Microwave digestion and alkali fusion assisted hydrothermal synthesis of zeolite from coal fly ash for enhanced adsorption of Cd(II) in aqueous solution

Abstract

A novel microwave digestion and alkali fusion assisted hydrothermal method was proposed to synthesize zeolite from coal fly ash and the zeolite product was studied for removal of Cd(II) from aqueous solution through batch experiments. The adsorbent was characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, surface area analyzer and zeta potential measurement. The results show that the synthetic zeolite was identified as faujasite. The optimum conditions for removal of Cd(II) are found to be: adsorbent dose of 0.5 g/L, pH 6, contact time of 90 min and initial concentration of 20 mg/L, the removal rate of Cd(II) is 98.55%. The experimental kinetic data agree well with the pseudo second-order equation; the Langmuir isotherm model is found to be more suitable to explicate the experimental equilibrium isotherm results than Freundlich, Dubinin-Radushkevich and Temkin models, and the maximum adsorption capacity of Cd(II) is found to be 86.96 mg/g. The thermodynamic parameters such as ΔG^Θ, ΔH^Θ and ΔS^Θ were evaluated and the results show that the adsorption of Cd(II) onto the as-synthesized zeolite is spontaneous, endothermic and feasible under studied conditions.

摘要

针对传统水热法合成沸石存在碱熔温度高、 反应时间长的问题, 提出了一种新的粉煤灰微波碱熔辅助水热合成沸石的方法, 并将沸石产品用于吸附水溶液中的 Cd(II)。 其合成工艺是将粉煤灰与氢氧化钠混合物置于微波箱式高温反应器中以 450 °C 反应 15 min, 碱熔产物加水在常温下磁力搅拌 4 h, 再在 90 °C 下静置 12 h, 过滤、 洗涤、 干燥得到沸石产品。 采用 X 射线衍射分析、 扫描电镜分析、 红外光谱分析、 比表面积分析和 zeta 电位分析表征吸附剂。 结果表明: 合成的沸石为八面沸石, 含量约为 93%, 比表面积为 75.72 m²/g, pH_zpc 为 3.80。 从水溶液中去除 Cd(II), 优化的吸附剂用量为 0.5 g/L, pH 6, 吸附时间为 90 min, 初始浓度为 20 mg/L。 在此条件下, Cd(II)的去除率达 98.55%。 Cd(II)在沸石表面的吸附符合伪二级动力学模型, Langmuir 等温吸附模型比 Freundlich 模型、 Dubinin-Radushkevich 模型和 Temkin 模型更适合于描述等温吸附过程, 最大的 Cd(II)吸附量为 86.96 mg/g。 热力学计算表明: Cd(II)在合成沸石表面的吸附为自发、 吸热过程。