Novel Palygorskite-based Adsorbents:Preparation And Adsorption Properties Towards Rare Earths From Aqueous Solutions

Rare earth, enjoying a reputation of "Industrial vitamin", have been massively used in petroleum, chemical industry, metallurgy, ceramics and permanent magnetic material. Especially, the high performance of rare earth-derived materials play the indispensable and irreplaceable place role in national defence industry and high-end technology products. Currently due to the coarse production of rare earth industry in mineral processing, smelting as well as the extraction separation, the wastewater that contains large amount of rare earth ions was discharged into environment directly. It not only lead to the waste of natural resources, but also result in heavy damages to water resources, soil and the production and living of human being. These realistic and serious problems are urgent to explore an effective method to recover rare earthioons from water. As one of the most cost-effective methods, the adsorption method has obvious advantages and broad prospects in wastewater treatment process, especially the polymer materials or environmental mineral materials have broad application prospects to be used as adsorbent in water treatment process. However, there still are many disadvantages of polymer adsorption materials, such as high cost, low mechanical strength, not ideal performance in reuse etc. Hence, more and more researches began to focus on the research of environment-friendly clay because of its abundant resources, cheap and good adsorb performance.Palygorskite is a kind of hydrous magnesium silicate mineral of layered chain structure with unique nanorods shape. Due to its characteristics of high specific surface area, strong polarity and easy to be modified, palygorskite is expected to become an effective and cheap adsorbent. But the natural palygorskite has limited adsorption capacity, so some modification is necessary to improve its adsorption capacity. There are abundant hydroxyl in crystal structure and broken edge surface of palygorskite, showing a high polar surface and reactivity, which should be available for chemical modification reaction sites. Based on this, some related functional modification can be designed for the better application in the enrichment and recovery of rare earth. Aiming at the resource waste and environmental pollution problems caused by the process of rare earth mining and metallurgy in China, this paper focus on the research of the preparation of novel palygorskite based adsorbent, the enrichment and recovery of rare earth and the adsorption mechanism in order to achieve the objective of palygorskite application extension, rare earth resources recovery and environmental protectionThis paper has prepared palygorskite-based adsorbents through organic and inorganic functionalization methods by using palygorskite sold in markets as raw materials. Heat treatment, acid activation and the single factor optimization trials have been conducted, three adsorbents (the sulfonic group modified palygorskite adsorbent, the poly(acrylic acid) brushs modified palygorskite adsorbent and The manganese oxide modified palygorskite adsorbent) was determined in this work. The physicochemical properties and active functional groups of the palygorskite-based adsorbents were explored and characterized by different techniques and the preparation mechanism was discussed. The adsorption of different rare earth ions by palygorskite-based adsorbents at different adsorption conditions (temperature, time and pH) were conducted. In addition, the adsorption mechanism of rare earth ion on palygorskite-based adsorbents was studied according to the parameters of thermodynamics and kinetics model. This present work provided novel technical process and theory for the realistic application of palygorskite.Innovation Points of the Research are listed as follow:(1) This paper proposed a kind of limited interface modification method on surface of palygorskite with silane coupling agent. By which, the hydrolysis and poly condensation reactions are limited at the interface area, leading to form a polysiloxane thiol network on the surface of palygorskite. Through combining the advantage of silane coupling agent modification on anhydrous and water conditions, at the same time overcoming the disadvantages of the both, this mothed can give some inspire for the efficient modification on the hydrophilic oxide.(2) Take advantage of the solvophobic property of molten photosensitizer (BP) in water, the liquid BP can be loaded on the organic-modified palygorskite quickly and efficient, which should be in favour of the grafting of polymer brushes on palygorskite by photo-initiation. Therefore, a weak acid type modified palygorskite adsorbent with high performance for rare earth ions has developed.(3) Atomic layer deposition technology (ALD) was introduced to the grafting of manganese oxide precursor on the surface of palygorskite in advance, next the material was roasted to form the uniform manganese oxide layer on substrate surface, the prepared composite has the feature of high specific surface of palygorskite, leading to a good adsorption performance for rare earth ions.The main research results of this paper are shown as follow:(1) This study investigated evolution structure and physical properties of palygorskite under heat treatment conditions, physical and chemical properties and acidizing theory of palygorskite in the acid activation conditions.The low temperature heat treatment to palygorskite had no effect to the palygorskite crystalline structure. With the heat treatment temperature increasing, crystallization water, constitution water dehydrated separately and the structure of palygorskite were destroyed, which meant its crystalline structure transform into amorphous. SEM results indicated that palygorskite remained good nano rod structure under different calcination temperature, however, with the increasing of the calcination temperature, draw ratio of the palygorskite crystal gradually reduced. Part of the palygorskite octahedral structure dissolved by using acidizing treatment. As the acid concentration and temperature increase, acidification process prolonged, octahedral cations completely dissolved and the crystal structure collapsed, at the same time, the crystal structure transformed into amorphous SO2. Under different acidulated condition, MgO in palygorskite showed a higher dissolution rate while Al2O3 and Fe2O3 showed a slow dissolution rate. The content of SiO2 increased with the dissolution of metal cations increasing, and they almost didn’t diminish, which indicated the stability of SiO2-The results of surface hydroxide radical on the acidified palygorskite showed that when the HCl solution concentration was 2 mol·L-1 and the reaction time was 6.5 h under 45℃, a large number of hydroxide radical could be obtained on the palygorskite surface, therefore, palygorskite was an ideal material for the preparation of mineral adsorption material.(2) The polysiloxane sulphur network structure was formed through polycondensation reaction by the silance coupling agent, which would be limited in the surface of palygorskite because of the water imbibition. And then the structure of the thiol group was oxidized into sulfonic group. The sulfonic group modified palygorskite adsorption materials were prepared and used for the adsorption study of rare earth ions.Research shows that with the amount of methylbenzene, silane coupling agent and deionized water increasing, the grafting ratio of SI-SH@PG increased at first and then decreased. When the amount of APG was 1.0 g, added 19.0 mL methylbenzene,2.3 mL K.H590 and 0.3 mL deionized water, the grafting ratio of SI-SH@PG reached its maximum value. the results of FE-SEM/EDS, FT-IR, XRD, BET and TG/DSC showed that the polysiloxane network successfully grafted to the surface of palygorskite. The adsorption experimental results of Ce3+, Eu3+, Dy3+ on SI-SO3H@PG showed that the adsorbing capacity increased at first and then keep constant with the increasing of pH and the prolongation of time. The adsorption capacity of SI-SO3H@PG will increase with the increase of temperature and concentration, until the saturated adsorption. The coexistence ions in solution is against to the adsorption of target ion, and the influence degree of interfering ions is different, the Ca2+ has the most effect on the rare earth adsorption. SI-SO3H@PG showd a strong ability combined with Ce3+ in the mixed rare earth solution. The Langmuir adsorption isotherm models were applied to the data of adsorption of rare earth ions by SI-SO3H@PG at 298.15,308.15 and 318.15 K, which instructed that the adsorption sites distributed uniformly on the material surface. The pseudo-second-order equation were fitted to describe the kinetics of Ce3+, Eu3+, Dy3+ adsorption on SI-SO3H@PG. The Weber-Morris model indicated the adsorption of Ce3+, Eu3+, Dy3+ on SI-SO3H@PG including two consecutive process:the outer surface diffusion and inner diffusion. The thermodynamic analysis showed that the values of △G0 were is negative and decreases with the increase of temperature, the values of △H0 and △S0 were positive, which meant that the process of adsorption of rare earth ions by SI-SO3H@PG were spontaneous, endothermic and an increase in disorder at the solid-liquid interface. SI-SO3H@PG remained a significant share of their adsorption capacity of rare earth ions after seven times regenerative adsorption, and the desorption rate remained above 90%. Adsorption mechanism study showed that the adsorption of rare earth ions by SI-SO3H@PG was mainly in the form of ion exchange.(3) The photosensitizer (BP) will adhere to the surface of the organic-modified palygorskite quickly and efficiently with using the solvophobic property of molten photosensitizer. Then the poly(acrylic acid) brush with carboxyl functional groups was successfully grafted on the surface of palygorskite using UV-initiated grafting. The pory(acrylic acid) brushs modified palygorskite adsorption materials were prepared and used for rare earth ions adsorption study.Experimental results show that, the grafting rate of PAA@PG presents a trend of reducing gradually after the first increase as the increasing of the dosage of the photosensitizer, monomer and inhibitor, while the grafting rate increased as the illumination time prolonged. The best grafting conditions of PAA@PG were 0.25 g MPS@PG,0.11 g BP,3.75 mL acrylic monomer,0.06 g inhibitor and keep the photo reaction for 39 min. The prepartion of PAA@PG has low equipment requirement, simple conditions, but the grafting effect is obvious. The results of XRD, FT-IR, FE-SEM/EDS, TG/DSC, BET and wetting contact Angle test revealed that poly(acrylic acid) was grafted to the surface of palygorskite successfully. The experimental results of the PAA@PG adsorption performance showed that the adsorption ability of Ce3+, Eu3+ Dy3+ by PAA@PG increased obviously with the increasing of the pH and the initial concentration, and the time to reach adsorption equilibrium extended with the increase of initial concentration. The coexistence ions in solution is also against to the adsorption of target ion for PAA@PG, especially in the presence of Ca2+. PAA@PG showed stronger binding ability to Eu3+in the mixed rare earth solution. The adsorption rule of rare earth ions by PAA@PG conform to the Langmuir model and the adsorption kinetics of different rare earthions were more suitable to pseudo-second-order kinetics model. The internal diffusion model studies showed that the adsorption of rare earth ions by PAA@PG spread outside earlier and within diffusion was given later, finally, reach the adsorption equilibrium. The PAA@PG adsorption thermodynamic studies showed that the value of △G0 were negative, which indicated that the adsorption process is spontaneous. In the seven times regeneration experiment, PAA@PG still maintained a high adsorption capacity towards rare earth ions and remained around 98% desorption ratio with the low concentration of HCl, which shows its good absorption performance. The adsorption mechanism of rare earth ions by PAA@PG were mainly in the form of forming complexes combine for adsorption.(4) The single molecule layer metal manganese oxide precursors were bonded on the surface of palygorskite by using Atomic Layer Deposition technology. Then through being calcined, the manganese dioxide layer was fromed uniformly. The manganese dioxide could be better play its adsorption with using the high specific surface area of palygorskite. The manganese oxide modified palygorskite adsorption material was prepared and using to the adsorption study of rare earth ions.Experiments show that, the adsorption performance of rare earthlons by MnO2@PG were improved with the dosage of manganese (II) acetylacetonate and the reaction time increasing. The best adsorption properties of optimal materials preparation condition were the amount of manganese (Ⅱ) acetylacetonate was 0.25 g, the reaction time was 8.0 h and the calcination temperature was 250℃. A variety of characterization results showed that the mo no molecular layer manganese oxide uniformly loaded on the surface of palygorskite through the use of ALD method after grafting and calcination. The adsorption effect of the experimental results showed that the adsorption of Ce3+, Eu3+, Dy3+ by MnO2@PG increased gradually stabilized after change firstly with the pH, initial concentration and temperature increasing. The effect of ionic strength experiment found that the absorption performance of rare earthions by MnO2@PG decreased with the presence of Ca2+ increasing. The MnO2@PG showed the better selective adsorption of Ce3+ in the mixed rare earth solution. The Langmuir adsorption isothermal adsorption model was fitted with the data of the Ce3+, Eu3+, Dy3+ adsorption by MnO2@PG at 298.15 K,308.15 K,318.15 K. The dynamic process of rare earth ions adsorption were suitable to pseudo-second-order kinetics model. Research results show that the diffusion model of Ce3+, Eu3+, Dy3+ adsorption process by MnO2@PG were composed external diffusion and internal diffusion. The results of thermodynamic analysis showed that the rare earth ions adsorption were spontaneous irreversible process because the values of △G0 were less than zero. The values of △H0 and △S0 were positive, which meant that the adsorption were endothermic and increased of the chaos on the solid-liquid interface degrees in the process. The adsorption capacity of three kinds of rare earth ions by MnO2@PG didn’t decline significantly after seven times repeated adsorption and desorption experiment, in which the desorption ratio of Ce3+, Eu3+, Dy3+ could remain at around 98%. The adsorption mechanism study results on MnO2@PG indicated that the prepared materials adsorbed rare earthions by some special chemical adsorption like ion exchange or coordination between the surrace of the hydration hydroxylation formed in the aqueous solution bonding with manganese oxide and rare earthions, which realized the adsorption rare earthions by MnO2@PG.The research results originated from this paper will be bound to rich modification technofogy of mineral, especially for their functionalization, and should be a beneficial reference for the preparation of modified mineral adsorption materials, adsorption mechanism and application. And these results, of course, will be valuable references to the research on functional mineral materials as well.