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Lead (Pb) and cadmium (Cd) are toxic heavy metals commonly found in aqueous environments. Biochar as a green adsorbent generated from biomass feedstock may be used for effective removal of these heavy metals. This study investigated the adsorption kinetics and isotherms of Pb 2+  and Cd 2+  in aqueous solutions at different pH by biochar prepared from banana stem and leaf (BSL-BC) at 400 °C. Characterizations using scanning electron microscope, X-ray diffraction, and Fourier-transform infrared spectroscopy showed that the synthesized BSL-BC had rough surface, porous structure, and oxygen-containing functional groups. The adsorption of Pb 2+  and Cd 2+  onto BSL-BC reached equilibrium in 8 h and 200 min, respectively, with faster adsorption attained at higher pH and the optimum pH occurred at 5 (Pb 2+ ) and 8 (Cd 2+ ). All adsorption kinetic data followed the pseudo-second-order rate model. The adsorption isotherm data of Pb 2+  and Cd 2+  could be well-described by the Langmuir and Freundlich models, respectively, whereas neither the Temkin or Dubinin–Radushkevich models provided satisfactory fitting results. The maximum adsorption capacities for Pb 2+  and Cd 2+  were 302.20 and 32.03 mg/g, respectively. The calculated mechanism contributions showed that complexation with oxygen-containing functional groups, ion exchange, mineral precipitation, and Pb 2+ /Cd 2+ -π coordination accounted for 0.1%, 8.4%, 88.8%, and 2.6% to Pb 2+  adsorption, and 0.4%, 6.3%, 83.0%, and 10.4% to Cd 2+  adsorption, respectively. Therefore, mineral precipitation was likely the major mechanism responsible for adsorption of both Pb 2+  and Cd 2+  by BSL-BC. The results suggest that the synthesized BSL-BC has great potential for adsorption of Pb 2+  and Cd 2+  from aqueous solutions.

Banana stem and leaf biochar as an effective adsorbent for cadmium and lead in aqueous solution