The optimization study of direct red 81 and methylene blue adsorption on NaOH-modified rice husk

Abstract

The objectives of this investigation are to obtain optimal adsorption conditions for direct red 81 (DR81) and methylene blue (MB) from aqueous solution on NaOH-modified rice husk (NaOH-RH) using response surface methodology (RSM). A Box–Behnken design (BBD) statistical experimental design was used to investigate the adsorption of DR81 and MB on NaOH-RH. For DR81, pH (4–7), initial concentration of dye (25–125 mg/l), adsorbent dose (2–6 g/l) and for MB pH (7–10), initial concentration of dye (25–125 mg/l) and adsorbent dose (0.25–0.75 g/l) were selected as independent variables. The results showed that the predicted values for both dye adsorptions were close to the experimental values and were in a good agreement, on the other hand, the R² values for DR81 (R² = 0.9685) and MB (R² = 0.9832) indicated that the regression is able to give a good predict of response for the adsorption process in the range studied. From the BBD predictions, the optimal conditions in the adsorption process for removing 54.04% of DR81 were found to be 25 mg/l initial dye concentration and 6 g/l adsorbent dose at pH 4. Also, removing 97.66% of MB was found to be 25 mg/l initial dye concentration and 0.595 g/l adsorbent dosage at pH 8.89.

Keywords:

• NaOH-modified rice husk
• Direct red 81
• Methylene blue
• Adsorption

Acknowledgment

This work was supported by Department of Environmental Health Engineering at School of Public Health, Tehran University of Medical Sciences, Tehran, Iran.