Quantification of Carbon Nanotubes in the Environmental Matrices by Using a Microwave Induced Heating Method

Carbon nanotubes (CNTs) are molecular-scale tubes of graphitic carbon with outstanding electronic, mechanical and thermal properties. The numerous industrial and commercial applications of CNTs are driving the growth of production of the nanotubes. As such, release of CNTs into the environment inevitably occurs. Therefore, quantification of CNTs in environmental matrices becomes critical to answer research questions regarding their environmental fate/transport and potential risks to environment and human health. However, most current analytical methods have some limitations on the environmental determination of CNTs. This dissertation aims to systematically explore the potential of an innovative microwave induced heating system for quantification of different types of CNTs in various environmental media. First, a microwave induced heating apparatus was established in our lab, connected with a custom-built temperature acquisition and control system. At a given microwave condition, a linear relationship between microwave induced temperature rises and mass of well-characterized CNTs (single-walled CNTs, multi-walled CNTs or carboxylated multi-walled CNTs) was successfully generated in quartz sand, agricultural soil and wastewater treatment plant sludge, respectively. Moreover, our results indicated that the presence of inorganic carbon, organic carbon and other carbon-based nanomaterials (fullerene, activated carbon or graphene oxide) in environmental samples did not affect the microwave measurements of CNTs when the microwave energy was high enough. The detection limits of the microwave method were determined for all three types of CNTs in different environmental matrices.Secondly, the impacts of heterogeneity of multi-walled CNTs (length, diameter and aggregation) on the microwave responses of multi-walled CNTs were investigated. We found that the microwave performances of multi-walled CNTs were independent of the length and diameter of the nanotubes, whereas the aggregated multi-walled CNTs were not able to convert the microwave energy to heat. To apply the microwave method to the aggregated multi-walled CNTs, a two-step pre-treatment process, involving elevated temperature exposure and acetone-based surfactant assisted dispersion, was proposed to suspend the agglomerated multi-walled CNTs that have been embedded in the environment before microwave quantification. With the suggested pre-treatment process, the microwave method could effectively quantify the aggregated multi-walled CNTs in environmental matrices. Thirdly, the simultaneous quantification of individual type of CNTs in environmental samples containing several types of CNTs was studied using Chemometrics based multivariate calibration methods. The environmental samples with two or three types of CNTs were exposed to varied microwave conditions to generate microwave induced temperature rises spectra. The relationship between the temperature rise spectrum and individual mass of CNTs was explored by such statistical tools as partial least square regression, least square-support vector machine and artificial neutral network. The performance and prediction ability of fitted models were evaluated in terms of R2, root mean square error of cross validation and root mean square error of prediction. Among three multivariate calibration approaches, the developed least square-support vector machine model showed best fitness and prediction results.Overall, this dissertation provides demonstration and insights for the development of a viable and low-cost quantification method for CNTs in environmental matrices, which could be beneficial for researchers and scientists in all CNTs-related studies.