Enhanced adsorption of Pb(II) onto modified hydrochar by polyethyleneimine or H3PO4: An analysis of surface property and interface mechanism

The corn straw-converted hydrochar (HC), HC modified with H3PO4(H3PO4-HC), and HC modified with polyethyleneimine (PEI) (PEI-HC) were prepared in this study for Pb(II) adsorption from an aqueous solution. SEM, TEM, XRD, Raman, FT-IR, BET and XPS analysis were carried out for the characterization of the as-obtained samples, and the Pb(II) adsorption capacities and mechanism onto them were analyzed. The results of SEM show that PEI loading on HC could cover the aggregates of the carbon spheres, altering the microscopic characteristic of HC. The lower I-D/I-G value of H3PO4-HC and no residual cellulose provided evidence to H3PO4-HC suffering a more thoroughly carbonization process than the original HC or PEI-HC. Both H3PO4-HC and PEI-HC exhibited excellent performance for Pb(II) adsorption with q(max) of 353.4 mg g(-1) and 214.0 mg g(-1), respectively, which are significantly higher than the adsorption capacity (32.67 mg g(-1)) of the original HC. And the dominant Pb(II) adsorption process onto the as-fabricated HCs was attributed to chemical reaction revealed by the results of XPS analysis. After the adsorption process, the binding energies of C-O and C=O or -NH-/-NH2 and -N= in the as-prepared HCs have been shifted, confirming that the surface functional groups had taken part in the Pb(II) adsorption. The main adsorption mechanisms of Pb(II) on H3PO4-HC are the ions exchange and hydrogen bonding, and that of PEI-HC are the coordination bonding and hydrogen bonding. This study put forward in-depth views on the adsorption mechanism of the modified HCs. This facilitates the development of new and efficient adsorbents for the removal of heavy metals from water.