Phosphates and metals are recovered for reuse, leaving clean water behind
In a city, managing where rainwater or snow-melt goes is an immensely complex challenge. While grassy and planted areas can absorb water, impervious materials like concrete, asphalt, and roofing materials cannot. As rainwater flows across these surfaces, it can collect any pollutants that might be present on them; sediments, fertilizer and pesticides, household chemicals, hydrocarbons from spilled fuel, animal waste, rubbish, and metals including zinc and lead. By the time runoff water makes it into storm drains and sewers, it may be heavily contaminated. As a result, many (but not all) urban areas now direct storm-water to a treatment plant before it is released into the environment.
Standard approaches to storm-water treatment are often slow and expensive, with some pollutants proving challenging to remove entirely. In addition, contaminants like phosphates and copper and zinc are finite resources, and are amongst our most valuable elements – phosphorus fertilizers underpin the world’s food supply, copper wires are found in every electrical device on the planet, and zinc is a vital ingredient in everything from paints and batteries, to medicine and steel. But they’re currently being washed away into urban drains.
An invention from materials scientists at Northwestern University aims to change the equation on storm-water treatment. They have developed a specialized spongy material that can not only suck up a range of pollutants, but also release them as desired, allowing them to be recovered for potential reuse.
They published their work this week in the American Chemical Society Journal ES&T Water.
A tiny sponge to clean up big spills
The group, led by Professor Vinayak Dravid, has been working on porous nanocomposite materials for environmental cleanup for years, patenting and commercializing them via a spin-out company called Coral Inventions. Previous iterations of the composite based on polyurethane have been used to clean up oil spills in a busy shipping port, treat wastewater from a large food factory, and filter lead from contaminated water down to undetectable levels.
The starting point for the latest version of their high-tech composite would look familiar to anyone who washes their dishes or cleans their homes – a pure cellulose sponge.
“We chose cellulose for several reasons, but the first is that it’s a sustainable material that can be made from waste,” says Kelly Matuszewski, a PhD student at Northwestern and lead author on the paper. Cellulose sponges are made from wood pulp, which is a by-product of the paper industry, or hemp fibers. This makes them biodegradable, unlike the colorful plastic sponges that adorn many a kitchen sink. They’re also robust, and have what’s known as hierarchical porosity – pores of many different sizes – which is important for the next part of the manufacturing process.
As Matuszewski describes in the paper, the team then dips the sponge into a wet slurry of iron oxide (magnetite) nanoparticles, which its pores eagerly soak up, before drying it in the oven. These steps are repeated several times until they’re left with an entirely black sponge that has nanoparticles deep within its porous structure. This combination of many, many tiny particles on and in a densely-porous sponge provides a huge surface area with which to capture pollutants.
“A lot of the time, storm-water has multiple contaminants in it – it’s never just phosphate alone,” says Matuszewski, speaking to me over Zoom. “So, I wanted to take the nanocomposite and try to introduce it to a more realistic contaminant stream; specifically for urban storm-water.” To test this, she pumped a mixture of water, phosphorous, cooper and zinc (at varying concentrations) through the coated cellulose sponge, and measured the levels of contaminants that remained in the water.
The sponge captured nearly 100 % of the phosphorous and copper in the water, as well as more than 80 % of the zinc. The pollutants were undetectable after passing through the sponge.
Recovering valuable minerals from the water
Cleaning the water was step one for Matuszewski. She also wanted to separate out the contaminants so that they could potentially be repurposed. For this, she used an approach called pH-Assisted Selective Extraction (pHASE). This involved taking the sponge nanocomposite – saturated with the pollutants it had absorbed from the water – and pumping an acidic solution (pH = 3) through it. This released the copper and zinc. Then a basic solution (pH = 11) was pumped through it, which released the phosphorous. In effect, she says, “we’re able to use pH as a lever to be able to selectively recover the metals and the phosphates into different waste streams. And then we can potentially turn that waste into wealth.”
The sponge is itself reusable too, as Matuszewski explains, “Once we regenerate it, and once we get those pollutants off it, we can then reuse the nanocomposite multiple times without seeing any decrease in its performance.” This, she says, is what makes this material attractive to the water industry, “emerging water technologies have to be incredibly cheap to make economic sense. Our process has that; a very low energy technique gets it [the sponge] ready to be reused again.”
Chicago’s stormwater drains
These experiments haven’t solely happened in an environmentally-controlled research lab. The Northwestern team is collaborating with StormTrap, a Chicago-based storm-water treatment equipment manufacturer. The three pollutants they’re investigating were specifically requested by StormTrap, and the company also defined the rate at which the contaminated water would need to be able to flow.
“They made it clear that they needed this to work at high throughputs, because in a storm drain, that’s how you avoid flooding,” says Matuszewski. “Thankfully, our platform is fine with water passing through it really, really fast.”
StormTrap is currently testing the nanocomposites in their own large-scale research system. There, water enters through a central inlet, before flowing through the sponges – arranged radially around a circular vessel – and onto the next stage in their process. “This is what the setup could look like in a real storm drain,” she says. The recovery stage would happen separately, with the nanocomposite sponges temporarily removed once a year to undergo a scaled-up version of the pH-based process that Matuszewski used in the lab.
There are some research questions that still need to be answered, particularly around the long-term performance of the sponge, and its behavior in different water chemistries, but Matuszewski seems optimistic, based in their results to date.
“Our group is focused on the circular economy model approach to the challenge of water scarcity. That means doing more than just making sure the water is clean; it’s also about reclaiming these non-renewable resources from that water so they can be reused.”
