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Better Potassium Processing Through Microfluidics

MIT Postdoctoral Associate Davide Ciceri yielding insights into microscale potassium release from feldspar ores.
MIT Postdoctoral Associate Davide Ciceri with microscope that can analyze thin-sections of samples of processed feldspar using both reflected and transmitted light. Photo: Denis Paiste, Materials Processing Center

Laboratory results on the release of ionic metals from minerals often do not match well with those obtained from field trials. MIT Postdoctoral Associate Davide Ciceri is exploring microfluidic techniques to reconcile the discrepancy.

"My project is looking at the fundamental science that could explain the differences that we observe between lab and field. As a team we are also investigating processing routes to improve, as much as we can, the release of potassium from feldspar," Ciceri explains.

His work fits into MIT metallurgy assistant professor Antoine Allanore's effort to develop potassium fertilizer from feldspar as an alternative source to potash, which currently is mined chiefly in the Northern Hemisphere. Brazilian mining company Terrativa is funding the research. (See related article.)

"The big picture is one of food security because the population in the world is growing, so we have to find ways to improve agricultural yields, also through the formulation of more efficient fertilizers," Ciceri says. Ciceri is writing a review article, which will include historical perspectives on the industrial production of potassium fertilizers.

Focusing on release of potassium from feldspar, Ciceri subjects the material to aqueous solutions in a microchannel and measures how much is released. Feldspar is an abundant mineral rock in Brazil, which now depends on imports for more than 90 percent of its need of potassium fertilizer.

To understand how potassium is released from feldspar, Ciceri starts with a thin section, 35 to 45 microns (millionths of a meter) thick, that has been separated from natural rock. He casts a mold from PDMS polymer, fabricates a tiny channel into the mold and binds it to the feldspar. He creates small holes in the mold for connecting capillary tubes that allow him to control fluid flow.

For his microfluidic experiment, Ciceri pushes an acidic solution through the channel, then measures the concentration of potassium released from the rock with a conductivity cell and ion sensitive electrodes.

After analyzing the amount of potassium released during the microfluidic experiment, Ciceri can remove the microfluidic mold and analyze the rock itself to study changes in its composition caused by exposure to the acid solution. He examines the rock with an optical microscope with both reflected and transmitted light. Microscopic analysis reveals grain structure and other facets of the material. Ciceri can also examine the feldspar with scanning electron microscopy, Raman spectroscopy and atomic force microscopy to reveal other characteristics.

The acidic solutions he uses in the lab are not representative of what occurs in natural soils, Ciceri explains. "It needs harsh treatment to release potassium, so that is where the processing part comes in. The rock by itself releases potassium slowly, so we are now processing the material in order to transform it into fertilizer that can more easily release potassium." Postdoctoral Associate Taisiya Skorina has developed a process based on crushing and chemically altering feldspar to produce a new compound, hydrosyenite, with a popcorn like texture in which potassium is contained in water-rich semi crystalline solids that make it more readily available. The chemical process is based on an alkaline treatment that does not lead to waste generation, a critical criteria for sustainable processes. (See related article.)

Experiments in a laboratory test tube or beaker do not fully replicate the microenvironment found in the soils, where both the size and shape of particles and pores, as well as the complex interactions with plants and microorganisms occur. "The way that a fluid behaves in a big system, like in a beaker, can be different from the way it behaves in a microchannel," Ciceri explains. Ciceri's work is about developing microfluidic devices that can better replicate soil conditions and complex interactions to validate on the bench top how the new material would perform under real world conditions. "The goal is to assess if the fluid behavior at the micro scale is the one that determines the overall result observed in the field," Allanore says. Ciceri and Allanore will present their microfluidic research at the World Soil Science Congress in Korea June 8-13, 2014.


Detail of microscopic image of feldspar subjected to acidic treatment. Thin line across middle has a width of 200 microns and shows the part of the feldspar exposed to the acidic solution. Shine and brightness resemble the gemstone opal. Image courtesy of Davide Ciceri.

If he can pursue the project long enough, Ciceri would like to add a biological component to the research, since microorganisms also play a role in weathering the rock in the natural environment. "The thing is that when do they do this, they do it in a small environment, in grains between the rocks or the soil particles," Ciceri explains. Although the amount of active molecules released by the bacteria in the soil microenvironment is small, their local concentration can be comparatively high. Conducting microfluidic studies would allow Ciceri to grow bacteria in the microchannel, reproducing more closely the natural environment, and assessing the leaching of potassium. That could yield insight into how much bacteria and fungi contribute to the discrepancy between what researchers see in the lab and what they see in the field.

A native of Italy, Ciceri received his doctorate in chemical and biomolecular engineering from the University of Melbourne in Australia. He earned bachelor's and master's degrees in chemistry from the University of Milan in Italy.

Written by Denis Paiste, Materials Processing Center

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Reconstituting feldspar for fertilizer 

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