Study Of Synthesis,Characteristics And Chromium Purification Performance Of Modified Zeolites Coated With Zn-LDHs

Nowadays,more and more wastewater containing heavy metals are discharged,and chromium pollution is one of the most common heavy metal pollution.Hexavalent chromium?Cr????has high toxicity,and it is difficult to be biodegraded,which leading to a Cr???accumulation in the human body through the food chain.More concerns have been paid to chromium pollution treatment.Constructed wetlands?CW?have the advantages of low energy consumption,easy to operate,low operation costs,and good removal efficiency.However,the substrates in CW often face the problem of limited adsorption capacity.Therefore,it is crucial to select a kind of efficient substrate which has high adsorption capacity,and can prevent the clog problem.Layered double hydroxides?LDHs?have a good adsorption effect on Cr???.However,LDHs are difficult to be applied into constructed wetlands due to its powder form.In present study,zeolites were selected as the original substrates,and four kinds of ZnAl-LDHs?Cl-LDHs?1:1?,Cl-LDHs?3:1?,CO₃-LDHs?1:1?,CO₃-LDHs?3:1??were synthesized and coated on original zeolites via co-precipitation method.The modified and original zeolites were filled into simulated purification experimental system to conduct Cr???removal study.Moreover,the effects of reaction conditions?dosage,pH,temperature,and coexisting ions?on Cr???adsorption with modified and original zeolites were also investigated.The results of Field Emission Scanning Electron Microscope?FE-SEM?showed that the apparent appearance of original zeolites had a significant change after ZnAl-LDHs coating modification.Energy Dispersive Spectrometer?EDS?results indicated that some typical elements of ZnAl-LDHs were detected on the surface of original zeolites after modification.The results of X-ray diffraction?XRD?and Fourier transform infrared?FT-IR?both suggested that the typical diffraction peak and functional groups of ZnAl-LDHs were detected in the coating samples of modified zeolites.All above results proved that ZnAl-LDHs were successfully synthesized and coated on original zeolites.In the purification study,it was found that different kinds of ZnAl-LDHs coating had promotion on Cr???removal performance.Moreover,Cl-LDHs?3:1?modified zeolites had the best Cr???removal effect.The study of reaction conditions further showed that Cl-LDHs?3:1?modified zeolites had the highest Cr???adsorption capacity,and the Cr???adsorption capacity of Cl-LDHs?3:1?modified zeolites reached the peak value at the condition of weak acid.The adsorption capacity of modified zeolites enhanced with increasing temperature,but original zeolites showed the opposite phenomenon.The effect of coexisting ions on Cr???removal with modified zeolites ranked from high to low:PO₄^3->SO₄^2->Mg²⁺>Ca²⁺.Meanwhile,Cl-LDHs?3:1?coating modification availably strengthened the regenerability of original zeolites.To sum up,it is effectual to synthesize and coat ZnAl-LDHs on the surface of original zeolites by co-precipitation method in alkaline condition.Among the modified and original zeolites,Cl-LDHs?3:1?modified zeolites showed the best Cr???removal performance,which suggested that Cl-LDHs?3:1?modified zeolites has the potential to be applied to remove Cr???in practical engineering of constructed wetlands.