Potential of rice husk as biosorbent for removal of Congo red: an optimization study

Abstract

Congo red is a physically and chemically stable dye and mainly used in textile industry. It has been discharged as constituent effluent and caused pollution to the environment. Rice husk was found abundantly in Malaysia and discarded as waste from rice milling process. Therefore, the main purpose of this research is to optimize the adsorption efficiency of rice husk as biosorbent to remove congo red. One factor at a time (OFAT) method was applied to investigate the effect of rice husk dosage, contact time and initial congo red concentration on the adsorption efficiency. The first parameter manipulated was the rice husk dosage (0.5 g to 2.5 g, 120 minutes, 15 mg /l), then contact time with optimal rice husk dosage (1 g, 30 minutes to 150 minutes, 15 mg/l) and lastly the initial congo red concentration with optimal rice husk dosage and contact time (1 g, 60 minutes, 5 mg/l to 25 mg/l). The surface of rice husk has been observed by Scanning Electron Microscopy (SEM) and it showed that the pores of rice husk has been filled by the congo red particles. The best OFAT results by using 1g of rice husk, 60 minutes and 20 mg/l of congo red condition. Next, Central Composite Design (CCD) of Response Surface Methodology (RSM) by using Design of Experiment (DOE) software has been used to optimize the parameters to obtain maximum efficiency. The parameters were fixed at certain range (0.5 g to 1.5 g of rice husk dosage, 40 to 80 minutes of contact time and 15 to 25 mg/l of initial congo red concentration). Based on the ANOVA report, the model was significant while the lack of fit was not significant. The 3D surface plots have showed that the optimum condition to reach maximum adsorption efficiency (83.52 %) were using 1.5 g of rice husk, 80 minutes of contact time and 25 mg/l of congo red solution. The validation test showed that the actual adsorption efficiency (81.83 %) closed to predicted value (82.86 %). Thus, rice husk has proven to be a biosorbent which can be used to remove congo red in wastewater.