Functions of natural multicomponent organic mixtures as emergent properties influenced by environmental contaminants
Date
2019-03-20
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Abstract
The primary focus of this study concerns the functions of natural multicomponent organic mixtures as emergent properties influenced by environmental contaminants. Two types of natural multicomponent organic systems were studied: biosolids and humic substances. The investigation of the mixtures interactions of the mixtures with environmental pollutants was performed both by experimental means and computational modelling. The current concern of possible soil pollution via application of municipal biosolids as fertilizers on agricultural land led us to investigate biosolids’ interaction with one of the most widely-spread nanoparticle contaminants present in wastewaters – ZnO NPs. Metal complexation by typical biosolids was explored using methods from the humics literature. Uptake of Zn from NPs in the biosolids was evaluated. Finally, the kinetics of release of Zn species are reported as a function of (i) pH and (ii) the presence of strongly binding ligands (e.g., ion-exchange resin promoting release). The investigation revealed that (i) metal binding sites of biosolids are analogs of humic substances, (ii) ZnO NPs do not survive in the digestion environment, and (iii) any ZnO NPs dissolve to aqueous Zn2+ in < 10 d. Kinetics of Zn in biosolids revealed that Zn release is a function of biosolid protonation. At pH 8, Zn is retained in the biosolids, whereas at pH 4.5, 10% of Zn is released from the biosolids. Adding a chelating resin to the system at pH 5.0 led to Zn release from the biosolids as per Noyes–Whitney kinetics, releasing 85% of the bound Zn in 360 h. Fifteen percent of Zn appeared to be irreversibly bound. These results encouraged us to investigate processes occurring at the molecular level. For this purpose, an approach that perceived natural multicomponent organic mixtures as labile aggregates with emergent functions, was used. The first step in this direction was the generation of computational models for two fractions of the well-studied and characterized natural multicomponent organic mixture, HS, Suwanee River Fulvic and Humic acids. Computational modeling was used to help understand processes and mechanisms on the molecular scale that occur in different fractions of HS, fulvic acids and humic acids, as they interact with organic pollutants. The importance of non-covalent interactions in the emergent functions of HS is highlighted. H-bonding, hydrophilic/hydrophobic surface areas and π-stacking interactions play a significant role in aggregation processes as well as in the sorption of environmental pollutants. In a highly hydrophilic system with small molecules (the SRFA-22 model) H-bonding is the main force that drives the aggregation process. On the other hand, in a highly aromatic and hydrophobic model with larger molecular fragments (SRHA-6) hydrophobic and π-stacking interactions dominate in the aggregation process. The chemical properties of contaminants significantly affect their mechanism of sorption by HS. The interaction of a polar pollutant, phenol, with HS occurs via H-bonding, whereas non-polar benzene interacts via hydrophobic and π-stacking interactions, resulting in much stronger sorption by HS and causing an additional structural rearrangement of the aggregates, making them more stable in the environment. The formation of inner/outer sphere metal-ligand complexes, metal binding sites, complex configurations, binding energies and aggregation/dissolution as emergent properties of HS were determined under various conditions in Chapter IV of the work. The results of computational modeling revealed that (i) the highest Cu2+ binding (55.6%) was by the SRFA-22 organic model, which represents low molecular weight fulvic acids. On the contrary, the highest amount of inner-sphere Mg-organic matter complexes (63.4%) was formed in SRHA-6 which has higher molecular weight constituents. Therefore, a correlation between the type of cation, system aromaticity and the extent of metal complexation is proposed. (ii) The increase of metal-ion concentration and/or decrease of water content resulted in an increase in the number of hydrogen bonds and more compact and stable aggregates with lower hydrophilic and higher hydrophobic surface areas in SRFA-22. However, in SRHA-6 the results varied due to the competition between metal binding, H-bonding and non-polar interactions in the structural arrangement of the aggregates. In general, the aggregation process, driven by metal complexation and water removal, resulted in the formation of more stable conformers, with lower potential energy, with the sole exception of SRHA-6-Cu. Finally, based on the obtained results, future research on expanding the modeling of emergent functions of HS influenced by environmental contaminants to the biosolids mixtures was proposed.
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biosolids, humic acid, fulvic acid, environmental contaminants, computational modeling, emergent properties
Citation
Vialykh, E. (2019). Functions of natural multicomponent organic mixtures as emergent properties influenced by environmental contaminants (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.