Arsenic Mobility and Chemical Transformation by Earthworms and Kentucky Bluegrass In Soil Mesocosms

Synchrotron X-ray fluorescence microprobe and X-ray absorption spectroscopy reveal changes in arsenic speciation within worm tissue (Lumbricus terrestrus) and grass shoots (Poa pratensis) following incubation in soil mesocosms after application of roxarsone or sodium arsenate, respectively. Earthworms were introduced to two 5cm(ID) x 30cm columns, each filled with artificial soil mixture containing quartz sand, kaolinite, and peat moss. A layer of the soil mixture contaminated with 85ppm arsenic as roxarsone occupied the 10-20cm depth interval. After 30 days columns were frozen with liquid nitrogen, cut into discs, and dried. Worm castings were observed on the soil surface and burrows extended 14-24cm deep. Selected soil discs analyzed at the Stanford Synchrotron Radiation Laboratory (BL 2-3) and the Advanced Photon Source (XOR/PNC 20BM) show vertical and lateral redistribution of arsenic. Most remained as roxarsone; however, worm tissue showed arsenic reduction to As(~0) and castings showed minor reduction that suggests a methylated As species. In a separate experiment, Kentucky Bluegrass, a common perennial rye grass that is used for grazing thoroughbred horses because of its high phosphorus content, was grown from seed in 10-gallon tanks. Sodium arsenate was applied to the soil surface following sprouting. After two days, grass plants were analyzed at SSRL. X-ray microprobe maps show arsenic throughout the root and shoot with highest concentrations at tips and junctions of lateral root branches. Arsenic on the root's outer surface remained As(V) but in the root interior and shoot most had been transformed to As(~0), suggesting conversion to arsenic-thiol complexes.