Plants are lovely to see – and they can raise your spirits, as well. But perhaps even more importantly: plants also play a very important role in ridding our surroundings of toxic substances. The world is becoming increasingly urbanised and dirty, in rapid tempo. In cities, smog forms a substantial problem, as do soil and water pollution. Luckily, though, green walls, green roofs and even green cities are on the increase and helping to purify our air, soil and water. The technical term for using plants as a means of cleansing the environment is phytoremediation.
Phytoremediation literally means: bringing our environment back into balance through the use of plants. It involves technologies which employ plants to purify the air, soil and water. Plants naturally absorb and remove contaminants. For example, a large number of plant species absorb volatile organic compounds from the air in our homes. They do this with their leaves and roots and the organisms in their (pot) soil, which in turn convert the compounds into food for the plants.
How does phytoremediation work?
There are a number a of ways in which certain plant species, together with the organisms in their soil, are able to cleanse their environment. They help remove contaminants in tandem with a number of different processes:
- Plants are able to lock up contaminants in their roots, stems and leaves (phytostabilisation).
- Contaminated portions of plants can in turn be removed and destroyed (phytoextraction).
- Plants and organisms in the soil are able to convert toxic substances into less toxic ones; this occurs either within the plants or in their root zone (translocation).
- With their juices, plants are able to attract certain bacteria to their root zone, where toxic substances in the soil are either reduced or locked up (phytodegradation).
- Plants and organisms in the soil are able to convert toxic substances in the air into less toxic gases, which are in turn released back into the air (phytovolatilisation).
- Plants are able to fix toxic substances onto their roots, where microorganisms, such as bacteria, in turn break them down and convert them into less toxic substances (stabilisation in the root zone).
- Some plants draw polluted groundwater upwards, so that it cannot seep deeper into the soil (stabilisation).
In practice: plants and green walls improve our interior environment
Within our homes, the phytoremedial value of plants lies primarily in their ability to purify the air. By means of phytovolatilisation, plants remove toxic, volatile organic compounds from the air and use them as nutrients. In this way, plants can contribute substantially to improving air quality in our homes and offices. Increasingly, interior landscape designers are creating green walls for a healthier environment in such venues as schools and airports.
Schiphol Airport. Photo: Marjolein Bezemer
Plants and the bacteria and moulds they work with are not just extremely proficient in cleaning up volatile substances, but also absorb fine particulates. Research conducted in Poland (2014) showed, amongst other things, that the spider plant (Chlorophytum comosum L.) absorbs fine particulates. Large particulates, as well as the finest and, to human beings, most dangerous particulates are absorbed by this plant via its leaves and their wax layer.
This short film shows how easy it is for everyone to employ phytoremediation in their own home.
Plants purify soil and water
Whilst indoor plants are deployed primarily to purify the air, plants are now also finding wide use outdoors to purify contaminated soil and (ground)water. Willowtrees (Salicaceae) and a range of species of mustard plant (Brassica), for example, have the ability to absorb such heavy metals as cadmium, nickel and lead from the soil. Indian mustard (Brassica juncea L.) and a number of Amaranth cultivars are even being used in Chernobyl and the surrounding area to absorb the radioactive substance caesium 137 from the soil. Plants are not only able to fix or convert heavy metals and radioactive material – they can also filter out low concentrations of pesticides, explosives and oil from the soil. At urban playground De Ceuvel in Amsterdam North, experiments are being carried out into the use of plants to remediate the heavily contaminated bottom of an old shipyard. Amongst the plants being tried out are: the broadleef cattail (Typha latifolia), the foxglove (Digitalis purpurea) and the black willow (Salix nigra).
Beer brewery De Koningshoeven (makers of La Trappe) in the Dutch province of North Brabant is in the advanced stages of plans to purify its waste water by means of its own plant-based installation. In cooperation with water board De Dommel, the brewery intends to construct a large glasshouse especially for this purpose. Tropical plants growing in the greenhouse are to be used to purify waste water and convert it into irrigation water. In this way, the brewery expects to reduce the amount of water used by 50%, as well as to demonstrate the efficacy of water purification through phytoremediation.
Plants purify the air outdoors
Plants are not just effective cleaners of the soil, water and indoor environment – they are also able to remove toxic substances from the air out of doors. Italian architect Stefano Boeri uses plants in his designs, with the aim of, amongst other things, improving the air quality in cities. The first of these ‘vertical forests,’ as Boeri refers to them, was a high-rise apartment building in Milan. Featuring 20,000 plants and 800 trees from more than a hundred different species, it can be compared to a piece of forest, two hectares in size, the only difference being that it is a building, with 1500 square metres of floorspace in the centre of a city. This vertical forest absorbs 40 tonnes of CO2 and 1.5 tonne of fine particulates each year. As a ’bonus,’ it also generates 90 tonnes of oxygen per year, lowers the outside temperature, functions as a sound barrier and stimulates biodiversity. A similar tower, by the same architect, is soon to be erected in Utrecht, appropriately dubbed Wonderwoods.
Photo: Wonderwood – Stefano Boeri Architetti
Stefano Boeri was recently commissioned to build an entire city in the form of a vertical forest, in China: in 2020, 30,000 people will be living in the green city, Liuzhou, together with more than a million plants.
It is of course not possible to put up a green city at once, everywhere. But it is possible to modify existing buildings by adding green roofs in order to purify the outdoor air. Research published in 2008 showed that plants on green roofs in Chicago were truly able to remove a portion of the toxic substances in the air. The scientists involved in the study investigated the air purifying capacity of 19.8 hectares of roof greenery. In one year, a total of 1,675 kg of airborne pollutants had been absorbed by the plants. The researchers also calculated that in excess of 2,046 tonnes of toxic substances could be filtered out of the air if all roofs in Chicago were covered with thick plant growth.
Phytoremediation is proving be a highly effective method for restoring balance to our environment, both indoors and out. Can we expect plants, green walls, green roofs and entire green cities to spring up everywhere in the future? For the sake of our health, and that of the earth itself, that would in any case be a splendid prospect!
Guest author: Marjolein Bezemer