Fate and transport of the engineered nanomaterials (ENMs) in aquatic systems has been increasingly concerned due to their potential human exposure and reported toxicity to the living organisms. When released into the water systems, the universal existence of naturally occurring colloids (NOC) and dissolved natural organic matter (NOM) will greatly alter the size and surface properties of ENMs depending on the solution chemistry, resulting in changes of their toxicity and bioavailability.
In our lab, we investigate the fate and transport of carboxylated multi-walled carbon nanotubes (COOH-MWCNT) in aquatic systems in the presence of a common clay, kaolinite, and aim to answer the following scientific questions:
1) How do kaolinite and various environmental compositions (solution pH, ionic strength, Ca2+ and NOM) impact the stability of CNTs in aqueous phase?
2) How do kaolinite, solution ionic strength and pH affect the subsurface transport of CNT in saturated porous media?
3) How does morphological information of particles help explain the mechanisms that cause the differences in the stability and mobility between CNT-only and CNT-kaolinite systems?
To answer these questions, we performed sedimentation experiments and column tests to simulate surface water and subsurface porous media environment, respectively. We also applied flow-cytometry for the morphological distribution information of particles. Results showed that COOH-MWCNT can aggregate with kaolinite and form both primary and secondary heteroaggregates with kaolinite depending on the solution chemistry and CNT: kaolinite ratio; The presence of kaolinite has important impact on the transport of COOH-MWCNTs through the formation of heteroaggregates, as well as competition for and blockage of attachment sites on mineral surfaces; And flow cytometer can detect from extremely diluted mixture samples the presence and extent of AFCNT homoaggregation and its heteroaggregation with kaolinite. Findings in this study provide fundamental understanding of stability and mobility of CNT in natural aquatic systems and proposed flow cytometry as an efficient tool in analyzing CNT-NOC heteroaggregations. They enable better environmental risk assessment and management of ENMs.