Evaluation of Walnut Shell and Eucalyptus Bark Extracts as Natural Pretreatment Agents for Methylene Blue Removal from Synthetic Wastewater

Dye-containing wastewater poses environmental risks and requires treatment alternatives that are based on renewable and low-cost materials. This study evaluated saline extracts obtained from walnut shells and eucalyptus bark as biocoagulants for apparent color removal from synthetic wastewater containing methylene blue as a model cationic dye at an initial concentration of 10 mg L−1. The effects of extraction time, extraction pH, and salt concentration were investigated using Response Surface Methodology and a three-factor Box–Behnken design. Apparent color removal was subsequently evaluated at biocoagulant dosages of 0.5, 1.0, and 1.5 mL per 100 mL of synthetic wastewater. Walnut shell and eucalyptus bark extracts were prepared using NaCl and KCl, respectively, and apparent color removal was evaluated after the interaction with the dye. Walnut-shell extracts achieved a maximum apparent color-removal efficiency of 47% after 24 h of extraction at pH 6, using 0.5 M NaCl and a dosage of 1.0 mL per 100 mL. Eucalyptus bark extracts achieved a maximum apparent color-removal efficiency of 48% after 12 h of extraction at pH 7, using 1.0 M KCl and a dosage of 1.5 mL per 100 mL. Although the maximum apparent efficiencies were similar, eucalyptus bark showed greater statistical consistency and better predictive model performance. Both residues demonstrated potential as renewable materials for apparent color removal, particularly as pretreatment agents or components of combined wastewater treatment systems.