Effect of Particle Size on Compost Analysis by Portable X-ray Fluorescence

Abstract

Portable X-ray fluorescence (pXRF) could be more widely used for analyzing organic amendments, because it allows for a fast and low-cost analysis and is easy to use, among other operational advantages. However, the heterogeneity of these materials causes deviations in the measurements made. In the present study, three composts different in their origin (municipal solid waste (MSW), sewage sludge, and alperujo (i.e., olive oil manufacturing) waste) and composition were used. The effect that the laboratory grinding process had on the results of their elemental analysis conducted with pXRF was studied, as well as the elemental composition of the compost granulometric fractions. The grinding process of the fractions larger than 0.5 mm caused higher concentration readings for the elements Si (up to 47%), Ti (up to 30%), Al (up to 27%), K (up to 26%), P (up to 24%), V (up to 18%), and Pb (up to 16%) and lower readings for Zn (up to −41%) and Cu (−64%) in the ground samples. The differences depended on the type of compost. However, in the complete compost samples, the grinding caused lower reductions: −34% in the Cu concentration of the MSW compost, −20% in the Zn concentrations of the MSW compost and the alperujo compost, and a 15.2% increase in the P concentration of the MSW compost. For most of the elements, grinding did not cause significant differences. Since several of the elements that presented wide variations are essential for the characterization of compost, it is recommended that in order to carry out measurements with this technique, a sample preparation process is carried out that guarantees its homogeneity.