'Phytoremediation' is the cleanup or control of wastes, especially hazardous wastes, using green plants. There are many types of phytoremediation, as shown in Table 1 , including the use of phreatophytes to control plumes of groundwater contaminants and contaminated vadose zones. Photoautotrophs, including vascular plants, green algae, cyanobacteria, and fungi, must be involved in the synthesis or maintenance of biomass, or in the direct metabolism, storage, detoxification, or control of contami nants. Glycosylation, occurring in plants and saprophytic fungi but not bacteria, is usually important in direct metabolism, detoxification, and accumulation or storage of pollutants by plants. Glycosylation is a sequestration of contaminant molecules by the addition of a glycosyl group to form a glycoprotein that plant cells can easily transport and store or transform. Not all applications of phytoremediation involve glycoproteins but the occur rence of glycosylation in pollutant transformations does distinguish whether the metabolism of organic con taminants or transformation of other contaminants is bioremediation or phytoremediation.
If heterotrophs are solely responsible for the metabo lism or mineralization of organic contaminants and the accumulation of metals and other elements using local accumulations of nonliving organic matter and oxidized inorganic compounds, these processes are part of the allied field of 'bioremediation'. However, when photoau totrophs are involved in treating contaminants by
• actively releasing organic matter during growth, main tenance, and senescence that increase the number and biomass of heterotrophs;
• selectively favoring specialized microorganisms that degrade or accumulate contaminants by pumping oxy gen into the root zone, releasing exudates, or depositing secondary metabolites during root die back in the rhizosphere to favor aerobic, facultative, or anaerobic organisms with enzymatic activity for the secondary products released or deposited; and • transporting pollutants into active microbial zones by evapotranspiration, blockage of flows, or other means.
These processes are a vital part of phytoremediation. Depending on the various interactions of photoautrophs and heterotrophic microbial communities and the con taminant transformations involved, these processes are known as 'phytostimulation', 'rhizo(sphere) degradation', 'rhizosphere bioremediation', or 'plant assisted bioreme diation' (see Table 1).
Distinction of bioremediation from phytostimulation is important in at least three cases. First, some hetero trophs sustainably derive carbon and energy from the degradation of organic contaminants. Second, anthropo genically synthesized organic or oxidized inorganic chemicals added to a contaminated site could temporarily free bioremediation from natural photo or chemoauto trophic synthesis long enough by cometabolism to achieve some cleanup. Third, chemoautrophs synthesiz ing biomass from inorganic compounds to provide organic carbon and energy for heterotrophs conceivably could be used in sustainable bioremediation. If any amendments and cofactors are obtained and added sus tainably, then these bioremediation processes are sustainable. The most common amendment is fertilizer, used primarily to bulk up plant biomass and thus increase microbial biomass and activity in the rhizosphere.
Redundant ecological engineering of both plant and microbial processes in remediation is usually the sustain able and successful approach. In practice, distinctions between phyto and bioremediation are only important for some specific contaminants at different sites. Different management approaches and techniques are required when microbial heterotrophy versus photoautotrophy dominates. Critical rates of pollutant control, uptake, storage, and metabolism, whether microbial or botanical,
Table 1 Types of phytoremediation ranked in terms of sustainability and applicability
Phytodegradation: phytoassimilation, phytotransformation, phytoreduction, phytooxidation, and phytolignification
Phytostimulation: rhizodegradation, rhizosphere bioremediation, and plant-assisted bioremediation
(1) Phyto- or solar pumping, phytohydraulic control, and phytohydraulic barriers (also biobarriers)
(2) Control of soil and landfill leaching
(3) 'Pump and tree', phytoirrigation, or other plant treatment ex situ
Aquatic and terrestrial plants take up, store, and biochemically degrade or transform organic compounds to harmless by-products, products used to create new plant biomass, or by-products that are further broken down by microbes and other processes to less harmful compounds. Growth and senescence enzymes, sometimes in series, are involved in plant metabolism or detoxification. Reductive and oxidative enzymes may be serially involved in different parts of the plant
Plant exudation, root necrosis, and other processes provide organic carbon and nutrients to spur soil bacteria growth by 2 or more orders of magnitude in number; stimulate enzyme induction and cometabolic degradation by mycorrhizal fungi and the rhizomicrobial consortium; provide diverse root zone habitat; and attenuate chemical movements and concentrations. Live roots transfer oxygen to aerobes, and dead roots may support anaerobes or leave aeration channels
Trees and other phreatophytes transpire large quantities to contain shallow groundwater plumes or contaminated soil leaching by reversing horizontal aquifer hydraulic gradients, or vertical soil moisture pressure gradients (infiltration and leaching minimized) both year-round or seasonally to fully or partially capture contaminants. Applications normally coupled with rhizo- and phytodegradation
Soils, sediments, wetlands, wastewaters, surface waters, groundwater, and air contaminated with chlorinated solvents (CCl4, trichloromethane, tetrachloromethane, HCA, PCE, TCE, DCE, and VC), methyl bromide, tetrabromoethene, tetrachloroethane, dichloroethene, atrazine, DDT, other Cl- and P-based pesticides, PCBs, phenols, anilines, nitriles, TNT, DNT, RDX, HMX, NB, picric acid, NT, nitromethane, nitroethane, and nutrients. Field demonstration: Iowa Army Ammunition Plant successfully restored using wetland plants (TNT and RDX). Proof of principle: (a) field - Populus spp. Carswell Air Force Base, Texas; Aberdeen Proving Grounds, Maryland; and using lysimeters at Tacoma, Washington (TCE); and (b) horseradish peroxidase pilot-tested in unit process to degrade phenols, aniline, and other aromatic contaminants in wastewater. Proof of concept: Rosa spp. cv. Paul's Scarlet (PCBs)
Soils and wetlands contaminated with crude oil, BTEX, other petroleum hydrocarbons, PAHs, PCP, perchlorate, pesticides, PCBs, and other organic compounds. Field proof of concept: BTEX, other hydrocarbons, PAHs, PCP, and TCE. Field tests: crude oil in wetlands of Spartina alterniflora and S. patens. Fungi: (1) field-scale tests: of white rot fungus degradation of BTEX and (2) proof of concept: for DDT, dieldrin, endosulfan, pentachloronitrobenzene, and PCP
Groundwater, vadose zone, wetlands, wastewater, and leachate contaminated with water-soluble contaminants (e.g., chlorinated solvents, MTBE, explosives, other organic contaminants, salts, and some elements)
(1) Field proof of principle: Populus spp. (TCE, PCE, MTBE, and CCl4)
(2) Concept not proven
(3) Proposed and undergoing testing: (a) pine (Pinus spp.) (TCE and by-products) and (b) Salix spp. (organic solvents, MTBE, petroleum hydrocarbons, and nutrients) (Numbers correspond to those in column 1)
Table 1 (Continued)
Brine volume reduction
Rhizofiltration: phytofiltration, blastofiltration, phyto- or biosorption, biocurtain, biofilter, contaminant uptake, and epuvalization
Phytovolatilization: biovolatilization and phytoevaporation
Brines pumped onto halophytes planted in wetlands that accumulate or excrete salt and the smaller volume residual brine transported and disposed of more economically
Compounds taken up, rapidly sorbed, or precipitated by roots (rhizofiltration) and young shoots (blastofiltration) or sorbed to fungi, algae, and bacteria (biosorption mainly to cell walls involving electrostatic attraction and formation of complexes). Marine algae possess large quantities of biopolymers (polysaccharides, uronic acids, and especially sulfated polysaccharides) that bind heavy metals. 10-60% dry weight of plant may be accumulated metals
Volatile metals and organic compounds are taken up, sometimes re-speciated (metals), and transpired. Some recalcitrant organic compounds are more easily degraded in the atmosphere but most multimedia transfers require a risk assessment before testing
Deep groundwater or oilfield brines. Wetland halophytes pilot tested in Oklahoma oilfield. No plant residuals: halophytes fed to cattle as a source of salt after toxicity testing of plants
Wetlands, wastewater, landfill leachates, surface water, and pumped groundwater contaminated with metals, radionuclides, organic chemicals, nitrate, ammonium, phosphate, and pathogens. Plant roots or shoots, aquatic plants, or algae, all live or dead, are added to or contained in wetlands, tanks, flowing water channels, or columns. Disposal of residuals unresolved. US practice is to dispose of residuals in hazardous waste landfills. Conceptually, metals sorbed to cell walls may be acid-extracted. Economic recovery of metals needs to be explored. Field proof of concept: sunflower (Helianthus annuus) at Chernobyl, Ukraine (Cs and Sr), and field pilot, Ashtabula, Ohio, for U. Proof of concept forphytosorption: aquatic plants (Salvinia spp. and Spriodela spp.) (Cr and Ni from wastewater and Pb, Cu, Fe, Cd, and Hg), algae (several metals), and marine algae (Sargassum Au: 40% of the algal dry weight). Proof of concept for rhizofiltration: sunflower (Helianthus annuus) and Indian mustard (Brassica juncea) (Pb, Cr, Mn, Cd, Ni, Cu, U(vi), Zn, and Sr)
Soils, sludges, wetlands, and groundwater contaminated with Se, tritium, As, Hg, m-xylene, chlorobenzene, tetrachloromethane, trichloromethane, trichloroethane, and other chlorinated solvents. Field proof of principle: Se from wastewaters and soil. Field proof of concept: tritium from groundwater. Current technical consensuses: (1) TCE volatilization has not proven significant to date but site risk assessments are required to be certain. The risk of volatilization of other organic pollutants has not been explored. (2) Transgenic plants volatize Hg in the lab but redeposition from the atmosphere makes field applications less feasible
Phytoextraction (including chelator induced): phytoaccumulation, phytoconcentration, phytotransfer, hyperaccumulation, and phytomining
Contaminants taken up with water by cation pumps, absorption, and other mechanisms and usually translocated above ground. Harvested shoots or roots put in hazardous waste landfills or could be smelted after volume reduction by incineration or composting. Hyperaccumulation is approximately 100 times normal plant accumulation of elements and is 0.01% by dry weight for Cd and other rare elements, 0.1% for most heavy metals, and 1% for Fe, Mn, and other common elements
Phytostabilization: biogeochemical stabilization, biomineralization, phytosequestration, and lignification
Enzymatically active plant material ground and slurried with wastewater, contaminated soil, or sediment
Contaminant translocated from soil or water into leaves and broken down by photolysis
(1) Revegetation to prevent erosion and sorbed pollutant transport
(2) Plants control pH, soil gases, and redox that cause speciation, precipitation, and sorption to form stable mineral deposits (effects of ecosystem succession unknown on long-term stability and thus sustainability)
(3) Humification, lignification, and covalent or irreversible binding of some organic compounds are expected
Extraction from soil of metals, metalloids, radionuclides, perchlorate, BTEX, PCP, short-chained aliphatic and other organic compounds not tightly bound to soils (although phytodegradation of inorganic and organic molecules is more sustainable). US practice is disposal of residuals in hazardous waste landfills but Ni smelting is feasible. Composting to reduce disposal volume conceptualized. Pilot field-testing eastern US: unproven at six sites with Pb using B. juncea but proven at two sites with Zn and Cd using Thlaspi caerulescens. Phytomining Ni: two US locations and testing in Albania and South Africa. Field proof of concept: Ni, Zn, Sr, Cs (see following warning), and Cd from long-term application of sludges using Brassicaceae hyperaccumulators in UK; Mariupol and Chernobyl regions, Ukraine; and Pennine Mountains, UK (plus Ag, Al, Co, Fe, Mo, and Mn). Failed two evaluations using chelators for Pb; thus questionable for Cr, Cs, and other tightly bound elements. New lab proof of concept now required for Pb and other tightly bound elements. Proof of concept: 1993-95 for Cd, Ni, Zn, Cu, Se, B, and other elements. Bench testing: at arid western US site for Cr, Zn, Hg, Ag, and Se using Salix x, Kochia scoparia, and Brassica napus and perchlorate using wetland halophytes
Lab proof of concept: wastewater, soil, or sediment contaminated with DNT and TNT
Proposed concept for soil, wastewater, wetlands, and groundwater contaminated with RDX
Soil, mine tailings, wetlands, and leachate pond sediments contaminated with metals, phenols, anilines, some pesticides, tetrachloromethane, trichloromethane, and other chlorinated solvents
(1) Extensive applications: revegetation grasses established for different metals dominated wastes in UK and US erosion prevention handbooks available for many countries
(2) Bench proof of concept: for stabilization of some pesticides, phenols, and anilines
(3) Lab proof of concept: for Pb and Cr6+(v:)a (the numbers correspond to those in column 2)
BTEX, benzene, toluene, ethyl benzene, and xylene; DCE, dichloroethane; DDT, dichloro diphenyl trichloroethane; DNT, dinitrotoluene; HCA, hexachloroethane; HMX, octahydro 1,3,5,7 tetranitro 1,3,5,7 tetraazocine; MTBE, methyl tert butyl ether; NB, nitrobenzene; NT, nitrotoluene; PAHs, polycyclic aromatic hydrocarbons; PCBs, polychlorinated biphenyls; PCE, tetrachloroethene; PCP, pentachlorophenol; RDX, hexahydro 1,3,5 trinitro 1,3,5 triazine; TCE, trichloroethene; TNT, 2,4,6 trinitrotoluene; VC, vinyl chloride.
define whether plant or microorganism management tech niques must be applied. When critical rates for microbial and botanical uptake and transformation are comparable, both techniques should be applied simultaneously for engi neering redundancy and ecological resilience.
One of the most significant advances in phytoremedia tion is that green liver metabolism is much more important in waste management than early biotechnology research revealed. Sandermann first coined the term 'green liver' to convey the great similarity between plant and mammalian sequestration and metabolism. So great is the similarity that many view plant metabolism more akin to mammalian metabolism than to bacterial metabolism. In fact, many fundamental metabolic processes first evolved in early cyanobacteria and bacteria and were carried forward, sometimes without evident purpose, into higher forms of life present today, including vascular plants and mammals. But for future xenobiotic and highly complex hazardous wastes, the most sustainable applica tions may need to concentrate on use of the most highly evolved enzymatic systems available only in plants and animals. In part, bacteria versus higher forms of life have evolved different survival strategies. Microbes are present in great numbers, almost ubiquitous on this planet, usually passively mobile, more adaptable, and capable of evolving rapidly. Thus, a toxic insult will kill many microorganisms but the species will usually survive, maybe even the rigors of outer space. If the die off is extensive or long term, new protections may evolve by selection of the fittest.
Plants are normally rooted in place and are much fewer in number. Thus, plants may have evolved greater numbers of metabolic proteins used to detoxify insults in place, than microorganisms evidently require for survival. Plants are different from animals in the lack of (1) an immediate flight response and (2) excretion of transfor mation products. Animals tend to excrete transformation products, whereas plants tend to accumulate some trans formation products in vacuoles or between layers of molecules in cell walls. Plant transformation products are accumulated and could be released into the environ ment upon death and lysis of plant cells.
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