Adsorption oxyanions

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

Some differences in arsenate and chromate adsorption on ODA-clinoptilolite and Pb-(Ag-linoptilolites) as well were recorded (Figs. 5 and 6). ODA-clinoptilolite exhibited more efficient arsenate and chromate removal from aqueous solutions than the inorganically exchanged modifications. However, silver exchanged clinoptilolite revealed higher capacity values for both oxyanions uptake than lead exchanged clinoptilolite did. This phenomenon supports preferred silver treated clinoptilolite utilization for specific water purification process even on the base of environmental acceptability. 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

Adsorption of simple inorganic anions, oxyanions and organic ions on iron oxides has been widely investigated (see Tables 11.1 and 11.2). Anions are ligands, i.e. they possess one or more atoms with a lone pair of electrons and so can function as the donor in a coordinate bond. Ligands may adsorb on Fe oxides either specifically or non specifically. 

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Trace metal adsorption on goethite is reportedly increased by the presence of sulphate ions (Balistrieri and Murray, 1982). Arsenate and antimony oxyanions enhance uptake of cobalt on ferrihydrite and hematite, respectively (Benjamin and Bloom,... 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

Phosphorus and arsenic are both group 15 elements. The main phosphorus oxyanion, phosphate (PO43- ), has the same charge and tetrahedral configuration as inorganic As(V), which allow phosphate to out compete As(V) for adsorption sites on clay minerals and various iron and aluminum compounds over a wide range of pH conditions (Stollenwerk, 2003), 85 (Gao and Mucci, 2001) (Su and Puls, 2003), 2582. Phosphate is also capable of desorbing arsenic from humic acids and some mineral surfaces (Stollenwerk, 2003), 91 (Mok and Wai, 1994), 111 (Lafferty and Loeppert, 2005), 2120. 

Hiemstra, T. and van Riemsdijk, W.H. (1999) Surface structural ion adsorption modeling of competitive binding of oxyanions by metal (hydr)oxides. Journal of Colloid and Interface Science, 210(1), 182-93. 

With water treatment technologies, adsorption refers to the removal of contaminants by causing them to attach onto the surfaces of solid materials (adsorbents or sorbents) (Chapters 2 and 3). Sometimes the adsorbed solute is called the adsorbate (Krauskopf and Bird, 1995, 145). Adsorption usually involves ion exchange (Eby, 2004, 345). For example, adsorbing arsenic oxyanions will replace other ions on the surface of the sorbent (Chapter 2). 

Charge distribution multisite complexation model (CD-MUSIC) A surface complexation model for explaining ion adsorption on the surfaces of adsorbents. Hiemstra and van Riemsdijk (1999) used the model to explain the adsorption of arsenate oxyanions on goethite. 

A prototypical example of Eq. 4.29 occurs with the specific adsorption of oxyanions by metal oxyhydroxides.9 Taking Mo04 as the oxyanion and goethite (a-FeOOH) as the metal oxyhydroxide, one can write down the reaction sequence (cf. Eqs. 3.53 and 3.56) ... 

Surface electric potential control (or surface charge control) of the rate of flocculation is possible for any adsorptive that forms a surface complex with suspended particles, as discussed in Section 6.1 and in Chapter 4 (cf. Table 4.2). Among these adsorptives for soil colloids are oxyanions, such as phosphate or oxalate, and transition metal cations. An expression analogous to Eq. 6.78 can be developed to define points of zero charge for any such adsorptive, as illustrated in Fig. 6.9.42... 

S. L. Cumberland and G. F. Strouse, Analysis of the Nature of Oxyanion Adsorption on Gold Nanomaterial Surfaces, Langmuir 18, 269-276 (2002). 

Commonly, arsenite exhibits low adsorption potential for oxides or clay edges because the former exhibits high pKas (9.22 and 13.52). Recall that maximum adsorption of an oxyanion by an oxide takes place at pH closest to its pKa. Arsenate adsorption is nearly at maximum in the pH range of 3 to 11 because it encompasses the range of its three pKa values, 2.22, 6.98, and 11.52 (see Chapter 4). 

Adsorption, 167-169 See Double layer effect Metals on organic matter, 137 Metals on clays, 167,171 Organic compounds, 181, 355 Oxyanions, 190 Inner-sphere coordination, 169 Outer-sphere coordination, 168 Models, 178,186 Constant capacitance, 186... 

The adsorption of oxyanions is important in electrochemistry because of the wide application of these ions in electrochemical systems of practical and fundamental interest. 

Figure 1 (a) Normalized XANES and (b) pre-edge peaks of Fe K-edge in Fe/en-MCM-41 before and after adsorption of oxyanions. 

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