Speciation dynamics of oxyanion contaminants (As, Sb, Cr) in argillaceous suspensions during oxic-anoxic cycles

Abstract

A B S T R A C TArgillaceous geological formations are considered promising repositories for waste containing inorganic contaminants. However, the sequestration capacity of an argillaceous natural barrier may change as a result of dynamic environmental conditions, in particular changes in redox state. Here, we imposed redox cycles to argillaceous suspensions amended with a mixture of the inorganic contaminants As(V), Sb(V), Cr(VI), by alternating 7-day cycles of sparging with either oxic or anoxic gas mixtures. During the redox cycles, we assessed the relative importance of different contaminant sequestration mechanisms under sterile and non-sterile conditions. The non-sterile experiments were carried out 1) with the argillaceous material as is, that is, containing the native microbial population, 2) with the addition of ethanol as a source of labile organic carbon, and 3) with the addition of both ethanol and a soil microbial inoculum. In order to capture the observed solid-solution partitioning trends, a series of mixed thermodynamic-kinetic models representing the dynamic conditions in the experiments were developed using PHREEQC. Parameter values for processes such as adsorption, which occurred in all experiments, were kept constant across all the simulations. Additional formulations were introduced as needed to account for the microbial detoxification and respiration of the contaminants in the increasing complexity experiments. Abiotic adsorption of As and abiotic reductive precipitation of Cr dominated sequestration of these two contaminants under all experimental conditions, whereas the presence of microorganisms was essential to decrease the aqueous Sb concentration. Once reduced, As(III), Sb(III), and Cr(III) were not reoxidized upon exposure to dissolved O 2 in any of the experiments. Neither the mineralogy nor the native microbiota catalysed contaminant oxidation processes. Thus, kinetically, the reduction of As(V), Sb(V), and Cr(VI) was irreversible over the experimental duration (49 days). The capacity of the argillaceous matrix to reduce and sequester the aqueous contaminants, without subsequent remobilization to the aqueous phase during the oxic periods, supports the use of argillaceous barriers for geologic waste storage.

A Critical Review of Resistance and Oxidation Mechanisms of Sb-Oxidizing Bacteria for the Bioremediation of Sb(III) Pollution

“…Hockmann et al (2014) reported that the transition to reducing conditions invoked by indigenous microbial activity at first led to the immobilization of Sb, as Sb(V) was reduced to Sb(III), which binded more extensively to iron (hydr)oxides, and the previously sorbed Sb(III) was gradually released into solution due to reductive dissolution of the iron(hydr)oxides if reducing conditions continued (Zhu et al, 2018;Burton et al, 2019). Markelova et al (2018) also reported that the reductive precipitation of Sb(III) appears to be mainly microbially mediated during oxic-anoxic condition. It was previously clearly revealed that a majority of Sb(III) was kinetically oxidized into Sb(V) on surface of soils under aerobic conditions and a smaller amount of Sb(III) was oxidized into Sb(V) under anaerobic conditions (Cai et al, 2016).…”

“…Therefore, studies of Sb(III) oxidation by Sb-oxidizing bacteria are relatively simple and controllable. However, the external soil environment contains a wide range of other components including high concentrations of nutrients such as nitrogen (N), phosphorus (P), potassium (K), and complex organic matter (including fulvic acid, humic acid, and low molecular organic matter) ( Biver and Shotyk, 2012 ; Wu et al, 2019 ); trace elements; and a range of other biotic and abiotic phases and ions ( Loni et al, 2020 ), such as iron/manganese (hydr)oxides ( Levett et al, 2020 ), associated metal ions (Ca 2+ , Mg 2+ , As 3+ , and Cr 3+ ) ( Markelova et al, 2018 ), SO 4 2+ /S 2– ( Park et al, 2018 ; Zhu et al, 2018 ), and PO 4 3+ ( Park et al, 2018 ), which affect the microbial oxidation of Sb(III). These chemical substances or ions may react with Sb(III) by complexation, adsorption, or induced catalysis, resulting in a greater degree of complexity in understanding and controlling the microbial oxidation process of Sb(III).…”

Monitoring and Management of Anions in Polluted Aqua Systems: Case Studies on Nitrate, Chromate, Pertechnetate and Diclofenac

“…In natural environments, oxoanions exist in a variety of species. Speciation depends on numerous factors including pH, electrochemical potential (Eh), redox conditions, electrolyte compositions, temperature and microbial activity [26,61]. For example, SO4 2is thermodynamically more stable than SO3 2at pH 6.5-8.5 and Eh 0.1-0.4V [26].…”

Effects of redox oscillations on the phosphogypsum waste in an estuarine salt-marsh system

“…The experiment was performed using a custom two part Pyrex® glass redox stat bioreactor system designed by Thompson et al (2006) and modified later by Parsons et al (2013); the system was also used in similar, recent studies (Markelova et al, 2018;Phan et al, 2018;2019) (Fig. 2).…”