Influence of clay minerals, used as supports in anaerobic digesters, in the precipitation of struvite

Abstract

This work studies the influence of clay minerals used as supports in anaerobic digesters on the precipitation of ammonium magnesium phosphate. The magnesium exchange cation present in the sample originally or from experimental saturation plays an important role in the precipitation of struvite crystal growth depending on the greater or lesser facility of its liberation to the medium. Vermiculite, and probably stevensite, influence the formation of struvite, transferring the magnesium cation only from their exchange positions, while sepiolite does so by contributing from its structure. In vermiculite, struvite crystal formation is on the edges or fractures of the laminas. The presence of other cations in the supports, such as exchange Ca 2+—either structural or as a component of accessory minerals, as in the case of stevensite—causes precipitation of calcium ammonium phosphate. In minerals with a high iron content, such as nontronite, iron phosphates are produced. The type of digester influences the size of the struvite crystals, these being larger in the semi-continuous digesters.