Sesquioxide surfaces

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

Hendershot,W.H. Lavkulich, L.M. (1983) Effect of sesquioxide coatings on surface charge of standard mineral and soil samples. Soil Sci. Soc. Am. J. 47 1252-1260... 

Phosphorus occurs in various soil fractions as soil minerals combined with Ca, Fe, Al, which are of low solubility bound to particle surfaces of, e.g. sesquioxides, calcite, to Al on humus surfaces in soil solution in the organic matter, primarily as esters. 

At ordinary temperatures rhodium is stable in air. When heated above 600°C, it oxidizes to Rh203, forming a dark oxide coating on its surface. The gray crystalline sesquioxide has a corundom-like crystal structure. The sesquioxide, Rh203, decomposes back to its elements when heated above... 

This substance, when laid upon a metallic surface of iron, copper, or zinc, attacks- these bodies by virtue of its excess of acid, being itself reduced to the state of a protosalt of iron, or rf the action be continued with exposure to air, to sesquioxide, thus —... 

I 3 The Use of Infrared Spectroscopic Methods Table 3.7 Surface OH groups on sesquioxides. 

Surface Hydroxyl Croups of Other Sesquioxides and of Spinel-Type Mixed Oxides... 

Adsorption. Monomeric silica can be removed from solution by adsorption onto surfaces of sesquioxide minerals (e.g., AljOa, FejOa) through pH-dependent reactions that apparently involve hydrogen bonding (see Jones and Handreck, 1967, and references therein) such reactions, especially with AI2O3, seem to exert a major control over the concentration of dissolved silica in soil solutions. 

---