Otimização da síntese de estruvita e seus análogos visando a recuperação de fósforo, magnésio, nitrogênio e potássio de águas residuais

Abstract

Domestic and industrial wastewaters, untreated or minimally treated, are rich in macronutrients that can cause eutrophication if they are released without treatment into the environment. Currently, domestic sewage has been studied as a material source for nutrient recovery, such as struvite precipitation. Struvite is an ammonium-magnesium phosphate hexahydrated (NH4MgPO4.6H2O) formed in equimolar proportions in alkaline aqueous systems. Depending on formation physicochemical conditions (ion concentration, pH and temperature), precipitation of both struvite and the named analogous struvites, with several possibilities of isomorphic substitutions, can occur. The present work aimed the synthesis, characterization, and optimization of struvite crystallization and its analogues for nutrient recovery from wastewater solutions containing magnesium (MgCl2.6H2O), phosphorus (K2HPO4) and nitrogen (NH4Cl); and also perform the mineralogical quantification of the struvite phases, the detailing of crystallochemical aspects, including the determination of the chemical formula of the best synthesis condition obtained using the combined Rietveld refinement method and Internal Standard of the Fullprof program with the FULL graphical interface. The statistical plans were carried out: I - Tree Diagram and II - 23 Full Factorial Design (DOE), duplicated with four central points, using the response surface methodology and the global desirability function to establish the most significant responses for struvite yield. The variables studied were pH (10, 11 and 12), phosphorus concentration (100, 200, and 300 mg L-1), and molar ratio Mg: P: N (1: 1: 1; 1: 1: 1.5; 1: 1: 2), with three response variables: (1) enthalpy variation (ΔH) of decomposition peaks of synthesis products by differential scanning calorimetry (DSC); (2) infrared percent transmittance (% T – IR) at 1073.17 position, which refers to the asymmetric stretching band of the P-O bond from phosphate (PO4 3-) and (3) variation of total mass loss Δm (%) in thermogravimetry (TG). The most significant factors were revealed (pH 10, P concentration = 300 mg L-1 and Mg:P:N molar ratio = 1: 1: 2). The most favorable condition for struvite precipitation, called condition 01 and duplicated (A, B, C, and D), was investigated in details. For this condition, the Rietveld refinement was performed. This technique proved that the generated struvites were of the analogous type with ammonium (NH4 +) substitution by potassium ion (K+). It was quantified an amount of 20.34% of struvite and 79.66% of analogous struvite (K-struvite). Mineralogical quantification was performed using the Rietveld refinement technique combined with the Internal Standard method, the obtained data were: 74.3% of analogous struvite, 16.6% of struvite, and 9.1% of amorphous material. Results were ratified by the analytical techniques XRF, SEM / EDS, DSC / TG, FTIR, and SSABET which proved to be efficient for the confirmation of the multiple phases identified in analogous struvites revealed by the Rietveld refinement.