Molecular Structure Analysis and Mercury Adsorption Mechanism of Iron-Based Modified Biochar

Abstract

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-Ox–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.