Solvent-free synthesis of carboxylate-functionalized cellulose from waste cotton fabrics for the removal of cationic dyes from aqueous solutions

In this work, the carboxylate-functionalized cellulose (CFC) from waste cotton fabrics (WCF) was studied for removal of cationic dyes from aqueous solutions. Instead of using conventional process for esterifying cellulose, which uses sulfuric or perchloric acid as catalysts, and methylene chloride as solvent, a novel mechanochemical approach which requires no catalyst or solvent in the reaction mixture was used. The prepared CFC with high concentrations of carboxyl groups ranging from 1.34 to 2.14 mmol/g was tested as adsorbent for the removal of methylene blue (MB) and rhodamine 6G (R6G) from aqueous solutions. The modified cellulose showed much higher adsorption capacity than that of unmodified WCF. The effect of operating variables such as concentration of carboxyl groups, solution pH, adsorbent dosage, temperature, and initial dye concentration on the adsorption capacity were investigated. The maximum adsorption capacity (185.63 mg/g for MB and 118.21 mg/g for R6G) was obtained at pH = 10 and temperature of 298 K. The experimental equilibrium data were analyzed by using Langmuir and Freundlich isotherm models and the results indicated that the adsorption behavior of both dyes could be described well by Langmuir isotherm. The value of standard enthalpy changes was −26.48 kJ/mol for MB and −42.16 kJ/mol for R6G, suggesting that the adsorption process is exothermic in nature. The results revealed that solvent-free mechanochemistry is an environmentally friendly method to prepare novel efficient absorbent for the removal of dyes from wastewater.