The influence of pH on the adsorption of lead by Na-clinoptilolite: Kinetic and equilibrium studies

Abstract

The objectives of this study were, firstly, to establish the mechanism by which modified clinoptilolite (in Na form) adsorbs lead ions and, secondly, to assess the extent of influence of pH on the adsorption capacity. To this end, the experimental data have been fitted by adsorption isotherms, thermodynamic and kinetic models. Based on the standard errors obtained during experiments, it was determined that the accuracy of prediction of the isotherm models considered for adsorption decreases in the order: Dubinin-Raduschkevich (4.63%, average normalised standard deviation error); Langmuir (7.90%); and Freundlich (15.98%). For the kinetic models, the accuracy of prediction decreases in the order: intra-particle Weber-Morris (with an average normalised standard deviation error of 5.53%); heterogeneous diffusion (5.67%); pseudo-second order kinetic model (10.47%); diffusion through the particle surface (41.73%); and the pseudo-first kinetic model (47.51%). The mechanism of adsorption of lead ions by Na-clinoptilolite was found to occur in a monolayer and heterogeneous surface. The pH of contact solutions played an important role owing to competition by hydrogen ions. As the pH of the solution decreased, the maximum monolayer adsorption capacity established theoretically, based on the Langmuir isotherm, also decreased. Thus, if, for instance, the pH decreases from 4 to 1, the maximum adsorption capacity decreases from 0.3569 to 0.1604 mol·kg -1. At high pH of the contact solution, the adsorption process occurs by ion exchange and at low pH; i.e., it is physical. The variation of the Gibbs free energy demonstrates that adsorption occurs spontaneously. The process was also observed to occur at a higher rate at low acidity. Diffusion through the internal structure of macro-and micropores is the stage taking place with the lowest speed during the adsorption process and plays an important role in the mechanism of adsorption. The intra-particle diffusion coefficient depends on pH, which can modify the shape and concentrations of the hydrated metal complexes in solutions, thus affecting the adsorption process. The decrease of pH from 4 to 1 resulted in a decrease of the intra-particle diffusion coefficient from 4.06·10 -11 to 1.96·10 -11 m 2 ·min -1. The film diffusion coefficients were found to be 10 times larger than the intra-particle coefficients, suggesting that diffusion to the external surface cannot be the rate-limiting step in the adsorption mechanism.

Sesame Husk as Adsorbent for Copper(II) Ions Removal from Aqueous Solution

“…The E value for Cu(II) on the sesame husk is 0.316 kJ/mol. The value of E is below 8 kJ/mol which indicates that physical adsorption is involved in the adsorption process [88] As seen in Table 3 This is in line with results reported by Cozmuta et al [174] who studied the adsorption of lead metal ions on Na-clinoptilolite and by Li et al [175] who studied the adsorption of Cu(II) ions on amino-functionalized magnetic nanoparticles. Table 4.…”

Disinfection and removal performance for Escherichia coli and heavy metals by silver-modified zeolite in a fixed bed column

“…Metal removal involved the mechanisms of both adsorption and ion exchange. This occurred via bulk diffusion of metal ions from the solution to the zeolite surfaces, intra-particle diffusion of metal ions through the zeolite macro pores and meso pores to the microcrystal surfaces where they replaced some of the exchangeable framework cations, adsorption on the internal surfaces of the zeolites and possibly precipitation on the surface of the zeolite due to increase in pH [2,22,[42][43][44]. There was an observed increase in the pH of effluent solutions from 5.5 to 6.30.…”

Physicochemical Conditions for Adsorption of Lead from Water by Rice Husk Ash

“…Under certain conditions, there is an increase in removal efficiency with increased pH (Chaouch et al 2014;Zeng et al 2015); at high pH, the adsorbent surface assumes a net negative charge, which favours the attraction and subsequent adsorption of positively charged ions. Cozmuta et al (2012) observed that lead complexes favoured at low pH have smaller hydrated radii and are therefore more mobile than the normal lead ion. Moreover, as pH increases, competition for adsorption sites ensues between the hydrogen ion and the lead ion, thus reducing adsorption efficiency.…”