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Abstract The Río Tinto mining area provides ideal conditions for studying the role of colloidal particles in concentrating. All the elements are present in higher concentrations respect to their content in common natural waters. 234U/238U activity ratio ranges between 2.286 ± 0.149 and 2.531 ± 0.151. Concentration of Fe and Al are in the order of 104–103 mg/L; Co, Zn and Cu values are three times lower than Fe and Al, but still much greater than in natural waters; U reaches values up to 40.73 μg/L. To evaluate the partitioning of these elements into the colloidal fraction, ultrafiltration (UF) experiments were made. A permeation model describing the relationship between concentration of the elements or isotopes in the permeate solution and the concentration factor (CF) was calculated. Experiments were carried out using CF from 1.5 to 25.0. The behaviour of investigated heavy metals and U can be predicted by a permeation model that provides similar permeation coefficient (P c) for all of them (P c = 0.95–0.97 for 50 kDa, P c = 0.86–0.88 for 10 kDa, and P c = 0.80–0.86 for 3 kDa). Retention for all the investigated elements occurs, resulting in up to 23% of the element associated to the 3 kDa colloidal fraction. The increase in U and heavy metals concentrations in the retentate fraction with the increasing CF is due to the retention of the colloidal fraction. The colloidal abundance of related metals increased with decreasing membrane MWCO even though the difference is small between 3 and 10 kDa fraction for some metals Characterization of colloidal material is needed to better understand the state of play in this context. Highlights • Partitioning of U and selected heavy metals in dissolved and colloidal fractions in AMD conditions were examined. • UF with distinct MWCO was used and a permeation model to quantify the behaviour of U and selected metals was applied. • Metal concentrations much higher than those in natural waters were found and high 234U/238U in the water samples were observed. • Little colloidal metals in the size fraction 3–10 kDa are present, likely due to the strong acidic conditions. • Low concentrations of colloidal U in the study area were measured. [ABSTRACT FROM AUTHOR]

Remediation of heavy metal-contaminated aquifers is a challenging process because they cannot be degraded by microorganisms. Together with the usually limited effectiveness of technologies applied today for treatment of heavy metal contaminated groundwater, this creates a need for new remediation technologies. We therefore developed a new treatment, in which permeable adsorption barriers are established in situ in aquifers by the injection of colloidal iron oxides. These adsorption barriers aim at the immobilization of heavy metals in aquifers groundwater, which was assessed in a large-scale field study in a brownfield site. Colloidal iron oxide (goethite) nanoparticles were used to install an in situ adsorption barrier in a very heterogeneous, contaminated aquifer of a brownfield in Asturias, Spain. The groundwater contained high concentrations of heavy metals with up to 25 mg/L zinc, 1.3 mg/L lead, 40 mg/L copper, 0.1 mg/L nickel and other minor heavy metal pollutants below 1 mg/L. High amounts of zinc (>900 mg/kg), lead (>2000 mg/kg), nickel (>190 mg/kg) were also present in the sediment. Ca. 1500 kg of goethite nanoparticles of 461 ± 266 nm diameter were injected at low pressure (< 0.6 bar) into the aquifer through nine screened injection wells. For each injection well, a radius of influence of at least 2.5 m was achieved within 8 h, creating an in situ barrier of 22 × 3 × 9 m. Despite the extremely high heavy metal contamination and the strong heterogeneity of the aquifer, successful immobilization of contaminants was observed in the tested area. The contaminant concentrations were strongly reduced immediately after the injection and the abatement of the heavy metals continued for a total post-injection monitoring period of 189 days. The iron oxide particles were found to adsorb heavy metals even at pH-values between 4 and 6, where low adsorption would have been expected. The study demonstrated the applicability of iron oxide nanoparticles for installing adsorption barriers for containment of heavy metals in contaminated groundwater under real conditions. • Ca. 150 m3 iron oxide suspension was injected into a contaminated aquifer for the immobilization of toxic heavy metals. • The permeable adsorption barrier of ca. 22 m × 9 m × 3 m did not change the hydraulic conductivity. • Downstream concentrations of dissolved heavy metals were strongly reduced. • Reduction of metal concentrations was observed at all pH conditions and even below pH 6. • Iron oxide nanoparticles have a great potential for in situ immobilization of a wide range of heavy metals in aquifers. [ABSTRACT FROM AUTHOR]