Enhanced lead adsorption by unbleached newspaper pulp modified with citric acid

A thermochemical method was performed to enhance lead adsorption of unbleached newspaper pulp. Unbleached pulp was obtained after mechanical treatment of waste newspaper and was chemically modified with citric acid (CA) at 90 °C for 90 min. The parameters under investigation were the pH of adsorption solution (3-9), the concentration of mechanically and chemically treated pulps (0.1-1 g), the concentration of CA (0.5 M and 1 M), the initial concentration of lead (20-1000 mg/l) and the pulp-lead solution contact time (0-24 h). The maximum adsorption capacity of mechanically treated newspaper pulp (MT-pulp) increased by 35% (25.71 mg/g) after modification with 0.5 M CA and 82% (34.6 mg/g) with 1 M CA. The modification process resulted in higher negative values of zeta potential and surface charge of mechanically treated pulp reaching the highest charge and maximum lead adsorption at pH = 6. The adsorption experimental data fitted well to the Langmuir isotherm model and inferred that adsorption and ion-exchange are major mechanisms for binding Pb2+ ions. Kinetic studies for three different concentrations of Pb2+ showed that the adsorption process involved two stages and the experimental data followed the pseudo-second order model. The results showed that CA-modified newspaper pulp is an effective adsorbent for the removal of Pb2+ from wastewater.