Document Type : Research Article
Authors
1 student phd
2 Professor, Dept. of Soil Science, University of Zanjan,
Abstract
Introduction:
Soil contamination with heavy metals poses a significant threat to both environmental and human health. Anthropogenic activities, including the use of chemical fertilizers and pesticides, industrial processes, wastewater disposal, and mining, contribute to the accumulation of heavy metals in soil. These contaminants can then be taken up by plants and enter the food chain, causing various health problems. Soil amendments such as biochar and activated carbon offer a promising strategy for reducing the mobility and bioavailability of heavy metals in soil. This study investigated the effectiveness of biochar and activated carbon derived from organic waste materials (wheat straw, walnut shells, and almond shells) in immobilizing lead (Pb), zinc (Zn), and cadmium (Cd) and promoting corn (Zea mays L.) growth in a greenhouse setting using contaminated soil.
Materials and Methods:
Three types of organic waste – wheat straw, walnut shells, and almond shells – were pyrolyzed at two temperatures (300 °C and 500 °C) under oxygen-free conditions for two hours to produce six types of biochar. The resulting biochars were then activated with phosphoric acid at their respective production temperatures, yielding six types of activated carbon. These organic waste materials, biochars, and activated carbons were added to a soil contaminated with lead, zinc, and cadmium at four application rates (0, 2.5, 5, and 10% by weight) in triplicate, 4.5 kg pots. The pots were incubated for one month under controlled temperature and humidity to achieve a relative equilibrium. Following incubation, the concentration of available heavy metals in the treated and control soils was measured. Corn was then planted in the pots, and at the end of the growth period, plant growth parameters (dry weight of shoots and roots) and heavy metal concentrations in plant tissues were determined. The data were analyzed using a completely randomized factorial design, and treatment means were compared to each other and to the control.
Results:
Increasing pyrolysis temperature resulted in increased biochar pH, electrical conductivity (EC), and ash content, while the percentage of organic carbon, C/N ratio, and cation exchange capacity (CEC) decreased. Activation with phosphoric acid lowered the pH, ash content, EC, and organic carbon content of the biochars, while increasing their CEC. Amending the soil with biochar significantly increased soil pH and EC, whereas activated carbon amendments decreased these parameters. All amendments (organic waste, biochar, and activated carbon) significantly reduced the concentration of available heavy metals in the soil. Activated carbon had the greatest effect on immobilization, while organic waste had the least. The highest dry weight of corn shoots and roots was observed in treatments containing activated carbon produced at 500 °C and applied at a rate of 5%. Conversely, the lowest concentration of heavy metals in corn tissues was observed in treatments with activated carbon produced at 500 °C and applied at a rate of 10%.
Conclusion:
This study demonstrates that activated carbon derived from organic waste materials can be an effective and sustainable method for remediating soil contaminated with heavy metals and promoting plant growth. However, the presence of detectable heavy metals in corn tissues following activated carbon application suggests that this approach may be best suited for soils with low to moderate contamination levels.
Heavy metals are persistent soil pollutants that pose significant risks to environmental and human health. The application of soil amendments such as biochar and activated carbon has been proposed as an effective strategy for reducing the mobility of heavy metals in soil. This study examines the impact of various organic wastes (wheat straw, walnut shells, and almond shells), along with the biochar and activated carbon derived from these wastes, on immobilizing heavy metals (lead, zinc, and cadmium) and promoting corn plant growth in contaminated soil under greenhouse conditions. Biochars were produced at two pyrolysis temperatures, 300°C and 500°C, and subsequently activated with phosphoric acid and the experimental treatments were added to a contaminated soil at four levels (0, 2.5, 5 and 10% by weight) and in three replicates The results showed that organic wastes, biochars, and activated carbons significantly reduced the concentration of available heavy metals in the soil at a probability level of 5%. Activated carbons had the most effect and organic waste had the least effect. The lowest concentrations of lead, cadmium, and zinc extractable with DTPA were observed with the 500°C activated carbon derived from wheat straw at a 10% application rate, with values of 1.6, 4.5, and 464 mg/kg soil, respectively, representing reductions of 99.46%, 83.67%, and 63.96% compared to the control treatment. This treatment also resulted in the lowest heavy metal concentrations in both the aerial parts and roots of the corn plants. Specifically, the lowest concentrations of lead, zinc, and cadmium in the aerial parts were 71.67, 490.67 and 1.67 mg/kg dry weight, respectively, and in the roots, they were 206, 1095 and 20 mg/kg dry weight, respectively. The highest dry weights of the aerial parts and roots were also observed with this treatment and a 5% application rate, with values of 5.76 and 1.84 grams per pot, respectively. The findings of this study suggest that activated carbon is an effective and sustainable method for remediating soils contaminated with heavy metals and enhancing plant growth.
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