Evaluating a method using pXRF for measuring sulphur along Ostlänken

Abstract

Ostlänken (The Eastern link) is going to be a new main railway in Sweden, between Södertälje and Linköping. The railroad will go through areas where there could be high sulphide concentration in the rocks, and during the construction, large amounts of rock is going to be excavated. Since sulphides reacts with water and oxygen, and causes acidification and leaching of metals, exposed sulphides in the excavated rocks can be very harmful to the local ecosystems. Trafikverket (The Swedish Transport Administration) has been working with SGU (Geological Survey of Sweden) to map the area and take samples of rocks with potentially high sulphur concentrations. However, current methods for analysing the samples are time consuming. The aim of this project is therefore to evaluate a new, quicker, method using an pXRF (Portable X-Ray Fluorescence). A pXRF sends out X-rays that excites electrons in the rock that then emits a characteristic X-ray which the instrument detects. The instrument has been used for detecting other elements, however measuring sulphur is challenging since it is a lightweight element with low characteristic energy. This means that the X-ray can’t penetrate very deep, and the sample size of the pXRF becomes quite small. The new method using a pXRF for sulphur concentration therefore needs to take this into consideration. Furthermore, the pXRF requires a calibration specific to each rock type to work. Therefore SGU samples sent to the laboratory for testing were also used for calibration of the instrument. During the calibration, different beam times of the pXRF was also tested in the laboratory figure out what would be optimal to gain most accurate results. Finally, during field work, the sites that SGU had sampled were revisited and measured using the pXRF to be able to compare the pXRF measurements with the results from the original laboratory work. Some additional measurements were made nearby the original sample sites to see how the results were affected by measuring on weathered rock surfaces. The results showed that the averages of the field measurements were close to the results of the powdered samples that had been analysed in a laboratory. This means that for the sites that were investigated in this paper the method is working. Each measurement series consisted of 30 pXRF measurements and results show large differences within the measurement series’ due to the heterogenous distribution of sulphides. Therefore, it is important to make many measurements. The method could be a useful tool for initial screening in an area that could have high sulphur concentrations to be able to focus other more time-consuming methods to areas that are of interest.