Uranium (U) and arsenic (As) of leachates from mine tailing dumps are a notorious cause of environmental concerns. Iron (Fe) oxides and metal reducing bacteria are well known immobilizers of U and/or As in unsaturated and saturated porous media. This paper describes investigations on geochemical controls of U fate under the influence of As, Fe and bacteria. Batch experiments were performed in glass test tubes using natural iron minerals and scrap metallic iron in setups designed to ascertain the kinetics and the influence of arsenic and iron. The effect of the background electrolytes NaCl or KCl and related ionic strengths were also considered. The experiments suggest prevalent role of the scrap iron’s corrosion products but also of those of potassium (K) and calcite. Similarly, four glass columns (40 cm height, 2.4 cm diameter) were packed in weight/weight proportion of 90% sand and 10% scrap Fe (0.2–0.8 mm). Two columns without metallic Fe were filled with 50% sand and 50% glass beads and 100% sand respectively. Five columns including a control were leached at 0.12–0.39 ml/min with a 0.01M KCl or NaCl background electrolyte spiked with 0.05mM of U (11.9mg/l) and As (3.7mg/l). Two columns were leached with Schneckenstein (Saxony, Germany) Uranium tailings leachate upgraded to 0.05 mM U and As and a parallel continuous feed of a solution of 1mM glucose. Effluents samples collected regularly for a total of 77 pore water volume (5 litres) show fixation of more than 95% of both U and As in all columns were scrap Fe was present. The control column, however, has shown an unexpected fixation of both U and As between 90–95% suggesting precipitation/co-precipitation of both U and As through sparingly soluble minerals such as Abernathyite KUO
O as predicted by PHREEQC (LLNL database). Thus, K may be an efficient and cost effective amendment for immobilizing both U and As in contaminated porous media through precipitation/coprecipitation mechanisms if As and U are present at high concentrations. However, the reduction of both arsenic and uranium is limited by the solubility product of Abernathyite maintaining uranium and arsenic concentrations in the range of tens of µg/L. Potassium as an essential intracellular nutrient might also play a double role of enhancing bacteria mediated biotransformation and immobilization of uranium and arsenic.