POTASSIUM ADSORPTION IN CALCAREOUS SOILS OF KURDISTAN REGION OF IRAQ

Sorption is one of the most chemical important processes, which determine nutrients availability in soil. Sorption isotherms provide sufficient information about soils sorption’s capacity, and it’s data can be used to determine thermodynamic sorption parameters. The aim of this studyis to evaluate the sorption of potassium onto some soils. An experiment was conducted with four calcareous soils of the Sulaimani province Kurdistan Region of Iraq by using the batch methods. 5g soil samples were equilibrated at 298±1 Kelvin with 50 ml of 0.01M CaCl2 containing 0 to 250 mg L -1 K as KCl. Suspensions were centrifuged, filtered, and concentration of K + in the clear extract solution was determined. Amount of K + sorbed by the soil was calculated from the difference between the initial and final concentration of K + in the equilibrium solution. Sorption of K + was evaluated using adsorption isotherms. The results showed that K + sorption was described by linear, Langmuir, Freundlich, and Temkin equations. Langmuir equation gave a better fit of equilibrium K adsorption when it has a higher R 2 and lowers SE. The data indicated that the maximum monolayer coverage capacity (b) from the Langmuir isotherm model ranged between (-113.63 to 2500) mg kg -1 . The negative values of (b) for studied locations soils indicated to potassium release instead of adsorption. The Langmuir isotherm constant (KL) ranged from (-0.01 to 0.01) L mg -1 . Maximum buffering capacity (MBC) is a capacity factor, which measures the ability of the soil to replenish K ion to soil solution that the ability of a soil to supply K to the soil solution. The value of MBC of the studied soils ranged from 0.453 to 23.75 mg kg -1 .The sorption processes for the forth locations are favorable and spontaneous because the value of RL was an equal one.


INTRODUCTION
Potassium is an essential element for crop production and productivity (43). Potassium plays a vital role in activates of over sixty (60 ) enzymes involved in the formation of carbohydrates, translocation of sugars, various enzyme actions, yield, quality parameters, tolerance to certain diseases, mechanisms to overcome abiotic stress, cell permeability, controls stomata opening, Also, K influences the microbial population in the rhizosphere. (35). Soil properties (physical, chemical, and biological), affect the availability of K (44). K adsorption in soils that is, the transformation of available K forms into unavailable ones, influences the effectiveness of fertilization in the soil-plant regime. The mechanism of adsorption of K in the soil is important because soils may contain widely variable pools of K that are potentially mobilized by chemical weathering of soil minerals (37). The equilibrium among the potassium retained by the interlayer sites controls the process of potassium adsorption by the surface and edge sites of the mineral crystal lattice and the potassium in the soil solution. The dynamic equilibrium is mainly affected by clay minerals types, pH, soil organic matter (SOM), hydroxide aluminum, soil moisture status, cation exchange capacity (CEC), fertilization and tillage system (29). The K adsorption in the soil is quite complex and may not be explained by the simple and single reaction. The amount of K can be adsorbed on the soil colloids depending on the amount and type of clay minerals up to 67%., and the high capacity of soils affecting negatively on the availability of K to crops (28). There is little information available relating to K adsorption characteristics of calcareous soils (18). To visualize the quantities of K adsorbed per unit soil mass, several equations or adsorption isotherms have been developed. Between the isotherms, Langmuir, and Freundlich adsorption isotherms are mostly employed and described the phenomena of quantities K adsorption per unit soil mass satisfactorily (8). The Freundlich equation is an empirical equation, which corresponds to a model of adsorption where the affinity term decreases exponentially as the amount of adsorption increases. Potassium adsorption was described well often by Freundlich equation over a limited range of concentration (7). The theory of Langmuir is restricted to cases where only one layer of the molecule can be adsorbed on the surface. Therefore, the non-conformity to the Langmuir model suggests the presence of several types of K sorption sites in the soils, each with different selectivity for K (18). According to the Freundlich equation, the energy of adsorption decreases as the amount of adsorption increases. The aim of this study was to determining K adsorption capacity, bonding energy and other adsorption parameters of Langmuir, Freundlich and Temkin isotherms of calcareous soils of the Sulaimani province Kurdistan Region of Iraq.

MATERIALS AND METHODS Sample collection and physicochemical analysis
The soils samples were taken at 0 to 30 cm depths from four locations that contained calcareous soils representative of the major agricultural soils of the Iraqi Kurdistan Region, Halabja, Said Sadiq, Chwarta, and Gerdjan. The soil samples were air-dried, crushed, and passed through a (2) mm, sieve for soil analysis and sorption studies. Selected physical and chemical properties of soils are present in Table 1.
The particle size distribution of the soil samples was determined by the pipette method, according to Gee and Bauder (15). The soil reaction (pH) and electrical conductivity (ECe) were measured for the soil saturation extract by a pH meter (HANA), model (HI 83141), and EC meter, model (HI 2314) respectively. Cation exchange capacity (CEC) of soil was obtained by saturating the soil sample with 1M ammonium acetate (NH 4 OAc) at pH 8.1 as an extraction solution according to the method described by Suarez (39). Organic matter (O.M.) content was determined by the method described by Nelson and Sommer (27). The total carbonate minerals in soil expressed as the carbonate minerals equivalent were determined by a rapid titration method, according to the method of Rayment and Higginson (32). The active carbonates or active equivalent carbonates (AEC), which was a fine particle size calcite, was estimated by the 0.5 M NH 4 oxalate method as described by Drouimeau (12).  The suspensions were then shaken with a horizontal flask shaker for 2 hours (180 rpm and 298 K) and overnighted to equilibrate. Each solution was centrifuged at 250 rpm for 5 minutes then filtered through a Whitman filter paper no. 42, and the filtrates were analyzed for K concentration by the flame photometer JENWAY model PFP7. The amount of adsorbed K (mg kg −1 ) was calculated following Vanderborght and Van Grieken (40) as: Where q e is the amount of K adsorbed from the solution (mg kg −1 ), C o is the initial concentration of K (mg L −1 ), C e is the concentration of K (mg L −1 ) at an equilibrium state, V is the volume of the solution (L), and W the weight of the soil sample (kg) used in the experiment. The data were computed using the linearized forms of the following equations: Langmuir, Freundlich, and Temkin Langmuir adsorption isotherm The Langmuir model supposes monolayer adsorption of solutes on homogenous sorption sites, the Langmuir equation has described the distribution of K between the solid solution interface equilibrium (25), Langmuir isotherm was the most commonly used linear expression to studied the relationships between the concentration of solute in the liquid phase and the solid phase at equilibrium conditions (21).
The well-known expression of the Langmuir model was used: Where qe = the amounts of adsorbed K per unit weight of soil (mg kg -1 ), and C e is the K concentration in solution at equilibrium (mg L -1 ), K L is a constant related to bonding energy or affinity constant of potassium to the soil (L mg -1 ), and b is the soil's maximum monolayer coverage capacity during K adsorption( mg kg -1 ). A plot of 1/q e vs. 1/C e gives a straight line with a slope of 1/bK L and intercept of 1/b R L the dimensionless constant called equilibrium parameter for Langmuir isotherm (22).
where K L = a constant related to bonding energy (L mg -1 ) and C 0 is the initial K concentration (mg L -1 ), R L values indicate the type of isotherm (20). R L value indicates the adsorption nature to be either unfavorable (R L > 1), linear (R L = 1), favorable (0 <R L < 1) or irreversible (R L = 0). The maximum buffering capacity (MBC) was calculated as the product of K L and b following Reyhanitabar et al. (34).

MBC=K L x b ……………… (4) Freundlich adsorption isotherm
Freundlich isotherm the earliest concepts the equation for adsorption isotherm (13). Freundlich isotherm is an empirical model, which can be applied to non-ideal adsorption on dissimilar surfaces along with adsorption multilayer, with the nonuniform distribution of adsorption heat and affinities over the heterogeneous surface (1). The empirical Freundlich equation based on sorption onto a heterogeneous surface is given as: Eq.(6) can be rearranged to obtain a linear form by taking logarithms Eq. (5): Log q e = log K F + 1/n log C e ……….(6) Where K F = the energy of sorption or distribution coefficient (mg kg -1 ), 1/n is the heterogeneity factor. n is the dimensionless constant, while 1/n represents the strength of adsorption in the adsorption process (41). If the Freundlich affinity value n is equal to 1, then the partition between the two phases is independent of the concentration. If 1/n is less than 1, the parameter indicates normal adsorption. Above 1, however, the parameter indicates cooperative adsorption (38). The Freundlich affinity value, n, is important for understanding sorption processes and the heterogeneity of the system. If the n value falls between 1 and 10, the sorption process is favorable (10 and 30). A more homogeneous system will have a 1/n value approaching unity, and a more heterogeneous system will have a 1/n value approaching zero (17).

Temkin isotherm
The Temkin isotherm contains a factor that taking into the account of adsorbent-adsorbate interactions and is simply a function of surface coverage (9). The Temkin isotherm is usually used for heterogeneous surface energy systems (nonuniform distribution of sorption heat) (5). The equation is stated as follows: The linear form of the Temkin equation is: q e = B ln A T +B ln C e …….……… (9) where q e = the mass of K adsorbed per unit mass of soil (mg kg -1 ), C e is equilibrium solution K concentration (mg L -1 ), A T = the Temkin isotherm equilibrium binding constant (L kg −1 ), and B is a constant related to the heat of sorption (J mol −1 ) calculated as:

Figure 2. Amount of K adsorbed as a function of different K concentration levels of studied soils Potassium adsorption study from graphical analysis of Langmuir, Freundlich and Temkin equation plots
The degree of accuracy of the sorption isotherms varied from soil to soil. The coefficient of determination (R 2 ) values in Table 2 indicate that the Langmuir equation has a better fit (Figure 3c) of equilibrium K adsorption data for Chwarta location than the other studied locations. This may be due to the high of clay content which had unlimited adsorption sited having a heterogeneous surface (19).

. Langmuir adsorption isotherms for (a) Halabja, (b) Said Sadiq, (c) Chwarta and (d) Gerdjan soils of Kurdistan Region of Iraq. Langmuir isotherm parameters
For Langmuir isotherm, it is assumed that the adsorption sites have equal affinities for molecules of the adsorbate. Therefore, the presence of adsorbed of molecules at an adjacent site (10). The maximum monolayer coverage capacity (b) from Langmuir isotherm model ranged between -113.63 and 2500 mg kg -1 (Table 2), the monolayer coverage capacity of the studied locations except for Chwarta were found to be below zero this may be due to the variation of organic matter content among the studied soils (Table 1). This result is harmonic with the result found by Ravikovitch et al. (31). Their result observed that the soils with a higher amount of OM per unit mass are likely to be covered by an extensive humic layer, sufficient to coat the core particle mineral surface, this indicated that the decline the value of monolayer coverage capacity of the soil. These negative intercepts suggest that the adsorption behavior of the tested systems does not follow the assumption of the Langmuir approach (6). The maximum adsorption (b) can be used to estimate the amount of fertilizer to be added to unfertilized soil (33). The soil of these locations also had no best fit for the coefficient of determination (R 2 ) of the Langmuir model (Figures 3 a, b and d). On the other hand, soil possessing best fit to Langmuir model (Chwarta location) (Figure 3c) indicated high monolayer coverage. These results in agreement with the results of Auge et al. (6). The Langmuir isotherm constant (K L ) ranged from -0.01 to 0.01 Lmg -1 . According to Mehandi and Taylor (24), smaller the K L values indicated that more amount of adsorbed K would be converted to non-exchangeable form either by the formation of crystalline K or by occultation through K ions. According to Del-Bubba et al. (11), K L is a measure of the affinity of the adsorbate for the adsorbent. Anderson and Wu (4) stated that a high K L value is an indication of the high clay contents of respective soil, and also shows the strength of bonding to clay minerals in the soil.  (Table 2). Al-Hassoon et al. (3) observed that diffusion coefficient (R L ) value of Cu adsorption in predominance soil orders (Entisols) Jadriya and Najaf of Iraq less than one which means that the reaction of the adsorbed copper is spontaneous. Maximum buffering capacity (MBC) of the studied soils ranged from 0.453 to 23.75 mg kg -1 , MBC is a capacity factor, which measures the ability of the soil to replenish K ion to soil solution as they tend to be decreased (33). Fried and Shapiro (14) stated that the ability of a soil to supply K to the soil solution an important factor in evaluating the K status of a soil.

Freundlich isotherm parameters
The coefficient of determination (R 2 ) value in Table 3 indicated that the Freundlich equation gave a better fit (Figure 4a) of equilibrium K adsorption data for Halabja location. This may be due to the high organic matter content in Halabja location, and soil organic matter may considerably favor a very fast initial adsorption of K (42)., for having sorption sites more accessible compared with the mineral components of the soil Sheikh-Abdulla et al. (36) found that the Freundlich isotherm is the best model to describe Mn adsorption in some calcareous soils at Sulaimani governorate.   (Table 3) may be taken as a measure of the extent of adsorption (41). Mburvi et al. (23) highlighted that the constant 1/n (Eq. 6) also represents the buffering capacities of soils. In this study, it ranged from 0.7096 to 2.665 mg kg -1 with a mean of 1.298 mg kg -1 . In the range, Chwarta soil had low 1/n values due to low sand proportion. The values of 1/n also indicate a heterogeneity parameter where smaller 1/n values reveal greater heterogeneity (10). Accordingly, Chwarta soil had high heterogeneity, and the parameter indicates normal adsorption for Chwarta location while for other locations, the adsorption process is cooperative ( Table 3). The n values indicate whether the sorption favorable or not (16). For favorable sorption processes, it lies between one and ten (16). The adsorption capacity, K f (Eq 6) represents the amount of potassium held on non-specific sites that ready to be released for uptake by plants during cropping season (23). It ranged from (0.013 to 501.086) mg kg -1 and had a mean of 15.033 mg kg -1 Temkin isotherm parameters The coefficient of determination (R 2 ) value in Table 4 indicated that the Temkin isotherm has a better fit (Figure 5b) of equilibrium K adsorption data from Said Sadiq location compare to other studied locations. This could be due to the high cation exchange capacity (CEC) of the soil and high amount of active lime content of Said Sadiq location, which may be related to the soil mineralogical composition of the soil for this location (Table  1). Al-Janabi (2) observed that high calcium carbonate content causes an increase in cation adsorption on active sites of calcium carbonate like Cu. Table 4. Temkin isotherm constants for the adsorption of potassium on the studied soils  From the Temkin plot shown in Figure 5 and Table 4, values of Temkin isotherm equilibrium binding constant, A T (L kg -1 ) ranged between 0.029 and 0.173. In this range, the soil of Said Sadiq location had a high binding constant and high coefficient of determination (Figure 5b). The values of b T ranged from 4.753 to 5.928 J mol -1 with the mean of 5.309 J mol -1 a positive value of b T indicates that the adsorption process is exothermic for studied locations. Furthermore, high values of constant related to the heat of sorption, B (J mol -1 ) indicate a strong interaction between adsorbent and adsorbate supporting a mechanism of ion exchange (10). The studied soils were varied from adsorption behaviors; all soils adsorbed the added K to a high degree. This indicated the high potential of studied soils to decrease crop availability of applied K. This indicated that crops in the studied soils were in K deficient condition and hence K recommendation should be one of the soil management choices.