Adsorption performance and mechanism of bentonite modified by ammonium bromide for gas-phase elemental mercury removal

Bentonite was modified with ammonium bromide to enhance its adsorption performance for the removal of elemental mercury. The adsorbents were characterized by N₂ adsorption/desorption, X-ray diffraction (XRD), elemental analysis, and Fourier transform infrared spectroscopy (FT-IR); the adsorption test was carried out in a laboratory-scale fixed-bed reactor. The results showed that the performance of sodium bentonite in mercury removal is only slightly higher than that of calcium bentonite; however, its performance in mercury removal can be greatly enhanced through the modification with ammonium bromide. The mercury removal efficiency reaches 97.7% over the sodium bentonite modified with ammonium bromide (Br-Ben/Na). High temperature may promote the removal of Hg⁰ and the mercury removal efficiency remains higher than 90% over the 10% Br-Ben/Na adsorbent for a long time at 140 ℃, suggesting that chemical adsorption played a dominant role in the adsorption process. Through the modification with ammonium bromide, the sodium ions were replaced with the ammonium ions added to the bentonite layers; the specific surface area of the modified bentonite is decreased, whereas the average pore size is increased. During the calcination activation process, the ammonium ions may combine with bentonite within the layers, forming the adsorption active centers, which promotes the reaction between Br^- and Hg⁰ and then enhance the adsorption performance of bentonite in the removal of mercury.