Mechanisms Of Phosphorus Removal By Substrates Coated With Zn-layered Double Hydroxides In Constructed Wetlands

Eutrophication caused by the phosphorus pollution has been received extensively concern.Constructed wetlands(CW)have the advantages of simple process,good remove performance and low operation costs.But the substrates of CW still face the problem of limited adsorption capacity and easy to saturation.Therefore,it is imperatively to select a kind of substrates which can efficiently remove the phosphorus,prevent the clog problem and extend the life cycles in order to cope with the high hydraulic load of pollutants and reduce the using land in constructed wetlands.Additionally,layered double hydroxides(LDHs)exhibit the properties of large surface area and high anion exchange capacity.However,the application in vertical-flow constructed wetlands(VFCW)and ecological restoration projects are still unsolved due to the powder form used to phosphorus adsorption.In present study,bio-ceramic and zeolites in CW were selected as the natural substrates in order to in-situ coated three kinds of different Zn-LDHs on the surface.Zn-LDHs(FeZn-LDHs,CoZn-LDHs,AlZn-LDHs)were synthesized by co-precipitation method in alkaline condition.The natural bio-ceramic and zeolites before and after modification were filled into the columns for the purpose of removing the phosphorus in municipal wastewater.Purification,isothermal adsorption,desorption and kinetic adsorption experiments were used to study the removal mechanisms of phosphorus by the Zn-LDHs modified bio-ceramic and zeolites.Scanning electron microscope(SEM)and X-ray fluorescence spectrometer(XRFS)results indicated that Zn-LDHs were successful loaded on the surface of the natural bio-ceramic and zeolites.Compared with the natural bio-ceramic and zeolites,the removal rates of phosphorus by the Zn-LDHs modified bio-ceramic and zeolites were greatly improved,especially for the Zn-LDHs modified zeolites which the improved removal rates were higher than the Zn-LDHs modified bio-ceramic.Simultaneously,the removal rates of the phosphorus by the three different Zn-LDHs modified bio-ceramic and zeolites followed the order of FeZn-LDHs > AlZn-LDHs > CoZn-LDHs.The maximum theoretical adsorption of phosphate by the Zn-LDHs modified bio-ceramic and zeolites were increased compared to the natural bio-ceramic and zeolites,indicating that the adsorption capacity of phosphate were enhanced after the Zn-LDHs modification.The kinetic adsorption of phosphate by the Zn-LDHs modified bio-ceramic and zeolites were better fitted to the pseudo-second-order model,except for the natural bio-ceramic and zeolites.The removal mechanisms of the particulate phosphorus by the FeZn-LDHs,AlZn-LDHs modified bio-ceramic and zeolites were physical interception and adsorption.However,the removal mechanisms of the particulate phosphorus by the CoZn-LDHs modified bio-ceramic and zeolites,as well as the natural bio-ceramic and zeolites,were physical interception.The removal mechanisms of the phosphate by the Zn-LDHs modified bio-ceramic and zeolites were physicochemical adsorption and chemical reaction.Moreover,the adsorption type of the phosphate by the bio-ceramic and zeolites were converted physical adsorption into chemical adsorption after the Zn-LDHs modification.