Starch-derived Material for Selective Precious Metal Recovery

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Gold jewel

Gold is a precious metal – and very useful as well as valuable. Image by Jastrow.

Researchers from York University developed a material – Starbon® – which can selectively recover precious metals such as gold, palladium and platinum.

The material is sustainable, being made from a renewable source (starch).

Researchers successfully used Starbon® loaded with the recovered metals as a catalyst, hence closing the precious metals loop.

Precious Metals

Precious metals are elements such as gold, relatively rare and with high commercial value.

Although we may associate them with jewels or ornaments, precious metals have many other important technological applications. These include catalytic converters in vehicles, catalysts in industrial processes, and electrical components in electronic devices.

The use of these metals has increased in recent years due to the development of low carbon technologies. Alternative / renewable energy, for instance, relies heavily on the use of some of these elements.

Critical Metals and Their Recovery

Because of this greater use, the supply of some precious metals could be at risk. In a recent report, the European Union compiled a list of elements classified as “critical.” The EU made this classification considering both the importance for the economy, and the risk for supplies. Examples of critical metals include palladium, platinum and rare earth elements.

difference between adsorb absorb

That’s not a typo – adsorb is a word that non-chemistry majors may confuse with absorb, but the two processes are not the same. Copyright image by Decoded Science, all rights reserved

One way to address this issue is the recovery of the metals. This, however, is often difficult as the wastewaters where these metals meet up contain many different elements. The recovery process, therefore, has to be based on a highly selective method.

Scientists have carried out a lot of research to develop effective recovery methods. The adsorption of the metals on the surface of an appropriate material is a possible solution.

Ideally, the material should adsorb the desired metal even in the presence of other species, with high efficiency. Moreover, the material should be made with a sustainable process, i.e. without using dangerous chemicals and using renewable sources.

New Adsorption Materials: Starbon ®

Scientists of the University of York (United Kingdom) made substantial progress in this field, as they developed Starbon ®, a new class of material which fulfils many of these criteria.

Decoded Science spoke to Ms. Andrea Muñoz Garcia, PhD student at the University of York involved in the research. She presented her work at the 19th Annual Green Chemistry and Engineering Conference, on the 15th of July 2015 in Maryland (US).

“We made our materials by trying to apply as much as possible green chemistry principles. In fact, we used starch as starting material, a renewable source which is readily available.

The synthesis involved different steps. First we expand the starch to create the porous structure – in fact we gelatinized it, then we cooled down the gel material, soaked it in ethanol, and finally removed the ethanol. Subsequently, the materials were drying using CO2 at supercritical conditions; to finish, the material was treated at high temperature (800 oC).”

The final obtained carbonaceous material – Starbon® – has a very high surface area (631 m2/g) and pores with an average dimension of 18.2 nm.

Gold solutions.

Other metals help to separate gold from wastewater. Image by Thejohnnler

High Adsorption Efficiency

To test Starbon® performance, Ms. Muñoz Garcia and her coworkers tested the removal of the Platinum Group Metals (PGM), i.e. gold (Au), palladium (Pd), and platinum (Pt).

Ms. Muñoz Garcia explains to Decoded Science:

“The PGM all have many important technological applications; their recovery, therefore, is something crucial for our future development. We tested Starbon® removal efficiency for these three metals in the presence of other species, to assess the selectivity of the material.

In fact, we used solutions containing Au3+, Pd2+, Pt2+ and, at the same time, nickel (Ni2+), copper (Cu2+), zinc (Zn2+) and iridium (Ir2+).

The results were very satisfactory, as Starbon® removed the PGM with very high efficiency (higher than 99, 90 and 80 % for Au3+, Pd2+ and Pt2+ respectively). At the same time, however, the removal of the other metals was much lower – almost none for Ni2+ and Zn2+, 9 % for Cu2+, and slightly higher for Ir2+ (31 %), which allows separation of the precious metals”

It is worth highlighting that, for Au3+, the removal efficiency is at least 30 times higher than for other adsorbing materials of natural origin (i.e. adsorbents made from coconut shells or peach stones).

Using the Material

Ms. Muñoz Garcia and her coworkers performed some preliminary tests to use the metal-loaded Starbon® as catalyst for the Heck reaction. This is a chemical reaction which normally is catalyzed by palladium nanopowders.

“The first tests gave good results, as we saw that the metal-loaded Starbon® was actually working as catalyst. We still have to optimize the performance of the catalyst, as we want to test them also on other reactions, and our future work will be focused on that; these data, however, showed us that the metal-loaded Starbon® has potential. In fact, the material is effective not just to recover the metals, but also to be used afterwards as catalyst.

In this way, we “close” the precious metal loop – we recover them from water streams and then we reuse them as catalysts – being supported in the same material employed for the recovery.

Prevent Waste and Sustainability

One of the green chemistry principles is to prevent waste generation. The work done by Ms. Munõz Garcia and coworkers indeed goes in that direction, as their material could be used to recover precious and critical metals and to reuse them as catalysts.

Moreover, as Starbon® is made from a renewable source (starch), its synthesis is sustainable; this again goes along with green chemistry principles.

Materials like this will help address the possible future shortage of precious metals, and help continue the technological development of our society.

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