Uranium (U) is a radioactive element that is economically important as fuel for nuclear power plants and in military applications. Its release into the environment has become a growing concern due to its toxicity to aquatic ecosystems. U can enter aquatic environments through several anthropogenic sources, including the mining and processing of U ores. Most studies on U release from mining have focused on U mine sites, where U is released from various components such as tailings dams, waste rock piles, ore piles, and exposed underground or open-pit workings. However, U can also be released from sites where other ore types, such as iron and base metal ores, are mined. A recent study by Dzimbanhete et al. (2024) [1] identified rocks with high U leaching rates in an open pit at an iron ore mining site in Northern Sweden. Mine water pumped from this pit contains U levels exceeding the Swedish maximum allowable concentration guideline for inland surface waters (HVMFS, 2019) [2]. This study builds on previous work [3] and [4], aiming to further investigate the weathering processes affecting two key rock types in the open pit using an isotope approach.

The U leaching of pegmatite (P) and trachyandesite (TA) rock samples with high U content (~44 and ~4.7 µg/g, respectively) collected from the open pit walls was investigated using both batch leaching and dynamic leaching. The leaching behavior was evaluated using elemental and isotopic analysis. For the batch leaching, samples were shaken with leaching solutions for 24 h, with aliquots taken at 6 different times to study time resolved concentration and isotopic changes. The first set of leaching solutions (12 different solutions per rock type) mimicked the type and concentration of ligand (CO32-, SO42-, NO3-, and Cl-) and pH previously determined in mine waters of the open pit [3]. The second set of leaching solutions (14 different solutions per rock type) evaluated the influence of acidic (H2SO4, HNO3) and alkaline (CO32-/HCO3-, PO43-) conditions as U is leached industrially either at acidic or alkaline pH. The dynamic leaching approach were based on column leaching, samples were leached with increasingly strong leachates sequentially (MQ-->0.5 M HNO3-->0.5 M H2SO4-->15.7 M HNO3) to differentiate the leaching of differently bound U containing phases within the rock samples. Elemental concentrations in each leachate solution were analyzed using sector field inductively coupled plasma mass spectrometry (ICP-SFMS) and isotopic composition (235U/234U and δ238U) analyzed using ICP-SFMS or multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS).

 In the first batch test, the highest U leaching efficiency (~5 and ~6% for P and TA, respectively) was observed by the CO32- ligand type and lowest for SO42- ligand type (~0.5 and ~0.7% for P and TA, respectively). The leaching efficiency increased with time for all ligand types, except SO42- where it decreased. In general, U leaching efficiency using the different leachates were similar for both P and TA. In the second batch test, the acidic conditions (up to ~94%) resulted in higher U leaching efficiency than alkaline conditions (up to ~28%) in the order H2SO4>HNO3>CO32-/HCO3->PO43- for both P and TA. Significant differences in U leaching efficiency were observed for P and TA for the different leaching solutions in the second batch test. Differences in U leaching efficiency between P and TA were large for the dynamic leaching, with most of the U leaching during the 0.5 M HNO3 step for P (~21%), while for TA it was during the latter 15.7 M HNO3 step (~42%). Generally, wide range of 235U/234U values and narrow range in δ238U values were observed, with 235U/234U values ranging between 10 to 295 and δ238U values mostly being close to the continental crust (−0.29‰ [5]). Overall, 235U/234U values in most cases had highest fractionation (Δ235U/234Uleachate−original_rock) for leachates with low U leaching efficiency (up to 154) and 235U/234U values close to the 235U/234U in the original rock for leachates with high U leaching efficiency. The change in 235U/234U values during the leaching is less consistent and in general more variable between different leaching solutions for P than for TA, indicating that TA mostly contains one type of U-bearing mineral and P contains several different U-bearing minerals having different 235U/234U ratios and U mobility. This study provides a better understanding of different U leaching behaviors and different possibilities to trace U pollution in future studies.