Adsorption of Potentially Toxic Metals from Mono and Multi-Metal Systems Using Groundnut and Shea Nut Shell Biochars

Background. Adsorption is a unique and promising method for the removal of trace metals from an aqueous environment using cost-effective and readily available biochars. Objective. The present study examined mono and simultaneous adsorption of cadmium (Cd), mercury (Hg) and lead (Pb) onto biochars produced at pyrolysis temperatures of 350 +/- 5 degrees C and 700 +/- 5 degrees C. Methods. Fifty mg/l of trace metal ions with 2 g/50 ml of adsorbent dosage were leached at constant room temperature of 24 +/- 0.5 degrees C in the laboratory with a constant contact time of 72 minutes. A total of 126 elutes were obtained from the batch experiments and conveyed to the Ecological Laboratory at University of Ghana for the analysis. Results. In the mono-component system of Cd, Hg and Pb, removal efficiency was almost 100% using groundnut, shea nut shell, and a combination of groundnut and shea nut shell biochars. The experiment showed that shea nut shell biochars have the strongest affinity for trace metal ions in the mono aqueous phase. In the binary system, the removal efficiency was over 99.60% for cadmium and 100% for mercury. The ternary experiment showed an order of adsorption of Pb2+ > Hg2+ > Cd2+ for Cd, Hg and Pb ions onto groundnut and shea nut shells biochars. Fast pyrolysis temperatures and some types of biochar showed a slight increase in the adsorption efficiency of metal ions, but the increase was not statistically significant (p > 0.05). Conclusions. The study revealed that the Langmuir adsorption isotherm was the best fit model for trace metal ion adsorption onto biochars in the batch experiment. The interactive effects of binary and ternary metal systems onto biochars are antagonistic and synergistic in nature. Based on these results, it is recommended that further competitive adsorption studies of these biochars should be undertaken for accurate estimation of adsorption in natural environments.