Phytoremediation

Heavy metal contamination of agricultural soils, aquatic waters, and ground water can pose serious environmental and health concerns (45). Experimentation into the phyotoextraction of copper from soils is limited (46). However, approximately 24 copper-hyperaccumulating plant species have been reported, including members of Cyperaceae, Lamiaceae, Poaceae, and Scrophulariaceae families (46). Reportedly, the only true copper-accumulating plants are from the central African countries of Za�re and Zambia (47,48). The political instability of these regions makes obtaining plant material for research experimentation difficult and has hindered the work in this area (47,48). Work by Morrison (49) with Za�rian copper-tolerant plants showed mint species (Aeollanthus biformifolius De Wild) to accumulate 3920 �g Cu g^-1 dry weight; figwort species, bluehearts, (Buchnera metallorum L.) to accumulate 3520 �g g^-1 dry weight; gentian species (Faroa chalcophila P. Taylor) to accumulate 700 �g g^-1 dry weight; and mint species (Haumaniastrum robertii (Robyns) Duvign. & Plancke) to accumulate 489 �g g^-1 dry weight (47,48). Rhodegrass (Chloris gayana Kunth.), African bristlegrass or forage setaria (Setaria sphacelata Stapf. and C.E.Hubb), two indigenous grass species, and oat (Avena sativa L.) were evaluated for copper soil extraction in Ethiopian vegetable farms irrigated with wastewater from a textile factory, water from the Kebena and Akaki Rivers, and potable tap water. The maximum copper concentration of these plants was only 10.4 mg kg^-1 dry weight. However, soil copper levels for the experiments ranged from 2.5 to 3.5mg kg^-1, and these low values may indicate low copper delivery from these irrigation sources (50).

Phytochelatins are peptides [(γ-Glu-Cys)_nGly] produced by plants in response to heavy metal ion exposure (51). These compounds function to complex and detoxify metal ions (52). A variety of metal ions such as Cu²⁺, Cd²⁺, Pb²⁺, and Zn²⁺ induce phytochelatin synthesis (47,48). In addition, cations Hg²⁺, Ag⁺, Au⁺, Bi³⁺, Sb³⁺, Sn²⁺, and Ni²⁺, and anions AsO₄^3- and SeO₃^2-, induce phytochelatin biosynthesis (52). Together with phytochelatin and metallothionein (cysteine-based proteins that transports metals) (53), internal coordination and vacuolar sequestration determine the tolerance of plant species and cultivars to heavy metals (18). No induction of phytochelatin synthesis was observed following exposure to Al³⁺, Ca²⁺, CO₂⁺, Cr²⁺, Cs⁺, K⁺, Mg²⁺, Mn²⁺, MoO₄^2-, Na⁺, or V⁺ (52). Copper phytochelatins have been isolated from common monkeyflower (Mimulus guttatus Fisch. ex DC) (54). Exposure of serpentine roots (Rauwolfia serpentina Benth. ex Kurz) to 50 μM CuSO₄ in hydroponic culture resulted in arrested plant growth for 10 h and rapid production of Cu²⁺-binding phytochelatins. Two days after treatment, 80% of the copper in solution was depleted from the nutrient solution, and the intercellular phytochelatin concentration reached a constant level, and normal growth resumed (52).

Some plants have shown a strong potential for hyperaccumulation of copper in their tissues. A population of aromatic madder (Elsholtzia splendens Nakai) collected on a copper-contaminated site in the Zhejiang providence of China demonstrated phytoremediation potential after the species was noted to accumulate 12,752 �g Cu g^-1 dry weight in roots and 3417 �g Cu g^-1 dry weight in shoots when cultured in nutrient solutions containing 1000 �M Cu²⁺ (55). Alfalfa shoots accumulated as much as 12,000 mg Cu kg^-1 (56). Roots of a willow species (Salix acmophylla Boiss.), an economically important tree which grows on the banks of water bodies, accumulated nearly 7 to 624 �g Cu g^-1 dry weight in response to increasing copper treatments in soil from 0 to 10,000mg kg^-1 (45). On three soils in Zambia, the roots of a grass species (Stereochlanea cameronii Clayton) accumulated 9 to 755 �g Cu g^-1 dry weight in response to a range from 0.2 to 203 �g Cu g^-1 in soil (57). Evidence suggests quantitative genetic variation in the ability to hyperaccumulate heavy metals between- and within-plant populations (58). Populations of knotgrass (Paspalum distichum L.) and bermudagrass (Cynodon dactylon Pers.) located around mine tailings in China contained 99 to 198 mg Cu kg^-1. These native grass populations were more tolerant to increasing CuSO₄ concentrations in solution culture than similar genotypes collected from sites containing much lower levels of copper in soil (2.55 mg Cu kg^-1) (59). Legumes, Lupinus bicolor Lindl. and Lotus purshianus Clem. & Clem., growing on a copper mine site (abandoned in 1955) in northern California showed greater tolerance to 0.2 mg Cu L^-1 in solution culture than genotypes growing in an adjacent meadow (60). Among ten Brassicaceae, only Indian mustard (Brassica juncea L.) and radish showed seed germination higher than 90% after 48 h exposure to copper concentrations ranging from 25 to 200 �M (18). As noted with other heavy metals, copper actually caused a slight increase in the degree of seed germination, possibly due to changes in osmotic potential that promote water flow into the seeds (18).

Copper toxicity limits have been established for grass species used to restore heavy metalcontaminated sites. Using sand culture, the lethal copper concentration for redtop (Agrostis gigantea Roth.) was 360 mg Cu L^-1, for slender wheatgrass (Elymus trachycaulus Gould ex Shiners) was 335 mg Cu L^-1, and for basin wildrye (Leymus cinereus A. Love) was 263 mg Cu L^-1, whereas tufted hairgrass (Deschampsia caespitosa Beauv.) and big bluegrass (Poa secunda J. Presl) displayed less than 50% mortality at the highest treatment level of 250 mg Cu L^-1 (61). Success has been shown with sodium-potassium polyacrylate polymers for copper remediation in solution and sand culture; however, the cost of application is often prohibitive. This polymer material at 0.07% dry mass in sand culture absorbed 47, 70, and 190 mg Cu g^-1 dry weight at 0.5�M, 1�M, 0.01 M Cu (as CuSO₄.5H₂O) in solution, respectively (62). In this experiment, the polyacrylate polymer increased the dry weight yield of the third and fourth cutting of perennial ryegrass (Lolium perenne L.) after 50 mg Cu kg^-1 was applied.