the upland soils(or paddy soils)studied the Ha/FA ratio of soil in Zhangpu with kaolinite as the predominant clay mineral was the lowest,and that of soil in Zhangpu with montmorillonite as the predominant clay mineral was the highest .the lowest.and that of soil in Zhangpu with montmorillonite as the predominant clay mineral was the highest It was concluded that the presence of montmorillonite favored the fromation and maturation of humic acid. 相似文献
8.
HU GUO-SONG 《土壤圈》1994,4(2):153-164
The study results of the effects of temperature and ionic strength on the adsorption kinetics of Pb2+ and Cu2+ by latosol, red soil and kaolinite coated with Mn oxide showed that Pbr" and Cur" adsorption by all samples, as a whole, increased with raising temperature. Temperature also increased both values of Xm (the amount of ion adsorbed at equilibrium) and k (kinetics constant) of Pb2+ and Cu2+ The activation energies of Pb2+ adsorption were kaolin-Mn>red soil>goethite and those of Cu2+ were latosol>red soil>kaolin-Mn>goethite. For a given single sample the activation energy of Cu2+ was greater than that of Pb2+. Raising ionic strength decreased the adsorption of Pb2+ and Cu2+ by latosol, red soil and kaolinite coated with Mn oxide but increased Pb2+ and Cu2+ adsorption by goethite. The contrary results could be explained by the different changes in ion forms of Pb2+ or Cu2+ and in surface charge characteristics of latosol, red soil, kaolin-Mn and goethite. Increasiclg supporting electrolyte concentration in-creased Xm and k in goethite systems but decreased and k in kaolin-Mn systems. All the timedependent data fitted the surface secondorder equation very well.] 相似文献
9.
X-ray photoelectron spectroscopy(XPS) and automatic titrimeter were used to study the relation bewteen pH and the transformation of the coordinate forms of P on goethite surfaces.The results showed that for a given P concentration,increasing the pH of suspension could cause a fast transformation of monodentate complexes of phosphate ions on goethite surfaces to binuclear ones,When lowering the pH,additional adsorption of P occurred and the binuclear complexes reverted slowly to the monodentate ones,The dissociation and association of protons of the sorbed P caused by pH changes was considered to be a major reason leading to the transformation of the coordinate forms of P on the surfaces.The stability of binuclear surface complex of P was greater than that of monodentate omplex.The possible reactions on the interface of goethite and solutions with pH changes,and the reasons causing the different stabilities of the two coordinate P complexes are discussed in the paper. 相似文献
10.
Experiments on proton dissociation from the surfaces of goethite, amorphous Al oxide, kaolinite and latosol were carried out, showing amphoteric behavior with reactions of proton dissociation-association on the surfaces and buffering capacity in such a sequence as amorphous Al oxide>latosol>kaolinite>goethite. Dissociation constants of surface proton, pKsa, are significantly correlated with surface charge density, which has been proved with an electrochemical model. The intrinsic constants of proton dissociation, Ksa(int), gained by extrapolation to zero charge conditions of plots of pKsa against ao, could be used to estimate the acidity strength of variable charge surfaces. The value of pKse(int) is 8.08 for goethite, 1.2 for morphous Al oxide, 6.62 for kaolinite and 5.32 for latosol. 相似文献
11.
Van Emmerik TJ Angove MJ Johnson BB Wells JD 《Journal of agricultural and food chemistry》2007,55(18):7527-7533
Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally <10%, with pH having at most a small effect. Sorption to montmorillonite was significantly greater, with approximately 50% of the initial CPF being removed from solution below pH 5. Above pH 5 the sorption decreased to about 30%. About 70% of CPF was sorbed to kaolinite and gibbsite after 30 min, whereas on montmorillonite only 50% sorbed in an initial rapid uptake (approximately 30 min) followed by slower sorption, with a maximum achieved by 24 h. Although CPF desorbed completely from kaolinite in methanol, only about two-thirds was desorbed from montmorillonite. CPF has only a weak affinity for the surfaces of kaolinite and gibbsite. In the case of montmorillonite, sorption is significantly stronger and may involve a combination of sorption to external surfaces and diffusion into microporous regions. At pH >6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating. 相似文献
12.
Sorption on the mineral matrix is an important process restricting the movement of dissolved organic matter (DOM) in soils. In this study, we aimed to identify the chemical structures responsible for the retention of DOM by sorption experiments with total DOM and acidic humic substances (AHS), containing humic and fulvic acids, on soil samples and minerals (goethite, ferrihydrite, and amorphous Al(OH)3). The AHS remaining in solution after sorption were studied by 13C nuclear magnetic resonance (NMR) analysis, and total DOM and AHS for bed on the surfaces of minerals by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy. The soil samples were taken from strongly sorbing Bw horizons of two Inceptisols rich in pedogenetic Fe (29 and 35 g kg ?1) and containing little C (7 and 22 g kg?1). The 13C-NMR spectra showed that sorption causes a preferential removal of aromatic and carboxyl C from the solution, whereas alkyl-C accumulates in the solution. No change was observed for O-alkyl C. The DRIFT spectra of sorbed total DOM and AHS showed a relative increase of the band intensity of carboxyl groups compared to DOM in the initial solution, confirming the importance of those groups for the sorption to mineral surfaces. The spectra also indicated reactions of carboxyl groups with metals at the mineral surfaces. The extent to which the carboxyl groups are bound depended on the surface coverage with DOM and the type of mineral. 相似文献
13.
Assessing the fixation and availability of sorbed phosphate in soil using an isotopic exchange method 总被引：3，自引：0，他引：3
An isotopic exchange method was used to characterize quantitatively the fixation and plant availability of phosphate previously sorbed by soils. In general, the exchangeability of the sorbed phosphate was much higher than its desorbability for both soils and clay minerals. Isotopic exchangeability of the sorbed phosphate increased with sorption saturation during the initial stage (15–60% saturation), but the increase was less with increasing saturation from 60–90% for all soils tested. Therefore a sorption saturation of 60% was recommended as the upper limit of P fertilization in terms of economical efficiency. For clay minerals, with increasing sorption saturation, the isotopic exchangeability of the sorbed P increased significantly for kaolinite and sesquioxides, but decreased for montmorillonite. Most of the phosphate sorbed by montmorillonite and kaolinite was found to be isotopically exchangeable, but only a small amount of the P sorbed by goethite could be exchanged. The P sorbed by Al oxide exhibited isotopic exchangeability between that of kaolinite and Fe oxide. The isotopically exchangeable phosphate pool could readily account for the P uptake of plants and the available P determined by some commonly used chemical methods, such as Olsen-P and Bray-P. 相似文献
14.
The sorption of metal ions (Pb2+, Zn2+ and Cu2+) and soil humic acids (HA) from aqueous solutions onto mineral particles (sand, calcite and clay) was investigated using a batch equilibrium system. The sorption reactions in two- component systems (heavy metals-mineral particles and humic acids- mineral particles), as well as interactions in three-component system (heavy metals-humic acids-mineral particles) were examined. Results showed that the presence of humic acids, dissolved or bound onto mineral surfaces, considerably influenced the fixation of heavy metals. The various effects, depending on mineral type, humic concentration and specific metal-ion, were observed in three- component system. Sorption of Cu2+-ions on all minerals studied rapidly increased as the concentration of dissolved HA increased. The amount of Pb2+-ions sorbed on sand slightly decreased, while on kaolin increased between 15 and 20%. Sorption of Zn2+-ions on all minerals studied decreased at pH 4. At pH 5.5 the sorption of Zn2+-ions onto calcite decreased, while on kaolin and sand increased as a function of the humic acid concentration giving the curve with maximum at c(HA) = 2.5 mmol C L-1. At pH 6.5 sorption onto kaolin and sand increased. This effect occurs as a result of the conditional stability constant of Zn-HA complexes increasing at higher pH which in turn promotes the chelation of Zn2+-ions to mineral- bound humic substances. The enhanced sorption of metal ions from the aqueous phase in three-component systems is not only the result of mineral sorption of free metals but also the result of chelation with HA sorbed on the mineral surface. 相似文献
15.
Sorption of dissolved organic nitrogen by acid subsoil horizons and individual mineral phases 总被引：10，自引：0，他引：10
Dissolved organic matter is important in translocation and export of nutrients from forest ecosystems. Its mobility in soil is restricted by sorption to mineral surfaces which depends on its chemical properties. Carboxyl and hydroxyl groups form strong bondings to mineral surfaces, whereas the role of N‐containing functional groups in the sorption process is less well understood. We examined in laboratory experiments the binding of dissolved organic matter from the forest floor to amorphous Al(OH)3, goethite, kaolinite, and illite and to subsoils in order to compare the sorption and desorption of dissolved organic C with that of dissolved organic N. The mineral samples were equilibrated with acidic solutions of organic matter at pH 4. In the equilibrium solutions organic C and N and their contribution to two operationally defined fractions, namely the so‐called hydrophilic and hydrophobic fractions, were determined. We measured neutral and acidic amino sugars to discover the nature of the binding of organic N. Within the hydrophilic and hydrophobic fractions, the sorption and desorption of organic C and N did not differ, indicating that there was no preferential binding of N‐containing compounds. The hydrophilic fraction contained more N and sorbed less than the hydrophobic fraction, and so the overall retention of organic N by the mineral phases and subsoils was smaller than that of organic C. Among the amino sugar compounds, muramic acid was preferentially removed from the solution, whereas the neutral amino sugars were sorbed similar to organic C. The results suggest that the sorption of N‐containing compounds is favoured by acidic groups and not by amino groups. 相似文献
16.
Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (Ø < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α‐FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X‐ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N2 adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N2‐accessible SSA of the goethite, mostly because micropores (Ø < 2 nm) were rendered inaccessible to N2. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (Ø < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze‐dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes. 相似文献
17.
Sorption of polycyclic aromatic hydrocarbons to mineral surfaces 总被引：1，自引：0，他引：1
Minerals contribute crucially to the retention of polycyclic aromatic hydrocarbons (PAHs) in subsurface environments. To investigate the sorption behaviour to mineral surfaces batch sorption experiments were conducted using three PAHs (phenanthrene, pyrene, benzo(a)pyrene) and three mineral sorbents that were representative of subsurface materials (quartz, goethite‐coated quartz, quartz‐montmorillonite mixture). Sorption kinetics showed an instantaneous, considerable PAH sorption to all minerals, except for phenanthrene sorption to quartz at small aqueous‐phase concentrations. Apparent sorption equilibrium was achieved after 4 hours of contact time. The sorption characteristics were fitted to six isotherm models by applying Monte Carlo simulation and nonlinear regression. Best‐fit models were obtained by a model discrimination approach. Phenanthrene and pyrene sorption were best described by the Freundlich isotherm model, with the exception of phenanthrene sorption to quartz (linear isotherm). Good fit results for quartz were also obtained for the combined linear‐Freundlich isotherm. Benzo(a)pyrene sorption to all minerals followed linear high‐affinity isotherms. In the case of phenanthrene and pyrene, the Monte Carlo simulations resulted in mean values with small standard deviations for the isotherm parameters, indicating a negligible influence of the experimental uncertainties on the accuracy of the fitted parameters. For phenanthrene, (i) linear isotherms to quartz and goethite‐coated quartz and (ii) a nonlinear concave‐shaped isotherm to quartz–montmorillonite, assuming a pore‐filling process to micropores formed by clay aggregates, were confirmed. For pyrene, nonlinear convex‐shaped isotherms to the mineral surfaces were assessed. A specific sorption affinity of pyrene to the goethite surface indicated a non‐covalent cation‐π interaction. Small sorption affinities to quartz–montmorillonite support an unfavourable partitioning into the adjacent water. 相似文献
18.
The stability and activity of phytases in the soil environment may be affected by their sorption on soil particle surfaces and by substrate availability with important consequences for P cycling and nutrient bioavailability. This work evaluated the interaction of phytases with goethite, haematite, kaolinite, montmorillonite and two oxisol clays and investigated how this interaction is affected when myo-inositol hexakisphosphate (InsP6) was sorbed on the mineral surfaces. phyA histidine acid phosphatases of fungal origin were used and their ability to release orthophosphate from the InsP6-saturated minerals was evaluated.The phytases showed a high affinity for the mineral surfaces, with a loss of enzyme activity generally being observed over 24 h (up to 95% of the initially added activity). The loss of phytase activity was dependent on the type of mineral, with kaolinite and montmorillonite showing the greatest effect. Retention of enzyme activity was higher with the two oxisol clays, suggesting that the heterogeneous nature of clay surfaces and the presence of endogenous organic matter may limit the inhibition caused by interaction with minerals.In the presence of mineral surfaces saturated with InsP6, the partitioning of enzyme activity between the solution and the solid phase was shifted more towards the solution phase, presumably due to the mineral surfaces being occupied by the substrate. However, phytases were not able to release any orthophosphate directly from InsP6-saturated goethite and haematite, and hydrolysed InsP6 that was desorbed from haematite. Conversely, in the case of kaolinite and of the oxisol clays, where desorption was limited, phytases appeared to be able to hydrolyse a small fraction of the InsP6 adsorbed on the surfaces. These findings suggest that the bioavailability of P from inositol phosphates is governed to a large extent by the mineral composition of soil and by competitive effects for sorption on reactive surfaces among inositol phosphates and phytases. 相似文献
19.
Studies of Cd and Zn sorption using Na-saturated kaolinite and montmorillonite, and low metal solution concentrations similar to those found in the environment, showed that metal sorption affinity (measured by K d values) decreased markedly with increasing surface metal loading for both layer silicates. For equilibrium solution concentrations <0.1 μmol L?1 for Cd, and < 1 μmol L?1 for Zn, both metals were sorbed with greater affinity by kaolinite than montmorillonite. These results were probably due to the higher proportion of weakly acidic edge sites present on kaolinite surfaces. In the case of Zn there was an affinity reversal for equilibrium solution concentrations > 1 μmol L?1, which was attributed to the permanent charge sites of montmorillonite. Cadmium ions were sorbed, by kaolinite, with greater affinity than Zn for equilibrium solution concentrations between 0.3 to 1.5 μmol L?1. This result was attributed to retention of these metal ions through electrostatic attraction by permanent charge sites present on the kaolinite used in this work. According of these results it seems that metal sorption by these layer silicates involves predominantly edge weak acid sites at lower surface coverages (higher affinity sites), and permanent charge sites at higher metal coverages (lower affinity sites). It was concluded that Cd and Zn sorption by those two layer silicates is greatly influenced by surface metal coverage, and results cannot be extrapolated from low to high surface coverages, and viceversa. 相似文献