Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels

Abstract The acidic environment has been confirmed to have significant influence on the biochar products through hydrothermal carbonization. To investigate the effect of H3PO4 on the surface properties and adsorption mechanisms of the biochar, fresh banana peels were used as feedstock and transformed into hydrochars under the catalysis of H3PO4 with concentration ranges from 0% to 50%wt. The addition of H3PO4 greatly impacted the physicochemical properties of the as-obtained hydrochars, such as carbonization degrees, pH values, amount of surface acidic functional groups and then further affected the adsorption mechanisms of lead ions on those hydrochars. Hydrochars catalyzed by H3PO4 with higher concentration showed higher degree of carbonization, lower value of pH and fewer amount of acidic functional groups, while lower content of H3PO4 could not completely catalyze the degradation reactions and resulted in larger quantity of intermolecular locked oxygen containing groups that could not be participated into the adsorption process. Among all six samples, hydrochar generated in 30%wt H3PO4 exhibited the best adsorption property of 241 mg g−1, possibly due to the largest concentration of the acidic functional groups on the surface rather than in the intermolecular structure of the carbon. It had been confirmed that the adsorption mechanisms were the combination of surface complexation, cationic exchange and part of precipitation. Hydrochars derived from fresh banana peel via hydrothermal carbonization catalyzed by phosphoric acid could be excellent adsorbents for lead removal in aqueous environments.