The effect of Calcium‚ Sodium, Potassium, and Ammonium cations on adsorption of Zinc in some calcareous soils of Chaharmahal -va- Bakhtiari Province

The adsorption is one of the most important chemical processes affecting the mobility and availability of Zinc (Zn) in the soils. Moreover, the presence of cations in the solution may affect Zn adsorption. Adsorption isotherms were used to determine the characteristics of Zn adsorption. The adsorption isotherms describe relation between adsorbed metal on sorbent and its concentration in equilibrated solution at a constant temperature. The calcium‚ sodium, potassium, and ammonium are the important cations in soil. In this study, the effect of the calcium‚ sodium, potassium, and ammonium on the Zn adsorption in 5 calcareous soils in Chaharmahal va- Bakhtiari was investigated.

An experiment was performed using completely randomized design to determine the characteristics of adsorption isotherms of Zn in the studied soils and in the presence of different electrolytes. Two g of each soil (in 3 replicates) placed into centrifuge tubes, then added 20 ml of solution ZnSO4 contains 25, 50, 75, 100, 150, and 200 mg / l of Zn in Ca(NO3)2, NaNO3, KNO3, and NH4NO3 electrolytes (at a concentration of 50 mM). The samples were shaken for 24 h and then were centrifuged (3000 rpm). The soil solution was separated and concentration of Zn in all extractants (in equilibrium) was determined using atomic absorption spectrophotometer (GBC model, 932). In the following, the concentration of adsorbed Zn was determined from the difference between initial and final concentration (equilibrium concentration). Then the Langmuir, Freundlich, and Linear equations were fitted to the data.

The results showed that concentration of adsorbed Zn in the all soils and all electrolytes increased by increasing Zn concentrations in the equilibrium solution. Amount of adsorbed Zn in the presence of Ca2+ was lower than the presence of Na+, NH4+, and K+. The Freundlich and Langmuir equations have the highest determination coefficient (R2) and the lowest standard error of estimate (SE). Therefore, these equations were able to describe the characteristics of Zn adsorption. The result of mean comparison showed that the maximum adsorption capacity (b in Langmuir equation) and adsorption intensity (n in Freundlich equation) were highest in the solution contains sodium, calcium, potassium, and ammonium, respectively. The coefficient related to the binding energy (k in Langmuir equation) and maximum buffering capacity (MBC in Langmuir equation) were highest n the solution containing potassium, ammonium, sodium, and calcium, respectively. The highest distribution coefficient was in solutions containing sodium, potassium, ammonium and calcium, respectively. Therefore, the highest and lowest affinity of Zn to adsorb onto soil solid phase was in the presence of sodium and calcium, respectively.

The results of this study illustrated that sodium can be decreased mobility of Zn in the calcareous soils more than the other cations.