Using lactic acid to separate oil and water

The Problem

Oil and water don’t mix. However, they’re still surprisingly difficult to separate and we spend a lot of money trying to do so. If you’ve ever heard of an oil spill, you’ve likely heard that they’re a pain to clean up. Oil is a severe environmental pollutant and can wreck habitats of all kinds.

In addition to large dramatic events like oil spills, we regularly separate oil from water in more mundane settings, like wastewater treatment. There’s a few common ways to do this, but they’re all expensive, relatively inefficient, and struggle with small oil droplets.

There’s been a growing push to use membranes as a replacement for the conventional separation processes. Membranes act like filters and separate out the water from the oil as the liquid is passed through them.

Unfortunately, many membranes are almost as environmentally unfriendly as the oil they’re trying to remove. Finding easily degradable but still cost-effective membranes is the key to using them for large-scale oil-water separations.

The Solution

This week’s scientists took a shot at oil-water separation by using a membrane made out of a modified lactic acid. That’s right, the same lactic acid that people say causes muscle soreness (spoiler; pretty sure that’s a myth).

Lactic acid is a small structure made of just a few atoms. However, when you join many lactic acid molecules together you can create a plastic-like material called polylactic acid, or PLA. PLA is commonly used in everything from food packaging to medical devices.

Given that it’s made from naturally occurring lactic acid, PLA is easy to degrade, environmentally friendly, and relatively inexpensive.

The only problem is that it sucks at separating oil from water.

The ideal oil filters let water pass but not oil. To do this, they’re typically very hydrophilic, meaning that water interacts well with them but oil doesn’t. Unfortunately, PLA isn’t strongly hydrophilic, meaning that both water and oil can get through filters made from it. Kind of a big problem when you want to use it to, uh, separate oil from water.

Of course, this week’s authors knew that, but were enthralled with the good qualities of PLA so decided to give it a try anyway. We love some stubborn scientists (we really do, science is tough).

The first thing they did was address the major issue of PLA letting oil through. To do this, they modified the typical PLA structure by adding in another material called PEO. This addition allowed the filter material to form hydrogen bonds with the surrounding water, making the filters much more hydrophilic.

It became a lot harder for oil to pass through the filters after this increased interaction with water. The addition of PEO took the PLA filters from slightly hydrophobic to superhydrophilic.

You can see this difference from the two graphs below. On the left side, the authors looked at how a water droplet behaves when you put it on the PLA membranes.

If the material is hydrophobic (aka a bad filter), the water beads up and doesn’t spread out. This translates to a high “contact angle” which is just a measurement of how beaded-up the droplet is. You can see this on the left image in the figure (the PLA only condition).

Once they added the PEO to their membrane, the water droplets spread out almost immediately when placed onto the materials. This indicates that the filters became hydrophilic and would no longer let oil pass through (both the H-PLA and H-PLA-AS conditions in the graph).

Additionally, more hydrophilic materials tend to take up water better. The graph on the right above shows that the more hydrophilic PLA/PEO membranes took up almost 3 times as much water as the PLA alone membranes.

Finally, they tested the ability of the PLA-PEO membranes to filter oil droplets from water.

They found that the PLA-PEO membranes could filter water nearly 6,200% faster than PLA alone (panel A in the top left), removed all but the smallest oil droplets (B, top middle), and worked consistently over the span of 4 weeks (C, top right). Their filters successfully separated an outstanding 99.6% of the oil from water.

Overall, this week’s scientists came up with a relatively simple fix that let the economically and environmentally friendly PLA be used as an oil filter membrane. This paper is on the more practical side of the science I read and has a great chance of making a real-world impact.

See you next week for more science,

Neil

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