Direct Blue 71 removal by tamarind shell-supported Phanerochaete chrysosporium: synergistic biosorption and biodegradation in batch and fixed-bed systems

Abstract

The increasing discharge of textile dyes into water bodies poses significant environmental and public health challenges. This study developed and evaluated a synergistic system to remove Direct Blue 71 (DB71), a recalcitrant triazo dye, from aqueous solutions by combining biosorption onto tamarind shell (Tamarindus indica, TS) with biodegradation by Phanerochaete chrysosporium (Pc) immobilized on TS. Both individual and combined systems (TS+Pc) were assessed in batch and continuous fixed-bed column configurations. FTIR and SEM-EDS analyses confirmed that TS has a porous, heterogeneous surface rich in hydroxyl, carboxyl, carbonyl, and amino functional groups; post-adsorption detection of sulfur, absent in the untreated material and attributable only to the dye's sulfonate groups, provided direct evidence of DB71 binding. The pseudo-second-order model best described biosorption kinetics. Maximum DB71 removal (69%) was achieved at pH 3 with a biosorbent dose of 20 g/L, with ~54% still removed at near-neutral pH 7, indicating effective operation without strong acidification. The equilibrium data in the concentration range studied were best fitted by the Freundlich model at 25 and 35 °C and by the Langmuir model at 40 °C, in line with adsorption on an energetically heterogeneous surface. Since no saturation plateau was obtained, the fitted Langmuir Q_o (13.66 mg/g at 25 °C and 15.73 mg/g at 40 °C) represents an operational capacity for this range rather than a monolayer capacity. Thermodynamic analysis indicated a spontaneous process (ΔG° = −33.3 to −35.5 kJ/mol), with uptake increasing with temperature. P. chrysosporium successfully colonized TS within 6–7 days and decolorized DB71 on both nutrient-rich and nutrient-free solid media. In batch experiments, the TS+Pc system achieved decolorization efficiencies of 98%, 76%, and 67% at DB71 concentrations of 10, 50, and 100 mg/L, respectively, substantially outperforming the fungus alone (68%, 49%, and 16%). In fixed-bed columns, inoculation of TS with Pc extended the breakthrough time from 25 to 82 min and increased the bed capacity at breakthrough from 2.51 to 8.39 µg/g, a 3.3-fold gain, with the Yoon-Nelson model providing an adequate operational description of the column dynamics (R² = 0.9928). These results demonstrate that tamarind shell functions simultaneously as a primary biosorbent and a nutrient-rich substrate that sustains fungal ligninolytic activity, yielding a synergistic, low-cost, and sustainable approach for treating azo dye–contaminated textile wastewater.

Keywords

Direct Blue 71 / tamarind shell / Phanerochaete chrysosporium / biosorption / biodegradation / fixed-bed column / azo dye / textile wastewater

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