Interaction of polar and nonpolar organic pollutants with soil organic matter: Sorption experiments and molecular dynamics simulation

Abstract

The fate of organic pollutants in the environment is influenced by several factors including the type and strength of their interactions with soil components especially SOM. However, a molecular level answer to the question "How organic pollutants interact with SOM?" is lacking. In order to explore mechanisms of this interaction, we have developed a new SOM model followed by carrying out molecular dynamics (MD) simulations in parallel with sorption experiments. The new SOM model comprises free SOM functional groups (carboxylic acid and naphthalene) as well as SOM cavities (with two different sizes), representing the soil voids, containing the same SOM functional groups. To examine the effect of the hydrophobicity on the interaction, the organic pollutants hexachlorobenzene (HCB, non-polar) and sulfanilamide (SAA, polar) were considered. The experimental and the theoretical outcomes explored four major points regarding sorption of SAA and HCB on soil. 1-The interaction depends on the SOM chemical composition more than the SOM content. 2-The interaction causes a site-specific adsorption on the soil surfaces. 3-Sorption hysteresis occurs, which can be explained by inclusion of these pollutants inside soil voids. 4-It was observed that the hydrophobic HCB is adsorbed on soil stronger than the hydrophilic SAA.Moreover, the theoretical results showed that HCB forms stable complexes with all SOM models in the aqueous solution while most of SAA complexes are accompanied by dissociation. The SOMcavity modeling showed a significant effect on binding of organic pollutants to SOM. Both HCB and SAA bind to the SOM models in the order of models with a small cavity > models with a large cavity > models without a cavity. Although HCB binds to all SOM models stronger than SAA, the latter is more affected by presence of the cavity. Finally, HCB and SAA bind to the hydrophobic functional group (naphthalene) stronger than to the hydrophilic one (carboxylic acid) for all SOM models containing cavity. For models without a cavity, SAA binds to carboxylic acid stronger than to naphthalene. 4

Adsorption process of polar and nonpolar compounds in a nanopore model of humic substances

“…Generally, nonpolar species will prefer binding in hydrophobic domains and micro(nano) cavities of HSs. Similar observations were recorded by Ahmed et al (2015).…”

“…FEP: free energy profile; HS: humic substance; MCPA: 2-methyl-4-chlorophenoxyacetic acid [Color figure can be viewed at wileyonlinelibrary.com] MCPA. A similar trend was observed in the work by Ahmed et al (2015). The authors performed FF-MD simulations to analyze the interactions of nonpolar hexachlorobenzene (HCB) and polar sulphanilamide (SAA) molecules with several models of HSs containing hydrophobic and/or hydrophilic domains.…”

“…In addition, factors such as pH, water content and natural inorganic cations in SOM have a strong impact on sorption of various species, making simulations even more complicated. Several models of HSs characterized by a relatively low number of atoms have been used previously to analyze the interactions, formation energies and thermodynamic properties of adsorption processes (Ahmed et al, 2015;Gros, Ahmed, Kühn, & Leinweber, 2017;Kubicki & Apitz, 1999;Schulten, Thomsen, & Carlsen, 2001;Tunega et al, 2016). QC methods are rarely used in the simulations of HSs as they are time consuming, which limits the number of atoms in the models.…”

Molecular simulation to discover rheological properties and soil‐binding ability of PHPA polymer on montmorillonite surface

“…In recent years, due to the improvement of computer power, MD simulation can be run in a long time and a larger material system 16 . Among them, its application in some special small‐scale geotechnical problems has become a research hotspot 17,18 . It has been used to explore the properties of soil minerals under poor geological conditions, such as expansive soil 19 and frozen soil 20 .…”

Surface Modification of Graphene Oxides by Plasma Techniques and Their Application for Environmental Pollution Cleanup

“…Among these methods, the sorption technique is most widely used because of its simple operation, low cost, high efficiency, and the fact that it can be applied in large scale in real applications. The elimination of organic pollutants, such as methyl orange, bisphenol A and bisphenol F, dyes, naphthalene, hexachlorobenzene, sulfanilamide, and inorganic pollutants, such as heavy‐metal ions Cr(VI), As(III), Pb(II), Cd(II), and Co(II) and some radionuclides such as 152 + 154 Eu(III), 90 Sr(II), 243 Am(III), 238 U(VI), and 137 Cs(I), using different kinds of natural clay minerals and man‐made nanomaterials has been studied extensively . The results showed that the sorption properties of various pollutants on natural minerals are mainly dominated by the surface properties of the solid particles and the environmental conditions such as pH, ionic strength, temperature, etc.…”