Enhanced Hg Removal from Aqueous Streams by Sulfurized Activated Carbon Products: Equilibrium and Kinetic Studies

The removal of Hg from contaminated aquatic media is of major importance, taking into consideration the highly toxic character of the element. One of the most promising water treatment technologies is adsorption by low cost adsorbents, such as activated carbon produced by agricultural byproducts. In this study, activated carbon in granular form (GAC) was produced using pistachio shells from Aegina Island (Greece). Two main GAC products have been synthesized. The first one was chemically activated using ZnCl2. The second one was further treated with Na2S in order to introduce S atoms on the functional groups. The effectiveness of synthesized GAC products for Hg removal was evaluated by conducting batch equilibrium and kinetic experiments. It was found that sulfurization was able to increase by a factor of more than 2 the adsorptive capacity of activated carbon. Namely the maximum adsorption capacity was 73 mg/g for the simple GAC and increased up to 166 mg/g for the S-modified product. The kinetics of adsorption was described with almost equivalent precision using the pseudo-first and the pseudo-second order models, a behavior which is often observed in adsorption experiments, depending on the experimental conditions. The value of activation energy E-A was found to be negative (- 18.8 kJ/mol) in the case of simple GAC and positive (8.27 kJ/mol), in the case of S-modified GAC, suggesting that Hg adsorption on the modified carbon follows a different mechanism, closer to chemisorption processes.