Enhanced removal of aqueous Cd(II) by a biochar derived from salt-sealing pyrolysis coupled with NaOH treatment

Activated biochar (A-BC550) that was prepared by salt-sealing pyrolysis (at 550 °C) and coupled with NaOH treatment was used to remove cadmium (II) from an aqueous solution. The FTIR spectroscopy and XPS analysis showed that the content of carboxyl and methoxy (CH₃sbnd Osbnd) groups was increased by the NaOH impregnation that occurred before pyrolysis. The complexation and precipitation that occurred played a leading role in the subsequent adsorption. Adsorption occurred on the surface of the biochar, and the adsorption equilibrium was achieved in approximately 10 min of reaction following the pseudo second-order kinetic model and the Redlich-Peterson isotherm model. The maximum adsorption capacity of A-BC550 was determined by the Sips isotherm model to be 216.2 mg g^-1 at pH = 7, which was 4.6 times that of the unmodified biochar without the NaOH treatment (47.13 mg g^-1). Thermodynamic parameters showed the increase of adsorption temperature was conducive to the adsorption process. This work provides a low-cost and environmentally friendly method to pyrolyze biochar. The A-BC550 is a favorable material for treating heavy metal-contaminated wastewater.