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