Biosorption of Pb2+ by original and protonated citrus peels: Equilibrium, kinetics, and mechanism

Biosorption can be an efficient low-cost process to remove toxic heavy metals from wastewater. This study investigated the uptake of Pb2+ by processed orange peels, a pectin-rich byproduct of the fruit juice industry. Potentiometric titrations showed a significantly higher negative surface charge of protonated peels compared to original peels, with acidic groups around pH 4, 6, and 10. FTIR spectra of peels were similar to those of pectin. The carboxylic group peak shifted from 1636 to 1645 cm−1 after Pb2+ binding, indicated the involvement of carboxyl groups in Pb2 binding. Depending on the particle size, equilibrium was achieved in 30 min to 2 h. The first-order model was inferior to second- or third-order models. The obtained rate constants were much higher for smaller particles, while the capacity was similar for all sizes. Low pH, increased ionic strength, or competing co-ions reduced Pb2+ binding at low sorbent dosages, but at high sorbent dosages removal remained above 90%. The Pb2+ uptake at 300 ppm was 2 mmol/g (40% of the dry weight). Due to high uptake, favorable kinetics and good stability, citrus peel biosorbents hold high promise for industrial applications.