Sorption Competition and Mechanisms of Desorption of Heavy-Metal Contaminants from Kaolin Clays Using Surfactant Solutions of Sodium Dodecyl Sulfate

Abstract

A new mechanism based on a combination of acid-base interactions and hydro- phobic attraction at the solid-water interface is proposed for the surfactant-enhanced washing of heavy-metal-contaminated kaolin clays. Batch washing mechanisms for kaolin clay suspension, which was chosen as a model system for the environmentally friendly separation of copper (Ⅱ) and nickel (Ⅱ) metal ions, were studied with respect to the sorption competition at the clay interface via acid-base interactions and hydrophobic attraction, transformation of the micellar structure of sodium dodecyl sulfate surfactants, and desorption of heavy metals off the particle surfaces. The sorption onto kaolin clays of heavy-metal ions from single- and binary-metal solutions was observed through isotherm tests conducted at 20℃ and pH 6.5. In binary systems, the adsorption of acidic Cu(superscript 2+) onto the basic surface sites of kaolin clays is four times higher than that of Ni(superscript 2+). Metal-spiked kaolin clays containing both 0.447 mg copper/g kaolin and 0.137 mg nickel/g kaolin were washed out less than 1% of each metal contaminant with water alone at pH 6.5. Experimental results of metal removal were compared under variation of operating conditions, including the surfactant dose, pH, temperature, time, and solid content. A single washing at pH 6.5 with 5000 mg/L of sodium dodecyl sulfate above the critical micelle concentration (cmc) could remove 60% of the nickel and 23% of the copper from kaolin clays. The combination of 5000 mg/L of basic surfactant and 10 mg/L of nitric acid at pH 5.5 was able to remove 63% of the nickel and 60% of the copper. It was then postulated that a three-step washing mechanism for metal removal using the sodium dodecyl sulfate surfactants could proceed through adsorption of basic, anionic surfactant molecules from dissociating micelles onto metal-spiked kaolin clay and self-reassembling into micelles via hydrophobic attraction. In this way, metal-surfactant complexes would be formed with acidic metal cations via acid-base interactions, followed by desorption of micellar metal-surfactant complexes from the kaolin clay into the solution via electrostatic repulsion.