An environmental waste remediation method is presented where porous cylindrical monoliths are interposed as septa between a solution contaminated with metal ions and a solution of a precipitant. Precipitant and contaminant diffuse and generate precipitate nano- and micro-particles inside the monoliths. Two types of silica sol-gel monoliths were tested. One class of materials was prepared following a conventional base-catalyzed route which yielded fragile silica monoliths with pore diameters on the order of 7-10nm. A second class of materials material consisted of templated silica macroporous monoliths that were cross-linked with diisocyanate. These materials had pore diameters on the order of microns and were mechanically extremely strong, having a Young modulus in excess of 400MPa. Both types of silica gel monoliths proved very versatile, and allowed to precipitate a wide variety of metal ions, including toxic metals such as Cd2+, and fission by-products such as lanthanides and Sr2+. The capacity of the gel monoliths was also very high, at least 20 times higher than the capacity of conventional derivatized gels. Most importantly, precipitation inside the macroporous gels was a factor 7-8 more rapid than in microporous gels. The results indicate that macroporous cross-linked sol-gel monoliths are a promising material for the development of efficient, mechanically strong filter elements for environmental remediation.