Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds

Soil contamination by heavy metals such as cadmium (Cd), copper and mercury has become a big concern particularly in metal plating plants, mining sites and surrounding areas as well as residential area and farmlands in the river downstream region neighboring these facilities. In some cases, heavy metals in soils leach into river water and then diffuse onto farmlands with irrigation, resulting in relatively low levels of heavy metals being spread into wider areas rather than being localized in high concentrations (Asami 1972). Therefore, when civil engineering methods including addition of topsoil and removal of contaminated soils are used for remediation of heavy metal contaminated soils, a large amount of uncontaminated fresh soil and large disposal areas are required, which creates a bottle neck for the remediation. Thus, the development of a new remediation technology to replace the conventional civil engineering technology is needed. The remediation technology for soils contaminated with harmful substances using plants is called phyto-remediation, and its potential has demonstrated by many researchers (Elizabeth 2005). However, this new remediation technology is in the initial stage even at present, except for some cases, e. g., the remediation of oil-spilled soil using Italian ryegrass (Kaimi et al. 2006) and that of Cdcontaminated paddy soil using rice plants (Honma et al. 2009) are currently being conducted, because no appropriate remediation plant and no remediation systems have been found and developed. Thus, weed species, possessing high adaptability to environment, have been pointed out as a suitable plant for soil remediation. Although research on phyto-remediation using weeds has just begun and there are many issues which need to be resolved, this remediation technique is expected to become a valuable technology for the alleviation of heavy metal contaminated soils in the near future. This chapter focuses on the potential of weed for remediation of Cd-contaminated soils. In order to better understand phyto-remediation by weeds, the rationale of using weeds for Cd remediation and the biological characteristics of weeds are explained. Herbaceous plants are classified into several groups such as crops, grasses, weeds and wild plants. Crops are plants that require artificial protection such as pest control, fertilization, watering and etc., ; on the other hand, weeds can thrive under severe growth conditions. For example, asiatic plantain (Plantago asiatica) in highly compacted soil areas, crabgrass (Digitaria cilliari) in dry regions, annual bluegrass (Poa annua) in cool wet regions, field horsetail (Equisetum arvense) in acidic soils, saltbush (Atriplex subcordata) in salt-accumulation areas, and broomsedge


Introduction
Soil contamination by heavy metals such as cadmium (Cd), copper and mercury has become a big concern particularly in metal plating plants, mining sites and surrounding areas as well as residential area and farmlands in the river downstream region neighboring these facilities.In some cases, heavy metals in soils leach into river water and then diffuse onto farmlands with irrigation, resulting in relatively low levels of heavy metals being spread into wider areas rather than being localized in high concentrations (Asami 1972).Therefore, when civil engineering methods including addition of topsoil and removal of contaminated soils are used for remediation of heavy metal contaminated soils, a large amount of uncontaminated fresh soil and large disposal areas are required, which creates a bottle neck for the remediation.Thus, the development of a new remediation technology to replace the conventional civil engineering technology is needed.The remediation technology for soils contaminated with harmful substances using plants is called phyto-remediation, and its potential has demonstrated by many researchers (Elizabeth 2005).However, this new remediation technology is in the initial stage even at present, except for some cases, e. g., the remediation of oil-spilled soil using Italian ryegrass (Kaimi et al. 2006) and that of Cdcontaminated paddy soil using rice plants (Honma et al. 2009) are currently being conducted, because no appropriate remediation plant and no remediation systems have been found and developed.Thus, weed species, possessing high adaptability to environment, have been pointed out as a suitable plant for soil remediation.Although research on phyto-remediation using weeds has just begun and there are many issues which need to be resolved, this remediation technique is expected to become a valuable technology for the alleviation of heavy metal contaminated soils in the near future.This chapter focuses on the potential of weed for remediation of Cd-contaminated soils.In order to better understand phyto-remediation by weeds, the rationale of using weeds for Cd remediation and the biological characteristics of weeds are explained.Herbaceous plants are classified into several groups such as crops, grasses, weeds and wild plants.Crops are plants that require artificial protection such as pest control, fertilization, watering and etc., ; on the other hand, weeds can thrive under severe growth conditions.For example, asiatic plantain (Plantago asiatica) in highly compacted soil areas, crabgrass (Digitaria cilliari) in dry regions, annual bluegrass (Poa annua) in cool wet regions, field horsetail (Equisetum arvense) in acidic soils, saltbush (Atriplex subcordata) in salt-accumulation areas, and broomsedge (Andropogon virginicu) in phosphate-deficient soils can grow vigorously under these conditions (Takematsu and Ichizen 1987, 1993, 1997).For plants, not only Cd contaminated soil but also various environmental factors such as low temperature, aridity, low sunlight (shade), nutrient deficiency (infertile soil), poorly drained soil, competitions between plants for water, nutrition, and light, and allelochemicals generated from plants are regarded as adverse growth conditions.Therefore, remediation plant (plant using for restoration of Cd pollution soils) must be able to grow under these adverse weather and soil conditions.On the other hand, several methods are considered in phyto-remediation, and the capability of remediation plants are depending on the approach considered.Unlike organic compounds, Cd cannot be degraded; therefore, absorption (phyto-extraction) and fixation (phytostabilization) are the most effective methods proposed for Cd-remediation.Phyto-extraction is the chemical removal method of Cd by absorption through the roots and accumulation in shoots, followed by plant harvesting.Phyto-stabilization is a method of retaining Cd on the adjacent surface of plant roots.Mulching, which prevents the run-off of Cd contaminated soil into the surrounding non-polluted area by the root system extending in soils, can also be considered for phyto-remediation technologies.Particularly in slopes, mulching with plants may be prior to Cd extraction from the contaminated soils.Among several screening studies on the remediation plants conducted, it is demonstrated that Athyrium yokoscense is highly tolerant to heavy metals (Nishizono et al. 1987); however, its biomass is extremely small to be valuable as a remediation plant.The plant species best suited for phyto-extraction require the ability to accumulate large amounts of Cd in their shoots, extension of their roots into soil, rapid growth and a long growing period.The plant species suitable for mulching require the ability to extend their roots into soil, a high LAI (leaf area index) value; plants with a high LAI value can reduce the physical strength of rainfall to scour soil, large biomass, rapid growth and long growing period.Furthermore, it is important that whether seeds and vegetative reproductive organs such as rhizomes and tubers can be inexpensively supplied in large quantities for the remediation plant.As compared with crops, weeds are generally superior in several points such as environment adaptability and Cd tolerance and accumulation, however, are inferior in seed supply (Table -1).Thus, in case when seeds of remediation plants (weeds) are difficult to obtain, top soil (seed bank) of non-pollution areas where weeds are densely grown, are available.As well as Cd tolerant weed species, Cd sensitive weed species are also useful for Cd remediation (Table -2); e. g., results of phyto-remediation at the pollution area can be evaluated by distribution and biomass of the Cd sensitive weed species such as Arenaria serphylliforia, Geranium carolinianum and Phseolus aureas.

Factors weeds Crops
Cd

Sensitivities of weeds to Cd
About 6,000 plant species are accounted as a weed in the world.Cd sensitivities weeds vary depending on species (Fig. 1 and Fig. 2).In this section, Cd sensitivity and Cd content of weeds are referred.
It is reported that Arabidopsis halleri and Thlaspi caerulescens are highly tolerant to Cd (Bert et al. 2003;Brown et al. 1995 pratensis is a long-lived grass that can survive more than a few decades; therefore, phytoremediation will be proceeded continuously once perennial weed species such as P. pratensis is introduced into the Cd pollution area. On the other hand, there are great variations on Cd tolerance among weed species.However, the tolerance can be predicted when they belong to the same family.Abe et al. (2006) reported that Gramineae and Compositae weed species were torelant, while Leguminoseae and Cruciferae weed species were sensitive according to the pot tests conducted  3 and Table-4).This result indicated that the Cd concentrations of soils in a certain areas could be predicted by analysis of weed vegetation.Among weed species belonging to Gramineae and Leguminosae species, appropriate Cd remediation plant and Cd indicator may be found.

Utilization of PGRs to Cd remediation by use of weeds
So far, various cultivation methods and materials have been used to control the Cd elution into soils (Turgut et al. 2004;Hattori et al. 2006); e. g., treatment of Calcium materials and deep flooding (rice paddy fields) have been used to diminish Cd elution into the free water in soils; on the other hand, drying of soil and treatment of chlorides and EDTA have been used to enhance the Cd elution.In addition to these materials, it is reported that certain plant hormones directly affect the growth of remediation plants.Abscisic acid suppresses a decrease in chlorophyll content caused by Cd in Brassica napus, while 28-homobrassinolide mitigates the growth inhibition caused by Cd in Cicer arietium.According to a hydroponic test using white Japanese millet (Echinochloa frumentacea) conducted by Abe et al. (2011), it was shown that HMI (3-hydroxy-5-methylisoxazole) at 2.5 × 10 3 mmol/l significantly reduced the growth inhibition (dry weight; % of control) of roots and shoots caused by Cd at 4.4 × 10 mmol/l from 4.2% to 23.9% and from 48.5% to 82.6%, respectively, while increasing Cd content (μg plant -1 ) from 0.32 ± 0.08 to 0.53 ± 0.04 in roots.This result indicates that Cd tolerance of remediation plants can be further enhanced by HMI. (Fig. 4, Fig. 5, Fig. 6) HMI, plant growth regulator (PGR) that shows similar action to those of plant hormones, has been used as rooting agent for paddy rice (Ogawa and Ota 1976) and thus it is thought that HMI can be also easily applied for Cd remediation with weeds, particular in the mulching.

Goal and approach for the restoration of Cd pollution soils by use of Weeds
In general, Cd pollution of soils are distributed over wide ranges, and recovery of vegetation at the pollution area and Cd extraction from the soils require a long time; thus it is important to make a grand design on goals and approach of the remediation of Cd pollution soils using weeds based on long term foresight.The remediation approach Fig. 7. Goal and approach of remediation of Cd pollution soils by use of weeds

Cd + HM1
depends on the not only Cd contamination levels in soil but also environmental and geographical conditions of the pollution areas, e. g., when the area is located in slopes, prevention of run-off of Cd contaminated soil by mulching should be preceded to Cd extraction.For the Cd remediation using weeds, several goals are suggested as follows; 1) prevention of Cd elution into the surrounding non-polluted areas; vegetation is restored by mulching with weeds and followed by introduction of grasses, turf grasses and shrubs as needed, but Cd extract is not involved in this case, 2) extraction of Cd from the pollution soils; Cd extraction is conducted by use of the remediation plants, and thereafter when postmitigated area is reused as a farmland, introduced remediation plants (weeds) should be controlled completely by herbicides and practical methods before land reuses (Fig. 7).As mentioned above, Cd remediation approach varies with goals, however, preliminary survey of Cd concentrations in soil, weather conditions (temperature, sunlight and precipitation by month), physicochemical characteristics of soils, drainage, fertility and slope angle are needed regardless of the approach and the goal to decide the remediation plant (weed species) and the remediation methods (phyto-extraction, phyto-stabilization and/or malching).Moreover, in some cases, repeated introduction of the remediation plants into the Cd pollution areaa may be required, because seed production of the plant is possibly be inhibited by Cd even when Cd tolerant plants are used.For example, it is reported that the number of seeds / plant of Portulaca oleracea var.sativa, which is tolerant to Cd, is inhibited by 65% at 20 mgkg -1 of Cd (Abe et al. 2008b).Use of hormone and PGR may increase in the seed production and biomass of the remediation plants, and those seedlings habituated in the Cd contaminated soils before introducing them into the pollution areas, may be useful for the remediation.

Conclusion
Weeds are highly adaptive to adverse environmental conditions compared to crops, which is a crucial factor to consider weeds for developing soil remediation technologies.However, it might be considerably more difficult to ensure the necessary seed and seedling supply of weeds compare to crops, which can hamper the development of potential remediation technique.Thus when weeds are used for restoration of Cd pollution soils, to develop an effective seed and seeding production systems may become a crucial requirement.Besides, the proposed phyto-remediation with weeds might be envisioned as a long term approach due to the time requirements of the approach.In general, there is a great tendency that Cd extraction is much spotlighted than other goals, however, supplying the soils abundant in organic matter which absorb Cd and keep it in the soils by transplanting of herbaceous plants including weeds; resulting in the prevention of the run-off of Cd contaminated soil into surrounding nonpolluted areas, may be efficient in case that Cd pollution is distributed over the extensive area.

Fig. 4 .
Fig. 4. Effect of HMI on Cd accumulation in shoot and root of Echinochloa frumentacea The error bars represent the standard deviation of the mean (n=3).The means followed by the same letter within a column are not significantly different by Tukey's multiple range test (p<0.05)

Table 1 .
Comparison of weeds and crops as a Cd remediation plants *: drought, cool, shade, wet, salinity and infertile land tolerance * Susuceptible weed species having ≤ 2 of I50 value (mg Cd kg -1 ): the concentrations of Cd causing a 50% reduction in fresh weight of shoot to that of the untreated plant grown under sand culture conditions Table 2. Susceptible weed species to Cd

Table 4
Oryza sativa (cv.Milyang) accumulate high concentrations of Cd (>100 mg kg -1 DW) in its roots but accumulate little Cd in their shoots, in contrary, Compositae plant, e. g., Bidens frondosa accumulate low concentration of Cd in the root but accumulate much Cd in its shoots.The Cd content ratio between roots and shoots (SR ratio) of C.
Abe et al. (2008a) of Caryophyllaceae, Cruciferae, Leguminosae, Compositae and Gramineae weed species to Cd3.Cd accumulation abilities of weedsHigh Cd accumulation ability as well as Cd tolerance is required for the remediation plants; however, there is no positive correlation between Cd accumulation and Cd sensitivity.Weed species with highly tolerant to Cd can be divided into two groups: weeds that absorb hardly Cd (Cd exclusion type) and weeds that absorb and detoxify Cd (Cd detoxaccumulation type).Abe et al. (2008a)revealed from a pot test that Cichorium intybus (77 mg kg -1 DW) and Matricaria chamomilla (64.4 mg kg -1 DW) can accumulate large amounts of Cd in their shoots; these accumulations are more than Polygonum thunbergii (56.2mg kg -1 DW) that is known as a hyper-Cd accumulator(Shinmachi et al. 2003) (Table-5).The mechanism involved in Cd accumulation by weeds are still not well understood; however, it can be predicted that the absorbed Cd is detoxified by forming chelates with phytochelatin, malic acid, and citric acid in plants.On the other hand, Gramineae plant, e. g.,

Table 5 .
Weed species which accumulate relatively high ( 30) and low ( 3 ) concentrations (mg kg -1 DW) of Cd in their shoots

Table 7 .
Weed species which accumulate relatively high ( 20 μg plant -1 ) content of Cd in shoots * shoot-root ratio : the ratio of Cd conc. in shoot and root in plant Table8.Weed species which have relatively high ( 2.0) and low ( 0.03) shoot-root ratios*