Geochemical speciation and dynamic of copper in tropical semi-arid soils exposed to metal-bearing mine wastes

- The hazardous effects of mine waste rocks at environmental conditions were studied;
- Acid drainage, metal concentration and copper geochemical in soils were evaluated;
- The acidity generated by sulphides oxidation is buffered by carbonates dissolution;
- Copper are mainly associated with carbonates, amorphous Fe oxides and sulphides;
- Plants and organic compounds may enhance carbonates dissolution and Cu2 + in soils.

The potentially hazardous effects of rock wastes disposed at open pit in three different areas (Pr: Ore processing; Wr: Waste rock and Bd: Border) of an abandoned copper mine were evaluated in this study, with emphasis on acid drainage generation, metal contamination and copper geochemical dynamics in soils. Samples of waste rock were analyzed by Energy dispersive X-ray fluorescence (XRF), scanning electron microscopy with microanalysis (SEM-EDS) and X-ray diffraction (XRD). Soil samples were analyzed to determine the total metal contents (XRF), mineralogy (XRD), pH (H2O and H2O2), organic and inorganic carbon, % of total N, S and P, particle size, and a sequential extraction procedure was used to identify the different copper fractions. As a result of the prevalence of carbonates over sulphides in the wastes, the soil pH remained close to neutral, with absence of acid mine drainage. The geochemical interaction between these mineral phases seems to be the main mechanism to release Cu2+  ions. Total Cu in soils from the Pr area reached 11,180 mg.kg− 1, while in Wr and Bd areas the values reached, on average, 4683 and 1086 mg.kg− 1, respectively, indicating a very high level of soil contamination. In the Pr and Wr, the Cu was mainly associated with carbonates and amorphous iron oxides. In the Bd areas, the presence of vegetation has influenced the geochemical behavior of copper by increasing the dissolution of carbonates, affecting the buffer capacity of soils against sulphide oxidation, reducing the pH levels and enhancing the proportion of exchangeable and organic bound Cu. The present findings show that the use of plants or organic amendments in mine sites with high concentration of Cu carbonate-containing wastes should be viewed with caution, as the practice may enhance the mobilization of copper to the environment due to an increase in the rate of carbonates dissolution.