The two main motivations for studying equations which describe adsorption of phosphate are to understand the processes involved, and to summarize many results by a few numbers. If more than one hypothesis about the process is tenable, appropriate statistical procedures should be used both to choose between them and to obtain the best summary of the results with a given equation. Equations most likely t o be successful in describing adsorption are those in which the affinity for adsorption decreases as the amount of adsorption increases. This effect is inherent in the process of phosphate adsorption and occurs because specific adsorption of anions increases the negative charge on the adsorbing surface. It is included in the complex equations of Bowden, and in simpler fashion in the Freundlich equation. Because the simpler equation can only approximately describe the true situation, a perfect fit over a wide range of concentrations should not be expected. Equations for which the affinity for adsorption is constant -such as the Langmuir equation -are not consistent with our knowledge of the adsorption process. This difficulty is not avoided by the multi-surface Langmuir equations: such equations may also be difficult to justify on statistical grounds. Introduction IN THIS paper, the term 'phosphate adsorption' is used to describe any process in which phosphate ions in solution react with atoms on the surface of soil particles. The extent to which this occurs is an important property affecting both the availability of phosphate to plants and the effectiveness of phosphate fertilizer. This property is commonly measured by shaking samples of the soil with phosphate solutions, measuring the change in phosphate concentration, and calculating the phosphate adsorbed. The information so gained is then summarized by plotting the calculated adsorption of phosphate against the observed solution concentration.
A model of phosphate reaction is constructed and its output compared with observations for the sorption and desorption of phosphate by soil. The model has three components: first, the reaction between divalent phosphate ions and a variable-charge surface; second, the assumption that there is a range of values of surface properties and that these are normally distributed; third, the assumption that the initial adsorption induces a diffusion gradient towards the interior of the particle which begins a solid-state diffusion process.The model closely describes the effects on sorption of phosphate of: concentration of phosphate, pH, temperature, and time of contact. It also reproduces the effects on desorption of phosphate of: period of prior contact, period and temperature of desorption, and soil : solution ratio. The model is general and should apply to other specifically adsorbed anions and cations. It suggests that phosphate that has reacted with soil for a long period is not 'fixed' but has mostly penetrated into the soil particles. The phosphorus can be recovered slowly if a low enough surface activity is induced.
Eight samples of goethite ranging in surface area from 18 to 132 m2 g-' were mixed with phosphate at a range of pH values for periods which ranged from 0.5 h to 6 weeks. The sample with a surface area of 18 m2 g-' had been hydrothermally treated to improve its crystallinity. Its rate of reaction with phosphate depended on pH but was complete within a day. Its maximum observed reaction was close to the theoretical maximum for surface adsorption of 2.5 pmol m-2. For the other samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2.5 pmole mP2. The duration and extent of the reaction depended on the crystallinity of the goethite. The results were closely described by a model in which the phosphate ions were initially adsorbed on to charged external surfaces. The phosphate ions then diffused into the particles. This was closely described using equations for diffusion into a cylinder.Samples of goethite which had been loaded with phosphate dissolved more slowly in HC1, and had a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HC 1 removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it was necessary to dissolve much more of the iron to obtain a similar removal of phosphate. Brief treatment with NaOH removed most of the phosphate from the hydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite. These results are consistent with the idea that phosphate ions were not only bound on external surface sites but had also penetrated into meso-and micro-pores between the domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sites for phosphate adsorption are therefore caused mainly by their location on either external or internal sites. Models that ignore this are incomplete.
The effects were measured of varying the pH and the concentration of adsorbing ion on adsorption of phosphate, citrate and selenite by goethite and on the charge conveyed to the surface. The ability of the model of Bowden et al. to describe these effects was investigated. The observed effects were closely described by the model, provided it was modified to permit the adsorbed ions to reside in a plane between the surface and the diffuse layer. The model requires that individual ionic species be considered. For phosphate and selenite, the divalent ion appeared to be the only ion adsorbed whereas for citrate it was the trivalent ion. The model also requires that adsorption depends on the electrostatic potential in the plane of adsorption. This potential decreases with increasing pH. Thus the effects of pH on adsorption were explained by changes in this potential, together with changes in the proportion of the ionic species present. Because adsorption made the surface more negative, it also decreased the electrostatic potential in the plane of adsorption. This made further adsorption more difficult, and as a result, adsorption at a constant pH did not follow the Langmuir equation. The model showed that the increase in negative charge as a result of adsorption was partly balanced by an uptake of protons by the surface. This was most marked at near-neutral pH and as a result the net charge per adsorbed ion was least.
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