Adsorption of phosphate

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

Hongshao Z, Stanforth R (2001) Competitive adsorption of phosphate and arsenate ongoethite. Environ Sci Technol 35 4753—4757 Hsia TH, Lo SL, Lin CF, Lee DY (1994) Characterization of arsenate adsorption on hydrous iron oxide using chemical and physical methods. Colloid Surface A 85 1-7... 

Since properties of both arsenate and phosphate species are very close, the adsorption of phosphate species was studied prior to that of arsenate. The phosphate adsorption was tested by the following two methods (i) and (ii). Hereafter, flow rate and volume of feed are given in space velocity (SV, h 1)... 

Behavior of Zr(IV)-loaded CRP200 in adsorption of phosphate species... 

Since properties of both phosphate and arsenate are very similar each other, the adsorption of phosphate was examined prior to the adsorption of arsenic species. Here, the feeding solution in the adsorption operation was 1 mM phosphate solution of pH3. Table 1 summarizes detailed experimental conditions and column performances during repeated adsorption-elution-regeneration cycles. Since supplied volumes of the feed are not constant (101 - 193 BV), it is not easy to judge the efficiency of the adsorption from total uptake of phosphate. Thus, removal of phosphate until 100 BV is listed at the last column of Table 1 as an index of the column performances. 

In runs from 2-1 to 2-4, the adsorption of phosphate was carried out by supplying the feeding solutions at SV 10 h 1. Leakage of phosphate from the column up to 100 BV of the feed is markedly depressed by repeating the adsorption-elution-regeneration cycle of the method (ii). In the runs from 2-2 to 2-4, the uptake until 100 BV of the feed is almost quantitative, and then the flow rate of the feed increased to SV 20 h1. Even at the higher flow rate of SV 20 h 1 (runs from 3-1 to 3-7), the column took up phosphate almost quantitatively up to 100 bed volumes of the feed. 

The final run 4-1 was the adsorption of phosphate from seawater to which 1 mM of phosphate was spiked. The concentration of NaCl in seawater is ca. 0.5 M, which is 500 times higher than the concentration of the spiked phosphate. Thus, it can be concluded electrolytes in seawater do not interfere with the adsorption of phosphate by Zr(FV) loaded CRP200 as in the case of arsenate uptake by Zr(IV) loaded phosphoric acid resin RGP.14... 

Figure 3 shows the pH dependence for phosphate adsorption on YAI2O3 for a phosphate concentration of 8 x 10 3 mol dm-3, at 1= 1.5xlO-2M, [P] = 30 g dm-3, and 25°C. It is well known that below the pHZpC in the absence of phosphate, both the surface- and C-potential in the -y—AI2O3 suspension are positive however in the presence of phosphate, the surface is negatively charged at pH values below the zpc (pH = 8.5 indicating specific adsorption of phosphate (see Figure 4). A mechanism to explain phosphate adsorption based on pressure-jump studies has been postulated by Mikami et al. (15)...

The retention of Cu by allophane is enhanced by phosphate regardless of the sequence of Cu and phosphate adsorption, although Cu has been found to have no effect on the simultaneous and subsequent adsorption of phosphate on surface bound Cu (32) ESR results suggest that the Cu binds to surface A10H groups of the allophane irrespective of the presence of phosphate and it was proposed that the enhanced Cu retention was the result of the formation of a ternary complex by the binding of phosphate to the axial position of the surface-bound Cu ion. 

The four layer model (Bowden et ah, 1980 Bousse and Meindle, 1986) also locates different adsorbing ions in different planes. It has been used to model adsorption of phosphate, citrate and selenite (Bowden et ah, 1980) and borate (Bloesch et al., 1987) on goethite and competitive adsorption of Ca and Cd on ferrihydrite (Cowan et al., 1991). 

Adsorption of phosphate is initially rapid and is followed by a slow stage (hours to days) that is more pronounced for less crystalline samples of Fe oxides (Barrow et ak, 1981 Torrent et ak, 1990 Nilsson et ak, 1992). The slow stage has been attributed to diffusion into micropores or grooves (Torrent, 1991 Strauss et ak, 1997) and into aggregates of particles (Anderson et ak, 1985 Willet et ak, 1988). Evidence for slow dif-... 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

Rietra et al. (2001) measured the simultaneous adsorption of phosphate and calcium on goethite over the pH range 4-11 and modelled the data with the CD-MU-SIC model. They concluded that the observed adsorption took place in response to electrostatic effects and that ternary adsorption was not involved. 

The interaction of phosphate with d and Na is shown in Figure 11.11. Upon adsorption of phosphate onto goethite, d adsorption dropped to zero, whereas adsorption of Na increased substantially (Nanzyo and Watanabe, 1981, 1982). It was suggested that adsorption of phosphate is followed by adsorption of additional cations at pH >7. 

As in pure systems, the adsorption of anions and cations on iron oxides is strongly pH dependent. This has to be kept in mind when an optimum pH is to be obtained with liming. The adsorption of phosphate, arsenate etc. increases as the pH falls below 7, whereas the adsorption of heavy metal cations rises as pH goes up (see eq. 11.18 11.19). Therefore, as soils become more acidic, heavy metals will be released into the soil solution. Conversely, liming soils has the opposite effect. 

Bowden, J.W. Nagarajah, S. Barrow, N.J. Posner, A.M. Quirk, J.P. (1980) Describing the adsorption of phosphate, citrate and selenite on a variable-charge mineral surface. Aust. J. Soil Res. 18 49-60... 

Madrid, L. De Arambarri, P. (1985) Adsorption of phosphate by two iron oxides in relation to their porosity. J. Soil Sci. 36 523-530... 

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... 

Different from the formation mechanism of titania nanotubes, Fe203 nanotubes are formed by a coordination-assisted dissolution process [95]. The presence of phosphate ions is the crucial factor that induces the formation of a tubular structure, which results from the selective adsorption of phosphate ions on the surfaces of hematite particles and their ability to coordinate with ferric ions. 

The amount of zinc(II) ions bound to the polymer was quite elegantly determined by the use of a zinc(II) selective fluorophore developed by the same group. [3] Adsorption of phosphates such as 5 -dAMP and 4-NPP, adenosine 3, 5 -cyclic-monophosphate (3, 5 -cAMP) and the corresponding dephosphorylated compounds, deoxyadenosine (dA) and 4-nitrophenol (4-NP) was studied. The polymer was stirred in a solution containing the studied guest in a buffered aqueous solution. The adsorption efficiency was determined by the decrease of the guest molecule concentrations by UV measurement. 

Krom, M.D., and Berner, R.A. (1980b) Adsorption of phosphate in anoxic marine sediments. Limnol. Oceanogr. 25, 797-806. 

Figure 4.7. Schematic of adsorption of phosphate by an iron oxide in an inner-sphere bidentate or monodentate mode.

Adsorption of phosphate ions by aluminum oxide (Experiment 124)... 

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