(209f) Life Cycle Ecotoxicity of Carbon Nanotubes

Carbon nanotubes (CNTs) comprise a promising and versatile class of materials with many potential environmental applications but with largely unknown environmental implications, with toxicity being the major concern driving research efforts in this area.  Much current work tests the behavior of CNTs under various environmental conditions, particularly their end-of-life fate and transport in aquatic environments.  Also important but less visible is work characterizing the impacts of CNT production using various synthesis methods, usually in terms of energy and material use.  Taking a life cycle approach, this project quantifies and compares the inherent environmental impacts of CNTs released into the environment with those related to their synthesis.  This was done using the single measure of aquatic ecotoxicity, which is the toxicity category most strongly supported by existing data.

As an emerging technology, CNTs are not included in any established life cycle assessment models.  Complicating the picture is the fact that CNT toxicity varies along multiple parameters, including size distribution, chirality, and functionalization, making it very difficult to establish a single toxicity factor for CNTs as a class.  Using published toxicity studies for single species from different trophic levels, we employed the USEtox model to develop a range of appropriate ecotoxicity characterization factors of approximately 1-10 mg/L (HC₅₀). USEtox is an integrated multi-media fate, transport, and toxicity model covering large classes of organic and inorganic substances.  Two distinct release, fate and transport scenarios were considered: a highly conservative “worst case” scenario and a “realistic” scenario that draws from exiting literature on CNT fate, transport, and ecotoxicity.  In terms of the production phase, a life cycle inventory of inputs and emissions was constructed by combining measured data for three different synthesis techniques (HiPco, CVD, and ablation) with the database of characterization factors provided with the USEtox model.  Current manufacturing trends for reagent recovery and reuse and pollution controls were taken into account. 

The results indicate that the ecotoxicity impacts of nanomaterial production processes are roughly equivalent to the ecotoxicity of CNT releases under the unrealistic worst case scenario, while exceeding the results of the realistic scenario by three orders of magnitude.  Ecotoxicity from production processes is dominated by emissions of metals from electricity generation.  Uncertainty exists for both production and release stages, and is modeled using a combination of Monte Carlo simulation and scenario analysis.  The results of this analysis underscore the contributions of existing work on CNT fate and transport, as well as the importance of life cycle considerations in allocating time and resources toward research on mitigating the impacts of novel materials, in particular green chemistry efforts that reduce production ecotoxicity by using benign reagents or by increasing yields.