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Proceeding Paper

Strategy of Phytoremediation for Sustainable Use of Arsenic-Rich Farmland †

by
Chang-Chao Chen
1,
Min-Siou Lin
1,
Pei-Cheng Cheng
2,
Chin-Yuan Huang
3,* and
Shu-Fen Cheng
3,*
1
Institute of Biochemical Technology, Chaoyang University of Technology, Taichung 41349, Taiwan
2
Institute of Environmental Engineering, National Sun Yat-Sen University, 70 Lienhai Rd., Kaohsiung 80424, Taiwan
3
Department of Environmental Engineering and Management, Chaoyang University of Technology, Tai-chung 41349, Taiwan
*
Authors to whom correspondence should be addressed.
†
Presented at the 2024 IEEE 4th International Conference on Electronic Communications, Internet of Things and Big Data, Taipei, Taiwan, 19–21 April 2024.
Eng. Proc. 2024, 74(1), 8; https://doi.org/10.3390/engproc2024074008
Published: 27 August 2024
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Abstract

:
Arsenic-rich groundwater causes arsenic accumulation in arable soils for irrigation. For such arsenic-contaminated farmland, phytoremediation is a feasible method in terms of cost and maintenance of soil. We studied arsenic-rich farmland planted with maize, Rotala rotundifolia, and the arsenic super-accumulating plant Pteris vittata and explored the arsenic absorption capacity of these plants to assess the effect of phytoremediation on arsenic-contaminated farmland. The arsenic removal by Pteris vittata was about 200 mg/m2 y. It would take about 90 years to reduce the soil arsenic below the regulatory standard of 60 mg/kg using Pteris vittata. Planting maize removed about 25.5 mg/m2 y of arsenic, and the arsenic concentration of the maize kernels was less than 0.2 mg/kg on a dry basis. It was below the standards of the animal food and human consumption limit. Pteris vittata needs to be planted first to rapidly reduce the bioavailable arsenic concentration in the soil. Subsequent planting of maize for remediating the soil while producing corn enables the sustainable production and utilization of farmland.

1. Introduction

Affected by geological factors, countries such as India, Bangladesh, Cambodia, China, Vietnam, Myanmar, the United States, Mexico, and Argentina are facing the problem of arsenic-rich groundwater [1,2,3]. Many areas in Taiwan such as Langyang Plain, Guandu Plain, Yunjianan Plain, and Pingtung Plain face the problem of arsenic-rich groundwater [4]. Oryza sativa is the most important food crop in Taiwan. In farmland with arsenic-rich groundwater, the soil accumulates arsenic due to long-term irrigation.
Long-term consumption of an arsenic-rich diet leads to chronic arsenic poisoning such as abnormal skin pigmentation, keratinization, skin cancer, and visceral cancer. In Taiwan, there have been cases of black foot disease caused by drinking arsenic-rich groundwater. The WHO recommends a limit of 10 μg/L for arsenic in drinking water [5]. In Taiwan, the edible limit of inorganic arsenic in rice is 0.35 mg/kg for brown rice and 0.2 mg/kg for white rice [6]. The total arsenic of other cereal crops is regulated, and the edible limit standard is 1.0 mg/kg (fresh weight). In order to prevent food crops from being rich in arsenic, many countries have established arsenic control standards for agricultural soils. The regulated value for rice fields in Japan is 15 mg/kg, and the farmland soil control standard is 60 mg/kg in Taiwan.
In Taiwan, there are many farmlands where the arsenic concentration exceeds the soil control standard due to groundwater irrigation. Physical or chemical technology, such as soil washing and soil solidification, costs considerably and destroys the characteristics of the soil. Phytoremediation does not destroy the productive properties of soil and is an ecologically and economically viable technology to remove contaminants. The techniques of phytoremediation include phytoextraction, phytostabilization, phytodegradation, phytoevaporation, and promotion of microbial decomposition in the rhizosphere. For arsenic-contaminated soils, phytoextraction is the most commonly used technique for phytoremediation. Through plant growth and metabolism, the arsenic in the soil is absorbed and accumulated in the plant. Then, through plant harvesting, the arsenic is removed [7]. The properties of plants as a phytoextractor include the following: (1) high tolerance to heavy metals, (2) high enough accumulation capacity for heavy metals and ease of harvesting, (3) accumulation of large amounts of biomass in contaminated soil, and (4) rapid root growth [8,9,10,11].
Pteris vittata L. also called Centipede Grass, is a certified arsenic-accumulating plant and has been used for the phytoremediation of polluted soils [12,13,14,15,16,17]. Pteris vittata L. grows on soils with arsenic content as high as 1500 mg/kg, and the accumulated arsenic concentration in leaves can reach 2500–22,630 mg/kg (dry basis). In non-arsenic-contaminated soil, arsenic accumulates up to 744 mg/kg in the above ground tissues of Pteris vittata, which is much higher than the arsenic content of general plants (<10 mg/kg) [12]. When the initial total arsenic concentration in the soil was about 63.9 mg/kg, the arsenic content in the soil decreased by 5 mg/kg after 7 months, and the arsenic reduction rate reached 7.84% [18]. In experiments of planting Pteris vittata L. in the southwest of England, the total arsenic concentration in the soil was 471 mg/kg, and the concentration of arsenic in the feather leaves was 4371 mg/kg. However, the biomass yield of Pteris vittata L. is low, with a yield of about 0.76 Mg/ha [15]. Kertulis-Tartar et al. [19] tested Pteris vittata L. in Florida with a mass yield of approximately 1.3 Mg/ha. Liu et al. [20] carried out a planting experiment of Pteris vittata L., and the aboveground biomass was lower than 2 Mg/ha. Excessive arsenic concentration in the soil inhibited the physiological activities of Pteris vittata L., reduced biomass yield, and caused a slow removal rate of arsenic from soil [21].
Arsenic is in a pentavalent form in aerated soil. The concentration of pentavalent arsenic in soil solution is very low (<1 μM) due to the strong adsorption of iron, manganese, and aluminum (hydroxide) oxides in soil and therefore reduces the bioavailability of arsenic in soil [22]. In anaerobic soils, arsenic is mainly in the trivalent form [23]. Soil immersed in water limits the transmission of oxygen, and due to the metabolism of facultative and anaerobic bacteria in the soil a reducing environment is created, resulting in the reductive dissolution of iron oxides and the release of arsenic into the soil solution. Aquatic environments enhance the bioavailability of arsenic in soil [24]. A variety of aquatic plants such as water hyacinths have a high absorption capacity for arsenic in water so can be used for arsenic removal in water [25]. Chen et al. pointed out that cattail had a strong tolerance to antimony and arsenic in slag, and the annual transfer amount of arsenic to cattail was 31.46 mg/m2; however, more than 98% of arsenic was accumulated in the root of cattail [26,27]. Lin et al. [28] studied the arsenic content of rice plants in the arsenic-rich area of the Guandu Plain in Taiwan. The average arsenic concentrations in roots, stems, and leaves of rice were 98.7, 3.49, and 0.074 mg/kg (dry weight), respectively. The arsenic content in the roots of rice accounted for about 96.8% of the whole plant, and that of the stems and leaves accounted for 3.1 and 0.1%. The cumulative concentration of arsenic in aquatic plants in roots was higher than that of stem and leaf. Rotala rotundifolia is an aquatic plant native to Taiwan. The studies pointed out that Rotala rotundifolia had a high translocation ability for arsenic, and the arsenic concentration in the stem and leaves reached 7822 mg/kg (dry weight). The lack of a translocation barrier in the root leads to a high accumulation concentration of arsenic in the stem and leaves [29].
Phytoremediation is the friendliest remediation technology for heavy metal contamination in farmland, but it is limited by the long time requirement. Even if hyperaccumulator plants are used, it often takes decades, hundreds of years, or even thousands of years to reduce the concentration of heavy metals in the soil through plant extraction to meet regulatory standards [30,31], making it difficult to promote phytoremediation technology. Therefore, phytoremediation must be sustainable and feasible only when the crops planted have a stable economic base. Meers et al. [32] put forward the concept of “Phytoattenuation”, arguing that the ability of plants to extract heavy metals must not be the main screening condition for the development of phytoattenuation. Matching with proper agronomic management, by planting crops to produce hygienic, safe, and economical products while the effect of soil pollution reduction is gradually achieved, makes phytoremediation sustainable and applicable. Mi et al. planted Pteris vittata L. and maize in soil with an arsenic content of 103.8 mg/kg. The accumulative arsenic concentration of Pteris vittata L. was 169.33–204.26 mg/kg, and that of maize stalk was 41.60–55.48 mg/kg [33,34]. Although the cumulative concentration of arsenic in maize is low, it has a high biomass yield with economic benefits. Cheng et al. planted maize on lead-contaminated farmland, producing 93.37 Mg/ha per year (in dry mass). If it is used as bioenergy, it produces 1545 GJ per hectare per year, which is equivalent to the combustion energy of 74 Mg of raw coal. An additional, about 25 Mg of feed corn could be produced per hectare per year [35].
An arsenic-contaminated farmland in Taiwan was selected for this study, growing the arsenic super-accumulator plant Pteris vittata L., the aquatic plant Rotala rotundifolia, and the food crop maize. The arsenic removal amount and economic benefits of three crops were compared, and the most feasible phytoremediation strategy for arsenic-contaminated farmland was determined.

2. Materials and Methods

2.1. Experiment Site

The farmland for the field experiment was located at No. 533, Ximenkou Section, Changhua City, Taiwan, the coordinates are X:201682, Y:2661575. Soil arsenic was accumulated from the long-term use of arsenic-rich groundwater for irrigation. Arsenic concentration decreased with the distance from the water inlet. The arsenic concentration in the topsoil was measured with a hand-held XRF on a 1 × 1 m grid before the experiment. For planting experiments, three soil arsenic concentration ranges were selected: a high-concentration area of more than 100 mg/kg, a medium-concentration area of 60–80 mg/kg, and a low-concentration area of below 40 mg/kg. After removing the weeds in each block, a rotary yak was used for stirring to make the soil arsenic concentration evenly distributed.

2.2. Soil Properties

Trial crops included Pteris vittata L., Zea mays L., and Rotala rotundifolia. Each plant was planted in three blocks in three concentration ranges, and the area of each block was 2 × 2 m. Each block was divided into four grids, and the soil in the center of the four grids was taken and mixed into a soil sample. The soil properties were analyzed, including soil texture, pH (the weight ratio of soil and reagent water was 1:1 mixed and stirred, and the supernatant liquid was taken for pH electrode analysis after standing still), organic matter content (Walkley–Black wet oxidation method and potassium dichromate titration method), soil conductivity (saturated soil paste extraction method), and total arsenic (decomposed organic acids with hydrogen peroxide, extracted with 9.6 M hydrochloric acid, and then reduced to arsine). Bioavailable arsenic concentration was analyzed using a 0.1 N hydrochloric acid extraction method and the sequential extraction method described by Wenzel et al. (2001). The dissolved fraction (F1) and the exchangeable fraction (F2) of Wenzel sequential extraction methods were extracted with 0.05 M (NH4)H2PO4 and 0.05 M NH4H2PO4, respectively [36]. The extracts were analyzed quantitatively by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Thermo CAP 6300, ThermoFisher, Waltham, MA, USA).

2.3. Planting Experiment

Pteris vittata L. were bred using spores. Then, seedlings with a height of about 10 cm were transplanted with a row spacing of 40 × 30 cm. Five plants were planted in each row and each experimental block had 4 rows (Figure 1a). Plants were collected after 9 months, in winter, when the plants began to wither, and the quality of each plant was measured. Three samples were randomly taken from each block and were separated into the aboveground tissues and the underground tissues, of which the arsenic content of the plants was measured. For maize, Tainan No. 24 hard corn was selected for the planting test. The seeds were germinated to about 3 cm before transplanting, being planted with a row spacing of 30 × 20 cm, 6 rows in each block, 8 plants in each row were planted (Figure 1b). The height and biomass of corn were measured when it was mature; about 120 days after planting. Three samples were randomly taken from each block, and the biomass and arsenic content of each part of the plant were measured by separating them into roots, stems, leaves, bract leaves, cobs, and kernels. Rotala rotundifolia was planted by cuttings. The stems of the plants were 5–8 cm long and with 3–4 stems in each clump, of which the row spacing is 25 cm × 20 cm. Rotala rotundifolia is an amphibious plant. In the planting period, the soil surface was kept flooded with a water depth of greater than 5 cm (Figure 1c). Three samples were randomly taken from each block 3 and 6 months after planting and were separated into aboveground and underground tissues to measure the arsenic content. Plants were harvested to measure the biomass after 6 months. The rhizosphere soil was taken from the three plants when they were harvested to analyze the bioavailable arsenic content.

3. Results

3.1. Soil Properties and Arsenic Concentration

The results of the soil characteristics analysis in the field test are shown in Table 1. The soil texture was sandy loam, with pH ranging from 6.58 to 6.81. The conductivity was 162 to 269 μS/cm, and the organic matter content was 3.06 to 3.22%. The total arsenic concentration of the soil in the high-concentration area (H) was between 102.2 and 108.7 mg/kg, with an average of 105.5 ± 3.3 mg/kg, In the medium-concentration area (M), the concentration was between 61.1 and 79.3 mg/kg, with an average concentration of 68.9 ± 9.4 mg/kg. The concentration in the low-concentration area (L) was between 28.8 to 38.7 mg/kg, and the average concentration was 34.1 ± 5.0 mg/kg. As for the concentration of bioavailable arsenic in the soil, which was extracted with 0.1 M HCl, it averaged 0.64 mg/kg in low-concentration soil; 0.72 mg/kg in medium-concentration soil; and 1.61 mg/kg in high-concentration soil. F1 and F2 of the Wenzel sequence extraction method represented the bioavailability. In the low-concentration soil the arsenic concentration was 5.4 mg/kg; in the medium-concentration soil it was 11.1 mg/kg; and in the high-concentration soil it was 21.0 mg/kg.

3.2. Pteris vittata L.

After breeding Pteris vittata L. with spores, it was transplanted to the field when it grew to about 10 cm. After 9 months of growth, winter had arrived and the plants began to wither; the plants were harvested and separated into the aerial tissues and the underground tissues (rhizome and adventitious roots). After air-drying at 70 °C, the plant biomass was measured (Table 2). The average weight of each plant in the high-concentration area was 108.4 ± 31.7 g; it was 135.1 ± 28.3 g in the medium-concentration area and 156.0 ± 45.7 g in the low-concentration area. Although there were no statistically significant differences (Fisher’s protected least significant difference test, LSD test, at p < 0.05), the average biomass decreased with increasing soil arsenic concentrations.
The arsenic content of the plants was measured at 3 and 9 months after planting and the results are shown in Figure 2. When Pteris vittata L. had grown for three months, the average arsenic concentrations in the underground tissues and shoots that were in the low-concentration area were 8.3 and 54.9 mg/kg; they were 54.6 and 101.5 mg/kg in the medium-concentration area and 62.2 and 164.4 mg/kg in the high-concentration area. After 9 months of growth, the average arsenic concentration in the underground tissues of Pteris vittata L. in the low-concentration area increased to 58.16 mg/kg, and the average arsenic concentration in the shoots was 47.5 mg/kg. The average concentrations of arsenic in the underground tissues and shoots in the medium-concentration area were 117.4 and 104.2 mg/kg, and they were 169.0 and 233.5 mg/kg in the high-concentration area. The analysis results showed that the average arsenic concentration in the underground tissues and shoots of Pteris vittata L. increased with increasing soil arsenic content. After 3 months, the arsenic content in the underground tissues of Pteris vittata L. continued to accumulate, but there were no significant differences in the accumulation of arsenic concentration in the shoots (at p < 0.05 by LSD test). Harvesting of aboveground tissues every 3 months after planting is therefore recommended.

3.3. Zea mays L.

For corn, in various soil arsenic concentration test areas, the plant height and growth (total biomass) were measured when the plants were mature. The statistical results are shown in Table 3. The average height of plants in the high-concentration area was 224.1 ± 21.1 cm; in the medium-concentration area was 218.0 ± 29.4 cm; and in the low-concentration area was 217.0 ± 39.1 cm. As for the average growth of each plant, it was 530.8 ± 43.7 g in the high-concentration area; 510.7 ± 88.5 g in the medium-concentration area; and 538.7 ± 108.1 g in the low-concentration area. The proportion of biomass (dry basis) of each part of the maize plants was similar in different experimental areas. On average, roots accounted for 11.3%; stems accounted for 25%; leaves accounted for 11.2%; husks accounted for 11.9%; the corn cobs accounted for 11.6%; and corn kernels accounted for 28.4%. The average yield of corn kernels per plant in the high-concentration area was 143.4 ± 46.3 g; it was 127.4 ± 51.2 g in the medium-concentration area and 178.8 ± 57.0 g in the low-concentration area. The results of the LSD test (at p < 0.05) showed that within the range of soil arsenic concentration of 28.8–108.7 mg/kg (in this study), there were no significant differences in corn plant height, growth, and corn kernel yield among the experimental groups, indicating that the soil arsenic concentration did not inhibit the growth of maize within this range. The analysis results of arsenic concentration in various parts of the maize from various concentration areas are shown in Figure 3. The concentration distribution of arsenic in maize plants was highest in roots, with an average of 3.330–4.673 mg/kg, followed by leaves with an average of 1.252–1.519 mg/kg; stems and bracts had similar concentrations, with an average of 0.307–0.47 mg/kg and corn kernel concentration was the lowest, where the average dry basis arsenic concentration was between 0.083 and 0.162 mg/kg. The fresh weight concentration should be less than 1.0 mg/kg, which is in line with the human consumption limit standard stipulated in Taiwan. The average arsenic content in roots, stems, and leaves increased with the higher soil arsenic concentration. However, there was no consistent trend in bract leaves, corn cobs, and corn kernels.

3.4. Rotala rotundifolia

Rotala rotundifolia were harvested 6 months after planting. The dry biomass yield per m2 was 138.8 mg in the low-concentration area; 175.6 mg in the medium-concentration area; and 188.9 mg in the high-concentration area (Table 4). The higher the soil arsenic concentration was the more the biomass increased. The biomass of Rotala rotundifolia was mainly distributed in the shoots, accounting for about 97.2%.
Figure 4 shows the arsenic concentration on a dry basis of Rotala rotundifolia 3 months and 6 months after planting. After 3 months of growth, the average arsenic concentration in the root of Rotala rotundifolia in the low-concentration group was 15.93 mg/kg; in the medium-concentration group it was 3.59 mg/kg and in the high-concentration group it was 7.26 mg/kg. After 6 months of growth, the concentration of arsenic in the roots of each experimental group increased significantly. The average arsenic concentration in the roots of the low-concentration group was the highest at 293.42 mg/kg. As for the medium-concentration group, it was 66.15 mg/kg, and for the high-concentration group it was 42.52 mg/kg. The results showed that the root arsenic concentration decreased with the increase in soil arsenic concentration. The average arsenic concentration in the stem and leaves 3 months after planting was 12.84 mg/kg in the low-concentration group, 5.60 mg/kg in the medium-concentration group, and 1.73 mg/kg in the high-concentration group. The average arsenic concentration in stems and leaves six months after planting was 57.32 mg/kg in the low-concentration group, 5.63 mg/kg in the medium-concentration group, and 3.74 mg/kg in the high-concentration group. The arsenic concentration in stems and leaves was the same as that in roots but decreased with the increase in soil arsenic concentration, and only the low-concentration experimental group showed a significant increase in arsenic concentration at six months after planting compared with three months. In addition, the increase of arsenic concentration in the stem and leaves of the medium- and high-concentration experimental groups was limited. The experiment results showed that a high arsenic concentration in the soil will inhibit the absorption of arsenic by Rotala rotundifolia.

3.5. Changes in the Bioavailable Arsenic Concentration in the Soil

We analyzed the changes in bioavailable arsenic concentrations in rhizosphere soils before and after the planting of three plants. The results of the 0.1 N hydrochloric acid extraction method and F1 and F2 of the Wenzel sequential extraction method are shown in Figure 5. Before planting, the average concentration of bioavailable arsenic in the low-concentration soil was 0.64 mg/kg. After planting, the bioavailable arsenic concentration in the rhizosphere soil of Pteris vittata L. was reduced by about 53%; that of maize was reduced by 52% and a 33% reduction was observed by planting Rotala rotundifolia. The average concentration of bioavailable arsenic in medium-concentration soil before planting was 0.72 mg/kg. The concentration decreased by 51% after planting Pteris vittata L., 26% after planting corn, and 31% after planting Rotala rotundifolia. The average concentration of bioavailable arsenic in high-concentration soil before planting was 1.53 mg/kg. The average concentration decreased by 38% after planting Pteris vittata L.; 29% after planting corn; and 25% after planting Rotala rotundifolia. The bioavailable arsenic concentration decreased the most after the planting of Pteris vittata L., and the higher the soil arsenic concentration, the lower the reduction rate.
The analysis results of the Wenzel sequence extraction method [24] are shown in Figure 5b. The total concentration of the soluble fraction and the exchangeable fraction before planting was 5.4 mg/kg in low-concentration soil, 11.1 mg/kg in medium-concentration soil, and 21.0 mg/kg in high-concentration soil. After planting, the bioavailable arsenic concentration was reduced, and the planting of Pteris vittata L. caused the highest reduction, reducing by 60–66%. The bioavailable arsenic showed a 23% to 57% reduction after planting corn and a 10% to 28% decrease after planting Rotala rotundifolia, which was the least of the three.

4. Discussion

The average biomass of the underground tissues of Pteris vittata L. accounted for 13% while shoots accounted for 87% of total biomass. The raw mass of the underground tissues and shoots of Pteris vittata L. was multiplied by the arsenic content (Table 2). On average, in the low-concentration area, 7.633 mg of arsenic was absorbed by each plant of Pteris vittata L.; in the medium-concentration area, 14.271 mg was absorbed and in the high-concentration area, 24.201 mg was absorbed. If the planting density is 40 × 30 cm, 63.6–201.68 mg of arsenic was removed per square meter over 9 months. About 13% of the plants were distributed in the underground tissues and about 87% were distributed in the shoots. If only the shoots are harvested, about 53.72–177.46 mg/m2 of arsenic could be removed.
The weight of arsenic absorbed by each plant of maize was calculated by multiplying the average biomass of each part of the maize by the arsenic concentration. The arsenic content was removed by planting corn based on a planting density with a row spacing of 30 × 20 cm shown in Table 3. In the high-concentration area, 8.163 mg/m2 of arsenic could be removed; in the medium-concentration area, 7.086 mg/m2 was removed, and in the low-concentration area, 6.052 mg/m2 was removed. The average corn planting period is about 120 days per round, and three rounds are planted per year. In one year, about 25.5 mg/m2 of arsenic could be removed in a high-concentration area, 21.56 mg/m2 could be removed in a medium-concentration area, and 18.2 mg/m2 of arsenic could be removed in a low-concentration area.
The amounts of arsenic that could be removed by planting Rotala rotundifolia were shown in Table 4. After 6 months of planting, the amount of arsenic removed was between 0.982 mg/m2 and 8.877 mg/m2, and the highest amount of arsenic removed was in the low-concentration test group. On average, about 80% of the arsenic was contained in the plant shoots of Rotala rotundifolia. If the aboveground tissues were harvested once every 6 months after planting, the amount of arsenic that could be removed each time would be between 0.680 and 7.732 mg/m2. Assuming that the stems and leaves that grow after the first harvest will have the same arsenic absorption capacity, harvesting twice a year could remove up to 1.36–15.5 mg/m2 of arsenic per year.
The arsenic concentration in the high-concentration soil was about 105.5 mg/kg, and approximately 200 mg/m2 of arsenic could be removed in 1 year by planting Pteris vittata L. The soil density was calculated as 1300 kg/m3, and the thickness of the remediation soil was calculated as 30 cm of topsoil. It was estimated that it would take at least about 90 years to reduce the soil arsenic concentration from 105.5 mg/kg to below the regulatory standard of 60 mg/kg by planting Pteris vittata L. Pteris vittata L. has no economic value, and it is not feasible to grow it for 90 years without production income.
Planting corn in high-concentration areas could remove about 25.5 mg/m2 of arsenic in one year; although the removal rate of corn is only one-eighth of the fern, the arsenic concentration of corn kernels was less than 0.2 mg/kg (dry basis). This not only meets the animal feed standard, but also meets the human consumption limit standard of 1.0 mg/kg (fresh weight concentration), which has production benefits. In addition, other parts of these plants, such as roots, stems, and leaves, had high biomass and could be used as biomass fuel. Maize is dry-farmed and requires less irrigation water during the growth process, which slows down the accumulation rate of arsenic in the soil.
The annual removal of arsenic from planting Rotala rotundifolia was about 15.5 mg/m2, which was lower than that of corn. Since it is an aquatic plant, a large amount of water is necessary in the planting process. If groundwater is used for irrigation, it may cause a higher concentration of arsenic to accumulate in the soil. Therefore, using Rotala rotundifolia for phytoremediation was not feasible.
Pteris vittata L. can rapidly reduce bioavailable arsenic in the soil. In the early stage of phytoremediation of arsenic-contaminated soils, Pteris vittata L. can be planted for the first 2–3 years to reduce the bioavailable arsenic concentration in the soil. Then, Zea mays L. can be planted. Zea mays L. is dry-farmed, so there is no need to irrigate a lot during growth. A small amount of arsenic can be removed through the high mass of Zea mays L. plants, preventing the continuous accumulation of arsenic in the soil. The concentration of arsenic in kernels can be lower than the limit for human consumption. It can be used as animal feed and human consumption with economic benefits.

5. Conclusions

Arsenic-contaminated farmland was remediated by phytoremediation with the application of the arsenic super-accumulator plant, Pteris vittata L.; however, the time required to reach the regulatory standard of 60 mg/kg is long. If there is no production for a long time, the remediation cannot be continued. Less water is required during the growth of maize, which slows down the accumulation of arsenic in the soil from irrigation with groundwater. Although the concentration of arsenic accumulated in maize plants was low, the amount of arsenic removed was considerable due to the high biomass of the maize plants. The content of arsenic in corn kernels was lower than the limit for human consumption, which allows for economic value. Maize can be used as a remediation crop on arsenic-contaminated farmland. Phytoremediation and production at the same time make the land use sustainable.

Author Contributions

Conceptualization, S.-F.C. and C.-C.C.; methodology, C.-C.C. and C.-Y.H.; experiment analysis, M.-S.L. and P.-C.C.; writing—original draft preparation, C.-C.C.; writing—review and editing, S.-F.C. and C.-Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Field planting of test plants. (a) Pteris vittata L.; (b) Zea mays L.; and (c) Rotala rotundifolia.
Figure 1. Field planting of test plants. (a) Pteris vittata L.; (b) Zea mays L.; and (c) Rotala rotundifolia.
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Figure 2. Arsenic concentrations in Pteris vittata L. 3 M: 3 months after planting; 6 M: 6 months after planting; root: the underground tissues; and shoot: the aerial tissues.
Figure 2. Arsenic concentrations in Pteris vittata L. 3 M: 3 months after planting; 6 M: 6 months after planting; root: the underground tissues; and shoot: the aerial tissues.
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Figure 3. Arsenic concentrations in various tissues of Zea mays L.
Figure 3. Arsenic concentrations in various tissues of Zea mays L.
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Figure 4. Arsenic concentrations in Rotala rotundifolia. 3 M: 3 months after planting; 6 M: 6 months after planting; root: the underground tissues; and shoot: the aerial tissues.
Figure 4. Arsenic concentrations in Rotala rotundifolia. 3 M: 3 months after planting; 6 M: 6 months after planting; root: the underground tissues; and shoot: the aerial tissues.
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Figure 5. Changes in bioavailable arsenic concentrations in rhizosphere soils before and after planting these three plants.
Figure 5. Changes in bioavailable arsenic concentrations in rhizosphere soils before and after planting these three plants.
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Table 1. Soil properties.
Table 1. Soil properties.
PropertiesUnitLMH
As concentrationmg/kg34.1 ± 5.068.9 ± 9.4105.5 ± 3.3
pH-6.586.616.81
ConductivityμS/cm245162269
Organic matter%3.233.223.06
Bioavailable As concentration0.1 M HClmg/kg0.640.721.61
Wenzel F1 + F2mg/kg5.411.121.0
L: low As-concentration soil; M: medium As-concentration soil; H: high As-concentration soil; and Wenzel F1 + F2: the total As concentration of the dissolved fraction (F1) and the exchangeable fraction (F2) of the Wenzel sequence extraction method.
Table 2. The average biomass and arsenic removal amount of Pteris vittata L. 9 months after planting.
Table 2. The average biomass and arsenic removal amount of Pteris vittata L. 9 months after planting.
Soil TypeAverage Biomass (g Per Plant, Dry Basis)As Removal Amount (mg)
UndergroundAbovegroundTotalPer PlantPer m2
PL20.4 ± 8.6135.6 ± 29.6156.07.6363.61
PM14.7 ± 5.7120.4 ± 34.1135.114.27118.93
PH17.2 ± 4.991.2 ± 29.1108.424.20201.68
PL: low As-concentration soil; PM: medium As-concentration soil; and PH: high As-concentration soil.
Table 3. Average biomass and arsenic removal amount of Zea mays L.
Table 3. Average biomass and arsenic removal amount of Zea mays L.
Soil TypeAverage Biomass (g Per Plant, Dry Basis)As Removal Amount (mg)
RootStemLeafB.LeafCobKernelTotalPer PlantPer m2
ZL60.4121.361.058.059.2178.8538.70.366.05
ZM59.0134.358.467.963.7127.4510.70.437.09
ZH58.7148.657.062.560.7143.4530.80.498.16
ZL: low As-concentration soil; ZM: medium As-concentration soil; and ZH: high As-concentration soil.
Table 4. Average biomass and arsenic removal amount of Rotala rotundifolia.
Table 4. Average biomass and arsenic removal amount of Rotala rotundifolia.
Soil TypeAverage Biomass (g/m2, Dry Basis)As Removal Amount (mg/m2)
RootShootTotalRootShootTotal
GL3.9 ± 0.3134.9 ± 23.0138.81.1447.7328.877
GM3.4 ± 0.4172.2 ± 13.1175.60.2250.9691.194
GH7.1 ± 1.3181.8 ± 39.8188.90.3020.6800.982
GL: low As-concentration soil; GM: medium As-concentration soil; and GH: high As-concentration soil.
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Chen, C.-C.; Lin, M.-S.; Cheng, P.-C.; Huang, C.-Y.; Cheng, S.-F. Strategy of Phytoremediation for Sustainable Use of Arsenic-Rich Farmland. Eng. Proc. 2024, 74, 8. https://doi.org/10.3390/engproc2024074008

AMA Style

Chen C-C, Lin M-S, Cheng P-C, Huang C-Y, Cheng S-F. Strategy of Phytoremediation for Sustainable Use of Arsenic-Rich Farmland. Engineering Proceedings. 2024; 74(1):8. https://doi.org/10.3390/engproc2024074008

Chicago/Turabian Style

Chen, Chang-Chao, Min-Siou Lin, Pei-Cheng Cheng, Chin-Yuan Huang, and Shu-Fen Cheng. 2024. "Strategy of Phytoremediation for Sustainable Use of Arsenic-Rich Farmland" Engineering Proceedings 74, no. 1: 8. https://doi.org/10.3390/engproc2024074008

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