| Coal gangue(CG)is an industrial waste material discharged during coal mining,processing and power generation.According to the survey,at this stage,China’s total coal consumption accounts for about 50%of the global total.2022 global coal production,although slightly reduced by the epidemic,China’s coal production increased by 1.2%in this year compared with the previous year.At present,the production of coal gangue in China has surged to more than 400 million tons/year.In the practice of coal mining,coal gangue as dumped in the wasteland of the "waste bank" is an important economic consideration in China,but also a major pollution problem.With the change of world pattern and the rapid development of Chinese economy,the reuse of bulk industrial solid waste byproducts is getting more and more attention from scholars at home and abroad.In this study,CG-LTA zeolite molecular sieve materials were synthesized by a simple and efficient method to solve the problem of domestic solid waste threat as well as to crack the situation that the supply of natural zeolite exceeds the demand.The prepared CG-LTA molecular sieve material was applied as an adsorbent to the removal of heavy metal ions in aqueous solution,and CG-LTA molecular sieve was also used as a soil conditioner to study its effects on soil physicochemical properties and enzyme activities.The results of the paper are as follows:(1)In this study,CG-LTA type zeolite molecular sieve with high crystallinity and high strength was successfully prepared by hydrothermal alkali-assisted activation technique.The effects of variations of reaction parameters such as n(SiO2)/n(Al2O3),n(NaOH)/n(SiO2),hydrothermal reaction time and hydrothermal reaction temperature on the results of product synthesis were considered comprehensively,and it was found that the addition of additional silicaalumina sources to the reactants with different reaction parameters helped to broaden the composition of gels in the zeolite interconversion method,which would lead to other structural features and applications of In this paper,the reaction conditions were finally determined as n(SiO2)/n(Al2O3)=1.5,n(NaOH)/n(SiO2)=5.0,hydrothermal temperature of 100℃ and hydrothermal time of 8 h.At this time,samples with LTA zeolite diffraction peaks were obtained,and the synthesized products had high crystallinity.The generated CG-LTA molecular sieve crystals are aggregated from small cubic crystals with an average size of 2-4 μm,with a smooth and regular surface,pHpzc is 4.0,and rich porous structure with a specific surface area of 24.28 m2/g.(2)In this paper,the ability of CG-LTA zeolite molecular sieve as an adsorbent to remove Cd(Ⅱ)and Cr(Ⅵ)heavy metal ions from aqueous solutions was evaluated.The effects of adsorbent content and initial pH of the solution on the adsorption effect were discussed.At the pH values of 1.0 and 6.0,respectively,CG-LTA molecular sieve showed good removal performance for both Cd(Ⅱ)and Cr(Ⅵ),and its effect on the removal of Cd(Ⅱ)was especially outstanding,and both were better than the adsorption of CG on Cd(Ⅱ)and Cr(Ⅵ).The fitting of adsorption kinetics and adsorption isotherm models revealed that the adsorption of Cd(Ⅱ)and Cr(Ⅵ)ions by CG-LTA molecular sieves both followed quasisecondary kinetic equations and conformed to the Langmuir isotherm model,indicating that the adsorption processes of Cd(Ⅱ)and Cr(Ⅵ)on the materials were chemisorption processes belonging to monolayer adsorption.The final adsorption mechanism revealed that the CG-LTA molecular sieve removes Cd(Ⅱ)and Cr(Ⅵ)ions mainly by redox,ion exchange and electrostatic interaction.(3)In this study,we investigated the effect of using synthetic CG-LTA molecular sieve as a soil amendment.In a 60-day experiment,we found that soil pH,salinity,exchangeable sodium,CEC,ESP,water-soluble HCO3-and Clcontents were reduced in the CG-LTA-treated group compared to the control group;whereas soil organic matter,nutrients,total nitrogen,soil enzyme activity,watersoluble SO42-,Ca2+ and Mg2+ contents were increased.CG-LTA molecular sieves as a silicate material,it helps to improve soil properties to reduce soil toxicity.In addition,CG-LTA zeolite molecular sieve is selective for major essential nutrients,including ammonium(NH4+),potassium(K+)and sulfate(SO42-),and its unique porous structure can reduce nutrient leaching,and its slow-release property will also improve water use efficiency and reduce environmental pollution caused by nitrate leaching.During the experiments,we also found that the elemental Si contained in CG-LTA molecular sieve can interact with soil organic matter to form monosilicicic acid and polysilicicic acid,thus having a high buffering capacity.In addition,monosilicicic acid can form complexes with organic and inorganic compounds and affect the activity of soil biota.In addition,the three-dimensional skeletal structure of CG-LTA molecular sieve leads to the formation of new water movement routes,and its addition can increase soil water infiltration rate and saturated hydraulic conductivity.In conclusion,CG-LTA molecular sieves can positively influence soil physicochemical properties,improve soil pore structure,enhance microbial activity and improve soil water storage capacity,and also improve soil structure by aggregating soil particles. |
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