Sulfide oxidation, chemical partitioning and environmental availability of iron and trace elements in abandoned mine wastes affecting a coastal wetland ecosystem.

Coastal wetland ecosystems are usually threatened by a variety of land uses and anthropogenic activities (urban, industrial, agricultural, etc.) that have the potential to cause multiple environmental impacts. The Domingo Rubio tidal channel is a Ramsar wetland site (480 ha) located in the estuary of Huelva (Spain) that is being subjected to hazardous effects of multiple pollution sources for decades. A technogenic soil (Spolic Technosol according to the WRB criteria), developed on sulphide-rich mine wastes left on the saltmarsh tidal channel, is one of the most important point-source pollution in the area. 

With the aim of understanding the environmental effects of the sulfide oxidation on the adjacent soil environment, a total of 24 samples were collected to determine the mineral composition (by XRD and SEM-EDS) and the multielement geochemical signature (by XRF and ICP-MS) of the technosol and soils surrounding the waste disposal site. 

The results showed that the soil derived from the mine wastes is extremely acid (pH values as low as 2) and oxidant (Eh values up to +707 mV), in contrast to the mean values of the nearby saltmarsh soils (pH 6.4 and Eh +378 mV). The technosol is mineralogically composed mainly of quartz, phyllosilicates (mica and kaolinite), feldspars, gypsum, amorphous or poorly-crystallized iron oxyhydroxides, jarosite [KFe₃(SO₄)₂(OH)₆], and water-soluble iron sulphate minerals such as ferricopiapite [Fe²⁺Fe³⁺4(SO₄)₆(OH)2·20(H₂O)]. Consistently, the major element oxides (in weight percent) of the technosol samples are SiO₂ (25-84%), Al₂O₃ (2.8-13%) and Fe₂O₃ (up to 30%). The chemical analysis by ICP-MS revealed high total concentrations of Pb (up to 8897 mg/kg), Cu (up to 2476 mg/kg), Zn (up to 1503 mg/kg), As (up to 452 mg/kg) and Cd (up to 5 mg/kg) in the surface layer of the technosol, while the concentrations of Cr, Co, Ni were found within the soil geochemical background.

Chemical partitioning of trace elements and the iron released from the pyrite oxidation, as determined by sequential extraction procedures (BCR scheme), showed that the metallic contaminants are largely associated with reducible phases, notably iron oxides and oxyhydroxides, which acted as scavengers for the heavy metals. According to the results obtained from single chemical extractions using deionized water, CaCl₂ (0.01 M) and EDTA (0.05M), the As concentration in the technosol leachates exceeded the SSL in both water and CaCl₂ extractions (61 and 59 mg/L, respectively), while the Pb concentration clearly exceeded SSL in EDTA extraction (525 mg/L), thus posing threats to ecological and human receptors. 

In conclusion, mineralogical and geochemical changes driven by acid and metal release from the abandoned sulphide-rich mine wastes have adverse environmental effects on the wetland ecosystem, and therefore there is a strong need to reclaim this highly degraded soil to a sustainable environmental quality.