Synthesis Of Waste Fly Ash Based Zeolite CHA And Its Adsorption Properties

Zeolite CHA,a class of hydrated aluminosilicates with CHA structure,exhibits unique gas adsorption properties as well as ion exchange properties due to the specific cage structures and three-dimensional pore systems.Its primary building units consist of silica tetrahedrons and alumina tetrahedrons,and its second building units include 4MR,6MR and 8MR.Several feasible methods,currently,are utilized in CHA synthesis,yet extra organic-structure-directing agents(OSDAs)or zeolite seeds are often required to assist the process.That not only enhances the difficulty in pollutant disposal but also complicates the process design and auxiliary equipment.Although some researches related to coal fly ash based zeolites have been reported,they usually focus on limited types so that reaction mechanisms and technology processes in zeolite CHA synthesis with waste fly ashes still remain to be further studied.In this investigation,waste ash based zeolite CHA was researched in aspects of synthesis optimization,gas adsorption and ion exchange.Both methods(hydrothermal synthesis,alkali fusion synthesis)and three different ashes were adopted in preparation of zeolite CHA,and obtained samples were characterized via XRF,XRD,SEM,IR,TG,etc.Adsorption/desorption data were conducted on a thermogravimetric analyzer(TGA)at several temperatures,and CO2/N2 adsorption isotherms were tested on a physical adsorption analyzer at 0?.Cu2+/Cd2+ adsorption performances in aqueous solution were collected by thermostatic oscillation experiments.It is indicated that the coal fly ash not only affords silicon and aluminum necessary in CHA synthesis,but also shows favorable characters as an ideal raw material.Alkali fusion hydrothermal method is feasible in preparing zeolite CHA and proved as an applicable synthesis in recycling different quality type ashes.Furthermore,characterization results verify that the zeolite CHA has acquired high qualities and inherits the heat stability of raw ash.The as-synthesized fly ash based zeolite CHA has possessed favorable CO2 adsorption properties.At 30?,the CO2 adsorption amount at 20min is 1.60mmol/g while the equilibrium absorption amount is 2.1Ommol/g,and the process could be well described with pseudo-second-order kinetic model.Moreover,the desorption rate is two times higher than adsorption rate,and 66.8%of adsorbed CO2 degas if the temperature rises from 60? to 120?,which reveals excellent renewabilities of zeolite CHA.The CO2 adsorption isotherm,at 0?,is similar with I-type and IV-type isotherms in BDDT,but the N2 adsorption isotherm is linear type and pore blockages appear in N2 adsorption.CO2/N2 adsorption amounts,at 0?,95kPa,are 2.24mmol/g and 0.13mmo1/g,respectively.Linear Langmuir equation and Toth equation fit the experiment results perfectly and parameters are similar with reports.However,shortages still exist such as high activation energy and nonuniform distribution of adsorption sites.The surface area calculated on the basis of Toth equation(353.48m2/g)is much higher than that acquired via BET at-196?(16.83m2/g),so surface areas of CHA are often greatly underestimated.Equilibrium times in adsorbing Cu2+/Cd2+ are less than 15min and Cue+/Cd2+ equilibrium adsorption amount are 6.17mmol/g and 6.45mmoL/g.Both processes could be simulated with pseudo-second-order kinetic model but it spends less time in adsorbing Cu2+.There comes no obvious promoting or inhibiting in bi-component system of Cue+/Cd2+ and all removal efficiencies are over 98%.Temperature increment influences CU2+ adsorption less yet strengthens Cd2+ adsorption.When the initial pH approaches 7,Cu2+ adsorption is inhibited but Cd2+ adsorption remains unchanged.As a whole,Cu2+/Cd2+ adsorptions on zeolite CHA are less sensitive to the pH value and failure phenomenon common in many adsorbents does not appear in this investigation.The fly ash based zeolite CHA exhibits stable qualities and reliable properties,and its synthesis is feasible and applicable in recycling of different type ashes.It reuses low-cost and widely-distributed waste fly ash and shows promising application in PSA.In addition,it could perfectly remove Cu2+/Cd2+ so that is also a valuable adsorbent in governing heavy-metal pollution.