A biocidal fluid purification cap (2) for a fluid container (1), preferably one storing purified water for use in laboratory environments and especially purified water for cell culture and water for molecular biology. The purification cap (2) includes a cap body (3) including an engagement feature (20) for removably attaching the cap body (3) to a mating engagement feature on a spout of the fluid container (1) to hermetically close a spout opening, at least one LED (7a) adapted to emit light in the UV-C range, electronic circuitry (15) for driving the LED(s) (7a), and a power supply for the electronic circuitry (15). The LED(s) (7a) is/are arranged in said cap body (3) so as to be separated from the environment by an UV-transparent window (6) provided in the cap body (3) such that light emitted from said LED(s) (7a) enters the opening of the spout when the cap (2) is attached to the spout of the container (1).