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The main purpose of this work is the study of the viability of a polyelectrolyte enhanced ultrafiltration (PEUF) process for the chromate recovery using poly(diallyldimethylammonium chloride) (PDADMAC) as water-soluble polymer. First of all, the interaction between PDADMAC and chromate ions was studied by potentiometric titrations of different poly(acrylic acid) (PAA) solutions in which its acidity constant (pKa) was determined using the Henderson–Hasselbach equation. This interaction is linked to the influence of the presence of PDADMAC and chromate in the PAA pKa value. Next, the operation conditions (pH, temperature and transmembrane pressure) that optimize the membrane hydrodynamic behaviour were studied with ultrafiltration runs in mode operation of total recirculation. Furthermore, working at the operation conditions determined in the previous stage and with solutions of PDADMAC and chromate, the influence of ionic strength on design parameters was studied. Finally, the viability of a chemical method to regenerate the polyelectrolyte, based on the addition of NaCl and subsequent ultrafiltration, was checked. We conclude that this method is not suitable, mainly because of the great amount of water spent in the diafiltration stage necessary to reduce the polyelectrolyte solution chloride concentration.

It has been demonstrated the existence of an interaction between PDADMAC and chromate anions by potentiometric titrations. Due to the concordance between characterization and ultrafiltration results, we conclude that the proposed semi-quantitative method, apart from been a simple approach, can be potentially applied to determine formation constants between cationic polyelectrolytes and anions and, therefore, be used as a useful tool in prediction and modelling of PEUF processes results. With respect to the ultrafiltration experiments results, on the one hand, it has been studied and optimised the chromate retention stage using PDADMAC as water soluble polyelectrolyte in a PEUF process. Chromate rejection coefficients up to 0.99 and permeate fluxes of around 200 L h−1 m−2 have been reached working at 50°C, 4 bar of transmembrane pressure and a PDADMAC and chromate concentration of, respectively, 0.1% w/w and 50 ppm. Furthermore, this polymer demonstrates very much higher affinity to bind chromate than chloride ions, even with a very much higher concentration of the latter and it does not lead to neither concentrationpolarization nor important fouling. These results demonstrate the complete technical viability of the PEUF process to retain chromate ions. On the other hand, it has been checked that the chemical regeneration stage by addition of NaCl is not a viable option to carry out the polymer regeneration. Two main reasons can be given to