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Phytoextraction: these plants can clean up pollution

Claude Grison
Director of the Laboratory for Bio-Inspired Chemistry and Ecological Innovations
Key takeaways
  • Mining and metallurgy activities severely degrade and erode soils, preventing vegetation from developing.
  • It is possible to restore ecosystems by using phytoextraction: certain plants extract metal elements from the soil and store them in their leaves.
  • This technique is inexpensive, creates a circular economy and allows for the rehabilitation of soils in an ecological restoration process.
  • However, phytoextraction has certain limitations, such as the total duration of the process or the natural capacities of plants.
  • The same process has been developed for water decontamination, using aquatic plants: this is rhizofiltration.

You work on depollution, can you tell us more about the processes you are developing? 

We restore ter­res­tri­al and aquat­ic ecosys­tems using plants that accu­mu­late metal­lic ele­ments. Min­ing and met­al­lur­gi­cal activ­i­ties severe­ly degrade soils, no veg­e­ta­tion can devel­op there. These bare soils are severe­ly erod­ed: dust loaded with metal­lic ele­ments flies away or is washed into water­cours­es, pol­lut­ing the sur­round­ing envi­ron­ment. Restora­tion is not lim­it­ed to decon­t­a­m­i­na­tion: the sus­tain­able rein­tro­duc­tion of suit­able plants is a pri­or­i­ty to lim­it the dis­per­sion of pollutants.

What plants are you talking about?

We have stud­ied sev­er­al ter­res­tri­al plants that are tol­er­ant and hyper-accu­mu­late metal­lic ele­ments: stinkweed (Noc­caea caerulescens), an eco­type of vul­nar­ia (Anthyl­lis vul­ner­aria) and trees such as Geis­sois pru­inosa and Gre­vil­lea meis­neri in New Cale­do­nia. Each of them car­ries out a nat­ur­al phy­toex­trac­tion process: they extract metal­lic ele­ments from the soil through their roots and trans­port them in the sap until they are stored in their leaves in very high con­cen­tra­tions. In the Gard region of France, the com­mon rag­weed stores more than 17,000 ppm of zinc. We esti­mate that if plant cov­er reach­es 70%, 27 kg of zinc per hectare can poten­tial­ly be extract­ed from the soil at each har­vest1. In New Cale­do­nia, har­vests of a sin­gle Gre­vil­lea meis­neri tree con­tain 2.5 kg of bio­mass con­tain­ing more than 10,000 ppm of man­ganese2.

What is the ecological benefit of these plants storing large quantities of metals?

Sev­er­al the­o­ries are being explored. In the case of Noc­caea caerulescens, it is that it does not resist com­pe­ti­tion well and is very quick­ly invad­ed by sur­round­ing plants. In these pol­lut­ed envi­ron­ments, it is the only one to sur­vive. In addi­tion, it clas­si­cal­ly defends itself from her­bi­vores by releas­ing tox­ic com­pounds called glu­cosi­no­lates. When it is rich in zinc, the plant cor­rel­a­tive­ly reduces the pro­duc­tion of glu­cosi­no­lates: this is an indi­rect way of pro­tect­ing itself from her­bi­vores3.

Why is it necessary to develop new rehabilitation solutions?

The avail­able solu­tions are not sat­is­fac­to­ry. One of them con­sists of con­fin­ing the pol­lu­tion by cov­er­ing the soil with mate­ri­als. How­ev­er, the pol­lu­tion con­tin­ues to spread into the water table as water infil­trates. The sec­ond solu­tion is very cost­ly: it con­sists of exca­vat­ing the con­t­a­m­i­nat­ed soil and treat­ing it chem­i­cal­ly in appro­pri­ate plants. This gen­er­ates a new waste prod­uct: the decon­t­a­m­i­nant asso­ci­at­ed with the metals…

Why are plant-based processes more satisfactory?

Phy­toex­trac­tion is effi­cient, inex­pen­sive and allows for the reha­bil­i­ta­tion of soils in an eco­log­i­cal restora­tion process: it is a long-term vision. And above all, it cre­ates a cir­cu­lar econ­o­my that does not gen­er­ate any waste. To take the exam­ple of New Cale­do­nia, the lit­ter from trees that hyper­ac­cu­mu­late man­ganese or nick­el is har­vest­ed and trans­formed into min­er­al mat­ter. The met­als are used as cat­a­lysts, called eco­cat­a­lysts. They replace those tra­di­tion­al­ly used for the syn­the­sis of drugs, for exam­ple, many of which are now being chal­lenged by Euro­pean chem­i­cal reg­u­la­tions (REACH)4. The eco-respon­si­ble use of these plants is at the heart of our devel­op­ments: this is the only way to sus­tain restora­tion efforts in the long term.

Doesn’t depollution by phytoextraction have certain limits?

If the objec­tive is to clean up the soil, it can take a long time: in the Gard region, the Ademe esti­mates that the total clean-up of the old decanta­tion basins would take 50 years. More­over, these tech­niques can­not be gen­er­alised: each plant is cho­sen in rela­tion to its nat­ur­al habi­tat. It is incon­ceiv­able to install a hyper-accu­mu­la­tive plant from New Cale­do­nia in main­land France. In Ore­gon, the estab­lish­ment of a species of Euro­pean ori­gin has led to a cat­a­stroph­ic sit­u­a­tion: it has become inva­sive. Final­ly, the pos­si­bil­i­ties are lim­it­ed by the nat­ur­al capac­i­ties of the plants. There are many species capa­ble of accu­mu­lat­ing nick­el, zinc, or man­ganese. On the oth­er hand, the capac­i­ties are lim­it­ed – if not impos­si­ble – for oth­er ele­ments such as arsenic, cobalt or copper. 

These lim­i­ta­tions have led us to devel­op a dif­fer­ent process for the depol­lu­tion of water. It is a very impor­tant resource to pre­serve, but it is pol­lut­ed by many human activities.

Can you tell us more about this new process?

It is based on rhi­zofil­tra­tion and biosorp­tion. We use aquat­ic plants capa­ble of seques­ter­ing met­als in their roots. They are very effi­cient: they have mol­e­c­u­lar anten­nae that cap­ture the nutri­ents dilut­ed in the water… and also the met­al pollutants. 

The use of dead plants is a major advance in water treat­ment: they retain their capac­i­ty for depol­lu­tion, but this makes the process scal­able. The roots are crushed and trans­formed into plant pow­der, then placed in a col­umn in which the water cir­cu­lates. The met­als are thus sequestered by the pow­der. We have demon­strat­ed good per­for­mance of these plant fil­ters to treat min­ing efflu­ents in France pol­lut­ed with zinc, iron and arsenic. The process is also adapt­ed to the chem­i­cal indus­try to cap­ture efflu­ents leav­ing reac­tors. In the lab­o­ra­to­ry, we are exper­i­ment­ing with the treat­ment of dread­ed organ­ic pol­lu­tants such as chlorde­cone, and our results are very con­clu­sive5.

Does the circular economy also have an important role to play in water decontamination?

Of course it does! Plant fil­ters loaded with metal­lic ele­ments are again trans­formed into eco­cat­a­lysts. The main advan­tage? Aquat­ic plants cap­ture cer­tain rare earths or ele­ments of inter­est such as pal­la­di­um. This is a strate­gic resource: Rus­sia is cur­rent­ly the largest pro­duc­er and many indus­tries use it mas­sive­ly (elec­tron­ics, auto­mo­bile, phar­ma­ceu­ti­cal). Recy­cling it has become a pri­or­i­ty, and plant fil­ters make it pos­si­ble6. With BioIn­spir, we mar­ket mol­e­cules of inter­est – fra­grances and sol­vents – that are 100% biobased, man­u­fac­tured using these eco­cat­a­lysts with­out chem­i­cal inputs. They are used in cos­met­ics, per­fumery, and fine chem­istry. Eco­catal­y­sis is an oppor­tu­ni­ty to revis­it chem­istry by lim­it­ing its envi­ron­men­tal foot­print to a min­i­mum7.

We have even tak­en the vir­tu­ous cir­cle a step fur­ther… Inva­sive alien plants are one of the main threats to bio­di­ver­si­ty in the world. Many of them are used in our process­es: Asian knotweed, water prim­rose, water let­tuce, etc. We har­vest them mas­sive­ly in the wet­lands of Occ­i­ta­nia to inte­grate them into our plant fil­ters. This rein­forces the sup­port for man­age­ment efforts and the non-pro­lif­er­a­tion of plant species that have become dangerous.

Interview by Isabelle Dumé
1The legu­mi­nous species Anthyl­lis vul­ner­aria as a Zn-hyper­ac­cu­mu­la­tor and eco-Zn cat­a­lyst resources, Env­i­ron. Sci. Pol­lut. Res. 2015, 22, 5667–5676, C. M. Gri­son, M.Mazel, A. Sell­i­ni, V. Escan­de, J. Biton, C. Gri­son
2Leaf-age effect: a key fac­tor to report trace-ele­ments hyper­ac­cu­mu­la­tion by plants and design appli­ca­tions, Env­i­ron. Sci. Pol­lut. Res. 2015, 22, 5620–5632, G. Los­feld, B. Fogliani, L. L’Huillier, C. Gri­son
3Iden­ti­fi­ca­tion of glu­cosi­no­lates in seeds of three Bras­si­caceae species known to hyper­ac­cu­mu­late heavy met­als, Chem­istry and Bio­di­ver­si­ty, 2017, Vol­ume 14, Issue 3, e1600311, S. Mon­taut, B. S. Gui­do, C. Gri­son, P. Rollin
4Eco-CaM­nOx: A Green­er Gen­er­a­tion of Eco­cat­a­lysts for Eco-friend­ly Oxi­da­tion Process­es, ACS Sus­tain­able Chem. Eng., 2019, 8, 10, 4044–4057, doi: org/10.1021/acssuschemeng.9b05444, C. Bihan­ic, S. Dilib­er­to, F. Pelissier, E. Petit, C. Boulanger, C. Gri­son
5Effi­cient removal of per­sis­tent and emerg­ing organ­ic pol­lu­tants by biosorp­tion using abun­dant bio­mass wastes, Chemos­phere, 2023, 213, 137307, P.-A. DeyrisF., Pelissier„  C. M.Grison, P. Hes­e­mann, E. Petit, C. Gri­son
6Eco­log­i­cal­ly respon­si­ble and effi­cient recy­cling of Pd from aque­ous efflu­ents using biosorp­tion on bio­mass feed­stock, J. Clean. Prod., 2021, https://​doi​.org/​1​0​.​1​0​1​6​/​j​.​j​c​l​e​p​r​o​.​2​0​2​1​.​1​26895, A. Gar­cia, P.-Al. Deyris, P. Adler, F. Pelissier, T. DumasY.-M. LegrandC. Gri­son
7Eco­catal­y­sis, a new vision of Green and Sus­tain­able Chem­istry, Cur­rent Opin­ion in Green and Sus­tain­able Chem­istry, 2021, 29, 100461. C. Gri­son, Y. Lock Toy Ki.

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