Effect of soil properties on gold- and platinum nanoparticle mobility

Abstract

Soils overlying buried gold (Au) and platinum (Pt) deposits are often Au/Pt-anomalous, widening exploration footprints. This may be a result of the dispersion of (bio)geochemically formed Au/Pt nanoparticles and their sorption/deposition by solid soil phases. However, the influence of soil properties on nanoparticle dispersion, sorption and partitioning are little understood. To assess the partitioning and sorption behaviour of Au- and Pt nanoparticles, soils were spiked with spherical, citrate-stabilized Au- and Pt nanoparticles (diameter: 10–12nm, final concentration 100μgkg−1). Soil slurries (1:10, w:w, soil:deionized water) were incubated at 22°C on an end-over-end shaker for 28days. Gold/Pt concentrations were measured in 0.45μm and 1kDa filtered solutions using inductively coupled plasma-mass spectrometry. Neither Pt nor Au were detected in 1kDa filtrates. Between 10.3 and 98.7wt% of Au/Pt was sorbed to solid soil phases. Clay, Corg, Mg, Mn, Fe and Ca contents of soils were significant predictors of Au/Pt-nanoparticle sorption, with clay content explaining >80% of variation within the Au sorption dataset. Partitioning of sorbed nanoparticles between solid phases was assessed using selective sequential extractions. In all soils 1–20wt% of Au/Pt was detected in watery extracts. A further 1–25wt% were associated with the easily remobilizable fraction. The majority of sorbed Au/Pt nanoparticles were sorbed to the organic matter (OM, 53–77wt% for Pt and 14–80wt% for Au) and the Fe/Mn-oxides (up to 42wt%). These results show that: i) Au- and Pt nanoparticles are stable and can remain highly mobile/dispersible in soil solutions, and ii) elevated clay, Corg and Fe/Mn-oxide contents decrease, whereas elevated sand contents and pH increase their mobility. Across all soils, OM and clays were the most important sorbent of Au/Pt nanoparticles, suggesting that the separation of clays and OM, and analyses of their Au/Pt-contents may enhance the contrast of geochemical footprints.