ABSTRACT
The potentials of Adansonia digitata root (ADRP) and stem powders (ADSP) and the stem activated carbon (ADSAC) for adsorption of Pb2+, Cd2+, Cu2+ and Co2+ from aqueous solutions were investigated. The activated carbon was prepared via chemical activation using ZnCl2 as the activating agent. Physico-chemical analysis of the adsorbents (ADRP, ADSP and ADSAC) showed the moisture contents (< 20 %), volatile matter (9.2 - 11.2 %), ash content showed < 5.0 %, fixed carbon (68 - 81.0 %), pore volume (3.0 - 4.0x10-3 m3 /g), bulk density (0.19 - 0.38 gml-1 ) and conductivity (221 - 301 µS/cm). ADSAC showed the highest values for all the physicochemical parameters determined except for moisture and ash content (5.2 % and 2.6 %). The FTIR analysis of the adsorbents (ADSAC and ADRP) before and after adsorption revealed that hydroxyl, carbonyl and amino groups were predominant on the surface of the adsorbents. The Scanning Electron Microscope (SEM) image revealed high porosity and irregular pores in the adsorbents indicating good adsorbents while the Energy Dispersive X-ray Spectrum showed carbon as the major element with 57.2 %, oxygen (36.4 %), and phosphorus (6.5 %) for ADSAC while ADSP showed 75 % oxygen, 13.4 % carbon, 0.9 % calcium and 9.8 % nitrogen. For ADRP, 53.0 % Nitrogen, 23.8 % carbon, 9.1 % calcium, 7.5 % potassium and 6.6 % magnesium were present. Batch adsorption was carried out to evaluate the optimal operational conditions such as initial concentration, pH, contact time, adsorbent dose, particle size and carbonization temperature on the removal of Pb2+, Cd2+, Cu2+ and Co2+. Removal of the metals decreased with increase in initial concentrations of the metal ions (0.5-40 mg/L) and in particle size of the adsorbents (32-250 µm) while removal of Pb2+, Cd2+, Cu2+ and Co2+ ions increased with increase in pH (2-5), contact time (30-90 min), adsorbent dose (0.1 - 0.4 g) and carbonization temperature (250 - 350 o C) for ADRP, ADSP and ADSAC respectively. The optimal conditions for the adsorptions are initial concentration of the metal ions = 0.5 mg/L, pH = 5, contact time = 90 min, adsorbent dose = 0.4 g, particle size = 32 µm and carbonization temperature = 350 o C. The competitive adsorption of Pb2+, Cd2+, Cu2+ and Co2+ mixed metal solutions showed that ADSAC was more efficient with % removal (75.0 - 99.9 %), ADSP (59.8 – 86.7 %) and ADRP (66.1 – 78.3 %). Desorption studies indicate that regeneration and recovery of the adsorbates are possible with acidic reagent with desorption above 90 % at 180 min while below 60 % at 180 min was recorded with basic reagent. Among the adsorption isotherms tested, Freundlich isotherm showed good fit to the adsorption of Pb2+, Cd2+, Cu2+ and Co2+ onto ADRP, ADSP and ADSAC while Langmuir isotherm indicate a favourable adsorption process between the adsorbents and metal ions in solution. Dubinin-Radushkevich isotherm revealed that the adsorption processes were physisorption having mean free energy value below 8 KJmol-1 (1.581 – 3.536 KJmol-1 ). The kinetics studies showed that Pseudo-second order provides the best fit to the experimental data for ADSAC while the presence of the boundary layer effect in intraparticle diffusion showed the existence of the surface sorption indicating that intraparticle diffusion was not the only rate-limiting step. Hence, the studied parameters showed that ADRP, ADSP and ADSAC can be effectively used as a low cost adsorbent.