Visible near-infrared diffuse reflectance spectroscopy and portable X-ray fluorescence spectroscopy for rapid compost analysis

Quantitative and qualitative evaluation of compost is necessary in order to provide consumers with basic knowledge about the product’s composition, and to protect public health and the environment by preventing the spread of contaminated material. Current methods for analysis of basic compost properties give accurate results but are time consuming and require numerous laboratory procedures. This study evaluated the use of visible near-infrared diffuse reflectance spectroscopy (VisNIR DRS) for organic matter (OM) determination and field portable X-ray fluorescence (PXRF) spectroscopy for determination of elemental composition of composted materials. These technologies were evaluated as alternatives to standard laboratory methods for their use in rapid in situ analysis. Thirty-six compost samples from a wide range of feedstocks were gathered and tested with VisNIR DRS and PXRF. For VisNIR DRS, the influence of sample moisture on scanning results was evaluated and the use of raw reflectance, first-derivatives, and second-derivatives of the reflectance spectra were compared. Partial least squares regression (PLS) and principal component regression (PCR) were used to build regression models of VisNIR DRS scans and lab measured OM. For PXRF, the influences of sample moisture, particle size, inter-elemental interactions, and OM on PXRF scanning results were investigated. Results from the VisNIR DRS study produced a promising r2 value of 0.82 and residual prediction deviation (RPD) value of 1.72 for the oven–dried first-derivative PLS model. Results indicate that VisNIR DRS shows great promise as a technique for analysis of OM content of dried compost samples, however further investigation with a larger sample set is necessary before VisNIR DRS can replace laboratory methods. Results of PXRF for elemental analysis were most promising for dried samples and for determining the elements Ca, Cr, Cu, Fe, K, Mn, P, and Zn. Arsenic detection was found to be greatly limited due to the influence of elevated Pb concentrations in the samples. Additionally, sample moisture, particle size, and OM were found to have varying influences on PXRF scan results for different elements. Compost elemental screening and definitive quantification of certain elements via PXRF is recommended by this study