Removal of gaseous elemental mercury by hydrogen chloride non-thermal plasma modified biochar.

Hydrogen chloride (HCl) non-thermal plasma was applied to introduce Cl active sites on biochar prepared from sorghum straw in this study. Surface modified biochar was then placed in flue gas with typical components to investigate its elemental mercury (Hg0) capture ability. To elucidate the adsorption mechanism & binding properties, samples were characterized by N2 adsorption, scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) and X-ray absorption near edge structure (XANES) analysis of Hg LIII-edge, Cl K-edge and S K-edge. Experimental results showed that HCl plasma modification successfully increased Cl active sites on biochar and greatly increased its mercury removal efficiency. Both HCl treatments (w/without plasma involvement) altered biochar's surface structure and layered structure generated. XANES spectra revealed that adsorbed-Hg on HCl-treated biochars mainly in the form of Hg+. Gaseous Hg0 was believed to heterogeneously react with chlorinated sites through electron-transfer and formed Hg2Cl2 compounds. With the presence of NO or SO2 in the system, adsorbed mercury existed on biochar mainly as Hg+. SO2 competed and inhibited the adsorption of Hg0; while NO promoted Hg0 removal capacity by increasing the active sites and enhancing the adsorption kinetics of adjacent Cl-containing sites.

Study on the effects of oxygen-containing functional groups on Hg0 adsorption in simulated flue gas by XAFS and XPS analysis.

The effect of physicochemical properties of activated carbon on adsorption of elemental mercury (Hg0) was investigated on a series of modified activated carbons. Heat treatment and benzoic acid impregnation were conducted to vary the oxygen functional groups on carbon surface. Hg0 adsorption experiments were run in a fixed-bed reactor at 140 °C. Surface characteristics of carbon samples were studied by N2 adsorption, Boehm titration, X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), respectively. The predominant mechanism of Hg0 removal was the formation of chemical bonds between Hg and various functional groups. Both XPS and XAFS analysis revealed that mercury bound on carbon surface was mainly in oxidation state. Under N2 atmosphere, the absorbed Hg was found as Hg2+, and coordinated to O atom. With the existence of HCl in simulated flue gas, Hg0 was bonded on Cl sites and HgCl2 was assumed to be the dominated form.