Study On The Purification Effect Of Several Microalgae And Submerged Plants On Heavy Metal Wastewater

The discharge of heavy metal wastewater is one of the pollution sources that affect water quality.The direct discharge or use of heavy metal wastewater can cause significant harm to the environment and human health.Traditional sewage treatment methods have high costs,are difficult to maintain,and may also bring secondary pollution.Moreover,algae and submerged plants are widely distributed,with diverse sources and species.The use of algae or submerged plants for the remediation of water bodies containing heavy metals has broad application prospects.This experiment screened out the species with better adsorption performance for Cu²⁺,Cr²⁺,Pb²⁺,Cd²⁺,and Hg²⁺in environments with different concentrations of heavy metals,including three microalgae,Chlorella vulgaris,Oocystis,and Scenedesmus obliquus,as well as three submerged plants,Ceratophyllum demersum,Myriophyllum verticillatum,and Vallisneria natans.Then set different temperatures,initial concentrations of heavy metals,and time gradients of experiments for microalgae and submerged plants,and detect the content of heavy metals in the solution.Analyze from the kinetic and thermodynamic aspects,and fit the experimental data with pseudo-first-order kinetic equation,pseudo-second-order kinetic equation,Langmuir isotherm equation,and Freundlich isotherm equation.The experimental conclusion is as follows:（1）The adsorption of Cd²⁺by Chlorella vulgaris can be enhanced by increasing its concentration at the same temperature.The adsorption efficiency of Cd²⁺at an initial concentration of 7.5 mg/L was 20.9%higher than that of 3 mg/L.The adsorption efficiency of Pb²⁺and Hg²⁺by oocysts shows a decreasing trend with increasing concentration.Compared with the initial concentration of 50 mg/L,the adsorption efficiency of Pb²⁺at an initial concentration of 150 mg/L decreased by 39.3%.Compared with the initial concentration of 5 mg/L,the adsorption efficiency of Hg²⁺at an initial concentration of 15mg/L decreased by 24.6%.（2）By constructing a kinetic model,it was found that the adsorption processes of microalgae and submerged plants were more in line with the pseudo-second-order kinetic model.The q_e of Chlorella vulgaris to Cd²⁺at different initial concentrations were 7.81mg/g,14.03 mg/g,17.83 mg/g,23.51 mg/g,and 24.14 mg/g,respectively.The q_e of Pb²⁺in Oocystis at different initial concentrations were 146.41 mg/g,216.92 mg/g,284.90 mg/g,290.70 mg/g,and 311.53 mg/g,respectively.The q_e of Hg²⁺at different initial concentrations was 16.42 mg/g,22.87 mg/g,28.10 mg/g,35.40 mg/g,and 41.93 mg/g.The q_e of Myriophyllum verticillatum to Pb²⁺at different initial concentrations were 8.01 mg/g,16.80 mg/g,21.88 mg/g,30.71 mg/g,and 33.70 mg/g,respectively.The q_e of Ceratophyllum demersum to Cd²⁺at different initial concentrations were 0.85 mg/g,1.23mg/g,1.53 mg/g,2.30 mg/g,and 2.40 mg/g,respectively.The q_e of Hg²⁺at different initial concentrations was 2.50 mg/g,3.18 mg/g,3.84 mg/g,4.80 mg/g,and 6.26 mg/g,respectively.（3）By constructing a thermodynamic model,it was found that the adsorption processes of microalgae and submerged plants were more in line with the Langmuir adsorption isotherm model.The q_m of Chlorella vulgaris to Cd²⁺at different temperatures was 71.01 mg/g,82.51 mg/g,and 60.35 mg/g,respectively.The q_m of Oocystis to Pb²⁺was407.09 mg/g,509.99 mg/g,and 630.08 mg/g,and the q_m of Hg²⁺was 56.08 mg/g,56.72mg/g,and 79.99 mg/g.The q_m of Myriophyllum verticillatum to Pb²⁺was 25.12 mg/g,26.47 mg/g,and 27.35 mg/g,while the q_m of Ceratophyllum demersum to Cd²⁺was 10.69mg/g,7.42 mg/g,and 3.68 mg/g.The q_m of Ceratophyllum demersum to Hg²⁺was 4.07mg/g,8.77 mg/g,and 13.52 mg/g.（4）The adsorption of heavy metals by microalgae is a rapid adsorption process,which is a spontaneous endothermic reaction,and the higher the temperature,the more spontaneous the better.Although the adsorption of heavy metals by submerged plants is also a spontaneous endothermic reaction,the higher the temperature,the worse the spontaneity,indicating that the adsorption of metal ions by submerged plants is different from that of microalgae.（5）The main functional groups that affect the adsorption of Pb²⁺,Cd²⁺,and Hg²⁺by microalgae are carboxyl,hydroxyl,and amide groups.The main functional groups that affect the adsorption of Cd²⁺by Chlorella vulgaris are-NH₂,C-H,C=O,-OH,and C-O-C in ester groups-NH₂ shifted from 3427 cm^-1 to 3429 cm^-1,and C=O shifted from 1648 cm^-1to 1652 cm^-1.And from the spectrum,it can be seen that there are more types of sugars on the cell wall of Chlorella vulgaris,while the content of lipids is relatively low.The functional groups that affect the adsorption of Pb²⁺and Hg²⁺by Oocystis are still-NH₂,C-H,C=O,-OH,and C-O-C in ester groups.When Oocystis adsorb Pb²⁺,-NH₂ shifts from3427 cm^-1 to 3421 cm^-1,C-H shifts from 2920 cm^-1 to 2925 cm^-1,and-OH shifts from 1249cm^-1 to 1238 cm^-1.When Oocystis adsorb Hg²⁺,-NH₂ shifts from 3427 cm^-1 to 3423 cm^-1,and C-O-C shifts from 1070 cm^-1 to 1046 cm^-1.