Comparative Study of Iron and Trace Element Mobilization during Fe-Oxide Bioreduction in Mine Tailings: a case study of Ensenada Chapaco (Chile) and Portman Bay (Spain)

Abstract

Bioreduction of Feoxides 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 (15wt%) 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 Elements (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 2000h. By contrast, the magnetite bioreduction lasts longer (3000h) in the column with Ensenada Chapaco tailings. This is because a lower magnetite content in the tailings (1wt%) 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 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.