Study On The Adsorption Of Heavy Metals On Microplastics In Water And Their Combined Toxicity To Chlorella Vulgaris

The environmental pollution caused by microplastics(MPs)is a growing global concern.It has been shown that MPs can act as carriers of heavy metals in water bodies,altering their environmental behaviour and biological effects,thus posing potential risks to human health and aquatic environmental safety.Therefore,it is important to study the sorption behaviour of MPs to heavy metals in water bodies and their combined toxicity to organisms.In this paper,the sorption behaviour of different species of MPs to two heavy metals,Zn(II)and Cu(II),was analysed under laboratory conditions,and the combined toxicity of MPs and heavy metal Zn(II)exposure to Chlorella vulgaris was further investigated using Chlorella vulgaris as the target organism.The main findings are summarised as follows.The paper first investigated the sorption behaviour of four different microplastics(PE,PVC,PS and PA)in the water column towards the heavy metals Zn(II)and Cu(II).The equilibrium adsorption magnitudes of the four MPs on Zn(II)were: PVC(0.416 mg/g)> PA(0.371 mg/g)> PS(0.319 mg/g)> PE(0.190 mg/g)and on Cu(II)were: PVC(0.247 mg/g)> PS(0.153 mg/g)> PA(0.151 mg/g)> PE(0.112 mg/g),and the equilibrium adsorption of Zn(II)was greater than that of Cu(II).the adsorption isotherms of Zn(II)on all four MPs could be fitted with the Freundlich adsorption model,and the adsorption of Zn(II)by MPs was a multilayer adsorption occurring on a heterogeneous surface.Except for PS,the adsorption isotherms of Cu(II)on the other three MPs can be fitted with the Langmuir adsorption model,indicating that there is no interaction between the adsorbed molecules and only monolayer adsorption can be formed on the surface of MPs.Secondly,the thesis investigated the single effects of the heavy metals Zn(II),microplastic PE and PVC on Chlorella.High Zn(II)exposure inhibited the growth of Chlorella and significantly increased the intracellular SOD activity and CAT activity,inhibited the photosynthetic activity of Chlorella and reduced the total chlorophyll content,as well as inhibiting the maximum photosynthetic efficiency and the efficiency of the exoxygenation complex.A single PE exposure also inhibited the growth of Chlorella,but the effect on the photosynthetic activity of Chlorella was not significant.High PE exposure significantly increased the SOD and CAT activities in Chlorella cells.PVC inhibited the growth of Chlorella to a greater extent than PE,with an EC50 of 6.19 mg/L at 72 h.Low PVC exposure significantly increased the SOD and CAT activities in Chlorella cells.PVC exposure significantly affected the photosynthetic activity of Chlorella and significantly reduced the maximum photosynthetic efficiency and oxygen release complex efficiency of Chlorella PSII.The maximum photosynthetic efficiency and the efficiency of the oxygen release complex of Chlorella vulgaris were significantly reduced.The combined exposure to PE and Zn(II)significantly inhibited the growth of Chlorella and also significantly reduced its photosynthetic activity.Quantitative results of apoptosis by flow cytometry showed that the combined effect of PE and Zn(II)increased the cell size and complexity of Chlorella cells,as well as disrupting the integrity of cell membranes and inducing early apoptosis.The combined effect of PE and Zn(II)significantly reduced the photosynthetic activity of Chlorella and also induced severe oxidative stress,but inhibited the antioxidant enzyme activity of Chlorella to a lesser extent than the single exposure of Zn(II),suggesting that PVC may The combined effect of PVC and Zn(II)increased the cell size and complexity of Chlorella cells,as well as disrupting the integrity of Chlorella cell membranes and inducing early cell apoptosis.