Expanding the bioscorodite process for As(III) wastewater remediation

Abstract

Arsenic (As), is the 20th most abundant natural element in the earth’s crust. Furthermore, an important source of drinking water contamination and undesirable element overproduced during the extraction of valuable metals from low-grade ores. To date, ferric arsenate, scorodite is the most appropriate carrier for long-term arsenic fixation in metallurgical processes. Previously, the biological scorodite crystallization (bioscorodite) from diluted As(V) streams demonstrated being cost-efficient and sustainable solution for the treatment of diluted As(V) solutions. Since As(III) is predominant in acid effluents and scorodite needs the As to be in the As(V) an oxidation previous oxidation step is required. In this work entitled Expanding the bioscorodite process for As(III) wastewater remediation, we explored at laboratory scale, the possibilities of arsenic immobilization from the treatment acid As(III)-bearing solutions through the biological oxidation and precipitation in a single unit process aiming to broaden the range of application of the bioscorodite concept. In this thesis, the proof of principle, reactor selection and operational conditions of the bioscorodite crystallization were studied. The main findings of this research demonstrated the feasibility of the simultaneous As(III) oxidation catalyzed by granular activated carbon (GAC) and biological Fe(II) oxidation and precipitation in a singles system leading as the main product the formation of highly stable scorodite crystals with low arsenic leachability. Furthermore, the predominance of thermoacidophilic archaea and identification of EPS like-structures indicated that scorodite formation is mediated by the microbial surface or by the exopolymeric organic components. The use of alternative Fe(II) sources such as pyrite and abundant mineral in hydrometallurgical processes was successfully tested indicating the potential of this cheaper source towards reducing the cost of the process. Besides the use of GAC as a cheaper catalyst and the use of cheaper alternative iron sources, other benefits of the proposed biological process include 1) the lower Fe/As molar ratio of the compact precipitate (close to 1), translating into a significant lower Fe consumption and a lower mass of the solid waste residue. 2) Zero cost related to the use of external seeds of stepwise neutralization since thermoacidophilic microorganisms mediate arsenic precipitation and, 3) the reusability of the GAC in the process. The work presented in this thesis demonstrated that the biological scorodite precipitation takes place simultaneously to the catalyzed As(III) by activated carbon oxidation, thereby providing a novel alternative for arsenic immobilization and safe disposal of As from acidic wastewater streams (i.e., metallurgical effluents).