**3. Assessment of phytoextraction**

#### **3.1. Advantages of phytoextraction**

had an average BLL of 60 μg/dL [16, 17]. Additionally, the soil near a car battery processing workshop in Kerman City, Iran, was found to contain 5, 780 mg/kg of lead [18]. The BLL of Indians near a residential area was 20–25 μg/dL, and the lead concentration of PM10 (or PM2.5)

Soil pollution from lead was not observed in the major cities in China, such as Beijing and Hong Kong; therefore, the BLLs of inhabitants in these cities were normal (4–5 μg/dL) [20-22]. The number of mining and metallurgy factories is rapidly increasing in China due to the increased consumption of lead-acid batteries. However, no formal reports on the lead con‐ centration and BLL have been reported in areas predominantly inhabited by mining and metallurgy factories, such as Zhejiang and Guangdong provinces. According to the WEB report [4], it is suspected that 100, 000 children are suffering due to lead toxicity. Therefore, in China, the areas predominantly inhabited by mining and/or metallurgy industries are thought to contain extremely high lead concentrations which are exhausted into the air, river, and soil. Another potential cause of soil pollution from lead is a firing range. One of the worst cases of soil pollution from lead at a firing range demonstrated more than 10 kg/kg of lead due to remnant lead alloy bullets. Therefore, lead pollution in firing ranges may be as harmful as in

**2.4. Effects of lead pollution on the health of inhabitants living near metallurgy and mining**

The BLL is an indicator of pollution from lead. Lead decreases the IQ value when the BLL is greater than 20 μg/dL [23]. A test to measure the ability of recognition in monkeys suggested that dysgnosia was observed in monkeys with BLLs of 10–13 μg/dL. Moreover, lead toxicity was observed when the BLL exceeded 40 μg/dL. According to recent studies, the BLL should

The main route of exposure for an elevated BLL is ingestion. The amount of lead that adult subjects ingest from food is generally 20–25 μg/kg and 5–10 % is absorbed. Approximately 100 mg of lead is present in the body. The sensitivity of lead in a child is much higher than in an adult because 40 % of the ingested lead is absorbed. A previous report demonstrated that when more than 5 μg/kg/day of lead was ingested in infants, 32 % of the lead was absorbed, although no accumulation was observed in infants who ingested less than 4 μg/kg/day. The WHO also suggested that the BLL was not increased in those who ingested less than 4 μg/kg/day [26]. The other most common route of exposure is polluted air. Approximately 40–50 % of lead taken in from the nose is absorbed by the lung. The relationship between the concentration of lead in the air and the BLL is shown in Figure 3 (based on the data from the study by Thomas et al. on pollution from lead gasoline [11]). The BLL was found to be strongly correlated with air

Safety standards are defined to keep the environment safe. In Japan, the lead concentrations

BLL observed in Japanese is 1–3 μg/dL, and the normal concentration of lead in the soil is 15– 30 mg/g. The lead concentrations in the areas near mining and metallurgy industries listed in

and 0.01 mg/L, respectively. Moreover, the normal

) [11, 19].

in the residential area was very high (10–14 mg/m3

252 Advances in Bioremediation of Wastewater and Polluted Soil

mining areas.

be maintained below 10 μg/dL [24, 25].

in the air and wastewater are below 1 ng/m3

**areas**

pollution.

The remediation of polluted soil is extremely costly. In the United States, ten billion dollars were invested to remediate soil pollution in the 1990s. According to an approximate calculation of the cost [27], more than 100 billion dollars would be necessary for remediation of polluted soil in the 2000s; thus, an inexpensive process was sought to decrease the investment. Phy‐ toextraction is a remarkable process where heavy metals can be absorbed from the soil and accumulated into plants at high concentrations without the use of expensive equipment. The advantages of phytoextraction also include increased safety and inexpensive running cost compared to physical and chemical methods, such as washing and solidification. Phytoreme‐ diation is an expanding market; in the United States and Japan, it is expected to result in 170 million dollars and 800 million yen, respectively.

The plants which contain the highest abilities of absorption are called "hyperaccumulators" and are the most suitable for the phytoextraction of heavy metals. With regard to lead, a hyperaccumulator is defined as a plant that is capable of accumulating greater than 1, 000 mg/ kg dry biomass (or 100 times more than other plants) and generally shows a high tolerance to heavy metals. Some prominent hyperaccumulators have been screened and identified [28]. *Stanleya pinnata*, for example, was found to accumulate 3, 000 ppm of selenium in its leaves when planted on soil containing 6 ppm of selenium [29]. *Rinorea niccolifera* was recently found in Western Luzon, Philippines, and could accumulate an unusually high amount (18, 000 ppm) of nickel by detoxifying with vacuoles [30]. *R. niccolifera* was the most prominent hyperaccu‐ mulator, because it was capable of absorbing heavy metals at a value several hundred times higher than other plants.

Additionally, an interesting tree was found in the Sabah Parks in Malaysia. The green sap of the tree contained high concentrations of nickel [31, 32] and could be continuously collected. When the tree was planted in soil containing wastes from nearby mines, a green nickel-rich sap containing 20 % nickel was collected from the trees. The ash of the burned sap additionally contained high concentrations of nickel (10–25 %), which corresponded to 200 kg/ha (2, 000 dollars/ton). Those studies suggest that phytoextraction may be applicable to soil pollution from nickel.

#### **3.2. Phytoextraction of lead by a hyperaccumulator**

Hyperaccumulators absorbing lead have been screened as well as hyperaccumulators for other heavy metals. The chief hyperaccumulators introduced in this section are shown in Table 1. *Thlaspi caerulescens* [33], kenaf [34], sunflower [35], *Cannabis sativa* [36, 37], *Tagetes minuta* L. [38], cabbage [39], *Brassica juncea* [40], *Acacia victoriae* [41], and buckwheat are superior hyperaccumulators compared to other weeds and crops, and the following plants are espe‐ cially remarkable. *A. victoriae* was found to be capable of accumulating 3, 580 mg/kg of lead from a 1, 000 mg/L solution of lead nitrate. *B. juncea* additionally showed a high lead tolerance and could accumulate a significantly high concentration of lead (34, 500 mg/kg). Furthermore, Shinshu buckwheat, an improved breed developed by Shinshu University, was capable of growing in soil containing more than 3, 000 mg/kg of lead and could accumulate 6, 000–10, 000 mg/kg of lead. Buckwheat may be the most suitable plant because it is obtained at a high yield (700 g/ha/year) [42].

Additionally, two types of pteridophytes, *Athyrium yokoscense* and *Pteris vittata*, showed high tolerance to lead. *A. yokoscense* is typically found around mine areas containing high concen‐ trations of heavy metals. It is known as an indicating plant to explore gold veins in Japan, as it lives in a cluster around areas of gold rubbish. One gametophyte of *A. yokoscense* was capable of accumulating high concentrations of lead (10, 000 mg/kg) and showed tolerance to extremely high concentrations of lead [43]. *P. vittata* has been shown to accumulate arsenic. A breed belonging to *P. vittata* could accumulate 16, 257.5 mg/kg of lead and grow in soil containing 92, 900 mg/kg of lead. Additionally, it accumulated 4, 829 mg/kg of lead when it was grown in mine soil for six months [44].

The two types of mosses *Scopelophila cataractae* and *Funaria hygrometrica* were also identified as hyperaccumulators. *S. cataractae*, which is known as "copper moss" in Japan, was found in soil containing high concentrations of copper and could accumulate copper selectively at the cell wall. Moreover, a breed belonging to *S. cataractae* could accumulate lead as well as copper. *F. hygrometrica* additionally showed a very high ability of absorbing lead [45, 46]. The reports by Riken (Japan) suggested that *F. hygrometrica* adsorbed 70 % of lead per dry biomass when an effluent containing lead was supplied to the column containing the moss [47]. An advantage of using moss is that it can grow at a fast rate without water. Thus, mosses may be suitable for areas in which there is little water.

when planted on soil containing 6 ppm of selenium [29]. *Rinorea niccolifera* was recently found in Western Luzon, Philippines, and could accumulate an unusually high amount (18, 000 ppm) of nickel by detoxifying with vacuoles [30]. *R. niccolifera* was the most prominent hyperaccu‐ mulator, because it was capable of absorbing heavy metals at a value several hundred times

Additionally, an interesting tree was found in the Sabah Parks in Malaysia. The green sap of the tree contained high concentrations of nickel [31, 32] and could be continuously collected. When the tree was planted in soil containing wastes from nearby mines, a green nickel-rich sap containing 20 % nickel was collected from the trees. The ash of the burned sap additionally contained high concentrations of nickel (10–25 %), which corresponded to 200 kg/ha (2, 000 dollars/ton). Those studies suggest that phytoextraction may be applicable to soil pollution

Hyperaccumulators absorbing lead have been screened as well as hyperaccumulators for other heavy metals. The chief hyperaccumulators introduced in this section are shown in Table 1. *Thlaspi caerulescens* [33], kenaf [34], sunflower [35], *Cannabis sativa* [36, 37], *Tagetes minuta* L. [38], cabbage [39], *Brassica juncea* [40], *Acacia victoriae* [41], and buckwheat are superior hyperaccumulators compared to other weeds and crops, and the following plants are espe‐ cially remarkable. *A. victoriae* was found to be capable of accumulating 3, 580 mg/kg of lead from a 1, 000 mg/L solution of lead nitrate. *B. juncea* additionally showed a high lead tolerance and could accumulate a significantly high concentration of lead (34, 500 mg/kg). Furthermore, Shinshu buckwheat, an improved breed developed by Shinshu University, was capable of growing in soil containing more than 3, 000 mg/kg of lead and could accumulate 6, 000–10, 000 mg/kg of lead. Buckwheat may be the most suitable plant because it is obtained at a high yield

Additionally, two types of pteridophytes, *Athyrium yokoscense* and *Pteris vittata*, showed high tolerance to lead. *A. yokoscense* is typically found around mine areas containing high concen‐ trations of heavy metals. It is known as an indicating plant to explore gold veins in Japan, as it lives in a cluster around areas of gold rubbish. One gametophyte of *A. yokoscense* was capable of accumulating high concentrations of lead (10, 000 mg/kg) and showed tolerance to extremely high concentrations of lead [43]. *P. vittata* has been shown to accumulate arsenic. A breed belonging to *P. vittata* could accumulate 16, 257.5 mg/kg of lead and grow in soil containing 92, 900 mg/kg of lead. Additionally, it accumulated 4, 829 mg/kg of lead when it was grown

The two types of mosses *Scopelophila cataractae* and *Funaria hygrometrica* were also identified as hyperaccumulators. *S. cataractae*, which is known as "copper moss" in Japan, was found in soil containing high concentrations of copper and could accumulate copper selectively at the cell wall. Moreover, a breed belonging to *S. cataractae* could accumulate lead as well as copper. *F. hygrometrica* additionally showed a very high ability of absorbing lead [45, 46]. The reports by Riken (Japan) suggested that *F. hygrometrica* adsorbed 70 % of lead per dry biomass when an effluent containing lead was supplied to the column containing the moss [47]. An advantage

higher than other plants.

254 Advances in Bioremediation of Wastewater and Polluted Soil

**3.2. Phytoextraction of lead by a hyperaccumulator**

from nickel.

(700 g/ha/year) [42].

in mine soil for six months [44].

Ornamental plants adapted for phytoextraction were also screened. However, one such plant, *Chlorophytum comosum*, could only accumulate 516 mg/kg when 1, 250 mg/kg of lead was supplied in the soil [48]. The advantage of using ornamental plants is that the flowers may be reused after remediation [48, 49]. Moreover, ornamental plants do not show a high tolerance to lead. Therefore, they may be readily adapted for soil pollution in urban places because the lead concentrations in these areas are relatively low and the plants can be used in landscaping.

Trees have interesting characteristics for phytoextraction. The advantage of using a tree is that the root is much deeper, and therefore, it can be applied to the remediation of soil at a depth of 3–5 m. Conversely, the disadvantages are a slow growth rate and low tolerance to lead, although the biomass per land area is high. To overcome these disadvantages, the best trees were screened [50-52] and a combination of aspen and rowan trees [53], a combination of *Ixora coccinea* and *Ficus benjamina* [54], and the use of ornamental trees and timber trees [55] were examined to enhance the remediation efficiency. Additionally, a field trial experiment was performed [56, 57] and the tree showing the highest ability for phytoextraction was *Acacia mangium*, which is widely used in an artificial forest in Malaysia because it is capable of growing well even in nutrient-poor soil and shows a high ability to adapt to its environment. It is known that the forest is the first stage in maintaining air and soil safety by capturing lead in the leaves [58]. The best candidate tree will be one that can effectively transfer lead to its leaves or sap, because leaves and sap can be easily and continuously collected.


**Table 1.** Amounts of lead absorbed by important hyperaccumulators

#### **3.3. Enhancement of the efficiency of phytoextraction using ethylenediaminetetraacetic acid (EDTA)**

Lead readily converts to lead(II) oxide (PbO) through contact with oxygen in the air. The resultant PbO slowly converts to inorganic salts of lead, such as Pb(NO3)2, PbSO4, and PbCO3, by acid rain containing nitrate and sulfate ions or water saturating carbon dioxide (containing carbonate ions). Table 2 shows the solubility of inorganic salts of lead [59]. The solubility of PbSO4 and PbCO3 is very low. Inorganic salts and free lead ions are adsorbed on particles in the soil by forming a complex with organic compounds contained in the particles [60, 61] and gradually changed to more insoluble compounds by reacting with phosphate. Therefore, the concentration of free lead ions in the soil is extremely low. The rapid absorption by plants is disturbed due to the low concentration of free lead ions or its inorganic salts. The amount of free lead ions must be increased by removing lead salts adsorbed in the soil particles for rapid absorption.

Many methods to increase the efficiency of phytoextraction have been reported [62], and the best method was to supply ethylenediaminetetraacetic acid (EDTA) to the soil. The complex formation constants (pK) of EDTA to Pb(II), Cd(II), Zn(II), and Fe(III) are 18.3, 16.6, 16.7, and 24.2, respectively. Therefore, the lead ions of inorganic salts (or PbO) in the soil positively conform the complex followed by the addition of EDTA (pH < 7) [63]. In a pilot experiment, when the soil containing 1, 935 mg/kg of lead was washed with water containing EDTA, 97 % of lead was extracted from the soil [64, 65].


**Table 2.** Solubility of lead and its inorganic salts in water. This table is based on the WEB data [59]

Moreover, the processing time and accumulation in phytoextraction can be drastically shortened and enhanced by EDTA. For example, *Scirpus Maritimus* L. could adsorb 80 % of lead in the root within 60 days when 5 mmol/kg EDTA was supplied [66]. Absorbed amounts of lead in *Zea mays* L. and *Pisum sativum* L. were enhanced to 120 times [67], and Indian mustard *Brassica juncea* had increased absorption when EDTA was supplied [68]. Kos et al. [38] investigated the effect of 5 mmol/kg EDTA or 10 mmol/kg ethylenediamine-N, N'-disuccinic acid (EDDS) on lead accumulation in various plants, and the results suggest that *Cannabis sativa* contained the best phytoremediation potential (26.3 kg/ha) and accumulated 1, 053 mg/ kg of lead. Furthermore, for cabbage (a high-biomass crop), the accumulation and treatment period was enhanced (5, 010 mg/kg) and shortened, respectively, when 3.0 mmol/kg EDTA was supplied for seven days [40]. When buckwheat was cultivated in soil containing 13, 032 mg/kg of lead and EDTA and citric acid were supplied for two months, 22, 363 mg/kg of lead was accumulated in the shoots and leaves [69]. These results suggest that EDTA is effective for increasing the absorption of lead in plants.

However, there are several disadvantages for the use of EDTA. One is the low degrading characteristic of EDTA [70]. EDTA supplied in the soil exists without degradation for a long period of time and slowly degrades to diketopiperazines, which are toxic compounds. To solve this problem, EDDS, which has a higher biodegradability than EDTA, has been used [71-73], although the complex formation constants (pK) of EDDS are lower than that of EDTA and an unfavorable exchange between lead and the others often occurred [67, 68]. Another disadvant‐ age is the elution of other metal ions following the use of EDTA. The addition of EDTA in the soil causes the leakage of important minerals (e.g., Mg and Ca) necessary for the plant growth, and the effluent containing toxic ions (e.g., Pb, Hg, and As) pollutes the groundwater [74]. Therefore, EDTA or EDDS must be applied to the soil at the lowest effective concentrations.

#### **3.4. Improved procedure for the phytoextraction of lead**

by acid rain containing nitrate and sulfate ions or water saturating carbon dioxide (containing carbonate ions). Table 2 shows the solubility of inorganic salts of lead [59]. The solubility of PbSO4 and PbCO3 is very low. Inorganic salts and free lead ions are adsorbed on particles in the soil by forming a complex with organic compounds contained in the particles [60, 61] and gradually changed to more insoluble compounds by reacting with phosphate. Therefore, the concentration of free lead ions in the soil is extremely low. The rapid absorption by plants is disturbed due to the low concentration of free lead ions or its inorganic salts. The amount of free lead ions must be increased by removing lead salts adsorbed in the soil particles for rapid

Many methods to increase the efficiency of phytoextraction have been reported [62], and the best method was to supply ethylenediaminetetraacetic acid (EDTA) to the soil. The complex formation constants (pK) of EDTA to Pb(II), Cd(II), Zn(II), and Fe(III) are 18.3, 16.6, 16.7, and 24.2, respectively. Therefore, the lead ions of inorganic salts (or PbO) in the soil positively conform the complex followed by the addition of EDTA (pH < 7) [63]. In a pilot experiment, when the soil containing 1, 935 mg/kg of lead was washed with water containing EDTA, 97 %

**Compound Solubility to water**

Pb 3.1 x 10-5 PbO 5.04 x 10-3 (α form)

Pb3(PO4)2 1.4 x 10-5 PbHPO4 2.187 x 10-2 Pb(NO3)2 54.3 PbCO3 7.269 x 10-5 PbS 6.77 x 10-13 PbSO4 3.836 x 10-3 Pb(OH)2 1.615 x 10-3 Pb(CH3COO)2 44.3

Moreover, the processing time and accumulation in phytoextraction can be drastically shortened and enhanced by EDTA. For example, *Scirpus Maritimus* L. could adsorb 80 % of lead in the root within 60 days when 5 mmol/kg EDTA was supplied [66]. Absorbed amounts of lead in *Zea mays* L. and *Pisum sativum* L. were enhanced to 120 times [67], and Indian mustard *Brassica juncea* had increased absorption when EDTA was supplied [68]. Kos et al. [38] investigated the effect of 5 mmol/kg EDTA or 10 mmol/kg ethylenediamine-N, N'-disuccinic acid (EDDS) on lead accumulation in various plants, and the results suggest that *Cannabis sativa* contained the best phytoremediation potential (26.3 kg/ha) and accumulated 1, 053 mg/

**Table 2.** Solubility of lead and its inorganic salts in water. This table is based on the WEB data [59]

**g/100g H2O (20 °C)**

absorption.

of lead was extracted from the soil [64, 65].

256 Advances in Bioremediation of Wastewater and Polluted Soil

Two devises have been proposed to enhance the concentration of free lead ions or its inorganic salts. One is an electro-phytoextraction process. Electro-phytoextraction is performed under an electric field and can enhance the performance. For example, the absorption in ryegrass was enhanced when 1.0 V/cm of DC electrical field was given to the soil in a vertical direction [75], and the absorption in *Brassica juncea* was enhanced when an electric field was given at four times over 30 V [76]. Additionally, when 1.0 V/cm of AC electrical field, as well as DC, was given to rapeseed (*Brassica napus*) and tobacco (*Nicotiana tabacum*), the absorption of Cd and Pb in the shoots was enhanced [77].

Another device used to improve phytoextraction is the use of acid [78, 79]. The concentration of free lead is increased when the pH of the soil is more acidic (near pH 5). The solubility of lead phosphate, which is an insoluble compound, was enhanced by a 0.15 M citric acid solution [80], and oxalic acid was the best at enhancing the solubility of pyromorphite (Pb5(PO4)3Cl), an insoluble phosphate [81, 82]. Because microorganisms can secrete various acids, such as acetic, citric, and lactic acids, the soil pH may be decreased by supplements of these microor‐ ganisms. The effects of urea [83] and other chelate compounds [84, 85], as well as the effect of acids, were examined. The cells of *Rhizobacteria* could enhance the concentration of free lead by secreting siderophores [86] and aided in plant growth by secreting indoleacetic acid (IAA), a plant growth factor. Such microorganisms are referred to as "plant-growth-promoting rhizobacteria" (PERG) [87, 88].

#### **3.5. Transgenic approach to improve the phytoextraction of lead**

The improvement of a hyperaccumulator via gene manipulation is the most effective way to enhance its ability. Transgenic plants have been energetically developed since the 1990s [89-92]. Transgenic plants which could increase the volatility of heavy metals or decrease the toxicity of heavy metals may be the best candidates because the remediation process can be continuously carried out without removing the plants. Transgenic *B. juncea*, for example, expressing the cystathionine gamma-synthase gene of *Arabidopsis thaliana* L. could convert selenium to volatile dimethylselenium [93, 94], and a plant expressing the methylmercury lyase gene decreased the toxicity by reducing methylmercury to mercury [95]. However, vaporization is not acceptable for lead because methylated lead diffuses into the air and exhibits a high toxicity as previously described.

Thus, the following two mechanisms have been proposed. One method is to enhance the number of compounds capable of combining heavy metals, such as metallothionein, gluta‐ thione, and phytochelatin. For example, the absorption efficiency of transgenic *B. juncea* expressing adenosine triphosphate sulfurylase, glutamyl-cysteine synthetase, and glutathione synthetase genes was 4.3 times higher than that in the wild plant [96]. Moreover, the accumu‐ lation in *Nicotiana glauca* expressing phytochelatin synthase was also enhanced [97]. The other mechanism is to obtain a high lead tolerance by enhancing the transport into the cell and vascular membranes. Higher tolerance and accumulation of Zn, Mn, and Cd were realized by the plants transformed with a zinc transporter (*ZAT* or *AtMTP1*), *ShMTP*, *CAX2*, *AtMHX* [89-91] or the *AtNramp, AtPDR8*, and *AtATM3* genes of ABC transporters [98, 99]. For lead accumulation, the following transgenic plants were studied: tobacco plants expressing the calmodulin-binding protein gene of *Nicotiana tabacum* (*NtCBP4*) [100] and *Arabidopsis* plants expressing the *ZntA* [101], which codes for the zinc transporter in *E. coli*, and an enhanced accumulation of lead, as well as other heavy metals, was observed. The yeast *YCF1* gene codes for a transporter of vacuolar storage of Cd/Pb. *A. thaliana* expressing the *YCF1* gene showed a high resistance to Cd and Pb and accumulated those heavy metals [102]. Transgenic poplar trees expressing the *YCF1* gene also developed a high resistance to Cd and Pb [103]. Moreover, a study conducted by Mizuno et al. showed that transgenic *A. thaliana* had longer roots (2.5 times longer) and a higher (3–14 times higher) accumulation of lead when the *FeMRP3* gene of buckwheat was expressed in *A. thaliana*.

#### **3.6. Assessment of efficiency of phytoextraction**

The author assessed the efficiency of phytoextraction in contaminated soil by lead. The most advantageous point of phytoremediation is its profitability. By the author's rough estimate, the income generated by the phytoremediation process is approximately 340, 000 dollars/ha for cases where it is assumed that (1) pollution is present at 1 m in depth and 10 g/kg of lead is contained in the soil (density: 1.7), (2) 100 % of the lead is extracted from the soil, and (3) the price of lead is 2, 000 dollars/ton. However, the approximation of the necessary expenses is much higher according to some reports and are estimated to be as high as 300, 000–5, 000, 000 dollars/ha (lowest estimation: 2, 500–15, 000) [104]. The difference in the costs suggests that further efforts are necessary to decrease the expenses in order to improve the application of phytoextraction.

The high necessary expenses of phytoextraction are due to the low yield per treatment period and the time-consuming posttreatment heavy metal recycling from the biomass. Even in buckwheat, which is one of the best hyperaccumulators for lead, the amount of lead absorption is only 20 kg/year (dual cropping), when the yield and adsorption ability are assumed to be 1 t/ha and 10 g/kg. Moreover, the plant absorbing lead must be dried, burned, and extracted to recycle the lead. The cost for those operations accounts for half of the total cost. Consequently, a hyperaccumulator with a fast growth time and a fast absorption rate, as well as a high accumulation ability, is required to overcome the problem of high necessary expenses.

Furthermore, the effect on the environment should be considered. Indigenous species do not necessarily have a superior ability for phytoremediation, although the use of indigenous species is acceptable [105]. The planting of a nonnative hyperaccumulator often changes the natural flora or may destroy the indigenous species because hyperaccumulators have an increased ability to adapt to the environment. This is particularly true for transgenic plants. Therefore, special consideration for the environment and a general consensus in the society are necessary.

In conclusion, further efforts to decrease the necessary expenses must be undertaken for the widespread use of phytoextraction, although phytoextraction is a remarkable procedure for recycling lead in the soil. For example, some weeds are capable of easily and rapidly repro‐ ducing even when most of the shoots and leaves are removed. The advantages of using a weed for phytoextraction include the following: (1) lead can be continuously obtained from the leaves and shoots, (2) the growth and absorption can be completed in a short period of time, and (3) the posttreatment is inexpensive. Therefore, the author suggests that the ideal phy‐ toextraction process includes the use of such a weed.
