Selenium is set to become of increasing importance, meaning research into treatment needs to connect with potential end uses to build value chains. Keith Hayward spoke with Piet Lens, co-editor of a new book on the element.
“Selenium is a very special element,” says Professor Piet Lens, of National University of Ireland Galway and IHE Delft, and co-editor of a new book on technologies for selenium treatment. His reason for saying this is that the desired nutritional range for selenium intake is quite narrow. “The difference between its essentiality and its toxicity is only an order of a magnitude of 10,” he says.
Stated values vary, but England’s National Health Service advises a daily intake of 75 μg a day for men, adding that “taking 350 μg or less a day of selenium supplements is unlikely to cause any harm” – indicating the concerns around the potential harm of intakes in excess of this.
“If, in our food, we have very little selenium, we get deficiency diseases,” says Lens, noting that this also applies for animals and plants – notably cattle and crops.
Geography and geology shape concerns around selenium. There are regions where soils have very little selenium. Lens mentions Poland, Finland and Hungary, as well as the west of China. “In Finland, selenium is added to fertilisers in order to have enough selenium intake in the daily nutrition of people,” he says.
At the other extreme, there are regions – such as the west of the USA, the north of India, and parts of Ireland – where the selenium builds in the food chain, especially because of elevated concentrations in soils and groundwater. “In the east of China, there is a band where huge values of selenium can be measured,” adds Lens. This can impact birdlife, for example. Lens explains that, in California, people noticed that cattle grazing on selenium-rich grasses were suffering from heart disease and even death. Consequently, the US Environmental Protection Agency (EPA) has developed very strong guidelines for selenium. “It’s really significant in these regions,” adds Lens.
Human activities can also contribute to problems. “A well-known example is that, in flue gases, selenium can be very problematic,” says Lens. In coal power generation, iron selenide in the coal leads to selenium release in the off-gas, following similar chemistry to sulphur. Pollutants are removed from flue gases by scrubbing, and the resulting wastewater can contain substantial amounts of selenium.
Traditional technology options
This indicates the potential need to use treatment technologies. Indeed, while regulation has been limited to date, Lens notes that the strong guidelines developed by the US EPA do include a 40 mg/l limit for coal mining wastewater. “There is a whole set of technologies – chemical and biological – that can be used to remove selenium,” says Lens.
As far as chemical techniques are concerned, Lens notes precipitation, membrane filtration, and reverse osmosis as key options. For biological treatment, options include use of up-flow anaerobic sludge bed reactors. “They are very efficient in reducing selenium to elemental selenium. That is a solid and, like that, we can remove it from the wastewater,” says Lens. Activated sludge treatment with a sequencing batch operation with an anaerobic zone is another option. “Then we can remove the selenium to zero or a low level, so that it is not toxic anymore,” he adds.
Such chemical and biological options can be seen as somewhat traditional approaches. Lens, though, emphasises that, even with these, the focus should be on the theme of the book series of which this book is part: ‘Integrated Environmental Technology’. These options need to fit into the wider picture of what other substances are present and how these are being dealt with, and to look to opportunities to recover and reuse pollutants.
Recovery and reuse connect with the need to create value. Here, Lens highlights a number of opportunities where selenium could be dealt with in a way that adds value. One is the area of biofortification – using natural processes to increase the level of selenium present in foods to be consumed in areas where diets are lacking in selenium. This could mean growing crops using selenium-rich waters or wastewaters, growing them in selenium-rich soils, or using fertilisers containing selenium. “Like that, we can then have an augmentation of the selenium content of the plants, especially in the seeds,” says Lens. “That can then be used as a natural food supplement, so that people get what they need as a nutritional requirement.”
Investigations covered in the book include work with high-rate algal ponds. “We have fed these with selenium, and then looked at whether the selenium content of the algae is enough when you use it as a fertiliser,” explains Lens. Pot experiments using dried algae applied around plant stems have looked at the potential for biofertilisation, while other investigations have included testing spraying of extracted selenium onto leaves as another route for fertiliser application.
This type of approach overlaps with the potential for phytoremediation. Here Lens explains that plants such as duckweed and Azolla can be grown on mine water, taking up the selenium. “Duckweed produces a lot of protein naturally, and then you have a protein that can be a food feed, while, at the same time, we have enrichment of the selenium,” he says.
Such potential uses may face barriers, such as the reluctance around the acceptability of foods somehow connected with waste streams. This means there are various factors to consider when looking at the opportunities to progress technologies, and Lens talks in terms of “weighting” and “balancing” of issues, an aspect he says is covered in the book.
A wider challenge is around how to make options viable, especially in a financial sense. “I know very few processes that are really economically viable at this moment,” says Lens. This is partly about the time needed to optimise processes. It is also about the availability of alternative resources. But Lens sees legislation moving in a direction that may change things. In particular, Europe is targeting a pollution-free future by 2050, and the need to cut carbon emissions is demanding different approaches. “These are huge drivers to foster the recovery of resources such as selenium,” he says.
Alongside this, Lens notes that people can be prepared to pay a premium for certain food products. “Selenium is a food supplement – people take tablets with yeast, enriched in selenium, and people pay a good amount of money for that,” he says. “If we can go into these niches of nutrition, there the breakpoints to get recovery with the high-value product are, for sure, much quicker to reach.”
It is with such higher-value end uses of selenium in mind that the book also includes chapters beyond the direct involvement of the water sector. One example is the use of selenium in medical applications.
“The applications for selenium nanoparticles in medicine are huge,” says Lens. This includes use in cancer treatment, for example. “Selenium can be used as a carrier for drugs,” he adds.
Lens explains that such applications can make use of the toxicity of the selenium. “It can be a toxicant itself in cancer-cell killing,” he says. Another application is in imaging. Here, quantum dots of selenides, such as cadmium selenide, that are just a few nanometres in size are engineered to emit light at different wavelengths, allowing them to be used to track the fate of injected substances, for example.
“If you have a product that you define… and then go back to the water treatment, the water treatment becomes your process to produce that”
This light emission presents a potential application in photocatalytic water treatment, and Lens says work at his institution is looking at use of copper sulphide and copper selenide for this. There is also potential to connect this light-emitting property of nanoparticles with the metabolism of bacteria, creating what Lens says are called ‘nanohybrids’. “That captured electron can be used in the metabolism of the bacterium. You can shift the microbial metabolism from what it can do normally and you can make them photosynthetic. Like that, you can get photosynthetic activity in non-photosynthetic organisms – it’s marvellous,” says Lens.
While some of these opportunities are beyond the water sector, Lens sees that it is important to connect up thinking across these potential value chains, as this will help build viable options and attract research funding. “If you have a product that you define – maybe quantum dots or new production techniques with these nanohybrids – and then go back to the water treatment, the water treatment becomes your process to produce that. That value chain has to be identified and worked towards,” says Lens.
Even with a value chain identified, a big task remains. As with other biorefinery-type approaches, this is about product recovery, such as the nanoparticles that are formed in a selenium wastewater treatment plant. “The downstream processing recovery of the products – that’s the major challenge ahead, and there I think a lot of funding is still needed,” says Lens. “The recovery of those high-value end products, that is still the major bottleneck.”
Asked to look ahead at how things will change over the next five years, Lens sees that there will be a greater need for treatment plants able to remove selenium. He also points to the growing need for high-quality protein, especially given the need for alternatives to animal cultivation. “There is a huge need for alternative protein, so I think that selenoprotein production from wastewater is the one that will be there in five years,” he says. “It’s not promised,” he adds, with a laugh, but this and another application area he expects to emerge – use of nanoparticles in photocatalytic water treatment – are both needed and relatively easy to do. “That’s something for the water sector and something where fundamental and more applied people can work together,” he adds. l
Environmental Technologies to Treat Selenium Pollution: Principles and Engineering
Piet Lens and Kannan Pakshriajan (Editors)
Selenium contamination of air, aquatic environments, soils and sediments is a serious environmental concern of increasing importance. Selenium has a paradoxical feature in bringing about health benefits under the prescribed level, but only a few-fold increase in its concentration causes deleterious effects to flora and fauna, humans and the environment.
This book presents the fundamentals of the biogeochemical selenium cycle and which imbalances in this cycle result in pollution. It overviews chemical and biological technologies for successful treatment of selenium contaminated water, air, soils and sediments. The book also explores the recovery of value-added products from selenium laden waste streams, including biofortification and selenium-based nanoparticles and quantum dots.
Integrated Environmental Technology Series
Available as an Open Access ebook