| As a high-ash combustible mineral,oil shale will generate a large amount of oil shale ash during pyrolysis and combustion.The resource utilization of oil shale ash is a necessary prerequisite for the sustainable development of the oil shale industry.Cadmium as a first-class pollutant,cannot be ignored.The development of inexpensive and highly efficient adsorption materials is the focus of research in this field.The research of this paper was to investigate the synthesis of zeolite from oil shale ash by alkaline fusion hydrothermal method for cadmium adsorption treatment.The investigations were performed about the controllable parameters,the formation mechanism,the adsorption performance of cadmium onto synthetic zeolites,and the adsorption mechanism.The high-silicon and low-aluminum oil shale ash and the low-silicon and high-alumina coal fly ash were used as the raw materials.The composite zeolites were synthesized by the alkaline fusion hydrothermal method through changing the mix ratios of oil shale ash and coal fly ash to adjust the silicon to aluminum ratio of the raw material composition and to optimize the pore size structure of zeolite,which improved the adsorption performance of cadmium.The main eonclusions drawn from this paper were as follows:? Synthesis of Na-X zeolite from oil shale ashes by alkaline fusion hydrothermal method(1)The Na-X zeolite was synthesized from oil shale ash by alkaline fusion hydrothermal method.The Na-X zeolite was synthesized from Longkou oil shale ash at the following conditions:NaOH fusion temperature 600?,ash to NaOH ratio 1:1.2g/g,solid-liquid ratio 1:4g/mL,crystallization temperature 80? and crystallization time 24h.And the Na-X zeolite was synthesized from Huadian oil shale ash at the following conditions:NaOH fusion temperature 600?,ash to NaOH ratio 1:1.5g/g,solid-liquid ratio 1:4g/mL,crystallization temperature 80? and crystallization time 12h.(2)The as-synthesized zeolite under optimal conditions,the SEM photograph showed that it had a typical octahedral morphology.The eharacteristic diffraction peaks of the as-synthesized zeolite were observed in the XRD spectrum,which was consistent with the spectrum of the standard card(PDF#12-0246:Na2Al2Si2.4O8.8·6.7H2O).The calculated Si/Al ratio in Na-X zeolite by NMR spectrums was Longkou ash zeolite 1.23 and Huadian ash zeolite 1.22.consistent with the observation from XRD.(3)The specific surface areas of Longkou ash zeolite and Huadian ash zeolite were 252m2/g and 313mZ/g,respectively.And the ion exchange capacity was 59.81mg/g and 61.12mg/g,respectively.Compared with the oil shale ash,the specific surface area was increased 70 times,the pore volume increased more than 10 times,and the ion exchange capacity was nearly 2 times.? Adsorption performance of cadmium by Na-X zeolite synthesized from oil shale ashes(1)The kinetics studies showed that Cd2+ adsorption onto both Longkou zeolite and Huadian zeolite was well fit by the pseudo-second-order kinetics model.The adsorption rate was mainly controlled by two steps.The initial stage was the extra-particle diffusion process.The second stage was the internal diffusion process.(2)The adsorption process of Cd2+ onto both Longkou zeolite and Huadian zeolite belonged to a monolayer chemical adsorption process.And the maximum monolayer adsorption capacity determined by the Langmuir adsorption isotherm was found to be 192.31mg/g and 189.49mg/g for Cd2+ adsorption onto Longkou zeolite and Huadian zeolite,respectively.(3)The Gibbs free energy change ?G was negative for the adsorption process of Cd2+ onto both Longkou zeolite and Huadian zeolite,indicating that the adsorption process was spontaneous.The enthalpy change ?H was positive for the adsorption process of Cd2+ onto these zeolites,indicating that the adsorption process was an endothermic process.(4)The adsorption mechanism was mainly ion exchange for Cd2+ adsorption onto both Longkou zeolite and Huadian zeolite by XPS.? Adsorption performance of cadmium by composite zeolites synthesized from combination oil shale ash with coal fly ash(1)The high-silicon and low-aluminum oil shale ash and the low-silicon and high-alumina coal fly ash were used as the raw materials.The composite zeolites were synthesized by the alkaline fusion hydrothermal method through changing the mix ratios of oil shale ash and coal fly ash to optimize the silicon to aluminum ratio of the raw material composition.When the oil shale ash mixing ratio was higher,the composite zeolite was X-type zeolite,and when the coal fly ash mixing ratio was higher,the composite zeolite was A-type zeolite.(2)The kinetics studies showed that Cd2+ adsorption onto oil shale ash/coal fly ash composite zeolites were well fit by the pseudo-second-order kinetics model.It indicated that the adsorption rate was proportional to the square of the remaining adsorption sites of the adsorbent for the Cd2+ adsorption onto oil shale ash/coal fly ash composite zeolites.(3)The adsorption process of Cd2+ onto oil shale ash/coal fly ash composite zeolites belonged to a monolayer chemical adsorption process.And the maximum monolayer adsorption capacity determined by the Langmuir adsorption isotherm was found to be 236.4lmg/g and 204.08mg/g for Cd2+ adsorption onto composite zeolites which were synthesized by Longkou ash 75%,coal fly ash 25%and Huadian ash 50%,coal fly ash 50%,respectively.Their adsorption capacities of Cd2+ were higher than that of other zeolites which was synthesized from single oil shale ash.(4)Under different adsorption temperature,the Gibbs free energy change ?G was negative for the adsorption process of Cd2+ onto oil shale ash/coal fly ash composite zeolites,indicating that the adsorption process was spontaneous.The enthalpy change ?H was positive for the adsorption process of Cd2+ onto these zeolites,indicating that the adsorption process was an endothermic process of Cd2+ onto oil shale ash/coal fly ash composite zeolites.(5)The adsorption mechanism of oil shale ash/coal fly ash composite zeolites removal Cd2+ was the same as that of single oil shale ash,that is ion exchange.The adsorption capacities of synthetic zeolites for Cd2+ were not only related to the specific surface area and average pore size of zeolites,but also related to the structure type of zeolites. |
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