English abstract
A novel environmentally friendly carbonaceous material was prepared via phosphoric acid treatment from Omani date palm leaflets. Dehydrated carbon was prepared using different concentrations of phosphoric acid at fixed parameters of temperature (150 °C), carbonization time (25 minutes) and the ratio of leaflets to acid (1 g leaflets: 4 mL acid). Carbon selection was based mainly on the sorption capacity of Pb2+ and Ag* from aqueous solutions. It was found that the carbon prepared under 13 M phosphoric acid, DC13, shows optimum sorption properties for Pb++ and Ag* and, thus, was selected for further sorption studies on both metals from aqueous solutions. The carbon produced possesses low surface area (7.8 m2 g:') based on nitrogen adsorption and it contains surface functional groups such as carboxyl and hydroxyl groups that are capable of metal sorption via ion exchange. Sorption of Pb-* and Ag* from aqueous solution onto the selected dehydrated carbon DC13 was studied at different time, pH, metal ion concentration, temperature and sorbent status (wet or dry). The metal ion removal was found to be pH-dependent. Sorption of both metals was low at low pH and increased with the increase in pH and attained a maximum at pH 5.0. Pb+* sorption was found fast reaching equilibrium within 1 hour while Ag* sorption was slow reaching approximate equilibrium within ~ 80 hours with better performance for the wet sorbent than for the dry one. Kinetics data for the sorption of both metals was found to follow pseudo-second order model. Sorption rate of both metals was enhanced with the increase in temperature. Activation energy, Eq, for Pb2+ sorption, was -- 11.0 kJ moll indicating a diffusion-controlled ion exchange process, however, for Agt sorption, E, was ~ 32 kJ mol' indicating a chemically controlled process that is involved with ion exchange process. Sorption capacity was much higher for Ag* than for Pb2+ with higher uptake for both metal ions by rising the temperature with sorption equilibrium data following well the Langmuir isotherm model indicating that the sorption generates monolayer formation. Thermodynamic parameters were calculated and showed that the sorption of metals on dehydrated carbon was spontaneous and endothermic in nature with an increased value of AHo for Ag* sorption. Sorption of Pb2+ and Ag* from binary mixtures of both metals was decreased as both metal ions compete for sorption active cites on the carbon surface. Ag* was reduced to elemental Ag on the sorbent surface and this was confirmed via the identification of Ag deposits on the sorbent surface by scanning electron microscopy and X-ray diffraction. However, no redox processes were observed for Pb2+ sorption. The peculiar behavior of the sorption of Ag* compared to that of Pb-* seems to be directly related to Ag* reduction on the sorbent surface unlike Pb2+
