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