As and S Speciation in a Submarine Sulfide Mine Tailings Deposit and its Environmental Significance the Study Case of Portmán Bay (Se Spain)

Abstract

The dumping of an estimated amount of 57 million tons of hazardous sulfide mine waste from 1957 to 1990 into Portmán’s Bay (SE Spain) caused one of the most severe cases of persistent anthropogenic impact in Europe’s costal and marine environments. The resulting mine tailings deposit completely infilled Portmán’s Bay and extended seawards on the continental shelf, bearing high levels of metals and As. The present work, where Synchrotron XAS, XRF core scanner and other data are combined, reveals for the first time the simultaneous presence of arsenopyrite (FeAsS), scorodite (FeAsO 4 ·2H 2 O), orpiment (As 2 S 3 ) and realgar (AsS) in the submarine extension of the mine tailings deposit. The occurrence of scorodite relates to the oxidation of arsenopyrite during sulfide ore processing at the old Portman’s floatation plant, whereas orpiment and realgar are associated to the dissolution of scorodite under moderately reducing conditions and the subsequent precipitation of As sulfides in the mine tailings. It is to be noted that the precipitation conditions for orpiment and realgar are far from those typical of near-surface environments, and hence the involvement of sulfate-reducing bacteria (SRB) is suggested. This view is supported by the presence of reduced organic sulfur compounds in the tailings. The reported precipitation of these two minerals, orpiment and realgar, has important consequences for As mobility and bioavailability as their formation reduces the release of As into the surrounding environment. Our work provides valuable hints on As speciation in a massive submarine mine tailings deposit, which is highly relevant for other Portmán’s Bay-like settings worldwide.

Keywords: Submarine mine tailings, Arsenic, sulfur, Speciation, XAS, environment