Abstract
Introduction: Cadmium is considered a critical heavy metal due to its high mobility in the environment, which poses serious risks to ecosystem health through its entry into the food chain. Accordingly, it is necessary to achieve efficient methods for stabilizing this element in contaminated soils. The present study aimed to synthesize and compare the performance of natural zeolite nanoparticles and microparticles and biochar in stabilizing cadmium in soil.

Materials and Methods: For this purpose, the incubation experiment was carried out in a completely randomized factorial design with three replications and a time period of two months. The structural characteristics of the particles produced through the planetary ball mill were determined using XRD, BET, SEM and FTIR analyses. The effect of different treatments (including zeolite and biochar in both nano and micro scales with levels of 1 and 3 wt%) on the cadmium adsorption and changes in its chemical forms was evaluated.

Results and Discussion: The findings indicated that all modifiers improved the cadmium fixation efficiency with increasing dosage. Also, particle size had a significant effect on the adsorption capacity; so that the performance of zeolite nanoparticles and biochar was superior to the micro versions. Among them, the 3% nanobiochar treatment showed the highest fixation potential with an efficiency of 95.27%. The study of cadmium chemical species showed that due to the high pH and the poverty of organic matter in the soil, the abundance of soluble, exchangeable and organic forms was lower and the carbonate form was the dominant species due to the high presence of calcium carbonate in the soil. Finally, the results confirmed that the use of modifiers, especially nanobiochar, in addition to reducing the availability of cadmium, leads to an effective redistribution of the chemical species of this element in the soil.

Conclusion: The finding of this study demonstrates that producing nano-sized zeolite and biochar via ball milling is an efficient and cost -effective strategy for enhancing cadmium stabilization cadmium in contaminated soils. FTIR analysis confirmed that mechanical size reduction of biochar does not alert the type of functional groups present; however, the number of active functional groups increase as particle size decreases. XRD results showed that mechanical milling reduces the crystallinity of natural zeolite, which contributes to improved reactivity. Nanozeolite through higher cation exchange capacity and specific surface area and nano biochar due to the high amount of functional groups, higher cation exchange capacity, and specific surface area reduces cadmium in the soil compared to micro particulate. Among the modifiers, Nano Biochar performed better. Also, each of the treatments had different effects on the distribution of cadmium chemical forms in the soil. Nano biochar at 3% application level, with the greatest reduction in exchangeable forms and carbonate and increasing the organic and residual fraction, has a greater effect on the redistribution of cadmium in the soil and by converting cadmium into inaccessible forms, partially reduces the concentration of mobile cadmium and reduces its availability. It can be concluded that the use of nano-biochars of corn produced at 400 °C at higher doses is useful for stabilizing cadmium in calcareous contaminated soils.