Adsorption on iron

Liu F, De Cristofaro A, Violante A (2001) Effect of pH phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite. Soil Sci 166 197-208 Livesey NT, Huang PM (1981) Adsorption of arsenate by soils and its relation to selected properties and anions. Soil Sci 131 88-94 Manceau A (1995) The mechanism of anion adsorption on iron oxides Evidence for the bonding of arsenate tetrahedra on free Fe(0, OH)6 edges. Geochim Cosmochim Acta 59 3647-3653. 

Fig. 1. The heat of adsorption on iron at — 196°C. as a function of surface covered.

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

The adsorption of ions on iron oxides regulates the mobility of species in various parts of the ecosystem (biota, soils, rivers, lakes, oceans) and thereby their transport betv een these parts. Examples are the uptake of plant nutrients from soil and the movement of pesticides and other pollutants from soils into aquatic systems. In such environments various ions often compete with each other for adsorption sites. Adsorption is the essential precursor of metal substitution (see Chap. 3), dissolution reactions (see Chap. 12) and many interconversions (see Chap. 14). It also has a role in the synthesis of iron oxides and in crystal growth. In industry, adsorption on iron oxides is of relevance to flotation processes, water pollution control and waste and anticorrosion treatments. 

Studies of adsorption on iron oxides have followed two main lines ... 

Anion adsorption on iron oxides is frequently described by this equation whereas cation adsorption data is often fitted to the Freundlich equation, i.e. 

Manceau, A. (1995) The mechanism of anion adsorption on iron oxides. Evidence for the bonding of arsenate tetrahedra at free Fe(0, OH) edges. Geochim. Cosmochim. Acta 59 3647-3653... 

Parfitt, R.L. Smart, R.S.C. (1977) Infrared spectra from binuclear bridging complexes of sulphate adsorbed on goethite (a-FeOOH). J. Chem. Soc. Faraday Trans. I. 73 796-802 Parfitt, R.L. Smart, R.S.C. (1978) The mechanism of sulfate adsorption on iron oxides. Soil Sci. Soc. Am. J. 42 48-50 Parfitt, R.L. (1980) Chemical properties of variable charge soils. In Theng, B.K. (ed.) Soils with variable charge. N. Z. Soc. Soil Sci., Lower Hutt. N. Z., 167-194 Parfitt, R.L. (1982) Competitive adsorption of phosphate and sulphate on goethite (a-FeOOH) A note. New Zealand J. Sci. 25 147-148... 

Peak, D. Sparks, D.L. (2002) Mechanisms of selenate adsorption on iron oxides and hydroxides. Envir. Sci. Techn. 36 1460-1466 Peak, D., Ford, R.G. Sparks, D.L. (1999)... 

Test of this proposal are under way, using bases of known Cb and Eg from Drago s tables, and measuring heats of adsorption on iron oxide and on silica from solutions in various hydrocarbons. 

The acidic sites on iron oxides are believed to be FeOH sites (32), much like the well-known SiOH sites on silica. Heats of adsorption on iron oxide of bases of known Cg and Eg, having appreciably different ratios of Cg to Eg ("hardness" or "softness"), allow estimation of the and for the acidic sites of iron oxide. Our initial studies were done by measuring adsorption isotherms at two or more temperatures (Figure 7) and from the temperature coefficient of the equilibrium constant K the enthalpy of adsorption was calculated. In Figure 7 the adsorption data is plotted as a Langmuir isotherm ... 

Lead can be found almost exclusively in the easily-reducible and in the residual fraction (Fig. 8-11). Its main binding forms are obviously the adsorption on iron and manganese hydroxides and mineral binding. The large increase in the lead concentration from Elsterberg to Greiz is not characterized by an increase of an individual binding form. 

Both type-4 and type-B adsorption on iron have a poisoning effect on the H2 and D2 exchange on iron catalysts at very low temperatures... 

Fig. 12.50. Model of iron surface plane covered by adsorbed hydrogen. (Reprinted from P. Kutej, J. Vosta, J. Pancir, J. Macak, and N. Hackerman, Electrochemical and Quantum Chemical Study of Dibenzylsulfoxide Adsorption on Iron J. Electrochem. Soc. 142(3) 833, 1995. Reproduced by permission of the Electrochemical Society, Inc.)...

The mechanism of adsorption has been extensively studied in order to evaluate its importance in the regulation of the concentration of certain species in natural waters. In particular, adsorption on iron and manganese oxides has been proposed as the chemical mechanism which controls the concentration of some trace metals in the world s oceans (], 2) and the enrichment of certain trace metals in ferromanganese nodules ( 3). ... 

The former is called molecular or nondissociated (e.g., CO) adsorption and the latter is called dissociative adsorption. Whether a molecule adsorbs nondis-sociatively or dissociatively depends on the surface. For example, CO imder-goes dissociative adsorption on iron and molecular adsorption on nickel. ... 

There is no particular a priori reason to suspect that oxygen production was any different from that in the modern ocean, except that there may have been constraints imposed by different availabilities of key nutrients such as phosphorus (Bjerrum Canfield 2002). Today, the availability of fixed nitrogen may constrain productivity on geological time scales (Falkowski 1997), but in the Archaean phosphorus removal by adsorption on iron oxides could have reduced P availability, significantly reducing productivity compared with today. 

Smith, R. W., and E. A. Jenne. 1991. Recalculation, evaluation, and prediction of surface complexation constants for metal adsorption on iron and manganese oxides. Envir. Sci. Technol. 25 525-31. 

Cowan, C. E., Zachara, J. M., and Resch, C. T. (1991). Cadmium adsorption on iron oxides in the presence of alkaline-earth elements. Environ. Sci. Technol. 25, 437-446. 

Cations and anions that adsorb by forming short directional bonds with the surface cannot be considered to be indifferent in character. These ions actually alter the surface charge by the very process of adsorption, and their bonding is classified as chemical adsorption (or chemisorption) in this text. Examples of chemisorption include copper and phosphate adsorption on iron oxides ... 

The equilibrium VSIP values were evaluated by the method proposed by Anderson and Hoffmann [22]. Benzoate adsorption on iron can involve Fe—C interactions through the carbon atoms of the >C=C< (from the aromatic ring) or >C=0 moieties and the resulting geometries imply a different polarization of the surface. However, in each case, the original VSIP and Slater orbital exponents define the open-circuit potential of the adsorbed ensembles, and all of them are compared through a parameterization based on the Fe—C bond. The open-circuit potential can be correlated to the experimental open-circuit value of the interface, that is, the electrode potential that results from the interaction between benzoate and iron. 

Figure 11-1. Adsorptive interaction of sulfur compounds with iron surfaces. Detection of sulfur retained adsorptively on iron surfaces by Auger spectroscopy. The sliding friction experiments were run at 0.5 cm/s, 0.981 N, 295 K. (a) Hydrogen sulfide. (b) Methyl mercaptan. From data by D. H. Buckley [8].

Goldblatt and Appeldoorn [43] prepared neutral tricresyl phosphate free of acidic contaminants by percolation through silica gel and compared its adsorption on iron powder with that of an acid phosphate. The phosphorus concentration in the carrier fluids was at the extremely low level of 15-30 ppm and the amount of iron powder was sufficient to remove 50% of the phosphorus in the solution if a monolayer of additive were ad-... 

As iron oxides / oxyhydroxide have a very high affinity for the adsorption of anions as well as cations under natural conditions the early diagenetic reactivity of iron is often of great significance for the behavior of compounds such as trace metals, phosphate and organic acids. The adsorption on iron oxides is caused by the... 

Similarly to the concept of the biographical nonuniform surfaces mechanism (7.97) can be used to derive a rate expression in supposition of lateral interactions. Assuming that surface species other than chemisorbed nitrogen are present on the surface in inferior quantities, which is backed by experimental evidences showing that nitrogen adsorption on iron catalysts proceeds at a rate approximately equal to that of ammonia synthesis, the equilibrium constant of step 2 in eq. (7.97) can be expressed, following the general treatment, as... 

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