Removal of phenol from an aqueous solution using carbon from African potato (hypoxis hemerocallidea) waste

The concentration of phenol in industrial wastewater has reached alarming proportions, reaching up to 7000 mg/L in some regions. Yet, the permissible level of phenol in drinking water is 0.3 mg/L. This study developed three low-cost adsorbents for removing phenol from water. The adsorbents were synthesized from (i) carbon derived from African potato waste (CAP), (ii) activation of the carbon with phosphoric acid (ACAP), and (iii) carbon activation with sodium hydroxide (BCAP). The BET results showed that the activated adsorbents had a mesoporous structure. SEM images for CAP showed that the sample had a rough surface texture and wrinkles, while ACAP and BCAP had amorphous particles of different shapes and sizes with porous surfaces. Batch adsorption experiments showed that increasing the temperature of the system impaired the uptake of phenol for all adsorbents, indicating that the adsorption process was exothermic. The equilibrium data indicated that the adsorption fits the Langmuir model, with predicted maximum adsorption capacities of 14.76, 61.39 and 71.55 mg/g for CAP, ACAP and BCAP, respectively. In addition, CAP fits the PFO model, while ACAP and BCAP fit the PSO model. The maximum adsorption capacity was achieved at a solution pH of 6. The time-contact effect trends showed three phases: fast adsorption (5-20 min), slow adsorption (20-60 min) and equilibrium (60-120 min). The values of △G o and △H o were negative, indicating that the uptake of phenol on the adsorbents was spontaneous and the removal was exothermic. The △H o value for CAP was -0.591 KJ/mol, indicating physisorption, while ACAP and BCAP were -86.66 and -50.22 KJ/mol, respectively, indicating chemisorption.