自然風化及加速風化下塑膠微粒的特性變化及重金屬吸附

Abstract

塑膠因成本低、易於加工及良好穩定性，被大量應用於食品包裝與容器產品上，然而當塑膠受到紫外線照射、熱氧化及生物活動等影響，會逐漸風化及降解，進而改變塑膠之物化性質並形成塑膠微粒。這些塑膠微粒可能成為環境污染物的載體，對環境及生態造成衝擊。 本研究探討「聚丙烯 (Polypropylene, PP)」、「低密度聚乙烯 (Low Density Polyethylene, LDPE)」及「高密度聚乙烯 (High Density Polyethylene, HDPE)」三種塑膠微粒的風化結果，實驗中以兩種風化方式比較，包含將塑膠微粒置於戶外，暴露於陽光下自然風化；同時將其置於長弧氙燈照射下加速風化。取樣分析兩種方式對三種塑膠微粒物化性質的影響，以瞭解自然風化與加速風化間的相關性與差異。 實驗結果顯示光氧化與熱氧化導致塑膠表面顏色改變、表面變為粗糙且出現破裂、表面形成官能基、平均粒徑變小、比表面積增加及表面電負性增強，因此形成風化塑膠微粒。三種塑膠微粒中，聚丙烯由於分子鏈存在較多支鏈，易在照射紫外線後發生斷鏈進而氧化，故平均羰基指數較高密度聚乙烯大。此外，加速風化時因溫度較自然風化時高，風化塑膠微粒之傅立葉轉換紅外線光譜的吸收峰值較大，並發生峰值偏移現象，這表明高溫可能促使不同於自然風化的化學反應發生。 本研究另外進行聚丙烯微粒吸附鉛的實驗，結果顯示聚丙烯表面會發生競爭吸附，因此吸附量隨pH值增加而增加，隨鹽度增加而減少。風化塑膠表面會促進鉛的吸附，在酸性環境中，風化聚丙烯微粒之吸附量比原始的增加156%。然而在高pH值時，鉛沉澱作用可能會干擾風化表面官能基的離子錯合，因此於鹼性環境中，風化聚丙烯微粒之吸附量反而降低。原始及風化聚丙烯微粒吸附鉛則較符合Langmuir等溫吸附模式。本研究探討塑膠在環境中的風化機制，並評估塑膠微粒對生態可能造成的危害。

Plastics are widely used in food packaging and container products due to their low cost, good processability, and stability. However, after experienced various weathering and degradation (e.g. UV irradiation, thermal effect, and biological activities), plastics may undergo characteristics changes and break down into microplastics. Microplastics can act as carriers of environmental pollutants, resulting in detrimental effects on the environment and ecosystems. In this study, three types of microplastics were investigated, including polypropylene (PP), low density polyethylene (LDPE), and high density polyethylene (HDPE). The microplastics were exposed to sunlight outdoors and UV irradiation from a long arc xenon lamp, respectively. Samples were collected and analyzed to evaluate the correlation and differences between the natural weathering and the accelerated weathering. The results indicated that photo-oxidation and thermal-oxidation contributed to the weathering of plastics, leading to changes in surface color, increase in surface roughness, formation of surface functional groups, reduction in mean particle size, increase in specific surface area, and enhancement of surface electronegativity. Among the microplastics, PP was more susceptible to the UV irradiation than HDPE. This might be attributed to the presence of more branches in the molecular chain of PP, making it more prone to chain scission and oxidation. Therefore, the average carbonyl index of PP was higher than that of HDPE. Additionally, due to the higher temperatures during the accelerated weathering than those during the natural weathering, the Fourier-transform infrared spectra of the weathered microplastics exhibited higher peak intensities and peak shifting of adsorption bands. The results indicated that heat might have induced chemical reactions different from those occurred during the natural weathering. Adsorption capacities of Pb on PP microplastics under different conditions were also investigated. The results showed that competitive adsorption of coexisting ions could occur on microplastics. Therefore, the adsorption capacities increased with an increase in pH, but decreased with an increase in salinity. Weathering could promote the adsorption of Pb on microplastics. Under acidic conditions, the adsorption capacities of the weathered PP microplastics increased by 156%, compared to those of the pristine ones. However, lead precipitation would probably interfere the complexation of Pb with the functional groups under alkaline conditions. Therefore, the adsorption capacities of the weathered PP microplastics decreased. Additionally, both pristine and the weathered PP microplastics exhibited a better fit to the Langmuir isotherm than to the Freundlich isotherm. In summary, weathering of plastics and the ecological risks of microplastics were investigated in this study.