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

Abstract

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.

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

$$\text{S}{}_{\mathrm{i}}\text{=}\text{Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{i}}\text{C}{}_{\mathrm{i}}\text{1+}\text{∑}\text{j}\text{k}{}_{\mathrm{j}}\text{C}{}_{\mathrm{j}}$$

C_i—cation concentration in solution, S_i—mount adsorbed per unit of adsorbent, Q_i—parameter of maximum adsorption, k_i—affinity Langmuir parameters (Murali and Aylmore, 1983). According to Van Riemsdijk et al. (1985), k_i is proportional to exp(−q_i/RT), q_i indicates the free energy change of adsorption of ion species i. The linearized Langmuir equation for monocomponent adsorption is:

$$\text{C}{}_{\mathrm{i}}\text{/S}{}_{\mathrm{i}}\text{=1/(Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{i}}\text{)+1/(Q}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{i}}\text{,}$$

or, written as a function of the distribution coefficient K_dⁱ=S_i/C_i

$$\text{1/K}{}_{\mathrm{<mtext>d</mtext>}}{}^{\mathrm{i}}\text{=1/(Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{i}}\text{)+1/(Q}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{i}}\text{,}$$

For binary adsorption, Murali and Aylmore (1983) wrote a competitive Langmuir-type equation

$$\text{1/K}{}_{\mathrm{<mtext>d</mtext>}}{}^{\mathrm{i}}\text{=1/(Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{i}}\text{)+(1/Q}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{i}}\text{+1/Q}{}_{\mathrm{i}}\text{(k}{}_{\mathrm{j}}\text{/k}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{j}}\text{,}$$$$\text{or}$$$$\text{1}\text{S}{}_{\mathrm{i}}\text{=}\text{1}\text{Q}{}_{\mathrm{i}}\text{+}\text{1}\text{k}{}_{\mathrm{i}}\text{Q}{}_{\mathrm{i}}\text{1}\text{C}{}_{\mathrm{i}}\text{+}\text{k}{}_{\mathrm{j}}\text{k}{}_{\mathrm{i}}\text{Q}{}_{\mathrm{i}}\text{C}{}_{\mathrm{j}}\text{C}{}_{\mathrm{i}}\text{,.}$$

Considering three cations C_i, C_j, and C_H⁺,

$$\text{1/K}{}_{\mathrm{<mtext>d</mtext>}}{}^{\mathrm{i}}\text{=1/(Qk}{}_{\mathrm{i}}\text{)+1/QC}{}_{\mathrm{i}}\text{+1/Q(k}{}_{\mathrm{<mtext>H</mtext>}}{}^{\mathrm{+}}\text{/k}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{<mtext>H</mtext>}}{}^{\mathrm{+}}\text{+1/Q(k}{}_{\mathrm{j}}\text{/k}{}_{\mathrm{i}}\text{)C}{}_{\mathrm{j}}$$$$\text{or}$$$$\text{S}{}_{\mathrm{i}}\text{=}\text{1}\text{Q}{}_{\mathrm{i}}\text{+}\text{1}\text{(Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{i}}\text{)}\text{1}\text{C}{}_{\mathrm{i}}\text{+}\text{1}\text{Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{<mtext>H</mtext>}}\text{k}{}_{\mathrm{i}}\text{C}{}_{\mathrm{<mtext>H</mtext>}}\text{C}{}_{\mathrm{i}}\text{+}\text{1}\text{Q}{}_{\mathrm{i}}\text{k}{}_{\mathrm{j}}\text{k}{}_{\mathrm{i}}\text{C}{}_{\mathrm{j}}\text{C}{}_{\mathrm{i}}\text{,}$$

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 isotherm

$$\text{S}{}_{\mathrm{i}}\text{=kC}{}_{\mathrm{i}}{}^{\mathrm{M}}\text{,}$$

$$\text{S}{}_{\mathrm{i}}\text{=k*(}\text{H}{}^{\mathrm{+}}\text{)}{}^{\mathrm{a}}\text{oc}{}^{\mathrm{b}}\text{C}{}_{\mathrm{i}}{}^{\mathrm{M}}\text{,}$$

Welp 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 as

$$\text{log}\text{S}{}_{\mathrm{i}}\text{=}\text{log}\text{k*+M}\text{log}\text{C}{}_{\mathrm{i}}$$$$\text{with}$$$$\text{log}\text{k*=β}{}_0\text{∑}\text{i=1}\text{M}\text{β}{}_{\mathrm{i}}\text{log}\text{x}{}_{\mathrm{i}}$$

S_i (mg kg⁻¹)—sorbed metal, k*, a, b, M—equation constants, C_i (mg l⁻¹)—metal concentration in solution, x_i—a soil experimental property, β_i—regression coefficient related to soil or solution property and β₀—the regression constant. Considering two competing cations, the equation can be written as

$$\text{S}{}_{\mathrm{i}}\text{=k*C}{}_{\mathrm{i}}{}^{\mathrm{M}}\text{C}{}_{\mathrm{j}}{}^{\mathrm{N}}$$

or with pH as a variable

$$\text{S}{}_{\mathrm{i}}\text{=k*(}\text{H}{}^{\mathrm{+}}\text{)}{}^{\mathrm{a}}\text{C}{}_{\mathrm{i}}{}^{\mathrm{M}}\text{C}{}_{\mathrm{j}}{}^{\mathrm{N}}$$

(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.