The textile industry uses vast quantities of synthetic dyes, and their release into wastewater poses significant environmental risks. Methyl orange (MO), a common anionic azo dye, is toxic and potentially carcinogenic, requiring effective treatment before discharge. Biochar, has gained attention as a cost-effective and sustainable adsorbent for removing pollutants from water. Researchers Aneek Krishna Karmakar, Fahima Khatun, Ahsan Habib, and Muhammad Saidur Rahman investigated the potential of biochar derived from puffed rice—a common food byproduct in Asia—for removing MO dye. Their findings were published in the Pakistan Journal of Science and Industrial Research Series A: Physical Sciences.

The team explored how factors like contact time, biochar particle size, initial MO concentration, and temperature influenced the adsorption process. They used Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) to examine the biochar’s chemical functional groups and physical structure, which are crucial for understanding how it captures the dye. The FTIR analysis confirmed the presence of various surface functional groups, like hydroxyls and carbonyls, which can act as binding sites for dye molecules. SEM images revealed a highly porous and irregular surface structure, providing a large surface area essential for effective adsorption.

Adsorption experiments showed that the puffed rice biochar quickly captured the MO dye, reaching equilibrium within about 60 minutes. Interestingly, smaller biochar particles (down to 170 µm) were more effective at removing the dye. This is likely because smaller particles offer a relatively larger external surface area and shorter paths for the dye molecules to reach internal pores. The initial concentration of MO also played a role; while the biochar efficiently removed dye at higher concentrations initially, its capacity became limited as the available adsorption sites filled up.

Temperature had a notable effect on MO removal. The study found that the adsorption process was exothermic and most spontaneous (requiring less energy input) at lower temperatures, specifically between 20°C and 30°C. At higher temperatures (tested up to 60°C), the Gibbs free energy became positive, indicating the process became non-spontaneous and less favorable. This suggests that lower temperatures promote adsorption, while higher temperatures might cause the dye to desorb from the biochar surface^15151515. Despite this, the researchers achieved nearly 96% dye removal under specific optimal conditions: a temperature of 40°C (313 K) and a particle size of 170 µm.

To better understand the adsorption mechanism, the researchers applied two common models: the Langmuir and Freundlich isotherms. The Langmuir model provided a better fit for the experimental data. This suggests that the MO dye molecules form a single layer (monolayer) on the surface of the puffed rice biochar at specific, finite adsorption sites, rather than forming multiple layers.

Overall, this research demonstrates that biochar made from puffed rice waste is an effective, low-cost adsorbent for removing methyl orange dye from aqueous solutions. Its porous structure and surface chemistry contribute to its high removal efficiency, especially when using smaller particle sizes and operating at temperatures around 20-40°C. By characterizing the material and optimizing process conditions, puffed rice biochar presents a promising, environmentally friendly option for treating textile wastewater, turning an otherwise low-value byproduct into a useful resource for environmental remediation.

Source: Karmakar, A. K., Khatun, F., Habib, A., & Rahman, M. S. (2025). Puffed Rice Biochar: Characterization and Adsorption Studies for Methyl Orange Dye Elimination. Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences, 68A(3), 235–251.