2. Mechanism of phosphorus adsorption in soils

#### 2.1 Specific adsorption of phosphorus by aluminium and iron oxides

Phosphate sorption is a term used to describe all the processes resulting in the removal of phosphate from soil solution, mainly by surface adsorption and precipitation [14]. Important soil factors that determine its capacity to retain phosphorus (P) are the presence of amorphous aluminium and Iron oxides compounds [15]. The process of adsorption of the phosphate by these compounds (i.e. aluminium and Iron oxides) which are also known as adsorbents is known as specific adsorption. The phosphate molecule or ion which is adsorbed is then known as adsorbate. Specific adsorption of ions can occur unto uncharged adsorbents and sometimes even unto surfaces bearing charge of the same sign as the adsorbent. Thus phosphate can be adsorbed unto surfaces of variable-charge minerals such as aluminium and iron oxides even at alkaline pH, where these adsorbents are negatively charged. Specific adsorption is characterised by formation of inner-sphere complexes, where no water molecules are interposed between the adsorbent and the adsorbate. The most important variable-charge minerals in the soil that adsorb P include aluminium oxides and iron oxides. The poorly ordered ("amorphous") iron and aluminium hydroxides possess very large specific surface area (SSA) which can be as high as 800 m<sup>2</sup> g<sup>1</sup> , and 10 times larger than the SSA of corresponding crystalline forms. Additionally, these sesquioxides have high singly coordinated surface hydroxyl density [16]. The reactive sites of these amphoteric AlOH and FeOH minerals are the hydroxyl groups exposed on the mineral surfaces. The kind of hydroxyl (OH) groups in which the oxygen of the OH is coordinated to one structural Iron (111) (Fe3+) ion (single – coordinated), are found to protonate and deprotonate in response to solution pH.

Processes and Factors Affecting Phosphorus Sorption in Soils DOI: http://dx.doi.org/10.5772/intechopen.90719

The single–coordinated OH groups are those surface hydroxyl groups onto which specifically adsorbable anions are adsorbed [7]. The single–coordinated hydroxyl groups can be quantitatively replaced or exchanged by the phosphate anions. This results in the formation of a binuclear or surface complex for the phosphate iron oxide system, where one phosphate ion occupies two surface sites. This is accompanied by a release of hydroxyl (OH) and H2O groups (Figure 1).

Aluminium oxides, iron oxides and clay silicates are well known phosphate adsorbents in soils [6]. According to Borggaard et al. [7], aluminium and iron oxides are the main phosphate adsorbents in sandy soils. Close correlations have been found between a soil's capacity to adsorb phosphate and the content of aluminium and iron oxides, in the soil suggesting these oxides to be the main phosphate

In acidic soils, P can be dominantly adsorbed by Al/Fe oxides and hydroxides, such as gibbsite, haematite, and goethite [9]. P can be first adsorbed on the surface

nonprotonated and protonated bidentate surface complexes may coexist at pH 4–9, while protonated bidentate innersphere complex is predominant under acidic soil conditions [10]. Clay minerals and Fe/Al oxides have large specific surface areas, which provide large number of adsorption sites. The adsorption of soil P can be

Phosphate is strongly adsorbed by the number of adsorption sites, which vary greatly among soils [11]. With further reactions, P may be occluded in nanopores that frequently occur in Fe/Al oxides, and thereby become unavailable to plants [10]. Therefore, the availability of soil phosphate as well as the soil solution concentration of phosphate will depend on the degree of phosphate saturation, rather than on the total phosphate content [12]. Phosphate saturation is the proportion of adsorption sites occupied by phosphate, which is normally taken as the ratio between adsorbed phosphate and the phosphate adsorption capacity (PAC) of

of clay minerals and Fe/Al oxides by forming various complexes. The

adsorbents in soils [8].

Sorption in 2020s

the soil [13].

be as high as 800 m<sup>2</sup> g<sup>1</sup>

46

deprotonate in response to solution pH.

enhanced with increasing ionic strength.

2. Mechanism of phosphorus adsorption in soils

2.1 Specific adsorption of phosphorus by aluminium and iron oxides

removal of phosphate from soil solution, mainly by surface adsorption and

[15]. The process of adsorption of the phosphate by these compounds (i.e. aluminium and Iron oxides) which are also known as adsorbents is known as specific adsorption. The phosphate molecule or ion which is adsorbed is then known as adsorbate. Specific adsorption of ions can occur unto uncharged adsorbents and sometimes even unto surfaces bearing charge of the same sign as the adsorbent. Thus phosphate can be adsorbed unto surfaces of variable-charge minerals such as aluminium and iron oxides even at alkaline pH, where these adsorbents are negatively charged. Specific adsorption is characterised by formation of inner-sphere complexes, where no water molecules are interposed between the adsorbent and the adsorbate. The most important variable-charge minerals in the soil that adsorb P include aluminium oxides and iron oxides. The poorly ordered ("amorphous") iron and aluminium hydroxides possess very large specific surface area (SSA) which can

crystalline forms. Additionally, these sesquioxides have high singly coordinated surface hydroxyl density [16]. The reactive sites of these amphoteric AlOH and FeOH minerals are the hydroxyl groups exposed on the mineral surfaces. The kind of hydroxyl (OH) groups in which the oxygen of the OH is coordinated to one structural Iron (111) (Fe3+) ion (single – coordinated), are found to protonate and

Phosphate sorption is a term used to describe all the processes resulting in the

precipitation [14]. Important soil factors that determine its capacity to retain phosphorus (P) are the presence of amorphous aluminium and Iron oxides compounds

, and 10 times larger than the SSA of corresponding

The three kinds of hydroxyl groups occurring on the goethite surface denoted (A) single-coordinated, (B) triple coordinated and (C) double-coordinated. Source: Borggaard and Elberling [6].

Figure 2. Examples of phosphate adsorption mechanisms. Source: Syers and Cornforth [17].

self-aggregation (clustering) and porosity seem to be important factors in controlling adsorption/desorption (irreversibility) of phosphate by iron oxides and thus by soils. Formation of iron phosphate coatings has, however, been rejected by others [9], who considered migration (diffusion) of phosphate into aggregated iron oxides,

Several investigations have shown the effect of pH on phosphate adsorption by soil and synthetic iron oxides [6, 23]. The pH effect on soil iron oxide adsorption seems to be less pronounced than on pure iron oxide adsorption. According to Borggaard [24], pH affects phosphate adsorption but the effect is limited for adsorption by soils in the pH range 4–8 in contrast to adsorption by pure iron oxides. For soils, increasing pH has been shown to either increase or decrease and to have no effect on phosphate adsorption [25]. Nwoke et al. [26] found that sorption of P decreased with increasing soil pH and this was attributed to increased negative charge on variable-charge colloids which cause electrostatic repulsion of the ionic P species from the surface. In contrast, Agbenin and Mokwunye [27, 28] reported an increase in sorption with increasing pH for some savannah soils. Agbenin [27] attributed this trend to the chemistry and retention of Ca2+, the predominant cation in savannah soils. Nevertheless, the pH effect on phosphate adsorption should not be exaggerated, since this effect is fairly small, particularly, over the pH range covering most soils,

and ancillary effects may therefore appear relatively important [29].

reduced soil phosphate adsorption significantly [31].

Organic matter may affect phosphate adsorption in two ways: Indirectly by inhibiting iron oxide crystallisation and directly by competing for adsorption sites [24]. Dissolved organic matter (fulvic and humic acids) has been shown to decrease phosphate adsorption by iron oxides and by soils, particularly at acid pH, indicating that dissolved organic matter can compete with phosphate for adsorption sites [30]. In the study by Sibanda [30], organic matter which was isolated from soils as humic and fulvic acids, was added in solution, and the background electrolyte was sodium chloride. Of seven naturally occurring organic compounds tested, only phytic acid

In contrast, the results of the study of influence of organic matter on phosphate

adsorption by aluminium and iron oxides in sandy soils clearly showed that organic matter has no direct influence on adsorption of phosphate by these soils [24]. According to these workers, the phosphate adsorption capacity changes with the amount of extractable aluminium and iron, irrespective of the organic matter content; even removal of the organic matter does not alter phosphate adsorption. In the study mentioned, there was no addition of organic matter, and calcium acetate was used as background electrolyte. Calcium flocculates organic matter, while sodium tends to disperse it. The interpretation of the results, therefore, could be that to act as a competitor, organic matter must be in solution; otherwise it has no direct effect on phosphate adsorption. In limed soils and in many cultivated soils the concentration of dissolved organic matter is considered to be very low. Interactions are known to occur between organic matter and the aluminium and iron oxides inhibiting their crystallisation, and thereby increasing their phosphate adsorption capacity [22, 32]. Soil organic matter, can indirectly affect soil phosphate adsorption capacity (PAC) by retarding crystal growth of poorly crystalline aluminium and iron oxides, which because of high specific surface areas

particularly ferrihydrite, to cause the slow reaction.

Processes and Factors Affecting Phosphorus Sorption in Soils

DOI: http://dx.doi.org/10.5772/intechopen.90719

3.2 Soil pH

3.3 Organic matter

have very high PACs [33, 34].

49

#### Figure 3.

The inner sphere formation of P in soil minerals (a) and the subsequent occlusion of adsorbed P (b). Source: Syers and Cornforth [17].

The precise nature of these reactions depends on pH which influences the proportions of hydroxyl (OH) and OH2 <sup>+</sup> groups on the solid surface and hence its surface charge.

If the adsorbed phosphate ions then diffuse into the solid, then they are "absorbed". Sorption covers the combined processes. Adsorbed phosphate may become trapped on the surface of soil minerals if any Fe or Al oxide coating is precipitated on the mineral. The trapped phosphate is then described as occluded (Figures 2 and 3).
