Preliminary study of pH effect in the application of Langmuir and Freundlich isotherms to Cu–Zn competitive adsorption
Introduction
Heavy metals contamination of soils is quantified based on total metal content. Soil trace elements exist as free or complexed ions in soil solution, and adsorbed at the surfaces of the clay, Fe, Al and Mn oxides and oxyhydroxides, Fe sulphides or complexed by organic matter (OM). They are also present in the lattice of secondary minerals such as clay minerals, sulphides, phosphates or carbonates, occluded in amorphous materials such as Fe and Mn oxyhydroxides, Fe sulphides and present in crystal lattices of primary minerals Lake et al., 1984, Tessier and Campbell, 1988.
Soil solution composition and concentration have strong effects on the sorption behaviour of heavy metals, but pH is by far the most significant state variable in that regard (Hornburg and Brümmer, 1993), followed by organic matter content and it is important to know their behaviour and interaction in soils at different pH. Although the essential mechanism to adsorption of these metals by oxyhydroxides of Fe and Mn is pH-dependent specific adsorption increasing with pH Tiller et al., 1984, Spark et al., 1995, heavy metals may be adsorbed by electrostatic attraction in the diffuse double layer passive to ionic exchange (non-specific adsorption).
The relation between the concentration of dissolved and adsorbed heavy metal may be expressed by the Freundlich and Langmuir isotherms. Mutual competition to the adsorption sites depresses heavy metals adsorption Christensen, 1989, Mesquita and Vieira e Silva, 1996, Mesquita and Vieira e Silva, 2000. Murali and Aylmore (1983) applied to multicomponent adsorption models, competitive Langmuir-type equations and Van der Zee and van Riemsdijk (1987) and Welp and Brümmer (1999) “extended” Freundlich isotherms.
In the Langmuir-type equation for multicomponent adsorption
Ci—cation concentration in solution, Si—mount adsorbed per unit of adsorbent, Qi—parameter of maximum adsorption, ki—affinity Langmuir parameters (Murali and Aylmore, 1983). According to Van Riemsdijk et al. (1985), ki is proportional to exp(−qi/RT), qi indicates the free energy change of adsorption of ion species i. The linearized Langmuir equation for monocomponent adsorption is:or, written as a function of the distribution coefficient Kdi=Si/CiFor binary adsorption, Murali and Aylmore (1983) wrote a competitive Langmuir-type equation
Considering three cations Ci, Cj, and CH+,
The first two terms on the right-hand side of , correspond to single species adsorption, and the last terms could be considered as interaction terms (Murali and Aylmore, 1983).
Van der Zee and van Riemsdijk (1987) suggested the inclusion of soil pH and organic C content (oc) in the Freundlich isothermWelp and Brümmer (1999) included other soil characteristics, such as dissolved organic carbon, total organic carbon, pH, pK1, ionic radii, and metal concentration in solution. Elzinga et al. (1999) applied Freundlich isotherms for Cd, Cu and Zn in soils; linearized asSi (mg kg−1)—sorbed metal, k*, a, b, M—equation constants, Ci (mg l−1)—metal concentration in solution, xi—a soil experimental property, βi—regression coefficient related to soil or solution property and β0—the regression constant. Considering two competing cations, the equation can be written asor with pH as a variable(oc), constant for each soil, can be included in k*; N, M—equation constants.
Both competitive Langmuir-type and extended Freundlich equations were applied to study Cu and Zn competitive adsorption at a variable pH attempting to contribute to the understanding of heavy metal behaviour in soils with acidification hazards, polluted by the use of sewage sludge with high content of Cu and Zn as an amendment or fertiliser.
Section snippets
Materials and methods
We worked with samples from the surface layer (0–20 cm) of two Dystryc Fluvisols (A, B), a Haplic Luvisol (C) and a Gleyic Podzol (D). Soil pH was determined in water (1:2.5 w/v soil/solution) by a potentiometric method (Van Lierop, 1990), organic matter (OM) by the Walkey–Black combustion method (Tan, 1996b), and CEC by a modified Melich method (Melich, 1948). Cu and Zn were determined by triacid digestion (Pratt, 1965) and the clay fraction by the pipette method (Tan, 1996a). Table 1 presents
Results and discussion
Table 2 presents the analytical data. These data fitted both Langmuir and Freundlich isotherms for monocomponent adsorption, although with different accuracies for each soil. The experimental data fitted better the shape of the Langmuir isotherms with high determination coefficients (Table 3, Fig 1).
Using simple and multiple regression analysis, the parameters of Langmuir-type and Freundlich-type equations were calculated considering Cu, Zn and H+ as competing cations both in the single and
Conclusions
Data from adsorption reactions fitted either Freundlich or Langmuir monocomponent isotherms, although experimental data fitted better the shape of Langmuir isotherms. Cu and Zn adsorption decreased when both cations were present in solution, due to competition to adsorbing sites.
The results predicted for Cu and Zn adsorption by monocomponent Langmuir equations and by the competitive Langmuir-type equation were not significantly different (Student's t-test P<0.05). However, the results obtained
Acknowledgements
The authors express their appreciation to M. Mouzinho Almadanim, R. Menino, O. Romero and F. Pedra for their help with the analytical work.
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