Preliminary study of pH effect in the application of Langmuir and Freundlich isotherms to Cu–Zn competitive adsorption

Assessing the behaviour of Cu and Zn in adsorption reactions in soils and the associated risks to soil and water pollution requires the use of sorption equations. Therefore, Langmuir and Freundlich isotherms were derived for Cu and Zn applied to soil samples of the surface layer (0–20 cm) of two Dystryc Fluvisols, a Gleyic Podzol and a Haplic Luvisol. The adsorption data obtained fitted both Langmuir and Freundlich isotherms. The goodness of fit was evaluated by the determination coefficient R2. The effect of pH on Cu–Zn competitive adsorption was quantified using competitive Langmuir-type equations as well as the extended Freundlich equations. The results obtained for Cu–Zn adsorption with the monocomponent Langmuir equation or with the competitive Langmuir-type equation were not significantly different according to the Student's t-test. Conversely, when Cu and Zn were added to the soil samples by the binary solution, the results from the Freundlich monocomponent equation were significantly different (Student's t-test) from those obtained with the extended Freundlich equation. The competitive Langmuir-type and extended Freundlich equations predicted for Cu and Zn both in single and binary solutions values in good agreement with the experimental data, confirming the interest of the introduction of a pH term.