Nitric acid modified powdered activated carbon for simultaneous adsorption of lead and phenol in aqueous solution

Powdered activated carbon (PAC) was modified with various concentrations of nitric acid (HNO3) to adsorb lead and phenol. The modification was carried out with HNO3 concentrations of 1 M (PAC-1), 3 M (PAC-3), and 5 M (PAC-5), respectively, and sodium hydroxide treatment was performed for neutralization. Oxygen functional groups such as phenolic hydroxyl, carboxy and lactone were produced after the HNO3 modification, and the surface area was decreased compared to unmodified PAC (PAC-W). A negative charge appeared in modified PACs due to the increased carboxyl groups, and PAC-5 exhibited the highest negative value as-34.56 mV. Batch adsorption was performed on both lead and phenol. Lead adsorption increased from 17.83 mg/g (PAC-W) to 28.70 mg/g (PAC-5) as HNO3 concentration increased. More lead was adsorbed in PAC-5 because of enhanced electrostatic attractions with increased oxygen functional groups. Conversely, phenol exhibited lower adsorption in PAC-5 (102.79 mg/g) than PAC-W (111.67 mg/g). Phenol adsorbed to PAC through it-it interactions, but reduced surface area and hydrophilic surface functional groups in modified PACs limited the adsorption of hydrophobic phenol molecules. In simultaneous adsorption, the lead and phenol concentration were fixed as 30 mg/L and 20 mg/L. The modified PACs exhibited a similar trend to single adsorption. However, the simultaneous lead and phenol adsorption on PAC-5 decreased to 26.74 mg/g and 84.13 mg/g, respectively. Lead ion reacted to oxygen functional groups while phenol occupied a less active site in PAC-5. The PAC-5 can be suitable for application in environmental decontamination due to the abundant functional groups.