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

Evgenios Kokkinos,Aggeliki Lampou,Dimitrios Karfaridis,Ioannis Kellartzis,Anastasios Zouboulis

doi:10.3390/w14010049

Abstract

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 Hgtotal legislative regulation limit (1 μg/L) in drinking water (denoted as Q1). 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 μg Hg/mg) compared with the biochar (0.108 μg 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.

Highlights

According to European Union policy, mercury is included in the list of priority pollutants for drinking water, as it presents high toxicity to living organisms
The need to address the total concentration of mercury (Hgtotal) is reflected in the extremely low Regulation Limit (RL) in drinking water, as clearly established by all international organizations
The binding energies 105.3 eV and 100.9 eV were related with Hg 4f5/2 and 4f7/2, respectively, indicating the successful MeHg adsorption, while the peak at 103.4 eV is attributed to the Si 2p core level [10,29]. Carbonaceous materials, such as those examined in this study, namely the commercial activated carbon (D45) and the sunflower seeds biochar (SF/KOH), are low-cost adsorbents and, the second one promotes sustainability, since it is produced by agricultural waste

Summary

Introduction

According to European Union policy, mercury is included in the list of priority pollutants for drinking water, as it presents high toxicity to living organisms. The need to address the total concentration of mercury (Hgtotal) is reflected in the extremely low Regulation Limit (RL) in drinking water, as clearly established by all international organizations. Its concentration in water should not exceed 1 μg Hgtotal/L (1 ppb) in order to be considered potable. This limit is the lowest among other toxic metals and, unlike other cases, it is accepted by both the European Union and the US Environmental Protection Agency (EPA). According to recent reports, the problem of MeHg environmental pollution is more intense in the Mediterranean region [3]

Methods

Results

Conclusion