Kanazawa University researchers have developed a protocol to efficiently purify palladium and silver ions from industrial waste. The protocol also converts the ions into pure metallic elements. This will help increase global stocks of valuable elements that are widely needed yet in scarce supply.

Many rare metals are in scarce supply, yet demand for use in chemical catalytic reactions, electronics, medical instrumentation, and other purposes continues to increase. As waste, many of these metals pollute the environment and harm human health. Ideally, we would recycle the metals from waste for reuse. Unfortunately, current recycling methods are usually some combination of complex, expensive, toxic, and wasteful processes – and ultimately inefficient.

In a study published in Chemical Engineering Journal, researchers from Kanazawa University reported a major improvement in recovering silver and palladium ions from aqueous acidic waste. Recovery of the metals in elemental, metallic form is straightforward – simply burn the extraction material and collect the remaining metal after further heating.

Graphic shows silver and palladium recycling steps in a new safer and low cost process. Image Credit: Kanazawa University. Click image for the largest view.

The researchers chemically modified ultrasmall particles of cellulose, an abundant and nontoxic biopolymer, to selectively adsorb silver and palladium ions at room temperature. Adsorption was nearly complete at an acidic pH with acid concentrations of around 1 to 13 percent by volume. Those are reasonable experimental conditions.

Lead author Foni Biswas explained, “The adsorbent selectively chelated the soft acid silver and palladium cations. Of the 11 competing base metals we tested, only copper and lead cations were also adsorbed, but we removed them with ease.”

Maximum metal ion adsorption was fast – e.g., an hour for silver. Maximum adsorption commonly requires many hours with other approaches.

Senior author Hiroshi Hasegawa explained, “Intraparticle diffusion did not hinder adsorption, which is an endothermic, spontaneous chemical process. Maximum metal adsorption capacities – e.g., 11 mmol/g for silver – are substantially higher than that reported in prior research.”

After the adsorption step, the researchers simply incinerated the cellulose particles to obtain elemental silver or palladium powder. Subsequent higher-temperature incineration converted the powder into pellets. Cyanide or other toxic extractants were not required. Spectroscopic analyses indicated that the final metal pellets were in metallic rather than oxide form.

Biswas noted, “We removed nearly all of the silver and palladium from real industrial waste samples. Obtaining pure and elemental metals proceeded as smoothly as in our trial runs.”

Palladium and silver are valuable metals yet natural supplies are increasingly limited. Future needs require that we recycle the metals that we already have in a practical manner. The research reported here is an important development that will avoid supply and distribution difficulties that will only increase in the coming years.

Well . . . The team’s last paragraph summed it up quite nicely and your humble writer heartily agrees. Now, lets hope the team keeps going to extract the rest of the list of rare metals in such a safe and effective manner. Its not a really long list and there is getting to be an immense amount of junked materials stacking up world wide.


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