Introducing
Your new presentation assistant.
Refine, enhance, and tailor your content, source relevant images, and edit visuals quicker than ever before.
Trending searches
Water Filtration w Ion Selectivity Based Membranes
2
Water Filtration with Ion Selectivity Based Membranes
Using Research performed by Samuel J. Lounder and Ayse Asatekin
Why A New Filtration Method is Necessary
Why?
- Current Water Filtration Methods are Not Sustainable
- Require a large power supply
- Lithium and Uranium are nonrenewable resources which limit:
- Lithium battery production
- Nuclear power generators
- Reverse Osmosis membranes have similar drawbacks:
- Very high water pressure which:
- Requires large amount of energy
- The membranes are easily compacted
The challenge:
- Synthesize a water filterable membrane that meets the following:
- Can separate ions of similar charge and size
- Fully scalable method
- Uses self-assembling synthetic co-polymers
- Made of materials that are low cost
The Goals
https://medium.com/swlh/how-small-can-we-go-33c6167cf681
Background Terms:
https://phys.org/news/2010-07-lessening-penalty-block-copolymer-nanostructures.html
Terminology & Abbreviations
- Nanoconfinement: Being trapped in a nanosized area
- Block Copolymers (BCPs): when two monomers cluster together to form 'blocks' of repeating units
- Self-assembling: Spontaneously form ordered and stable structures
- Zwitterions (ZI): a molecule/ion having positively & negatively charged groups
- Cations: positively charged ions
- Anions: negatively charged ions
Specific to This Discovery
Process/Experiments
- Selectivity: the ratio of desired results to undesired
- Rejection %: the amount of ions that went through the membrane (i.e. get rejected by the system)
- Partition Coefficient: The ratio of a substances concentration of 1 phase to that in a 2nd phase
- Sorption: Absorption and adsorption in a single process
- Permeabiliity (P): measurement of the flux/flow of a target substance/particle through the material
- Diffusivity (D): the rate that things can spread
The interactions control ion separation and diffusion rate
What We Know About Biological Ion Channels (BICs)
- Capable of ion selectivity that can lead to new applications of membranes.
- Utilizes the favorability and strengths of specific functional groups
- So they can separate similarly sized ions
Applying Basic Understandings
What Do BICS Show Us?
- BIC small pore diameters creates a nanoconfinement
Smaller pore size increases selectivity!
Amplifying selectivity
Synthetic Replication
- Block Co-polymers (BCPs)
- Have a synthetic ultra-filtration membranes
- However, the pore sizes (>2 to 3 nm) are too large to separate ions
Block Co-polymers
Why BCPs?
- Low Cost & Scaleable
- Self-assembling
- Pore size is modifiable through different mechanisms
Random Zwitterionic Amphiphilic Co-polymers
- Randomly self-assembling zwitterions (ZI) that utilize ZI interactions and hydrophobic repeated units
- The ZI interactions form nanochannels that are surrounded by a hydrophobic phase
Zwitterionic Amphiphilic Co-polymers (r-ZACs)
- Polymerization cross-links the hydrophobic area
Prevents the nanochannels from swelling
Thin Film Composites (TFC)s
- Common in water filtration systems due to their porous nature
TFCs
- Typically suffer from compaction under pressure
- Tighter fitting structure that results in a lower porosity
- The higher the pressure, the greater the compaction
https://www.mdpi.com/2077-0375/8/3/68/htm
Increased Performance of TFCs Do To Structural Integrity
- r-ZAC membranes lessen the compact of the polymers as they provide structural support, with the pores not swelling, the pore size remains effective
Personally drawn while refrencing figures within sources
Combining TFCs with r-ZACs
Schematic Showing the ZI interactions between the TFC and r-ZACS
Cross Section image of TFC membrane. Top layer is r-ZAC support
ZAC-X (Highly cross-linked r-ZACs
- Self-assembled membrane system lined with a surplus of ZI groups
ZI-anion interactions provide more salt separation and permeation rates than ZI-cation interactions
Cross-linked r-ZACs
- Higher structural integrity for the TFC membranes and obstruct the flow of the charged ions
Sodium & Chloride Salt Rejection
- ZAC-X has more ZI groups within the narrow nanochannels
- Forces the confinement of permeating ions causing more ZI interactions to take place
- High concentrations lead to interaction based selectivity
Rejection Test
Weak hydrate ions (ClO4) were favored over strong (SO4-)
Sodium vs Chloride Solutions
Rejection not determined by hydration strength
Rejection increased with hydration strength
Rejection Test
- Weaker Poly-cation interactions have limited impact on selectivity
Salt Partition Coefficient
- eq. of the line is y=mx+b with the slope being the Salt Partition Coefficient (Ks)
Ks follows the ZI-anion interaction
Sorption to Determine Selectivity
- The preference for salt is rarely observed in membrane system and suggests more frequent interactions
Supported by the more narrow pores caused by ZAC-X
- Faster permeation rates were observed for salts more in favor of ZI-Anion interactions
Sorption
Measuring Permeability to Determine Selectivity
- The permeability of alts over a large range of concentrations
- For lower concentrations (20 to 60 mM):
- P increased with concentrations for all salts except NaClO4
- For the highest concentration (500 mM):
- P increased for all, but NaClO4 barely changes
Permeability (Ps)
Since NaClO4 behaved differently, it can be speculated that the permeation mechanism NaClO4 changes slightly at high concentrations
Diffusivity (Ds) using Ks & Ps
- For a ZI-complexed ion to diffuse, it must break the temporary bond with the ZI-cation
- Interactions occur as follows:
Suggested Binding Strengths
Diffusivities (Ds)
Are the interactions strong enough?
- NaCl permeated faster than NaF through ZAC-X
- Ideal for removal of fluoride from drinking water
- ZAC-X membranes show no irreversible blockage or change in channel size even with challenging feeds
Chloride-Fluoride Separation
ZAC-X completes the challenge:
- Scalable
- Self-assembling
- Low Cost
- Need less maintenance
- Extremely low risk of compaction
Does Selectivity Decrease in Complex Feeds?
- Mixed salt filtration was compared to single
- Flux rate was not affected nor was the rejection rate in the mixed solutions
Mixed Salt Filtration Test
NaCl does not inhibit the Fluoride selectivity
KEY RESULTS
Cross-linked r-ZAC membranes have a very high potential to provide a cost, effective, sustainable source of clean drinking water
Observing BICs showed the nanoconfiment Increases selectivity due to more interactions
r-ZACs are auto-assembling co-polymers that are cost efficient and scalable
Conclusion
Cross-linked r-ZACs (ZAC-X) provide structural support to the TFC membranes which prevents compaction
ZI-anion interactions in the nanochannels lead to enhanced salt ion separation based on the salt partition coefficient
ZAC-X can remove Fluoride without the need of remineralization since NaCl has a higher permeability and doesn't interact with other ions
The mechanisms of the salt ions with the ZI groups are still to be determined, but its likely due to size of the bonds within the anions
Figures not labeled are from:
Sources
- Lounder, Samuel J., and Ayse Asatekin. “Interaction-Based Ion Selectivity Exhibited by Self-Assembled, Cross-Linked Zwitterionic Copolymer Membranes.” Proceedings of the National Academy of Sciences, vol. 118, no. 37, 2021, pp. 1–7., https://doi.org/10.1073/pnas.2022198118.
- Lounder, Samuel J., and Ayse Asatekin. “Interaction-Based Ion Selectivity Exhibited by Self-Assembled, Cross-Linked Zwitterionic Copolymer Membranes.” Proceedings of the National Academy of Sciences, vol. 118, no. 37, 2021, pp. 1–7., https://doi.org/10.1073/pnas.2022198118.
Questions?