Article pubs.acs.org/JPCA
Influence of Organic Chemicals on Water Molecule Bridges in Soil Organic Matter of a Sapric Histosol Pavel Ondruch, Jiri Kucerik, Zacharias Steinmetz, and Gabriele E. Schaumann* Institute for Environmental Sciences, Workgroup of Environmental and Soil Chemistry, University of Koblenz-Landau, Fortstr. 7, 76829 Landau, Germany S Supporting Information *
ABSTRACT: Water molecules in soil organic matter (SOM) can form clusters bridging neighboring molecular segments (water molecule bridges, WaMBs). WaMBs are hypothesized to enhance the physical entrapment of organic chemicals and to control the rigidity of the SOM supramolecular structure. However, the understanding of WaMBs dynamics in SOM is still limited. We investigated the relation between WaMBs stability and the physicochemical properties of their environment by treating a sapric histosol with various solvents and organic chemicals. On the basis of predictions from molecular modeling, we hypothesized that the stability of WaMBs, measured by differential scanning calorimetry, increases with the decreasing ability of a chemical to interact with water molecules of the WaMBs. The interaction ability between WaMBs and the chemicals was characterized by linear solvation energy relationships. The WaMBs stability in solvent-treated samples was found to decrease with increasing ability of a solvent to undergo H-donor/acceptor interactions. Spiking with an organic chemical stabilized (naphthalene) or destabilized (phenol) the WaMBs. The WaMBs stability and matrix rigidity were generally reduced strongly and quickly when hydrophilic chemicals entered the soil. The physicochemical aging following this destabilization is slow but leads to successive WaMBs stabilization and matrix stiffening.
1. INTRODUCTION Soil organic matter (SOM) is a crucial component of soil that assures most soil functions such as water retention, storage, the availability of plant nutrients, and the immobilization of xenobiotics.1−5 These functions are a result of the supramolecular structure of SOM, which is a heterogeneous and polydisperse mixture of compounds stabilized by intermolecular forces such as hydrogen bonds, hydrophobic interactions, and electrostatic effects.6−8 The result of this enormous diversity of molecules and attractive forces is the formation of microdomains in a matrix heterogeneous on both the molecular scale and the nanoscale. The partially weak and dynamic interactions render the supramolecular SOM structure extraordinarily flexible toward changes in environmental conditions.6,7,9 An important factor for the formation and stiffening of the supramolecular SOM structure is water. It can interact with hydrophilic domains,10 hydrate functional groups of SOM, or be trapped in hydrophobic microvoids.11 Moreover, water can modify the location and conformation of individual SOM segments,12 and growing evidence suggests that clusters of water molecules can bridge molecular segments of SOM,13−17 resulting in a physical stabilization of the supramolecular network. Such water molecule bridges (WaMBs) have also been hypothesized to physically immobilize organic molecules by preventing them from entering or leaving specific regions of the SOM if they are not able to disrupt the WaMBs.15,18,19 The © XXXX American Chemical Society
physical entrapment is expected to be directly linked to the lifetime of a WaMBs. Therefore, it is very important to understand the nature and stability of a WaMBs and how these are affected by environmental conditions, time, and the presence of soil constituents (e.g., cations or organic compounds). The disruption of WaMBs is connected to a sudden increase in the molecular segment mobility of the SOM.20,21 The disruption is referred to as a WaMBs transition because it is a process associated with an abrupt increase in the heat capacity at temperature T* typically occurring at 40−75 °C.22,23 Thereby, T* indicates the stability of the WaMBs.23 By analogy to classical glass transitions, the change in heat capacity (ΔC) during the WaMBs transition can be interpreted as an increase in segment mobility in the SOM matrix during the WaMBs transition. ΔC analyzed over a broad range of different soils showed its relation to the SOM content and quality.24 In contrast, the WaMBs stability, expressed by T*, is not a specific property of the respective organic matter but of the status of the WaMBs, which dynamically reacts to the sample history.20,21,25,26 This can be seen also in tabulated data for both ΔC and T* measured in various soils and can be found here.24 In the mentioned study, T* varied between 51 and 67 Received: October 9, 2016 Revised: February 25, 2017 Published: March 2, 2017 A
DOI: 10.1021/acs.jpca.6b10207 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A °C and ΔC varied between 0.13 and 0.34 J/(gSOM °C), with no clear dependence on soil type, texture, or SOM content. The WaMBs transition can be detected by means of differential scanning calorimetry (DSC), and it is only a partially reversible process that reappears after a certain number of days of SOM storage.19,23,25,27−30 In addition to DSC, NMR spectroscopy and relaxometry can be applied to study WaMBs and their involvement in SOM dynamics. The application of 1H NMR allows us to distinguish mobile water molecules and water involved in WaMBs.23 The application of 1H and 2H NMR confirmed that water can act as a plasticizer on long-chain aliphatic soil components.31 Although the use of NMR techniques provides valuable information about WaMBs, they cannot be used to study WaMBs stability. Therefore, we focused on the use of DSC in this study. Previous studies14,17 revealed that WaMBs formation depends on the local distribution of functional groups.27−29 In addition, the size and stability of the WaMBs and their stiffening effect on the supramolecular SOM structure are expected to depend on the distance between neighboring functional groups and the number of water molecules present close to the functional groups. For example, molecular modeling showed that distances of 1.0−1.3 nm can be bridged by a WaMBs consisting of 14 water molecules and can reduce the energy of the system by 10−20 kcal/mol.13 It can be assumed that a distance-specific number of water molecules are required to form the optimally stabilized WaMBs, which suggests that WaMBs can be destabilized by either an excess or a deficiency of water molecules.29 Also, multivalent cations can form associations between WaMBs and cation bridges, with the consequence of a significant extension of the distance, which can be bridged.23,28 For example, 30 water molecules can lead to Al3+-WaMBs clusters bridging distances of up to 3 nm.29 In addition, environments formed by polar solvents were predicted to reduce the stability of WaMBs, whereas nonpolar solvents were expected to stabilize them.14 However, this prediction from modeling has up to now not been verified experimentally for real soil samples. It will be of high importance for our understanding of the nature and rigidity of WaMBs-stabilized SOM systems. In this study, we aimed to understand the relation between WaMBs properties and the physicochemical properties of their environment. We hypothesized that the addition of compounds to SOM, which have the ability to interact with water molecules, will destabilize the WaMBs and thus reduce the WaMBs transition temperature.32 We especially expected that molecules being able to interact via H-donor/acceptor interactions will destabilize WaMBs whereas apolar molecules will increase the WaMBs stability as compared to the untreated state. Furthermore, we hypothesized that the extent to which the mobility of the molecular segments is reduced by the WaMBs, is directly determined by the WaMBs stability. Along with this hypothesis, the change in heat capacity (ΔC) during the WaMBs transition is expected to be influenced in a similar manner and by the same interaction parameters as T*. To test these hypotheses, we spiked a sapric histosol with either naphthalene or phenol using up to nine solvents varying in type and degree of intermolecular interactions on the basis of linear solvation energy relationships (LSERs).33,34 By using pure solvents and solvents containing naphthalene or phenol as the spiking chemical, we aimed at distinguishing the influences of a short contact time with the solvents from influences
resulting from the lasting presence of a chemical being able to undergo H-donor/acceptor interactions to a greater (phenol) or limited extent (naphthalene). To analyze the long-term impact of solvent treatment on WaMBs, we investigated the WaMBs thermal properties in selected samples after aging for 30 days, and their time development was monitored for selected solvents during a period of 89 days.
2. MATERIALS AND METHODS 2.1. Materials. A sapric histosol collected from Totes Moor (Fuhrberg, Germany) served as an organic, clay-free model soil for the investigation of WaMBs stability in SOM without interference by SOM−mineral interactions. After sampling, the soil was air-dried to constant weight and homogenized. Crusts and aggregates were destroyed using a spatula and were sieved to obtain particles
