Absorbent materials filter toxic chemicals from water

Absorbent materials filter toxic chemicals from water

Researchers at the Department of Energy’s Oak Ridge National Laboratory are tackling a global water challenge with unique absorbent materials designed to target not one but two toxic, heavy metal pollutants for simultaneous removal.

ORNL’s Santa Jansone-Popova of the Chemical Sciences Division, and Ping Li, now at Elementis Global, have discovered an adsorbent material with high selectivity for chromium and arsenic in real conditions where water resources contain many chemically similar elements.

Results published in Small demonstrated the new material captures chromium and arsenic in a balanced 2-to-1 ratio. The fundamental advance creates a synergy between chromium and arsenic capture. The more chromium the material grabs, the more arsenic it can also remove.

“It is rare for an adsorbent to capture two pollutants simultaneously. It is also rare for them to work quickly and efficiently in realistic scenarios to address the broad range of water conditions worldwide,” Jansone-Popova said.

Chromium and arsenic

Chromium and arsenic are two of the most dangerous pollutants found in drinking water worldwide. Both are toxic and can cause adverse health effects, including cancer. Low levels pose significant risks to living organisms because doses build up with each exposure. They can gradually reach harmful amounts. These elements occur naturally. However, their presence in the environment has increased with industry and urbanisation as byproducts of broad mining, production and manufacturing. Releases impact air, soil and water, but drinking water is the most common route of exposure.

In water, these metals dissolve to form chromate and arsenate salts. They are chemically similar to beneficial minerals naturally present, including phosphate, sulphate, nitrate and bicarbonate. Chromate and arsenate are highly mobile in water and can have far-reaching impacts. They do not degrade and are permanent in the environment without intervention. Targeted approaches are needed to separate these metals from harmless mineral salts vital to the ecosystem.

Comments from the researchers

Jansone-Popova is part of an ORNL group specialising in studying absorbent materials designed to target specific elements and bind them to a surface. Adsorbents have broad applications in helping recover precious metals or remove environmental pollutants.

“They are one of the most promising water treatment options. They are affordable, easily deployed and can work quickly to filter water supplies. However, they need to be tailored for practical use in cleanup scenarios,” Jansone-Popova said. “The challenge is to design absorbent materials that can effectively isolate trace amounts of harmful elements very similar to the bulk chemical species found in water.”

In adsorbent design, selectivity is key. Because a material’s surface offers limited real estate, the goal is to grab only targeted elements. They should capture as much as possible before the adsorbent fills up and needs to be replaced or recycled. Poorly selective materials lack the precision to single out targets in mixed environments, such as water, where similar elements compete for space.

Absorbent material research

Jansone-Popova previously led the design of an adsorbent with high selectivity for chromate that works rapidly and in the presence of competing species to decontaminate water. A study published in Environmental Science and Technology showed the novel material decreased chromate concentrations 100-fold within one minute (1 part per million to 10 parts per billion) and achieved an order of magnitude below allowable limits set by the U.S. Environmental Protection Agency.

Ping Li’s team builds on the approach to develop a framework for capturing chromate and arsenate with one material.

“Our starting material is highly effective at capturing chromium in its most toxic form, hexavalent chromium, but the approach was not designed to be selective for arsenic,” Li said. “As this reaction happens, however, the material changes, creating a platform for new chemistries.”

Researchers modified the original structure to reduce captured chromium-6 into a less toxic state, chromium-3. Chromium-3 also has the benefit of providing an anchor point to bind arsenate. The new system enables a chemical reaction that forms stable chromate-arsenate clusters strongly bonded to the surface. The result traps the toxins permanently because they will not wash off or detach from the filter material without intentional removal by chemical processing.

Absorbent materials trap toxic pollutants

“We leveraged the efficient capture of hexavalent chromium to introduce a new architecture that could also bind with arsenic,” Li said.

Once used as an additive in pressure-treated lumber, Chromate arsenate inspired the structure.

The team has patented the structure and is working with collaborators to expand the approach to other environmental pollutants.

“Fundamental discoveries like these can help us reduce toxic pollutants in the environment and meet regulatory goals for clean water,” Jansone-Popova said.

The journal article is published as “Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process.”

The work has supported the Office of Science and used the Computer and Data Environment for Science resources at ORNL.

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