Investigation of manufactured carbon in the environment

Abstract

The stability, strength, and good electrical conductivity of graphite raise interest in designing materials with graphite-like structures. Biochar and carbon nanotubes (CNTs) are two commonly used materials with a graphite-like structure. Methods for their quantification in environmental samples are needed to assess exposure risk and perform full life cycle assessments. Use of biochar in agriculture and carbon sequestration raises the need for its quantification in soil. Difficulty lies in the presence of natural soil organic matter. In order to differentiate the two species, thermogravimetric analysis (TGA), chemical oxidation with total organic carbon analysis (TOC), and nuclear magnetic resonance (NMR) were applied to analyze two soil samples, two biochar samples, and their mixtures. Results suggest that 13C cross polarization/ magic angle spinning NMR (13C CP/MAS NMR) was the most effective quantification method of the three investigated. Although a detection limit was not determined, biochar was detected in soil at 0.5 wt% using NMR. The high analysis cost of NMR likely will limit its widespread application. The potential ecological impact of CNTs calls for reliable nanometrology that is applicable to ng/L concentrations. Single particle inductively coupled plasma mass spectrometry (spICP-MS) was shown to have the needed elemental specificity and low detection limit. By monitoring intercalated catalyst residual metals in CNTs, micro-second dwell time spICP-MS (s-spICP-MS) was successful for mass quantification and particle number counting. The method was applied to track CNT released from polymer nanocomposites under UV radiation. The proportionally lower release of CNT as compared to polymer mass loss indicated an accumulation of CNTs in the residual polymer. The s-spICP-MS was also applied to biological organisms (D. magna) and sediment water extracts. CNTs associated with Daphnia magna were measured at aqueous exposure concentrations in the low g/L concentration range. Association of CNTs with D. magna was influenced by the presence of a surfactant. Analysis of 3 different SWCNTs spiked into sediments at low mg/kg range confirmed the detection capability of spICP-MS, even in complex environmental samples. While there is room for improvement in the s-spICP-MS methodology, it appears to be a promising approach to examine CNT environmental fate.