Combining single-particle inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy to evaluate the release of colloidal arsenic from environmental samples.

Detection and sizing of natural colloids involved in the release and transport of toxic metals and metalloids is essential to understand and model their environmental effects. Single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) was applied for the detection of arsenic-bearing particles released from mine wastes. Arsenic-bearing particles were detected in leachates from mine wastes, with a mass-per-particle detection limit of 0.64 ng of arsenic. Conversion of the mass-per-particle information provided by SP-ICP-MS into size information requires knowledge of the nature of the particles; therefore, synchrotron-based X-ray absorption spectroscopy (XAS) was used to identify scorodite (FeAsO4·2H2O) as the main species in the colloidal particles isolated by ultrafiltration. The size of the scorodite particles detected in the leachates was below 300-350 nm, in good agreement with the values obtained by TEM. The size of the particles detected by SP-ICP-MS was determined as the average edge of scorodite crystals, which show a rhombic dipyramidal form, achieving a size detection limit of 117 nm. The combined use of SP-ICP-MS and XAS allowed detection, identification, and size determination of scorodite particles released from mine wastes, suggesting their potential to transport arsenic. Graphical abstract Analytical approach for the detection and size characterization of As-bearing particles by SP-ICP-MS and XAS in environmental samples.

Colloidal mobilization of arsenic from mining-affected soils by surface runoff.

Scorodite-rich wastes left as a legacy of mining and smelting operations pose a threat to environmental health. Colloids formed by the weathering of processing wastes may control the release of arsenic (As) into surface waters. At a former mine site in Madrid (Spain), we investigated the mobilization of colloidal As by surface runoff from weathered processing wastes and from sediments in the bed of a draining creek and a downstream sedimentation-pond. Colloids mobilized by surface runoff during simulated rain events were characterized for their composition, structure and mode of As uptake using asymmetric flow field-flow fractionation coupled to inductively plasma mass spectrometry (AF4-ICP-MS) and X-ray absorption spectroscopy (XAS) at the As and Fe K-edges. Colloidal scorodite mobilized in surface runoff from the waste pile is acting as a mobile As carrier. In surface runoff from the river bed and the sedimentation pond, ferrihydrite was identified as the dominant As-bearing colloidal phase. The results from this study suggest that mobilization of As-bearing colloids by surface runoff may play an important role in the dispersion of As from metallurgical wastes deposited above ground and needs to be considered in risk assessment.

Arsenic speciation in the dispersible colloidal fraction of soils from a mine-impacted creek.

Arsenic and iron speciation in the dispersible colloid fraction (DCF; 10-1000 nm) from an As-rich mine waste pile, sediments of a streambed that collects runoff from waste pile, the streambed subsoil, and the sediments of a downstream pond were investigated by combining asymmetrical-flow field-flow fractionation (AsFlFFF)/inductively-coupled plasma-mass spectrometry (ICP-MS), transmission electron microscopy (TEM) and X-ray absorption (XAS) spectroscopy. Calcium, Fe and As (Fe/As molar ratio ∼ 1) were the main components of the DCF from waste pile. TEM/EDS and As and Fe XAS analysis revealed the presence of nanoparticle scorodite in this same DCF, as well as Fe nanoparticles in all samples downstream of the waste pile. Arsenic and Fe XAS showed As(V) adsorbed onto nanoparticulate ferrihydrite in the DCF of downstream samples. Micro-X-ray fluorescence indicated a strong correlation between Fe and As in phyllosilicate/Fe(3+) (oxi) hydroxide aggregates from the sediment pond. Fractionation analysis showed the mean particle size of the DCF from the streambed sample to be smaller than that of the streambed subsoil and sediment ponds samples. These results show that an important and variable fraction of As may be bound to dispersible colloids that can be released from contaminated soils and transported downstream in natural systems.