Column adsorption and regeneration tests

Column adsorption tests of the removal of heavy metal ions were carried out at 25 °C using a polyethylene column (Omnifit Co., UK) with an inner diameter of 15 mm and a length of 130 mm containing BC@MnO₂-26.6. Three milliliters of wet BC@MnO₂-26.6 powder (4.5 g) were packed in the column. At the top and bottom of the column, quartz sand (diameter ~0.2 mm; does not adsorb heavy metal ions) was packed to prevent the loss of adsorbents and control the flow. Four synthetic wastewaters containing 75 mg L⁻¹ Pb(II), 60 mg L⁻¹ Cd(II), 40 mg L⁻¹ Cu(II) or 40 mg L⁻¹ Zn(II), respectively, were used as influents in the column adsorption tests. Pb(II), Cd(II), Cu(II) and Zn(II) were removed from synthetic wastewater by four separate column beds packed with the same mass of BC@MnO₂-26.6 powder. The breakthrough points were set as 0.1 mg L⁻¹ for Pb(II), 0.01 mg L⁻¹ for Cd(II), 1.5 mg L⁻¹ for Cu(II), and 5.0 mg L⁻¹ for Zn(II), as the permitted maximum concentrations by China’s Quality Standard for Ground Water (GB/T 14848-2017). The influents continuously pumped through the column at a rate of 60.0 mL h⁻¹ using a calibrated syringe pump (Longerpump Co., China) for 10 h. Effluent samples were collected regularly using a BSZ-100 fraction collector (Huxi Analysis Instrument Factory Co., China) and subjected to AAS. Next, 0.1 mol L⁻¹ HNO₃ solution was added (30.0 mL h⁻¹ for 5 h in the upwards direction) to exhaust the column, followed by rinsing with 120 mL of distilled water. The procedure for the first adsorption-desorption cycle was used for the following seven cycles, and the total adsorption time was 10 h.

To investigate the feasibility of BC@MnO₂-26.6 for removing heavy metal ions from wastewater, we collected sewage from the sewage discharge port of an electroplating factory in Yangzhou. The wastewater was used directly without treatment. The pH and chemical oxygen demand (COD) were 4.51 and 864.7 mg L⁻¹, respectively. The concentrations of Pb(II), Cd(II), Cu(II), and Zn(II) in the wastewater were 25.6, 57.1, 11.3, and 14.7 mg L⁻¹, respectively.