Sorption of lead and silver from aqueous solution on phosphoric acid dehydrated carbon

Dehydrated carbon was prepared from date palm leaflets via the chemical treatment with phosphoric acid. Carbon preparation was investigated by varying the acid concentration at 150 C and selection criterion was based on the maximum removal of Pb²⁺ and Ag⁺ from aqueous solution. 13 M phosphoric acid was selected for the preparation of dehydrated carbon referred as DC13 that shows maximum removal for both metals from aqueous solution. DC13 was tested for the sorption of Pb²⁺ and Ag⁺ at different pH, contact time, metal concentration, temperature and sorbent status (moistened or dry). Maximum sorption was obtained at initial pH 5.0 for both metals. Equilibrium was reached in ∼1 h for Pb²⁺ sorption and ∼80 h for Ag⁺ sorption with higher activation energy for Ag⁺ sorption (31.9 kJ mol^-1) than that for Pb²⁺ sorption (∼11.0 kJ mol^-1). Moistened carbon showed better performance than the dry carbon and sorption capacity using the Langmuir isotherm was 41.5 and 37.9 mg g^-1 for Pb²⁺ sorption and 312.5 and 285.7 mg g^-1 for Ag⁺ sorption on moistened and dry carbons, respectively. Sorption of Pb²⁺ and Ag⁺ from a binary mixture showed a decrease in their uptake. The peculiar behavior of metal sorption is related to the chemical reduction of Ag⁺ to Ag ⁰ on the carbon surface, however, no chemical reduction was involved in Pb²⁺ sorption.