Thiol-Functionalization Carbonaceous Adsorbents for the Removal of Methyl-Mercury from Water in the ppb Levels

Mercury is a highly toxic pollutant of major public health concern, and human exposure is mainly related to the aqueous phase, where its dominant form is methyl-mercury (MeHg). In the current work, two carbon-based adsorbents, i.e., a commercial activated carbon and a sunflower seeds' biochar, were modified by the introduction of thiol-active groups onto their surfaces for the MeHg removal from natural-like water in ppb concentration levels. The examined thiol-functionalization was a two-step process, since the raw materials were initially treated with nitric acid (6 N), which is a reagent that favors the formation of surface carboxyl groups, and subsequently by the thiol surface bonding groups through an esterification reaction in methanol matrix. The adsorbents' capacity was evaluated toward the Hg-total legislative regulation limit (1 mu g/L) in drinking water (denoted as Q(1)). The respective isothermal adsorption results revealed an increased affinity between MeHg and thiol-functionalized materials, where the commercial carbon showed slightly higher capacity (0.116 mu g Hg/mg) compared with the biochar (0.108 mu g Hg/mg). This variation can be attributed to the respective higher surface area, resulting, also, to higher thiol groups loading. Regarding the proposed mechanism, it was proved that the S-Hg bond was formed, based on the characterization of the best performed saturated adsorbent.

Automated summary

Mercury is a highly toxic pollutant that poses a major threat to public health. This study successfully removed low concentrations of methyl-mercury from water by modifying two carbon-based adsorbents with thiol-functionalization. The commercial activated carbon exhibited higher adsorption capacity, possibly due to its larger surface area and higher thiol loading.