Sorption of natural organic matter to carbon nanotubes

Over the past decade, carbon nanotubes (CNTs) have gained increasing attention due to their unique properties, including strong sorption affinity toward organic contaminants. The production of CNTs has been increasing, making the materials release in the aquatic environment inevitable. Interactions with natural organic matter (NOM) are known to affect the fate and behavior of CNTs. First, NOM can significantly disperse CNT suspensions. A link between stabilization and sorption of NOM to CNTs was often proposed to explain discrepancies observed for different types of NOM and CNTs. However, studies investigating both processes are scarce and the link was never tested systematically. Studies focusing on the sorption of NOM to CNTs generated contradictory results and the sorption mechanisms involved, as well as the impacts of NOM and CNTs properties, remain poorly understood. We believe that accounting for the heterogeneity of NOM is necessary to unravel the mechanisms linking the processes of sorption and dispersion. Second, NOM can greatly decrease the exceptional sorption potential that CNTs exhibit towards organic contaminants. Competition and pore blockage mechanisms are generally proposed. Until recently, the effects of NOM on CNTs dispersion and sorption potential were studied separately, providing only a partial evaluation of the interactions involved. Preliminary work considered the two processes simultaneously and demonstrated that dispersion has major effects on the sorption potential of CNTs and that the effect of NOM greatly depends on the dispersion status. The links between NOM sorption to CNTs and consequences on CNTs dispersion and sorption potential remain however poorly studied and understood. With the purpose of filling this knowledge gap, we propose to systematically study the process of sorption of NOM to CNTs by accounting for the preferential sorption of NOM fractions to CNTs. The resulting NOM fractionation will be characterised (both in terms of chemistry and size) for a series of NOM and CNTs covering a range of properties. Comprehensive data on sorption will be combined to an extensive characterisation of CNTs dispersion and sorption potential for model organic contaminants. The project will provide a mechanistic and quantitative understanding of the interactions involved in the three-phase system, and allow linking processes that were up to now, studied individually. The overall aim is to elucidate the mechanisms of interactions between NOM and CNTs, and assess the consequences in terms of dispersion and sorption behavior towards organic contaminants. The main hypothesis is that fractionation of NOM occurs upon sorption to CNTs. Specific objectives are to systematically evaluate (i) the influence of NOM features (chemical and molecular weight) on its sorption to CNTs, (ii) the influence of CNTs characteristics (dispersion and surface chemistry) on sorption of NOM and (iii) the consequences on the CNTs sorption behavior towards organic contaminants.

In the project 'Sorption of carbonaceous materials to carbon nanotubes' we analyzed interactions between natural and anthropogenic compounds that are abundant in natural surface waters and soils - and consequently come into contact with each other. One group of these compounds are carbonaceous materials. Carbonaceous materials are an encompassing term for carbon-based materials of different origins and applications; these include chars, soot, or graphite. Due to the variety of input sources, carbonaceous materials are virtually omnipresent in the environment. Once they are introduced to surface waters they will inevitable come into contact with natural organic matter. Natural organic matter is a complex mixture of compounds originating mainly from the decomposition of plants and waste products of animals, ubiquitous in both aquatic and terrestrial systems. When studying the interactions between carbonaceous materials and natural organic matter the main challenge comes from their very nature: both are complex in structure, composition, and properties. So far, it was shown that natural organic matter does not attach uniformly to carbonaceous materials. It was established that there are preferentially adsorbed fractions that are fairly large and generally have aromatic character. But, this study revealed that sorption is governed by much more intricate combination of the molecule size and its chemical properties. This investigation also demonstrated the importance of the surface properties of the carbonaceous materials on interactions with other compounds present in the environment. The study analyzed interactions of soot - a carbonaceous material distributed via airborne transport - with organic contaminants. During the atmospheric transport, soot can undergo transformations that impact its surface properties. It was shown that these transformations can have strong implications on the behavior of these materials once they are deposited in surface waters, including their adsorption affinity for organic contaminants. Carbonaceous materials usually strongly bind organic contaminants, affecting their transport and fate. However, here it was shown that upon transformation of soot during its atmospheric transport its ability to sorb organic contaminants decreases. Furthermore, the investigation demonstrated that the adsorbed contaminants can influence the aggregation of the carbonaceous materials. Organic contaminants make particles like biochar more prone to aggregation; together, they form larger clusters. This is an important finding: accumulated biochar particles reduce the risk of uncontrolled transport of the pollutant in the environment, after it is bound to the biochar particles. This project gave powerful insight into interactions of carbonaceous materials with natural organic matter and contaminants in the environment. Results can be applied to real-life problems and solutions, such as crucial contaminant control.