Glyoxal-modified chestnut tannins as sustainable coagulants for textile wastewater treatment

Developing sustainable treatment technologies for textile effluents is essential to ensure safe wastewater discharge and protect freshwater resources. Coagulation–flocculation is widely applied due to its simplicity and low cost, but it is still largely limited to the use of conventional chemical coagulants. Natural alternatives, particularly tannin-based coagulants, have gained attention over the past decade. However, their production typically relies on formaldehyde, a toxic reagent, highlighting the need for safer options. Glyoxal has emerged as a promising, eco-friendly substitute. In this study, chestnut shell (CS) tannins were aminomethylated with glyoxal and NH 4 Cl to confer cationic properties, confirmed through zeta potential and compositional assessments. The coagulation performance of the synthesized products was compared with Tanfloc SG, formaldehyde-modified CS tannins, and aluminum sulfate, showing similar effectiveness in contaminant removal. Seven coagulants (G1–G7) were prepared by varying tannin, NH 4 Cl, and glyoxal ratios. Coagulants produced with 1:1:4 and 1:2:4 M ratios (T:NH 4 Cl:Glyoxal) achieved 33–61% color reduction in batch experiments and 23–39% in continuous flow tests at 500 and 250 mg·L −1 , respectively, in real textile effluents. However, at these dosages, the coagulants increased the initial nitrogen concentration and the dissolved organic carbon (DOC) because of their organic nature and the use of NH 4 Cl during their production. These findings demonstrate the potential of glyoxal-modified tannins as alternatives to conventional chemical coagulants for textile effluents, following further optimization and validation. • Glyoxal was proposed as an alternative reagent for producing natural coagulants. • Glyoxal offers several economic and environmental advantages over formaldehyde. • Glyoxal-based coagulants were effective in removing color from textile wastewater. • Formaldehyde yielded a more efficient coagulant at lower dosages.