Enhanced adsorption for aqueous lead (II) by red mud-modified biochar via microwave-assisted hydrothermal carbonization and K2CO3 activation: Performance and mechanism

• Adding RM improved S BET , pore volume and magnetic strength of MHC and AMHC. • Magnetization strength of AMHC-C5R5 was 30.85 emu/g and was benefit for separation. • RM in AMHC made Pb(II) adsorption keep spontaneous at higher concentration. • –COOH, –OH, Fe 3 O 4 , Fe 2 O 3 promoted electrostatic adsorption between Pb(II) and AMHC. • Adsorption mechanism included precipitation, ion exchange, pore filling etc. In this study, red mud-modified biochar (AMHC) derived from low-cost corncob (CC) and red mud (RM) was successfully prepared by microwave-assisted hydrothermal carbonization (MHTC) and K 2 CO 3 activation. The effect of RM and K 2 CO 3 activation on the characteristics of AMHC for lead (Pb) (II) was innovatively investigated. The results showed that adding RM could promote the characteristics of AMHC obviously, and when the mass ratio of CC to RM was 8:2, AMHC exhibited the highest special surface area of 591.91 m 2 /g and the highest total pore volume of 0.49 cm 3 /g. And when the mass ratio of CC to RM was 5:5 (AMHC-C5R5), AMHC exhibited the highest magnetic strength of 30.85 emu/g, which was beneficial for separating of them from aqueous solutions by magnetic separation. Moreover, the results of batch adsorption experiment also showed that AMHC-C5R5 exhibited a maximum adsorption capacity of 350.28 mg/g at 45 °C, and this process was dominated by chemisorption. Interestingly, for AMHC without added RM, the adsorption process was non-spontaneous when the initial concentration of Pb (II) exceeded 200 mg/L, while for AMHC-C5R5, the adsorption was still spontaneous until the initial concentration of Pb (II) reach 500 mg/L. Furthermore, the results also indicated that –COOH, –OH, Fe 3 O 4 and Fe 2 O 3 on the surface of AMHC could promote electrostatic adsorption between Pb (II) and AMHC. And the adsorption mechanisms included not only ion exchange and electrostatic interaction, but also precipitation, complexation, and pore filling as well.