**2.5 Changes of sorption isotherm coefficients with supporting electrolytes**

Although all sorption data fitted Freundlich isotherms with R<sup>2</sup> values greater than 0.9, the Freundlich coefficients varied with the type of supporting electrolytes (**Table 3**). For both 0.01 M KCl and fertilizer mixture, the Freundlich isotherm constant was significantly lower (*p* < 0.05) than for 0.005 M CaCl2 and significantly greater (*p* < 0.05) than for deionized water (**Table 4**). Although the same equivalence was used for K+ and Ca2+, sorption was greater for 0.005 M CaCl2 than


**Table 3.**

*Average sorption isotherms of five replicates showing variabilities in Freundlich sorption coefficients.*


**Table 4.**

*Comparisons of Freundlich sorption coefficients and sorbed phosphorus concentrations.*

for 0.01 M KCl and fertilizer mixture likely due to the influence of charge (+2) on Ca2+ that reduce the electrostatic repulsion effect between phosphate and the soil surface. A similar trend (Ca+2 > K+ ) for P sorption was identified in other sorption studies [36]. Phosphorus sorption was also reported to increase with increase in background electrolyte concentration [37]. The sorption characteristics of P (KH2PO4) prepared in deionized water was lower than for 0.01 M KCl and fertilizer mixture probably due to significantly lower K+ concentration that contributes less to ionic strength than in the latter two. The greater Freundlich coefficients for Bh were greater than A horizon because of the greater total carbon, oxalate iron, and oxalate aluminum that enhance greater P sorption. Soils with greater free aluminum and iron were associated with greater P sorption by different researchers [38–40].

### **2.6 Trends in sorption kinetics data for different supporting electrolytes**

**Figure 1** shows the sorbed P concentrations as a function of initial concentrations for a 24 h time step with no significant difference in sorbed concentrations for 0.01 M KCl and fertilizer mixture. **Figure 2** shows the trends in relative solution concentrations (C/C0) and sorbed concentrations (P). While the relative solution concentrations decreased over time, sorbed concentrations increased over time. For both 0.01 M KCl and fertilizer mixture, sorbed P kinetics data were significantly lower (*p* < 0.05) than for 0.005 M CaCl2, and significantly greater (*p* < 0.05) than for deionized water (**Table 4**). Sorption was fast for the first hours due to the presence of high P affinity sorption sites on the exchange sites and gradual for the following hours (**Figure 2**). A fast P sorption first phase followed by a steady phase was also documented in other studies [2, 15, 41].

**Figure 1.** *Sorbed P (S) concentration as a function of initial concentrations (C0) for A horizon of Immokalee soil.*

**35**

**3. Summary/conclusions**

**Figure 2.**

**Acknowledgements**

providing the necessary funds.

The results presented in this chapter suggest that if another nutrient is applied with P in the field, the P sorption behavior should be studied with the applied fertilizer mix, and P prepared in recommended supporting electrolyte as well. The sorption characterization with the two scenarios will help in identifying the appropriate sorption characteristics (sorption isotherm coefficients and kinetics

*Relative solution (C/C0) and sorbed (S) concentration as a function of time for A horizon of Immokalee soil.*

The authors are grateful to Florida Department of Environmental Regulation for

constants) used for predicting P movement and P management options.

*Sorption of Phosphorus from Fertilizer Mixture DOI: http://dx.doi.org/10.5772/intechopen.80420* *Sorption of Phosphorus from Fertilizer Mixture DOI: http://dx.doi.org/10.5772/intechopen.80420*

*Advanced Sorption Process Applications*

surface. A similar trend (Ca+2 > K+

**Table 4.**

mixture probably due to significantly lower K+

was also documented in other studies [2, 15, 41].

for 0.01 M KCl and fertilizer mixture likely due to the influence of charge (+2) on Ca2+ that reduce the electrostatic repulsion effect between phosphate and the soil

*NS, no significant difference (α = 0.05); S, significantly greater or lower (α = 0.05).*

*Comparisons of Freundlich sorption coefficients and sorbed phosphorus concentrations.*

**Comparisons A-Immokalee A-Margate Bh Bw** Fertilizer mixture versus deionized water S S S S Fertilizer mixture versus 0.005 M CaCl2 S S S S Fertilizer mixture versus 0.01 M KCl NS NS NS NS

tion studies [36]. Phosphorus sorption was also reported to increase with increase in background electrolyte concentration [37]. The sorption characteristics of P (KH2PO4) prepared in deionized water was lower than for 0.01 M KCl and fertilizer

ionic strength than in the latter two. The greater Freundlich coefficients for Bh were greater than A horizon because of the greater total carbon, oxalate iron, and oxalate aluminum that enhance greater P sorption. Soils with greater free aluminum and iron were associated with greater P sorption by different researchers [38–40].

**Figure 1** shows the sorbed P concentrations as a function of initial concentrations for a 24 h time step with no significant difference in sorbed concentrations for 0.01 M KCl and fertilizer mixture. **Figure 2** shows the trends in relative solution concentrations (C/C0) and sorbed concentrations (P). While the relative solution concentrations decreased over time, sorbed concentrations increased over time. For both 0.01 M KCl and fertilizer mixture, sorbed P kinetics data were significantly lower (*p* < 0.05) than for 0.005 M CaCl2, and significantly greater (*p* < 0.05) than for deionized water (**Table 4**). Sorption was fast for the first hours due to the presence of high P affinity sorption sites on the exchange sites and gradual for the following hours (**Figure 2**). A fast P sorption first phase followed by a steady phase

**2.6 Trends in sorption kinetics data for different supporting electrolytes**

*Sorbed P (S) concentration as a function of initial concentrations (C0) for A horizon of Immokalee soil.*

) for P sorption was identified in other sorp-

concentration that contributes less to

**34**

**Figure 1.**

**Figure 2.**

*Relative solution (C/C0) and sorbed (S) concentration as a function of time for A horizon of Immokalee soil.*

## **3. Summary/conclusions**

The results presented in this chapter suggest that if another nutrient is applied with P in the field, the P sorption behavior should be studied with the applied fertilizer mix, and P prepared in recommended supporting electrolyte as well. The sorption characterization with the two scenarios will help in identifying the appropriate sorption characteristics (sorption isotherm coefficients and kinetics constants) used for predicting P movement and P management options.

## **Acknowledgements**

The authors are grateful to Florida Department of Environmental Regulation for providing the necessary funds.

*Advanced Sorption Process Applications*
