Calcite modification of agricultural waste biochar highly improves the adsorption of Cu(II) from aqueous solutions

Calcite-modified biochar was developed as an inexpensive adsorbent for heavy metal immobilization. The biochar was prepared by pyrolysis of coconut shells under a N2 atmosphere at 600 °C and then modified by mixing with calcite in an aqueous solution. The surface area of the modified coconut biochar (CAL/BC) was relatively small (9.32 m2·g−1). The CAL/BC surface had CO, CO32- and CC functional groups. The maximum adsorption capacity of Cu(II) on CAL/BC was 213.9 mg·g−1 at 25 °C, and the removal efficiency was maintained at 87.7% even after four adsorption–desorption cycles. The adsorption process was described well by the pseudo-second order model (R2 = 0.9445–0.9976) and Langmuir adsorption model (R2 = 0.9908–0.9934), which meant that monolayer and chemical adsorption dominated. The dominant adsorption mechanisms of Cu(II) on CAL/BC were surface complexation, precipitation, and ion exchange. This study suggests that biochars prepared from two inexpensive materials (calcite and coconut shells) can be used as an adsorbent for effectively removing heavy metals from simulated aqueous solutions.