Algae turns heavy metals green

JCU chemical engineering academic Dr. Madoc Sheehan, Associate Professor Kirsten Heimann, and past JCU chemical engineering students Hannah Palma and Emily Killoran have successfully grown microalgae biofilms in tailings dam water at Queensland Nickel.

Dr. Sheehan, who is a chemical engineering sustainability expert, said the scientists used algae to turn a problem into a solution.

“Microalgae biofilms grow on the surface of wet rocks and on the sides of boats and marine structures. It’s better known as biofouling and it’s an expensive and time-consuming problem for boat owners and marine operators.

“It grows by using energy from the sun while simultaneously consuming CO₂, nutrients and dissolved metals, and it can absorb other metals that are dissolved in water.”

The Queensland Nickel site contained dissolved metals such as cobalt, nickel and manganese, yet the team found they could grow algae at rates similar to commercial algae operations and clean the water at the same time.

Assoc. Prof. Kirsten Heimann is an industrial algal biotechnologist and ecotoxicologist. She said the water can be more easily re-used by industry once it’s cleaned, and poses less of a danger to the reef.

“Algae grown in the tailings water reduced the nitrogen content and the heavy metal content of the water, lowering its pollution potential. It absorbed the dissolved metals as it grew, to the extent that when harvested at least 10% of the algae’s dry weight was made up of pollutants,” she said.

Dr Sheehan said algae grown in Queensland Nickel tailings water was 40% carbohydrate, making it suitable for converting into a biofuel such as ethanol.

“This means the energy obtained by turning the algae into biofuel could potentially be re-used to harvest and grow the algae in the first place, making the process cheap and effective.”

“Conservative estimates show the growing area required to reduce the heavy metal concentration in the tailings water by 75% would be less than twice that of a full-size football field,” he said.

The next step is for the scientists to take their promising laboratory results and test them out on a full-scale industry tailings dam.

Dr Sheehan said there was a lot of potential for this type of nature-based technology.

“Eventually we’ll be able to clean mine waste water using algae, concentrate the heavy metals, sequester CO₂ from the atmosphere and turn it all back into energy. Using nature in this way is a step toward more sustainable industry, with less impact on threatened ecosystems such as the Great Barrier Reef.”

Assoc. Prof. Heimann said “We can see the day that mine waste waters become a valuable future resource, rather than the economic cost and environmental burden that they currently are.”

Both student researchers involved in this work have graduated and are now employed on sustainability projects.

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