Adsorvente de nitrogênio amoniacal a partir do amianto crisotila e ácido fosfórico

Abstract

Adsorbent of ammonium nitrogen from chrysotile and phosphoric acid was synthesized. It was used molar proportions of chrysotile:H3PO4 1:1, 1:2 and 1:3 that was named as methods 1, 2 and 3, respectively. In method 1, the reagents were mechanically stirred by 6 hours at 50 °C and the solid resultant was washed with water. In methods 2 and 3, the reactants were held in contact at 90 ºC until the water of the reactive medium evaporate, and the solids were thermally treated at 105 ºC until constant weight, and then were washed with water and heated at 150 °C for 6 h. This temperature was determined from an endothermic peak observed by differential scanning calorimetry (DSC). It was observed that the best solid was obtained by method 3 due its highers mass gain and adsorption capacity and lower solubility. This solid was characterized by Scanning Electron Microscopy with X-ray Dispersive Spectroscopy (SEM-EDX), Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric Analysis (TGA) and pHPZC determination. These analysis indicated the formation of the hydrated magnesium phosphate Mg3(PO4)2.22H2O which after calcination converts to magnesium pyrophosphate (Mg2P2O7), mixed with amorphous silica from the decomposition of chrysotile. In the kinetic adsorption tests it was verified that the process is favored at pH 10 following a pseudo-first order model with activation energy of 8329 J mol-1, indicating physical interaction. The Hill Sigmoidal model was the best fit for the isotherms, demonstrating two phenomena: complexation until the inflection point KH between 10 and 35 mg L-1 and then the adsorption with qmax = 19,6 mg g-1. The thermodynamic parameters ΔH = 12.182 J mol-1 and ΔS = 75.42 J mol-1 K-1 and the negative values for ΔG evidencing that the adsorption is spontaneous, endothermic and there is an affinity between the adsorbent and the adsorbate. In the tests with effluent were noted the removal of 85% of N-NH3, 24% of COD, 47% of color, 50% of turbidity, and pH between 7.5 and 8.5. After the adsorption procedure there was a mass loss of 5% of the material, which does not refer to the loss of magnesium since this metal ion was not detected in the resulting solutions. In the reuse adsorption test the adsorption capacity was mantained after two turn overs. Therefore, the adsorbent synthesized by method 3 proved to be efficient for the removal of N-NH3 in solution and in ETE effluent, making it an attractive application for chrysotile asbestos.