Impact of gut passage and mucus secretion by the earthworm Lumbricus terrestris on mobility and speciation of arsenic in contaminated soil

► Passage through the earthworm gut increased water soluble arsenic. ► 56-Day aged casts still contained more than nine times greater water soluble As. ► Changes were due to increases in As(V) mobility, with no change in As(III). ► Dilute mucus reduced As mobility due to As-amino acid-Fe-OH ternary complexes. ► Impact of earthworms on As mobility should be considered in risk assesment.