The Responses Of Plant And Characteristics Of Pollutant Removal In Subsurface Flow Wetland Under Sodium And Calcium Salt Stress

Sodium and calcium salts can destroy the osmotic balance in the root zone of the constructed wetland,thus inhibiting the growth of plants,resulting in dehydration and inactivation of microorganisms,and affecting the removal effect of pollutants in the constructed wetland.Wetlands with different plants and different depths have different root zone environments,and the advantages and disadvantages of facing sodium and calcium salt stress are not clear,and the mechanism has not been fully clarified.To this end,calamus and White palm were used as cultivated plants in wetlands.By comparing the correlation between physiological response,root zone environment characteristics and wetland pollutant removal effect of calamus and white palm in two depths of wetlands under sodium and calcium salt stress,the effects of wetland depth and wetland plant changes on wetland treatment of sodium and calcium salt wastewater were discussed.In order to provide theoretical support for the optimization of the design and operation of subsurface flow wetland for the treatment of saline wastewater.The main conclusions are as follows:（1）Under metal salt stress,the response of calamus and White palm is:the deterioration of appearance,the decrease of chlorophyll content,the slow growth of shoot,the blackening of root system under high osmotic pressure,and the maintenance of plant growth by the way of outwards salt from stem.The deterioration of plant appearance in0.6-CW was weaker than that in 0.1-CW.In other words,from the perspective of plant growth,plants in 0.6-CW were more resistant to metal salt stress,and white palm was more resistant to metal salt stress.（2）Under metal salt stress,chlorophyll content of calamus and White palm decreased,FAS content and MDA content increased,that is,metal salt affected the photosynthesis of plants,broke the osmotic balance in plants,resulting in lipid peroxidation of plant cell intima;However,0.5%NaCl or 50 mmol/LCaCl₂ did not cause serious damage to Calamus and white palm within the tolerance threshold range,while1%NaCl or 100 mmol/LCaCl₂ exceeded the tolerance threshold of calamus and white palm,causing irreversible effects on both calamus and white palm.（3）Under metal salt stress,the total antioxidant capacity of Calamus and White palm increased with time,but the total antioxidant capacity of White palm was higher than that of Calamus,that is,the antioxidant system of white palm plant was more sensitive and stronger than that of Calamus.（4）In 0.1-CW,the DO was slightly higher,the ORP was lower,the abundance of facultative heterotrophic proteobacteria was higher,and the removal effect of organic matter was better than 0.6-CW.Metal salts inhibit plant growth,reduce DO content and microbial diversity in the environment,resulting in poor TOC removal effect.（5）The dissolved oxygen content of 0.1-CW was higher than that of 0.6-CW,which was consistent with previous studies of the research group.The aerobic nitrification in0.1-CW was more thorough due to the larger root content of white palm and the higher DO content of white palm wetland.Under metal salt stress,the growth of plants is inhibited,and the root system appears black and necrotic phenomenon at the highest concentration,which leads to the decrease of DO content in wetlands,thus inhibiting the aerobic nitrification process in wetlands.（6）The DO value in 0.6-CW is less than 0.1-CW,that is,0.6-CW provides a better anaerobic environment for anaerobic denitrification,but the ORP value in 0.1-CW is less than 0.6-CW,which makes 0.1-CW more suitable for the growth of anaerobic denitrification bacteria.Therefore,the removal effect of NO₃^--N in 0.1-CW is not as good as 0.6-CW.However,the NO₃^--N removal rate of the two can reach more than 90%.Most ORP in 0.6-CW is in the range of 100 m V～200 m V,and aerobic microorganisms can grow.Most ORP in 0.1-CW is in the range of-200 m V～0 m V,and facultative anaerobic microorganisms can grow.（7）The removal mechanism of P was different among different plants.Acorus calamus was the dominant species in the process of phosphorus removal,and the phosphorus removal effect of 0.1-CW was better than 0.6-CW in acorus calamus wetlands.The higher the concentration of NaCl,the stronger the reducibility in the environment,and the inhibition of phosphorus removal in the wetland becomes more serious over time.However,the higher the concentration of CaCl₂,the weaker the reducibility in the environment.Therefore,long-term CaCl₂ treatment can promote the removal of P in the wetland.This study also indirectly indicates that substrate adsorption is not the main way of phosphorus removal in all wetlands.