Improved carboxylate density hydrochar by alkylation of surface phenol for adsorption of cationic dye in aqueous solution

Abstract

Methylene blue (MB) adsorption in aqueous solution studied using rice straw derived hydrochar (RSH) and carboxylate functionalized hydrochar (RSHC) as adsorbent. RSHC with improved carboxylate functional group density was prepared by selective functionalization of phenolic oxygen functional groups present on RSH surface. The RSH and RSHC were characterized by ultimate and proximate analysis, Boehm Titration, ¹³C CP-MAS SS NMR, XPS, SEM, BET surface area and zeta potential analysis. Batch adsorption experiments were optimized for MB adsorption were, adsorbent dose of 0.2 g/L, pH 8 and 7, respectively for RSH and RSHC, with 3.5 h contact time at 298 K. Nonlinear fit to Langmuir, Freundlich, Temkin, d-R models and Sips isotherm models, showed good fit with R²>0.95. RSHC (Q_m = 434.6 mg/g) showed four-fold increase in MB adsorption as compared to the pristine hydrochar RSH (Q_m =111.6 mg/g). Contact time study showed good nonlinear fit to pseudosecond order model indicating a strong interaction between hydrochar adsorbents and MB. Thermodynamics of MB adsorption (288–308 K) onto the surface of RSH and RSHC, is a spontaneous process with ∆G° -23.85 and -28.31 kJ/mol, respectively. RSH in characterized by enthalpy driven adsorption characteristic of proton-MB exchange, whereas in case of RSHC, K⁺–MB exchange is entropically driven process.