Investigation of some aspects related to the degradation of polyamide membranes: aqueous chlorine oxidation catalyzed by aluminum and sodium laurel sulfate oxidation during cleanin

The investigations are related to some problems that arose in a demineralization water plant located in southern Brazil: membrane degradation, and decline of membrane performance after cleaning. The water pretreatment of this plant utilizes aluminum sulfate in order to coagulate suspended solids, and gaseous chlorine proceeds the disinfection. The chemical cleaning procedure of the membranes adopted in the industrial process uses two solutions, an acid and an alkaline prepared with sodium hydroxide and sodium laurel sulfate. This paper details bench-scale experiments used to determine the harmful effects of aluminum in catalyzing free-chlorine polyamide membrane oxidation. The study of the effect of the alum on chlorine membrane oxidation was undertaken by comparison with that when iron is present since both metals have several similarities. The experiments were carried out in a RO unit, utilizing commercial polyamide membranes and feed solutions with 500 ppm of aqueous chlorine with different concentrations of aluminum or iron (0.05, 0.1 and 0.26 ppm). This high chlorine concentration was used in order to evaluate its effect on membranes in a short period of time. Permeate flux and salt retention were measured during the experiments to determine the effect of chlorine on the performance of polyamide membranes. The experiments with residuals of alum or iron were performed under the same operating conditions and the results obtained compared qualitatively. Additional experiments were conducted to evaluate the effect of sodium laurel sulfate during the cleaning procedure of non-degraded and degraded membranes, and it was observed that the performance of degraded membranes diminished after cleaning.

Oxidation of membranes is a complex phenomenon due to the large diversity of substances present in the feed water, which may interfere to some extent. The results of this work demonstrate that users should pay attention to membrane limitations in order to improve the lifetime of membranes—not only with aqueous chlorine, but with all oxidants that could be used. The following conclusions are drawn from the experimental results obtained: C Although the exact reaction mechanism was not investigated, residuals of chlorine and aluminum and/or iron in the feed water of a RO system can increase the oxidation potential of aqueous chlorine, which means that they had the reaction rates were increased, increasing the permeate flux and reducing the retention of the selective layer. C Although aluminum is not a transition metal, the results obtained in the experiments in this work showed an influence slightly superior to the degree of oxidation by free chlorine than when residuals of iron are present. C The oxidation rates by chlorine increased as the residual concentration of iron and aluminum became higher. The membranes were more rapidly oxidized with higher metal concentration. Thus, the oxidation rates were proportional to the quantity of iron and aluminum present, indicating the suggested catalytic effect of these metals in the oxidation process. C The ability of SLS to increase the damage of membranes was examined. This demonstrated strong effects on degradation and shows that surfactants should be used carefully.