Tropical soils characterization at low cost and time using portable X-ray fluorescence spectrometer (pXRF): Effects of different sample preparation methods

Abstract

Portable X-ray fluorescence spectrometer (pXRF) has been recently adopted by the Soil Science community for uses in both field and laboratory, obtaining the total content of several chemical elements in a few seconds. Sulfuric acid digestion is an expensive and time-consuming laboratory analysis that provides contents of Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 and P 2 O 5 , important for soil studies. Due to few pXRF studies in tropical soils, this work aimed to compare contents of Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 and P 2 O 5 obtained by pXRF with sulfuric acid digestion results, and to evaluate the effects of varying forms of preparing soil samples and scanning with pXRF on the resulting values in Brazilian soils. Soils were scanned in five conditions in-field (in situ) and in laboratory, evaluating varying sample preparation methods, particle sizes and soil moisture. Four pXRF scanning operational modes were tested. Linear regressions were adjusted between results of pXRF and sulfuric acid digestion. Equations were validated with an independent set of samples. Statistical analyses compared the methods of preparing the samples. Adequate linear models reached R 2 of 0.99 and 0.89 for Fe 2 O 3 and TiO 2 , respectively. Validation promoted R 2 greater than 0.97 and RMSE and ME close to zero for both oxides. Statistical differences of pXRF results were found among the methods of preparing samples. pXRF spectrometer has great potential to obtain Fe 2 O 3 and TiO 2 content rapidly and economically with high correspondence with laboratory results of sulfuric acid digestion analysis. Varying methods of preparing the samples promote differences in the results of pXRF.Index terms: Soil chemical properties; proximal sensor; sulfuric acid digestion analysis; soil properties prediction; oxides. RESUMOO espectrômetro portátil de fluorescência de raios-X (pXRF) foi recentemente adotado pela Ciência do Solo, para uso em campo e laboratório, para obtenção do conteúdo total de vários elementos químicos em poucos segundos. A digestão com ácido sulfúrico é uma análise laboratorial cara e demorada que fornece teores de Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 e P 2 O 5 , importantes para estudos sobre solos. Devido aos poucos estudos sobre o pXRF em solos tropicais, este trabalho objetivou comparar os teores de Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 e P 2 O 5 obtidos pelo pXRF com os resultados de digestão com ácido sulfúrico e avaliar os efeitos de diferentes formas de preparo de amostras de solo e leitura com o pXRF sobre seus resultados para solos brasileiros. Os solos foram submetidos a leituras com o pXRF em cinco condições, em campo (in situ) e em laboratório, avaliando variados métodos de preparo de amostras, tamanhos de partículas e umidade do solo. Quatro modos de operação do pXRF foram testados. Regressões lineares foram ajustadas entre os resultados do pXRF e digestão com ácido sulfúrico. As equações foram validadas com um conjunto independente de amostras. Análises estatísticas compararam os métodos de leitura de amostra...

Tropical Soil Toposequence Characterization via pXRF Spectrometry

“…The correlations between elemental contents obtained by pXRF and soil properties provide new insights of relationships (positive or negative correlation) previously less explored, mainly in quartzite‐derived soils, such as: Rb, Zn, K 2 O with silt content; Sr and Ti with soil fertility properties; Zn with fine sand content; and Zr and clay content. Others already reported in Brazilian soils (Silva et al, 2017, 2018b; Teixeira et al, 2018) include: CaO and exchangeable Ca 2+ , Mg 2+ , Al 3+ , pH, and other soil properties; SiO 2 and sand (positive) and clay (negative) content; and Al 2 O 3 with clay content. The relationships between the elemental contents obtained by pXRF may contribute to studies focusing on the understanding of the geochemical behavior of some elements, such as Ca with Sr, and K and Zn with Rb.…”

“…Thus, this equipment allows for fast, low cost, environmentally friendly, and in‐field identification and quantification of several elements from the periodic table (Weindorf et al, 2014; Stockmann et al, 2016; Ribeiro et al, 2017). It has proven useful for investigations related to soil genesis and horizon variability (Stockmann et al, 2016; Weindorf et al, 2012a, 2012b; Silva et al, 2018a, Sharma et al, 2014, 2015; Zhu et al, 2011), parent material spatial variability (Mancini et al, 2019), soil property prediction and mapping (Silva et al, 2017; Duda et al, 2017; Pelegrino et al, 2018), and for other pedological and environmental studies (Rouillon and Taylor, 2016; Silva et al, 2018b; Suh et al, 2016).…”

Atomic Spectrometry Update – a review of advances in environmental analysis

“…In a review (49 references) on the use of pXRFS for the characterisation of tropical soils, attention was drawn 220 to the various factors that can influence results such as soil moisture content, particle size distributions, spectral interferences, scanning through plastic bags used for sampling and the scanned area and penetration depth of X-rays. Some of these factors were further evaluated 221 in a follow-up study from the same research group. Whereas results for Fe 2 O 3 and TiO 2 correlated with those obtained following H 2 SO 4 digestion, those for SiO 2 and Al 2 O 3 did not.…”

Using pedological knowledge to improve sediment source apportionment in tropical environments

“…Sediment elemental composition was determined by X-ray fluorescence (XRF), using a portable XRF spectrometer equipped with a 50 kV/100 μA X-ray tube. XRF technology has been increasingly used for quantifying soil geochemistry, given that it provides a non-destructive method with rapid results and no chemical waste generation (Ribeiro et al 2017;Silva et al 2017). The analysis allows for the quantification of the following 45 elements: Ag, Al 2 O 3 , As, Au, Ba, Bi, CaO, Cd, Ce, Cl, Co, Cr, Cu, Fe, Hf, Hg, K 2 O, La, MgO, Mn, Mo, Nb, Ni, P 2 O 5 , Pb, Pd, Pt, Rb, Rh, S, Sb, Se, SiO 2 , Sn, Sr, Ta, Th, Ti, Tl, U, V, W, Y, Zn, Zr.…”