Observations on the Fujiwara test for the determination of oxidation in polyamide membranes

The Fujiwara test has been applied for several years to determine if polyamides have been damaged by the combination of halogenated compounds with the polymer network of the aromatic polyamide of reverse osmosis and nanofiltration membranes. In principle, this test is qualitative and, depending on how it is carried out, can even produce erroneous results. The method is essentially based on the reaction of pyridine in a strong alkaline environment, which in the presence of halogens, produces a pink and in some cases, an intense red coloration. The objective of this article is to highlight some aspects of the Fujiwara method that should be taken into account in order to minimise analytical errors. In addition, the article briefly examines the conversion of this qualitative method into a quantitative method for the quantification of halogens combined with the polyamide of membranes.

At seawater or brackish water reverse osmosis plants where aromatic polyamide membranes are used, chlorine is normally employed as a disinfectant. The chlorine is usually dosed in the form of sodium hypochlorite and pH adjustment is generally carried out to obtain the most appropriate concentration of hypochlorous acid (HCIO) and hypochlorite ion (ClO-). In addition to the halogenated compounds produced by chlorine, the most important of which are trihalomethanes (THMs) and chloromines, a small quantity of free chlorine always remains and this persists through the entire pretreatment process until it reaches the membrane racks.

This chlorine remaining in the water is removed by means of sodium bisulphite (NaHSO3) or sodium metabisulphite (Na2S2O5). Chlorine dioxide (ClO2) is also used as a disinfectant and produces by-products such as chlorite ion (ClO2-) and, in some cases, chlorate ion (ClO3-). The ClO2 can be reduced using sodium bisulphite. However, the chlorite and chlorate ions are not easily reduced by the sodium bisulphite.

Enric Palacios Doñaque. Technical Consultant R&D&i Dept. Acciona Agua

Article published in: FuturENVIRO November 2013