#phytoremediation

Bioremediation

Which plant family is extensively used for phytoremediation?

Brassicaceae also called Cruciferae mustard family dominates phytoremediation literature due to hyperaccumulation capacity, rapid growth biomass 5 to 20 tons per hectare, high seed production and genetic tractability with Arabidopsis thaliana as model enabling molecular characterization. Genera include Thlaspi caerulescens now Noccaea accumulating zinc up to 30,000 and cadmium 3,000 mg per kg, Brassica juncea for lead up to 10,000, selenium, chromium and uranium, Alyssum bertolonii and murale hyperaccumulating nickel above 10,000 mg per kg in serpentine soils, and Brassica napus for radionuclides. Underlying traits involve overexpression of metal transporters ZIP ZNT1 and HMA4 P-type ATPase critical for xylem loading confirmed by QTL mapping and transgenic overexpression enhancing accumulation 3-fold, plus histidine chelation and antioxidant glutathione-ascorbate cycle for reactive oxygen scavenging. Fast doubling and extensive roots allow multi-cropping. Compared to Poaceae grasses used for rhizosphere oxidation, Fabaceae for nitrogen fixation, and Malvaceae minor, Brassicaceae provides optimal compromise between accumulation intensity and biomass, supported by genome resources and transformation protocols enabling engineering via overexpressing HMA4 or MTP1.

Ref: Baker 1997 Brassicaceae hyperaccumulators New Phytol; Peer et al. 2006 Brassica phytoremediation Plant Biology; EPA Brassicaceae plant family guide.

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Bioremediation

Rhizofiltration is mainly used for removal of contaminants from:

Rhizofiltration specifically uses plant root systems grown hydroponically in greenhouse to develop extensive fibrous mass before transfer to contaminated water to filter, adsorb and absorb contaminants from groundwater, effluents and radioactive ponds. Dense mats of sunflower Helianthus annuus accumulating uranium and cesium, Indian mustard Brassica juncea removing lead and chromium, and bulrush Scirpus for selenium provide 10 to 20 square meters surface per plant coated with mucilage and biofilm containing phytochelatins that precipitate and chelate metals via ion exchange. Mechanism involves physical filtration of suspended particles, apoplastic uptake and symplastic transport via IRT1 transporter, vacuolar sequestration and microbial degradation in rhizosphere for organics. Water flows with retention hours to days achieving efficiencies above 90 percent for radionuclides Cs-137, Sr-90, lead at 0.1 to 10 mg per L. After saturation indicated by breakthrough curves, plants harvested, dried and ashed for disposal, making technique suited to low to moderate strength wastewaters rather than highly contaminated soils where metals less mobile and root contact limited.

Ref: Dushenkov et al. Environmental Science Technology 1995 Rhizofiltration of radionuclides; EPA Rhizofiltration overview; PubMed water phytoremediation 1997.

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Bioremediation

Phytoextraction refers to:

Phytoextraction also termed phytoaccumulation focuses on removal of inorganics heavy metals nickel, zinc, cadmium, lead and metalloids arsenic and selenium from soils via plant uptake into harvestable shoots. Hyperaccumulators Alyssum bertolonii accumulating nickel up to 10,000 mg per kg, Thlaspi caerulescens Noccaea concentrating zinc above 30,000, Brassica juncea for lead achieve concentrations 10 to 500 times soil via high-affinity transporters ZIP ZNT1 and HMA ATPase, chelation with citrate, malate, histidine and sequestration in leaf vacuoles via MTP1 and HMA3. Shoots harvested at flowering when accumulation peaks, biomass ashed at 500 Celsius or processed for phytomining metal recovery via smelting, progressively depleting soil inventory over sequential cropping 5 to 20 seasons. Unlike phytostabilization immobilizing metals in situ reducing leaching or phytovolatilization releasing to atmosphere, phytoextraction physically extracts and concentrates metals into manageable biomass offering sustainable low-disturbance alternative to excavation but requiring long timeframe and careful handling to prevent food chain entry, often combined with soil amendments enhancing bioavailability.

Ref: Baker & Brooks 1989 Terrestrial higher plants hyperaccumulation; Chaney et al. Current Opinion Biotechnology 1997 phytoextraction; EPA extraction guidance.

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Bioremediation

Phytoremediation uses:

Phytoremediation integrates botanical processes with rhizosphere microbial ecology for contaminant removal, transformation, containment or detoxification using living plants and their associated microbiome. Roots actively exude low-molecular-weight organic acids such as citrate, malate, oxalate, sugars like glucose, amino acids and flavonoids that function as chemoattractants recruiting plant-growth-promoting rhizobacteria including Rhizobium, Pseudomonas fluorescens, Burkholderia and mycorrhizal fungi like Glomus that colonize root surface and interior, forming beneficial biofilm carrying catabolic genes for pollutant mineralization. Plant contributions encompass uptake of water-soluble organics via transpiration stream, translocation, sequestration in vacuoles mediated by tonoplast transporters, enzymatic transformation via cytochrome P450 monooxygenases, glutathione S-transferases, dehalogenases and peroxidases, plus hydraulic control preventing plume migration through high evapotranspiration rates 5 to 10 liters per day per mature poplar. Associated microbes enhance bioavailability through biosurfactant production, siderophore mediated iron competition, and mineralize petroleum hydrocarbons to carbon dioxide while plant supplies habitat and oxygen via aerenchyma. Animals do not actively contribute to primary mechanism; purely microbial approach excluded because phytoremediation definition inherently requires plant component providing photosynthetically driven solar remediation engine.

Ref: Pilon-Smits Phytoremediation Annual Review Plant Biology 2005; EPA Phytoremediation overview 2000; PubMed plant-microbe interaction.

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