The Potentials Of Adansonia Digitata Root And Stem Powders And Stem Activated Carbon As Low-Cost Adsorbents For The Removal Of Heavy Metals From Aqueous Solutions

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 oC) 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 oC.

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.