Adsorption of Congo red and malachite green using H_3PO_4 and NaCl-modified activated carbon from rubber (Hevea brasiliensis) seed shells

摘要

Cationic dyes are toxic environmental pollutants that need to be mitigated from the aqueous environment. In this study, Hevea brasiliensis seed shells (HBSS) activated with H_3PO_4 (PHBSS) and NaCl (SHBSS) were explored for the removal of Congo red (CR) and malachite green (MG) from their aqueous solutions. The adsorbents were carbonised at 300 °C for 3 h and characterised by FTIR, SEM and XRF. Maximum removals for CR and MG were observed at pH 2 and pH 10, respectively. The uptake of both dyes was optimum at 0.3 mm particle size, 2 g/L adsorbent dosage and 100 mg/L initial dye concentration. The adsorption capacity of PHBSS and SHBSS to CR was 55.87 and 50.51 mg/g, respectively, while for MG was 58.48 and 56.82 mg/g, respectively. The study observed that the uptake of both dyes by both adsorbents was best fit to the Freundlich isotherm and the pseudo-second order kinetic model. The thermodynamics modelling revealed that the process was spontaneous and endothermic. The mechanism of CR and MG uptake was by a combination of electrostatic attraction, π-π electron-donor interaction, hydrogen bonds and pore diffusion. Furthermore, H_3PO_4 was a better activating agent for HBSS than NaCl given the current application.