Siderite dissolution coupled to iron oxyhydroxide precipitation in the presence of arsenic revealed by nanoscale imaging

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 h 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.