Sarra Gaspard

Activated carbons (ACs) are widely used in the purification of drinking water without almost any knowledge about the adsorption mechanisms of the persistent organic pollutants. Chlordecone (CLD, Kepone) is an organochlorinated synthetic compound that has been used mainly as agricultural insecticide. CLD has been identified and listed as a persistent organic pollutant by the Stockholm Convention. The selection of the best suited AC for this type of contaminants is mainly an empirical and costly process. A theoretical study of the influence of AC surface groups (SGs) on CLD adsorption is done in order to help understanding the process. This may provide a first selection criteria for the preparation of AC with suitable surface properties. A model of AC consisting of a seven membered ring graphene sheet (coronene) with a functional group on the edge was used to evaluate the influence of the SGs over the adsorption. Multiple Minima Hypersurface methodology (MMH) coupled with PM7 semiempirical Hamiltonian was employed in order to study the interactions of the chlordecone with SGs (hydroxyl and carboxyl) at acidic and neutral pH and different hydration conditions. Selected structures were re-optimized using CAM-B3LYP to achieve a well-defined electron density to characterize the interactions by the Quantum Theory of Atoms in Molecules approach. The deprotonated form of surface carboxyl and hydroxyl groups of AC models show the strongest interactions, suggesting a chemical adsorption. An increase in carboxylic SGs content is proposed to enhance CLD adsorption onto AC at neutral pH conditions.

Activated carbon (AC) is employed in drinking water purification without almost any knowledge about the adsorption mechanism of persistent organic pollutants (POPs) onto it. Hexachlorocyclohexane (HCH) is an organochlorinated contaminant present in water and soils of banana crops production zones of the Caribbean. The most relevant isomers of HCH are γ-HCH and β-HCH, both with great environmental persistence. A theoretical study of the influence of AC surface groups (SGs) on HCH adsorption is done in order to help to understand the process and may lead to improve the AC selection process. A simplified AC model consisting of naphthalene with a functional group was used to assess the influence of SGs over the adsorption process. The Multiple Minima Hypersurface (MMH) methodology was employed to study γ-HCH and β-HCH interactions with different AC SGs (hydroxyl and carboxyl) under different hydration and pH conditions. The results obtained showed that association of HCH with SGs preferentially occurs between the axial protons of HCH and SG's oxygen atom, and the most favorable interactions occurring with charged SGs. An increase in carboxylic SGs content is proposed to enhance HCH adsorption onto AC under neutral pH conditions. Finally, this work presents an inexpensive computer aided methodology for preselecting activated carbon SGs content for the removal of a given compound.

Turbinaria turbinata brown seaweeds were tested as carbon electrode material in symmetric, electrochemical supercapacitors. The electrochemical properties of the carbon materials were characterised for their application as supercapacitors using cyclic voltammetry, galvanostatic charge/discharge method and electrochemical impedance spectroscopic analyses. Our initial results showed that the optimal behaviour was obtained for the sample prepared by pyrolysis at 800 °C. The average surface area of the carbon was 812 m2/g. Electrochemical tests with an organic electrolyte gave the following interesting results: a capacitance of 74.5 F/g, a specific series resistance of 0.5 Ω cm2 and an ionic resistivity of 1.3 Ω cm2. These results show the promising capacitive properties of carbon derived from seaweeds and their application in electrochemical supercapacitors.

Abstract From the 1960s to the 1990s, the large-scale production of banana in the French West Indies required an intensive use of chlorinated pesticides, such as chlordecone (Kepone), resulting in the diffuse contamination of soil and surface waters in the banana-producing areas. For this reason, drinking water plants were equipped with filters containing commercial activated carbons, being one of the challenges to find local adsorbents for the sustainable management of water treatment plants. In this paper, the adsorption of chlordecone (CLD) on activated carbons (ACs) prepared from sugar cane bagasse is studied, aiming to understand the mechanism of CLD adsorption on the AC surface. First, textural, acido-basic and chemical characteristics of the ACs were determined by thermal desorption, X-ray photoelectron and Boëhm studies. Adsorption isotherms of CLD show that the adsorption capacity increases with the amount of carbon and acidic groups at the AC surface whereas basic groups, hydroxyl and ether groups are detrimental to adsorption. The adsorption capacity is maximized at a solution pH level equal to the pHpzc of the considered AC. From Temperature programmed desorption studies, it is proposed that chlordecone adsorption mechanism onto ACs is mainly governed by interaction with carboxylic groups. These results were correlated to molecular modeling studies of CLD interactions with surface functional groups of AC. The models of preferential positions, corresponding to minimal value of association energy, of interactions between CLD and AC functional groups were obtained.