Disclosed is a separation device and a method for separating charged particles from a liquid stream. The separation is effected by establishing an ion concentration gradient across the direction of the liquid stream by the introduction of a gas which when contacted with the liquid, in a reversible reaction, forms a soluble ionic species. A concentration gradient is maintained across the direction of the liquid stream which in turn induces separation of charged particles within the liquid stream due to the effect of diffusiophoresis. The device operates using little or no power, and dispenses with the need for filtration media or separation membranes. The device and method is adaptable to any of a number of separation processes, including biological separation processes, water purification and industrial processes.