More than one-third of the world’s accessible fresh water is used for public or industrial purposes, resulting in widespread contamination by toxic chemicals, flushed-away medicines, radioactive molecules, and hydrocarbons from oil-and-gas operations.
Both industry and academia are looking at new ways to clean up contaminated water resources. Researchers at the University of Minnesota, for example, have recently announced the creation of a nano-sized “silica sponge” that contains oil-eating bacteria.
Using a unique electrospinning technique, mechanical engineering professor Alptekin Aksan and microbiology professors Larry Wackett and Michael Sadowsky, all members of the University of Minnesota’s BioTechnology Institute, have successfully embedded living, oil-eating bacteria within porous silicon fibers. The oil contaminants pass through the pores of the fibers and are consumed by the active bacterial cells. The fibers can be spun to create a spongy material that can be a mitigation resource for treating contaminated water or oil spills.
Design Challenges Abound
“For engineers, this approach is a multi-scale transport problem, since one needs to consider phenomena from the macro-scale all the way down to the micro-scale, including the chemical reactions in the cytoplasm of the cell, at the cell-gel wall interface, and the quite interesting mechanisms that are at play at those small scales—such as exclusion of specific ions and salts from small pores due to interface effects,” says Aksan.
Other challenges include designing a matrix that is strong enough to protect the embedded microbial factories and can be handled, stored, transported, processed, and re-cycled safely. Designing the silica sponge is also an interesting manufacturing challenge because there are several methods (extrusion, electrospinning, molding, emulsion) that can be used to produce different products for different purposes.
Bacteria may someday clean leftover frack water. Image: umn.edu
For example, although electrospinning has been widely used to manufacture products in other fields, it has only recently been considered for environmental remediation. These electrospun fibers must be highly permeable and have small pore sizes, high specific surface areas, and good pore interconnectivity. Silica is an excellent material for bioencapsulation because of its low cost, mechanicalrobustness, manufacturability, thermal and pH stability, and chemical inertness. However, fabrication of silica nanofibersvia traditional electrospinning methods requires organic solvents that may be toxic to the encapsulated bacteria.
To eliminate this problem, Aksan and his team developed a microfluidic timer coupled with a coaxial electrospinning system to produce nanofibrous membranes with a bacterium-containing core and a silica-based shell surrounding the core. Solution parameters (viscosity, conductivity, and surface tension) and processing parameters (applied voltage, working distance, solution flow rate, needle diameter, and core needle protrusion) were carefully monitored and controlled. This allowed the nanofibers to be electrospun continuously into a fabric with a very high loading efficiency (up to 40 grams of bacteria per 100 grams of matrix material).
Biology and engineering have collaborated for decades, notes Aksan, but the focus has always been on biomedicine and biomedical engineering for human health.
“What is unique about our approach is that we can show that biology can be used in industry, as well in large scales, to solve problems of significance such as pollution control and bioremediation at very low cost and in a ‘green’ way,” he says. “The beauty of the approach is that we use naturally occurring microbes and design/engineer a system that couples them to existing industrial processes and increases their efficiency and longevity, with absolutely minimum levels of external energy supplied.”
Continuous encapsulation of bacteria in water-insoluble, biocompatible, high-porosity nanofibrous membranes, as well as maintaining the activity of the encapsulated bacteria, are very important steps for scaling up production of bioreactive membranes to be used in industrial wastewater cleaning applications. Aksan hopes to further develop his silica-sponge technology to include gas-phase reactions and the production of valuable chemicals (instead of degrading them).
Bioremediation, he stresses, is a multi-disciplinary science that represents collaboration among mechanical engineers, biochemists, and microbiologists. Aksan and other professors at the BioTechnology Institute co-supervise the research projects of five or graduate students from mechanical engineering, microbial engineering, and biochemistry who ultimately become experts in the field of bioremediation engineering. “We have tremendous industrial and institutional support and it is tremendously rewarding that people from around the world come here to learn the techniques we have developed,” he says.
Mark Crawford is an independent writer.
What is unique about this approach is that we can show that biology can be used in industry, as well in large scales, to solve problems of significance such as pollution control and bioremediation, at very low cost and in a ‘green’ way.
Alptekin Aksan, University of Minnesota