**2. Sorption experiments and trends in sorption data**

## **2.1 Determination of soil properties**

The soil samples used for sorption experiments were air dried, passed through 2-mm sieve and first analyzed for pH, total carbon, oxalate extractable iron, oxalate extractable aluminum, and exchangeable calcium. Particle size distribution of the

soil samples was also determined. A soil to water solution ratio of 1:2 was prepared, and the soil pH was measured using a standardized pH meter (model: AR15; manufacturer: Fisher Scientific) [33]. The combustion method with the element analyzer (Carbo-Erba NA 2500 instrument (Model: NA 2500; manufacturer: CE instruments, Italy) was used to measure total carbon. The inductively coupled plasma (ICP) (model: Optima 700 DV; manufacturer: Perkin Elmer) was used to analyze for oxalate iron and aluminum after extraction with oxalate solution [34]. Exchangeable calcium was also analyzed using ICP after extraction with 0.2 M NH4Cl [35]. Particle size distribution was determined by hydrometer method [35] .

#### **2.2 Determination of sorption isotherms**

An example of P sorption experiment that involved using KH2PO4 fertilizer prepared in 0.01 M KCl, 0.005 M CaCl2, and deionized water and fertilizer mixture (NH4NO3, KH2PO4, and KCl) prepared in deionized water was used for this study. The fertilizer rates, 50 kg P2O5 ha<sup>−</sup><sup>1</sup> , 200 kg N ha<sup>−</sup><sup>1</sup> , and 200 kg K2O ha<sup>−</sup><sup>1</sup> , applied to sugarcane fields were used to prepare the fertilizer mixture. Potassium chloride and calcium chloride were used because they are commonly used to conduct P sorption experiments with the assumption that the solutions' ionic strength and pH are close to those of the crop fields. The concentration, 0.005 M for CaCl2 and 0.01 M for KCl were used to attain the equivalences of Ca2+ and K<sup>+</sup> . Deionized water was used because irrigation water is used to provide the necessary plant moisture.

The two sandy soils, Margate (sandy, siliceous, hyperthermic Mollic Psammaquents) and Immokalee (sandy, siliceous, hyperthermic Arenic Alaquods) used for the experiment contribute the most to the greatest percentage of soils used for sugarcane production in Southwestern Florida. Five soil samples of each of the soil horizons, A and Bh for Immokalee soil and A and Bw for Margate soil, were used to represent the varying soil properties (e.g., total carbon, iron, and aluminum). The soil samples were sampled from two sugarcane fields each of 12 ha located in Hendry County, southwestern Florida (26.75° N, 80.93° W).

The initial P concentrations (C0) used for the experiment ranged from 8 to 60 mg L<sup>−</sup><sup>1</sup> . The soil to solution ratio of 1:2 (10 g of soil and 20 mL of solution) was used and equilibrium solution concentration was analyzed after 24 h of shaking. Blanks where soil was shaken with only 0.01 M KCl, 0.005 M CaCl2, and deionized water were also included in the experiment, and the blank equilibrium concentrations were subtracted from the treatment sample equilibrium concentrations. The experiments were conducted at room temperature (25°C). Before analyzing the solution concentrations, soil solutions were centrifuged at 5000 rpm for about 20 min and filtered using 42 Whatman filter. The spectrophotometer (HACH DR/4000U) was used to analyze solution P at a detection wavelength of 880 nm.

Sorbed P (S) was equal to V/M (C0 − Ce) where V, M, C0, and Ce are volume of solution, mass of soil, initial solution P concentration, and equilibrium P concentration, respectively. Sorption data for all the supporting electrolytes were fitted to Freundlich isotherm [sorbed (S) versus equilibrium solution (C) concentration]. The Freundlich sorption isotherm is represented by Eq. (1).

$$\mathbf{S} = \mathbf{K}\_{\mathbf{f}} \mathbf{C}^{\mathbf{N}} \tag{1}$$

**33**

**Table 3.**

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

**2.3 Sorption kinetics experiments**

data trends over a 24 h period.

**2.4 Selected soil properties**

97.0%, 6.8, 15.2 g kg<sup>−</sup><sup>1</sup>

97.5.0%, 8.3, 11.2 g kg<sup>−</sup><sup>1</sup>

114.4 mg kg<sup>−</sup><sup>1</sup>

and 2.0 cmolc kg<sup>−</sup><sup>1</sup>

equivalence was used for K+

Immokalee soil

Immokalee soil

concentrations (C0), 19, 29, and 38 mg L<sup>−</sup><sup>1</sup>

coefficients.

to coefficients for 0.01 M KCl, 0.005 M CaCl2, and deionized water. The paired t-test was used to identify significant differences in Freundlich sorption

The sorption of P has been assumed as a kinetic process [15, 28]. Three initial

The average percent sand, pH (1:2 soil:water volume), total carbon, oxalate iron, oxalate aluminum, and exchangeable Ca for A horizon of Immokalee soil were

tively. The average percent sand, pH (1:2 soil:water volume), total carbon, oxalate iron, oxalate aluminum, and exchangeable Ca for A horizon of Margate soil were

tively. The average percent sand, pH (1:2 soil:water volume), total carbon, oxalate iron, oxalate aluminum, and exchangeable Ca for Bh were 87.5%, 6.8, 39.7 g kg<sup>−</sup><sup>1</sup>

sand, pH (1:2 soil:water volume), total carbon, oxalate iron, oxalate aluminum, and

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

**CaCl2**

Margate soil A S = 7.2 C0.6 S = 24.6 C0.3 S = 5.4C0.5 S = 7.3 C0.6

Margate soil Bw S = 10.3C0.3 S = 28.0 C0.2 S = 6.7C0.4 S = 9.5C0.3

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

A S = 4.8 C0.5 S = 13.5 C0.4 S = 2.7C0.8 S = 4.6C0.5

Bh S = 19.1C0.6 S = 79.0C0.2 S = 13.1C0.8 S = 21.1C0.6

, and 5.4 cmolc kg<sup>−</sup><sup>1</sup>

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

Although all sorption data fitted Freundlich isotherms with R<sup>2</sup>

, 280.4 mg kg<sup>−</sup><sup>1</sup>

, 307.3 mg kg<sup>−</sup><sup>1</sup>

, 234.0 mg kg<sup>−</sup><sup>1</sup>

, 661.1 mg kg<sup>−</sup><sup>1</sup>

, 305.0 mg kg<sup>−</sup><sup>1</sup>

exchangeable Ca for Bw were 97.2%, 8.4, 3.9 g kg<sup>−</sup><sup>1</sup>

, respectively.

**Soil Horizon 0.01 M KCl 0.005 M** 

kinetics experiment. The A horizon of Immokalee soil was used for sorption kinetics experiment, and the soil to solution ratio of 1:2 (10 g of soil and 20 mL of solution) was used. Solution concentrations were analyzed after 4, 8, 12, and 24 h. The paired t-test was used to identify significant differences in relative concentrations (C/C0) and sorbed concentrations between fertilizer mixture and supporting electrolytes. R-software was used for statistical analyses. Graphs of relative concentrations (C/C0) and sorbed concentrations (S) versus time were plotted to show the

were used for conducting sorption

, and 3.6 cmolc kg<sup>−</sup><sup>1</sup>

, and 6.6 cmolc kg<sup>−</sup><sup>1</sup>

, 149.0 mg kg<sup>−</sup><sup>1</sup>

and Ca2+, sorption was greater for 0.005 M CaCl2 than

**Deionized water**

, respectively. The average percent

, 89.0 mg kg<sup>−</sup><sup>1</sup>

values greater

**Fertilizer mixture**

, respec-

, respec-

,

,

where S is the amount of P sorbed (mg kg<sup>−</sup><sup>1</sup> ), C is the solution P concentration (mg L<sup>−</sup><sup>1</sup> ), Kf is the Freundlich sorption coefficient (LN, kg−<sup>1</sup> mg1−N), and N is an empirical constant. The coefficients for fertilizer mixture were compared

*Advanced Sorption Process Applications*

**2.2 Determination of sorption isotherms**

this study. The fertilizer rates, 50 kg P2O5 ha<sup>−</sup><sup>1</sup>

ha<sup>−</sup><sup>1</sup>

and K<sup>+</sup>

60 mg L<sup>−</sup><sup>1</sup>

wavelength of 880 nm.

necessary plant moisture.

soil samples was also determined. A soil to water solution ratio of 1:2 was prepared, and the soil pH was measured using a standardized pH meter (model: AR15; manufacturer: Fisher Scientific) [33]. The combustion method with the element analyzer (Carbo-Erba NA 2500 instrument (Model: NA 2500; manufacturer: CE instruments, Italy) was used to measure total carbon. The inductively coupled plasma (ICP) (model: Optima 700 DV; manufacturer: Perkin Elmer) was used to analyze for oxalate iron and aluminum after extraction with oxalate solution [34]. Exchangeable calcium was also analyzed using ICP after extraction with 0.2 M NH4Cl [35]. Particle size distribution was determined by hydrometer method [35] .

An example of P sorption experiment that involved using KH2PO4 fertilizer prepared in 0.01 M KCl, 0.005 M CaCl2, and deionized water and fertilizer mixture (NH4NO3, KH2PO4, and KCl) prepared in deionized water was used for

, applied to sugarcane fields were used to prepare the fertilizer mixture. Potassium chloride and calcium chloride were used because they are commonly used to conduct P sorption experiments with the assumption that the solutions' ionic strength and pH are close to those of the crop fields. The concentration, 0.005 M for CaCl2 and 0.01 M for KCl were used to attain the equivalences of Ca2+

The two sandy soils, Margate (sandy, siliceous, hyperthermic Mollic

located in Hendry County, southwestern Florida (26.75° N, 80.93° W).

The Freundlich sorption isotherm is represented by Eq. (1).

where S is the amount of P sorbed (mg kg<sup>−</sup><sup>1</sup>

Psammaquents) and Immokalee (sandy, siliceous, hyperthermic Arenic Alaquods) used for the experiment contribute the most to the greatest percentage of soils used for sugarcane production in Southwestern Florida. Five soil samples of each of the soil horizons, A and Bh for Immokalee soil and A and Bw for Margate soil, were used to represent the varying soil properties (e.g., total carbon, iron, and aluminum). The soil samples were sampled from two sugarcane fields each of 12 ha

The initial P concentrations (C0) used for the experiment ranged from 8 to

Sorbed P (S) was equal to V/M (C0 − Ce) where V, M, C0, and Ce are volume of solution, mass of soil, initial solution P concentration, and equilibrium P concentration, respectively. Sorption data for all the supporting electrolytes were fitted to Freundlich isotherm [sorbed (S) versus equilibrium solution (C) concentration].

S = KfC<sup>N</sup> (1)

), Kf is the Freundlich sorption coefficient (LN, kg−<sup>1</sup>

is an empirical constant. The coefficients for fertilizer mixture were compared

), C is the solution P concentra-

mg1−N), and N

was used and equilibrium solution concentration was analyzed after 24 h of shaking. Blanks where soil was shaken with only 0.01 M KCl, 0.005 M CaCl2, and deionized water were also included in the experiment, and the blank equilibrium concentrations were subtracted from the treatment sample equilibrium concentrations. The experiments were conducted at room temperature (25°C). Before analyzing the solution concentrations, soil solutions were centrifuged at 5000 rpm for about 20 min and filtered using 42 Whatman filter. The spectrophotometer (HACH DR/4000U) was used to analyze solution P at a detection

. The soil to solution ratio of 1:2 (10 g of soil and 20 mL of solution)

. Deionized water was used because irrigation water is used to provide the

, 200 kg N ha<sup>−</sup><sup>1</sup>

, and 200 kg K2O

**32**

tion (mg L<sup>−</sup><sup>1</sup>

to coefficients for 0.01 M KCl, 0.005 M CaCl2, and deionized water. The paired t-test was used to identify significant differences in Freundlich sorption coefficients.
