Tailoring CaCO3 microstructure to improve trace phosphate removal from water

The phosphorus raw material is in crisis, and the demand for P-fertilizer production is increasing, therefore different sources are sought. The trace concentration of phosphorus in treated domestic wastewater is a potential point of recovery as an alternative source of phosphate solids. In this study, we synthesized CaCO3 with a specific microstructure with an enhanced kinetics for the removal of trace concentrations of phosphate from a model solution. The crystalline morphology of the materials was tailored by the addition of organic additives into the CaCO3 precipitation solution. We synthesized CaCO3 by mixing Na2CO3 and CaCl2 water solutions and different concentrations of citrate (from 3 to 152 mM, equal to 1.2, 11.3 and 50.6 % citrate/Ca2+) as morphology modifiers. Samples were characterized by SEM, TEM, XRD, and nitrogen physisorption. The crystalline structure of all materials corresponds to calcite crystals; however three main properties: the crystalline habit, crystallite size, and surface area, change in function of the citrate concentration used in the synthesis. Particularly, CaCO3 synthesized in a 50 % citrate/Ca2+ (sample C3) consists of agglomerates of porous peanut-shaped calcite crystals, with a crystallite size of 28 nm, and a surface area of 29.8 m2/g. Selected samples were tested for the removal of phosphate from water in batch and column experiments. In batch experiments, sample C3 was able to reduce 80 % of the phosphate within minutes from a solution with an initial concentration of 2.4 mg P/L, using a CaCO3 dose of 1.3 g/L. Sample C3 was tested for the removal of phosphate in a fluidized bed column system. The spent material showed calcium phosphate precipitates as verified by XRD characterization, revealing a surface pre-cipitation removal mechanism.