Removal and Recovery of Gaseous Elemental Mercury Using a Cl-Doped Protonated Polypyrrole@MWCNTs Composite Membrane

Due to the restrictions on mercury mining, recovering the mercury from mercury-containing waste is attracting increasing attention. This study successfully achieved the removal and recovery of gaseous elemental mercury (Hg⁰) by using membrane technology. A novel composite membrane of Cl-doped protonated polypyrrole-coated multiwall carbon nanotubes (Cl-PPy@MWCNTs) was fabricated in which MWCNTs acted as the framework to support the active component Cl-PPy. The morphology, structure, and composition of the prepared membranes were determined by field emission scanning electron microcopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, etc. The composite membrane exhibited an excellent performance in Hg⁰ removal (97.3%) at a high space velocity of 200,000 h^-1. The dynamical adsorption capacity of Hg⁰ was 3.87 mg/g when the Hg⁰ breakthrough reached 10%. The adsorbed Hg⁰ could be recovered/enriched via a leaching process using acidic NaCl solution; meanwhile, the membrane was regenerated. The recovered mercury was identified in the form of Hg²⁺, with a recovery efficiency of over 99%. Density functional theory calculations and mechanism analysis clarified that the electrons of Hg⁰ transported to the delocalized electron orbits of protonated PPy and then combined with Cl^- to form Hg₂Cl₂/HgCl₂. Finally, we first demonstrated that the analogous protonated conductive polymers (e.g., polyaniline) also possessed good Hg⁰ removal ability, implying that such species may offer more outstanding answers and attract attention in future.