Phytoextraction: these plants can clean up pollution

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.

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.

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.

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…

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.

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.

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.

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