Chemistry 143 



This isotherm implies that energy of adsorption decreases linearly with 

 increasing surface coverage; therefore, a plot of A' against In C or log C 

 should be linear. Data from one experiment are plotted in this format in 

 Figure 4-22. The other two, more frequently used isotherms assume either 

 a constant binding energy (Langmuir) or an exponentially decreasing 

 energy of adsorption with increasing surface saturation (Freundlich). 



The Temkin equation gives a good fit over 3 orders of magnitude of 

 phosphate concentration (Figure 4-22). Thus for this sediment, the 

 equilibrium phosphate concentration is proportional to the antilog of the 

 quantity of sorbed inorganic phosphorus; the less phosphate present, the 

 more strongly it is held. 



The meager literature on phosphate Temkin isotherms allows only a 

 very limited comparison with the Pond B isotherm. The critical parameter 

 for this comparison is the isotherm slope dX{d log C) ~ ' =2.3 RT Xm b \ 

 because with three unknowns in the isotherm equation the two that give 

 the slope, X„ and b, cannot be separated. Thus, the slope is the only term 

 available for comparison against the Temkin isotherms of other sediments 

 or soils. The slope expressed either as the differential or the ratio of the 

 constants is essentially a measure of the phosphate buffering intensity of 

 the sediment. That is, it is a measure of the tendency of a solution in 

 contact and in equilibrium with a sediment to resist a log C change 

 resulting from addition or withdrawal of sorbed phosphate. Bache and 

 Williams (1971) presented either Temkin isotherm slopes or graphs from 

 which the slope could be calculated for four soils, at least three of which 

 were cultivated (presumably fertilized) soils. These slopes ranged from 65 

 to 310 {fig P g"')/(log Mg P liter '), much less than the 690 calculated for 



rrT| 1 1 — I M I I 



"T — I — I I I I I 1 1 1 1 — I I I I III 1 — I — I — Mill 



I02 I03 



C, /ig P liter"' 



J I I 1 1 1 ii I ' ' I 1 1 III 



4 in5 



10' 



10= 



FIGURE 4-22. Results of a sediment-phosphate sorption ex- 

 periment in Pond B (0 to 10 cm). Data are plotted as a Temkin 

 isotherm. The intercept gives the position of a one-point PSI 

 determination: 1490 (\xg P g') ^ (log 537)(C in ytg P liter') = 

 545. 



