Adsorption Characteristics of Zn2+ Ions onto Different Aggregate Size Fractions of Some Calcareous Soils and their Relationship with Soil Properties

Soils are exposed to potentially toxic trace elements (PTEs) such as zinc (Zn) through various ways. Adsorption of Zn in soils is one of factors affecting its availability and mobility. Soil aggregates are units of soil structure very important for adsorbing and retaining of PTEs. In this study, characteristics of the adsorption of Zn were studied in the aggregates of five calcareous soils. Soil aggregates were separated using the dry sieving into four size fractions: 4 to 2 and 2 to 0.25 mm (macroaggregates) and 0.25 to 0.053 and <0.053 mm (microaggregates). Then, adsorption of Zn was studied in different aggregate size fractions using an electrolyte solution calcium chloride (10 mM) containing Zn. Also, properties, such as pH, cation exchange capacity (CEC), calcium carbonate equivalent (CCE), free iron oxides, organic carbon, and amounts of total and available of Zn were determined in different aggregate size fractions. Results showed both the Langmuir and Freundlich equations are applicable to describe the adsorption of Zn in various aggregates. Adsorption capacity (qm in Langmuir model), distribution coefficient (K-F in Freundlich model), and adsorption rate (n in Freundlich model) of the Zn in the microaggregates were higher than in the macroaggregates. At the same time, adsorption energy (K-L in Langmuir model) and, therefore, maximum buffering capacity (MBC in Langmuir model) in the microaggregates were lower than in the macroaggregates. Thus, the amount and rate of Zn adsorption in the microaggregates was higher than in the macroaggregates, but it retained with lower energy. The results of regression study indicated that CEC and CCE are the most effective soil properties on the coefficients of adsorption isotherms. The results of this study revealed that the microaggregates (<0.25 mm) have a higher potential to contaminate the environment in comparison with the macroaggregates.