Removal of hazardous azopyrazole dye from an aqueous solution using rice straw as a waste adsorbent: Kinetic, equilibrium and thermodynamic studies

• We use rice straw as a waste adsorbent for the removal of hazardous azopyrazole dye.
• The equilibrium data fitted well with the Freundlich model.
• The kinetic data tends to fit well in the pseudo-second-order kinetics model with high correlation coefficients.
• The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process.
• Results are of marked significance to the water treatment industries.

In this research, activated carbonmade from rice straw (ACRS) was synthesized simply by a low cost and nontoxic procedure and used for the adsorption of hazardous azopyrazole dye. The effect of different variables in the batch method as a function of solution pH, contact time, concentration of adsorbate, adsorbent dosage and temperature were investigated and optimal experimental conditions were ascertaine. Surface modification of ACRS using scanning electron microscopy (SEM) was obtained. More than 75% removal efficiency was obtained within 75 min at adsorbent dose of 0.5 g for initial dye concentration of 30–100 mg L−1 at pH 3. The experimental equilibrium data were tested by the isotherm models namely, Langmuir and Freundlich adsorption and the isotherm constants were determined. The kinetic data obtained with different initial concentration and temperature were analyzed using a pseudo-first-order and pseudo-second-order equations. The activation energy of adsorption was also evaluated and found to be +13.25 kJ mol−1 indicating that the adsorption is physisorption. The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process. The results indicate that ACRS could be employed as low-cost material for the removal of acid dyes from aqueous solution.

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