Evaluation of the effect of ultrasonic waves on nitrate removal from aqueous solutions using Zinc- and Iron- coated activated carbon

Nitrate contamination in water represents a significant threat to both public health and the environment. While ultrasonic technology has emerged as an eco-friendly approach with potential for enhancing nitrate removal, its full capacity remains underexplored. This study aims to assess the effectiveness of ultrasound in improving nitrate removal from aqueous solutions using biochar derived from rice straw, modified with Fe³⁺ and Zn²⁺ as cationic bridges. Conducted at the water quality laboratory of Sari Agricultural Sciences and Natural Resources University, the experiments revealed that iron-coated biochar treatments (BF and BFU) exhibited outstanding performance in nitrate removal. Ultrasound application significantly enhanced nitrate removal efficiency, with the combination of ultrasonic waves and iron-coated biochar (BFU) achieving a maximum adsorption capacity (q_m) of 3.664 mg/g, which surpassed non-sonicated treatments (BF: 3.345 mg/g) and reduced equilibrium time by 92% (from 60 min to 5 min). Furthermore, ultrasonic treatment improved the performance of Zn²⁺-coated biochar (BZU), boosting removal rates by more than 25% through cavitation-induced particle fragmentation and enhanced mass transfer. Mechanistic analysis indicated that ultrasound facilitates the homogenization of the adsorption surface, favoring Langmuir-type monolayer adsorption (R² > 0.95), while the cationic bridges (Fe³⁺/Zn²⁺) strengthened electrostatic interactions with nitrate ions. Under optimized conditions, the combination of ultrasound and cation-modified biochar achieved over 90% nitrate removal, presenting a promising, energy-efficient, and sustainable solution for water treatment. These findings demonstrate the potential of ultrasonic-assisted, cation-modified biochar as a highly effective strategy for mitigating nitrate contamination in water systems.