Insight into the Effect of Al2(SO4)3 on the Mechanism of Fluorine Adsorption by Saline-Sodic Soils

Fluorine pollution in saline-sodic soils has become an important environmental problem, but little is known about the interaction between aluminium sulfate (Al-2(SO4)(3)) and fluorine when Al-2(SO4)(3) is used to improve saline-sodic soils. It is vital to clarify the effect of Al-2(SO4)(3) on fluorine adsorption by saline-sodic soils, which will provide theoretical support for evaluating the risk of fluorine pollution in the improved saline-sodic soils. Batch adsorption experiments were conducted to investigate the impacts of different saline-sodic soils with Al-2(SO4)(3) on fluorine adsorption, and the mechanisms of adsorption were characterized by scanning electron microscopy, specific surface area and pore size analyzer, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Fluorine adsorption decreased with increasing soil salinity, and Al-2(SO4)(3) addition increased the adsorption of fluorine in soils. Among the three saline-sodic soils, Al-2(SO4)(3) contributed greatly to fluorine adsorption by mildly saline-sodic soils, with a range of 40.84-69.70%. Fluorine adsorption isotherms could be properly described by the Langmuir and Freundlich models. At higher fluorine equilibrium concentrations, adding Al-2(SO4)(3) first and then adding fluorine, the adsorption amount of fluorine was greater for soils. Both the soil porosity and the numbers of aggregate were increased after adding Al-2(SO4)(3). Functional groups such as siloxy group, hydrocarbyl group, hydroxyl group and hydroxyaluminum group were involved in the reaction process. Fluorine adsorption on saline-sodic soils refers to the ligand exchange and electrostatic adsorption, and fluorine ion replaces hydroxyl group on the soil surface. This work may provide a theoretical reference for the changes in fluorine chemical behavior when Al-2(SO4)(3) ameliorates saline-sodic soils.