Phytoremediation Mechanisms Reference Bioremediation book Chapter 2 Description

Phytoremediation Mechanisms Reference: Bioremediation book, Chapter 2

Description of mechanisms involved in bioremediation Multiple mechanisms involved in bioremediation of soil, water & air Mechanisms dealt How contaminants contact with plant system (rhizosphere & transportation processes) Mechanisms interrelated & depends on plant physiological processes driven by solar energy, rhizospheric processes and other available precursors. Cleanup of organics and inorganics

Description of mechanisms involved in bioremediation Phytosequestration Phytodegradation Phytovolatilization Phytostabilization Phytoextraction Rhizofiltration Rhizoremediation Sequester - 은퇴하다 - isolate or hide away Phyto-sequestration 1) Phytochemical complexation Reduce the contaminant mobility & Prevent migration to soil, water & air 2) Transport protein inhibition 3) Vacuolar storage

Phyto-sequestration 1) Phytochemical complexation 2) Transport protein inhibition 3) Vacuolar storage Phytochemical complexation in root zone Phytochemicals exuded 분출 된 in root zone Complexation Contaminant immobilize 안 정화 or precipitation Combination of individual atom groups, ions or molecules to create one large ion or molecule Reduce the fraction of contaminant

F 1 - Soluble & exchangeable Soil Particles

Phyto-sequestration 1) Phytochemical complexation 2) Transport protein inhibition on root membrane Prevent contaminants enter plant 3) Vacuolar storage Transport proteins associated with root irreversibly 비가 역적 bind (not able to altered) & stabilize 안 정화 contaminants

Phyto-sequestration 1) Phytochemical complexation 2) Transport protein inhibition Vacuolar storage in root cells Transport proteins transfer contaminants between cells Preventing translocation to xylem Contaminants sequestered in vacuoles of root cells 3) Vacuolar storage But, plant cells compartment (“vacuole”) Storage

Description of mechanisms involved in bioremediation Phytosequestration Phytodegradation Phytovolatilization Phyto-degradation or Phyto-transformation Phytostabilization Phytoextraction Rhizofiltration Rhizoremediation

Phyto-degradation or Phytotransformation Uptake of contaminants by plant root 1) Contaminant breakdown by plant root 3) Metabolization 2) Mineralization 광물 Through various internal enzymatic reactions & metabolic processes

Phyto-degradation or Phytotransformation Contaminant Impeded 방해 (delay or prevent) by phytosequestration and/or rhizodegradation Then contaminants (partially or negligibly (less)) pass through rhizosphere Depends on Concentration & composition of contaminants Plant species Then contaminant subject to biological processes within plant Soil conditions Dissolved in transpiration stream & phytoextracted

Plant internal reactions by producing enzymes Oxygenases in plants Degrade hydrocarbons Plants Catalyze 촉매 Nitroreductases Reduce & breakdown Explosives 폭발물 trinitrotoluene (TNT), 1, 3, 5 -trinitroperhydro -1, 3, 5 - triazine (RDX) 1, 3, 5, 7 -tetranitro -1, 3, 5, 7 -tetrazocine (HMX High melting explosive)

Plant enzymes metabolize or mineralize contaminants Converted to CO 2 & water Endophytic symbiotic 공생 bacteria Methyl bacterium populum lives within poplar plant Mineralize RDX & HMX Further, oxidation & reduction cycle operating during photosynthesis offers additional contaminant breakdown potential Poplar plant

Phytosequestration Sequester - 은퇴하다 - isolate or hide away

Phyto-volatilization Reference: Bioremediation book

Phytovolatilization Volatilization of contaminants Leaf stomata Plant stems Chemical characteristics Henry’s constant & vapor pressure Henry’s Law Dictate the ability of organic contaminants to volatilize At a constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Henry’s constant C = k. Pgas C - solubility of a gas at a fixed temperature in a particular solvent (m. L gas/L) K - Henry's law constant Pgas - partial pressure of the gas (Atm)

Transportation of food & nutrients (Sugars & amino acids) from leaves to storage organs Water & mineral transport from roots to aerial parts

Volatilization of contaminants Henry’s constant & vapor pressure Trichloroethene (TCE) Some cases Parent contaminant in soil Along the transpiration pathway Rhizodegradation and/or phytodegradation Phytovolatilized Transpiration is the process of water movement through a plant and its evaporation from aerial parts

Poplars Uptake and phytovolatilization of trichloroethene (TCE) or its breakdown products in poplars

Inorganics Similar to above Mercury Alter the chemical speciation Phytovolatilize safe levels of less toxic elemental mercury to atmosphere Volatilized by tobacco plants Take up highly toxic methylmercury Once volatilized Many chemicals recalcitrant 고집 불통 (not control) in Subsurface environment React rapidly with atmospheric hydroxyl radicals, an oxidant formed during the photochemical cycle

Growing trees & other plants take up water & contaminants. Phytovolatilization Converted (by methylation to the volatile dimethyl Se) into nontoxic forms & volatilized Contaminants pass Plants leaves & volatilize into atmosphere at low concentrations Plants genetically altered due to mercury movement through a cell into air Se by Brassica plants Used only for contaminants that highly volatile Elemental Hg in air less risk than Hg forms in the soil Hg & Se, taken by roots, converted to non-toxic forms & volatilized from roots, shoots, or leaves

PHYTOSTABILIZATION— Plants control movement of toxins from the site either by Controlling or erosion 부식 Movement by or water by binding them tightly to their roots Rendering them unavailable The contaminants are not removed from the site Thus harmless

Phytostabilization Holding of contaminated soils and sediments in place by vegetation & immobilizing toxic contaminants in soils

Establishment of Rooted Vegetation Hydraulic control (Phytohydraulics) Prevents windblown dust by human from hazardous waste sites Large volume of ground water transpired 나오다 by plants. Prevents migration of leachate towards groundwater or receiving waters.

Phytostabilization Applicable to metal contaminants at waste sites where metals do not degrade Low level or high level of contaminants Capturing in situ at sites Phytoremediation groundcovers Soil/Sediment Stabilization Infiltration Control Phytostabilization

a) Soil/Sediment Stabilization: Soil & sediment mobilized by Uncontrolled water flows Phytostabilization Natural barrier 장벽 & resistance to erosion & leaching + minimize NPS pollution Blowing wind Erosion” or Leaching 침출 Migrate of contaminants non-point source (NPS) pollution Grasses Infusion 주입 mechanism of plants roots into soil Herbaceous species Fibrous rooted plants depths 3060 cm for upland species & < 30 cm for wetland species Halophytes (dry land plants) & hyperaccumulators also used

b) Infiltration Control: Mechanism used in Landfill covers Surface water recharge of groundwater plumes Water interacting with waste Infiltration Control prevent this mixing

Phytostabilization covers for infiltration control know as Evapotranspiration or water-balance Rain intercept 차단 by Plants or Vegetative covers • Prevent infiltration • Take up & remove rain water from soil • Minimize the percolation 여과 (runoff) with waste • Called as Phytohydraulics mechanism • Time-dependent or climate-dependent processes - successfully remove water from the system Infiltration Ground water

Vegetation cover Seed mixes or mixed communities of plants/trees They access the stored water Create intercepting canopy (upper layer of mature tree)

When minimizing infiltration Create an anaerobic zone underneath the phytostabilization cover Some cases, subsurface conditions Methanogenic (methane-producing) conditions Not good for Phytostabilization due to landfill gas While methane may or may not toxic to plants, gas in vadose zone restrict the oxygen transport needed for root cell respiration Anaerobic zone Also landfill gas diffusion to surface stopped due to landfill cover So, these gases controlled through other methods Waste Methanogenic Condition

Phytoremediation groundcovers Densely rooted groundcover Applied to surface soils Groundcovers is 30 -60 cms below ground surface; however, depths down to 1. 5 meters Recalcitrant compounds PAHs, PCBs, & persistent organic pollutants (less mobile, soluble, biodegradable, & bioavailable) Used for metals, salts & radionuclides

Phyto-extraction 추출

Ability of plants to take up contaminants into the roots and translocate them to the aboveground shoots or leaves

For extraction Toxicant must be dissolved in the soil water Come in to contact with roots by transpiration Or Uptake through vapor adsorption by root in vadose zone

Contaminant adsorbed Contaminant dissolve in transpiration water or actively taken up by plant transport mechanisms

Absorption Storage (Or) Contaminant metabolized through phytodegradation mechanisms or phytovolatilized in transpiration stream existing in plant via Lignification (Or) Sequester (isolate) into cell vacuoles of aboveground tissues (as opposed to in root cells) as part of phytosequestration. Converted to by-products of plant biomass (Covalent bonding of chemical or its byproducts into lignin of plant)

Factors affect Plant uptake For organic chemicals Hydro-phobicity Polarity Sorption properties Solubility Octanol-water partition coefficient, log Kow Quantify molecule’s hydrophobicity Defined as ratio of chemical's concentration in the octanol phase to its concentration in the aqueous phase of a two-phase octanol/water system.

1 -octanol 2 -Octanol is a colorless liquid used in the manufacture of perfumes, disinfectant soaps and to prevent foaming

Octanol-water partition coefficient, log Kow values between 1 and 3. 5 Enter into plants Organic membrane plant root consisting lipid bilayer Lipids make root partially hydrophobic while the bilayering aspects make it also nonpolar Therefore, hydrophobic chemicals

log Kow > 3. 5 Chemicals not soluble in water Not sufficiently soluble in transpiration stream (or) Bound strongly to surface of roots Not easily translocate into plant xylem

Chemicals soluble in water Not sufficiently sorbed by roots Log Kow <1. 0 or Actively transported through plant membranes due to their high polarity

Susceptible to phytoextraction Short chain aliphatic chemicals Benzene Log Kow = 2. 13 Toluene Chlorinated solvents Log Kow = 2. 73 Xylene (BTEX) Log Kow = 3. 15 Ethylbenzene Log Kow = 3. 15 Vapor uptake pathway into plants identified for chlorinated solvents such as perchloroethene (PCE, “tetrachloroethene”).

Root concentration factor (RCF) Ability of plant to take chemical from the soil or groundwater into its roots RCF = Ratio of concentration in root (mg/kg): concentration in external solution (mg/L) Transpiration stream concentration factor (TSCF) Chemical translocating from soil to shoots Ratio of Xylem concentration (mg/L): concentration in external solution (mg/L)

RCF & TSCF Values depend on Soil properties Plant species Chemical partitioning RCF & TSCF Values Higher values Indication of enhanced contaminant uptake by plants & vary directly with log Kow of chemical Contaminants in solution with the highest TSCF contained a log Kow in the range of 1– 3. 5

For inorganics Salts, metals & radionuclides Redox state Uptake & translocation to above ground tissues depends on Chemical speciation in soil, sediment or groundwater Plant species

Readily bioavailable inorganics Moderately bioavailable Easily bioavailable As, Cd, Cu, Ni, Se & Zn Co, Fe & Mn Cr, Pb & U Several of these constituents, often considered as environmental contaminants in sufficient concentration, are also essential plant nutrients.

Hyperaccumulators Plant absorb unusually large amounts of metals compare to other plants & ambient metal concentration Halophytes (dry land plants) Able to accumulate 1, 000 mg/kg (dry weight) of a specific metal Some metals or metalloids, concentration 10, 000 mg/kg Tolerate & accumulate large quantities of salt (Na. Cl, Ca & Mg chlorides) Take up & exude the excess salt through stomata Hyperaccumulators & halophytes selected to grow at site based on the metals or salts naturally present, forming their own niche through evolution.

Remediation aspects Plant harvested using conventional agricultural methods Enhance phytoextraction Chelating agents EDTA (for Pb & radionuclides)

Availability of uranium & 137 Cs enhanced by Mobilize target contaminants to deeper soil or groundwater Citric acid & ammonium nitrate Enhancing risks

Halophytes 염생 식물 Take up saline water & exude excess salt through stomata back onto the ground as a means to create the niche Some plants produce & exude specific phytochemicals directly into the soil environment Alter chemistry & speciation of chemicals Planting known crop to accumulate metals, metalloids or, radionuclides and then harvesting the crop contaminant is recovered. Promote mobilization & uptake into plant Enhancing the uptake of essential nutrients through the release of acidic phytochemicals

Rhizofiltration

Rhizofiltration Use of plant roots Absorb Concentrate Hazardous compounds heavy metals or radionuclides Aqueous solutions Precipitate

Hydroponic cultivation 수경법 Plants rapidly remove HM from water Concentrate in roots & shoots Rhizofiltration Effective in contaminated wetlands 습지 Water allowed to contact with roots Contaminants (Pb, Cr(III) U, Ar) sorb strongly by roots

Densely growing vegetation plants roots sorbe large quantities of Pb & Cr from soil water or from water dense vegetation

Shallow lagoons 얕은 석호 engineered as wetlands & maintained as facultative microbial systems with low dissolved oxygen in the sediment Groundwater or wastewater pumped through this system remove contaminants by rhizofiltration. Success in wetlands.

Acid mine drainage Long-term utilization of wetland 습지 plants & sulfate-reducing conditions Increase in p. H & decrease in toxic metals Bamboo 대나무 successful with Acid Mine Drainage

Root systems & sediments in wetlands Facultative (aerobic & anaerobic zones) facilitates sorption and precipitation of toxic metals. Facultative : bacterium live in absence as well as in the presence of atmospheric oxygen. Ion exchange

Harvested plants containing heavy metals disposed of or treated to recycle the metal Plants with high biomass production Wide variety of metal removal capacity Rhizofiltration many benefits than other phytoextraction techniques Including low cost & minimal environmental disruption

Continuous flow system Circulates contaminated water Through specially designed plant containment units Periodically, older plants harvested & replaced

Experimental evidence showing nonlinear kinetics 비선형 동역학 of disappearance of metals from solution suggests that several different mechanisms, of differing speeds, operate simultaneously.

Surface absorption by roots Surface absorption Fastest & prevalent mechanism depends on physicochemical processes (ion exchange, chelation) and can even take place on dead roots. 탄산

Chelation 킬레이트 : ions & molecules bind metal ions

Surface absorption Primary mechanism for removing metals from waste streams by roots - Biosorption Absorb large quantities of heavy metals. Plant root used living or dead Microbial, fungal or other biomass

Other slower mechanisms also occur in rhizofiltration Surface absorption Biological processes Precipitation of metal from solution by plant exudates Intracellular Deposition Translocation uptake in vacuoles to shoot Rhizofiltration effective for dilute concentrations of contaminants in large volumes of water (radionuclide decontamination)

Rhizoremediation

Well established rhizoremediation processes (a) Sequestration or immobilization or retention of toxicants within a confined area (b) Removal of (c) Destruction/degradation contaminants from of organic pollutants by plantsoil/waste water microbial association Soil at the site of their release (or) in contaminated soil placed in a landfill Used a, b and c - individually (or) in combination

Partial immobilization of water soluble contaminants removed by plant transpiration 증발. Soil Root Stem Leaf evaporated by transportation process This process removes soil water contaminants otherwise contaminant leaching & move to groundwater. Removal of toxic metals from contaminated soil occurs when inorganic ions are taken up by plant roots and translocated 이 동 시 키 다 through the stem to aboveground plant parts. Soil microflora of plant roots (rhizosphere zone) is involved in xenobiotic metabolism.

Catabolic activity within rhizosphere by bacteria & fungi using enzymatic 효소 expression Organic chemicals released from both living and dead roots Contaminants direct & indirect degradation by root physiology & biosynthetic (or anabolism) pathways Potentially occur at the lowest depth of root penetration, a special feature of plant remediation.

Rhizodeposition & Root exudates Roots Deposit High amounts of photosynthetically derived hydrocarbons into surrounding soil Annually, plants transfer 40– 90 % of the net fixed carbon (as primary and secondary metabolites) to roots.

Organic compounds as rhizodeposits Exudates Secretions Mucigel Plant mucilages 식물 점액 Root lysates All organic substances Stimulation of microbial degradation of contaminants in root zone by maintain gas exchange & soil moisture Mucigel - slimy substance that covers the root cap

No single plant or microbe Contaminants immobilization, removal & destruction Maximum uptake of all toxic metals or faster degradation of all organic contaminants Successful treatment Combination of plant species with appropriate remediation properties Rhizosphere microorganisms, closely associated with roots, termed Plant Growth Promoting Rhizobacteria (PGPR). Rhizosphere communities (bacteria and fungi) active against specific contaminants

Microbes help to plant Rhizosphere microbes play significant roles in recycling of plant nutrients, maintenance of soil structure, detoxification of noxious chemicals, and control of plant pests. Plant help to microbes Plant root exudates provide nutrition to rhizosphere microbes for increasing microbiological activity Stimulate plant growth & reduce metal toxicity in plants

Plant Growth Promoting Rhizobacteria (PGPR) and Arbuscular Mycorrhizal Fungi (AMF) have gained prominence all over the world to treat soil (Figure). Mycorrhizal fungal networks connect the roots of the same or different plant species, provide pathway for nutrient transfer. Associated plant growth promoting rhizobacteria foster rhizoremediation of inorganic and organic pollutants.

Bacteria, yeast and fungi Specific microbial populations Naturally select contaminants Several orders of magnitude Food & energy

Contaminant Degradation, metabolization, or mineralization rate depends on bioactivity of proteins & enzymes from soil organisms. However, contaminant breakdown is often limited by the availability of electron acceptors or donors, cometabolites, inorganic nutrients, plant vitamins and hormones, p. H, and/or water. Co-metabolism Simultaneous degradation of two compounds Degradation of second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate).

Plants & soil microbes in the rhizosphere Symbiotic relationship Nutrients Plants microbes Healthier soil environment Specifically, plants loosen soil and transport oxygen and water into the rhizosphere. Endosymbiont bacterium or fungus

Plants exude phytochemicals Sugars Alcohols Carbohydrates Primary food sources (carbon) for microbes But, plant exuded allelopathic phytochemical suppress other plants growth in the same soil. Providing the healthier soil environment. Plants protected from competition, soil pathogens, toxins, and other chemicals

Microbial populations higher in vegetated soil compared to unvegetated soil Rhizodegradation (phytostimulation, rhizosphere biodegradation, or plant assisted bioremediation/degradation) Breakdown of contaminant by increasing bioactivity using plant rhizosphere environment to stimulate the microbial populations Organic contaminants can be remediated converted to source of food and energy for the plants or soil organisms. Specific proteins & enzymes produced by soil organism break down the contaminant.