Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment

Abstract

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

Unique Geochemistry of Arsenic-Contaminated Groundwater and Corresponding Mitigation Efforts in Southern Nepal

“…A recent study has shown that intermitted flow without exposure to air leads to the formation of mixed Fe(II,III) phases and magnetite, which are denser than Fe(III) phases and thus maintain better porosity in the filter and higher corrosion rates. 60 Figure 2 shows an old filter with the outlet lower than the nail bed (red) and a filter filled with new nails and sand and a raised outlet (green).…”

“…Formation of denser mixed Fe(II,III) phases and particularly of magnetite in constantly immersed nail beds can maintain a better permeability. 60,61 A slightly higher TDS in the postmonsoon season is indicative of an increased water−rock interaction during high-precipitation seasons and caused by an increased particle size in the groundwater: such particles can also lead to a decline in the porosity of the nail bed and subsequent clogging.…”

“…In the SONO filters, the reactive iron layer (CIM) between two sand layers never completely dries, which results in black corrosion products (predominantly magnetite). A recent study has shown that intermitted flow without exposure to air leads to the formation of mixed Fe­(II,III) phases and magnetite, which are denser than Fe­(III) phases and thus maintain better porosity in the filter and higher corrosion rates …”

“…The in situ generated Fe­(III)­hydr­(oxides) are larger in volume than their parent Fe(0), which leads to a decline in the initial porosity of the nail bed containing Fe(0), so the pores will be progressively filled causing a loss of permeability in the porous and permeable media. Formation of denser mixed Fe­(II,III) phases and particularly of magnetite in constantly immersed nail beds can maintain a better permeability. , A slightly higher TDS in the postmonsoon season is indicative of an increased water–rock interaction during high-precipitation seasons and caused by an increased particle size in the groundwater: such particles can also lead to a decline in the porosity of the nail bed and subsequent clogging.…”

Coupling High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy with Microfluidic Cells for in Situ Monitoring of Tungsten Nanoparticles Redox Transformation

“…However, whatever the speed at which spectra are acquired, the signal-to-noise ratio of the spectra represents a real limitation, and in all cases, the sample is constantly evolving during acquisition. The integration of microfluidic chips with XAS has emerged as a promising approach, opening up new avenues for studying, in situ and operando, chemical, physicochemical, and biological systems on a microscale. − Indeed, there has been recently an increasing use of microfluidic cells in environmental applications as water treatment, carbon storage, phosphate immobilization, etc . These miniaturized experimental devices, commonly referred to as “laboratories on chips,” offer ideal microplatforms to study complex reaction kinetics, diffusion phenomena, and reaction mechanisms under controlled flow conditions, such as mixing, diffusion, and concentration gradients .…”