How rusting iron removes arsenic from water
Nov 16, 2022
In many regions of the world, groundwater is contaminated with arsenic of natural origin. The harmful substance can be filtered out of water with the help of iron. Eawag researchers have for the first time made visible exactly what happens in this process in a new type of experimental set-up.
by Isabel Plana, EAWAG
“The reactions between arsenic and iron are usually investigated in suspensions in which the filter material is floating in the water. However, the results do not show which processes take place in the pore space, i.e. the cavities between the individual solid particles of a filter.+ The researchers from the W +T department were particularly interested in how arsenic removal is influenced by the periodic operation of a water filter, i.e. when water flow and water accumulation alternate.
The work groups of Andreas Voegelin, Joaquin Jimenez-Martinez, Stephan Hug and Michael Berg wanted to explore this question in a joint experiment that combined all their expertise. Environmental engineer Jonas Wielinski, who did his doctorate at Eawag, took on the challenging tasks and developed an experimental set-up that reproduces the conditions in an arsenic filter as well as possible and makes them visible. “Our objective was to observe and understand the geochemical processes in the pore space between iron particles and sand grains on the scale of micrometres,” says Wielinski, who is now a postdoctoral researcher at Carnegie Mellon University in the USA.
Miniature filters under the microscope
Alternation between water flow and water accumulation favours filtration
After the end of the experiment, the filter model was analysed by X-ray microscopy to determine the type and distribution of the iron oxides and the arsenic bound to them. By combining these results with the colour changes observed in the optical microscope, the researchers were able to understand in detail the dynamic formation and transformation of the iron oxides in the filter and their effect on arsenic removal.
“With this new experimental set-up, we were able to visually demonstrate how the distribution of the iron and quartz sand grains and the water flow through the filter influence the spatial and temporal sequence of the arsenic removal,” says Wielinski. In particular, the alternation between water flow and water accumulation had a positive effect on the filter performance. “A realisation that is useful in the further optimisation of such filters,” he notes and adds: “The set-up developed in this study also has great potential for researching other biogeochemical processes in porous media such as groundwater aquifers or soils."
Wielinski, J.; Jimenez-Martinez, J.; Göttlicher, J.; Steininger, R.; Mangold, S.; Hug, S. J.; Berg, M.; Voegelin, A. (2022) Spatiotemporal mineral phase evolution and arsenic retention in microfluidic models of zerovalent iron-based water treatment, Environmental Science and Technology, 56(19), 13696-13708,
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