Adsorption of Lead, Cadmium, and Nickel on Benchmark Soils of Pakistan

Abstract

Adsorption and desorption reactions at the solid phase–solution interface, play a significant role in controlling metal concentrations in soil solution and metal translocation to plants. Five predominant benchmarks soils of Pakistan were characterized for their lead (Pb), cadmium (Cd), and nickel (Ni) adsorption parameters by constructing equilibrium adsorption isotherms. The adsorption parameters (maximum adsorption capacity and affinity coefficient) were calculated by fitting measured adsorption data to Langmuir, Freundlich, and Redlich–Peterson adsorption models. Thermodynamic properties (Gibbs free energy—ΔG°, enthalpy—ΔH°, and entropy—ΔS°) were measured by equilibrating soils with Pb, Cd, and Ni solution at 25 ± 2 and 45 ± 2°C. The results revealed that the Langmuir and Redlich–Peterson adsorption models described metal adsorption data equally good. Langmuir’s predicted Pb, Cd, and Ni adsorption demonstrated better correlation with measured adsorption than Freundlich’s model. According to the maximum Pb, Cd, and Ni adsorption calculated by Langmuir and Redlich–Peterson models and Freundlich sorbate affinity to sorbent, the studied soils were formed the following sequence: Kotli > Miranpur > Gujranwala > Shahdara > Rasulpur. The magnitude of metal adsorption (b and q_mon) and affinity (K_f) was highly correlated with the clay, soil organic matter, and iron and aluminum contents and cation exchange capacity. The measured metal adsorption in different soils followed the order: Pb > Cd > Ni. The magnitude of maximum Pb sorption (calculated by Langmuir and Redlich–Peterson models) was approximately two and four times higher than that for Cd and Ni, respectively. Similarly, the Freundlich’s sorbate affinity was also higher for Pb than for Cd and Ni. Thermodynamic parameters revealed that the Pb, Cd, and Ni adsorption reactions were spontaneous and exothermic in nature, and the process was dominated by physical adsorption.