Type-wide biochars loaded with Mg/Al layered double hydroxide as adsorbent for phosphate and mixed heavy metal ions in water

This study discussed the adsorption of mixed heavy metal ions (Cu²⁺, Co²⁺, Pb²⁺) and phosphate ions by ten pristine biochars and those with precipitated Mg/Al layered double hydroxide (LDH). The pristine biochars have adsorption capacities of 6.9–13.4 mg/g for Cu²⁺, 1.1−9.7 mg/g for Co²⁺, 7.8−20.7 mg/g for Pb²⁺, and 0.8−4.9 mg/g for PO₄³⁻. The LDH-biochars have markedly increased adsorption capacities of 20.4–25.8 mg/g for Cu²⁺, 8.6−15.0 mg/g for Co²⁺, 26.5−40.4 mg/g for Pb²⁺ with mixed metal ions, and 13.0−21.8 mg/g for PO₄³⁻. Part of the Mg ions but Al ions are released from the LDH-biochars during adsorption, counting less than 7.2% of the adsorbed ions. The pristine biochars have specific adsorption sites for Cu²⁺ and Co²⁺, separate Pb²⁺ sites related to ether groups on biochar, and areal-dependent sites for PO₄³⁻. There is no universal adsorption mechanism corresponding to mixed metal ion adsorption for individual pristine biochar involving different contributions of C–O–C, C–O–H, and C[dbnd]O groups and graphitic-N, pyrrolic-N, and pyridine-N groups. The LDH complexes with hydroxyl and carbonyl groups of biochar, and the LDH interacts with biochar's ether groups, which contributes to metal adsorption, against the conception that the biochar is merely a carrier of LDH as adsorbents.