Optimization Of Mercury Removal Efficiency In Vertical Subsurface Flow Constructed Wetlands

Mercury pollution in the environment,which has four characteristics of long-term existence,long-distance transmission,high biological toxicity and bioaccumulation,has become an increasingly serious global environmental problem.Because of the special physical and chemical properties of mercury,it can enter various ecosystems through sedimentation mechanism,and all kinds of water bodies are also threatened by mercury pollution due to human production activities.As a commonly used sewage treatment process,constructed wetlands have a good removal effect on heavy metals.In current study,based on the constructed wetland mercury removal efficiency,the comparison and selection of wetland plants and wetland fillers were carried out to build an optimized wetland system,and the operation conditions of the system were analyzed.(1)Four kinds of wetland plants including Canna indica L.,Thalia dealbata Fraser,Phragmites australis L.and Typha latifolia were selected,and the plant species with the best mercury enrichment and transfer efficiency were screened out through the hydroponic culture experiment with mercury containing nutrient solution,the result showed that: the four kinds of wetland plants grow well in the three concentrations(51.67±3.84 μg/L,108.34±5.63 μg/L,221.42±7.31 μg/L)of mercury-containing hydroponic nutrient solution,and their absorption of mercury was mainly enriched from the root system,and the enrichment capacity was: Thalia dealbata Fraser > Canna indica L.> Phragmites australis L.> Typha latifolia;Part of the mercury in the root system of the plants was transferred to the leaves through the roots and stems,and the transfer capacity was: Thalia dealbata Fraser > Canna indica L.> Typha latifolia >Phragmites australis L,about 52.8%-64.9% of mercury was trapped in the root system;The amount of mercury absorption and transfer by wetland plants was roughly positively correlated with the concentration of mercury in the culture fluid and the cultivation time.(2)Three kinds of fillers including gravel,zeolite,ceramsite were selected as wetland fillers to construct four plant-free vertical submerged constructed wetlands with three single fillers and one combined filler.Mercury-containing influent test was carried out to select the best assembly method of wetland filler for purification effect,the result showed that: the four plant-free vertical subsurface flow constructed wetlands with different fillers(CW-1 gravel,CW-2 zeolite,CW-3 ceramsite,and CW-4combined filler)had the total mercury(THg)removal rate of 63.6%-80.5% for different influent concentrations(54.49±3.71 μg/L,107.38±5.65 μg/L,216.12±8.95 μg/L),the THg purification capacity of the 4 wetlands was: CW-4> CW-2> CW-3> CW-1;The removal rate of particulate mercury(PHg)was 62.5%-85.1%,the PHg purification capacity was CW-4 > CW-2 > CW-3 > CW-1;The removal rate of dissolved mercury(DHg)was 63.3%-74.8%,the DHg purification capacity was CW-4 > CW-2 > CW-1 >CW-3.(3)The optimized wetland system CW-5 was constructed with the selected Thalia dealbata Fraser and combined filler,and the mercury removal efficiency of CW-5 and the influence of HRT and temperature on the mercury removal efficiency were analyzed through the mercury containing influent operation test.The results showed that: at three influent water concentrations(58.84±4.26 μg/L,114.74±6.82 μg/L,224.75±9.47 μg/L),comparing with CW-4 with the same filler structure but no plants,the THg,PHg and DHg removal rate of CW-5 increased by 7.6%-8.8%,4.9%-6.0%,16.7%-19.3%respectively,and the contribution rate of plants to the removal of this three forms of mercury was 8%,5% and 15%-20% respectively;Within the set HRT range(12 h-72h),the removal rate of THg was roughly positively correlated with HRT(74.7%-84.0%),the effect of HRT on PHg removal was not significant(76.5%-82.4%),but significant affecting the removal of DHg(61.4%-94.5%),and the most cost-effective HRT for CW-5 was 24 h;In the test temperature range(5 ℃-35 ℃),the removal rates of THg,PHg and DHg were roughly positively correlated with temperature,which were 70.5%-94.7%,72.2%-94.3%,42.3%-97.2% respectively.The current study provided an alternative for the ecological restoration of Hg contaminated water.