posted on 2024-01-22, 09:03authored byZhoujie Wang, Qiuyi Lu, Chaopeng Liu, Huadong Tian, Jingyi Wang, Lei Xie, Qi Liu, Hongbo Zeng
The
mobility and distribution of heavy metal ions (HMs) in aquatic
environments are significantly influenced by humic acid (HA), which
is ubiquitous. A quantitative understanding of the interaction mechanism
underlying the adsorption and retention of HMs by HA is of vital significance
but remains elusive. Herein, the interaction mechanism between HA
and different types of HMs (i.e., Cd(II), Pb(II), arsenate, and chromate)
was quantitatively investigated at the nanoscale. Based on quartz
crystal microbalance with dissipation tests, the adsorption capacities
of Pb(II), Cd(II), As(V), and Cr(VI) ionic species on the HA surface
were measured as ∼0.40, ∼0.25, ∼0.12, and ∼0.02
nmol cm–2, respectively. Atomic force microscopy
force results showed that the presence of Pb(II)/Cd(II) cations suppressed
the electrostatic double-layer repulsion during the approach of two
HA surfaces and the adhesion energy during separation was considerably
enhanced from ∼2.18 to ∼5.05/∼4.18 mJ m–2. Such strong adhesion stems from the synergistic metal–HA
complexation and cation−π interaction, as evidenced by
spectroscopic analysis and theoretical simulation. In contrast, As(V)/Cr(VI)
oxo-anions could form only weak hydrogen bonds with HA, resulting
in similar adhesion energies for HA–HA (∼2.18 mJ m–2) and HA–As(V)/Cr(VI)–HA systems (∼2.26/∼1.96
mJ m–2). This work provides nanoscale insights into
quantitative HM–HA interactions, improving the understanding
of HMs biogeochemical cycling.