Adsorption Characteristics of Direct Violet Dye by Natural Rubber Chips

Abstract

The study examined the adsorption behaviour of Direct Violet 51 (DV 51) dye from aqueous solution onto natural rubber chips. The effects of various operational parameters such as adsorbent dose, contact time, pH and dye concentration were investigated using the batch adsorption technique. The natural rubber sheet (thickness 2mm) was prepared by compression moulding and cut into 5 mm square pieces. The point of zero charge (pHpzc) of the rubber chips was determined using the pH drift method. Equilibrium adsorption data were analysed by means of Langmuir and Freundlich isotherms, and adsorption data by spectrophotometer. It was found that the optimum adsorbent dose was 3g and optimum contact time was 120 minutes. The highest removal efficiency was found at pH 7. The explanation may be understood in terms of the pHpzc of the rubber chip (pHpzc=7.45), at which point the rubber surface is neutral. The surface charge of the rubber chip is therefore positive when the solution pH is below pHpzc. Because of its basicity, Direct Violet therefore tends to be adsorbed onto the surface of natural rubber chips. In addition, at pH 7 common salt (NaCl) was also found, which is believed to reduce the repulsion between the rubber chips and the dye, allowing the dye to diffuse more easily on to the surface of the rubber chip fibres, thereby improving their dyeability. The adsorption data correlated well with the Langmuir isotherm model, suggesting the monolayer adsorption behaviour.