Oxides sorption

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

KEYWORDS Tailings, antimony, Fe-oxyhydroxides, oxidation, sorption... 

Alumina, alkaline-earth oxides, mixed oxides (spinels), rare-earth oxides, and lanthanide ores are known additives capable of sorbing S-impurities. The properties of these materials can be manipulated to produce catalysts capable of reducing up to -80% S-emissions and meet the refiner needs. It is, however, unlikely that these systems will be capable of satisfying the more stringent environmental S-emission standards expected in the future. Details of the reaction mechanism by which additives and promoters catalyze the oxidative sorption of S-impurities and details of catalyst deactivation have not yet been proposed. This work could provide useful information to help design more efficient S-transfer catalysts. The catalytic control of S-emissions from FCC units has been described in detail in two papers appearing in this volume (46,47) and in the references given (59). 

Sorption of polyvalent ions increases very rapidly with pH on oxides and hydrous oxides Because natural materials generally contain oxides or at least adsorbing groups that behave like oxides, sorption of polyvalent nuclides at intermediate pH will tend to follow behavior typical of oxides Thus, a strong dependence of sorption on salt concentration may not be observed This lack of dependence on salt concentration does not preclude ion exchange as a mechanism however, since this type of behavior follows directly from equilibrium equations ... 

Meyer R., Palmer D., Arnold W., and Case F. (1984) Adsorption of nuclides on hydrous oxides. Sorption isotherms on natural materials. Geochem. Behavior Disp. Radioact. Waste, 79—94. 

Fine colloidal or soluble inorganic contaminants are removed from water by physical-chemical methods of waste water cleaning. The main methods used are coagulation, oxidation, sorption, extraction and electrolysis methods. 

Kaputskii, F.N. and Bashmakov, l.A. (1973) Role of die degree of oxidant sorption in the nitrogen oxide cellulose system. Vestsi Akkad Naviik BSSR Ser Khim, 4, 29-32. 

Removal of NOx from stack gas presents some formidable problems. Use of liquid scrubbers such as those employed to remove SOj from stack gas is not very effective for NOx removal because of the low solubilities of nitrogen oxides. Sorption onto solids followed by destruction of the sorbed gases has been tried. Catalytic reduction and decomposition of nitrogen oxides are employed in automobile catalytic converters (see Section 8.8) and may be applicable to stack gas, although sulfur gases and particles in stack gas may interfere and poison the catalysts. Another possibility is the use of biofilters in which microorganisms held on support media metabolize NOx Section 8.9). 

An oxidant-sorption procedure was used by Ojio and Miyata [338] for the fabrication of P(Py)/PVA (Poly(Vinyl Alcohol)) composites. PVA, of M 22,000, was dissolved with FeClj (the oxidant) in water, a film cast from this solution onto a PET film substrate. This oxidant-saturated host-polymer film was then exposed, in a desiccator at low temperature and in a deoxygenated atmosphere, to monomer (pyrrole) and water vapor for 0.5 to 24 h, with the resulting composite films, ca. 2 jam thick, dried in vacuum. Conductivities and transmission values saturated at about 1 h exposure time, to ca. 1 S/cm and 40% ( 550 nm) for a 70 30 w/w ratio PVA/FeClj. Transmission of the film was down to ca. 55% within 0.5 h, (with conductivity ca. 0.1 S/cm) for the same PVA/FeClj ratio. Higher PVA/FeClj ratios (90 10, 95 5) gave not only higher transmissions with minimal conductivity reduction, but also more homogeneous films as evidenced by SEM. 

Lafosse [342] described an oxidant-sorption method for synthesis of P(Py)/Teflon (PTFE) composites. A commercially available, surfactant-stabilized PTFE emulsion is mixed with aqueous Fe(III)-tosylate (oxidant-cum-dopant). Pyrrole is then added to this, yielding a finely divided P(Py)/PTFE composite after several h, precipitated with ethanol. SEM analysis showed the composite to be particles of 0.2 pm diameter which are presumed to be PTFE spherules coated with P(Py), rather than a homogeneous blend. Nevertheless, the composite showed conductivity of ca. 10 S/cm at a percolation threshold of ca. 16 v/v% P(Py), and acceptable microwave absorption properties. A similar emulsion method has been used by Sun and Ruckenstein [343] to synthesize P(Py)/Synthetic Rubber composites, with FeCl3 as oxidant, and several solvents (aqueous/organic combinations) and several nonionic surfactants being employed. Conductivities of the composites were ca. 3 S/cm, Tensile Strengths ca. 10 MPa, and Elongation at Breaks 38% to 166%. 

The conducted researches of complexing processes of noble metals on a sulfur-containing CMSG surface formed the basis for development of sorption-photometric, sorption-luminescent, soi ption-atomic-absoi ption, sorption-atomic-emission and sorption-nuclear-physic techniques of the analysis of noble metals in rocks, technological objects and environmental objects. Techniques of separation and detenuination of noble metals in various oxidation levels have been proposed in some cases. 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Stauffer TB, MacIntyre WG. 1986. Sorption of low-polarity organic compounds on oxide minerals and aquifer material. Environ Toxicol Chem 5 949-955. 

Rates of hydrolysis may be influenced by the presence of dissolved organic carbon, or organic components of soil and sediment. The magnitude of the effect is determined by the structure of the compound and by the kinetics of its association with these components. For example, whereas the neutral hydrolysis of chlorpyrifos was unaffected by sorption to sediments, the rate of alkaline hydrolysis was considerably slower (Macalady and Wolf 1985) humic acid also reduced the rate of alkaline hydrolysis of 1-octyl 2,4-dichlo-rophenoxyacetate (Perdue and Wolfe 1982). Conversely, sediment sorption had no effect on the neutral hydrolysis of 4-chlorostilbene oxide, although the rate below pH 5 where acid hydrolysis dominates was reduced (Metwally and Wolfe 1990). 

The sensor s fnnction rests on the property of lead dioxide to sorb nitrogen oxides (NOJ, forming lead nitrate. In this case the lead nitrate concentration is proportional to the nitrogen oxide concentration in the air. The sorption of the nitrogen oxides is a reversible process The Pb02 is regenerated completely within a few minutes when exposed to clean air (free of nitrogen oxides). 

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