Phosphorus Sorption Isotherms

The adsorption isotherms of soils are usually measured by mixing a known amount of soil with a solution containing a known phosphorus concentration. The mixtures are equilibrated for a fixed period (usually 24 h) at a constant temperature under continuous shaking. A wide range of soil-to-water ratios are used in batch isotherm experiments. Phosphorus not recovered in solution is assumed to be adsorbed on solid phase. Phosphate retained by soils is calculated as follows  

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

Equation (9.17) is used primarily to calculate the phosphorus lost (adsorbed) or released (desorbed) in relation to phosphorus added to soil. These calculations do not account for the amount of native soil phosphorus in the adsorbed phase. The total amount (native + added) of phosphorus retained by soil can be calculated as follows  

A number of empirical equations have been used to describe these relationships (Berkheiser et al., 1980). The most commonly used phosphorus sorption isotherm equations are as follows. 

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From a conceptual standpoint, the phosphorus cycle would be relatively simple to understand and model if phosphorus sorption isotherms could simply be added to ecosystem models, parameterized using the abundance of phosphorus sorption data from the scientific literature, and allowed to run alongside the carbon and nitrogen submodels that are so familiar to the ecological community. Unfortunately, there are two central issues that make this approach untenable. First, phosphorus sorption dynamics are not straightforward, and simple representations fail to capture... 

Example Problem Suppose that a deep and fairly homogeneous soil is going to be used to dispose of septic tank waste water that contains 10 mg/liter of soluble phosphorus in the form of phosphate. A sorption isotherm is measured for the soil, from which the sorption capacity of the soil is estimated at Qm = 200 mg P/kg soil. The 600 liters of waste water that is generated each day is evenly disposed of over a 70 square meter area. Estimate the depth of phosphate movement in the soil after ten years, and make a decision whether a shallow well (10 meters deep) at the edge of the disposal area could become contaminated by phosphate. 

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. 

Single-point isotherms were initially used by Bache and Williams (1971) to determine phosphorus sorption capacity of a wide range of soils. The approach involves measuring phosphorus sorption at one fixed concentration after hxed equilibration period (18 h). The phosphorus sorption index (PSI) (Bache-Williams index) is calculated as follows ... 

Quantity/intensity relationships are often used to describe soil capacity to buffer phosphorus concentration in soil pore water. The quantity (0 refers to the amount of phosphorus adsorbed on soil surface, whereas intensity (/) refers to the concentration of P in soil pore water. This ratio can also be viewed as partition coefficient (K ), as indicated by liner sorption isotherms. The ratio expressed as either QII or is influenced by various physicochemical properties of soils, including clay content, high concentration of Fe and Al oxides, CaCOj content, organic matter content, pH, and redox potential. 

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. 

Phosphorus sorption maximum as estimated by sorption experiments and Langmuir sorption isotherms = 1000 mg P/kg... 

The sorption isotherms are established experimentally starting mostly with dry products. The initial humidity of the air with which the product is in equilibrium should be brought to extremely low values using either concentrated sulfuric add or phosphorus pent-oxide, so that the moisture content of the product is close to zero at the beginning. The product is then exposed to successively greater humidities in a thermostatically controlled atmosphere. Sufficient time must be allowed for equihbrium between the air and solid to be attained. Using thin slices of the... 

Anionic dyes readily adsorb on starch (Tables XXXIX-XLIII).802 However, there is evidence that this rule is violated in the case of potato starch.802-803 The simplest intepretation of this fact has been presented in terms of the phosphate groups present in potato starch. A relationship exists between the phosphorus content in potato starch and the sorption capacity of that starch for Methylene Blue. The sorption follows the Langmuir isotherm. Extrapolation of the result to 0% phosphate leads to the conclusion that no Methylene Blue would be adsorbed on phosphate-free starch.804 Methylene Blue is better adsorbed on potato starch, but is also... 

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