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甲基橙(methyl orange, MO)是一种水溶性偶氮染料,染料释放到天然水体中将严重影响水生生物生长,危害人类健康,具有致畸、致癌和致突变的作用[1]。MO在水中可电离为Na+与有机阴离子,并在微生物的作用下,产生芳香胺类中间体产物,随着食物链传播,在生物体内富集,加剧了人体健康的危害[2]。众所周知,塑料是一种可塑性强、化学稳定性高的高分子材料, 广泛应用于服装、包装、电子等各类产品中[3-6]。但是塑料的危害并没有引起人们的足够重视,80%的塑料产品没有经过有效处理就流入垃圾填埋场和自然环境中,并在重力作用、生物作用、水力作用、天气作用以及人类活动等外界驱动力的作用下,形成粒径<5 mm 的微塑料[7]。微塑料具有不规则的表面结构、较大的比表面积以及对疏水性污染物较强的亲和力,使其能够作为环境污染物的载体,影响污染物在环境中的迁移转化[8-10]。
环境中的微塑料容易受到光照作用而发生老化,老化后的微塑料会产生新的含氧基团(如羟基、羰基等),增加对有机污染物或重金属的吸附能力[11]。BHAGAT等[12]通过研究发现紫外(ultraviolet light, UV)老化增加了微塑料对有机污染物的亲和力;LI等[13]研究发现PE/PS/PA三种微塑料在UV老化后对Cr(VI)的吸附增强。但由于UV条件下微塑料老化速率较低,限制了人们研究微塑料在环境中与污染物的迁移转化。因此,关于微塑料的实验室加速老化技术逐渐被研究和开展。光催化技术是实验室加速微塑料老化的一种有效手段,常见的光催化剂有 TiO2、ZnO、Cds、H3BO3等,不同的光催化剂对PVC老化的影响如表1所示,其中 TiO2因具有活性高、热稳定性好、成本低等特点,使用最为广泛[14],但TiO2光催化降解过程中光激发产生的电子-空穴对的复合会导致催化活性的降低。由于O3本身及其在水中分解产生的自由基具有一定的氧化能力,可以达到加速老化的目的[15-16],因此将光催化和O3结合可以有效增强单一光催化技术的氧化能力,利用TiO2/UV协同老化过程中产生的 e−可与O3反应生成氧化性更强的·OH,抑制了电子-空穴对的复合,使光催化性能得到进一步提高[17]。综上所述,本文以PVC微塑料为研究对象,采用TiO2/UV/O3协同老化的方法对PVC进行加速老化实验,研究微塑料表面形貌及微观结构的变化,以及PVC对MO的吸附行为,为研究环境中的微塑料在污染物迁移转化过程中所起到的作用提供参考。
TiO2/UV/O3协同老化对微塑料吸附甲基橙的影响
Effect of TiO2/UV/O3 collaborative aging on adsorption of methyl orange by microplastics
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摘要: 为了探究协同老化后的微塑料与有机污染物的相互作用机制,以PVC作为研究对象,采用TiO2/UV/O3协同老化方式,对比考察了老化前后PVC对甲基橙(MO)的吸附性能。结果表明,随着老化的进行,PVC颗粒表面碎片化加深,粒径明显减小,Zeta电位值降低,并出现了新的含氧官能团。原始PVC对MO的吸附符合准一级动力学模型,而老化后的PVC对MO的吸附符合准二级动力学模型,且主要的吸附模式均为液膜扩散和颗粒内扩散。动力学拟合结果表明老化前的PVC对MO的吸附以物理吸附为主,而老化后的PVC对MO的吸附以化学吸附为主。老化前后的PVC对MO的吸附均符合Freundlich等温吸附模型,表明MO与微塑料之间的相互作用是在非均匀表面上的多层吸附。以上研究结果可为微塑料携带有机污染物在环境中的迁移转化的行为提供参考。Abstract: To explore the interaction mechanism between microplastics and organic pollutants after collaborative aging, PVC was taken as the research object and TiO2/UV/O3 collaborative aging method was adopted to compare the adsorption characteristics of MO on PVC before and after aging. The results showed that with the aging process, the surface fragmentation of PVC particles was deepened, their particle size decreased obviously, their Zeta potential decreased, and new oxygen-containing functional groups appeared. The adsorption of MO on the original PVC conformed to the quasi-first-order kinetic model, while the adsorption of MO on the aging PVC conformed to the quasi-second-order kinetic model, and the main adsorption modes were liquid film diffusion and intra-particle diffusion. The difference of kinetics indicated that the adsorption of MO on PVC before aging was mainly physical adsorption, while the adsorption of MO on PVC after aging was mainly chemical adsorption. The adsorption of MO on PVC before and after aging conformed to Freundlich isothermal adsorption model, indicating that the interaction between MO and microplastics was the multi-layer adsorption on the non-uniform surface. Therefore, this can provide a reference for studying the migration and transformation behavior of organic pollutants carried by microplastics in the environment.
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Key words:
- microplastics /
- collaborative aging /
- adsorption /
- methyl orange
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表 1 不同光催化剂对PVC的降解实例
Table 1. Examples of degradation of microplastics by different photocatalysts
表 2 MO在老化前后PVC上的吸附动力学参数
Table 2. Adsorption kinetics parameters of MO on PVC before and after aging
样品 qe/(μg·g−1) 准一级动力学 准二级动力学 qe/(μg·g−1) K1 R2 qe/(μg·g−1) K2(×10−5) R2 PVC 115.522 115.455 0.143 0.989 122.886 161 0.948 O3-PVC-T0.5 212.314 195.531 0.454 0.775 217.416 211 0.871 O3-PVC-T1 250.798 242.483 0.275 0.972 258.5 200 0.927 O3-PVC-T1.5 275.791 255.491 0.289 0.813 277.505 158 0.902 O3-PVC-T2 323.091 290.845 0.217 0.907 329.7 89.726 0.979 表 3 MO在老化前后PVC上的内扩散模型参数
Table 3. Internal diffusion model parameters of MO on PVC before and after aging
样品 第1阶段 第2阶段 第3阶段 C1 Kw1 R12 C2 Kw2 R22 C3 Kw3 R32 PVC −45.69 49.65 0.99 56.48 9.20 0.82 84.85 3.13 0.98 O3−PVC-T0.5 −224.87 252.53 0.99 93.32 27.58 0.98 162.16 5.23 0.79 O3−PVC-T1 −146.8 220.33 0.99 120.69 28.91 0.69 228.49 4.99 0.87 O3−PVC-T1.5 −94.84 151.89 0.87 151.17 20.04 0.46 210 4.3 0.70 O3−PVC-T2 −42.77 116.78 0.92 131.35 32 0.99 258.75 6.68 0.91 表 4 老化前后的PVC对 MO的吸附等温线拟合参数表
Table 4. The fitting parameters of PVC adsorption isotherm towards MO before and after aging
样品 Henry Langmuir Freundlich Kd×10−3
/(L·g−1)R2 Qm/(μg·g−1) KL R2 KF/(μg·g−1) n R2 PVC 6.601 0.948 401.811 0.010 0.952 6.268 0.790 0.969 O3-PVC-T0.5 3.338 0.976 506.753 0.010 0.989 8.031 0.780 0.996 O3-PVC-T1 4.306 0.971 579.578 0.015 0.949 11.069 0.764 0.993 O3-PVC-T1.5 4.934 0.928 604.564 0.017 0.914 15.421 0.715 0.968 O3-PVC-T2 5.587 0.900 633.214 0.011 0.947 19.181 0.691 0.987 -
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