城市污泥中微塑料赋存对其热解生物炭性质的影响

EFFECT OF MICROPLASTICS ON THE PROPERTIES OF BIOCHAR PYROLYZED FROM SEWAGE SLUDGE

储瑶竹

11849064

硕士

环境科学与工程

唐圆圆

2020-05-28

2020-06-10

哈尔滨工业大学

深圳

在城市生活污水中含有大量的微塑料，经处理后污水中大部分微塑料会沉降到污泥中，有研究发现，每千克干污泥中微塑料的含量可达到几千甚至上万个，是沉积物中微塑料含量的数十甚至数百倍。据最新统计，我国剩余污泥年产量在5665万吨（湿重）左右，因此污泥中微塑料的产生量巨大。对城市污泥的主要处理处置方式包括填埋、焚烧、热解等，其中热解可将污泥转为生物炭并应用于环境修复中而引起了广泛关注。因为微塑料的热解特性，所以微塑料的大量赋存势必会对污泥热解行为及生物炭的产品特性产生一定影响。而目前尚未有研究探讨微塑料在污泥热解中的作用行为及机理，因此，本课题将采集城市污泥，选用聚氯乙烯（Polyvinyl chloride，PVC）、聚对苯二甲酸乙二醇酯（Polyethylene terephthalate，PET）等代表性微塑料，研究微塑料影响下的污泥热解行为及生物炭性质，系统研究不同含量、种类微塑料对污泥热解行为的影响机制，深入探讨微塑料对污泥生物炭产量以及碳组分性质的影响。研究发现，与污泥原样生物炭相比，微塑料共热解后生物炭中的产量、碳组分性质（包括碳含量、C/H比、碳结构、碳滞留及碳长期稳定性）等都会产生影响，影响程度随着微塑料含量增加而加剧。在生物炭的产量方面，所有生物炭产量均随热解温度增加而下降。由于微塑料在200°C未发生分解，微塑料的存在可使生物炭产量有所增加；然而在300-700°C范围内，微塑料发生热解反应，所以产量会较污泥原样减少，且PET比PVC微塑料共热解后的生物炭产量下降了2.0%左右。对碳含量占比而言，污泥生物炭中碳含量占比随着热解温度的升高先下降后平缓，而微塑料共热解后生物炭中碳含量占比随温度先升高后下降至平缓。200-300°C时，微塑料系列生物炭中碳含量占比有所上升，并在300°C达到最大值。同时，微塑料系列生物炭中碳含量占比均高于同一温度下污泥生物炭。此外，生物炭中的C/H含量比均随着温度的上升而增加，PVC系列最高，PET次之，原样最低，表明微塑料的存在，尤其是PVC微塑料的存在，会加剧生物炭中芳香化程度。基于生物炭产量及碳含量占比，对于生物炭中碳滞留率进行了计算，结果表明碳滞留率均随温度上升而下降。其中，PVC系列生物炭在200-700°C时的碳滞留率高于污泥原样；而PET系列生物炭在200-300°C内碳滞留率高于污泥原样，400-700°C碳滞留率低于污泥原样。同时分析了微塑料对于生物炭产品中碳结构以及碳稳定性的影响。随着热解温度上升，生物炭中碳结构的有序性、碳稳定性都有所增加。研究对拉曼光谱中代表碳结构的D峰和G峰进行分析，表明微塑料的存在加剧了生物炭中碳结构的有序化进程。与碳滞留率相似，300-600°C内，PVC系列生物炭的碳长期稳定性均高于污泥生物炭。而PET生物炭300°C时碳长期稳定性优于污泥生物炭，但在400-600°C内，碳长期稳定性弱于污泥生物炭。

Most of the microplastics are always settled and accumulated in the sewage sludge after municipal wastewater treatment. The occurrence of microplastics can reach thousands or tens of thousands particles in per kilogram dry sludge. With annual production of 56.65 million tons (wet weight) in China, the sewage sludge has become one of the critical environmental issues. At present, there are various treatment and disposal methods for the sewage sludge, including landfill, incineration, pyrolysis, etc. Pyrolysis can change the sewage sludge into biochar potentially for environmental remediation, and therefore, the pyrolysis technology together with the sludge-derived biochar has attracted wide attention. Microplastics will affect the pyrolysis behavior of the sewage sludge and the characteristics of the biochar product, due to the abundant occurrence and unique pyrolytic properties of the plastic particles. However, as the effect of microplastics on sewage sludge treatment is an emerging topic, there have been only very few studies reported on this topic especially on the mechanisms of microplastics’ impact on sludge pyrolysis. Therefore, this study conducted a preliminary research on the effect of typical microplastics on the pyrolysis of the sludge and the properties of the sludge biochar. Specially, Polyvinyl chloride (PVC) and Polyethylene terephthalate (PET) were selected as representative microplastics. Through a series of experiments, this study revealed the variation in biochar production, and carbon properties with change in pyrolysis temperature as well as microplastics types and contents. The biochar production rates of all sample series were decreased with elevated pyrolysis temperature. Compared with the raw sludge, the presence of microplastics in the sludge can enhance biochar production rate after 200C, which might be caused by remaining of undecomposed microplastic in biochar. However, at higher temperature range (300-700C), the existence of microplastics during pyrolysis resulted in the reduction of biochar production. In detail, compared with the production rate of biochar containing PVC microplastics, the biochar production rate with PET microplastic is reduced by about 2.0%. The carbon content kept decreasing with increasing temperature for biochar production from the raw sludge. However, for the biochar derived from the mixture of sludge and microplastics, the carbon content in the biochar products increased first until reaching the maximum value at 300C, and then decreased gradually with temperature rising. Besides, at the same temperature, the carbon content in the biochar pyrolyzed from the sludge with microplastics is higher than that in the biochar from the raw sludge. Moreover, the C/H ratio of all biochar samples was increased with the elevated pyrolysis temperature. Meanwhile, the C/H ratio of PVC-sludge biochar was the highest, which was followed by PET-biochar, the raw sludge biochar with the lowest value, indicating that the presence of microplastics, especially PVC, can aggravate the aromatization of carbon in the biochar. Furthermore, the carbon retention rate was calculated from the biochar production rate and carbon content. It was found that the carbon retention rate of all sample series was decreased with temperature going up. The presence of PVC microplastics increased the carbon retention rate in biochar at 200C. For biochar pyrolyzed at 300-700C, the PVC series showed higher carbon retention than the raw sludge series. Aside from PVC-sludge biochar, the carbon retention rates of PET-sludge series were also higher than those of the raw sludge series pyrolyzed at 200-300C, but lower at 400-700C. In addition, the effects of microplastics on carbon structure and stability of the biochar products were further analyzed. For all sample series, the carbon structure became more orderly and the stability of carbon increased in the biochar pyrolyzed at increasing temperature. The existence of microplastics aggravated the ordering process of carbon structure in the biochar. After analyzing the D and G Raman peaks for carbon structure, the results showed that the existence of microplastics enhanced the ordering process of carbon structure in biochar. Similar to the carbon retention rate discussed above, the carbon stability of the PVC-sludge biochar is higher than that of raw sludge biochar at 300- 600C. The carbon stability of 300C pyrolyzed PET-sludge biochar is better than that of the raw sludge biochar, but is weaker at higher temperature range (400-600C).

微塑料 城市污泥 生物炭 热解 碳组分 PVC PET

microplastics sewage sludge biochar pyrolysis carbon PVC PET

中文

联合培养

南方科技大学

储瑶竹. 城市污泥中微塑料赋存对其热解生物炭性质的影响[D]. 深圳. 哈尔滨工业大学,2020.