The
elemental composition, molecular skeleton vibration mode, and
carbon-chain structure of iron-based modified biochar demercuration
materials were studied on a microscopic scale using a variety of characterization
methods. A three-dimensional molecular structure monomer model of
iron-based modified biochar with defective carbon rings doped with
high-valence metals was constructed. The reaction path of Hg0 adsorption on the surface of iron-based modified biochar was studied.
The activation energy barrier and the rate-determining step of Hg0 adsorption on the surface of iron-based modified biochar
were determined. Then, two reaction mechanisms and the corresponding
bonding mechanism of Hg0 adsorption on modified biochar
were proposed. In addition, the feasibility of regenerating biochar
with different load ratios was verified and the regeneration reaction
mechanism of inactivated biochar at different adsorption sites was
revealed. The results show that the molecular structure of iron-based
modified biochar is dominated by polycyclic aromatic carbon, and its
molecular formula is C45H24O12NFe.
Adsorption sites on iron-based modified biochar surface are not unique.
A heterogeneous oxidation reaction occurs between Hg0,
a Lewis base, and modified biochar, a Lewis acid. Hg-O-Fe-O
x
–1 and the complex Hg-OM are
the main products of Hg0 oxidation. Oxygen vacancies with
electrons are the chemical adsorption sites, and Fe3+,
lattice oxygen, and chemisorbed oxygen are the main oxidation sites
on the modified biochar. The coupling of these four constituents enables
the adsorption and oxidation of Hg0. Inactivated biochar
can be regenerated by supplementing the lost lattice oxygen or chemisorbed
oxygen with more oxygen. The maximum mercury adsorption efficiency
decreased to 90% of the primary mercury adsorption efficiency. This
study quantitatively revealed the mercury adsorption mechanism of
iron-based modified biochar and the regeneration mechanism of inactivated
biochar, laying a foundation for further improvements in the mercury
adsorption efficiency of metal-modified biochar.
Based on a comprehensive study of the pyrolysis characteristics, pore structure, surface chemical functional groups, organic carbon framework structure, microcrystalline morphology, and lattice characteristics of walnut shell biochar for mercury adsorption, the structural characteristics of biochar were analyzed, and a model of the monomeric molecular structure of biochar was constructed. In addition, the density functional theory of quantum mechanics was introduced into the gas−solid adsorption reaction of elemental mercury with biochar. By quantitatively studying the adsorption energy, adsorption height, and Mulliken population number of the adsorption system, the adsorption mechanism of Hg 0 by biochar was revealed to provide a theoretical basis for future mercury removal methods. The results showed that walnut shell biochar consisted mainly of C, H, O, and N. Aromatic carbon was the main component of the biochar molecular structure, while aliphatic carbon was linked to aromatic structural units. The molecular structure model was mainly composed of aromatic structures, including one methyl, four hydroxyl, and eight carbonyl groups. The molecular formula was C 55 H 37 NO 14 , and the molecular weight was 935. The adsorption of Hg 0 by biochar was mainly via chemical adsorption, and the adsorbed Hg 0 could stably exist on the biochar surface. The Hg 0 adsorption process on the biochar surface mainly depended on the charge of the adsorption site. An adsorption site with a negative charge and with a high number of charges facilitated the adsorption of elemental mercury on the biochar surface. Moreover, the charge of the ortho atom adjacent to the adsorption site had a substantial influence on the adsorption activity of the adsorption site.
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