Abstract
The adsorption of Pb(II) and Cu(II) onto Fe3O4@Mg2Al-NO3 Layered Double Hydroxide (LDH) as a function of Fe3O4@Mg2Al-NO3 LDH concentration was studied. An adsorbent concentration effect ( Cs effect), namely adsorption isotherm declines as adsorbent concentration ( Cs) increases, was observed. The experimental data were fitted to the adsorption models including the classic Freundlich model, the metastable-equilibrium adsorption theory, the flocculation model, the power function model, and the surface component activity model. The results show that the Freundlich-type metastable-equilibrium adsorption equation, the power function model, and the Freundlich-surface component activity equation can adequately describe the Cs effect observed in the batch adsorption tests as all the correlation coefficients ( R2) of the nonlinear plots are higher than 0.96. In other words, their intrinsic parameters simulated from the experimental data are independent of Cs value. It is considered that the Freundlich-surface component activity equation is the best model to describe the Cs effect of the studied adsorption systems by Akaike Information Criterion evaluation criterion.
Highlights
In studies of adsorption at the solid–liquid interface, an anomalous phenomenon of ‘‘adsorbent concentration effect’’ (Voice and Weber, 1985) or ‘‘solids effect’’ (O’Connor and Connolly, 1980; Voice et al, 1983) (Cs effect), namely adsorption isotherm declines as adsorbent concentration increases, has been observed since the 1980s
The nonlinear plots for various Cs values show a unique curve for each adsorption system, and their correlation coefficients (R2) are higher than 0.97. These results indicate that the power function model can adequately describe the Cs effect observed in the two systems
The adsorption of Pb(II) and Cu(II) onto Fe3O4@Mg2Al-NO3 Layered Double Hydroxide (LDH) in the studied conditions is subject to the Cs effect in which the adsorption isotherm declines as the adsorbent concentration increases
Summary
In studies of adsorption at the solid–liquid interface, an anomalous phenomenon of ‘‘adsorbent concentration effect’’ (Voice and Weber, 1985) or ‘‘solids effect’’ (O’Connor and Connolly, 1980; Voice et al, 1983) (Cs effect), namely adsorption isotherm declines as adsorbent concentration increases, has been observed since the 1980s. The classic adsorption model cannot explain this anomalous phenomenon because it was derived on the assumption that the adsorption equilibrium constant is independent of the adsorbent concentration (Cs). Because the substances for which adsorption isotherm apparently declined with increasing Cs include inorganic and organic adsorbates in freshwater and marine sediments, quarts, clays and clays minerals, and digested sewage sludge, it seems unlikely that experimental artifacts could explain widespread agreement of a Cs effect for so diverse a set of adsorbates, adsorbents, and investigators (McKinley and Jenne, 1991). It can be concluded that the Cs effect is an experimental fact not an artifact, and there should be a universal reason that causes the Cs effect it is still not clear
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