Biosorption of methylene blue by de-oiled algal biomass: equilibrium, kinetics and artificial neural network modelling.

Abstract

The main objective of the present study is to effectively utilize the de-oiled algal biomass (DAB) to minimize the waste streams from algal biofuel by using it as an adsorbent. Methylene blue (MB) was used as a sorbate for evaluating the potential of DAB as a biosorbent. The DAB was characterized by SEM, FTIR, pHPZC, particle size, pore volume and pore diameter to understand the biosorption mechanism. The equilibrium studies were carried out by variation in different parameters, i.e., pH (2–9), temperature (293.16–323.16 K), biosorbent dosage (1–10 g L−1), contact time (0–1,440 min), agitation speed (0–150 rpm) and dye concentration (25–2,500 mg L−1). MB removal was greater than 90% in both acidic and basic pH. The optimum result of MB removal was found at 5–7 g L−1 DAB concentration. DAB removes 86% dye in 5 minutes under static conditions and nearly 100% in 24 hours when agitated at 150 rpm. The highest adsorption capacity was found 139.11 mg g−1 at 2,000 mg L−1 initial MB concentration. The process attained equilibrium in 24 hours. It is an endothermic process whose spontaneity increases with temperature. MB biosorption by DAB follows pseudo-second order kinetics. Artificial neural network (ANN) model also validates the experimental dye removal efficiency (R2 = 0.97) corresponding with theoretically predicted values. Sensitivity analysis suggests that temperature and agitation speed affect the process most with 23.62% and 21.08% influence on MB biosorption, respectively. Dye adsorption capacity of DAB in fixed bed column was 107.57 mg g−1 in preliminary study while it went up to 139.11 mg g−1 in batch studies. The probable mechanism for biosorption in this study is chemisorptions via surface active charges in the initial phase followed by physical sorption by occupying pores of DAB.