Iron-Coated Pine Bark as Biosorbents for Textile Wastewater Treatment: A Sustainable Approach

Dyes are widely used in textile processing and are frequently discharged without adequate treatment, posing risks to aquatic ecosystems through reduced water quality, toxicity to organisms, and long-term environmental degradation. To address the need for sustainable remediation solutions, this study investigated the use of pine bark (Pinus pinaster), an abundant forestry byproduct, as a low-cost biosorbent for textile dye removal. Powdered (<0.5 mm) and granular (>1 mm) bark fractions were washed, dried, and modified through iron impregnation (10 wt.% Fe) via sonication in an FeCl3·6H2O solution, with one iron-coated variant subsequently subjected to thermal treatment at 400 °C under nitrogen (1 h) and hydrogen (3 h). Adsorption performance was evaluated using synthetic effluents containing Sirius Blue, Astrazon Red, and Sirius Yellow, individually and as a ternary mixture (80 mg/L each), with added NaCl and NaHCO3 to simulate realistic conditions. Thermally treated granular iron-coated bark showed the highest removal efficiency, achieving >90% dye elimination within 24 h without detectable iron leaching, along with strong iron retention (~80%) and a 53% thermal-treatment yield. Maximum adsorption reached 15.51 mg/g at 5.0 g/L, while lower adsorbent doses increased capacity (26.8 mg/g) but reduced overall removal (~83%). Kinetic analysis was dose-dependent: the pseudo-first-order model provided the best fit at 5.0 g/L, reflecting the rapid approach to equilibrium, whereas the Elovich model fitted best at 2.5 g/L (R > 0.99), consistent with heterogeneous surface interactions under limited adsorbent availability. These results demonstrate the potential of thermally treated iron-coated pine bark as an efficient and sustainable biosorbent for textile wastewater treatment.