Phosphorus adsorption

While phosphorus export from agricultural systems is usually dominated by surface runoff, important exceptions occur in sandy, acid organic, or peaty soils that have low phosphorus adsorption capacities and in soils where the preferential flow of water can occur rapidly through macropores (Sharpley et al., 1998 Sims et al., 1998). Soils that allow substantial subsurface exports of dissolved phosphorus are common on parts of the Atlantic coastal plain and Honda, and are thus important to consider in the management of coastal eutrophication in these regions. 

IV. Yttrium basic carbonate and oxide on hematite. J. Coll. Interf Sd. 126 645-649 Ainsworth, C.C. Sumner, M.E. (1985) Effect of aluminum substitution in goethite on phosphorus adsorption II. Rate of adsorption. Soil Sd. Soc. Am. J. 49 1149-1153... 

Dissolved metals other than calcium have a minor effect on the distribution of phosphorus between the water column and sediment in this fluvial system. The two principal metals of potential interest, iron and aluminum, are present in Genesee River water almost entirely in the particulate phase ( ). Dissolved concentrations of these metals are below the detection limit (less than 50 ug/1). Iron and aluminum minimum detectable dissolved concentrations were used to estimate the saturation levels of the corresponding phosphate minerals. These calculations suggest that both iron and aluminum phosphate minerals are substantially below saturation levels. The solid surfaces exhibited by iron and aluminum hydrous oxides (as particulate material in the water column) undoubtedly serve as sites for phosphorus adsorption and incorporation in the fluvial system. Data presented for the oxalate extraction procedure in Table III demonstrate the importance of phosphorus binding by hydrous metal oxides. 

Bjerrum, C. J., and Canfield, D. E. (2002). Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides. Nature 417, 159—162. 

FIGURE 9.10. Phosphorus adsorption data plotted according to the Langmuir equation. (After J. K. Syers, M. G. Browman, G. W. Smillie, and R. B. Corey. 1973. Soil Set. Soc. Am. Proc. 37 358-363.)... 

Forster, J.C. and Zech, W. (1 993) Phosphorus status of a soil catena under Liberian evergreen rain forest results of P NMR spectroscopy and phosphorus adsorption experiments. Zeitschrift fur Pfianzenernahrung und Bodenkunde 1 55, 51-55. 

Barrow (1993) successfully applied this model to phosphate adsorption by taking into account pH and electrolyte concentration. The use of this equation to describe organic phosphorus adsorption can be important as well, due to the high charge density of molecules such as myo-inositol hexakisphosphate, but the complexity of organic phosphorus compounds and the different mechanism of adsorption can make the model difficult to apply. 

Fig. 6.8. Electric potential ( >) at the particle surface and zeta potential (Q at the shear plane before and after phosphorus adsorption on oxides at initial pH lower than the point of zero charge.

Adsorption It refers to the movement of soluble inorganic phosphorus from soil pore water to soil mineral surfaces, where it accumulates without penetrating the structure (Figure 9.16). Phosphorus adsorption capacity of soil increases with clay content or minerals. Adsorbed phosphorus maintains equilibrium with phosphorus in soil pore water. 

Desorption It refers to the release of adsorbed inorganic phosphorus from the mineral surfaces into soil pore water. Depletion of phosphorus from soil pore water results in the release of phosphorus from mineral surfaces until the new equilibrium is reached. The balance between phosphorus adsorption and desorption maintains the equilibrium between solid phases and phosphorus in soil pore water. This phenomenon is defined as phosphate buffering analogous to pH buffering. 

With time, some of the adsorbed phosphorus diffuses into solid phase (absorption) where it forms discrete phosphate minerals. This process occurs very slowly over timescales of days to months or years. Decrease in quantity of phosphorus on solid surfaces results in more sites for soil pore water phosphorus adsorption. These conditions can reduce soil pore water phosphorus concentration, resulting in a new equilibrium. 

A sorption isotherm describes the eqnilibrinm relationship between the concentrations of adsorbed and dissolved species at a given temperatnre. Becanse soil scientists have adapted and modified these fnnctions and nsed them to describe phosphate adsorption from solution, they have proved to be less than ideal. Phosphorus adsorption increases with increasing soil pore water phosphorus concentration, nntil all sorption sites are occnpied. At that point, adsorption reaches its maximum, as indicated by Similarly, an incremental decrease in soil pore water phosphorns concentration resnlts in desorption of phosphorns from the solid phase. At low pore water phosphorus concentration, the relationship between adsorption and soil pore water phosphorus concentration is linear. The intercept on y-axis (Fignre 9.22), as indicated by Sq, snggests that phosphorns is adsorbed at soil pore water phosphorns concentrations approaching near-zero levels. If phosphorus is added to soil at concentrations lower than that of phosphorns in soil pore water, then the soil tends to release phosphorns nntil new eqnilibrinm is reached. Soils adsorb only when added phosphorus concentrations are higher than the concentration of phosphorns in soil pore water. 

FIGURE 9.22 A typical phosphorus adsorption isotherm showing amount of phosphorns adsorbed or desorbed in relation to phosphorus concentration in soil pore water. 

FIGURE 9.26 Phosphorus adsorption characteristics of soils from mineral and peat-based wetlands. (Adapted from Richardson, 1985.)... 

FIGURE 9.27 Phosphorus adsorption characteristics of soils from northern Everglades (Water Conservation Area 2A) (Clark, 2002). 

The log-log plots are insensitive within the range of experimental data, and the equation contains two empirical constants, and N. Because the Freundlich equation is empirical in nature, it provides very little insight into the mechanism regulating phosphorus adsorption process. 

FIGURE 9.28 Relationship between phosphorus adsorption maxima estimated using Langmuir isotherm equation and saturation maxima estimated from single-point isotherm. (From Reddy et al., 1998a.)... 

Phosphorus adsorption typically occurs at low concentrations and reaches saturation level once all potential sorption sites are occupied. However, if the concentration of soil pore water is increased beyond the capacity of soil to adsorb phosphorus, the precipitation reactions may be involved in retaining phosphorus. When sorption isotherms are measured at high concentrations (Figure 9.26), it is hard to differentiate between adsorption and precipitation reactions. An example of potential precipitation reactions at high solution concentrations is shown in Figure 9.29. 

Adsorption and precipitation are the retention mechanisms for inorganic phosphorus. Adsorption initially happens very quickly, and then the rates decrease. Inorganic phosphorus sorption by soils is described by several models Langmuir, Freundlich, and linear... 

The study of the phosphorus adsorption in the Changjiang River Estuary sediments showed that the sediments were buffers to phosphate and the balance time of phosphorus adsorption (desorption) was about 6 h. The saturated adsorption quantity of phosphorus was about 600 pg/g and the saturated desorption was about 126.37 pg/g. The apparent adsorption heat of fine sediments (AH) was 47.59 kJ/mol. The adsorption of phosphate was in accordance with Freundlich s isothermal equilibrium and it was a heat absorption reaction. The contents of sand and mud in sediments had some influence on adsorption and the adsorption would decrease with the increase in sand or mud content. In general, the increase in pH favors adsorption. Moreover, the increase in salinity does not favor adsorption but favors desorption. The research shows that the release of phosphorus in the sediments of the Changjiang River Estuary... 

BLOOM P.R. 1980. Phosphorus adsorption by an aluminium-peat complex. Soil Science Socity of America Journal, 45, 267-272. 

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