| Controlling of the input flux of Cd into farmland in irrigation water is an important way to reduce the Cd pollution in farmland soil and the accumulation of Cd in rice.In this paper,three common wetland plants,the barracuda,Myriophyllum sp,and Hydrilla verticillata,were selected to building a microcosms and a in situ?irrigation water purification pilot?wetland system for comparing the remove efficiency of Cd.The remove efficiency of Cd was studied under static and dynamic conditions in the microcosms and the effects of different culture environments,Cd concentration,HRT on the simulated purification system was also investigated.Furthermore,the removal ability and enrichment of Cd in different plants over time in the microcosms system was analyzed.The constructed in situ wetland system was built by“sanding pool+first-class plant pond+secondary plant pond+third-stage surface flow constructed wetland system+adsorption tank”.To explore thepurification effect of three kinds of wetland plants on the removal of irrigation water with excessive Cd(the average concentration of full amount Cd is about 6.65 g·L-1)in typical mining areas in Hunan province and the resistance control effect of rice Cd.The influence of the concentration of water in and out and the dry and wet deposition flux on the purification effect of the system and the adsorption capacity of wetland plants in the purification system was also explored.The results show that:1)The three aquatic plants have higher static removal rate for Cd in hydroponic environment and close to maximum removal efficiency at 12h.The two sediment culture environments have better removal rate than hydroponic environment,and the highest removal rate can be It reached 98.6%.The three plants had great adsorption capacity for cadmium,and concentration in the barracuda grass was up to 89.9 mg·kg-1,and the concentration of Cd 2+in the water after several plants treated under different culture environments was below In 5?g·L-1?GB 5084-1992 irrigation water quality standard?,and 3plants are selected for one-third of the biomass mixed cultivation conditions had better effect.In the dynamic experiment,the effluent Cd concentration of each simulated wetland system in different culture environments was significantly reduced to below 3?g·L-1.HRT had a effect on the purification efficiency of cadmium.The removal capacity was as follows:HRT24 h>HRT12 h;In order to avoid the risk of water quality substandard,multi-stage constructed wetlands are a good choice.The removal of Cd in the sediment culture environment has a better effect.Considering of the experimental design,the removal effect is:non-polluting sediment environment>moderately polluted sediment environment>hydroponic environment.?2?The three aquatic plants of barracuda,Myriophyllum sp.,and Hydrilla verticillata have great removal ability for Cd in irrigation water.The average removal rate of total and soluble Cd in irrigation water after system purification is as high as 70%and 91%respectively.It can effectively reduce the cadmium in the wetland system and prevent the migration of Cd into the paddy field.The concentration of Cd in the wetland inflow easily fluctuated with rainfall,but the removal effect on the wetland system was not affected.There was a linear positive correlation between rainfall and Cd wet deposition flux,and a linear negative correlation with Cd dry deposition flux.During the monitoring period?April 16,2017-September 17,2017?,the total input of the Cd of the 3-stage purification system was428.46 g,of which the dry sedimentation settlement was 20.52 g,the wet deposition settlement was 57.60 g.The system effectively intercepted Cd accounts for 87.94%of the total input.Dry and wet deposition is one of the sources of external Cd input,which has no significant effect on the operation of the purification system.In the 2017-2018,under the normal operation of wetlands,the Cd content in rice roots,stem sheaths,leaves,chaff and brown rice in the irrigated area after purification decreased significantly by 32?27%and31?26%,38?44%,48?49%,41?30%respectively,compared with that in the untreated irrigated area.?3?The Cd content in the sediment of each unit in the wetland purification system gradually decreased with the flow of water direction;and increased with sampling time,the variation of Cd content in the sediment ranged from 1.15 to 38.58 mg·kg-1,and the enrich rate of Cd is relatively fast near the water inlet.The content of Cd in the belowground part of barracuda grass is up to 125.88 mg.kg-1.The content of Cd in the aboveground part is lower,the highest is 9.16 mg.kg-1.The enrichment and transport coefficient ranges are 8.24-34.96and 0.036-0.176 respectively.It indicated that most of the Cd in the barracuda grass was fixed at the root,and the upward transport was less,and the concentration of Cd in the stem and leaf was less changed.Barracuda has the potential to purify Cd in farmland irrigation water.Foxtail algae and Hydrilla verticillata showed a slow downward trend with water flow direction in each sampling.By the last sampling time,the Cd concentration in the two plants was 32.25 mg·kg-1.6.94 mg·kg-1 decreased to 1.19 mg.kg-1 and 1.5mg.kg-1 through water flow direction which indicating that it has strong ability to adsorb Cd. |
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