Phosphorus adsorption—desorption

FIGURE 9.29 A typical phosphorus adsorption isotherm showing phosphorus adsorption, desorption, and precipitation in relation to phosphorus concentration in soil pore water. 

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... 

Mechanistic Multiphase Model for Reactions and Transport of Phosphorus Applied to Soils. Mansell et al. (1977a) presented a mechanistic model for describing transformations and transport of applied phosphorus during water flow through soils. Phosphorus transformations were governed by reaction kinetics, whereas the convective-dispersive theory for mass transport was used to describe P transport in soil. Six of the kinetic reactions—adsorption, desorption, mobilization, immobilization, precipitation, and dissolution—were considered to control phosphorus transformations between solution, adsorbed, immobilized (chemisorbed), and precipitated phases. This mechanistic multistep model is shown in Fig. 9.2. 

Catalytic properties Phosphorus is known to have deactivation effects for some automotive catalysts and the formation of CeP04 has been identified in phosphorus contaminated catalysts (Uy et al., 2003). Nanocrystalline LaP04 would act as Lewis acid in a catalytic process, which could be determined by a temperature-programmed ammonia adsorption/desorption process (Onoda et al., 2002 Rajesh et al., 2004, 2007). In addition, the rare earth phosphate NCs could act as supports for example, Pd, Pt, or Rh supported on RPO4 show excellent catalytic reduction of NO into N2 and O2 (Tamai et al., 2000), and gold supported on RPO4 shows catalytic activity and stability for CO oxidation. 

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. 

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. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

Figure 7 Characteristic reactions involving phosphorus in the sediments of Toolik Lake, Alaska. The primary processes controlling porewater phosphorus concentrations are adsorption to and desorption from iron oxyhydroxides and the precipitation of authigenic vivianite...

Mikami et al. (56) concluded from a detailed kinetics investigation that the phosphorus species adsorb electrostatically rather than covalently on the alumina at low phosphorus contents. They calculated the intrinsic value of the adsorption and desorption rate constants at 25°C ... 

As for phosphate, the adsorption reaction of organic phosphorus is not readily reversible, although some phosphorus can pass to the solution depending on the time of desorption, the solution-to-soil ratio, and temperature (Barrow, 1983). As solution characteristics play an important role in desorption, different approaches using free water, dilute electrolyte solutions, or chemical extractants with variable pH and ionic composition have been used to quantify the amount of phosphorus desorbable from minerals and then available to plants (Frossard et al, 1995). Desorption of inorganic and organic phosphorus was found to increase with pH (Cabrera et al, 1981 Celi et al, 2003 Martin et al, 2003), with the percentage of phosphorus saturation (Parfitt, 1979 He et a7., 1991, 1994 Martin... 

Addition of soluble inorganic phosphorus to soil increases the soil pore water phosphorus concentration. This results in rapid adsorption of phosphorus onto soil surfaces to maintain equilibrium. Soil s capacity to adsorb additional phosphorus dictates the concentration of phosphorus in soil pore water. These adsorption processes occur within a short time period. When soil particles become saturated with phosphorus, there is an increase in phosphorus concentration in soil pore water. Reaction kinetics are on the order of minutes to hours to reach sorption equilibrium. Figure 9.21 illustrates a two-step process in which rapid phosphate exchange takes place between soil pore water and soil particles or mineral surface (adsorption) followed by slow penetration (absorption) of phosphate into solid phase. Similarly, desorption of phosphorus can also... 

When inorganic phosphorus is added to soil, adsorption process continues until a new state of equilibrium is reached. Adsorption equilibrium is said to be achieved when the concentration of phosphorns in soil pore water does not change between measurements made at different time intervals. If snfhcient time is allowed, and biological activity is inhibited, the system eventually reaches equilibrium. At equilibrium, the rates of forward reaction (adsorption) and reverse reaction (desorption) are the same. Adsorption data at eqnilibrinm are presented in the form of an adsorption isotherm as shown in Fignre 9.22. 

If added solution phosphorus concentration is lower than the concentration of phosphorus in soil pore water (e.g., rainwater), then the soil releases or desorbs phosphorus until new equilibrium is maintained. For any given soil, at some critical concentration, net adsorption equals zero, which means that adsorption equals desorption and the system is at equilibrium as indicated by EPCg (equilibrium phosphorus concentration). At this point, soil exhibits maximum capacity for buffering phosphorus in soil pore water. In this region, the system reattains equilibrium conditions, even if soils are loaded with or depleted of phosphorus in soil pore water. If the water entering a wetland has a phosphorus concentration below EPCq, then that soil releases phosphorus or serves as a source of phosphorus to the water column or soil pore water. If the water entering a wetland has phosphorus concentration higher than EPCq, then that soil adsorbs or retains phosphorus or serves as a sink for added phosphorus. 

FIGURE 9.24 Influence of phosphorus loading on soil EPCq (equilibrium phosphorus concentration at which point adsorption equals desorption) (Clark, 2002). 

Soil s capacity to adsorb phosphorus is regulated by the EPCq, at which point adsorption equals desorption. Each soil buffers a threshold concentration. If the water entering the soil has a concentration of P > EPCg, that soil will have a tendency to adsorb phosphorus. If the water entering the soil has a concentration of phosphorus [Pg.402]

An adsorption isotherm was constructed from integration of TPD peak desorption features for mass 31 (P) associated with increasing e qposure time to calcium phosphate solution (Figure 8). The rapid uptake of phosphorus that begins at approximately 200 min. is a result of the large desorption feature at 1200 K (55). The rapid rise at early e qposure times is primarify a result of the peak at 950 K and may be considered a result of residual salts accumulated at the sur ce, de ite the rinsing procedure. 

Figure 9 shows several TPD spectra of mass 31 (P) at different exposure times to calcium phosphate solution (55). Phosphorus desorption from alumina riiows broad features from 450-700 K, the residual salt peak at 980 K, and a high tenq>erature feature, vAach occurs between 1400 and 1560 K. Variation in the tenq>erature associated with the high tenq)erature desorption feature may be a result of variations in heating rate or thermocouple placement. Unlike the titania spectra, the large feature at 1200 K is absent. In addition, at least one new feature can be observed at approximately 450-700 K Figure 9b shows the corre onding calcium and potassium desorption features. The ratio of calcium and potassium ions to pho horus is conq)arable, but for longer exposures, this ratio decreases dramatically. For a 30 hr. e q>osure, the calcium and potassium to phosphorus ratios drop to less than 1% of the solution concentration. Integration of several phosphorus desorption ectra provide the data for an adsorption isotherm shown in Figure 8. Onset of rapid uptake of pho hate is observed between 20-25 hr.

Ethylene conversion to acetic acid can be increased in at least two ways. One is making the catalyst surface more acidic, which will strengthen ethylene adsorption on the surface to give more time for its oxidation to acetic acid. A more acidic surface will also facilitate desorption of acetic acid thus reducing its surface over-oxidation to carbon oxides. This approach has been realized by adding small amounts of P, B and Te to the MoVNb oxide catalyst. Table 11.2 presents the results of adding phosphorus. 

The effect of pH on heavy metal ion adsorption capacity was studied by previous researchers using the shake flask experiments. Eric and Roux used the shake flask experiment to study the influence of pH on the heavy metal ion binding onto a fimgus-derived bio-sorbent in the year, 1992. Also the evaluation of the effect of the hydrochloric acid concentration on the adsorption of platinum group metal ions onto chemically modified chitosan was done by Inoue et al., using the shake flask experiment [85]. Depending upon the type of P complexation with the surface such as monodentate, bidentate mononuclear, and bidentate binuclear the phosphorus desorption is potentially controlled. These complexes can be either non protonated or protonated depending on the suspension pH [184]. 

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