Adsorption of lead by chemically activated carbons from three lignocellulosic precursors

Guava seeds, Dindé stones and Tropical almond shells are chemically activated using sulfuric acid with an impregnation ratio of 1:1 then carbonized at 600 °C. The three activated carbons (CAC G, D and A) are characterized by TPD/MS, Boehm titration, DRIFT, SEM, N₂ and CO₂ adsorptions at 77 and 273 K respectively and later used for lead sorption. The experimental sorption data, kinetics and equilibrium, are adjusted with different models according to different theoretical foundations to interpret the sorption process. The kinetics is well described by the Elovich model showing the sorption is heterogeneous. The parameters α and β are calculated. The results show that the kinetics are positively influenced by the ultramicropore and mesopore volumes of the carbons. The equilibrium is well fitted by the Redlich Peterson model, compatible with heterogeneous adsorption sites. The parameters K_S,RP, Qmax and β are calculated. K_S,RP is positively influenced by the sum of acid groups of the carbons. The optimum sorption conditions are 30 °C, initial pH 5–6, adsorbent dose 0.5 g L⁻¹ and initial lead concentration 20 mg L⁻¹ for CAC D and CAC G and 5 mg L⁻¹ for CAC A. New suggestions are given to explain the influence of the operating conditions on the sorption results. The thermodynamics parameters ΔG°, ΔH° and ΔS° are calculated and show the sorption is favorable for the three carbons, it is exothermic and less random for the carbons from Almond and Dindé. A mechanism involving the OH, SH groups and the π electrons of the carbon is proposed to explain the sorption results. The activated carbons from Almond, Dindé and Guava adsorb maximum amounts of lead of 4.5, 7 and 11 mg g⁻¹ respectively; they have the highest β Elovich parameter, ΔH° and ΔS° values from the literature.