RESUMO
Aluminum-based coagulation has long been regarded as a reliable and cost-effective process for the defluoridation of industrial effluents. However, such a well-recognized viewpoint is challenged by the underestimation of fluoride levels in treated effluents. Herein, we developed a systematic protocol to distinguish different fluoride species, including free F-, exchangeable fluoride (EF), and nonexchangeable fluoride (NEF). We demonstrated that EF forms complexes with octahedral aluminum (AlO6) on the surface of polyaluminum and can be exchanged with (1,2-cyclohexylenedinitrilo)-tetraacetic acid (CDTA). However, NEF is incorporated with tetrahedral aluminum (AlO4) at the core of polyaluminum, as confirmed by 19F/23Al NMR and ESI-MS analysis, and cannot be exchanged with CDTA due to steric hindrance. Increasing the aluminum coagulant dosage effectively reduced free F- levels in photovoltaic and electroplating effluents to below 1 mg/L. However, the total fluoride content, with over half in the form of EF and NEF, was above 2 mg/L, exceeding the discharge limit regulated by many local governments of China. Furthermore, both EF and NEF can gradually transform to free F- in natural waters. Our findings indicate that aluminum-based coagulation inevitably accompanies the formation of substantial amounts of EF and NEF, compromising its defluoridation efficiency toward industrial effluents.
Assuntos
Alumínio , Fluoretos , Alumínio/química , Fluoretos/química , Purificação da Água/métodosRESUMO
Metal oxides (MOs) having Mg and Al with Mg/Al ratios of 1, 2, 3, and 4 were synthesized via calcination of the layered double hydroxides (LDH). The X-ray diffraction analysis revealed that all the MO consisted of periclase (MgO) crystallite with comparable crystallinity regardless of the metal ratio. According to the 27Al magic-angle spinning nuclear magnetic resonance, the phase transformation from LDH to MO upon calcination facilitated the evolution of the Al3+ ions with unsaturated coordination at the surface of MO. The specific surface area values of MOs were not significantly different from each other, ranging between 100 and 200 m2/g, suggesting that the metal ratio did not strongly influence the porous structure of MO. The temperature-dependent desorption of ammonia demonstrated that the Lewis acidity of the Al-rich MOs was the largest with an Mg/Al ratio of 1, attributed to the efficient exposure of the surface-active site Al3+-O2- pairs. The acidity of heterogenous Al-rich MOs significantly increased with the exposed tetrahedral Al site on the surface and dramatically diminished when the molar ratio (Mg/Al) was over two.