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Science: Cleaning mining waste using microorganisms

Published on
November 23, 2019

Using a combination of chemistry and biology, PhD researcher Silvia Vega developed an effective method to convert toxic arsenic, present in mining waste, into scorodite. These are stable iron-arsenate crystals that can be stored safely. The method is environmentally friendly, cost-effective and helps to deal with the increasing problem of arsenic-containing mining waste.

Copper and gold mining and the subsequent processing of these metals is associated with the production of large quantities of waste, containing a variety of metals, including arsenic. This waste is successively stored in so-called mine tailings. These consist of a mixture of ground waste rock and effluent containing chemicals. However, the arsenic present may easily leach into the environment, poisoning water supplies. Globally, arsenic pollution affects millions of people. ‘I am originally from a mining town in Chile, and have experienced the effect of arsenic pollution specially on drinking water which can’t be directly consumed from the tap’, Vega says. ‘And this problem will increase in the future, since only rock containing small amounts of copper is left. Mining these low-grade ores results in even more arsenic waste.’

Disadvantage

To make mine tailings less toxic before disposal, arsenic must be oxidized, commonly using a powerful oxidant, like hydrogen peroxide. Thus, toxic arsenite, As3+, is oxidized to the less toxic arsenate, As5+. However, these processing costs are high and therefore, mining waste usually remains untreated. As a result, it poses a serious hazard to people and environment. ‘Technologies for better arsenic management, before disposal, are desperately needed’, Vega states. ‘So, we developed a
method to convert arsenic waste in a stable form, allowing safe and long-term storage.’

Extreme versatility

But to reduce the toxicity of mining waste is a complex task due to the complex mixture of chemicals present and the high acidity. Besides the toxic As3+, they also contain high amounts of other metals, including iron (Fe2+). Microorganisms could potentially play a role in breaking down or modifying this complex mixture of metals, due to their extreme versatility to grow on all kinds of different substances. Hence, Vega aimed at finding suitable microbes to play a key role in the conversion of the toxic As3+ into the non-toxic As5+ form. But, in addition to be able to grow on chemicals,
such as Fe2+ present in mine tailings, the microbes should also be able to thrive in this acidic environment while they overcome the toxicity of
As3+ present. Eventually, the scientist found iron-oxidizing microorganisms that were able to survive at high temperatures and high acidity. The researchers found that the addition of activated carbon (AC) to the process reduced the toxicity of arsenite present, allowing the microbes to effectively oxidize the Fe2+ into Fe3+. The toxic As3+ was oxidized into As5+ with AC as a catalyst, followed by the precipitation of iron-arsenate, or scorodite (Fig. 1). These greenish crystals are non-toxic and can safely be stored for a long time without the arsenic leaching out.

Vega scheme.jpg

Fig. 1. Schematic representation of the reactions involved
in converting toxic arsenic (As III) into scorodite crystals. 

Although, the arsenic and iron could be oxidized with only AC present, no scorodite crystals were formed when the microorganisms were absent. But when Vega added the iron-oxidizing microorganisms, scorodite crystals were formed. Clearly, the microbes played a crucial role in crystal formation. The scientist thinks that organic material from these microbiota, so-called EPS, might serve as starting point for scorodite crystal formation, similar to a condensation nucleus that is a starting point for the formation of water droplets.

Costly chemicals
Vega showed the proof of principle to effectively deal with arsenic waste using a combination of chemical and microbiological conversions in a cost-effective way: there is no need to use costly chemicals. Recently, she successfully scaled up her method from a 250-milliliter volume to a laboratory reactor, containing nine liters. And the principle of the new method can possibly also be used to recover other metals from low-grade ores or mine tailings still containing small amounts of valuable metals. Vega: ‘Since low-grade mining becomes more and more common, every little bit of recovered metal counts for a cost-effective operation.’

Vega Scorodite.jpg

Fig.2. Electron-microscopic image of scorodite crystals.

This research was funded by Chilean government (CONICYT) and Paques BV

Selected publication:

Vega-Hernandez S., Weijma J., Buisman C.J.N., 2019. Immobilization of arsenic as scorodite by a thermoacidophilic mixed culture via As(III)-catalyzed oxidation with activated carbon. J. of Hazard. Mater. https://doi.org/10.1016/j.jhazmat.2019.01.051