Study on the adsorption kinetics of natural dye by colored paper fiber raw material

To ensure stable quality of colored cigarette paper dyed with natural dyes, real-time kinetic data of dye adsorption onto cellulosic fibers remains insufficient due to limitations of traditional offline sampling methods. In this work, brown natural dye was used as the adsorbate, and an online UV-Vis spectral monitoring system was developed to achieve continuous, non-destructive detection of the full adsorption process without intermittent sampling. The effects of initial dye concentration and fiber type on the adsorption behavior and mechanism were systematically investigated. All experiments were conducted at 20 °C with a pulp consistency of 0.2% (w/v), and the initial dye concentration ranged from 10 to 80 mg/L. Pseudo-first-order (PFO) was applied for data fitting. The adsorption process exhibits two successive stages: a water absorption-swelling induction period and a dye adsorption stage. A competitive adsorption mechanism between water molecules and dye macromolecules on fiber surfaces is proposed to explain the induction period. The induction time decreases exponentially with increasing initial dye concentration, following (y = 6,998*e^-0·⁰⁴⁷ˣ) with (R² = 0.987). Within 30-80 mg/L, the equilibrium adsorption capacity (q_e) increases linearly with dye concentration. For hemp pulp fiber at 80 mg/L, (q_e) reaches 1.1184 mg/g, the PFO rate constant (k₁) is 0.0012 s^-1 with (R² = 0.9980). The PFO model shows better fitting performance for all systems, indicating a diffusion-controlled physisorption process. Hardwood pulp reaches equilibrium at ~1,500 s with the fastest initial rate, while softwood and hemp pulp require over 10000 s. This work provides fundamental kinetic data and theoretical support for formula design and process optimization of online dyeing for colored cigarette paper.