The effect of biochar-modified layered double hydroxide (LDH) on uptake of heavy metals from landfill leachate and industrial wastewater by basil plant

Introduction

The increasing use of effluents for irrigation, particularly in regions facing water scarcity, poses a substantial threat to agricultural sustainability due to the high level of heavy metals. These metals can accumulate in soil and plants, posing risks to food safety and human health. To address this issue, researchers have explored various remediation techniques, including the use of layered double hydroxides (LDHs). The LDHs can potentially capture the heavy metals through multiple mechanisms. These include ion exchange, where positively charged layers attract and bind negatively charged metal ions. Surface complexation involves the formation of complexes between metal ions and hydroxyl groups on the LDH surface. The intercalation allows larger metal ions to enter the interlayer space and interact with anions. In addition, the precipitation of metal hydroxides within the interlayer space can further immobilize heavy metals. These mechanisms, acting synergistically, contribute to the remarkable adsorption capabilities of LDHs for heavy metal removal. However, the efficiency of LDHs can be further enhanced by incorporating biochar into their structure. Biochar, a carbonaceous material produced from the pyrolysis of biomass, possesses a high surface area and porosity, enhancing the adsorption capacity of LDHs. The combination of LDHs and biochar (BCLDH) creates a synergistic effect, resulting in a more efficient and sustainable remediation approach. Therefore, the aim of this study is to investigate the effect of LDH and BCLDH on reducing the uptake of some heavy metals, including lead, cadmium, nickel, and zinc, in basil plants during irrigation with landfill leachate and industrial wastewater.

Materials and Methods

The LDH and BCLDH were synthesized using the co-precipitation method. The landfill leachate was sampled from the Sari landfill site, and the industrial wastewater was collected from Sari Industrial Park No. 1. A split-plot design with 3 replications was used for this study, where the main factor was the type of irrigation water, including two types: landfill leachate and industrial wastewater. The sub-factor consisted of treatments with LDH and BCLDH amendments, arranged in a completely randomized design with 7 treatments as follows: 1- Control, 2- LDH 0.25%, 3- LDH 0.5%, 4- LDH 1%, 5- BCLDH 0.25%, 6- BCLDH 0.5% , and 7- BCLDH 1%. After mixing various amounts of LDH and BCLDH with 6 kg of dry soil, 10 basil seeds were planted in each pot. The irrigation method was based on weight loosing, meaning that before planting the seeds, the pots were first saturated with tap water according to their pore volume, and the weight of each pot after draining excess water was recorded as the initial weight. Two weeks after seed germination, irrigation with landfill leachate and industrial wastewater was conducted over 8 weeks based on the plants' evapotranspiration and the reduction in pot weight (after reducing the available water up to 75%). Finally, after harvesting the plants, the levels of heavy metals in the soil and plants were measured.

Results and Discussion

The results indicated an increase in soil pH up to 7.8 following the application of 1% BCLDH, probobly due to the effective role of biochar in raising soil pH. Morever, the results showed that the LDH and BCLDH amendments in irrigation with industrial wastewater had no significant effect on the levels of heavy metals in the soil and the basil in the roots, and shoots. In contrast, both application of LDH and BCLDH on irrigation of landfill leachate successfully minimized the metal levels in the soil and plant organs, with rate of BCLDH 1% showing the best performance. The lowest concentrations of lead, cadmium, and nickel in the soils were measured in the BCLDH 1% treatment, with values of 2.44, 0.18, and 1.64 mg/kg, respectively. Moreover, this treatment in landfill leachate irrigation reduced the concentrations of lead, nickel, and zinc in the shoots by 75.5%, 62%, and 36.4%, respectively. This effect is attributed to the clear role of LDH and its contribution in increasing soil pH, cation exchange, electrostatic adsorption, and the precipitation of metals into insoluble compounds in the soil. In addition, the modification of LDH with biochar improved its structure, increased its surface area, and enhanced its porosity, thereby reducing pore diameter. This led to the increased surface adsorption capacity, enhanced electrostatic adsorption, and more effective removal of heavy metals, which reached its peak in the BCLDH 1% treatment.

Conclusion

The present study focused on the beneficial role of LDH and BCLDH amendments in reducing the uptake of heavy metals (lead, cadmium, nickel, and zinc) by basil plants under irrigation with landfill leachate and industrial wastewater. The results do not support the hypotheis on use of these amendments for industrial wastewater irrigation, which is likely due to the inefficiency of amendments at low concentrations of heavy metals. On the other hand, the results demonstrated the high efficiency of BCLDH in reducing the uptake of heavy metals by basil during landfill leachate irrigation. The lowest levels of heavy metals were observed in the BCLDH 1% treatment in soil, roots, and shoots. This outcome highlights the successful synthesis of biochar on LDH, which improved the structure and increased the metal adsorption capacity of BCLDH. . In addition, the BCLDH 1% treatment increased soil pH, electrostatic adsorption, and cation exchange, thereby the reducing metal uptake by basil. Given that the uncontrolled use of effluents may lead to excessive accumulation of metals in the edible parts of plants and heightened concerns about human health, the application of BCLDH emerges as a effective approach for significantly improving plant performance and reducing heavy metal uptake and its associated consequences. However, before making the final recommendations, futhur studies should be conducted at the field scale, over multiple years, and using different plant species to ensure the effective and reliable outcomes.