Adsorption and desorption of divalent mercury (Hg2+) on humic acids and fulvic acids extracted from typical soils in China

A series of batch equilibration experiments were conducted to assess the adsorption and desorption of divalent mercury (Hg2+) by humic acids (HAs) and fulvic acids (FAs) extracted from black soil and red soil in China. The Fourier transform infrared (FTIR) spectroscopy, cross-polarization (CP) with magic-angle spinning (MAS) 13C nuclear magnetic resonance (NMR) spectroscopy and scanning electron microscopy (SEM) were jointly adopted to characterize the humic substances (HSs) samples and HSs–Hg complexes. The FTIR spectra showed that the adsorption of Hg2+ mainly acted on O–H, C–O and CO groups of HAs and FAs. The NMR spectra indicated that HAs are higher in Paraffin and carbonyl C content compared with corresponding FAs, while FAs exhibit higher methoxy C, O-alkyl C and carboxyl C contents. The SEM images revealed the different surface structures of HAs, FAs and HSs–Hg complexes, which explains Hg2+ complexation phenomenon on HSs. The study also showed that HAs have higher adsorption capacity for Hg2+ than those of FAs. Hg2+ adsorption isotherms could be well fitted with both Langmuir and Freundlich equations. The desorbed percentages of all HSs samples were less than 1%, which indicated their high binding strength for Hg2+. Furthermore, HAs samples have a lower desorption ratio than FAs, HAs can play a more important role in pollution control of Hg2+ in environment.