Effect Of Surface Oxidation Research Articles

Hydrogen permeation in iron-chromium-aluminum (FeCrAl) alloys and the effects of microstructure and surface oxide

Iron–chromium–aluminum (FeCrAl) class alloys are candidates for use as cladding for accident-tolerant fuels and moderators. In this context, hydrogen isotope permeation in FeCrAl alloys is an important material property. In the present work, the apparent permeability, effective diffusivity, and apparent solubility of hydrogen in the FeCrAl alloys C26M and Kanthal D (KD) were measured with gas-driven hydrogen permeation. Permeation measurements were conducted at temperatures of 400 to 700 °C and at gas-driven pressures from 1 to 100 kPa. In particular, the effect of grain size on hydrogen transport was studied with KD samples with three different microstructures: nanocrystalline (NC), ultra-fine grained (UFG), and coarse-grained (CG). The UFG and NC specimens had higher apparent activation energies (73.4 kJ mol-1 and 65.2 kJ mol-l, respectively) for hydrogen permeability than the CG sample (46.9 kJ mol-1). An aluminum oxide layer formed on the primary- and secondary-side surfaces of all samples subjected to permeation experiments which demonstrated the propensity of FeCrAl alloys to form these innate oxide permeation barriers.

Mechanisms of interface electrostatic potential induced asphalt-aggregate adhesion

The electrostatic potential at the interface of the asphalt molecular and aggregate oxides is the primary factor generating the adhesion between the asphalt-aggregate contact surfaces. However, the route of electrostatic potential affecting the asphalt-aggregate adhesion is yet to be determined at the molecular level. This study proposed that the electron accumulation at the interface of the asphalt molecular and aggregate oxides generates a significant electrostatic potential hence to induce the adhesion between the asphalt and aggregate in asphalt pavement. A microscopic molecular dynamics (MD) simulation incorporated with macroscopic adhesion work experiments is conducted to analyze the interface adhesive performance between the asphalt and aggregate. The adhesion work experiment demonstrated the effect of aggregate surface oxides on asphalt adhesion at the macro level and further verified the simulation results. The result demonstrated that a notable electron accumulation can be observed by MD at the interface of asphalt molecular and multiple oxides which generates the electrostatic potential. The strong electrostatic attractions and hydrogen bonds between alkaline oxides (MgO, CaO, CaCO3) and asphalt molecules enhance the relative concentration distribution, diffusion coefficient, and adhesion of asphalt on the aggregate surface. The significant electrostatic potential difference at the interface of the asphalt molecular and the oxide enables a strong adhesion between them due to the near-surface failure potential. Besides, the negative charge accumulation on both the benzene ring structure and oxygen atoms in the asphalt molecules results in poor adhesion at the interface of asphalt molecules and multiple oxides. It was found that a high correlation exists between the MD prediction and experimental testing results. Thus, this research provides a valuable tool for guiding the pairing and selection of asphalt with various minerals for pavement design, construction, and maintenance agencies.

Impact of aging of primary and secondary polystyrene nanoplastics on the transmission of antibiotic resistance genes in anaerobic digestion

Sewage sludge is a significant reservoir of nano/microplastics (NPs/MPs) and antibiotic resistance genes (ARGs). Research has revealed that NPs/MPs may exert an inhibitory effect on anaerobic digestion (AD) of sludge. Moreover, NPs/MPs can influence microbial community diversity and composition, potentially increasing ARGs dissemination. The morphological changes to NPs/MPs surface due to aging contribute to modifying hydrophobic properties. To date, there is limited comprehension regarding how various surface properties of NPs influence ARGs dissemination during AD. This study investigated the impact of primary aged/non-aged and secondary aged/non-aged polystyrene nanoplastics (PSNPs) on ARGs and mobile genetic elements (MGEs) propagation during AD. The findings indicated that the UV-aging process resulted in surface oxidation and distinct morphological characteristics in both primary and secondary PSNPs, while the surface oxidation effect was more pronounced in the secondary aged PSNPs. High concentrations (150 μg/L) of primary and secondary PSNPs inhibited methane production, with secondary PSNPs causing greater inhibition by 16 to 20 % compared to control. In contrast, low concentration (25 μg/L) had negligible or slightly positive effects on methane production. PSNPs at 150 μg/L reduced total VFA concentration, indicating an inhibitory effect on the fermentation step in the AD process. Primary and secondary PSNPs exhibited changes in EPS characteristics. ARGs abundance was enriched in reactors amended with PSNPs, with the highest abundance of 8.54 × 105 copies/g sludge observed in the secondary aged PSNPs (150 μg/L) reactor. Reactors exposed to aged PSNPs exhibited a relatively higher abundance of ARGs compared to reactors exposed to non-aged PSNPs. Exposure to PSNPs increased the microbial community diversity within the digesters and triggered the enrichment of Comamonadaceae and Syntrophaceae, belonging to Proteobacteria phylum. On the other hand, archaeal communities tended to shift towards hydrogenotrophic methanogens in PSNPs reactors. The correlation analysis showed that Comamonadaceae were positively correlated with the majority of ARGs and intl1. A positive correlation was observed between MGEs and most ARGs, suggesting that the increased proliferation of ARGs under PSNPs exposure may be linked to the abundance of MGEs, which in turn promotes the growth of hosts carrying ARGs. These findings suggest that aged and non-aged NPs could substantially impact the spread of ARGs and MGEs, which also led to notable alterations in the composition of the microbial community. Overall, this study provides valuable insights into the multifaceted impacts of PSNPs with various characteristics on AD processes, microbial communities, and ARGs proliferation, highlighting the urgent need for comprehensive assessments of NPs pollutants in the environment.

Effect of surface oxidation on the performance of silicon nanoparticles

Effect of surface oxidation on the performance of silicon nanoparticles

Effect of Surface Oxidation and Crystal Thickness on the Magnetic Properties and Magnetic Domain Structures of Cr2Ge2Te6.

van der Waals (vdW) magnetic materials, such as Cr2Ge2Te6 (CGT), show promise for memory and logic applications. This is due to their broadly tunable magnetic properties and the presence of topological magnetic features such as skyrmionic bubbles. A systematic study of thickness and oxidation effects on magnetic domain structures is important for designing devices and vdW heterostructures for practical applications. Here, we investigate thickness effects on magnetic properties, magnetic domains, and bubbles in oxidation-controlled CGT crystals. We find that CGT exposed to ambient conditions for 5 days forms an oxide layer approximately 5 nm thick. This oxidation leads to a significant increase in the oxidation state of the Cr ions, indicating a change in local magnetic properties. This is supported by real-space magnetic texture imaging through Lorentz transmission electron microscopy. By comparing the thickness-dependent saturation field of oxidized and pristine crystals, we find that oxidation leads to a nonmagnetic surface layer that is thicker than the oxide layer alone. We also find that the stripe domain width and skyrmionic bubble size are strongly affected by the crystal thickness in pristine crystals. These findings underscore the impact of thickness and surface oxidation on the properties of CGT, such as saturation field and domain/skyrmionic bubble size, and suggest a pathway for manipulating magnetic properties through a controlled oxidation process.

Ti compound coating of carbonyl iron particles with controlled surface conditions using a peroxotitanium acid solution for stable magnetic particles

The surface coating of carbonyl iron particles (CIPs) deposited under various surface conditions was examined using a peroxotitanium acid (PTA) solution including H2O2 to obtain highly stable magnetic particles. Upon adsorption on the CIP surfaces, peroxotitanium complexes dehydrated and transformed into TiO2 with hydroxides (TiO2–OH), and the coating morphology significantly changed depending on the surface conditions of the CIPs. The volatile surface impurities that remained on raw CIPs inhibited the adsorption of the complexes, leading to insufficient coating. As oxidation treatment using a boric acid aqueous solution was performed to examine the effects of surface oxides on the coating, the treatment increased the oxide layer thickness and specific surface area of the raw CIPs. During the coating of oxidized CIPs, active surfaces were dissolved in a PTA solution, and γ-FeOOH plate particles were redeposited on the particle surfaces because H2O2 remaining in the PTA solution initially oxidized the active surfaces and then depleted. The results indicated that the treatment with the PTA solution was not suitable for oxidized CIPs with increased specific surface area. A simple dry treatment under vacuum conditions decreased surface impurities on raw CIPs without increasing their specific surface area; consequently, their surfaces were uniformly and sufficiently coated with TiO2–OH. The uniform surface coating decreased the saturation magnetization of the CIPs by only 0.98% and improved their oxidation onset temperature from 369.8°C to 437.1°C, making them suitable for practical use.

The Effects of Surface Oxidation and H-Termination Processes Applied to Si Using Electrolytic Hydrogen Peroxide Solution to The Produced Cu/p-Si Schottky Contact Parameters

By using electrolytic hydrogen peroxide (H2O2) solution, oxidation and H-termination processes were applied to the p-Si crystal surface, which will be used for Cu/p-Si Schottky contact production, in a selective and controlled manner. Before the oxidation and H-termination processes, the p-Si(100) wafer used in this study was subjected to conventional chemical cleaning, and ohmic contact was made using pure aluminum (99.99%) metal on its back surface. The p-Si/Al with ohmic back contact was divided into three parts. A rectifying contact was immediately made to the front surface of one of them by using pure copper (99.98%) metal and called the REF (Reference) sample. The front surface of one of the remaining two p-Si/Al parts was oxidized, and the front surface of the other was H-Terminated. Rectifier contacts were made for both using pure copper (99.98%) metal and were named MIS (metal-insulator-semiconductor) and SP (surface passivated), respectively. Current-voltage (I-V) measurements of Schottky diodes of REF, MIS, and SP samples were performed at room temperature and in the dark. From the obtained data, the ideality factor (n), barrier height (Fbo), and series resistance (Rs) values of the samples were determined. As a result of the investigations, it was observed that the surface oxidation and H-Termination processes caused a decrease in the rectification factor and Fbo values of MIS and SP samples. These interesting situations were interpreted by the double-layer theory, which Bardeen predicted could exist on the surface of a semiconductor crystal and contribute to its work function.

Intermolecular interactions in graphene and oxidized graphene nanocomposites

The spatial arrangement of graphene (G) or graphene oxide (GO) nanoplate agglomerations will significantly impact the material properties of graphene-based polymer nanocomposites. Therefore, we investigated the stability of G and GO agglomerates without the presence of a polymer network using molecular dynamics (MD) simulations, which was not considered in our previous work (Reil, 2022) [15]. G and GO nanoplates, often procured in powder forms, display a significant volume difference by visual observation. To this end, MD simulations were performed to model G or GO agglomerations in the powder form, including the effects of varied plate dimensions and localized surface oxidation. G plates on average released 231 kcal/mol upon agglomerating into aligned stacks, closely matching results in the literature. Simulations of GO resulted in agglomerations of clusters of plates rather than aligned stacks, releasing on average 16 % less energy than G. The clustered arrangement of GO plate agglomerates in simulations occupied a volume approximately 7x that of the aligned stack G plates, closely replicating the experimentally determined volume ratio of 10x. This research could yield further insight into the behavior of G and GO when in the presence of various polymer networks, which could be different than in isolation.

Hydrogen peroxide electrosynthesis with nickel foam electrodes: influence mechanism of surface oxidation

AbstractBACKGROUNDNickel foam (NF) is widely used as battery electrode matrix and electrochemical process cathode. However, the surface of NF is prone to oxidation, which may affect its ability to reduce oxygen and produce H2O2, but the influence mechanism is still unclear. In this paper, surface oxidation of the NF was regulated by pickling and drying pretreatment, and the surface oxides were characterized by X‐ray photoelectron spectroscopy (XPS) and energy dispersive X‐ray spectrometry (EDS). The NF was used as the substrate, graphite powder as the catalyst, and PTFE as the binder to prepare graphite‐coated NF composite electrodes. The effect of NF surface oxidation on their ability to generate H2O2 was investigated.RESULTSThe results demonstrated that after pickling and drying, the NF surface generated a large amount of NiO, which increased the charge transfer resistance of the NF by 20.3%, and accelerated further reduction decomposition of H2O2. The rotating ring‐disk electrode (RRDE) tests further confirmed that NiO can reduce the two‐electron oxygen reaction selectivity, which was detrimental to H2O2 electrosynthesis. The graphite‐coated NF composite electrode with pristine NF as the substrate can produce 869.1 mg L−1 of H2O2 and maintain relatively good reusability.CONCLUSIONOverall, the surface oxidation of NF is not conducive to H2O2 electrosynthesis, and it is necessary to prevent oxidation during electrode preparation. This study provides a valuable reference for the preparation of NF electrodes and can promote the further study of in situ H2O2 electrosynthesis. © 2023 Society of Chemical Industry (SCI).

Investigating on the microstructure and optical properties of Au, Ag and Cu implanted TiN thin films: The effects of surface oxidation and ion-induced defects

In the present paper, the effects of metal ion implantation on the structural and optical properties of TiN thin films have been investigated. TiN films of 170 nm thickness were grown by d.c. reactive sputtering on Si (100) wafers and then irradiated at 5 × 1016 ions/cm2 with either Au, Ag, or Cu ions by using two different energies per each implanted metal. The results showed that as deposited TiN crystallizes in the form of a fcc cubic structure, with the crystallites preferentially oriented along the (111) plane. For all implanted layers, the cubic structure was preserved, but compared to as deposited TiN the crystallites were smaller and the lattice was contracted. These changes were correlated with the depth distribution of Au, Ag and Cu ions and assigned to implantation-induced damage that was larger when higher ion energies were used. High-resolution XPS spectra of the surface of as deposited sample showed the coexistence of TiN, TiO2 and TiOxNy phases and this was related to the surface oxidation of the films due to the exposure to air. After implantation, the results were almost similar for all metals, showing an increase in TiO2 contribution and the formation of pure metallic Au and Ag phases, while copper is in the Cu2+ state, which is attributed to Cu(II)-oxide and Cu(OH)2. The microstructural characteristics including defect formation, changes in crystallite size and lattice contraction, and also growth of different metallic phases during implantations were correlated with the findings of the optical characterization of the implanted films. For the as deposited film we found an energy gap of 2.91 eV, which was lower than the value typical for TiN. After implantation the gap was shifted to higher energies, while at the visible part of the region, the existence of additional energy levels, at photon energies below 2.9 eV was observed. Besides, all implanted films showed degraded photocatalytic activity compared to as deposited TiN, among which Cu-implanted samples exhibited the best photocatalytic performances. The lower photocatalytic activity of Au and Ag implanted films compared to Cu implantations was ascribed to larger structural defects and the formation of less favorable electronic states.

Effect of surface oxidation on the interfacial and mechanical properties in graphite/epoxy composites composite bipolar plates

Epoxy resin-reinforced graphite composites have found extensive application as bipolar plates in fuel cells for stationary power supplies, valued for their lightweight nature and exceptional durability. To enhance the interfacial properties between graphite and epoxy resin (EP), surface oxidation of graphite was carried out using diverse functional groups. Experimental assessments illustrated that the composites with graphite oxide resulted in heightened mechanical strength and toughness compared to pristine graphite, which could be attributed to the excellent interface connection. Moreover, these composites displayed remarkable conductivity while simultaneously retaining their mechanical attributes. Furthermore, molecular dynamics simulations outcomes unveiled that the inclusion of oxygen-containing functional groups on the graphite surface augmented the interfacial energy with EP, and the interface morphology between graphite and resin exhibited heightened stability throughout the stretching process. This simple and effective technique presents opportunities for improving composites interfaces, enabling high load transfer efficiency, and opens up a potential path for developing strong and tough composite bipolar plates for fuel cells.

High temperature oxidation of cold spray Cr-coated accident tolerant zirconium-alloy cladding with Nb diffusion barrier layer

Cr-Nb bilayer coatings on Optimized ZIRLO™ light water reactor (LWR) fuel cladding tubes were developed using cold spray deposition technology to increase the limiting operational temperature (i.e., binary Cr-Zr eutectic temperature, 1332 °C) of Cr-coated Zr-alloy system. The properties of the dual-layered coatings were improved by varying the type of Cr powder and post-annealing of the cold sprayed Nb layer. High temperature oxidation behavior of Cr-Nb-coated cladding at 1200 °C and 1425 °C in flowing steam were compared with those of Cr-coated cladding and uncoated cladding. At 1200 °C, both Cr-coated cladding and Cr-Nb coated cladding exhibited outstanding oxidation resistance and a continuous intermetallic compound layer (1 – 3 μm thickness) of predominantly Zr-Cr and Cr2Nb formed at the Cr/Zr and Cr/Nb interface, respectively. At 1425 °C, the Cr-coated Zr-alloy cladding showed enhanced oxidation due to rapid consumption of Cr coating by interdiffusion and liquid phase formation, and consequent destabilization of the outer protective Cr-oxide layer. However, the Nb diffusion barrier layer between the Cr coating and Zr-alloy substrate prevented the melting and the outer Cr coating provided excellent oxidation resistance. For this bi-layered system, a diffusion layer devoid of intermetallic phases was observed at the Nb/Zr interface due to the miscibility of the two metals. Reaction pathways that include both surface oxidation and interdiffusion effects within the coating-substrate system have been proposed based on experimental results.

Effect of Surface Oxide on Decarburization Reaction in the Austenitic Steel S304HCu

In the present work, the high-temperature decarburization of the austenitic stainless steel S304HCu in Ar–4%H2–xH2O at temperatures between 1000 and 1150 °C was investigated. The focus was on determining parameters that influence the decarburization rate during heat treatment. Thermogravimetric experiments were performed with simultaneous measurements of CO release from the sample using a mass spectrometer. The variation of the experimental parameters included the composition of the atmosphere during pre-oxidation and decarburization, the temperature and the sample thickness. The results indicate that the loss of carbon from the specimen occurs faster through reaction of Cr2O3 with C from the steel compared to the reaction of C at the steel surface with H2O, provided that in the former case CO can easily evaporate. The overall decarburization process is governed by competing reactions; the destruction of chromium oxide by C and the formation of oxide scale by water vapour. The relative rates of these two reactions determines whether decarburization will occur or whether formation of a protective oxide stops the release of CO, the latter being promoted at lower temperatures and high water vapour contents. A dense, sufficiently thick chromia scale retards the onset of decarburization but does eventually not stop the carbon loss. Indications were found that the destruction of a pre-formed chromia scale by carbon involves a two-step process whereby presence of hydrogen is important.

Unveiling the failure mechanism on creep response of a casting Ni-based superalloy in thin-wall thickness

With the improvement of thermal efficiency and the lightweight tendency of engine blades, Ni-based superalloy is widely used owing to its excellent performance in high-temperature atmospheres. This work studied the effects of surface oxidation, internal environmental attack, and matrix damage on the failure mechanism in a thin-walled casting Ni-based superalloy at 980 °C/160 Mpa. At the edge of the fracture, the sample suffered a severe environmental attack, resulting in the oxidation-affected zone forms. However, the loss of effective bear area induced by surface damage could not be the main reason for the sample's failure. At the interior of the matrix, voids were preferably initiated at the interface of MC carbides. As the increase of creep deformation, dynamic recrystallization (DRX) occurred at the tip of the voids, which increased the transverse grain boundaries and promoted crack propagation. Moreover, the DRX provided a short penetration path for the nitrogen, causing internal nitridation with AlN and Ti(Ta)N to precipitate. EBSD analysis confirmed that nitrides induced significant dislocations to accumulate at the boundaries of nitrides/γ, accelerating the failure of the sample.

Deuterium transport behavior of 15-15Ti stainless steel for nuclear structural material in Gen-IV reactors

As a new type of nuclear reactor, the Gen-IV reactor attracts increasing interest from the industry. Tritium is produced in lead-cooled fast reactors (LFR), sodium-cooled fast reactor (SFR), and other Gen-IV reactors. It is necessary to study the tritium permeation behavior of 15-15Ti stainless steel as a candidate cladding and structural material in Gen-IV reactors. In this study, gas-driven permeation experiments were conducted to measure the high-temperature deuterium transport behavior parameters of 15-15Ti stainless steel. The results showed that the deuterium permeation behavior of 15-15Ti and 316L stainless steels is similar. Additionally, this study discussed the effects of microstructures and surface oxidation on the permeability of stainless steels. This study will aid in evaluating tritium permeation in lead-based fast reactors and other Gen-IV reactors and whether tritium permeation barrier or tritium-removal devices are necessary.

In English

Various 2D carbons demonstrate significant effects of surface oxidation, heating, suspending–drying, cryogelation, swelling, and adsorption of polar and nonpolar compounds on the morphological, structural, and textural characteristics. Heating at 120–150 °C could result in collapse of pores not only between carbon sheets in stacks but also between neighboring stacks; therefore, the specific surface area (SSA) decreases by a factor of 30–100 for preheated graphene oxides (GO). According to the TEM and XRD data, the GO structure is rather amorphous, since only small X-ray coherent scattering regions demonstrate a certain order giving broad XRD (001) and (002) lines. In the Raman spectra, the D line (disordered defect structures with sp3 hybridized C atoms) intensity for GO is similar to that of the G line (ordered structures with sp2 hybridized C atoms). The graphite oxide (GtO) structure, which is closer to that of graphite than that of GO, is characterized by intensive G and low D lines, and the main XRD peak at 26.4° (characteristic for graphite) is broadened similar to the XRD peak of GO at 10°. Despite the GO stacks have a tendency to collapse upon heating, the collapsed stacks can be swollen not only in water (strongly) but also in liquid nitrogen (relatively weakly). Therefore, the use of GO in aqueous media can provide great SSA values in contact with the solvent and solute molecules. This could provide high efficiency of the GO use for purification of wastewater, separation of solutes, etc. MLGO produced from natural flake graphite as a precursor (flakes < 0.2 mm in size) using a modified method of ionic hydration and freeze–drying is characterized by typical light brown color, low bulk density, flexible sheet stacks easily collapsed, but its interaction with water results in strong swelling. Interaction between the carbon sheets in preheated MLGO is strong and nonpolar molecules, such as benzene, n–decane, poorly penetrate between the sheets, i.e., intercalation adsorption is small. However, water molecules can effectively penetrate (this is rather intercalation adsorption resulting in swelling) between the sheets, but the swelling effect of water adsorbed from the gas phase could be weaker than that in the aqueous suspensions. Thus, the proposed synthesis method of MLGO using natural graphite is effective and appropriate for preparation of the materials for various practical applications.

Zinc dialkyldithiophosphates adsorption and dissociation on ferrous substrates: An ab initio study

Zinc dialkyldithiophosphates (ZDDPs) have been commonly used as anti-wear additives in the automotive industry for the past 80 years. Despite their widespread use, a general agreement on their primary functioning mechanism is still lacking. The morphology and composition of the ZDDPs phosphate-based tribofilm, which is essential for its lubricant functioning, have been widely studied experimentally. However, the formation process and the relevant driving forces are still largely debated. In particular, it is unclear whether the stress-induced molecular dissociation occurs in the bulk oil or on the substrate. In this work, we employ ab initio density-functional theory simulations to compare ZDDP fragmentation in vacuum and over a reactive substrate, considering the effects of surface oxidation on the dissociation path. To do so, we developed a computational protocol to study the effects of shear stress on molecules. Our results show that the molecular dissociation is endothermic in the absence of a supporting substrate, while in the presence of an iron substrate, it becomes highly energetically favoured. Moreover, the presence of the substrate changes the reaction path, inducing the detachment of organophosphorus units from Zn-S ones. At the same time, surface oxidation reduces the molecule–substrate interaction. These findings provide valuable insights into the early stages of the formation of phosphate-based tribofilms.

Effects of Surface Oxidation on the Magnetic Properties of Fe-Based Amorphous Metal Powder Made by Atomization Methods

Effects of Surface Oxidation on the Magnetic Properties of Fe-Based Amorphous Metal Powder Made by Atomization Methods

Elucidating the effects of surface and internal oxidation on material properties of carbon black

This paper investigates the effects of surface and internal oxidation of carbon black (CB) using different oxidizing media, such as ozone, hydrogen peroxide, nitric acid, and potassium permanganate, on CB's porosity, crystallite, and functional groups. The study uses various techniques, such as thermogravimetric analysis (TGA), scanning electron microscopy, N2 adsorption, Raman and infrared spectroscopy, X-ray diffraction (XRD), and zeta potential analysis, to analyse the samples. The results show that internal oxidants cause higher changes in the CB's crystallinity and porosity dynamics than surface oxidation, while surface oxidation predominantly occurs at the CB surface interface. The XRD and fractal analysis of the pore network after oxidation indicate that internal oxidation of the crystallites that make up the CB matter reduces the interlayer distance by compacting their pore networks. This study provides a quantitative analysis of the crystallinity and CB porosity dynamics, which can help in the development of highly porous CB that can act as substitutes for higher grades of CB used in polymer products as reinforcement, activated carbons, or as high-end materials in the development of super-capacitors for battery applications.

Effect of surface oxidation in air and water on hydrophobicity and floatability of a bituminous coal

Effect of surface oxidation in air and water on hydrophobicity and floatability of a bituminous coal