Elemental analysis of biochar-based fertilizers via portable X-ray fluorescence spectrometry

Abstract

A complete elemental characterization of biochar samples is needed prior to their use for agronomic and/or environmental purposes. Elemental analyses of biochar are still a challenge mainly due to some difficulties to achieve a total digestion of the samples for clearly identifying and quantifying elements of interest. The hypothesis of this study is that X-ray fluorescence spectrometry will be a useful, fast, and accurate alternative method to obtain the total elemental composition of biochars. Thus, this work aimed to evaluate the performance of pXRF for the determination of P, K, Ca, Mg, S, Na, Al, Fe, Mn, Zn, and Cu in 31 biochar-based fertilizers, comparing these results with the standard method of biochar samples digestion (modified dry-ashing method — MDA). Moreover, the biochar samples were also characterized via wavelength dispersive X-ray fluorescence (WDXRF) for comparison with portable X-ray fluorescence (pXRF) spectrometry. For all elements and biochar samples, adequate linear regression adjustments between methods were achieved. For the lightest elements (P, Ca, Mg, S, Na, and Al) the concentrations obtained via MDA were higher followed by WDXRF and pXRF. Conversely, for the heaviest elements (Fe, Mn, Zn, and Cu), XRF results (pXRF and WDXRF) were higher than MDA. Significant linear regressions between methods were obtained, mainly for plant micronutrients (Fe, Mn, Zn, and Cu), suggesting that standard-curves can be obtained in order to develop pXRF applications like a “Biochar Mode” by the manufacturers. Portable X-ray fluorescence spectrometry showed encouraging results allowing a rapid and environmentally-friendly characterization of biochar-based fertilizers.