Mineralogical characterization of tobacco stalk ash

The biomass ash generated from the smoldering combustion of tobacco stalk (stem, petiole, and midrib), during the production of "tuibur," a "smokeless" tobacco product of Northeast India (NEI), is dispersed in soil. The tobacco stalk ash (TSA) was examined. Despite a common mineralogy, owing to smoldering combustion, TSA has exhibited different microstructures and chemical compositions. Widespread deposition of fine to ultrafine aggregates of ash over the inorganic mineral crystallites was observed. At the end of smoldering combustion, none of the volatile gases were released during the crystallization as manifested by smooth-faced nanoscale inorganic mineral crystallites. The presence of calcite, potassium chloride, potassium aluminum silicate, magnesian calcite, and calcium silicate hydrate as sub-micron scale crystallites in TSA is consistent with the high calcium content and lower silicon content of tobacco stem/midrib. The poorly resolved signals with broadened diffraction peaks and the lower peak intensities reflect the nanoscale attributes and poorly crystallized silicate mineral phases of TSA. Two polymorphs of calcium carbonate, i.e., calcite and aragonite, were observed. The concentrations of potential hazardous elements (PHEs) including Cd (400 ± 10 μg/kg), Cr (10.1 ± 0.05 mg/kg), Pb (2.3 ± 0.12 mg/kg), and Ni (16.8 ± 0.80 mg/kg) in the bulk of ash were determined. Thus, the disposal of TSA can cause the contamination of PHEs and toxic organic pollutants on the soil and in groundwater, posing negative environmental risks.