The Department of Defense (DoD) is responsible for managing all of their sites with contaminated sediments. A diverse suite of organic contaminants exhibiting a wide range of chemical properties, toxicities, and concentrations are present at these sites. These include hydrophilic compounds such as munitions constituents (e.g., TNT and RDX) and their metabolites (e.g., ADNTs, DANTs and TNX), as well as hydrophobic compounds including pollutant classes like dioxins and PCBs. Accurately determining the level of these contaminants, especially in the freely-dissolved sediment pore water phase, is critical to the effective management of these sites but not a trivial task given system complexity. Although passive sampling devices are relied upon to assist in such measurements, there are critical gaps in existing devices that limit their applicability to the mixtures of contaminants most relevant to DoD’s interests.

The principal objective of this project was to develop new and innovative nanomaterials to overcome traditional hurdles in existing passive sampling devices (e.g., multi-target sampler for chemically diverse compounds), thereby improving the DoD’s ability to characterize the distribution and concentrations of pollutants at their contaminated sites. 

Using electrospinning, researchers synthetized seven electrospun nanofiber mats (ENMs) from polymers [ethylene-vinyl-acetate (EVA), polyacrylonitrile (PAN), polyethylene terephthalate (PET), polymethylmethacrylate (PMMA), polystyrene (PS), polyvinyl acetate (PVAc), and polyvinylidene fluoride (PVDF)] and investigated their performance as next-generation passive sampling materials. Given the ease of material fabrication provided by electrospinning, researchers were able to tune the physical and chemical properties of polymeric ENMs to produce robust materials with improved selectivity and enhanced sorption capacities toward several DoD relevant target pollutants. ENM performance was tested in homogeneous (i.e., single phase) aqueous systems as well as heterogeneous (i.e., dual phase) systems containing model soils against targets including aniline and nitrobenzene (as models for TNT and RDX) and a selection of ten PCB congeners and dioxin. These chemical targets were selected because of their wide range of physical-chemical properties (e.g., log Kow values spanning 8 orders of magnitude) and relevance as polar munitions constituents and hydrophobic compounds commonly encountered at contaminated DoD sites. 

Generally, aqueous uptake experiments with ENMs revealed very fast rates of partitioning with equilibration times less than 1 d. Equilibrium partition coefficients (L/kg) for ENMs ranged from 0.72 to 2.8 log units for aniline and nitrobenzene, with evidence suggesting uptake via partitioning into the bulk nanofiber (i.e., absorption) and specific binding interactions (e.g., hydrogen-bonding and Coulombic interactions) contributing to, and potentially controlling, polar target uptake. PCBs and dioxin also exhibited very fast equilibrium uptake (achieved in < 18 h), with equilibrium partition coefficients for ENMs ranging from 3.2 to 6.4 log units. Collectively, the rates and partition coefficients measured for the best performing ENMs often exceeded partition coefficients achieved with commercially available passive sampling materials (e.g., low-density polyethylene and PDMS glass fiber), particularly for polar analytes. Across a range of experimental conditions (e.g., variable pH, analyte concentration, and complex analyte mixtures), little change in ENM performance was observed. Researchers also found promising performance in heterogeneous systems with model soils, where the optimal ENM, polystyrene, not only yielded reproducible measurement of nitrobenzene pore water concentration, but also allowed for greater ease of handling by minimizing unwanted polymer-soil organic matter interactions. Building upon these promising results, further efforts improved performance through fabrication of novel polymer composites and surface-chemical functionalized ENMs. For example, it was demonstrated that carbon nanotubes functionalized with carboxylic acid functional groups (which are deprotonated at pH 7) or the inclusion of anionic surfactants could be used to promote uptake of aniline (by as much as 1 log unit in partition coefficient), a fraction of which is positively charged under our experimental conditions. Further, integration of silver nanoparticles with select ENMs could be used to impart biocidal activity, thereby slowing biofouling during application. Researchers also fabricated novel multilayer ENMs, in which layer-by-layer combinations of different polymers impart multi-target capabilities (e.g., simultaneous uptake of polar and hydrophobic species) and greater ease of application and handling in complex media (e.g., protective surface layers that limit fouling). 

The results supported the initial hypothesis that electrospun nanofiber mats represent next-generation passive sampling materials that can be easily modified to enhance compound selectivity, sorption capacities, and improve field applications. Ultimately, the results of this project will serve to catalyze the production of innovative nanoenabled materials with the potential to expand the use and increase the reliability and performance of passive sampling devices. This may in turn lead to improved site characterization, where the small ENM material footprint should allow better spatial resolution of data and their fast rates of uptake should enable better temporal resolution of data. Further, there is great potential for the rapid scale up and transition of this technology to the commercial marketplace because electrospinning is already an industrially viable fabrication process for non-woven polymers. Ultimately, these outcomes also pave the way for future research that will examine a broader suite of DoD relevant chemicals, aim to further enhance ENM capacity, selectivity, and functionality, and scale-up and prototype an ENM-based passive sampler for field deployment and testing.