Abstract
Abstract. An approach for nanoscale 3-D tomography of organic carbon (OC) and associated mineral nanoparticles was developed to illustrate their spatial distribution and boundary interplay, using synchrotron-based transmission X-ray microscopy (TXM). The proposed 3-D tomography technique was first applied to in situ observation of a laboratory-made consortium of black carbon (BC) and nanomineral (TiO2, 15 nm), and its performance was evaluated using dual-scan (absorption contrast and phase contrast) modes. This novel tool was then successfully applied to a natural OC–mineral consortium from mountain soil at a spatial resolution of 60 nm, showing the fine structure and boundary of OC, the distribution of abundant nano-sized minerals, and the 3-D organo-mineral association in situ. The stabilization of 3500-year-old natural OC was mainly attributed to the physical protection of nano-sized iron (Fe)-containing minerals (Fe oxyhydroxides including ferrihydrite, goethite, and lepidocrocite), and the strong organo-mineral complexation. In situ evidence revealed an abundance of mineral nanoparticles, in dense thin layers or nano-aggregates/clusters, instead of crystalline clay-sized minerals on or near OC surfaces. The key working minerals for C stabilization were reactive short-range-order (SRO) mineral nanoparticles and poorly crystalline submicron-sized clay minerals. Spectroscopic analyses demonstrated that the studied OC was not merely in crisscross co-localization with reactive SRO minerals; there could be a significant degree of binding between OC and the minerals. The ubiquity and abundance of mineral nanoparticles on the OC surface, and their heterogeneity in the natural environment may have been severely underestimated by traditional research approaches. Our in situ description of organo-mineral interplay at the nanoscale provides direct evidence to substantiate the importance of mineral physical protection for the long-term stabilization of OC. This high-resolution 3-D tomography approach is a promising tool for generating new insight into the interior 3-D structure of micro-aggregates, the in situ interplay between OC and minerals, and the fate of mineral nanoparticles (including heavy metals) in natural environments.
Highlights
Three main mechanisms for soil organic carbon (SOC) stabilization have been proposed: (1) chemical stabilization as a result of chemical or physiochemical binding between SOC and soil minerals, namely organo-mineral complexation; (2) physical protection, which occurs predominantly at the microaggregate level and is built on top of the chemical organo-mineral complexation; and (3) biochemical stabilization in the form of recalcitrant SOC compounds (Six et al, 2002)
Figure 3. 3-D tomography illustration of laboratory-made black carbon (BC) and the mineral nanoparticle consortium observed at −45◦ (a, d), 0◦ (b, e), and +45◦ (c, f) azimuthal viewing angles under absorption contrast (a, b, c) and phase contrast mode (d, e, f)
In summary we have developed a high-resolution 3-D tomography approach using dual-scan modes and successfully applied it to study the in situ interplay of OC and minerals in a laboratory-made and natural OC–mineral consortium at the nanoscale
Summary
Three main mechanisms for soil organic carbon (SOC) stabilization have been proposed: (1) chemical stabilization as a result of chemical or physiochemical binding between SOC and soil minerals (especially clay and silt in current opinions), namely organo-mineral complexation; (2) physical protection, which occurs predominantly at the microaggregate level and is built on top of the chemical organo-mineral complexation; and (3) biochemical stabilization in the form of recalcitrant SOC compounds (Six et al, 2002). High-resolution synchrotron-based TXM has been demonstrated as a powerful tool for understanding the internal 3-D structure of particles down to nanometer scale, due to its large penetration depth and superior spatial resolution (Kuo et al, 2011; Wang et al, 2015). This technique has been successfully applied to reveal the discrete threedimensional micro-aggregation structure of clay (kaolinite) in natural aqueous environment and generated remarkable tomography that revealed precise inter-particle structure (Zbik et al, 2008). Clay particles with a diameter below 500 nm were clearly visible and their pseudohexagonal symmetry was recognized in three-dimensional details
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