Phosphate species, 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). 

The oldest, most well-established chemical separation technique is precipitation. Because the amount of the radionuclide present may be very small, carriers are frequently used. The carrier is added in macroscopic quantities and ensures the radioactive species will be part of a kinetic and thermodynamic equilibrium system. Recovery of the carrier also serves as a measure of the yield of the separation. It is important that there is an isotopic exchange between the carrier and the radionuclide. There is the related phenomenon of co-precipitation wherein the radionuclide is incorporated into or adsorbed on the surface of a precipitate that does not involve an isotope of the radionuclide or isomorphously replaces one of the elements in the precipitate. Examples of this behavior are the sorption of radionuclides by Fe(OH)3 or the co-precipitation of the actinides with LaF3. Separation by precipitation is largely restricted to laboratory procedures and apart from the bismuth phosphate process used in World War II to purify Pu, has little commercial application. 

Adsorption processes may be particularly important in influencing species concentrations, since the arsenic present in the pore waters will probably be in equilibrium with arsenic adsorbed on solid surfaces. Arsenic in any species measured in pore waters may be only a fraction of the total amount of that species present in the sediments, the rest being adsorbed to or incorporated into particulate matter. Thus, it is important to study the sorptive characteristics of each of the arsenic species in the sediments. In the Menominee River sediments studied, the four oxygenated arsenic species (arsenate, arsenite, monomethyl arsonic acid and cacodylic acid) are often present together and competing among themselves and with phosphate for the same sorption sites. The competitive adsorptive characteristics of the species could greatly influence... 

Sorption of Phosphate and Arsenic Species by Anaerobic Sediments... 

The sorption of phosphate and the arsenic species on the twelve soils used in Wauchope s experiment correlated well with both the clay content and the iron content of the soils. The Menominee River sediments, however, were anaerobic, so iron should have been present as Fe(0H)2 rather than the Fe(0H)3 which was, presumably, present in Wauchope s alluvial soils. 

The effect of sorption of HC03 on ferrihydrite in the model also requires discussion. With the derived complexation constants, HCOs was, together with phosphate, the dominant species on the weak sites of the ferrihydrite surface. Sorption had no effect on the groundwater concentrations of HC03, but the high HCOs concentrations did affect the sorption of the other surface... 

Inspecting Table 4, it can be noted that arsenite is much less sorbed than arsenate in groundwater without phosphate. However, in the presence of P04 the sorption of arsenate is diminished to a very small quantity due to negative charging of the ferrihydrite surface. The effect is greater for arsenate than for arsenite because arsenate is sorbed as a negative species, while arsenite forms a neutral surface complex. The HCOs surface complex also is mainly neutral, and the ion floods the weak sites of ferrihydrite and... 

Consider now adsorbed molecular or ionic species that are, practically speaking, immobilized in the soil. Unless the soil is extremely acid, metals such as Cu, Cr, and Pb fall into this category. Also, certain anions such as phosphate bond so strongly on minerals that they too behave as immobile elements. The property that all of these ions have in common is that their sorption isotherms are not reversible within a time scale relevant to soil processes the adsorption (forward) isotherm is usually approximated closely by a Langmuir function of the strong-affinity type, but the desorptioii (backward) isotherm deviates markedly from the adsorption isotherm. This kind of nonequilibrium behavior, depicted in Figure 9.6, is sometimes referred to as hysteresis. Possible reasons for hysteresis in chemisorption are discussed in Chapter 4. 

Effect of Phosphate. The pH 7 buffer solution used to maintain the pH of the Pu(IV) solutions contained KH0PO4. Because phosphate ions are known to complex Pu(IV) (10), a study was conducted to determine if adjustment of the phosphate concentration would alter the nature of the colloidal species and thereby affect the sorption onto silica. 

Pu(HP04)44 in 2M nitric acid. The formation of the phosphate complex, in effect, increases the average negative charge of the ionic or colloidal plutonium species. As in the case of the bicarbonate complex, this should reduce sorption onto the negatively charged silica surface. However, as is shown in Table V, the sorption increased. In another experiment in which the phosphate concentration was varied between zero and 1.25 X 10 2M, there was no significant difference in the sorption constants. 

The diversity of reactions which actinides can undergo in natural waters is pres ted schematically in Figure 22.9. Complexation by anions such as hydroxide, carbonate, phosphate, humates, etc. determine the species in solution. Sorption to colloids and suspended material increases the actinide concentration in the water while precipitation of hydroxides, phosphates, carbonates, and/or sorption to mineral and biological material limit the amount in the solution phase. 

A sorption isotherm describes the eqnilibrinm relationship between the concentrations of adsorbed and dissolved species at a given temperatnre. Becanse soil scientists have adapted and modified these fnnctions and nsed them to describe phosphate adsorption from solution, they have proved to be less than ideal. Phosphorus adsorption increases with increasing soil pore water phosphorus concentration, nntil all sorption sites are occnpied. At that point, adsorption reaches its maximum, as indicated by Similarly, an incremental decrease in soil pore water phosphorns concentration resnlts in desorption of phosphorns from the solid phase. At low pore water phosphorus concentration, the relationship between adsorption and soil pore water phosphorus concentration is linear. The intercept on y-axis (Fignre 9.22), as indicated by Sq, snggests that phosphorns is adsorbed at soil pore water phosphorns concentrations approaching near-zero levels. If phosphorus is added to soil at concentrations lower than that of phosphorns in soil pore water, then the soil tends to release phosphorns nntil new eqnilibrinm is reached. Soils adsorb only when added phosphorus concentrations are higher than the concentration of phosphorns in soil pore water. 

Finally, there is the question of sorption of both arsenic species to mineral phases. Arsenate adsorbs strongly to a number of common minerals, such as ferrihydrite, goethite, chlorite, and alumina, which constrains its mobility in aquifers, soils, and sediments. This is a complicated, pH-dependent phenomenon (36). Phosphate has chemical properties similar to arsenate and is a common anion... 

Fig.4.13 Recording of a typical FI profile for sorbent absoiptiometric optosensing of sorption of phosphomolybdate on C 8 sorbent (L), reduction of the sorbed species by ascorbic acid (R). and elution by methanol (E). The flow is momentarily stopped during reduction. A are signal heights related to phosphate content in samples. 1, 2. 3, signals corresponding to 0, 25, and 50 /ig P P BLK, blank SC, signal generated by Schlieren effects [60].

Perhaps the best methods for demonstrating the existence of adsorption in a soil are optical, magnetic resonance, and X-ray photoelectron spectroscopy, which give direct evidence for the presence of adsorbed species. These methods currently are under development for application to soils extensive calibration with well-characterized, reference soil minerals. Until this calibration is completed, it is possible to use kinetics data to make an operational distinction between adsorption and precipitation. This strictly empirical method of analyzing sorption data can be illustrated with the important case of o-phosphate reactions. 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

Identification of the specific species of the adsorbed oxyanion as well as mode of bonding to the oxide surface is often possible using a combination of Fourier Transform Infrared (FTIR) spectroscopy, electrophoretic mobility (EM) and sorption-proton balance data. This information is required for selection of realistic surface species when using surface complexation models and prediction of oxyanion transport. Earlier, limited IR research on surface speciation was conducted under dry conditions, thus results may not correspond to those for natural systems where surface species may be hydrated. In this study we review adsorbed phosphate, carbonate, borate, selenate, selenite, and molybdate species on aluminum and iron oxides using FTIR spectroscopy in both Attenuated Total Reflectance (ATR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) modes. We present new FTIR, EM, and titration information on adsorbed arsenate and arsenite. Using these techniques we... 

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