An adsorption and kinetic study of lac dyeing on silk

Adsorption and kinetic studies of lac dyeing of silk were investigated using, as optimal conditions, a pH of 3.0, a material to liquor ratio (MLR) of 1:100, an initial dye concentration of 450 mg/L and 60 min contact time. The progress of dye adsorption was monitored at λmax 487 nm by visible spectroscopy. The experimental data fitted well to the Langmuir and Freundlich isotherms with a high correlation coefficient (R2). The pseudo second-order kinetic model was indicated with the activation energy of 47.5 kJ/mol. It is suggested that the overall rate of lac dye adsorption is likely to be controlled by the chemical process. The values of the enthalpy (ΔH#) and entropy of activation (ΔS#) were 44.7 kJ/mol and −175.7 J/mol K, respectively. The free energy of activation (ΔG#) at 30 °C was 97.9 kJ/mol. The activation parameters with and without erythrolaccin in the lac dye were similar, consistent with erythrolaccin only having a slight effect on the lac dyeing of silk. The free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) terms for the lac dyeing were also determined, and the negative values of ΔG° and ΔH° obtained indicated that the lac dye adsorption process is a spontaneous and an exothermic one.