Effective elimination of hexavalent chromium and lead from solution by the modified biochar with MgMn 2 O 4 nanoparticles: adsorption performance and mechanism.

Heavy metal pollution is one of the environmental problems that need to be solved urgently. The adsorption method is thought as the most effective and economical treatment technology. Nature biochar usually showed unsatisfactory adsorption capacity due to its relatively small adsorption capacity and slow adsorption rate. The metal of Mn has been widely applied in the modification of biochar, which could effectively improve the adsorption capacity of biochar. However, leaching of Mn 2+ on the adsorbent materials would appear during the adsorption process. And it would increase the risk of secondary pollution. The multifunctional binary modified biochar could improve the adsorption capacity of environmental pollutant removal. In addition, it could also act as a metal support carrier, reducing the risk of secondary pollution. A novel effective biochar loaded by Mg-Mn binary oxide nanoparticles (MgMn 2 O 4 @Biochar) was prepared and applied for the Cr(VI) and Pb(II) removal in aqueous solution. The characteristic of MgMn 2 O 4 @Biochar was analyzed by SEM, TEM, FTIR, and XRD. The irregular and somewhat flaky shaped particles of different shape and sizes clustered on the surface of MgMn 2 O 4 @Biochar appeared. Abundant functional groups of O-H, -C-OH, C-O, and C-OOH could be observed on the surface of MgMn 2 O 4 @Biochar. The elements of Mg and Mn elements besides of C, O, and Si elements were presented on the surface of MgMn 2 O 4 @Biochar. The wt% of C, O, Mg, Mn, and Si were 42.82%, 48.99%, 2.83%, 4.44%, and 0.93%, respectively. The operational parameters had an important influence on adsorption capacity of Cr(VI) and Pb(II) removal. The results showed that the adsorption capacity of MgMn 2 O 4 @Biochar for Cr(VI) and Pb(II) would reach 33.5 mg/g and 536 mg/g, respectively, within 360 min. Additionally, the adsorption processes of Cr(VI) and Pb(II) in solution could be described with pseudo-second-order. For Cr(VI), the Langmuir model was suitable to the adsorption process. However, the adsorption process of Pb(II) in solution could be described with Freundlich model. Furthermore, it could be concluded that the possible mechanism of Cr(VI) and Pb(II) removal by MgMn 2 O 4 @Biochar was physical adsorption, surface complexation reaction, and electrostatic adsorption.