Effective adsorption of Pb(II) from wastewater using facile enclosed pyrolysis strategy for defect-rich lignite-based carbon-coated zero-valent iron

In recent years, nanoscale zero-valent iron has been widely applied to treat the Pb(II)-laden wastewater. And the catalytic pyrolysis is an important way to convert coal into carbon based functional materials. However, it still remains challenging for designing a novel carbon-based composite structure to effectively control nanoscale zero-valent iron oxidation and agglomeration problems. For this purpose, a defect-rich lignite-based carbon-coated nano-zero-valent iron structure (AC4) was synthesized by the one-step pyrolysis of lignite catalyzed by K2FeO4 in a closed environment. During the pyrolysis process, K2FeO4 not only promoted the formation of defective carbon and zero-valent iron, but also formed a special structure of carbon-coated iron. The larger the ratio of K2FeO4, the easier it is to generate adsorption sites. Adsorption experiments showed that AC4 exhibits a large adsorption capacity. The higher adsorption amount was due to the synergistic effect between zero-valent iron and defective carbon. The Langmuir isotherm model fitted the true experimental data successfully and the best adsorption amount of Pb(II) was 1118.54 mg/g at 298.15 K and pH 5. The true experimental situation was more accurately fitted by the pseudo-second-order model and chemisorption was the main reason for the adsorption. From a thermodynamic point of view, adsorption was an endothermic reaction and a spontaneous process. This work demonstrates that lignite resources can be converted into high value-added adsorbents.