Iron-based Materials To Strengthen Nitrogen And Phosphorus Removal Performance Of Constructed Wetlands

Constructed wetland is a low-cost and eco-friendly wastewater treatment technology that has been widely used in various types of wastewater treatment,but in practical engineering,the performance of traditional constructed wetland is limited by factors such as carbon source and wetland structure,resulting in limited efficiency of nitrogen removal and phosphorus removal.Previous research by the group found that iron-based materials,represented by sponge iron,have been shown to be effective in improving the efficiency of nitrogen removal in wastewater treatment,and have the ability to enhance phosphorus removal.However,little research has been conducted on the effect and mechanism of iron-based materials on the enhanced performance of different types of constructed wetlands for nitrogen and phosphorus removal,and whether the addition of iron-based materials will cause stress to wetland plants needs to be further investigated.Therefore,it is important to investigate the enhancement mechanism and effect of iron-based materials on the performance of constructed wetlands in nitrogen and phosphorus removal for further application of constructed wetlands in wastewater treatment.In this study,the adsorption performance of two iron-based materials,sponge iron and pyrite,was compared with conventional fillers through adsorption kinetics and adsorption isotherm experiments.Two wetland types,tidal constructed wetland and aeration baffled constructed wetland,were adopted to investigate the effect of the type of iron-based materials,the dosage and the operation mode on nitrogen and phosphorus removal.The role of plants in the removal of nitrogen and phosphorus in constructed wetlands and the stressing effect of iron-based materials on wetland plants were discussed,and the mechanism of enhanced removal of nitrogen and phosphorus in constructed wetlands by the two iron-based materials was investigated.The results of the study showed that:（1）Through adsorption kinetics and isotherm experiments,both Langmuir and Freundlich equations can better describe the adsorption process of NH₄⁺-N by gravel,sponge iron,pyrite,ceramsite,and volcanic rock,and the adsorption process of TP by the five fillers is more consistent with Langmuir isotherm adsorption model.Volcanic rock,sponge iron and ceramsite had stronger adsorption capacity and larger theoretical adsorption amounts of 0.267 mg/g,0.228 mg/g and 0.192 mg/g,respectively.From the simulation results,it can be seen that sponge iron,ceramsite and volcanic rock had stronger theoretical adsorption capacity and larger adsorption amounts of 0.254 mg/g,0.206 mg/g and 0.195 mg/g.（2）The removal effects of pyrite and sponge iron tidal constructed wetlands on nitrogen and phosphorus were investigated at 1:3,1:5 and 1:7 dosing rates and different inundation and emptying times,and in the tidal constructed wetlands the pyrite wetlands at1:7 had better removal effects on TIN,the sponge iron wetlands had better removal effects on TP.The best TIN removal rate was obtained when the F/R was 24/24.The comprehensive experimental results concluded that pyrite is more suitable as a filler for tidal constructed wetland,and the optimal operation method is 24h/24h for F/R and 1:7 for the optimal dosage.（3）In the aeration baffled constructed wetland,the best removal of TIN was achieved with a sponge iron to gravel mass ratio of 1:5.The best removal of TIN was achieved at HRT of 36h and the best removal of TP was achieved at HRT of 18h.The optimum dosage of sponge iron to gravel mass ratio of 1:5 was used to obtain the best denitrification and phosphorus removal efficiency at HRT of 36h.（4）The results of nitrogen and phosphorus removal at different stages and sequential batch experiments,as well as the monitoring of environmental indicators such as p H and dissolved oxygen and high-throughput sequencing results,we inferred the mechanism of action of iron-based materials in the process of nitrogen and phosphorus removal in artificial wetlands.The hydrogen precipitation corrosion and iron-carbon microelectrolysis processes accompanying the redox process of sponge iron enriched the iron-reducing bacteria represented by Desulfuromonas and the autotrophic denitrifying bacteria represented by Hydrogenophaga,ultimately realising the biological denitrification process with iron intervention.The process of phosphorus removal by iron sponge was mainly achieved by adsorption and precipitation;the addition of pyrite enriched the autotrophic denitrifying bacteria represented by Thauera and Ellin 6067,and also increased the abundance of heterotrophic denitrifying bacteria of SWB02,achieving a mixed nutrient denitrification process combining autotrophic denitrification and heterotrophic denitrification,while generating precipitation for the removal of nitrogen and phosphorus.The wetland was used for the removal of nitrogen and phosphorus.In terms of the effect of the wetland on plant stress,the plants were most severely stressed in the wetland with the highest iron dosing,and the stressing effect of the iron-based material on the plants gradually decreased with decreasing iron dosing.