Preying on Pollutants: Technology imitates life to generate cleaner water

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The sea anemone Actinia is a deceptive organism, in both name and appearance. For one, it is known as the “flower of the sea” for its colorful ring of tentacles. It also adopts a sessile lifestyle, preferring to lie and wait for food rather than go looking for it. When an unassuming plankton or fish passes by and stimulates its tentacles, the marine predator stuns it, abruptly retracts its tentacles, and gracefully encapsulates its catch.

In a classic example of biomimetics–the imitation of natural systems for the purpose of solving human problems–a team of researchers from Yale and Peking University has designed a coagulant that can capture contaminants in water via a shape eversion process reminiscent of Actinia’s feeding process. Due to its unique structure, the innovative technology can trap many different types of contaminants in a single step. This versatile feature can potentially enable tremendous increases in water purification efficiency.

Water and Energy: A Modern Compromise

This innovation comes at a crucial time, as it promises to revolutionize the way we treat water. One in nine people worldwide do not have access to potable water. Water scarcity is a multifaceted problem without a clear solution, and cleaning up water with modern water purification systems has a high energetic and economic cost. Water and wastewater facilities are responsible for approximately thirty-five percent of a typical US municipal energy budget. Today, in order to supply consumers with clean water, facilities release over forty-five million tons of greenhouse gases annually–an output equivalent to that of more than six million cars.

Current water treatment systems feature an extensive number of steps to ensure complete purification, because each step cannot remove all contaminants at once. Water contains many types of contaminants, each with drastically different chemical properties–think bacteria, harmful chemicals, and organic matter.

One popular purification method is coagulation, in which positively charged coagulants, when added into water, bind to negatively charged dissolved organic carbon (DOC) and carbon-based remains of organisms. The added coagulants and organic matter clump together into larger particles, then settle by gravity to the bottom of the basin so they can be more easily removed from the water. Another common step, sedimentation, allows heavier particles to settle, and a further chlorination step kills remaining microorganisms. Each of these methods requires specific technologies and tools, and the entire package together accounts for the high energy costs incurred today.

The newly developed technology developed at Yale and Peking University, aptly named Actinia-like micellar nanocoagulant (AMC), streamlines water purification in two ways: one, it decreases the number of steps; two, it maximizes energy efficiency. If AMC enters the water purification industry, it could well eliminate multiple types of contaminants in one step, an optimal solution to lowering both the time and cost of cleaning water.

Form Follows Function

Last month, the collaborative project published results demonstrating that AMC could capture a spectrum of different contaminants in a single step. Conventional coagulants, such as aluminum sulfate, can typically only remove bulky organic matter. AMC inherits its special properties from Actinia. Like the sea anemone, the coagulant features a flexible outer shell that can be everted–turned outward/inside-out–to reveal an inner core, which has contrasting chemical properties. The positively charged outer shell binds to DOCs and other organic substances, while the negatively charged inner core contains organic micelles that capture smaller micropollutants. Notably, AMC can also extract nitrates, an excess of which can be harmful to humans. 

Both inner and outer surfaces can be accessed by altering the pH–a scale from zero to fourteen that corresponds to a solution’s acidity–of the surrounding solution. When stored in acidic conditions with a pH lower than four, the coagulant maintains its stable form, with the inorganic shell completely shielding the organic core. Raising the pH around AMC is akin to stimulating the tentacles of the Actinia–in wastewater above pH four, AMC changes its configuration to reveal the organic core. “Shape eversion of the coagulant adds additional functionality. Coagulants neutralize the charges on the organic matter and cause them to clump together so they can settle. But when the core is exposed, it absorbs additional contaminants that other coagulants would not be able to,” said Ryan DuChanois, a graduate student in the Elimelech group, who worked on the project.

Another unique characteristic of the AMC is its ability to self-assemble. Much like the phospholipid bilayers that make up cell membranes, the AMC contains both hydrophobic and hydrophilic elements that rearrange in an aqueous solution. During the synthesis of AMC, hydrophobic carbon chains gather inwards, driven away from water, to form the organic core, and hydrophilic ionic complexes, comprising ammonium, silicon, and aluminum, form the outer shell. The researchers closely regulated the ratios of the reactants to achieve the product’s desired shape. Thanks to AMC’s optimized and stable build, it does not aggregate in solution and instead maintains its size and shape even after one year of storage.

Having synthesized the particles, the researchers had to perform characterization to confirm and optimize its structure. However, one significant challenge of this project was the nanoscale of the coagulant; specialized equipment was required to observe the structure and function of the miniscule AMC particles. Researchers confirmed the spherical shape of AMC using transmission electron microscope (TEM) images and approximated its size with dynamic light scattering (DLS) measurements. For both types of technology, a beam of electrons, or light, is transmitted through the molecule to map out its structure. The caveat is that these methods only work when the molecule being observed is stationary. This is not the natural working state of the AMC. As such, the researchers had to turn to other technologies to learn about AMC’s mobile functionality.

To observe how AMC self-assembles, researchers conducted molecular dynamics simulations. These computational simulations, among other visualization techniques, predict and explain how a coagulant will behave when exposed to different types of wastewater. “[Simulations] show why the shell is removing certain contaminants and why the core is removing certain [other] contaminants. You can pinpoint the exact mechanisms, like electrostatic and hydrophobic interactions,” DuChanois said.

Comparing Coagulants

Once the structure and function of AMC were confirmed and better understood, researchers progressed to the field research portion of the project. Jar tests–in which treatment parameters, such as dosage, mixing rate, and aeration time, are altered to determine how a coagulant will behave with specific contaminants–were used to simulate full-scale water purification processes. These were performed to compare the efficiency of AMC and conventional coagulants. Efficiency was determined by two factors: the resulting contaminant concentration, as well as turbidity, a quantitative measurement of a liquid’s murkiness.

AMC edged out other commercially used coagulants tested in the study–including a polymer called polyDADMAC, aluminum sulfate, and iron(III) chloride–with an average efficiency of over ninety percent. While all of the coagulants exhibited similar efficiency in removing turbidity, AMC was more successful at lowering DOC, phosphorous, and nitrate concentrations. Furthermore, while traditional coagulants demonstrated negligible removal of nitrate, AMC’s nitrate removal efficiency exceeded ninety percent.

Other key contaminants the researchers considered were organic micropollutants and certain pharmaceuticals. Commonly, water is contaminated by micropollutants from residue left behind by personal care products, hormones, and pesticides. Because these particles are nonbiodegradable, they persist in wastewater treatment discharges and enter the environment.

Conventional coagulants perform poorly in this category. Instead, the existing methods of removing micropollutants are ozonation and ultrasound. These approaches are energy-expensive, time-consuming, and work for only their specific class of contaminants.

In this study, the conventional coagulants demonstrated removal efficiencies between zero and sixty percent. AMC outshone all of them, achieving removal efficiencies of over ninety percent for all tested micropollutants. This makes AMC all the more promising as a future water purification technique because it offers an entire functional package. “The real benefit is that if you can remove many different types of contaminants in one step instead of using many steps, you can simplify the process, use less money, and take up less land. Overall, the process becomes more efficient,” DuChanois said.

The Big Picture

Ultimately, this project demonstrates how adaptations which species develop to face problems in nature can inspire technological innovations to solve human problems. Encouraged by the success of AMC, the researchers hope to continue applying their novel ideas to other areas of water treatment and materials science. “Coagulation has been in use for centuries and nothing has really changed. You would think at this point in time, after thousands of people have thought about this, that we would have perfected the water treatment process. But there’s always opportunity to innovate,” DuChanois said. Perhaps more innovations, building on the success of the AMC, will lead to effective solutions to the abiding challenge of energy-efficient water purification.