Effect of Biochar and Activated Carbon from Organic Waste on the Immobilization of Heavy Metals (Pb, Zn, Cd) and Corn Plant Growth in Contaminated Soil

Introduction Soil contamination with heavy metals significantly threatens both environmental and human health. Anthropogenic activities, including chemical fertilizers and pesticides, industrial processes, wastewater disposal, and mining, contribute to the accumulation of heavy metals in soil. Plants can then taken up these contaminants 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, biochar, and activated carbons were added to soil contaminated with lead, zinc and cadmium at four application rates (0, 2.5, 5, and 10% by weight) in triplicate, 4.5 Kg Pot-1. 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 the control.   Results and Discussion 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 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-1 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-1 dry weight, respectively, while in the roots, they were 206, 1095, and 20 mg kg-1 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 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.