南方小城镇生活垃圾热解焚烧灰渣中微塑料与重金属的赋存特征

Occurrence characteristics of microplastics and heavy metals in pyrolysis incineration residues of small towns in Southern China

采用密闭限氧热解方法处理生活垃圾正成为小城镇广泛应用的垃圾处理方式.本文选择南方地区江西省典型小城镇的生活垃圾热解焚烧处理厂作为研究区,以垃圾热解焚烧后堆放地的灰渣样品及其周边环境灰渣土、表层土为研究对象,采用浮选分离、扫描电镜、ICP-MS、BCR三步连续提取法等方法研究微塑料的丰度等特征及重金属Cu、Cd、Pb、Zn、Cr等元素的不同形态赋存特征.结果表明,不同来源样品中微塑料的丰度存在一定差异,不同垃圾厂灰渣中的微塑料丰度范围为131.00—176.00 n·kg-1dw(每kg干重土样中微塑料的数量),均显著高于周边环境中灰渣土、表层土中的微塑料丰度;微塑料的形态类型主要以碎片类(38.5%)为主,其次为薄膜类(17.6%)、纤维类(26.7%)和发泡类(17.2%);颜色及比例分别为黑色(29.4%)、白色(12.2%)、透明色(20.8%)、蓝色(12.4%)、红色(25.3%)等5种;不同大小粒径微塑料丰度所占比例分别为:>5 mm(11%)、2.5—5 mm(13%)、1—2.5 mm(19%)、0.5—1 mm(24.2%)、≤0.5 mm(32.8%),随着微塑料粒径的减小,微塑料丰度在各类样品中的数量呈增加趋势.扫描电镜与能谱(SEM-EDS)分析结果发现,大多微塑料边缘均具有明显撕裂痕迹、表面有较多突起及一定划痕等特点,重金属Cu、Cd、Pb、Zn、Cr等元素在不同微塑料表面均有吸附.不同形态重金属的占比也存在显著差异,5种重金属元素有效态所占比例依次为Pb(88.27%)>Cd(73.48%)>Zn(55.69%)>Cu(38.8%)>Cr(38.02%).微塑料的丰度与重金属的赋存相关性分析结果表明,垃圾热解灰渣中的微塑料与重金属具有一定相关性,不同特征的微塑料与重金属含量间的相关性具有一定差异,其中黑色微塑料、粒径<0.5 mm的微塑料与大多数重金属元素间存在着显著相关性(P<0.05).

Closed oxygen-limited pyrolysis is becoming a widely used waste treatment method in small towns.In the present study,we selected some typical municipal solid waste incineration plants in Jiangxi Province in Southern China as the research areas,and collected the ash samples which from the stacking ground after pyrolysis of refuse and its surrounding environment soil with ash slag and surface soil as the research materials.The flotation separation method,scanning electron microscopy method,ICP-MS and BCR three-step continuous extraction methods were used to study the abundances of microplastics and the occurrence characteristics of different forms of heavy metals such as Cu,Cd,Pb,Zn and Cr.The results showed that there were some differences in the abundance of microplastics in samples from different sources.The abundance range of microplastics in ash from different garbage factories was from 131.00 n·kg-1 dw to 176.00 n·kg-1 dw(the amount of microplastics in dry soil samples per kg),which was significantly higher than that in the soil with ash slag and surface soil in the surrounding environment.The morphological types of microplastics were mainly debris(38.5%)followed by film(17.6%),fiber(26.7%)and foam(17.2%).The main colors of microplastics were black(29.4%),white(12.2%),transparent(20.8%),blue(12.4%)and red(25.3%).The proportions of the microplastics abundances with different particle sizes were>5 mm(11%),2.5—5 mm(13%),1—2.5 mm(19%),0.5—1 mm(24.2%)and<0.5 mm(32.8%).The abundance of microplastics in various samples increased with the decrease of the particle size of microplastics.The results of scanning electron microscopy and energy dispersive spectroscopy(SEM-EDS)analysis showed that most of the microplastics had obvious tear marks,more protrusions and some scratches on the surface,and all kinds of heavy metals such as Cu,Cd,Pb,Zn and Cr were adsorbed on different surfaces of microplastics.There were also significant differences in the proportion of heavy metals in different forms,and the effective states of the five heavy metal elements accounted for Pb(88.27%)>Cd(73.48%)>Zn(55.69%)>Cu(38.8%)>Cr(38.02%).The results of correlation analysis showed that the abundance of microplastics and the occurrence of heavy metals in pyrolysis residue of the garbage were corelated and the correlation between the content of heavy metals and microplastics with different characteristics was different.There was a significant correlation(P<0.05)between black microplastics and most heavy metal elements,and also between microplastics with particle size less than 0.5 mm and most heavy metal elements.