Alkali treatment of cellulose II fibres and effect on dye sorption

To understand the effect of alkali treatment on sorption behaviour of cellulose II fibres, samples were continuously pre-treated using NaOH over a concentration range of 0.0–7.15 mol dm−3, with varying tension; treated substrates were dyed with hydrolysed C. I. Reactive Red 120. Greatest adsorption of dye occurs for cellulose II fibres treated with 2.53 and 3.33 mol dm−3 aqueous NaOH solution. Correlation to sorption isotherms is most closely associated with a Langmuir type isotherm, but correlation to the Freundlich isotherm is still significant, indicating sorption via a combination of Langmuir and Freundlich isotherms. Adsorption energy (ΔG0) increases with increasing NaOH concentration to a maxima between 2.53 and 3.33 mol dm−3 NaOH and then decreases with further increase in NaOH concentration. Equilibrium dye sorption shows good correlation with water sorption as assessed by the reactive structural fraction (RSF) theory. Theoretical monolayer capacity (q0) increases with increasing NaOH concentration to a maxima at 3.33 mol dm−3 NaOH and then decreases with further increase in NaOH concentration; q0 is significantly in excess of the number of available specific sites (–COO−Na+) in the substrate, indicating non-site-specific interactions, more typical of a Freundlich isotherm. Pores in the fibre significantly affected by alkali treatment (<20 Å diameter) and accessibility of dye (14 Å) sorption into those pores account the differences observed herein; maximum qe, q0 and ΔG0 are observed for cellulose II fibre treated with 2.53–3.33 mol dm−3 NaOH as this concentration range affects the greatest increase in accessible pore volume in the fibres.