Removal of Cu²⁺ and Ag⁺ from aqueous solution on a chemically-carbonized sorbent from date palm leaflets

A chemically-carbonized sorbent was prepared from date palm leaflets by sulphuric acid treatment at 170°C. Carbonization took place via the dehydration effect of the hot sulphuric acid producing a carbon with reduction property. Sorption of Cu²⁺ and Ag⁺ from aqueous solution was investigated in terms of pH, contact time, metal concentration and temperature. A peculiar behaviour was found for the sorption of the two metals on the produced carbon. Sorption of Cu²⁺ was fast, reaching equilibrium within ∼2 h, whilst Ag⁺ sorption was slow and required ∼60 h to reach equilibrium. Activation energy (E_a) for Cu²⁺ sorption was ∼16.1 kJ/mol indicating a diffusion-controlled ion exchange process; however, E_a for Ag⁺ sorption was ∼44.3 kJ/mol indicating a chemically controlled process. Equilibrium sorption data were tested for the Langmuir and Freundlich equations. Sorption capacity appears to be much higher for Ag⁺ than for Cu²⁺ with increased uptake, for both metals, when increasing the temperature (25-45°C). Ag⁺ was reduced to elemental silver on the sorbent surface and this was confirmed using scanning electron microscopy and x-ray powder diffraction; however, no reduction processes were involved in Cu ²⁺ sorption. This paper discusses the sorption mechanism.