Effects of selected properties of woodchar and bonechar on lead and Cadmium adsorption

Abstract

The choice of materials used for clean-up of contaminated soils is mostly based the adsorption capacities of such materials. Biochar, a pyrolysed product of organic materials was employed in Lead (Pb) and Cadmium (Cd) sorption studies. Fourier transform-infrared (FT-IR) spectrometer was  employed to determine the presence of functional groups in the biochar samples that may likely influence the sorption of the heavy metals. Biochar samples were subjected to electronic scanning to reveal the micro-structures of the biochar particles using a scanning electron microscope. Batch experiments were carried out to determine the adsorption isotherm that best fits Cd and Pb ions. FT-IR spectroscopy performed on the biochars  detected the characteristic peak values and the functional groups contained by the biochars. The spectroscopy revealed that the functional groups were dominated by C-H, O-H, S=O, C=C and C-O groups of alkane, alcohol, carboxylic, sulphate, alkene and vinyl ether. Electronic scanning of the  biochar samples revealed uneven surfaces for bonechar and well developed mesopores for woodchar. The Langmuir isotherm gave maximum adsorption values of 6.39 and 0.17 for Pb ions onto woodchar and bonechar; 0.05 and 0.042 mg g-1 for Cd ions onto woodchar and bonechar respectively. Langmuir adsorption isotherm model was best fit with high R2 values (0.97 and 0.99) and were obtained from woodchar and bonechar sorption of Pb. Woodchar was observed to have significantly (p<0.05) higher sorption capacity for Pb than Cd when compared to bonechar. The n values obtained from the Freundlich adsorption isotherm indicated that the adsorption process was a chemical phenomenon. This study showed that biochar especially those made from plant sources can be used as a promising adsorbent for remediating Pb contaminated soils.

Keywords: Adsorption, Isotherms, Biochar, Lead, Cadmium