Speciation, mobilization, and bioaccessibility of arsenic in geogenic soil profile from Hong Kong

The behaviour of arsenic (As) from geogenic soil exposed to aerobic conditions is critical to predict the impact of As on the environment, which processes remain unresolved. The current study examined the depth profile of As in geologically derived subsoil cores from Hong Kong and investigated the mobilization, plant availability, and bioaccessibility of As in As-contaminated soil at different depths (0–45.8 m). Results indicated significant heterogeneity, with high levels of As in three layers of soil reaching up to 505 mg/kg at a depth of 5 m, 404 mg/kg at a depth of 15 m, and 1510 mg/kg at a depth of 27–32 m. Arsenic in porewater samples was <11.5 μg/L in the study site. X-ray absorption spectroscopy (XAS) indicated that main As species in soil was arsenate (As(V)), as adsorbed fraction to Fe oxides (41–69% on goethite and 0–8% on ferrihydrite) or the mineral form scorodite (30–57%). Sequential extraction procedure demonstrated that 0.5 ± 0.4% of As was exchangeable. Aerobic incubation experiments exhibited that a very small amount (0.14–0.48 mg/kg) of As was desorbed from the soil because of the stable As(V) complex structure on abundant Fe oxides (mainly goethite), where indigenous microbes partly (59 ± 18%) contributed to the release of As comparing with the sterilized control. Furthermore, no As toxicity in the soil was observed with the growth of ryegrass. The bioaccessibility of As was <27% in the surface soil using simplified bioaccessibility extraction test. Our systematic evaluation indicated that As in the geogenic soil profile from Hong Kong is relatively stable exposing to aerobic environment. Nevertheless, children and workers should avoid incidental contact with excavated soil, because high concentration of As was present in the digestive solution (<0.1–268 μg/L). - Highlights: • The soil cores from Hong Kong exhibit high level of arsenic (up to 1510 mg/kg) with significant heterogeneity. • Arsenic in the soil was mainly arsenate (>92%), primarily as adsorbed species on Fe oxides or the mineral form scorodite. • Only 0.14–0.48 mg/kg of As was released from soil during aerobic incubation tests. • An average of 59% of the released As was contributed by the soil indigenous microbe. • The bioaccessibility extraction test indicated probable As toxicity (268 μg/L) with incidental soil ingestion. - Geogenic arsenic is mainly arsenate (>98%), primarily as adsorbed species on Fe oxides, with low leachability in soil under aerobic conditions.