Microwave-assisted pyrolysis with chemical activation, an innovative method to convert orange peel into activated carbon with improved properties as dye adsorbent

摘要

Microwave-assisted pyrolysis with chemical activation was developed and optimized to transform orange peel into activated carbon (AC) desirable for use as a dye adsorbent. The orange peel was first carbonized via microwave-assisted pyrolysis to produce a biochar, which was then activated and converted into AC via chemical impregnation coupled with microwave-assisted pyrolysis. The process parameters involved was optimized to maximize the yield of AC and its adsorption efficiency on malachite green dye using response surface methodology adopting central composite design. The use of microwave-assisted pyrolysis provided a fast heating rate and short process time in converting orange peel into AC, recording a heating rate of up to 112 °C/min in a process taking about 25 min, representing a method that is potentially faster and more energy efficient compared to that shown by the method commonly performed using conventional heating source (≥1 h). The results showed that AC with the highest yield (87 wt% of biochar) and optimal adsorption efficiency (28.5 mg of dye/g of AC) can be obtained by performing chemical impregnation at an impregnation ratio of 1:1 coupled with microwave-assisted pyrolysis under microwave irradiation (heating) for 5 min using 550 W of microwave power. The addition of chemical activation with alkali metal hydroxides resulted in the production of AC with improved properties. The AC showed a highly porous structure containing high content of fixed carbon (83 wt%) and high BET surface area (1350 m 2 /g). The adsorption–desorption isotherm showed a combination of Type I and Type II isotherms, which indicates the presence of microporous-mesoporous structure, thus exhibiting a characteristic of improved pores accessibility and high adsorption capacity. Combined with the detection of low ash (3.2 wt%) and moisture content (5 wt%), the AC shows great promise as a high-grade dye adsorbent with high adsorption capacity and potentially increased durability since a low moisture content could increase the rate of adsorption of dye contaminants and a high ash content could promote undesirable catalytic reactions and reduce the adsorption capacity and reactivation efficiency of AC. The recovery of AC with improved properties and the desirable process features (fast heating rate, short process time) suggest the great potential of this method as an alternative for the treatment and recovery of fruit peel.