Potential of L-tyrosine and L-tryptophan modified silica nanoparticles as adsorbents for Pb(Ⅱ) ions: kinetics and thermodynamic investigation

摘要

The search for novel adsorbents to lower the concentration of toxic species in polluted waters is ongoing globally. Organic ligands such as l-tyrosine (Tyr) and l-tryptophan (Trp) are constituted by electron-donors (i.e., oxygen and nitrogen atoms), and these ligands can greatly enhance the sorption capabilities of nanoparticles for toxic metallic ions. The effect of parameters influencing adsorption efficiency of Pb(II) by two adsorbents bearing Tyr (Tyr-SiO2) and Trp (Trp-SiO2) groups, including aqueous phase pH, amount of adsorbent, stirring time and initial concentration of the metal ions were assessed and discussed. Under optimum experimental conditions (0.2 g of the adsorbents, solution pH 5 and 1,000 rpm stirring rate), Tyr-SiO2 showed a capacity of 7.60 mg/g after 10 min. Under the same conditions, the capacity presented by Trp-SiO2 was 6.92 mg/g after 60 min. It was found that the adsorbed Pb(II) ions can be recovered by acid backwashing of the loaded nanoparticles Tyr-SiO2 and Trp-SiO2 with 5 mL of nitric acid 0.1 mol/L or hydrochloric acid 1 mol/L. The calculated thermodynamic parameters indicated that the adsorption process based on Tyr-SiO2 adsorbents was endothermic and controlled by entropy changes, while process based on Trp-SiO2 adsorbent was exothermic and controlled by changes in enthalpy. The adsorption of Pb(II) ions onto the studied adsorbents was well described by Langmuir model. Pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion kinetic models were examined during the investigation of the kinetics of adsorption of Pb(II) ions by Trp-SiO2. This investigation indicated that the process is well described by pseudo-second-order kinetics. Although both the studied adsorbents showed good adsorptive capacity with respect to that reported for some other adsorbents, a higher adsorptive capacity, more rapid kinetics, and the ability to remove Pb(II) ions at lower pH values, were the superiority of Tyr-SiO2 in comparison with Trp-SiO2.