摘要
随着我国城市化进程的加快,城市污水处理率逐年提高,城市污水处理厂的污泥产量也急剧增加。未经恰当处理处置的污泥进入环境后,直接给水体和大气带来二次污染。污泥农用是最好的消纳污泥的方法,也是适合我国国情处置方法,而污泥中往往含有一些有害物质,特别是重金属元素,成为污泥农用的限制因子,所以如何处理污泥中所含的重金属成为污泥农用要解决的关键问题。本实验旨在寻找一些环境友好的去除污泥中重金属的方法。
在国内外前人研究的基础上,通过比较分析生物淋滤法和化学法去除污泥中重金属的相关因素及机理,本研究分别选用了植物可吸收利用的铁元素:硫酸高铁(Fe~(3+))和易于在环境中降解的有机酸:柠檬酸(CTA)为浸提剂和萃取剂对两种不同污泥[分别取自长沙某污水处量厂(W1)和东莞市某污水净化厂(W2)]中的重金属进行了相关的实验研究。
试验结果表明:在室温,污泥浓度20g·L-1条件下,向污泥中投加硫酸铁能导致污泥酸化和污泥中重金属的溶出。向W1中投加按Fe~(3+)计算为1.5 g·L-1硫酸铁时,其pH值降至3.02,Cd,Cu,Zn溶出率分别可达到82.5%、70.9%、89.0%;向W2中投加4.5 g·L-1Fe~(3+),其pH值降至3.19,Cd,Cu,Zn的溶出率分别达到80.0%、88.2%、92.1%。在相同pH条件下,Fe~(3+)比硫酸能更有效浸出污泥中重金属;用Fe~(3+)浸提污泥中重金属能达到与生物淋滤法同等的溶出效果,并缩短了反应时间。
柠檬酸(CTA)对东莞市污水净化厂污泥中重金属的萃取实验结果表明:柠檬酸浓度、固液比与污泥中重金属的溶出量呈正相关性,即浓度越高,固液比越大,重金属的溶出量越多;而萃取时间对重金属的溶出在初始阶段呈正相关,随着时间延长,重金属的溶出量有所下降。CTA对污泥中的四种金属Pb、Cd、Cu和Zn均表现出较强的萃取能力。在室温条件下,用0.2 mol·L-1的CTA溶液萃取24h后,各金属的去除率分别达到56%、85.2%、65%、88.5 %,CTA不仅能萃取易溶态的金属,如交换态、吸附态、有机结合态和碳酸盐态的金属,还能萃取部分以硫化物结合态金属和少量残渣态的金属。
With the development of city and increase of treatment ratio of wastewater, a large amount of sludge has been produced. The sludge directly enter into environment will cause secondary pollution to waters, air, soil. Land application of sludge on agricultural lands, forest and disturbed lands was considered a best method to treat municipal sludge. Unfortunately, pollutants such as toxic organic compounds, heavy metals and pathogens in the sludge, especially heavy metals limit its use on land application. Now, the main problem is how to remove or control the heavy metals in sludge effectively and economically. The aim of this study is to look for some new methods which can remove heavy metals fast and effectively and are friendly to environment.
Through analyzing the mechanism and factors which affect the removal of heavy metals from sludge by bioleaching and chemical method, In this work the ferric sulfate and citric acid were used to remove heavy metals in sludge.
In the first experiment , The removal of heavy metals from two different kinds of sewage sludge(W1&W2) was conducted by using ferric sulfate at room temperature and sludge solid concentration is 20g·L-1. It was found that the addition of ferric sulfate to the sludge causes the acidification of the tested sludge and the dissolution of heavy metals from the sludge. For W1, the best amount of ferric ion added was 1.5g·L-1 with a low pH value 3.02 and high removal percents of Cd,Cu,Zn which are as follows: 82.5%、70.9%、89.0% respectively; while for W2 the best amount of ferric ion is 4.5g·L-1 and the pH values is 3.19 and the removal percents of heavy metals——Cd,Cu,Zn are 80.0%、88.2%、92.1% respectively. Ferric ion dissolved Cd,Cu,Zn more effectively than sulfuric acid at same pH value. The removal percents of Cd, Cu and Zn using ferric sulfate was equivalent to that by bioleaching and shorted the experiment time.
In the second experiment, CTA was used to treat heavy metals of sludge(W2). The results indicated that both CTA concentration and sludge-liquid ratio have a positive relation with the elution quantity of heavy metals, while just at the early phase of extraction, the extracting time has a positive relation with the elution quantity, with the time prolonging, there is a little decrease in the quantity of extracted metals. And CTA can effectively extract Pb, Cd, Cu, Zn from sewage sludge. At room temperature, the remove percent of Pb, Cd, Cu, Zn can reach 56%,85.2%,65%,88.5%, respectively, by extracting with 0.2 mol·L-1CTA and a sludge-liquid ratio 1:20. CTA can extract not only exchangeable metals, adsorbed metals, organically bound metals and metal carbonates, but also some metal sulfides, and a little residual metals.
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