Research paperIncongruent weathering of Cd and Zn from mine tailings: A column leaching study

The weathering of discharged mine tailings can contaminate groundwaters, rivers and floodplains with potentially toxic Cd and Zn, depending on tailings mineralogy, storage, dispersal and climatic conditions. The mechanisms of long-term tailings weathering and its influence on waste piles and floodplain environments were assessed by a column leaching experiment that incorporated tailings and soil from Potosí, Bolivia, and modelled 20 cycles of wet and dry season conditions over three calendar years. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modelling were carried out to determine the processes responsible for the leaching of Cd, Fe, S and Zn. Over this period, approximately 50 to 95% of the original Cd and 50 to 60% of the Zn were leached from the columns. Large amounts of leached Cd and Zn at the beginning of the experiment are attributed to the dissolution of soluble sulphate minerals present in the original tailings and formed after the first wetting of the columns. The Zn/Cd mass ratios of the tailings and soil, initially 429 and 400, respectively, vary considerably over the course of the experiment. Low values (between 220 and 300) in the early cycles are attributed to preferential weathering of Cd-rich wurtzite [Zn,Fe)S] and sequestration of Zn in preference to Cd in secondary Fe phases forming in the columns. In the middle cycles, dissolution of secondary Fe(OH)3 under low pH (< 3) conditions, and of ferroan (Cd-poor) sphalerite [Zn,Fe)S], releases Zn and raises the Zn/Cd ratio to 550-600 in the tailings-only columns and up to 1500 in the mixed tailings-soil columns. The very high ratios in the latter are also ascribed to the formation of low molecular weight organic ligands that have high affinity for Zn over Cd. In the later column-cycles, Zn/Cd ratios return to near-initial values, due to the weathering of Fe-poor sphalerite and secondary Fe phases, and the declining preference of Zn over Cd in the soil organic acids under the strongly acidic conditions prevailing in the columns. The formation and dissolution of secondary soluble sulphate minerals also play a role in Cd and Zn cycling, especially at the beginning of the experiment.

Research Highlights► Cd and Zn undergo incongruent weathering from Bolivian mine tailings. ► Dissolution of Cd-rich wurtzite contributes to low Zn/Cd during early weathering. ► Dissolution of Fe(OH)3 and ferroan sphalerite raises Zn/Cd as weathering progresses. ► At pH ~ 2 weathering of low-Fe sphalerite and Zn-rich soil organic acids lowers Zn/Cd. ►Formation and dissolution of soluble sulphates also releases Cd and Zn.