Mono and competitive adsorption of Zn, Cu and Mn in different calcareous soils

Soil carbonate is considered an effective adsorbent to trace element retention. Many researchers have studied the preferential adsorption of those heavy metals on calcareous soils. In 2021-2022, an experiment was conducted in agriculture college laboratories to assess the mono and competitive of three trace elements (Cu, Zn, Mn) on three calcareous soils that differ in their CaCO 3 content. Calcareous representative soil samples were collected from different Nineveh governorate and north Iraq locations. The adsorption experiment was carried out using the batch method by equilibrating 2.5 gm soil with 25 ml of a solution containing concentrations of (1.5, 10, 20, 30, 40, 50, 60 and 70 mg. L -1 ) of all traced elements in the same concentrations. Sorption isotherms were characterized using linear Langmuir and Freundlich equations. Results showed huge differences in sorptions capacities and other studied parameters. All studied soils showed a high maximum adsorption capacity Q max and strength binding for Cu than Zn and Mn. On the basis of Q max and distribution coefficient values (Σkd) for each studied soil and element, the selectivity sequence was as follows Cu>Zn >Mn. Gibbs free energy (-ΔG) values were decreased as the sorption capacity decreased too. In mono and ternary adsorption system, Langmuir isotherms were of H-type whereas Freundlich isotherms were of C-type. The soil properties such as CaCO 3 , pH, Clay, C.E.C, and O.M were significantly related to trace elements adsorption.


Introduction
Calcareous soils are widely spread in arid and semiarid regions, including Iraq, and form about (15-35%) of the total Iraqi area (Fadhal and Ismaeal, 2023;Noori and Ismaeal, 2023).Calcium carbonate in calcareous soils dominantly controlled soil chemical characteristics such as high pH value, cation exchange capacity, and soluble calcium (Mesquita et al., 2000;Noori and Ismaeal, 2023;Fadhal and Ismaeal, 2023).Increasing pH value expose trace element to several reactions on carbonate active surfaces like adsorption retention, precipitation, and complexation and leads to reducing those elements' bioavailability in soil solution (Osman et al., 1980;Al-Jumaily et al., 2022;Qadir and Al-Obaidi, 2024).
Zinc, Copper, and Manganese are important nutrient elements for plant growth and at the same time have more affinity to adsorb and interact with amorphous oxide, organic acid's functional groups, and on carbonate surfaces leading to deficiency symptoms on plant tissues (Dandanmozd and Hosseinpur, 2010;Obaid et al., 2023).The Knowledge of the kinetics of adsorption-desorption reactions is essential to understanding the interaction of the elements with soil properties and their fate in the soil profile (Yang et al., 2021;Al-Janabi et al., 2023).
Adsorption-desorption studies can be well documented by computing different adsorption isotherms (Rahman et al., 2021).The Langmuir and Freundlich equations and calculation of distribution coefficient (Kd) are useful tools for comparing the adsorption capacity of different soils or materials for any metal when measured under the same conditions (Iyengar and Raja, 1983).Industrial and municipal wastes are contributed directly with major polluted sources which usually generate a solution of multi-components (Gomes et al., 2001).
In this situation several heavy metals cations will compete with each other on soil components consequently, competitive adsorption or mobilization by the soil becomes of major importance in determining heavy metals' fate in soil (Brümmer et al., 1986).Competitive adsorption of heavy metals is key to predicting the mobility or retention of any metal in polluted Soil using the Kd parameter (Zhang and Zheng, 2007).Many researchers have found that the coexistence of heavy metals reduces their adsorption tendency on the solid phase (Serrano et al., 2005;Li et al., 2013;Gui et al., 2006).
Soil carbonate is an effective material for heavy metal retention because it has a higher negative surface charge with a wide surface area and functional carbonate groups, which have high affinity to adsorb heavy metals with different mechanisms.Several studies have been conducted on the preferential adsorption of heavy metals in calcareous soils (Jalali and Moharrami, 2007).The objective of this study is to evaluate the effect of carbonate levels in calcareous soils on the adsorption sequence of three heavy metals Cu, Mn, and Zn.

Material and Methods
Three samples of representative surface soil were collected at depth (0-20 cm) from different locations in Mosul, Nineveh Governorate, Iraq (Table 1, Figure 1) for the years 2021-2022.Samples were dried, crushed, and passed through a 2 mm sieve, then the chemical and physical analyses were conducted.Soils belong to Haplocalcids, Vertic Haplo Calcids (Soil Survey Staff., 1999).Chemical and physical characteristics were determined according to methods of soil analysis (Page et al.,1982;Al-Qattan and Al-Khafagi, 2023).
Organic matter was determined by dichromate oxidation and the cation exchange capacity was obtained by treating the soil with neutral 1N NH4OAC and 1N NH4OANa.Electrical Conductivity (EC) and soil (pH) was measured using 1:2.5 soil-to-water suspension.Soil textures were determined according to Bouyoucos (1962).The absorption experiment was carried out in a plastic stoppered tube with a 50 ml volume.The sorption isotherms were obtained for each soil sample by equilibrating 2.5 g soil with 25 ml of a solution containing concentrations of (1.5, 10, 20, 30, 40, 50,60, and 70 mg L −1 ) of all trace elements in the same concentrations.
The solutions were prepared by using sulfate salts CuSO 4 .5 H 2 O, and ZnSO 4 .H 2 O, and MnSO 4 .H 2 O of these metals in a background of 0.01 M of CaCl 2 .Soil suspensions were shaken for 2 hours and allowed to dynamically equilibrate for 24 hours at room temperature (25°C).The trace element concentration in the equilibration solution was measured by a GBC 933plus atomic absorption device.Sorbed metal by soil was calculated with the following equation 1 (Dandanmozd and Hosseinpur, 2010).Where: M = is the concentration of adsorbed metal (mg kg -1 ).Ci = is the initial concentration of metal in equilibrated solution (mg L -1 ).
V = is the solution volume ml.W = is the weight of the soil (gm).Cf= is the final concentration of metal in equilibrium solution after filtration (mg L -1 ).Polyphenol Freundlich and Langmuir equations (Giles et al., 1974) were applied to fit the data from isotherm studies.qe = Kf Ce 1/n (2) qe= (K L qm Ce )/(1+ K L Ce) (3) Where: qe = is the amount of adsorbed metal concentration at equilibrium( mg kg -1 ) Ce = is the concentration of the metal in solution at equilibrium (mg L -1 ).qm = is the maximum adsorption of metal on soil (mg Kg -1 ), KL (L Kg -1 ), KF (L Kg -1 ).
For every stage of the experiment, the distribution of each metal between sorbent and solution following equilibration was expressed in terms of the distribution coefficient (Kd) (Sprynskyy et al., 2011): Kd = qe/Ce (4) Gibbs' differential molar free energy for the specific adsorption was calculated using the following equation (Sprynskyy et al., 2011): dG0 = −RT lnKd (5) Where: dG0 = is the thermodynamic adsorption potential (the Gibbs' differential molar free energy of adsorption, kJ/mol).R = is the universal gas constant (8.314J/mol K) qm = is the maximum adsorption of metal on soil (mg Kg -1 ), KL (L Kg -1 ) ,KF (L Kg -1 ) T= is the adsorption absolute temperature (298 K).Joint distribution coefficients (KdΣsp) were also calculated for each soil to establish the preference order of adsorption of the metals by the soil (Jalali and Moradi, 2013).
Where: Cj = adsorption, Cj, solution are the concentrations of metal j in the soil (mg kg −1 ) and in the solution (mg l −1 ) .
The essential features of the isotherm can be expressed in terms of a dimensionless constant separation factor (RL) that can be defined by the following relationship (Anirudhan and Radhakrishnan, 2008).
RL= 1/1+KL Ci (7) Where: Ci = is the initial concentration (mg/L).Kl= is the Langmuir equilibrium constant (L/mg).The value of the separation parameter RL provides important information about the nature of adsorption.The value of RL indicated the type of Langmuir isotherm to be irreversible (R=0), favorable (0<R,<1), linear (R=1), or unfavorable (R>1).It can be explained apparently that when kL > 0, the sorption system is favorable (Chen et al., 2008).After collecting the data, the analysis was done with spss program for the t-test, and the data were divided into two groups.The test was done with probability levels of 0.01 and 0.05 to know if had significance or not (Johnson and Bhattacharyya, 2019).

Results and Discussion
Some physiochemical properties of selected soils are shown in Table 2. Calcareous soils were neutral, slightly alkaline, and low in organic matter and Ec values ranged from 0.73 -1.23 ds m -1 .The calcium carbonate equivalent was 230.9, 306.2, 379.8 gm kg -1 for low, moderate, and high CaCO3 contents, respectively.C.E.C ranged from 20 -25 Cmolec Kg -1 and clay content varied from 135 -492 gm kg -1 .Figure 2 shows Langmuir curves and Figure 3 shows Freundlich curves for a single Cu, Zn, and Mn adsorption system on calcareous soils with different CaCO3 content.A high R2 (0.80 -0.97) or determination coefficient indicates that both equations adequately described heavy metals adsorption but the Langmuir equation was closer to experimental results than Freundlich by higher R2.All curves for both equations are somewhat vertical and indicated a positive relationship between initial concentrations and adsorbed metals on the soil solid phase.
Sorption is a term used to describe metals removed from solution by selling absorbent in different mechanisms like retention, occlusion reaction, and complexation (Serrano et al., 2005).Heavy metals adsorption is influenced by soil properties like pH, C.E.C, Organic matter, content, CaCO 3 , Clay minerals, and heavy metals properties and concentrations (Srivastava et al., 2005) Langmuir and Freundlich parameter data are shown in Table 3.
Maximum adsorption capacity values of single adsorption (Q max ) for Zn, Cu, and Mn were (114.94,628.93 and 126.58 μg.gm -1 ) respectively.The differences in Qmax values between adsorbed metals are attributed to soil and metal properties.Mesquatta et al., (2000) mentioned that at the same pH value, more Cu than Zn was adsorbed due to differences in hydrolysis constant values for Cu and Zn compounds (Spark et al., 1995).Besides Cu and Zn could be adsorbed and precipitate as M(OH + ) (elements hydroxides) on carbonate surfaces.Mn Q max value was more than the results obtained in the present study, which could be attributed to the capacity of calcareous soils for Mn retention and Mn hydrolyzed species Mn(OH + ) which has a high affinity on carbonate surfaces (Karimian and Ahangar, 1998).
As calcium carbonate in studying soils increased, Qmax values for adsorbed metals were increased as well, except Cu in moderate CaCO 3 content.While in soils with high CaCO 3 , Cu Q max (628.93 mg kg -1 ) increased compared to that in moderate CaCO 3 content (175.44 mg kg -1 ).Adsorption capacity for Zn (114.94 mg kg -1 ) and Mn (and 126.58 mg kg -1 ) decreased compared to that in the moderate CaCO 3 content, 156.25 and 138.89 mg kg -1 , respectively.This may be due to Ca +2 activity in soil solution which shifts absorbent zero-point charge (PHz) to the left and reduces soil metal selectivity coefficient due to the metal hydrated ion species and their ability to compete for Ca +2 adsorption on specific and non-specific sites (Baghenejad et al., 2016) .
Maximum buffering capacity (MBC) values of studied soils for Cu, Zn, and Mn ranged between (10.67, 12.59, 14.93 L gm -1 ).The differences in the (MBC) quantity values among studied elements are pretend to the different values of Qmax and bonding energy for the studied elements because (MBC) in Langmiur equation obtained by the equation (Boning energy* Maximum adsorption),this parameter indicates the soil capacity resistance to the change in soil solution concentration (AL-jumaily et al., 2022).This parameter can be related to soil properties such as C.E.C , pH , clay mineral and metal concentration.
The parameter (RL) used to predict adsorption favorite in the batch adsorption process (Equation 7).This parameter is derived from the Lungmiar equation to predict the adsorption nature and whether the sorption system is favorable or unfavorable.Favorable (0 < RL <1), unfavorable (RL), irreversible (RL = 0), and liner (RL = 1) (Chen et al., 2008).The values of RL for Cu, Zn and Mn range between (0.437-0.895) for the studied calcareous soils with different CaCO3 content indicating that all metal adsorption is favorable.
The adsorption reaction of studied metals Cu, Zn, and Mn in a single sorption system with different calcareous soils is spontaneous and exothermic as values of (-dG°) are negative.From table (3) it is clear that adsorption (-dG°) for studying metals is between (-4.86 to -7.30 KJ.mole -1 ).Adsorption standard free energy is related to the energy of adsorption bonding.So, the values of the free energy increased as Kd values increased.The range of (-dG°) values indicated that the type of adsorption is physical because physical adsorption is characterized by free energy values up to 40 KJ mole -1 (Sprynskyy et al., 2011).
The most important parameter in the Freundlich equation is the (K F ) distribution coefficient, which is defined as, adsorped metal quantity on the absorbent solid phase to the metal concentration in the liquid phase at equilibrium.This parameter may be considered as an affinity or strength bond of metal on a solid phase and can be used to predict the adsorption or the mobility of heavy metals (Morera et al., 2001).Revista de Agricultura Neotropical, Cassilândia-MS, v. 11, n. 3, e8543, July/Sep., 2024.The values of (K F ) for studying metals in different calcareous soils for a single adsorption system ranged from 4.85 -29.59 L.gm -1 (Table 3).The highest (K F ) values for Cu, Zn, and Mn, were obtained in the highest CaCO 3 , soil, while (K F ) values in other soils for the same metals were decreased as CaCO 3 , decreased too.Besides the effect of soil factors on metals retention and mobility, such as the nature of clay minerals, organic matter, and pH, many researchers have reported that soil carbonates controlled the adsorption of heavy metals in calcareous soils (Compton and Pritchard, 1990;Maftoun et al., 2002;Elzinga and Reeder, 2002).
The Freundlich equation's slope indicates the adsorption tensity and the reciprocal of this parameter may be used as an adsorption favorite indicator (AL-Degs et al., 2006).A slight slope (n<1) indicates a favorable sorption process over the range of study concentrations, while a steep slope (n>1) indicates the favorite at high concentrations (Frimmel and Huber, 1996).For all studied metals in calcareous soils, (n) values are less than (1), which indicates favorable sorption at the range of experimental concentrations (Alghamdi and Alasmary, 2022).
Figure 4 shows the competitive adsorption of study metals.The interaction of metals in soil solution affects the sorption-absorption phenomenon of each other, leading them to be either fixed or mobile.Table 4 shows Langmuir isotherms study calcareous soils for Cu, Zn, and Mn in a triple ion solution system.Isotherms of these adsorption curves (Table 4) correspond soil affinity to the adsorbate metals at low concentrations which decrease at higher concentrations of added metals due to the saturation of available sorption sites, (Shaheen et al., 2017;Sipos, 2021).Heavy metals adsorption data in soil systems are very important to know the fate of any element in soil solution and to understand the process of soil-heavy metals interaction to establish useful models which can be applied to risk evaluation (Vega et al., 2006;Yang et al., 2021).
Table 4 shows the values of Lungmiur parameters in the triple ion system for Cu, Zn, and Mn in different calcareous soils.The Qmax values in triple competitive adsorption ranged between (27.7-212.77mg kg -1 ) with the sequence order Mn < Zn < Cu for studying soils with different carbon content.The higher adsorption selectivity for copper over zinc with the soil adsorbing complex was reported by Flogeac et al., (2007), he mentioned that because the electric negativity of cu is more than Zn.The values of Qmax for Cu, Zn, and Mn in a triple competitive system are less than Qmax values in a mono adsorption system.Because heavy metal adsorption is depressed by metal competition (Mesquita, 1996).Morera et al., (2001), found that the co-existence of heavy metals reduces their adsorption tendency in the solid phase.
The values of (R L ) parameter in the Langmuir equation are very close and range between (0.495-0.437).These values indicated that the competition of studied metals has the same adsorption favorite in calcareous soils where R L values ranged between (0< R L <1) according to Chen et al., (2008).Adsorption-free energy of study metals in calcareous soils has been calculated using (Equ 5).
Revista de Agricultura Neotropical, Cassilândia-MS, v. 11, n. 3, e8543, July/Sep., 2024.4) Gibbs free energy, of studying metal ranges between (-3.99 to -6.37 KJ.mole) and increased as Kd values increased within the physical adsorption limit.The values of the adsorption energy differ from one to another depending on metal affinity and soil CaCO 3 content in the following order Mn <Zn < Cu.The exothermic characteristics of the sorption process for Zn and Cu in a competitive system have been reported by Sprynskyy et al., (2011).The soil binding energy coefficient varied as CaCO 3 content varied between study soils and ranged from 0.077 -0.3878 L gm -1 , and all studied soils showed a higher adsorption affinity for Cu than other metals.
Binding strength obtained from sorption isotherms should be considered only qualitatively because they are related to the change of free energy for different adsorption species.(Van Riemsdijk et al.,1986).According to Serrano et al., (2005), the high values of binding energy are related to specific metal adsorption on high-energy surfaces with low dissociation constants while the lower values of bending energy are related to metal sorption on low-energy surfaces with high dissociation constants.(Adhikari and Singh, 2003).
K Lb in the Lungmiur equation represents maximum buffering capacity (MBC) according to the assumption, at low concentrations x/c = K Lb , since x/c is the ratio of adsorbed metal to the metal concentration in equilibrium solution, it is termed the distribution coefficient and is designated by (Kd).(Iyengar and Raja, 1983;AL-jumaily et al., 2022).MBC values for the Langmuir equation in the triple competitive system for Zn, Cu, and Mn in studying calcareous soils ranged between (7.96-28.74L gm -1 ).The variations in MBC values are related to the variation in Qmax and binding energy values.Mn has a higher MBC value due to the multiple ion species in natural soil pH, competing as monovalent cations or nucleated as MnCO 3 species on CaCO 3 surfaces (McBride, 1979).
Freundlich adsorption isotherms for Cu, Zn, and Mn in a triple competitive system by selecting calcareous soils are shown in figure (5).Sorption isotherms for these metals illustrate differences in shape and amount adsorbed due to variations in soil properties.Metal sorption isotherms were fitted to C-type sub-group f isotherms according to Giles classification which indicates the adsorption of hydrated molecules.Frundlich parameter K F or distribution coefficient values for Cu, Zn, and Mn in selected soils ranged between (1.315-2.015L gm -1 ) (Table 4).
K F values for Cu were higher than Zn and Mn indicating that Cu was the most retained cation while Mn was the lowest K F value indicating that in the competition this element is easily exchanged and substituted by Cu and Zn.The higher affinity of selected soils for Cu adsorption is probably due to a high number of active sites on CaCO 3 surfaces with high specificity for Cu (Jalali and Moradi, 2013).The slope or (n) value in Freundlich isotherm represents adsorption tensity or adsorption favorite.(n) values for Zn, Cu, and Mn ranged between (0.813-0.977).
That indicates the favorite of study metals adsorption at high concentrations according to Frimmel and Huber (1996).K d isotherms parameters obtained from the relationship between adsorbed metal and metal concentration in equilibrium solution.The determination coefficient K d values of studying metals ranged between (0.66-0.89) and K d values of studying metals ranged between (5-13 L gm -1 ).The order of adsorption metals according to K d values was Cu> Zn> Mn on selected calcareous soils.Mckenzie (1980) reported that the higher affinity of soils for Cu adsorption may be due to the existence of a greater number of active sites with high specificity for Cu.
Depending on the results obtained for Q max , K d , and K F in this study for both mono and triple adsorption system, the selected sequence found was Cu>Zn> Mn.This sequence may follow the order of the electronegativity of these metal cations which is Cu, 1.9 Zn 1.6 Mn 1.5 (Shaheen et al., 2013;Yatim et al., 2018) mentioned that electronegativity is a very important factor in determining metal cations chemisorb with the highest preference.Table (5) shows the simple linear correlation coefficient between Q max , ƩK d , and some soil properties.
Soil properties influenced the maximum adsorption of Cu, Zn, and Mn due to the high significance of simple linear correlation except for Mn.Mn had a weak significance related to Ec and O.M (r = 0.51 and 0.55 -p<0.05).The highest correlations were between Q max of Zn, Cu, Mn, and CaCO 3 , which teaches the high affinity of these metal ions to carbonate surfaces.Generally, the linear correlation coefficient between Q max and soil properties may suggest the adsorption of hydrolyzed ion species formation according to the high correlation between max and pH.Also, there was a highly significant correlation between ΣK d and soil properties at p <0.05.For a competitive adsorption system, the most strongly soil properties related to Cu, Zn, and Mn adsorption were CaCO 3 , Clay, C.E.C, and pH.

Conclusions
Calcareous soils have a high affinity towards trace element ions adsorption.Detailed research of mono and competitive sorption processes between three trace elements (Cu, Mn, and Zn) was performed on three calcareous soils with different CaCO 3 content using sorption isotherms.Trace elements sorption was described satisfactorily by the Langmuir and Freundlich isotherms.In the mono and competitive adsorption system, Langmuir sorption isotherms were of H-type whereas Freundlich isotherms were of Ctype.As determined by Qmax and distribution Coefficient ƩKd, Cu was sorbed in greater amounts with high binding strength compared to Zn and Mn.
Generally, the selectivity sequence for studied metals was Cu>Zn> Mn in all studying soils because of the specificity adsorption of Cu by Clay, CaCO3, and Organic matter.Trace element adsorption capacity in studying soils depends significantly on the soil properties.The values of Gibbs free energy, decreased as the maximum adsorption capacity decreased too, and correspond to the physical adsorption and indicate exothermic sorption process.The correlation analysis for mono and competitive adsorption showed a high correlation between studying metals adsorption and soil properties.The most strongly related to Cu, Zn, and Mn adsorption were Clay, CaCO3, C.E.C, and pH.

Authors' Contribution
Hudhaifa Maan Al-Hamandi: analysis and interpretation of data and he is the corresponding author, guided the research; Mijbil Mohammad Aljumaily: writing a manuscript, analysis and interpretation of data.Mohammed.Ali.Al-Obaidi: acquisition of data, author of the idea, guided the research.

Figure 1 .
Figure 1.Soil sampling locations in Mosul, Iraq

Table 1 .
The coordinates of the soil sampling locations in the study areas using GPS.

Table 2 .
Soil physicochemical properties of the studied soil(Mosul, Iraq)

Table 3 .
Parameters of Langmuir, Freundlich, and linear isotherms for the sorption of mono elements of the studied soil (Mosul,

Table 4 .
Parameters of Langmuir, Freundlich, and linear isotherms for the sorption of ternary elements of the studied soil (Mosul, Iraq ) elements of the studied soil(Mosul, Iraq ) Maximum adsorption capacity, K L = Langmuir equilibrium constant , MBC= Maximum buffering capacity, K F = Distribution Coefficient, Kd= Determination Coefficient.As shown in Table(