Optical Spectroscopy of Hydrothermally Treated Soil for Organic Matter Monitoring

Abstract

ABSTRACT Hydrothermal treatment is popular in reforming biomass for light-emitting carbon dots for multiple applications, but until now no work has been done on the hydrothermal treatment of soil organic matter (SOM) for this transformation. This work translates the knowledge from the chemical synthesis of carbon dots to the monitoring of SOM by a facile hydrothermal treatment of soils without the usage of toxic chromate chemicals: hydrothermally treated soils produce light-emitting carbon dots whose light absorbance and photoluminescence (PL) was attempted to correlate with the content of SOM. The light absorbance of the carbon dots was found to be correlated very well (R 2 around 0.7) with the SOM by Walkley-Black acid digestion, but the PL was poor in describing the SOM. The absorbance at 250 nm related to the π-π* transition was less affected by pH or but can be reduced by high concentration ammonia. While the absorbance at wavelengths longer than 300 nm was variable, but it still can be used for SOM quantification. The fundamental optical properties are explored via PL and PL excitation spectroscopies to provide an overview of the monitoring of SOM using facile light-absorbance method.