Nickel sorption in selected calcareous soils of Iran

The presence of heavy metals in the environment is causing ever-growing concern due to their toxicity, carcinogenicity and non-biodegradability. Nickel may enter and accumulate in agricultural soils through various ways. One of the most important chemical processes which affects the behavior and bioavailability of heavy metals is sorption of metals from liquid phase on the solid phase. This process controls the concentration of metal ions and complexes in the soil solution and can influence their uptake by plant roots. Sorption isotherm is a useful method to study metals retention in soils as it gives much useful information about sorption capacity of soils. The aim of the present study was to investigate the sorption properties of Ni by calcareous soils of Iran and its relation with some soil properties to better understand the fate of this metal in calcareous soils. Nickel (Ni) sorption characteristics in 20 calcareous soils of Iran were investigated in a range from 5.87 to 751.23 mgL-1 Ni concentration in solution using sorption isotherms of Langmuir, Freundlich and Redlich–Peterson. The “Curve Expert” software was used to fit the models of sorption isotherms. The high coefficient of determination (r2) and low standard error of estimate (SEE) were used as a criterion for selecting the best model. The Ni sorption isotherms could be fitted by the Freundlich model (r2=0.92-1.00; mean 0.99). The dimensionless parameter n that may indicate the sorption site energy distribution for different soils was between 2.07 and 3.15 and average of 2.46. Also the coefficient of determination and standard error of estimate showed that adsorbed behavior of Ni by calcareous soils can be described by Langmuir adsorption equation (r2=0.94-0.99; mean 0.98). The maximum sorption of studied soils for Ni ranged from 1369 to 5378, with average of 360 mg/kg. The coefficient of bonding energy (KL) in different soils was between 0.008 and 0.029 with an average of 0.015 L/mg that indicate narrow range of this parameter among the studied soils. The distribution coefficient (Kd) ranged from 22.6 to 122.5 with average of 54.8 L/kg. According to the coefficient of determination (r2=0.94-1.00; mean 0.99) and standard error of estimate (SEE= 47.9-163.3; mean 96.9), the Redlich–Peterson equation achieved the best fit to the data of Ni sorption in studied soils. The maximum Ni sorption (qmax) correlated with organic matter (r2=0.50, P