Effects of Activated Carbon Surface Chemistry Modification on the Adsorption of Mercury from Aqueous Solution

Mercury (Hg), a naturally occurring element, is toxic and can lead to negative health impacts for humans and ecosystems. Activated carbon adsorption is effective in treating Hg-laden aqueous effluent for safe discharge. Two modifications of commercially available activated carbon were investigated: iron impregnation to allow for magnetic sorbent recapture and wet chemical oxidation to enhance aqueous Hg capture. The modified carbons were characterized by nitrogen adsorption-desorption, XRD, pHpzc, vibrating sample magnetometry, elemental analysis, and total acidity titration. The 3:1 C:Fe magnetic powdered activated carbon (MPAC) retained a high surface area of 790 m2 /g and was 95% magnetically recoverable, with the iron present primarily as maghemite. The characteristics of the surface oxygen modified carbons varied based on the nature of the modifying reagent and its concentration. The modified carbons were applied to trace level Hg solutions (100 μg/L). The 3:1 MPAC achieved the highest adsorption capacity, reaching 91% Hg removal with 2% volatilized and 84% adsorbed. Adsorption occurs primarily as chemisorption, thus allowing for non-hazardous residuals disposal until reaching a loading of greater than 12 800 μg Hg/ g MPAC. Surface area and point of zero charge were identified as primary variables influencing adsorption in this system. Hg(II) adsorption was strongly correlated with oxygen content of the C(O)-modified activated carbons. Carbons with the highest oxygen content achieved the highest Hg(II) removal. Contrary to expectations, a strong correlation with oxygen content was not seen in Hg(0) adsorption. Rather, these data best fit a four variable model that identified surface area, pore volume, pHpzc, and oxygen content, with the pHpzc being the primary variable influencing results. Using the standardized EPA TCLP protocol, it was found that no carbons leached Hg at levels requiring disposal as a hazardous waste at the experimental loading rate. Kinetic models indicated both physisorption and chemisorption adsorption mechanisms. Hg speciation and binding mechanisms was predicted using sorbent and matrix characteristics. The use of sequential chemical extraction to verify these operational binding mechanisms was unsuccessful due to extraction inefficiencies and phase transformation