Flocculation studies of precipitated calcium carbonate induced by cationic polyacrylamides (C-PAMs) were carried out using light diffraction scattering (LDS). The effect of both polymer charge density and concentration on the flocculation process and on flocs density was investigated. As expected, results show that high charge density C-PAM induces flocculation by bridging and patching mechanisms simultaneously, while medium charge density C-PAM acts mainly according to the bridging mechanism. Consequently, the mass fractal dimensions of the flocs produced by high charge density C-PAM are higher. Results also show the effect of flocculant concentration: flocculation rate decreases and denser flocs are obtained as flocculant concentration increases. The results obtained so far allowed a preliminary quantitative evaluation of flocculation kinetics. In the flocculation curve, two regions corresponding to different kinetics were identified: a first region dominated by particle aggregation and a second region dominated by flocs stabilization. Therefore, LDS is considered a useful tool to evaluate flocculants performance. A strategy was developed that resulted in the use of LDS to retrieve, in a single test, information on the evolution with time of flocs dimension and structure, flocs resistance and flocculation kinetics. All the tests were performed under turbulent conditions similar to the ones prevailing in process equipment.

In papermaking, mill water closure may result in a significant increase of inorganic salts in the white water. The effect of these contaminants on the performance of retention aids was evaluated, in this study, through flocculation of Precipitated Calcium Carbonate (PCC) with three very high molecular weight cationic polyacrylamides (C-PAM) of medium charge density and with different degrees of branching. Furthermore, flocs resistance and reflocculation capacity was also investigated when 2 different types of shear forces were applied. Tests were carried out in distilled water and in industrial water containing a high concentration of cationic compounds. The flocculation, deflocculation and reflocculation processes were monitored by evaluating particle size distribution using a light diffraction spectroscopy technique. Additionally to the effect of the cationic content of the medium, the influence of the flocculant dosage and degree of polymer branching were also studied. The effect of these parameters on the flocs structure was estimated by determining both the mass fractal dimension and the scattering exponent of the aggregates. The results show that the presence of inorganic salts affects significantly the performance of the polyelectrolytes. The flocculation kinetics is faster but the required flocculant dosage is higher when the suspending medium is industrial water. The cationic entities affect also the flocs structure because they reduce the reconformation of the polymer during flocculation. Additionally, in industrial water, flocs become more resistant and this effect is more pronounced as the flocculant branching decreases. In the case of the linear polymer, this effect is not so obvious because reconformation is less pronounced due to its molecular structure. Reflocculation capacity of flocs is very reduced both in distilled and industrial water.

In a previous paper we have shown the added value of using LDS to monitor flocculation. It can supply, simultaneously, information on flocs size and structure and enlighten flocculation kinetics and mechanisms. In this paper, LDS is applied to study deflocculation and reflocculation processes of precipitated calcium carbonate (PCC) induced by cationic polyacrylamides, when different types of shear forces are applied. LDS can detect the influence of polymer characteristics and concentration as well as of the type of shearing, on flocs resistance and reflocculation degree, which depend on flocs structure and on the type of bonds between particles. As expected, flocs formed by bridging mechanism reflocculate with difficulty while flocs formed by patching reflocculate to a higher degree. Flocs resulting from reflocculation are more compact than the original ones, as assessed by the mass fractal dimension. Reflocculation is also lower when the flocs are submitted to superficial shearing than when they are submitted to sonication. Shearing induced by sonication is sufficient to break down the flocs in many fragments while the increase of pump speed only detaches particles by erosion, at the flocs surface, where bonds are weaker. Results prove that LDS is useful to monitor deflocculation and reflocculation processes and to predict floc resistance under different conditions. Moreover, the whole study demonstrates the benefit of using LDS for a complete evaluation of flocculants performance in the different stages of flocculation: aggregation, stabilization, deflocculation and reflocculation.

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, Eucalyptus bleached pulp and a cellulosic pulp with high lignin content (∼4.5 wt%) obtained from Eucalyptus 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) Grenda et al.Evaluation of Anionic and Cationic Pulp-Based Flocculants 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 technique, applying the different PELs produced and for a range of PEL concentration. The results obtained through this monitoring procedure allowed to discuss the possible flocculation mechanisms involved in the process. The results obtained with the bio-PELs were compared with those obtained using synthetic PELs, commonly applied in effluents treatment, polyacrylamides. The developed bio-PELs can be competitive, eco-friendly flocculation agents for effluents treatment fro...

The present work aimed at evaluating the toxicity and genotoxicity of two organic (vesicles composed of sodium dodecyl sulphate/didodecyl dimethylammonium bromide-SDS/DDAB and of monoolein and sodium oelate-Mo/NaO) and four inorganic (titanium oxide-TiO₂, silicon titanium-TiSiO₄, Lumidot-CdSe/ZnS, and gold nanorods) nanoparticles (NP), suspended in two aqueous media (Milli Q water and American Society for Testing and Materials (ASTM) hardwater), to the bacteria Vibrio fischeri (Microtox test) and Salmonella typhimurium-his⁻ (Ames test with strains TA98 and TA100). Aiming a better understanding of these biological responses physical and chemical characterization of the studied NP suspensions was carried out. Results denoted a high aggregation state of the NP in the aqueous suspensions, with the exception of SDS/DDAB and Mo/NaO vesicles, and of nanogold suspended in Milli Q water. This higher aggregation was consistent with the low values of zeta potential, revealing the instability of the suspensions. Regarding toxicity data, except for nano TiO₂, the tested NP significantly inhibited bioluminescence of V. fischeri. Genotoxic effects were only induced by SDS/DDAB and TiO₂ for the strain TA98. A wide range of toxicity responses was observed for the six tested NP, differing by more than 5 orders of magnitude, and suggesting different modes of action of the tested NP.