Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

Abstract

In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu(II) and organic acid, the system pH, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. Phthalic acid affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with pH. The adsorption of Cu(II) induced by organic acids increased with increasing pH and reached a maximum value at approximately pH 4.5, and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing pH; it reached a maximum value at pH ∼5.25 for control and salicylic acid systems and at pH ∼5.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils.