Removal Efficiency And Accumulation Of Cadmium In Irrigated Water By Multistage Surface Flow Constructed Wetland

In order to reduce the Cd pollution in the paddy soil and reduce the risk of rice Cd pollution,the effect and mechanism of surface flow constructed wetland to remove Cd in irrigation water were explored.The experiment selected three wetland plants of Pontederia cordata,Myriophyllum verticillatum,and Wild Zizania to build a Cd micro-polluted irrigation water purification simulated surface flow wetland system（hereinafter referred to as simulated wetland）.The effect of wetlands on the removal of Cd in the water,and the effects of different influent Cd concentration and hydraulic retention time on the purification effect of the system were analyzed.Also,a pilot project of farmland irrigation water Cd purification artificial wetland system（hereinafter referred to as artificial wetland）was built for study.And long-term continuous monitoring was carried out to study its removal efficiency of Cd in irrigation water,and the rule of temporal variation of Cd enriched in wetland plants and sediments was analyzed.On the other hand,the Cd flux of the inflow and outflow of the artificial wetland and the total amount of Cd intercepted by each unit of the system were estimated.The main results are as follows:（1）In each treatment of a single-stage wetland,Myriophyllum verticillatum with Pontederia cordata mixed culture had better removal efficiency under different water inlet conditions;When the influent Cd concentration is 10μg·L^-1,under HRT12h and HRT24h,each single-stage wetland system can make the Cd concentration in water lower than the national standard GB 5084-1992 irrigation water quality standard.At HRT12h,the effluent quality of each single-stage wetland system did not meet the standard(>5μg·L^-1).Among the three-level simulated wetland treatment,the T2 treatment with three plants paired and connected in series is the best.At the influent concentration of 10 and 20μg·L^-1,both HRT12h and HRT24h can make the effluent Cd concentration lower than 5μg·L^-1,the effect of continuous removal of water Cd is better In a simulated wetland system,the sediments are used for heavy metals Cd in water The amount of entrapment is higher than that of plants,and it plays a major role in the entrapment of cadmium in water.The adsorption of Cd by sediments may be mainly due to the effect of organic components,and-COOH and-OH are effective functional groups for Cd adsorption.（2）Each unit in the"plant pond+artificial wetland+adsorption pond"purification system can effectively reduce the Cd concentration in the irrigation water and significantly reduce the suspended Cd and soluble Cd concentrations which the removal rates was 86.58%and 90.37%,respectively;The concentration of suspended particulates in the irrigation water inlet was significantly positively correlated with the concentration of suspended Cd,and the flow velocity of the water was significantly positively correlated with the concentration of suspended particulates.The wetland system had a good removal effect on suspended particulates,with an average removal rate of 81.54%.During the eleven-month monitoring period,the total input Cd flux of the purification system was about 954.95 g,and the effective interception of Cd was about 868.52 g.Specifically,sorting the interception of Cd from high to low by each unit as followed:first plant pond（544.64 g）>thrid free wetland with a adsorption tank（167.62 g）>second plant pond（156.26 g）（3）The three wetland plants in the artificial wetland purification system can effectively enrich trace Cd in the irrigation water,and the concentration as a whole gradually decreases with the flow of water.During the monitoring period,the maximum concentration of Cd in the belowground and aboveground of Pontederia and Wild Zizania.were 164.18,105.56,8.45,and 5.61 mg·kg^-1 respectively.The maximum Cd total enrichment values were reached in October and September,which were 6.26 and 6.13 mg·m^-2,respectively The maximum Cd concentration of Myriophyllum was 69.08 mg·kg^-1 and Cd total enrichment values reached 59.47 mg·m^-2 in September;The Cd concentration in the sediments of the wetland system is generally on the rise and the increase rate in the sediments of the first plant ponds is relatively fast.The Cd morphology of the sediments in the system is susceptible to changes in the environment.Compared with the beginning and end of monitoring during the year,the concentration of Cd in sediments mainly increases in a weakly acid-soluble form.