Adsorption Properties Of Heavy Metal Ions And Environmental Risks Control By Different Types Of Spirulina Platensis

Due to its high bioaccumulation and toxicity,heavy metals have focused global attention on the pollution of heavy metals in water.Lead and cadmium are two very toxic heavy metals that are common in heavy metal polluted water.Eutrophication is another extremely challenging problem in water pollution.Especially,algae outbreaks in urban water supply reservoirs are very easy to cause the filter material of the water supply pipe network to block and possibly penetrate into the water supply network.Therefore,drinking water treatment plants use polyacrylamide（PAM）to remove microalgae,but will produce microalgae sludge waste containing PAM.The resource utilization of microalgae waste is an effective treatment method for proper treatment to avoid secondary pollution.In this paper,Spirulina platensis,which is common in the water bodies of Northwest China,was selected as the research algae species.Spirulina platensis was collected by PAM to simulate the microalgae sludge waste in drinking water plants.The microalgae sludge was prepared into microalgae powder by traditional high temperature oven drying and vacuum freeze drying.At the same time,it was compared with pure algae powder that had not undergone PAM flocculation under the same drying conditions.The adsorption effect and adsorption mechanism in the simulated lead and cadmium single system and binary system wastewater was explored.The use of traditional algae powder will lead to an increase in the content of dissolved organic matter in the water body,resulting in potential environmental risks.Ultrasonic crushing and vacuum freeze-drying methods were used to improve the preparation technology of algae powder to explore its adsorption performance and risk control The results are as follows:（1）Oven-dried-PAM-Spirulina platensis powder（Op）,vacuum freeze-dried-PAM-Spirulina platensis powder（Vp）,oven-dried Spirulina platensis powder（O）,and vacuum freeze-dried Spirulina platensis powder（V）had a high adsorption capacity for Pb²⁺and Cd²⁺.Moreover,the adsorption capacities of these four algae powders were better than those of the majority of the other adsorbents studied recently for removing both Pb²⁺and Cd²⁺,and the adsorption effect of the two freeze-dried algae powders was more significant.By contrast,the affinity of Pb²⁺and algae powders was stronger than that of Cd²⁺.Vp and V had a better adsorption effect on Pb²⁺,while Cd²⁺and Op had a better binding effect.When 0.5 g/L algae powder was added to a Pb²⁺solution with a p H value of 5.0,the adsorption effect was the best at 25℃.For Cd²⁺,7.0 was the optimal adsorption p H value,and other adsorption conditions were the same as Pb²⁺.The adsorption behavior of four algae powders in Pb²⁺and Cd²⁺solutions followed a pseudo-second-order kinetic model.The Langmuir model could well fit the adsorption process of four algae powders for Pb²⁺.For Cd²⁺,except that Op was suitable for the Freundlich model,the Langmuir model fitted the remaining three algae powders better.The order of the maximum adsorption capacity（Q_m）of the four algal powders calculated by the model to Pb²⁺is:V>Vp>Op>O,and the order of Cd²⁺is Op>O>Vp>V.（2）In the binary system of Pb²⁺and Cd²⁺,the optimal adsorption conditions for Pb²⁺and Cd²⁺by four kinds of algae powder are 0.5 g/L dosage,solution p H 5.0 and25℃.Due to competitive adsorption,compared with the single system,the adsorption capacity of Pb²⁺and Cd²⁺of the four algal powders in the binary system has decreased.In addition,when the initial concentration is higher than 10 mg/L,the decrease in Cd²⁺adsorption in the presence of Pb²⁺was greater than the decrease in Pb²⁺adsorption in the presence of Cd²⁺.The initial metal concentration in the binary system had a significant effect on the Q_b/Q_sratio,indicating that Pb²⁺and Cd²⁺had competitive adsorption.The adsorption process followed the pseudo-second-order kinetic model and the Langmuir model.（3）Vp belonged to mesoporous and macroporous materials,while the remaining three algae powders were all macroporous materials.Scanning electron microscopy showed that Op cells and O cells had adhesions,and Vp cells and V cells after vacuum freeze-drying had better dispersion and the integrity of Vp cells was better than Op cells.Energy dispersive X-ray analysis confirmed that Pb²⁺and Cd²⁺had been attached to the algae powders separated from the metal solution.The hydroxyl groups,amino groups,carboxylates,ph Ohates and amides on the surface of the four algal powders materials participated in the adsorption process.（4）Compared with traditional drying algae powder Op,the vacuum freeze-dried-PAM-Spirulina platensis powder（Vp）,ultrasonic crushed-oven drying Spirulina platensis powder（UOp）and ultrasonic crushed-vacuum freeze-dried PAM-Spirulina platensis powder（UVp）can effectively control DOC residual in water caused by the adsorption of Pb²⁺by the algae powders.The DOC residual in water increased with the increase of the dosage of algae powder and the extension of time.Compared with Op,the three modified algae powders had better biosorption effect on Pb²⁺,and the time required to reach adsorption saturation was shorter.Specifically,the maximum biosorption of Pb²⁺by UVp is 582 mg/g,which was more than twice that of Op.It reached adsorption saturation within 40 min,11 h shorter than O.The Langmuir isothermal model can fit the adsorption of Pb²⁺by four algae powders.The preliminary application opinions on the treatment of heavy metal wastewater with Vp,UOp and UVp were given:0.5 g/L Vp staying in Pb²⁺solution for 2 h is the best method.In this case,the DOC residue in water is only 3.45 mg/L and the adsorption capacity of Pb²⁺is 85.89 mg/g;0.5 g/L UOp stays in the wastewater for 1 h.The DOC residue is only 10.88 mg/L and the adsorption capacity of Pb²⁺reaches 95.10 mg/g;0.5g/L UVp stays in Pb²⁺solution for 40 min.The DOC residue is 9.82 mg/L,the adsorption capacity of Pb²⁺reaches 98.66 mg/g.