Predicting soil arsenic pools by visible near infrared diffuse reflectance spectroscopy

Rapid and cost-effective analysis of soil solid As phases would be an invaluable tool in studying polluted soils and predicting soil As mobility. Analysis of soil solid As phases has commonly used sequential extraction; however, the approach is time consuming, destructive, and costly. Several studies have established the viability of using visible near infrared diffuse reflectance spectroscopy (VisNIR DRS) for elemental data analysis of soil, sediment, and other matrices. This pilot study used VisNIR DRS spectral data for rapidly predicting total As and five different solid As phases (Mg, PO₄, Ox, HCl and org pools). A total of 200 surface soil (0–15 cm) samples were collected from arable lands surrounding a polluted landfill site and scanned via VisNIR DRS. The raw reflectance spectra were preprocessed using three spectral transformations for predicting soil total As and five extracted pools using partial least squares regression (PLSR). Quantitatively, better accuracy was produced by PO₄ (Validation R² = 0.72, RPIQ = 3.39) and org (Validation R² = 0.93, RPIQ = 4.81) pools along with total As (Validation R² = 0.88, RPIQ = 3.54) using the first derivative of original reflectance values. Both qualitative spectral analysis and PLSR coefficients indicated that prediction of soil As and its phases were dependent on their close association with spectrally active soil organic matter, clay minerals and Fe/Al-oxides.