Biochar modification can enhance the properties associated with porosity and functional groups and has been identified as an effective way to improve adsorption capacity. Modified corncob biochars pretreated by different contents of MgCl₂ are obtained through slow pyrolysis; then, this work explores the adsorption abilities towards heavy metals. After modification, biochars mainly impregnate carbon surface with Mg(OH)₂ and MgO particles. The best mass ratio of magnesium (Mg)/feedback is 15% for modified biochar, and 15%Mg-BC has much higher (2.36–9.34 times) metal sorption capacity than pristine biochar. Batch adsorption experiments show that copper (Cu(II)) and lead (Pb(II)) on adsorbents follow a pseudo-second-order model and cadmium (Cd(II)) follows an intraparticle diffusion model. The adsorption isotherm of Cu(II) fits well with the Langmuir model, and Pb(II) and Cd(II) fit with the Freundlich model. In the binary system, modified biochar still effectively removes metals, but the sorption capacity of metals decreases rapidly because of competitive sorption. The main adsorption mechanisms of metals include surface precipitation, cation π-banding, complexation, and ion exchange. The results show that Mg-modified biochar may be an environmentally friendly material for reducing the water pollution of heavy metals.

Activated carbon@Fe₃O₄ composites with good electromagnetic wave absorbing performances in the low frequency range were synthesized via a hydrothermal method.