Phosphorus is a component of the complex nucleic acid structure of plants, which regulates protein synthesis. Phosphorus is, therefore, important in cell division and development of new tissue. Phosphorus is also associated with complex energy transformations in the plant. Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering, and also hastens maturity (Adhami et al., 2007).
Dominant P species in soil solution that plants take up are H 2PO4 – or HPO 4 – in the pH range of common soils. In acidic soils H 2PO4- ions dominate and in alkaline soils HPO4- ions. Presents P cycle in modern eco-system where maximum quantity of soluble P is governed by input from organic and inorganic sources and solid phases through precipitation and dissolution processes. Now at any one-time absolute quantity of soluble P is low compared to the plant requirement (Delgado and Torrent, 2000) and without fertilizer addition the natural soil P reserves are not enough to support intensive crop cultivation. For optimum crop production use of fertilizer P becomes essential to replenish soil soluble P continuously.
Plant available soil phosphorus level in soil is generally too low for sustained crop yield therefore, application of inorganic fertilizer P becomes necessary for commercial crop production as in soils, plants take up less than 20% of applied P and the remaining is immobilized (Bürkert et al., 2001). The poor mobility of soil inorganic P is thought to be due to low dissolution rate of inorganic P species, sorption of phosphate ions onto iron oxides in acidic soils and onto calcium carbonate in alkaline soils P becomes fixed in the soil matrix (Hinsinger, 2001).
Calcareous soils have some level of iron oxides, either as discrete mono-mineral phases or as aggregates with other phases as coatings on other mineral particles. So, phosphate sorption in calcareous soils may also be controlled by the presence of iron oxides like that in acidic soils (Manojlovic et al., 2007).
Iron oxides are the most abundant metallic oxides in soils. Iron oxides occur in most soils. They are present in most soils of different climatic regions as very fine particles in one or more different mineral forms and at variable levels of concentrations. They may occur as dispersed throughout the soil in horizons with a friable or loose consistency or concentrated in discrete horizons or morphological features as mottles, nodules, etc. Because iron oxide surfaces have a strong affinity for the oxyanions of P and for transition metals, they play a significant role in the environmental cycling of these elements (Kämpf et al., 2002). Term of “iron oxides” is used which refers to oxides, hydroxides and oxyhydroxides of iron (Bigham et al., 2002). Out of the 14 iron oxides, 10 are known to occur in nature. The most widespread iron oxides in soils, having high thermodynamic stability are crystalline goethite and hematite.
Specific adsorption by phosphate on iron oxides
At hydroxylated surfaces, a positive or negative charge is created by the adsorption or desorption of H + or OH- ions, which is balanced by an equivalent amount of anion through specific adsorption. Sorption of P occurs through ligand exchange on variable charge surfaces by the exchange of OH- on the surface for phosphate ion. There is covalent bond between the metal ion and the phosphate ion. Phosphate is considered to sorb mainly as an inner-sphere complex, which means that the sorption takes place at specific coordination sites on the oxides or hydroxides. Phosphorus is more strongly surface-associated through covalent bonds formed by ligand exchange with oxide surfaces OH groups compared to SO4, which is a non-specifically bound oxyanion and is weakly surface-associated due to electrostatic interaction (Yuji and Sparks, 2001). The sorption reaction is strongly non-reversible and the sorbed phosphate is mostly unavailable for plant uptake. The precise nature of these reactions depends on pH (Bertrand et al., 2001).

Review of literature
There is a close relationship between phosphate and iron oxides in many soils. Part of the phosphate associated to iron oxides occurs in ‘occluded’ forms that are released only when iron oxides are reduced by dithionite treatment. Several forms of occluded phosphate are likely to be present in individual particles and aggregates of iron oxides, but their relative significance has not been established. Several investigations suggest that data on occluded phosphate can help elucidate some aspects of soil genesis (Torrent, 2000).
In various operating conditions it was investigated that there is a potential use of iron oxide particles as adsorbents for removal of residual phosphorus from the secondary effluent discharged from activated sludge treatment. Substantial phosphorus removal was achieved with IOPs including ferrihydrite, goethite, and hematite, though the adsorption capability of phosphorus depended on the types of IOPs tested. Adsorption isotherm tests helped to understand the behaviour of the adsorptive properties of phosphorus on different IOPs. Phosphorus removal efficiencies for secondary effluent remained constant or further increased over the wide pH range of below neutral to alkaline values, in contrast to those for a synthetic phosphorus solution. This could be in close association with the interaction of phosphorus with Ca and Mg ions present in secondary effluent leading to chemical precipitation, such as apatite, dolomite, calcite, and brucite. The reactivity of IOPs with phosphorus at neutral pH appeared to be almost independent of the existence of background inorganic species, such as inert salts, alkalinity, and hardness. The phosphorus removal efficiency was compared during regeneration and recovery of used IOPs using centrifugation and microfiltration (MF) methods. Microfiltration following IOP adsorption was found to be efficient enough to accomplish significant phosphorus removal during continuous adsorption suggesting that the combination of IOP adsorption with MF would be attractive as a tertiary treatment alternative (Kang at el., 2002).
Continuous crop expansion has led to a growing demand for phosphate fertilizers. A knowledge of the dynamics of phosphorus, and its interaction with iron oxides and organic matter, can be useful to develop effective strategies for sustainable management, especially in a scenario of increasing shortage of mineral phosphate resources. Chintala et al., 2014 reviewed the relationship of phosphate to iron oxides and organic matter, and its effect on phosphorus availability. Crops typically obtain phosphate from weathered minerals and dissolved fertilizers. However, the amount of phosphorus present in the soil solution depends on the extent to which it is adsorbed or desorbed by iron oxides, which may be influenced by interactions with organic matter. Therefore, systems for fertilizer recommendation based on methodologies considering interactions between soil components such as oxides and organic matter, and the phosphorus sorption capacity resulting from such interactions (e.g. residual P analysis), may be more reliable to ensure efficient, rational use of phosphate (Chintala, et al., 2014).
Wang et al., 2012 investigated that there are also some factors such as pH, solution ion composition, and the presence of natural organic matter play an important role in the effectiveness of phosphorous adsorption by iron oxides. With the help of different mechanisms and adsorption experiments, the net macroscopic effect of single and combined factors can be better understood and predicted. In the present work, the relative importance of the above-mentioned factors in the adsorption of phosphate was analysed using modelling and comparison between the model prediction and experimental data. The results showed that, under normal soil conditions, pH, concentration of Ca, and the presence of NOM are the most important factors that control adsorption of phosphate to iron oxides. The presence of Ca not only enhances the amount of phosphate adsorbed but also changes the pH dependency of the adsorption. An increase of dissolved organic carbon from 0.5 to 50 mg L can lead to a >50% decrease in the amount of phosphate adsorbed. Silicic acid may decrease phosphate adsorption, but this effect is only important at a very low phosphate concentration, in particular at high pH.

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Wang, L., V. Riemsdijk, T. Hiemstra. 2012. Factors controlling phosphate interaction with iron oxides. J. Environ. Qual. 3: 628-635.


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