MIT researchers designed a device that quickly recovers drinking water from an atmospheric water harvesting material. The system uses ultrasonic waves to shake the water out of the material, recovering water in minutes.
It’s possible, but not as much risk as other factors. I would expect the sorbent needs to be replaced mostly due to atmospheric exposure than the sonication. Sonication could accelerate environmental degradation. This conclusion would vary by the sorbent used.
Risk 1: degradation of the sorbent supramolecular structure.
The paper uses a hydrogel made from polyacrylamide and lithium chloride (PAM-LiCl) that is placed on top of a sonicator and typically treated <10 min at a time. I’m not familiar with PAM-LiCl hydrogels specifically, but many polymeric hydrogels have self-healing properties for the super molecular structures that could reform after disruption by something like sonication or shear-stress. This is addressed in the supplementary info of the paper and the conclusions sections say they didn’t see the structure break down from the treatment based on SEM images. I did not look at this figure to double check that statement.
Risk 2: degradation of the polymer/molecular structure.
PAM is relatively resistant to the mechanical and temperature stresses it would be exposed to under these conditions, so I expect little risk from the sonication treatment. To assess that, we should consider the nature of that treatment.
They designed their sonicators themselves, so I can’t directly compare them to what I’m familiar with using, but the frequency they use is relatively high (>100 KHz) with relatively low power (1.5 W) compared to the types of sonicators used to break apart nanomaterials and the energy from the treatment does not appear to be focused, meaning the energy transfer is spread throughout the material relatively well. At higher frequencies, sonication treatment is not particularly well suited to breaking apart materials. You would want closer to 40 KHz to effectively break apart materials in the microparticle to nanoparticle range (typical size range for hydrogel supramolecular structures). The power used here is also relatively low and unfocused, so even if the frequency was lower, it wouldn’t have much power to break apart the sorbent material a d the energy is well distributed. Any focal points created could experience higher rates of degradation if they exist within the material.
If the treatment is not risky, then the main risk for PAM degradation is exposure to the atmospheric environment. This would be necessary for water extraction, but could expose PAM to conditions where it would begin to degrade or become contaminated and lower the water extraction efficiency. Acidic or alkaline conditions, exposure to oxidizers, free radicals, etc. would be the main contributors to degradation. Exposure to all of these would increase with time exposed to the atmosphere (and oxygen).
Conclusion:
My guess is that the sorbent would eventually need to be replaced, but even if the sonication doesn’t contribute to this, exposure to the atmosphere will be a big factor in contamination/degradation of the sorbent material. What sonication treatment would likely do is contribute to speeding this degradation up. I could also see something surprising happening like the sonication process protecting the material by helping clear contaminants that lead to faster degradation via “washing” with the water pulled from air too, so maybe it could even be beneficial.
The authors discussed applying this to other sorbents, so my answer would change with different materials, but where it would be viable to apply this technology, you could engineer the sonication to be minimally fatiguing to the sorbent.
I’m curious, do ultrasonics also fatigue the capture material, shortening its lifespan?
It’s possible, but not as much risk as other factors. I would expect the sorbent needs to be replaced mostly due to atmospheric exposure than the sonication. Sonication could accelerate environmental degradation. This conclusion would vary by the sorbent used.
Risk 1: degradation of the sorbent supramolecular structure.
The paper uses a hydrogel made from polyacrylamide and lithium chloride (PAM-LiCl) that is placed on top of a sonicator and typically treated <10 min at a time. I’m not familiar with PAM-LiCl hydrogels specifically, but many polymeric hydrogels have self-healing properties for the super molecular structures that could reform after disruption by something like sonication or shear-stress. This is addressed in the supplementary info of the paper and the conclusions sections say they didn’t see the structure break down from the treatment based on SEM images. I did not look at this figure to double check that statement.
Risk 2: degradation of the polymer/molecular structure.
PAM is relatively resistant to the mechanical and temperature stresses it would be exposed to under these conditions, so I expect little risk from the sonication treatment. To assess that, we should consider the nature of that treatment.
They designed their sonicators themselves, so I can’t directly compare them to what I’m familiar with using, but the frequency they use is relatively high (>100 KHz) with relatively low power (1.5 W) compared to the types of sonicators used to break apart nanomaterials and the energy from the treatment does not appear to be focused, meaning the energy transfer is spread throughout the material relatively well. At higher frequencies, sonication treatment is not particularly well suited to breaking apart materials. You would want closer to 40 KHz to effectively break apart materials in the microparticle to nanoparticle range (typical size range for hydrogel supramolecular structures). The power used here is also relatively low and unfocused, so even if the frequency was lower, it wouldn’t have much power to break apart the sorbent material a d the energy is well distributed. Any focal points created could experience higher rates of degradation if they exist within the material.
If the treatment is not risky, then the main risk for PAM degradation is exposure to the atmospheric environment. This would be necessary for water extraction, but could expose PAM to conditions where it would begin to degrade or become contaminated and lower the water extraction efficiency. Acidic or alkaline conditions, exposure to oxidizers, free radicals, etc. would be the main contributors to degradation. Exposure to all of these would increase with time exposed to the atmosphere (and oxygen).
Conclusion:
My guess is that the sorbent would eventually need to be replaced, but even if the sonication doesn’t contribute to this, exposure to the atmosphere will be a big factor in contamination/degradation of the sorbent material. What sonication treatment would likely do is contribute to speeding this degradation up. I could also see something surprising happening like the sonication process protecting the material by helping clear contaminants that lead to faster degradation via “washing” with the water pulled from air too, so maybe it could even be beneficial.
The authors discussed applying this to other sorbents, so my answer would change with different materials, but where it would be viable to apply this technology, you could engineer the sonication to be minimally fatiguing to the sorbent.
Sources:
The nature paper didn’t want to load for me, so I used arxiv for the paper High-efficiency atmospheric water harvesting enabled by ultrasonic extraction)
Polyacrylamide degradation and its implications in environmental systems