Renard, and three co-authors, reported in 2016 that siderite, the iron carbonate mineral, occurs in several geological environments and contributes to both the global iron and CO2 cycles. Under crustal conditions, this mineral may dissolve,releasing iron that becomes oxidized and then precipitates in the form of iron oxides and oxyhydroxides that have a high affinity for pollutants, such as arsenic. The process of siderite dissolution, dissolved iron oxidation, and oxyhydroxide precipitation is coupled in time and space. Here, we study the entire process using time-lapse in-situ atomic force microscopy.
Natural siderite crystals were dissolved at room temperature in acidic aqueous solutions in the presence or absence of arsenic. The dissolution process, whose rate could be measured at a nanometer scale, occurred by the nucleation and growth of etch pits, the retreat of step edges, and the deepening of cleavage steps. Precipitation of iron oxyhydroxide phases coupled to siderite dissolution was imaged in-situ. Nucleated particles have an initial height of 1-2 nm after 1 minute reaction and then grow with time into aggregate precipitates 130-220 nm wide and up to 80 nm high after 24 hours of reaction. Ex-situ stirred-flow reactor measurements confirm the same sequence of siderite dissolution and iron oxyhydroxide precipitation. The arsenic is adsorbed by iron oxyhydroxides and its presence does not change significantly the rate of dissolution-precipitation of the overall process. Results provide a basis for understanding and quantifying the interactions between reduced-iron minerals and aqueous-phase oxidants, as well as potential sequestration of toxic elements such as arsenic.
The authors concluded that the coupled process of siderite dissolution in acidic aqueous fluids, in the presence of arsenic, was studied in-situ using atomic force microscopy. The main results are:
- Dissolution of siderite occurs by the nucleation of etch pits and step edge retreat, as observed previously (Duckworth and Martin, 2004; Tang and Martin, 2011), and by a third mechanism identified by the authors herein, the widening and deepening of cleavage planes.
- Coupled to siderite dissolution, iron oxyhydroxide precipitates as nanometer size particles on the surface (heterogeneous nucleation) and primary nanoparticles are then self-assembled into rounded aggregates that coalesce to form larger particle aggregates, probably amorphous, 130-23 nm wide and up to 80 nm high that cover the carbonate surface. This process reduces the surface area available for dissolution and may passivate (coat) the siderite surface. Goethite and a weak Raman peak at the position of ferrihydrite were identified from flow-through reactor experiments.
- The presence of arsenic, even at high concentration, does not significantly modify the siderite dissolution process. Arsenic preferentially adsorbs onto the iron oxyhydroxide precipitates formed from the interface-coupled dissolution and precipitation process occurring under acidic aqueous conditions, typically occurring in acid mine drainage environments.
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