Insights into the removal of Cd and Pb from aqueous solutions by NaOH–EtOH-modified biochar

The enhancement of surface functional groups on biochar is essential to improving its toxic metals adsorption capacity. In this study, we used two solutions: NaOH(aq) and NaOH dissolved in absolute ethanol (C2H5OH) to dope the surface of powdered oiltea camellia shell. Then, the shells were pyrolyzed at 500 °C for 120 min to obtain NaOH-modified biochar (SBC), NaOH–C2H5OH-modified biochar (EBC), and the undoped sample (BC). After modification, the special surface area, O-containing functional groups, especially C=C were improved. The specific surface area of EBC and SBC (91.76 and 56.83 m2g−1, respectively) were superior to that of the BC (36.98 m2g−1). With NaOH–C2H5OH, more C=C groups were introduced via elimination reaction, enhancing its adsorption properties. EBC consistently showed higher adsorption efficiency than BC for Cd(II) and Pb(II). Moreover, the spectroscopic analysis revealed that Cd(II)-π, Pb(II)-π interactions were the predominant mechanisms for the toxic metals' collection on the EBC, each contributing 75.45% and 58.60%, respectively. Besides, the O-functional groups onto EBC surface bonded easily to Pb(II) than Cd(II) in the mixture solution. Therefore, EBC produced in the NaOH–C2H5OH system, as efficient and cost-effective biochar, is promising for toxic metals adsorption.