Abstract
The influence of nitrogen-containing surface groups (SGs) onto activated carbon (AC) over the adsorption of chlordecone (CLD) and β-hexachlorocyclohexane (β-HCH) was characterized by a molecular modelling study, considering pH (single protonated SGs) and hydration effect (up to three water molecules). The interactions of both pollutants with amines and pyridine as basic SGs of AC were studied, applying the multiple minima hypersurface (MMH) methodology and using PM7 semiempirical Hamiltonian. Representative structures from MMH were reoptimized using the M06-2X density functional theory. The quantum theory of atoms in molecules (QTAIM) was used to characterize the interaction types in order understanding the adsorption process. A favorable association of both pesticides with the amines and pyridine SGs onto AC was observed at all pH ranges, both in the absence and presence of water molecules. However, a greater association of both pollutants with the primary amine was found under an acidic pH condition. QTAIM results show that the interactions of CLD and β-HCH with the SGs onto AC are governed by Cl···C interactions of chlorine atoms of both pesticides with the graphitic surface. Electrostatic interactions (H-bonds) were observed when water molecules were added to the systems. A physisorption mechanism is suggested for CLD and β-HCH adsorption on nitrogen-containing SGs of AC.
Highlights
Chlordecone (CLD, C10Cl10O), and hexachlorocyclohexane (HCH, C6H6Cl6) chlorinated pesticides were extensively used in Martinique and Guadeloupe until the beginning of the 1990s to prevent the propagation of the banana weevil (Cosmopolite sordidus), which attacks the roots of the banana tree, resulting in the contamination of soil and surface waters [1,2]
Because the adsorption of CLD and β-HCH occurs in aqueous solution and they co-exist in real water, it is important to understand the role that water molecules play in the interaction between both pollutants with the nitrogen-containing surface groups of activated carbon (AC)
The application of the quantum theory of atoms in molecules (QTAIM) analysis allowed us to characterize the topology of the electron density (ρ) and its Laplacian (∇2ρ) at the bond critical points (BCPs) and, to describe the molecular interactions and to classify them according to generally accepted criteria
Summary
Chlordecone (CLD, C10Cl10O), and hexachlorocyclohexane (HCH, C6H6Cl6) chlorinated pesticides were extensively used in Martinique and Guadeloupe until the beginning of the 1990s to prevent the propagation of the banana weevil (Cosmopolite sordidus), which attacks the roots of the banana tree, resulting in the contamination of soil and surface waters [1,2]. The persistence of CLD in the environment can be explained by its poor biodegradability [5,6] due to its chemical structure and high steric hindrance. Chlordecone has a high bioaccumulation factor in organisms, is highly lipophilic, persists in the environment and organisms, and has a broad range of toxicities, including neurotoxicity. The islands of Martinique and Guadeloupe show the highest prostate cancer diagnosis rates in the world, which could be related to pesticides exposure [3]
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