Chlorophyllin-immobilized chitosan/xanthan gum composite as an adsorbent and visible-light photosensitizer for the removal of genotoxic azo, anthraquinone and triphenylmethane dyes.

The discharge of genotoxic synthetic dyes, particularly from complex tannery wastewater, poses severe threats to aquatic ecosystems and public health. To overcome limitations of conventional single-mechanism treatments, a novel dual-action adsorbent and visible-light-responsive photosensitizer was synthesized by immobilizing sodium copper chlorophyllin (5.0% w/w) within a cross-linked chitosan/xanthan gum polyelectrolyte complex. Characterisation confirmed an amorphous, mesoporous framework with a specific surface area of 40.32 m2 g-1 (after lyophilization) and enhanced thermal stability up to 280 °C. Diffuse Reflectance Spectroscopy showed chlorophyllin incorporation narrowed the direct band gap from 4.50 eV to 3.68 eV, facilitating efficient visible-light harvesting. The composite exhibited sequential dark adsorption and visible-light photocatalytic degradation of six structurally diverse dyes present in leather tanning effluents. Adsorption followed pseudo-second-order kinetics and the Langmuir isotherm, achieving a maximum monolayer capacity of 205.55 mg g-1 for Malachite Green at 298 K. Under visible-light irradiation (λ > 420 nm, 150 mW cm-2), pre-concentrated dyes underwent rapid oxidative degradation (k = 0.0209 min-1 for Acid Black 210 at 25 mg L-1), mainly through continuous Type II singlet oxygen (1O₂) generation (0.080 min-1, ∼2.3-fold higher than superoxide). Overall removal reached 96.9% for Acid Black 210 and remained >85% after 10 cycles with only 11.5% cumulative photosensitizer leaching. Simulated tannery wastewater treatment achieved 85.4% Total Organic Carbon mineralization, 66.4% Cr3+ removal, and marked toxicity reduction (Vibrio fischeri inhibition: 91.4%-10.4%; Daphnia magna mortality: 100%-10.5%). These findings establish a scalable, sustainable remediation material outperforming biopolymer-based adsorbents and photocatalysts for detoxifying and mineralizing recalcitrant dye-contaminated industrial effluents.