Enhanced adsorption of aqueous Pb(II) and Cu(II) by biochar loaded with layered double hydroxide: Crucial role of mineral precipitation

To improve heavy metal adsorption efficiency and investigate interaction mechanisms by biochar-based materials, MgAl-layered double hydroxide (MgAl-LDH) was loaded on the coconut shell biochar (BC) obtaining the composite (BC-LDH) for adsorptive decontamination of Pb(II) and Cu(II) in water. The removal capacities of BC-LDH (294 and 38.6 mg/g) was higher than BC (13.3 and 11.4 mg/g) and LDH (126 and 22.7 mg/g) for aqueous Pb(II) and Cu(II). The interaction mechanisms between BC-LDH and heavy metals were classified as mineral precipitation, cation exchange and isomorphic replacement, electrostatic attraction, and surface complexation according to the characterization results, and the corresponding adsorption capacity were named as Qpre, Qexc, Qele, and Qcom. The quantitative analysis indicated that the contributions of the above mechanisms of BC-LDH for Pb(II) and Cu(II) followed the order of Qpre > Qele > Qexc and Qpre > Qexc > Qele, respectively. The Qpre values reached 76.26% and 45.0%, suggesting the mineral precipitation was the dominant mechanism. The cation exchange and isomorphic replacement (44.18% for Cu(II)), as well as the electrostatic attraction (12.46% for Pb(II)), were also key contributions to the interaction mechanisms. Compared with BC, the Qpre increased and Qele decreased significantly, certified the combination of BC and LDH provided a kind of viable composite in heavy metal wastewater treatment.