Adsorption Performance And Mechanisms Of Fluoride On The Akaganeite And Its Aluminum Oxide Complex

Fluorine is an essential element for the mineralization of human tissues and plays an important role in bone health and the formation of tooth enamel.However,excessive intake of fluorine can cause serious health problems such as dental fluorosis and calcium and phosphorus metabolism disorders.The main source of human intake of fluorine is drinking water,so it is of great significance to control the fluorine content in drinking water sources and other water bodies.The adsorption method is widely used in fluorine pollution control due to its low cost,high efficiency,reproducibility,and simple operation.The cost of adsorbent is currently the focus of the development and promotion of adsorption technology.As a ubiquitous substance in nature,iron oxide has many types and complex structures,and has a good adsorption performance for fluoride.However,the fluoride adsorption performance and mechanism of the akaganeite with a porous structure is still unclear.Therefore,this paper chooses akaganeite as the adsorbent to study the adsorption for fluoride.At the same time,it studies the influence of the commonly used flocculant hydrolysate of aluminum chloride on the adsorption for fluoride by akaganeite,revealing the adsorption and removal for fluoride by different hydrolysates.The mechanism provides a scientific basis for the research and development of low-cost adsorbents.In this paper,akaganeite with high purity and well-arranged pore structure was synthesized by the hydrolysis method.The synthesized sample is a spindle-shaped and needle-shaped particle with a length of about 200 nm and a diameter of about 30nm,with a specific surface area of 50.434 m²/g.The results of static adsorption experiments show that the akaganeite has good adsorption performance for fluoride in the p H of 4～10.The adsorption process conforms to the Langmuir model and the pseudo-first-order kinetic model.The adsorption amount can reach 23.86 mg/g.Adsorption process is a spontaneous exothermic reaction,and the adsorption rate is mainly controlled by physical adsorption.Common anions and cations in nature(NO₃^-、SO₄^2-、SiO₄^2-、Na⁺、Mg²⁺、Ca²⁺)have little effect on adsorption.The results of XPS analysis indicated that when akaganeite adsorbed fluoride,part of the fluoride exchanged with the surface hydroxyl groups of the adsorbent,and part of the fluoride exchanged with the chloride in the pores,forming new Fe-F bond during the adsorption process.The bond is accompanied by electrostatic adsorption.Secondly,aluminum hydroxide was prepared by direct hydrolysis at room temperature.After characterization,it was determined that the synthesized sample was composed of two aluminum oxides,and the sample particles varied in size from1μm to 20μm.The results of static adsorption experiments show that the aluminum hydroxide has good adsorption performance for fluoride in a wide p H range.The adsorption process conforms to the Langmuir model.The maximum theoretical adsorption capacity is 31.82 mg/g,which is an ideal fluoride adsorbent.The adsorption process accords with the pseudo-second-order kinetic model,which is an endothermic reaction dominated by chemical adsorption.XPS analysis results show that fluoride form Al-F bonds with aluminum hydroxide and Al-O-F bonds are fixed on the surface.Finally,seven kinds of Fe-Al complex were prepared by changing different conditions,and it was found that the composition of the Fe-Al complex had nothing to do with the molar ratio and the order of addition,but was only related to the temperature.The Fe-Al complex hydrolyzed at 70℃has the characteristics of akaganeite,and the Fe-Al complex hydrolyzed at 25℃has the characteristics of aluminium hydroxide.Using the two complex obtained by hydrolysis at two temperatures as adsorbents to carried out adsorption experiments and it was found that the maximum adsorption amount of Fe-Al complex for fluoride at 70℃and 25℃was 37.81 mg/g and 73.39 mg/g,respectively.The complexes generated under the two conditions all adsorb fluoride through the formation of Fe-F bonds,Al-F bonds and Al-O-F bonds,and there is exchange of fluoride with chloride and hydroxyl groups at the same time.