A study published in Applied Water Science by Jae-In Lee, Jun-Sik Jo, Youn-Jun Lee, Chang-Gu Lee, Young-Man Yoon, and Seong-Jik Park investigated a sustainable method for treating highly contaminated industrial wastewater. They successfully converted Perilla frutescens stems (PFS), an abundant agricultural byproduct commonly discarded or burned in fields, into a functional biochar adsorbent (PFSB) for the removal of hazardous cadmium (Cd). This material valorization offers dual advantages: mitigating air pollution from agricultural residue burning and addressing heavy metal contamination in industrial effluents, thereby promoting a scalable and cost-efficient solution aligned with circular economy practices.

The researchers systematically analyzed biochars produced at various pyrolysis temperatures, ranging from 300°C to 750°C. The biochar synthesized at 450°C (PFSB-450) demonstrated the most advantageous physicochemical characteristics, including a high Carbon/Nitrogen (C/N) ratio, low Hydrogen/Carbon (H/C) ratio, well-developed porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More, and a rich presence of oxygen-containing functional groups. As the pyrolysis temperature increased, the carbonization process intensified, resulting in an increased carbon content and a reduction in hydrogen content, which led to a lower H/C ratio. Lower H/C ratios indicate greater aromaticity and a more stable aromatic structure. PFSB samples also contained major elements such as potassium (K), calcium (Ca), and phosphorus (P), with their concentrations increasing with higher pyrolysis temperatures. These elements, along with carboxyl and hydroxyl groups, contribute significantly to the subsequent adsorption mechanisms.

Batch adsorption tests confirmed PFSB-450’s superior performance, achieving a peak Cd adsorption capacity of 75.74 mg/g. This performance is comparable to or greater than that of many other biochar-based adsorbents reported in the literature, notably without requiring chemical modification or intricate processing steps. The adsorption kinetics were best characterized by the Elovich and pseudo-second-order (PSO) models, indicating that chemisorption, the formation of chemical bonds between the adsorbent and cadmium ions, was the dominant rate-limiting step. Furthermore, isotherm data favored the Langmuir model, suggesting that Cd adsorption primarily occurs as a monolayer onto a surface with uniform adsorption energy under favorable conditions.

Mechanistic investigation using X-ray photoelectron spectroscopy (XPS) revealed a multi-step process for Cd removal. Firstly, ion exchange was identified, where Cd²⁺ ions replaced native cations like K⁺ and Ca²⁺ on the biochar surface through electrostatic interactions. Secondly, the appearance of new peaks in the Cd 3d spectra confirmed the formation of chemical complexation bonds, specifically Cd-O and Cd-π interactions, indicating that oxygen-containing and aromatic functional groups on the PFSB-450 surface actively bound the Cd²⁺ ions. The strong influence of these chemical and electrostatic forces, rather than surface area alone, was supported by the finding that PFSB-750, which had the highest specific surface area, did not demonstrate a substantial improvement in Cd removal. The PFSB-450 material offers notable cost-effectiveness, with a calculated production cost of approximately 11.8 USD per kilogram, making it an affordable and scalable option for water treatment, especially since its limited reusability supports its primary use as a single-use adsorbent without complicated regeneration needs.

To ensure real-world applicability, response surface methodology (RSM) was applied to optimize the treatment of authentic zinc smelter wastewater, which had a high initial Cd concentration of 94.9 mg/L. The RSM analysis showed that adsorbent dose (X1​), wastewater pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More (X2​), and reaction time (X3​) had statistically significant impacts on Cd removal, while reaction temperature (X4​) was insignificant. The positive correlation with dose is due to the increased availability of active sites , while higher pH renders the biochar surface more negatively charged, enhancing electrostatic attraction for Cd2+ ions. Under the optimal predicted conditions—a dose of 39.97 g/L, pH 6, reaction time of 7.94 h, and temperature of 23.16°C—the PFSB-450 achieved a maximum Cd removal efficiency of 80.84% in the actual industrial wastewater.

Source: Lee, J.-I., Jo, J.-S., Lee, Y.-J., Lee, C.-G., Yoon, Y.-M., & Park, S.-J. (2025). Mechanistic insights and process optimization of perilla frutescens stem biochar for cadmium removal from zinc smelter wastewater. Applied Water Science, 16(2).