Iron colloidal transport mechanisms and sequestration of As, Ni, and Cu along AMD-induced environmental gradients

Colloids are common in mine waters and their chemistry and interactions are critical aspects of metal(loid)s cycling. Previous studies mostly focus on the colloidal transport of metal(loid)s in zones where rivers and soil profiles receive acid mine drainage (AMD). However, there is limited knowledge of the colloid and the associated toxic element behavior as the effluent flows through the coal waste dump, where a geochemical gradient is produced due to AMD reacting with waste rocks which have high acid-neutralization effects. Here, we investigated the geochemistry of Fe and co-occurring elements As, Ni, and Cu along the coal waste dump, in aqueous, colloidal, and precipitate phases, using micro/ultrafiltration combined with STEM, AFM-nanoIR, SEM-EDS, XRD, and FTIR analysis. The results demonstrated that a fast attenuation of H⁺, SO₄^2-, and metal(loid)s happened as the effluent flowed through the waste-rock dump. The Fe, As, Ni, and Cu were distributed across all colloidal sizes and primarily transported in the nano-colloidal phase (3 kDa-0.1 μm). An increasing pH induced a higher percentage of large Fe colloid fractions (> 0.1 μm) associated with greater sequestration of trace metals, and the values for As from 39.5 % to 54.4 %, Ni from 40.8 % to 75.7 %, and Cu from 43.7 % to 56.0 %, respectively. The Fe-bearing colloids in AMD upstream (pH ≤ 3.0) were primarily composed of Fe-O-S and Fe-O-C with minor Al-Si-O and Ca-O-S, while in less acidic and alkaline sections (pH ≥ 4.1), they were composed of Fe-O with minor Ca-O-S. The iron colloid agglomerates associated with As, Ni, and Cu precipitated coupling the transformation of jarosite, and schwertmannite to ferrihydrite, goethite, and gypsum. These results demonstrate that the formation and transformation of Fe-bearing colloids response to this unique geochemical gradient help to understand the natural metal(loid)s attenuation along the coal waste dump.