Interaction of polar and nonpolar organic pollutants with soil organic matter: Sorption experiments and molecular dynamics simulation
Graphical abstract
Introduction
Soil is a complex heterogeneous mixture of mineral and organic matter in addition to water and air. Despite the low percentage of the soil organic matter (SOM) in most soils, it has a substantial influence on soil properties and agricultural productivity. At the current state of knowledge, SOM is regarded as highly polydisperse, complex macromolecular system, characterized by multiple constituents and functional groups, voids and variable rigidity (Schulten and Schnitzer, 1998, Schaumann, 2006a, Schaumann, 2006b, Senesi and Loffredo, 1999, Senesi et al., 2009). SOM consists of carbohydrates, peptides, alkyl aromatics, N-heterocyclic compounds, phenols, sterols, lignins, lipids, hydrocarbon, bound and free fatty acids, nitriles, suberin to name just the most important compounds (Schulten and Leinweber, 1999). SOM largely governs soil functions, one being the interaction of pollutants in soil.
Persistent organic pollutants (POPs) are among the most environmentally hazardous compounds. Due to their hydrophobicity, they are resistant to environmental degradation through chemical, biological, and photolytic processes and are tending to accumulate in soil and ground water (Ritter et al., 1995). They are ubiquitously distributed in the environment, having long life times of up to years and decades in the soil (Jones and de Voogt, 1999). Additionally, polar organic pollutants, such as pharmaceuticals and personal care products, have been recognized as emerging soil pollutants in the past decades. They reach soil through contaminated wastewater and biosolids as well as excreta from medicated livestock used as fertilizer (Boxall et al., 2004, Jjemba, 2006). In general, they are mostly polar and ionizable compounds, bearing various functional groups (Thiele-Bruhn, 2003). The fate of the different types of pollutants in the environment is influenced by several factors. The most important one is the type and strength of interactions of the pollutant to soil components especially SOM (Senesi, 1993). Consequently, the physical and chemical properties of the soil as well as the pollutant play a pivotal role for the nature of pollutant–soil interaction.
The extent and concentration dependence of pollutant–SOM interaction can be investigated via adsorption experiments of the pollutant on soil (Senesi, 1993). Adsorption of hydrophobic pollutants on soils is described by two stage kinetics (Weber et al., 1991, Chen et al., 2004). The vacant sites are filled up in a rapid initial stage, while diffusion of the hydrophobic pollutants into SOM is a slow rate-limited process (Chiou et al., 1983). In contrast, the mechanism of sulfonamide adsorption may include hydrogen bonding, van der Waals forces, cation exchange, cation bridging, and surface complexes (MacKay and Canterbury, 2005, Lertpaitoonpan et al., 2009, Senesi, 1992) in addition to hydrophobic interaction. In general the sorption of polar compounds and especially sulfonamide is strongly affected by pH and ionic strength (Ter Laak et al., 2006, Gao and Pedersen, 2005, Kurwadkar et al., 2007). Since adsorption experiments yield information that can only be correlated statistically to soil properties, combination of molecular modeling and computational chemistry is a complementary promising approach to develop an atomistic understanding of the binding of pollutants to soil (Gerzabek et al., 2001, Schaumann and Thiele-Bruhn, 2011).
Due to the heterogeneity and complexity of SOM composition, still its modeling is a complicated task and the existing models have not been introduced without triggering criticism. There are different hypotheses concerning the SOM principal structural organization (Schaumann and Thiele-Bruhn, 2011), i.e. macromolecular vs. supramolecular (Piccolo, 2002, Sutton and Sposito, 2005, Schaumann, 2006a, Schaumann, 2006b). The first hypothesis for a polymeric SOM model has been developed by Schulten and coworkers on the basis of bio- and geochemical, NMR-spectroscopic and mass spectrometric analyses (Schulten and Schnitzer, 1995, Schulten and Leinweber, 2000, Schulten, 2002). It was established by creating a network of long chain alkyl structures including aromatic rings (Schulten et al., 1991). Moreover, some carbohydrate and protein units can be inserted into this model in its internal voids and on its surfaces (Schulten and Schnitzer, 1993, Schulten and Schnitzer, 1997). This modeling approach could be criticized because of the huge number of possibilities for combining all SOM compounds and functional groups together into a single macromolecule. These models have been used for molecular mechanics calculation of the interaction of POPs as well as polar organic pollutants with SOM. Results showed that hydrophobic chemicals interact through hydrophobic interactions, e.g. π–π bonds and alignment of aromatic rings, while polar sorbates interact mostly through H-bonds and van der Waals forces (Senesi, 1992). Moreover, trapping in microvoids of the flexible SOM structure was identified as a relevant mechanism for the immobilization of both polar and hydrophobic organic sorbates that is often, though not necessarily combined with additional binding forces (Schulten and Leinweber, 2000). For example, computational modeling confirmed that entrapment in voids of SOM is crucial for the sorption of pharmaceutical antibiotic sulfonamides in soil (Schwarz et al., 2012, Thiele-Bruhn et al., 2004). However, the role of voids does not always match with experimental findings (e.g., Wang and Xing, 2005) and needs further investigation.
Another approach is the description of SOM as a set of SOM functional groups (Aquino et al., 2007, Aquino et al., 2009). Here, a deficiency could arise from modeling of SOM by a small number of functional groups only. In order to overcome such problems, recently Ahmed et al., 2014a, Ahmed et al., 2014b have developed an advanced approach for SOM modeling based on an experimental SOM characterization by different analytical techniques (Ahmed et al., 2012). This model includes a large test set of separate representative systems covering the most relevant functional groups as well as analytically determined compound classes. The validity of this model has been checked by experimental adsorption of hexachlorobenzene on well-characterized soil samples (Ahmed et al., 2014a). In these studies it was stressed that further improvement and testing of this model will be needed, especially in case of explicit solvation by water molecules.
The objective of the present study was to model the sorption of the hydrophobic hexachlorobenzene (HCB) and of the polar pharmaceutical sulfonamide sulfanilamide (SAA) by molecular dynamics simulations to explore general aspects of the interaction of both the hydrophobic and hydrophilic organic pollutants to SOM. This has been accomplished by using the above selected, representative sorptive systems of the previously developed SOM model test set (Ahmed et al., 2014a, Ahmed et al., 2014b), now, for the first time, incorporating the effect of void-restricted interactions and molecular dynamics simulations in the presence of water.
Section snippets
Adsorbates
Two organic compounds from different organic pollutant categories were selected in the current study, i.e. HCB with logKOW of 4.89 and SAA with logKOW of − 0.62. The three-dimensional geometries for both HCB and SAA are given in Fig. 1. Results on soil adsorption of HCB have been recently published (Ahmed et al., 2014a), hence, information on sorption experiments is focused on SAA adsorption. SAA was obtained from Sigma (Taufkirchen, Germany) with purity of ≥ 99.0%.
Soil site and sampling
Three topsoil samples were
Sorption experiments
The long-term different treatments resulted in different organic carbon (OC) contents of the soils increasing in the order “U” < “StmII” < “StmI” (Table 1), which agrees with previous reports on this experiment (e.g., Schmidt et al., 2000). Sorption of the polar sulfonamide SAA to these three soil samples was best-fitted using the Freundlich isotherm (r2 ≥ 0.85; Table 1). Due to the pH of the soil samples, sorption was fully restricted to the neutral SAA molecule (neutral species fraction ≥ 99.99%).
Summarizing discussion
In summary, we have introduced a new SOM model and demonstrated how it can be used to describe the interaction of organic pollutants with SOM. The focus has been on two organic pollutants (HCB and SAA), which differ in their polarity. Experimental evidence was presented which showed that the interaction of SAA with SOM depends more on the chemical composition of SOM than on the SOM content. Moreover, it was confirmed that SAA obeys a site-specific adsorption on the soil surfaces. Furthermore,
Acknowledgment
This project was funded by the Deanship of Scientific Research (DSR) King Abdulaziz University, Jeddah, under Grant No. RG/18/34. In addition, this contribution is considered as a continuation of work that was supported by the Interdisciplinary Faculty (INF), University of Rostock, Germany. Therefore, the authors would like to acknowledge with thanks DSR and INF support for Scientific Research.
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