Comparative evaluation of agricultural waste derived agrochar in removing Rhodamine B from aqueous solutions

Dye pollution from textile, paper, and leather industries poses environmental risks due to the persistence and toxicity of dyes, while conventional treatment methods are often costly and generate secondary waste. This study evaluates the novel agrochar derived from agricultural residues (straw, cow dung, sludge, food waste, and lawn grass) as a low-cost adsorbent for Rhodamine B removal. Non-activated agrochar (NAAC) was produced by carbonization and microwave irradiation, while activated agrochar (AAC) underwent additional concentrated H₂SO₄-assisted sonication. Batch kinetic experiments examined the influence of pH (2 to 10), contact time (0 to 360 min), dye concentration (10 to 70 mg/L), adsorbent dosage (0.5 to 5 g), and temperature (13-53 °C) on agrochar performance. NAAC achieved 66% removal with maximum adsorption capacity (q _max ) of 2.035 mg/g, whereas AAC reached 96% removal with q _max of 48.35 mg/g at early saturation and adsorption equilibrium time. Langmuir isotherm best described AAC adsorption (R² = 0.973), indicating monolayer coverage. BET analysis showed a drastic increase in surface area from 3.18 to 262 m²/g and pore volume from 0.0044 to 0.43 cm³/g after activation, enhancing mesoporosity. XRD confirmed the amorphous nature of NAAC, while AAC exhibited partial structural ordering along with crystalline peaks from acid-stable mineral phases, which attenuated after dye loading. FTIR analysis revealed that hydroxyl, carbonyl, and aromatic groups facilitated adsorption through hydrogen bonding, electrostatic attraction, and π–π interactions with dye molecules. These results establish activated agrochar as a sustainable and highly efficient adsorbent for dye-laden wastewater.