Scientists have tested many ways to remove microplastics from water, but most come with major drawbacks.

In many cases, the process creates contaminated material that has nowhere useful to go.

Filters can pull particles out, but the sludge still needs storage or disposal. The plastic just moves from one location to another.

To address this issue, researchers built a system where the algae that captures microplastics also turns them into something new.

The removal and manufacturing happen in the same tank, and the whole process runs on the oily compound that gives orange peels their scent.

Algae that cling to plastic

The research was led by Dr. Y. Dai in the College of Engineering at the University of Missouri (Mizzou). The system is called RUMBA, short for Remediation and Upcycling of Microplastics by Algae.

Today’s wastewater plants can pull out visible chunks of plastic that float to the surface.

The smallest fragments slip past every filter and end up in drinking water, lakes, and rivers. There’s been no effective way to retrieve them.

The team’s idea was to let the algae do the work. Scientists already knew that cells can trap plastic, causing the clumps to sink so the biomass can be collected.

The challenge was understanding the chemistry behind the process.

Citrus chemistry explained

The researchers did not choose a random alga for the job. Instead, they genetically modified a fast-growing strain of cyanobacteria called Synechococcus elongatus.

The strain pumps out limonene, the natural oil that gives oranges their citrus smell.

Limonene does not stay inside the cells. It migrates to the cell surface, where it makes the algae repel water.

Because most plastics also repel water, the algae stick to the plastic when the two are mixed together, pulling it downward.

Algae prove effective

Particles between 200 and 800 nanometers wide – far too small to see without a microscope – were mixed in with the engineered cells.

Within an hour, 91.4% had settled to the bottom of the container. Nothing comparable happened with the unmodified strain.

This discovery wasn’t subtle. Electron microscopy images show plastic specks crowded at the seams where modified cells clung together.

Specialized chemical imaging picked up the limonene signal sitting in those exact spots – right where the plastic clusters formed.

When the team added a detergent-like chemical that broke up the water-repelling surfaces, the entire effect collapsed.

Strong evidence suggested that the limonene coating was the agent doing the work.

Real-world cleanup results

A clean laboratory tank is one thing, but real wastewater is murkier and thick with dissolved organics and competing particles.

The team tested the system with samples from a local treatment plant and a campus lake.

The engineered algae cleared roughly 90% of polystyrene from both water types at 500 or 800 nanometers. Removal still hit around 80% for the smallest 200-nanometer particles.

Other plastics responded too. PET, the polymer contained in soda bottles, and polyethylene, the plastic in most grocery bags, both clung to the cells under the microscope.

Solving multiple problems

From there, the researchers tested further. The modified cells were allowed to grow inside wastewater for several days, feeding on whatever was already there.

Over five days, the algae stripped 97.5% of the nitrate and nearly all of the ammonia from one sample. When a bit of growth media was added, phosphate removal climbed close to 100 percent.

Meanwhile, the same cells kept grabbing microplastics, hauling out as much as 88.6% in extended runs.

One tank completed three jobs: nutrient cleanup, plastic capture, and biomass production. Pulling those particles out before they are deposited into a drinking glass is incredibly important.

A recent paper tied microplastics found in human artery plaque to higher rates of heart attacks, strokes, and death.

From waste to bioplastic

The plastics collected through earlier methods were often stored away or dumped in landfills, trading one environmental problem for another.

In contrast, Dai’s team treated the algae-plastic sludge as raw material. They processed the sediments and then ran mechanical tests on the resulting films.

The composite stretched 2.3 times farther than pure polystyrene, and it absorbed 2.2 times more energy before it broke.

The films had a faint golden-green tint from chlorophyll and carotenoids in the biomass. They contained less pull strength but were more flexible and tougher than pure plastic.

Until this study, no one had paired microplastic capture with bioplastic upcycling in a single workflow. The captured pollution now becomes a feedstock instead of creating a disposal problem.

Scaling the technology

The early numbers are promising. An open-pond version could produce bioplastic at $3.58 per kilogram. This number is near current bioplastic prices.

Furthermore, the process runs on renewables and removes more carbon dioxide than it emits. Every other bioplastic method adds emissions.

Dr. Dai already runs a 100-liter bioreactor at Mizzou nicknamed “Shrek,” currently used to scrub industrial flue gas. Bigger versions, she noted, could eventually be installed into city wastewater plants.

“By removing the microplastics, cleaning the wastewater and eventually using the removed microplastics to create bioplastic products for good, we can tackle three issues with one approach,” concluded Dr. Dai.

The study is published in the journal Nature Communications.

NOTE – This article was originally published in Earth and can be viewed here