Thermodynamics And Mechanism Study On Removal Of Lead And Fluoride Ions From Aqueous Solution By Adsorption Onto Attapulgite

Since high lead and fluoride concentrations in water cause great damage to aquatic animals, plants and human body, it is essential that there are technologies for controlling the concentrations of lead and fluoride in aqueous emissions. One effective and economic approach is to use alternative adsorbents. Extensive investigations have been carried out to identify suitable adsorbents in recent years. For its low costs and great performance in the adsorption of many pollutants, attapulgite has been obtaining increasing scientific attention.The attapulgite used in this study is from Xuyi in Jiangsu province. Adsorption of lead ions by natural attapulgite and fluoride ions by three modified attapulgites with magnesium and aluminum salts are conducted by batch experiments. Based on the physical and chemistry properties of the attapulgite, the thermodynamics and mechanism of the adsorption processes have been mainly studied.With the software Microsoft Excel, two-parameter Langmuir and three-parameter Redlich-Peterson isotherms are examined as the best-fitting models for the experimental data by the non-linear method. An increase in temperature results in a higher lead or fluoride loading per unit weight of the adsorbent, suggesting that both the two adsorption processes are spontaneous and endothermic. Besides, the two are both favorable adsorption systems. The maximum adsorption capacities for attapulgite/lead and modified attapulgite/fluoride adsorption system are 114.0mg/g and 44.0mg/g in the temperature 313K respectively. According to the monolayer adsorption theory of the Langmuir isotherm, the specific surface area of attapulgite towards Pb²⁺ binding is 17-19m²/g.The XPS test for the surface of the attapulgite and changes of the pH value in the aqueous solutions before and after the reaction prove that there exist electrostatic gravitations in the attapulgite/ lead system but also precipitation of the lead(II). Modified attapulgite with a mass ratio attapulgite:MgCl₂·6H₂O:AlCl₃·2H₂O = 2:1:2 has higher adsorption efficiency for fluoride ions. The agreement of the adsorption capacities and changes of the hydroxy ion concentration indicates that anion exchange reactions occurr in the adsorption process.The experimental results suggest that nature attapulgite is a suitable adsorbent for removing lead ions and the modified attapulgite suitable for fluoride ions, which can be widely applied in the water treatment projects.