Evaluation of arsenic sorption and mobility in stream sediment and hot spring deposit in three drainages of the Tibetan Plateau

Enrichment of arsenic (As) in sediment (12-227 mg/kg) in the upstream tributaries of the Indus and Brahmaputra Rivers originating from the Tibetan Plateau where hot springs were abundant has been found. Sandy sediment samples from the Nu-Salween River, Lantsang-Mekong River and Jinsha-Yangtze River also originating from the Tibetan Plateau were collected in July, 2012 and were found to contain 6.8 mg/kg to 30.5 mg/kg As (average17.3 ± 6.5 mg/kg, n = 12). Deposits collected within 1 m of two hot springs in Changdu, Tibet displayed significantly higher As levels: 263.7 mg/kg in LD-2 with 65% quartz and 101.8 mg/kg in NuD-1 with 82% calcite. To evaluate the valence states of As and also the phases responsible for sorption, X-Ray Absorption Spectroscopy (XAS) was employed to analyze the two hot spring deposits and three river sediment samples: coarse sand NuD-6 (As 14.6 mg/kg), fine sand NuD-4 (As 17.8 mg/kg), and silty sand LD-1 (As 30.5 mg/kg). The X-ray absorption near edge spectrum (XANES) data indicate that 70% of As from 3 samples in the Nu-Salween River drainage (NuD-1, NuD-4 and NuD-6) is As(V) or arsenate, with the rest being As(0) or As-Fe sulfides. The proportion of As(V) is 90% for 2 samples in the Lantsang-Mekong River drainage (LD-1 and LD-2). Linear combination fit of the iron extended X-ray fine structure spectrum (EXAFS) show that 3 samples from the Nu-Salween River contain 20% ferrihydrite and 10% goethite without any hematite being detected but 2 samples from the Lantsang-Mekong River contain <10% ferrihydrite, 20% goethite and with 30-60% of hematite. Concentrations of reductively leached As and Fe are correlated (Pearson correlation coefficient 0.673), with an average value of extracted As being 1.7 ± 0.6 mg/kg (n = 8) and 4.3 ± 2.0 mg/kg (n = 3) for the Nu-Salween and Lantsang-Mekong river, respectively. Parameters from the Langmuir isotherm fit to sorption experiments of As(III) and As(V) onto three river sediment samples were used to estimate “sorbed” As concentrations in river sediment in equilibrium with the average river water As concentrations. The “sorbed” As concentrations were 0.8 mg/kg and 2.8 mg/kg for the Nu-Salween and Lantsang-Mekong drainage, respectively. Taken together, the data suggest that this pool of “sorbed” As in river sand, likely to have a geothermal As component, remains largely particle-bound in the oxic and circumneutral riverine environment during transport; it is subject to mobilization once buried in the floodplain areas down gradient.