Insight into the adsorption of Pb2+ ions onto Opuntia ficus indica cladodes

Abstract

Opuntia ficus indica cladodes (OFIC) particles were prepared from the raw material through a cost-effective treatment and used as an adsorbent for the removal of Pb2+ ions from aqueous solutions under different experimental conditions. The adsorbent was characterised by means of ATR-FT-IR spectroscopy, potentiometric titration, pHpzc, and SEM-EDX techniques. Based on preliminary single batch adsorption experiments at different pH values, kinetics and isotherm adsorption studies were carried out fixing the pH at 5.0. Both the adsorption kinetics and thermodynamics were studied in batch without ionic medium, and in the presence of NaNO3 and NaCl background salts at different ionic strengths, in the 278.15–318.15 K temperature range. The experimental conditions of the Pb2+ ions solution played an important role in both the affinity and the adsorption capacity of OFIC. Kinetic and isotherm adsorption data were tentatively subjected to regression analysis using diverse mathematical approaches. The pseudo second order and the Langmuir models provided satisfactory description of the kinetics and the isotherms adsorption data, respectively. Adsorption equilibrium was achieved within 150–200 min. The highest qm value (121 mg g−1) was reached at pH = 5.0, at T = 284.15 K. The adsorption process showed an activation energy of 24 kJ mol−1, being endothermic and with a positive entropy change. The adsorption process was found to be reversible, and the adsorbent could be reused for at least 4 cycles, with a regeneration efficiency of 82.6 % at the fourth adsorption/desorption cycle. Breakthrough curves showed comparable adsorption behaviour under both equilibrium and non-equilibrium conditions.