Eco-Friendly and Sustainable Remediation of Copper- and Zinc-Contaminated Farmland †
1
Institute of Biochemical Technology, Chaoyang University of Technology, Taichung 41349, Taiwan
2
Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
3
Department of Environmental Engineering and Management, Chaoyang University of Technology, Taichung 41349, Taiwan
*
Authors to whom correspondence should be addressed.
†
Presented at the 2024 IEEE 6th International Conference on Architecture, Construction, Environment and Hydraulics, Taichung, Taiwan, 6–8 December 2024.
Eng. Proc. 2025, 91(1), 13; https://doi.org/10.3390/engproc2025091013
Published: 21 April 2025
Abstract
:Copper and zinc are metals commonly used in industry. However, improperly disposed copper and zinc pollute soil seriously. In farmland where the concentrations of copper and zinc exceeded regulatory standards and farming has been banned for many years, we measured the copper and zinc concentrations in soil. The copper concentration ranged from 30.2 to 1082.3 mg/kg, while the zinc concentration was between 200.2 and 3335.3 mg/kg. To explore the correlation between the concentration of copper and zinc in soil and plants and plant growth, Pennisetum was chosen as the test crop. The economic and carbon reduction benefits of planting Pennisetum in copper- and zinc-polluted farmland were also investigated. The results indicated that the concentration levels of copper and zinc were not significantly impacted, and neither was the growth of Pennisetum. Farming Pennisetum produces a total of about 1100 tons of biomass per hectare per year. The income per hectare was about USD 48,000 per year. Pennisetum captures 578.8 tons of carbon every year, equivalent to 2124.2 ton-CO2e. When used as fuel, it provides 23,649 GJ of bioenergy. Therefore, Pennisetum is an appropriate plant for the green and sustainable remediation of polluted soil.
1. Introduction
Copper and zinc are commonly used in industry. Electroplating wastewater, sludge, and discarded wires and cables contain high concentrations of copper and zinc which pollute soil when disposed of inappropriately. Taiwan has regulatory standards for copper and zinc in agricultural soil, which are 220 and 600 mg/kg, respectively. When the concentrations of copper and zinc in soil exceed regulatory standards, farming is banned for remediation.
Different crops have different absorption capabilities of copper and zinc in soil. Vegetables have stronger absorption abilities, while food crops have weaker absorption abilities. When the concentrations of copper and zinc in soil are too high, their toxicity prevents plants from growing, which reduces yields. However, copper and zinc are essential nutrients and trace elements for plant growth and the human body [1,2]. In farmland polluted by copper and zinc, the quality and yield of crops are significantly reduced. In soil that is not heavily polluted, crops can clean the soil and increase productivity again through phytoremediation.
Hou et al. planted four herbaceous energy plants, namely switch grass, barley, Arundinoideae, and Pennisetum, in lightly heavy metal-polluted land in the suburbs of Beijing [3]. The biomass yield of Pennisetum was the highest at 59.22 t/hm2, and the accumulation rates of arsenic, mercury, copper, lead, and cadmium were 23.12, 0.35, 1132.62, 95.18, and 6.07 g/hm2, respectively. Pennisetum showed a high absorption capacity for copper. Pennisetum is a perennial giant grass that produces a biomass yield of up to more than 200 t FW/hm2 [4].
Zou et al. studied the accumulation of lead, cadmium, copper, and zinc in Pennisetum [5]. The accumulated contents of the four metals in the leaves were 29.51, 19.60, 182.8, and 178.2 μg/plant, respectively; in the stems, 8.89, 17.99, 61.68, and 205.4 μg/plant; and in the roots, 8.45, 2.18, 36.35, and 47.94 μg/plant. It was also shown that Pennisetum accumulated copper and zinc the most. Ko conducted pot experiments using native Taiwanese plants with energy potential, including Pennisetum, miscanthus, rapeseed, and water barnyard grass, in soil contaminated with copper, nickel, and chromium [6]. Pennisetum maintained the largest biomass and removed more heavy metals than other plants. It removed 11.99–27.30 mg of nickel, 26.69–53.82 mg of chromium, and 27.34–92.52 mg of copper per square meter. Pennisetum also extracted copper and zinc, but they were adsorbed on the root surface. The more extensive the root system, the more copper and zinc are retained, and the less they are transmitted to the shoots [7].
Pennisetum belongs to the Poaceae family and the genus Pennisetum and is a perennial grass plant. Its scientific name is Pennisetum alopecuroides L. Pennisetum is propagated by cuttings. It has strong regeneration ability, wide adaptability, and adaptability to various soil types [1,8,9]. Loose, fertile, neutral sandy loam with good drainage performance is the best soil for Pennisetum to grow in. There are few pests and diseases which affect its growth. It grows quickly and has a high yield. It can be harvested every 8 to 10 weeks. Its green leaves are rich in chlorophyll, β-carotene, multivitamins, phenolic compounds, and minerals, and they are the main feed for herbivores. Pennisetum is also used as an ingredient and material for field coverings, papermaking, and bio-alcohol production [10]. Pennisetum’s photosynthesis pathway is different from that of C3 plants as it is a C4 plant. After carbon dioxide is fixed, Pennisetum produces organic acids that absorb more carbon dioxide, generate more energy, and have a higher carbon fixation capacity than C4 plants. Therefore, C4 plants including Pennisetum play an important role in climate control [9].
This study was conducted on farmland in Tainan, Taiwan, where copper and zinc exceeded regulatory standards, and farming has been prohibited for many years. Using Pennisetum as a test crop, the correlation between soil and plant copper and zinc concentrations and plant growth was explored, and the economic and carbon reduction benefits of planting Pennisetum in copper- and zinc-contaminated soil were investigated.
2. Materials and Methods
The farmland was located at Land No. 732 and 733, Xingnong Section, Tainan City. It was contaminated by waste disposal, and the major pollutants were copper and zinc. The copper concentration was in the range of 30.2–1082.3 mg/kg, while that of zinc was 200.2–3335.3 mg/kg. Pennisetum Taiwan Livestock No. 2 was planted for experiments. This variety has the characteristics of high plant height, up to more than 200 cm; high yield; high sweetness; and excellent palatability. It is mechanically harvested. It is resistant to pests and diseases and does not require pesticides. Pennisetum was planted with a root segment of about 30 cm in this study. The planting row and spacing were 40 and 30 cm. In blocks A, B, C, and D, we planted 60 Pennisetum plants in each block (Figure 1).
When planting Pennisetum, the soil samples around each plant hole were collected to analyze the concentrations of copper and zinc. A total of 40 samples were taken from the farmland. Along with the soil concentration, the concentrations in plant roots, stems, and leaf mass were also analyzed.
The soil samples were treated using the aqua regia digestion method to extract and measure the concentrations of copper and zinc. An inductively coupled plasma spectrometer (ICP-OES) was used for the analysis. Soil properties, including soil texture, pH, conductivity, organic matter, and fertility, were also analyzed. The soil size was measured using the Baumann hydrometer method, and the texture was determined according to the soil texture triangle diagram of the USDA soil taxonomy.
Soil pH and conductivity were measured using electronic testers with electrodes. After mixing the soil and deionized water at a ratio of 1:5 (w/v), a pH meter and a conductivity meter were used to measure soil pH and conductivity. Soil organic matter was analyzed using the Walkey–Black wet oxidation method. In this method, a hot mixture of potassium dichromate (K2Cr2O7) and concentrated sulfuric acid (H2SO4) is used to oxidize the organic matter in the soil. After measuring organic carbon concentration, it is converted into soil organic matter content. Soil fertility was measured using Menlike No. 3 extract by extracting available phosphorus, potassium, calcium, magnesium, iron, manganese, copper, and zinc and measuring their concentrations using an ICP-OES.
Statistical analysis was conducted using Statistical Product and Service Solutions (SPSS) software (IBM SPSS Statistics 22.0). The correlation coefficient (r) was used to examine the linear correlation between variables. A Pearson correlation coefficient was calculated using an equation of the common variation in two variables (X and Y) divided by their standard deviation. When the r value is close to 0, the correlation is weak, while an r close to 1 indicates a significant correlation.
3. Results and Discussion
3.1. Analysis Results
The farmland soil showed a heterogeneous distribution of soil composition due to the disposal of unknown waste in the past. The results of soil property analysis were shown in Table 1. Block A soil samples showed a higher sand content; Block B soil samples had a higher clay content as they consisted of a sandy clay loam; Block C and D soil samples were sandy loam. The pH values ranged from 7.32 to 7.91. Conductivity ranged from 238 to 749 μS/cm. The organic matter content was between 6.18 and 13.9%. The phosphorus content was in the range of 52.1–125.4 mg/kg; potassium was in the range of 86.8–205.1 mg/kg; calcium was in the range of 1220.0–1789.7 mg/kg; magnesium was in the range of 152.8–248.4 mg/kg; iron was in the range of 87.9–346.4 mg/kg; and manganese was in the range of 34.8–73.9 mg/kg. The contents of the fertility components generally were in an appropriate range for farming.
3.2. Correlation Analysis of Copper and Zinc Concentration in Soil and Plant Growth
The copper concentration in the soil samples ranged from 30.2 to 1082.3 mg/kg. The lowest zinc concentration was 200.2 mg/kg, while the highest was 3335.3 mg/kg. The tallest Pennisetum plant measured 287.0 cm, while the shortest stood at 140.0 cm. The copper concentrations in the rhizosphere soil were in the range of 211.5 and 568.2 mg/kg; the zinc concentrations were in the range of 1055.2 and 1872.4 mg/kg. The highest biomass of Pennisetum roots was 1744.0 g, and its soil copper and zinc concentrations were 172.4 and 889.4 mg/kg. The highest biomass yield in the shoots of Pennisetum was from the same plant with the highest root yield, which was 18,500 g. The distributions of soil copper and zinc concentrations in the plant rhizosphere, plant height, and growth mass are shown in Figure 2.
The Pennisetum plant height did not vary with soil copper and zinc concentrations. The correlation coefficient between the plant height and soil copper concentration was 0.061, indicating no significant correlation (Table 2). The correlation coefficient between plant height and soil zinc concentration was 0.116. The correlations between the biomass of the plant roots and stems and leaves and soil copper concentration were −0.056 and −0.072, respectively. The correlations between the biomass of the root stems and leaves and soil zinc concentration were −0.072 and −0.038. Pennisetum showed no significant correlation between soil copper and zinc concentrations and plant growth. Chen et al. planted Pennisetum in moderately cadmium-contaminated soil and found no remarkable plant growth inhibition at any stage [11].
3.3. Correlation Analysis of Copper and Zinc Concentrations in Soil and Parts of Pennisetum
The relationship between Cu and Zn concentrations in various parts of Pennisetum and soil Cu and Zn concentrations was explored (Figure 3). The highest Cu concentration in the roots was 346.1 mg/kg, while the lowest was 5.73 mg/kg. The corresponding soil copper concentrations were 115.9 and 30.2 mg/kg. The highest stem copper concentration was 2.43 mg/kg, the lowest was 0.82 mg/kg, and the corresponding soil copper concentrations were 568.2 and 526.6 mg/kg, respectively. The highest leaf copper concentration was 1.48 mg/kg, the lowest was 0.48 mg/kg, and the corresponding soil copper concentrations were 275.9 and 185.0 mg/kg, respectively. The highest zinc concentration in the roots was 312.0 mg/kg, the lowest was 33.3 mg/kg, and the corresponding soil zinc concentrations were 3058.1 mg/kg and 200.2 mg/kg, respectively. The highest zinc content in the stems was 34.3 mg/kg; the lowest was 12.0 mg/kg. The soil zinc contents were 1872.4 mg/kg and 309.3 mg/kg, respectively. The highest zinc content in the leaves was 11.2 mg/kg; the lowest was 3.58 mg/kg. The soil zinc contents were 1450.0 mg/kg and 766.0 mg/kg, respectively.
The copper concentration in the stems and leaves of Pennisetum plants varied by approximately threefold, while the maximum difference in the roots was sixtyfold. Similarly, the maximum zinc concentrations in the stems and leaves varied threefold, while the maximum concentrations in the roots varied tenfold. The large difference in the concentration in the roots indicated that the roots were not easily cleaned and did not retain soil. The correlation coefficient between the zinc concentrations in the roots of Pennisetum and in the soil was 0.333. The correlation coefficients of copper concentration in the stems and leaves and soil were 0.262 and 0.078; the correlation coefficients of zinc concentration in the stems and leaves and soil were 0.079 and 0.204. The soil copper and zinc concentrations did not correlate significantly with the plant parts.
3.4. Benefits of Pennisetum on Farmland
Copper and zinc are essential nutrients for animal growth. There are different standards for feeds for animals. In the National standard CNS 3027 of Taiwan, copper content for formulated feeds for livestock and poultry must be 35–150 ppm [12]. For zinc, the recommended content is 100–140 ppm [12]. According to the limit standards for homemade feed, the copper content must range from 25 to 60 ppm, while the zinc content must range from 100 to 130 ppm. The edible parts of Pennisetum are the stems and leaves. The highest zinc concentrations in this study were 34.3 mg/kg in the stems and 11.2 mg/kg in the leaves. The highest copper concentrations were 2.43 mg/kg in the stems and 1.48 mg/kg in the leaves. The copper and zinc concentrations in the stems and leaves of Pennisetum met the feed limit standards as shown in Table 3.
Pennisetum can be used for the phytoremediation of copper- and zinc-polluted soil as it can provide about 1100 tons of shoots per hectare per year. The feed price is USD 0.04 per kilogram, and the annual income can be up to USD 0.4 million. The average carbon contents of the stems and leaves of Pennisetum are 40.53 and 42.89%. Combustion heat values are 16.31 and 17.25 KJ/g. By planting Pennisetum, 578.8 tons of carbon per hectare per year can be fixed, which is equivalent to 2124.2 tons of carbon dioxide. When used as fuel, Pennisetum provides 23,649 GJ as a biomass energy source.
4. Conclusions
Pennisetum activates farmland and provides production benefits in farmland polluted by copper and zinc. It can be used for the phytoremediation of polluted soil and the reduction in the cost of physical and chemical remediation and carbon emissions. In the growth process, Pennisetum absorbs carbon dioxide, and the harvested plants can be used as bioenergy. It captures carbon and is used as a green bioenergy source. Pennisetum is appropriate to be used for an eco-friendly and sustainable remediation strategy.
Author Contributions
Conceptualization, S.-F.C. and C.-C.C.; methodology, C.-C.C. and C.-Y.H.; experimental 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 this 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.
Planting Pennisetum and its growth.
Figure 2.
Correlation between soil copper and zinc concentrations and Pennisetum plant growth.
Figure 3.
Correlation between soil copper and zinc concentrations and copper and zinc concentrations in various parts of Pennisetum.
Figure 3.
Correlation between soil copper and zinc concentrations and copper and zinc concentrations in various parts of Pennisetum.
Table 1.
Soil properties of samples in farmland.
| Block | Particle Size Distribution (%) | Soil Texture | pH | Conductivity (μS/cm) | Organic Matter Content (%) | ||
|---|---|---|---|---|---|---|---|
| Sand | Silt | Clay | |||||
| A | 96 | 2 | 2 | sandy soil | 7.84 | 366.0 | 10.81 |
| B | 64 | 15 | 21 | sandy clay loam | 7.32 | 238.0 | 6.18 |
| C | 80 | 9 | 11 | sandy loam | 7.51 | 749.3 | 9.36 |
| D | 64 | 17 | 19 | sandy loam | 7.91 | 396.0 | 13.90 |
| Block | Soil Fertility Elements (mg/kg) | ||||||
| Phosphorus | Potassium | Calcium | Magnesium | Iron | Manganese | ||
| A | 125.4 | 101.3 | 1351.0 | 248.4 | 182.4 | 73.9 | |
| B | 62.4 | 88.7 | 1220.0 | 152.8 | 346.4 | 40.3 | |
| C | 52.1 | 86.8 | 1250.3 | 240.3 | 146.4 | 70.6 | |
| D | 72.5 | 205.1 | 1789.7 | 224.9 | 87.9 | 34.8 | |
Table 2.
Correlation coefficients between soil Cu and Zn concentrations and the growth of Pennisetum.
Table 2.
Correlation coefficients between soil Cu and Zn concentrations and the growth of Pennisetum.
| Heavy Metal | Pennisetum | |||||
|---|---|---|---|---|---|---|
| Height | Biomass | Cu and Zn Concentration | ||||
| Root | Shoot | Root | Stem | Leaf | ||
| Cu | 0.061 | −0.056 | −0.072 | −0.021 | 0.262 | 0.078 |
| Zn | 0.116 | −0.013 | −0.038 | 0.333 | 0.076 | 0.204 |
Table 3.
Maximum concentration of copper and zinc in edible parts of Pennisetum and feed limit standards.
Table 3.
Maximum concentration of copper and zinc in edible parts of Pennisetum and feed limit standards.
| Feed Limit Standards | Copper (ppm) | Zinc (ppm) | |
|---|---|---|---|
| CNS 3027 (Formulated feeds, for livestock and poultry) | 35–150 | 100–140 | |
| Limit standards for self-made feed (ROC) | 25–60 | 100–130 | |
| Pennisetum | Stem | 2.43 | 34.3 |
| Leaf | 1.48 | 11.2 | |
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MDPI and ACS Style
Chen, C.-C.; Cheng, P.-C.; Huang, C.-Y.; Lin, M.-S.; Cheng, S.-F. Eco-Friendly and Sustainable Remediation of Copper- and Zinc-Contaminated Farmland. Eng. Proc. 2025, 91, 13. https://doi.org/10.3390/engproc2025091013
AMA Style
Chen C-C, Cheng P-C, Huang C-Y, Lin M-S, Cheng S-F. Eco-Friendly and Sustainable Remediation of Copper- and Zinc-Contaminated Farmland. Engineering Proceedings. 2025; 91(1):13. https://doi.org/10.3390/engproc2025091013
Chicago/Turabian StyleChen, Chang-Chao, Pei-Cheng Cheng, Chin-Yuan Huang, Min-Siou Lin, and Shu-Fen Cheng. 2025. "Eco-Friendly and Sustainable Remediation of Copper- and Zinc-Contaminated Farmland" Engineering Proceedings 91, no. 1: 13. https://doi.org/10.3390/engproc2025091013
APA StyleChen, C.-C., Cheng, P.-C., Huang, C.-Y., Lin, M.-S., & Cheng, S.-F. (2025). Eco-Friendly and Sustainable Remediation of Copper- and Zinc-Contaminated Farmland. Engineering Proceedings, 91(1), 13. https://doi.org/10.3390/engproc2025091013
