Preparation Of Magnetic Zeolites For The Absorption Of Nickel In Aqueous Solution

Adsorption is a kind of green,simple and efficient method for wastewater treatment.In recent years,a variety of new adsorbents are developed aimed at the nickel removal in wastewater.Among them,magnetic adsorbent which is based on magnetic separation technology attracts attention due to its quick separation from solution in the external magnetic field.In this paper post-modification and in-situ synthesis method were used to functionalize the commonly used adsorbent zeolite.Four kinds of magnetic zeolites were prepared.The characterization of the materials,the adsorption properties of adsorping nickel in aqueous solution,the regeneration and the stability of the adsorbents,the preparation principle and mechanism of adsorption etc.were systematically studied.The main research contents and results were summarized as follows:（1）Artificial zeolite（60⁸0 mesh）was magnetically modified by chemical deposition method.The artificial zeolite was added to the forming of magnetic Fe₃O₄process which was called the chemical coprecipitation in the adhesion of polyacrylamide.In this way magnetic zeolite FZP was synthesized.SEM photographs showed that FZP was completely coated by Fe₃O₄,Fe₃O₄ particles were evenly distributed on the surface of FZP and aggregated to form a cage like structure,the particle size distribution of FZP was relatively wide,the shape was irregular and the surface structure was flocculent,porous and loose.The obtained FTIR spectrum peaks mainly included typical Fe-O stretching vibration spectrum,the skeleton of zeolite’s vibration spectrum,the lattice water and surface hydroxyl groups spectrum,indicating that Fe₃O₄was successfully loaded onto the surface of artificial zeolite and magnetic zeolite was formed.XRD patterns showed that FZP had an ideal crystal structure conbined artificial zeolite and Fe₃O₄ and the average grain size of the sample was 36.7nm.The magnetic hysteresis loop obtained from VSM test showed that FZP had super-paramagnetism,the saturation magnetization was 6.6844 emu/g,the residual magnetization was 0.0911 emu/g and the coercivity was 5.229 G.From BET method the specific surface area of FZP was 4.749 m²/g.And from BJH method the total pore volume of FZP was 0.004085 cm³/g and the average pore diameter was 34.41 nm.The magnetic adsorbent was applied to the adsorption of nickel in aqueous solution.The effects of pH,temperature,time,concentration of nickel solution and dosage of adsorbent were investigated.The results showed that when the dosage of FZP was 0.8g/L pH=7 t=6 h and T=298 K,the Langmuir monolayer saturated adsorption capacity was 70.06 mg/L in the nickel adsorption process by FZP.The calculated thermodynamic parameters[ΔG^θ=-19.81 KJ/mol,ΔH^θ=-24.57 KJ/mol,ΔS^θ=-92.23 J/（mol·K）]showed that the exothermic process was favorable and spontaneous in low temperature.For the wastewater containing nickel with an initial concentration of 20 mg/L,the effluent concentration was below 1.0 mg/L when the dosage of FZP was 1.2 g/L,meeting the national standard for wastewater discharge guideline.（2）Magnetic zeolite RAG was successfully prepared by combining the raw materials-cheap magnetic substance red mud and artificial zeolite（100 mesh）using glutaraldehyde as crosslinking agent.SEM pictures showed that the size distribution of RAG was not even,the surface was rough,the shape of particle was mostly spherical,the size was different,the shape of the red mud in RAG was basically unchanged and the crystal form of zeolite was obvious,forming close complex with red mud.The infrared spectrum of RAG showed the skeleton characteristic vibration peaks of artificial zeolite.The results of XRD characterization showed that the main crystal phase of RAG wasγ-Fe₂O₃,the average grain size was 36.7 nm.It was showed that the red mud was covered by the cross-linked artificial zeolite,and the synthesized RAG kept the structure and property of red mud and artificial zeolite.VSM test showed that the saturation magnetization of RAG was 32.156emu/g,the residual magnetization was2.1986 emu/g and the coercivity was 104.52G.From BET method the specific surface area of RAG was 41.79 m²/g.And from BJH method the total pore volume of RAG was0.00962 cm³/g and the average pore diameter was 9.208 nm.The static adsorption experiments showed that it took 3 h for RAG to reach nickel adsorption equilibrium when pH was 7.And the maximum adsorption capacity was 23.27 mg/g at 298 K when the dosage was 2.0 g/L.The calculated thermodynamic parameters[ΔG^θ=3.26 KJ/mol,ΔH^θ=100.33 KJ/mol,ΔS^θ=319.38 J/（mol·K）]showed that the endothermic process was not spontaneous and both its enthalpy and entropy were increasing.For the wastewater containing nickel with an initial concentration of 20 mg/L,the effluent concentration was below 1.0 mg/L when the dosage of RAG was 8.0 g/L,meeting the national standard for wastewater discharge guideline.（3）Alkaline fusion clinker was obtained by dry process-calcining red clay and fly ash with alkali,then the magnetic zeolite RFS was synthesized by further hydrothermal method.SEM photographs showed that its particle size was relatively uniform,and the dispersion was good,and the surface was in the structure of evacuating spongy,there were a lot of open holes;and compared to raw materials the morphology of RFS was obviously changed which indicated the zeolite framework structure of RFS was reformed.FTIR spectra absorption vibration peak of RFS and NaX zeolite was very consistent,and the emergence of the infrared characteristic peak ofγ-Fe₂O₃,indicating the synthesis of NaX type magnetic zeolite.In the XRD spectrum of RFS,besides the main crystalline phaseγ-Fe₂O₃ from red mud,the characteristic diffraction peaks matched well with standard NaX zeolite.This can be determined that the main crystalline component was NaX type zeolite.And the average grain size was 21.7 nm.VSM test showed that the saturation magnetization of RFS was12.986 emu/g,the residual magnetization was1.5395 emu/g and the coercivity was 166.57 G.From BET method the specific surface area of RFS was 70.89m²/g.And from BJH method the total pore volume of RAG was 0.1028 cm³/g and the average pore diameter was 5.800 nm.The best experimental conditions for the adsorption of nickel by RFS were as follows:the dosage was 1.2 g/L,pH=7,t=6 h,T=298 K.And the Langmuir monolayer saturated adsorptioncapacitywas66.27mg/L.Thecalculatedthermodynamic parameters[ΔG^θ=-29.54 KJ/mol,ΔH^θ=95.83 KJ/mol,ΔS^θ=409.56 J/（mol·K）]showed that the endothermic process was favorable and spontaneous in high temperature.For the wastewater containing nickel with an initial concentration of 20 mg/L,the effluent concentration was below 1.0 mg/L when the dosage of RFS was 1.0 g/L,meeting the national standard for wastewater discharge guideline.（4）Through pickling the impurity of fly ash was removed.Magnetic fly ash was formd by combining the treated fly ash and washed red mud using glutaraldehyde as cross-linking agent,and then treated with alkali solution,added with NaAlO₂ in order to adjust silica alumina ratio.After hydrothermal reaction,magnetic zeolite MAZ was synthesized.SEM photographs showed that the particle size of MAZ was uniform and homogeneously dispersed,clear crystal particles were showed on the surface,clear lattice fringes were visible in the grain boundaries and there were many spherical crystal particle with smaller diameter.It was showed that zeolite particles were formed and had good crystallinity.The infrared characteristic peaks of NaA zeolite andγ-Fe₂O₃ were appeared in the infrared spectrum of MAZ.It was inferred that NaA type magnetic zeolite was formed.The XRD patterns of MAZ agreed with standard NaA type zeolite,indicating that MAZ had NaA zeolite crystal structure.And the preparation process didn’t destroy the crystal structure ofγ-Fe₂O₃.The average grain size of MAZ was 30.4nm.VSM test showed that the saturation magnetization of MAZ was 16.56 emu/g,the residual magnetization was1.2382 emu/g and the coercivity was 198.43 G.From BET method the specific surface area of RFS was 71.21 m²/g.And from BJH method the total pore volume of RAG was 1.005cm³/g and the average pore diameter was 5.644 nm.The experimental results showed that the adsorption of nickel by MAZ reached equilibrium in 3 hours when pH was 7.And the maximum adsorption capacity was63.32 mg/g at 298 K when the dosage was 1.2 g/L.The calculated thermodynamic parameters[ΔG^θ=-0.190 KJ/mol,ΔH^θ=-0.437 KJ/mol,ΔS^θ=-0.841 J/（mol·K）]showed that the exothermic process was favorable and spontaneous in low temperature.For the wastewater containing nickel with an initial concentration of 20 mg/L,the effluent concentration was below 1.0 mg/L when the dosage of RFS was 1.8 g/L,meeting the national standard for wastewater discharge guideline.（5）The adsorption of nickel by four magnetic zeolites added with the increase of pH and the adsorption process followed pseudo-second-order kinetics and Freundlich isotherm models indicating that the adsorption process may be heterogeneous chemical adsorption;In the competitive adsorption with cobalt four magnetic zeolites showed good adsorption effect towards cobalt;The desorption with acid under ultrasonic were not good;The materials showed excellent anti air,anti acid and anti alkali performance;Magnetic response ability were also good.The mechanism of remove nickel ions in aqueous solution by magnetic zeolites included adsorption and ion exchange.