Removal of nodularin from brackish water with reverse osmosis or vacuum distillation

Abstract

Reverse osmosis and vacuum distillation are commonly used for desalination when purifying drinking water from sea or brackish water. The ability of these methods to remove nodularin, a hepatotoxin produced by the cyanobacteria Nodularia spumigena, was investigated. For this purpose, natural brackish water was contaminated with toxic algal material. Nodularin concentrations of raw and purified water were determined after a solid-phase extraction procedure employing high-performance liquid chromatography. In the case of reverse osmosis (Freshwater R MT 5000B) the initial nodularin concentration was 10.0 μg/litre, but, after a concentration programme which included recycling the wastewater and discharging the purified water, the final nodularin concentration after 19.5 h was 32.3 μg/litre. The nodularin concentration of the purified water always remained below the limit of quantitation, which was 0.5 μg/litre, but, by the end of the experiment, when both the nodularin and salt concentrations had increased threefold, traces of nodularin were detected. The specific conductivity of the purified water increased simultaneously. In the case of vacuum distillation, the nodularin concentration of brackish raw water was 70 μg/litre and the test was performed using a Rotavapor R-114 instrument at 60°C to simulate an existing purification method. No nodularin was detected in the distillate. Both reverse osmosis and vacuum distillation removed nodularin effectively. When brackish water is to be purified by reverse osmosis, conductivity measurement can be used to monitor the effectiveness of the purification result.