Evaluation of Anionic and Cationic Pulp-Based Flocculants With Diverse Lignin Contents for Application in Effluent Treatment From the Textile Industry: Flocculation Monitoring

In wastewater treatment, flocculation is a widely used solid/liquid separation technique, which typically employs a charged polymer, a polyelectrolyte (PEL). Polyelectrolytes features, such as charge type, charge density and molecular weight, are essential parameters affecting the mechanism of flocculation and subsequent floc sedimentation. The effectiveness of the process is also influenced by the characteristics of the system (e.g., type, size, and available surface area of suspended particles, pH of the medium, charge of suspended particles). Thus, a good understanding of the flocculation kinetics, involved mechanisms and flocs structure is essential in identifying the most adequate treatment conditions, having also into consideration possible subsequent treatments. In this study, bleached pulp and a cellulosic pulp with high lignin content (~4.5 wt%) obtained from wood waste were used for bio-PELs production. Firstly, a pre-treatment with sodium periodate increased the pulps reactivity. To produce cationic cellulose the oxidation step was followed by the introduction of cationic groups in the cellulose chains, through reaction with Girard's reagent T. Applying different molar ratios (0.975 and 3.9) of Girard's reagent T to aldehyde groups led to cationic PELs with diverse charge density. On the other hand, to obtain anionic cellulose a sulfonation reaction with sodium metabisulfite was applied to the intermediate dialdehyde cellulose-based products, during 24 or 72 h, and anionic-PELs with diverse features were obtained. The developed water soluble, anionic and cationic bio-PELs were characterized and tested as flocculation agents for a textile industry effluent treatment. Initially, jar-tests were used to tune the most effective flocculation procedure (pH, flocculant dosage, etc.). Flocculation using these conditions was then monitored continuously, over time, using laser diffraction spectroscopy (LDS). Due to the small size of the dyes molecules, a dual system with an inorganic complexation agent (bentonite) was essential for effective decolouration of the effluent. Performance in the treatment was monitored first by turbidity removal evaluation (75-88% with cationic-PELs, 75-81% with anionic-PELs) and COD reduction evaluation (79-81% with cationic-PELs, 63-77% with anionic-PELs) in the jar tests. Additionally, the evolution of flocs characteristics (structure and size) during their growth and the flocculation kinetics, were studied using the LDS tec