Influence of solution matrix on the adsorption capacity of NaOH-modified Carica papaya seed for Congo red and malachite green removal

Abstract

Underutilized agro-based waste Carica papaya seed (CPS) was processed into a base-modified adsorbent for the remediation of malachite green (MG)- and Congo red (CR)-contaminated water. CPS was treated with 300 mL of 0.1 M NaOH for 24 h at room temperature, and the treated adsorbent was denoted as BCPS. BCPS was characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and pH point of zero charge (pH_pzc). Batch adsorption studies were carried out as functions of contact time, adsorbent dose, solution pH, initial adsorbate concentration and temperature. Adsorption isotherm, kinetic and thermodynamic studies were also performed. FTIR analysis indicated surface groups including O--H, C≡C, C=C, C≡N, C--O, C--C and C--H; O--H, C=O, C≡N and C--H were implicated in MG and CR adsorption. The pH_pzc of BCPS was 6.2. BCPS showed higher adsorption capacity for MG than CR under most experimental conditions, except pH, where the optimum values for MG and CR adsorption were pH 9 and pH 3, respectively. Maximum adsorption capacities for MG in single and binary systems were 42.37 and 7.62 mg/g, respectively, whereas those for CR were 6.95 and 5.58 mg/g, respectively. Langmuir and pseudo-second-order models best described the isotherm and kinetic data, respectively, and the adsorption processes were thermodynamically feasible.