Struvite-K crystal growth inhibition by citric acid: Formation of complexes in solution and surface adsorption effects

Abstract

Controlled precipitation of struvite-K is of interest in the treatment of kidney stones, for phosphorous and potassium recovery from wastewaters, and in the chemistry of magnesium phosphate cements. The effect of citric acid in the growth of struvite-K from solution has been studied at room temperature and pH = 9 under different conditions of energy of mixing and concentrations of the reactants. The time-evolution of pH during the precipitation reaction evidenced that the citric acid increased induction time and decreased the apparent growth rates of crystals. Chemical speciation analysis, measurements of citrate concentration, zeta potential and X-ray photoelectron spectroscopy indicated that crystallization is controlled by the formation of complexes with magnesium ions in solution and adsorption of citric acid at the growing surfaces. In analogy with Ca-carbonates and Ca-phosphates, the latter process impairs formation of nuclei of critical size. Crystal habit is dictated by supersaturation conditions and selective adsorption at crystal faces in reason of their residual charge density; consequently, alteration in the crystal shape is observed, and the average size of crystals and the amount of precipitate are reduced.