The interstitial location of selenium and arsenic in rocks associated with coal mining using ultrasound extractions and principal component analysis (PCA)

► Selenium is closely associated with 14 Å d-spacing 2:1 clays (chlorite, montmorillonite, vermiculite). ► As is closely associated with illite, a 2:1 clay. ► As and Se can reside in the octahedral or tetrahedral positions in the clay mineral matrix. ► As and Se are less likely to be found in kaolinite, a 1:1 clay.