Preparation of high-capacity macroporous adsorbent using lignite-derived humic acid and its multifunctional binding chemistry for heavy metals in wastewater

Lignite , a low-quality fossil fuel with high moisture and ash content, leads to considerable carbon emissions during electricity generation. Such carbon emissions routinely result in smog. Humic acids (HAs) derived from lignite as by-products can be used to prepare sulphonated HA resin (SHAR) to adsorb heavy metals from wastewater. SHAR exhibits an excellent adsorption capacity because of its high ligand density and abundant macroporous structure. The adsorption capacities of SHAR for Pb 2+ , Cd 2+ , and Cu 2+ can reach 621, 366, and 141 mg/g, respectively, which are considerably higher than those of many reported adsorbents. Multifunctional binding chemistry of SHAR for heavy metals in water was comprehensively investigated. Ion exchange, complexation, adsorption precipitation, and π-electrons interaction between SHAR and heavy metals were experimentally confirmed. In the solution with multiple metal ions (Cu 2+ , Pb 2+ , and Cd 2+ ), synergistic or competitive adsorption among these heavy metal ions on SHAR was controlled using solution pH. The adsorption selectivity of SHAR to lead was proved by real wastewater , and the highly efficient adsorption was associated with S- and O-containing multi-dentate ligands structure. In this study, a green utilisation of lignite and a promising candidate for the purification of heavy metal polluted water were proposed.