Bioreduction of Fe-oxides in mine tailings deposited under marine conditions releases Fe and associated trace elements (e.g., Ti, Ni, Cd, Pb), leading to contamination of the marine environment. Sea-tailings disposal (STD) along the northern coast of Chile (Ensenada Chapaco) and along the eastern coast of Spain (Portman Bay) results in an adverse impact on the environment. This paper focuses on bioreduction under marine conditions. To this end, two column experiments were carried out with samples from Portman Bay and Ensenada Chapaco. Lactate (i.e., organic matter source) was supplied during the experiments. The results obtained are compared with those from batch experiments performed under similar conditions. In the column filled with Portman Bay tailings, the high content of magnetite (15 wt%) in contact with water gives rise to a large magnetite surface area and abundant Fe(III), which results in a high release of Fe(II) and trace metals (TE). Since Fe(II) adsorbs onto the magnetite surface reducing the availability of Fe(III), the magnetite bioreduction and the consequent TE release decrease after 2000 h. By contrast, the magnetite bioreduction lasts longer (3000 h) in the column with Ensenada Chapaco tailings. This is because a lower magnetite content in the tailings (1 wt%) provides a smaller reactive surface area yielding less Fe(III). Consequently, the concentrations of Fe(II) and TE in the output solutions are lower, which slows down the Fe(II) adsorption onto magnetite. This results in a a longer magnetite bioreduction. Bioreduction is regulated by the availability of Fe(III) in both columns. It is inferred that the bioreduction rate diminishes as a function of time and increases as a function of soluble Fe(II) concentration. Moreover, the concentrations of TE released from the two bioreduced tailings exceed the elemental concentrations under marine conditions.

We would like to thank Natàlia Moreno, Rafael Bartrolí and Mercè Cabanas (IDAEA), David Artiga, Maite Romero and Xavier LLovet (SCT-Barcelona University) for analytical assistance. This work has been financed by the CGL2017-82331-R (Spanish Ministry of Economy and Competitiveness) and PID2020-119196RB-C22 (Spanish Ministry of Science and Innovation) projects, the 2021 SGR 00308 project (Catalan Government) and the Chilean Government through the Research Fund for Fishery and Aquaculture (Fondo de Investigación Pesquera y de Acuicultura; FIPA) of SUBPESCA (FIP 2015-11 project). IDAEA-CSIC is a Severo Ochoa Center of Excellence (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S funded by MCIN/AEI/10.13039/501100011033).

Peer reviewed