We developed a strategy based on reaction-diffusion coupling to achieve selective precipitation from a multicomponent feedstock solution. As proof-of-concept, we demonstrated this approach for a solution of mixed metal salts, namely Mn-Co-Ni chlorides; an important problem in the context of critical materials recovery from recycled electrodes. The solution mixture is placed in a cylinder on top of an agarose hydrogel layer loaded with reacting counterions, in this case sodium hydroxide (Figure 1). As the metal ions diffuse into the gel, crystallization begins to take place in regions of high supersaturation, which locally depletes the ions, to be subsequently replenished by diffusive flux. This interplay of diffusion, nucleation, and growth kinetics results in a spatial unfolding of unique nonequilibrium conditions along the length of the reactor. We observe that the chemical composition of the precipitates showed a gradient along the length of the reactor, ultimately producing almost pure manganese (hydr)oxide beyond a sharp boundary of other mixed phases (Figure 3, unpublished). Note that this separation was accomplished without the use of complex membranes, binding agents, high temperature processing, or even electric fields. The metal ion mixture was simply placed on top of a hydrogel loaded with sodium hydroxide and allowed to 'develop" such that the various metal oxides were formed in order of their precipitation rates as they diffuse into the gel.