Abstract: A waste stream contaminated with mercury is purified by flowing the stream cyclically and/or sequentially through at least one electrolytic cell provided with an anode (11) and a cathode (10) made of tin, copper, silver or alloys thereof and optionally comprising other alloying element(s). The cathode (10) is a microporous, high surface area body through which the electrolyte can pass, with the solution sweeping the microporous surfaces. After cycling through one cell or cell module (1) with cathodes of given microporosity, the solution is passed to the next cell or cell module with cathodes of finer miocroporosity. The microporous surfaces are periodically cleaned by counter-current flushing, preferably followed by a rest period. The waste stream may be an acidic waste stream containing organic mercury compounds.