Kinetics and thermodynamics of nickel sorption to calcium–palygorskite and calcium–sepiolite: A batch study

• Sorption of Ni onto palygorskite and sepiolite is temperature-dependent.
• Sorption of nickel to palygorskite and sepiolite is an endothermic reaction.
• The Ni sorption to sepiolite is more spontaneous than that to palygorskite.
• Activation energy for Ni sorption to sepiolite is lower than that to palygorskite.

Knowing thermodynamic features of sorption/desorption of metals to/from soil minerals is a very efficient tool to assess and control the fate of these chemicals in the environment. In this batch experiment, we assessed the sorption of nickel (Ni) onto palygorskite and sepiolite at temperatures of 25, 35 and 45 °C. Nickel sorption onto both minerals at the three temperatures was adequately described by the Langmuir sorption model (R2 > 0.97 for palygorskite and R2 > 0.84 for sepiolite). Increasing the temperature from 25 to 45 °C caused an increase in both the maximum adsorption capacity of the solid phase (qm) and the Langmuir sorption coefficient (KL), indicating the enhanced sorption potential and stronger bonding of Ni to the minerals. Positive values of enthalpy (ΔH) revealed the endothermic nature of Ni sorption and negative free energy ΔG° indicated that sorption of Ni to palygorskite and sepiolite under standard conditions (25 °C) is spontaneous. Time-dependent sorption data at the three temperatures were adequately described by the first order kinetic model (R2 > 0.93 for both minerals). The rate constant of the reaction and the quantity of Ni sorbed on both minerals at equilibrium increased with increasing temperature. However, the lower activation energy (Ea) for sepiolite compared with palygorskite indicated that Ni was more readily sorbed onto sepiolite.