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 degrees 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 degrees 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 Cu2+ sorption. This paper discusses the sorption mechanism.