Effect of nanobiochar on the kinetics and isotherm of cadmium adsorption in calcareous soil

Introduction

Soil pollution with heavy metals such as cadmium has become a global environmental concern. Adsorption is widely used as a remediation methods due to its advantages, including cost-effectiveness and high performance for metal ions. Biochar is one effective way to improve soil properties and act as a sorbent for various toxins. In particular, nanobiochar, with its unique properties such as high specific surface area, has the potential to be one of the most effective soil amendments. Nanobiochar has gained attention in recent years due to its unique properties and potential applications in various fields. Its high specific surface area and small particle size make it an effective adsorbent for pollutants in contaminated soil and water. Nanobiochar has demonstrated the ability to adsorb a wide range of pollutants, including heavy metals, organic compounds, and nutrients, making it a promising material for environmental remediation. Despite the potential applications of nanobiochar, more research is needed to fully understand its properties and potential applications. The field of nanobiochar is still new, and many questions remain unanswered. Also, the importance of nanobiochar in remediating polluted calcareous soils is not well known. The aim of this study was to investigate the effect of nanobiochar made from reed on the adsorption isotherms and kinetics of cadmium in polluted calcareous soil.

Materials and Methods

For this research, a composite soil sample was collected from a depth of 0-30 cm from the farm of the Faculty of Agriculture, Shahid Chamran University of Ahvaz. The soil sample was air-dried and passed through a 2 mm sieve, then some physical and chemical characteristics of the soil were measured. To prepare biochar, the raw common reed was collected, air-dried, and passed through a 2-mm sieve and common reed was soaked in 0.5 M FeCl2. The raw and Fe soaked common reed biomass were then pyrolyzed at 500 °C for 3 h with a heating rate of 6 °C min−1 in an electrical furnace under anaerobic conditions. Nanobiochar was prepared using a ball milling method, and its properties were measured. To investigate the effect of the adsorbents on the isotherm and kinetic of Cd adsorption in soil, an incubation experiment was conducted with three types of adsorbents, including common reed biochar, iron-modified common reed biochar, and common reed nanobiochar at three application rates of 0, 0.5, and 1% in three replicates for 60 days at a temperature of 25°C. At the end of the incubation period, some soil properties were measured, and the isotherms of cadmium adsorption at different concentrations (0, 5, 10, 20, 30, 40 and 50 mg L-1) was determined. The kinetics of cadmium adsorption in 40 mg L-1 Cd concentration at different times (0.5, 1, 2, 4, 8, 12, 24 and 48 hours) were also studied. Then, the experimental data were fitted using nonlinear adsorption and kinetic models, and the model parameters were calculated.

Results and Discussion

The results showed that reducing the size of biochar to the nano-scale increased the specific surface area and cation exchange capacity. The application of nanobiochar at a 1% application rate increased the percentage of soil organic matter compared to other treatments. Soil trested with nanobiochar at 0.5% and 1% application rates increased soil organic matter by 33.02% and 83.02% compared to the control treatment, respectively. Nanobiochar had the greatest effect on the adsorption of cadmium in the soil. The Langmuir model provided a better fit to the experimental data than the Freundlich and Temkin models. The maximum adsorption capacity of cadmium (qmax) increased with the use of adsorbents, and the highest adsorption capacity was observed in soil trested with nanobiochar at a 1% application rate. The maximum adsorption capacity of soil trested with nanobiochar at a 1% application rate for Cd was 1048 mg kg-1 of soil. The pseudo-second-order kinetic model provided the best fit to the kinetic adsorption data due to its high coefficient of determination (R2) and low standard error (SE). The adsorption capacity of cadmium (qe) increased with the use of reed biochar, iron-modified reed biochar, and nanobiochar compared to the control treatment, with higher increase observed at the 1% applicathan rate compared 0.5%. Additionally, the rate constant K2 showed a higher surface adsorption rate of cadmium in soil treated with nanobiochar compared to the other treatments.

Conclusion

In general, the study findings confirmed that the application of nanobiochar produced by ball milling is more effective in the adsorption of cadmium in polluted calcareous soils. The results showed that the amending cadmium polluted soils with nanobiochar had a better effect compared to biochar treatments. Nanobiochar at a 1% application rate was the best treatment in this study for improving soil properties and increasing Cd adsorption due to its unique properties (high surface area and improved adsorption capabilities). As a result, the application of nanobiochar as an adsorbent has environmental and economic advantages. Application of nanobiochar as a strong and effective adsorbent to adsorb cadmium from the soil can be considered as an effective solution to protect the environment and human health.