Study on the effect of KMnO4−modified humic acid residues on the removal of Hg(Ⅱ)/Pb(Ⅱ) from solution

Abstract

As a solid waste, a coal−based humic acid residue (HAS) is less researched for its resource utilization. HAS is advantageous to the adsorption of heavy metals in water bodies because of its high adsorption capacity and low cost. In this study, HAS was modified using KMnO4 solution to improve the adsorption of Hg(Ⅱ)/Pb(Ⅱ), while the modification process was optimised by response surface methodology (RSM) and named K−HAS. The Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X − ray photoelectron spectroscopy (XPS) and X − ray diffraction (XRD) were jointly adopted to characterize the HAS and K−HAS. The batch sorption experiments were used to study the adsorption characteristics of external factors such as initial concentration, pH, and reaction time on the adsorption of Hg(Ⅱ)/Pb(Ⅱ) by K−HAS, and the isothermal adsorption model and kinetic model were used to fit the behavior of Hg(Ⅱ)/Pb(Ⅱ) adsorption by K−HAS. The results show that the pseudo−second−order model and the Langmuir adsorption isotherm model can better describe the adsorption behavior of K−HAS on Hg(Ⅱ)/Pb(Ⅱ), with the maximum adsorption amounts of 128.40 and 342.43 mg·g−1, respectively; there is competitive adsorption between Hg(Ⅱ) and Pb(Ⅱ), and Pb(Ⅱ) has a stronger competitive ability, as the concentration of Hg(II) increased, the adsorption of Pb(II) by K-HAS decreased by 20.29% and the adsorption of Hg by K-HAS decreased by 22.77% as the concentration of Pb(II) increased; the thermodynamic parameters indicate that the adsorption of Hg(Ⅱ)/Pb(Ⅱ) by K−HAS is a spontaneous heat absorption reaction. The adsorption mechanism of K−HAS for Hg(Ⅱ)/Pb(Ⅱ) mainly consisted of precipitation and complexation, which contributed 58.7% and 20.6% to Hg(Ⅱ) and 50.7% and 27.5% to Pb(Ⅱ), respectively. In addition, the site energy distribution (SED) shows that Hg(Ⅱ)/Pb(Ⅱ) preferentially occupy the high−energy adsorption sites and then gradually occupies the low−energy sites and that the K−HAS surface becomes more and more inhomogeneous as the solution temperature increases. The experiments demonstrate that K−HAS is effective in the adsorption of Hg(Ⅱ)/Pb(Ⅱ), and the investigation of the adsorption mechanism can be useful for further studies.