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中国作为一个水资源匮乏的国家,人均水资源占有量仅为世界人均水资源占有量的25%。其中,地下水作为第2大淡水资源,已成为我国多数城市的供水水源。但由于人类在农业生产过程中对氮肥的过度使用以及工业中含氮废水的不达标排放等原因[1-2],地下水中的硝酸盐污染不断加剧,呈现出污染范围不断扩大、程度不断加深的趋势[3]。HAN等[4]对中国的52个地下水系统数据进行汇总,发现部分含水层中(36个浅层含水层中的25个,37个深层或岩溶含水层中的10个)的地下水硝酸盐污染水平已远远超过了美国环境保护署公布的最大污染量(10 mg·L−1
${\rm{NO}}_3^ - $ -N)。同时在不同的接触浓度和接触时间条件下,水中硝态氮对人体有着不同程度的危害,轻微时会使血红蛋白失去输氧能力,从而使人体产生缺氧症状,严重时导致消化道系统发生癌变,并且也会造成类似水体富营养化的环境问题,从而产生严重的经济损失[5-6]。因此,地下水中的硝酸盐污染的治理将是一个亟待解决的问题。长期以来,人们对地下水硝酸盐的去除做了大量的研究工作,总结出了许多可行的防治措施,这些方法主要包括生物修复技术、物理化学修复技术(离子交换、反渗透RO、电渗析法)和化学修复技术[7-11]。其中,物理化学处理技术只是将污染物转移或浓缩,并没有将污染物除去,因此,在地下水水质修复的应用中受到限制。近年来,零价铁在有毒重金属、有机氯化物的去除过程中展现出优良的性能,因而被广泛应用,并得到大量推广[12-13]。但是随着零价铁去除地下水硝酸盐研究的不断深入,发现其在实际的运用中仍存在许多问题,主要包括:纳米零价铁粒径较小且带有一定的磁性,极易在水体中发生团聚,失去了原有的迁移性;在利用纳米零价铁去除硝酸盐时,纳米零价铁在还原的过程中会在其表面形成氢氧化铁副产物,从而抑制反应的进行[14]。因此,对零价铁进行改性,以获得更稳定的性能尤为重要。针对以上问题,国内外学者已经提出了许多关于纳米零价铁的改性方法,活性炭负载零价铁的复合材料表现出优良的硝酸盐去除性能。此外,修宗明等[15]还发现活性炭与铁粉在溶液中可形成铁炭原电池(铁为阴极,炭为阳极),其产生的电场可以促进硝酸根的还原,从而大大促进硝酸盐的去除。
本研究以玉米秸秆作为生物炭的原料,采用液相还原法研究了不同制备条件(溶剂体系、生物炭与零价铁的质量比例)对零价铁-生物炭复合材料(ZVI-BC)的影响,以优选制备条件;通过严格控制厌氧环境和模拟地下水环境,对在优选制备条件下制备的复合材料去除地下水中硝酸盐的效果进行了评估,同时考察了不同实验条件对硝酸盐去除效果的影响;从动力学、氮平衡等方面探讨了零价铁-生物炭复合材料还原硝酸盐的可能途径;将生物炭的吸附性能与零价铁的还原性能结合,为零价铁-生物炭复合材料在地下水硝酸盐的治理中提供参考。
零价铁-生物炭复合材料对地下水中硝酸盐的去除
Nitrate removal from groundwater by zero-valent iron-biochar composites
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摘要: 为了解决我国日益严重的地下水中硝酸盐污染问题,采用液相还原法制备了零价铁-生物炭复合材料(ZVI-BC)。研究了不同制备条件对ZVI-BC合成的影响,并通过X射线衍射(XRD)、电子扫描显微镜(SEM)、透射电镜(TEM)和X射线光电子能谱(XPS)对优选制备条件下所合成的复合材料进行了表征分析。在严格控制厌氧环境的基础上,考察了初始pH、初始硝酸盐浓度、复合材料投加量和反应温度对复合材料去除水中硝酸盐效率的影响,并从动力学、氮平衡等方面初步探讨了零价铁-生物炭复合材料还原硝酸盐的途径。结果表明:复合材料的优选制备条件为聚乙烯吡络烷酮K30(PVP)作溶剂、ZVI∶BC=1∶0.1,通过对在优选制备条件下得到的零价铁-生物炭复合材料的SEM表征,发现成链状连接的零价铁颗粒负载在层状分布的生物炭表面,其对硝酸盐具有良好的去除效果;初始硝酸盐浓度越大,则反应初期硝酸盐去除速率越慢,硝酸盐的去除率越低;硝酸盐去除率随复合材料投加量的增加而有所升高;反应体系中的pH对复合材料降解硝酸盐的影响不大;反应体系的温度越高,硝酸盐去除速率越快;硝酸盐去除过程不能单纯的使用反应级数来表示,硝酸盐反应的最终产物主要为氨氮。生物炭负载零价铁有效的减少了零价铁的团聚,进一步促进了硝酸盐的去除,在地下水硝酸盐的去除中具有良好的应用前景。Abstract: In order to solve the problem of increasingly serious nitrate pollution in groundwater of China, zero-valent iron-biochar composites (ZVI-BC) were prepared by liquid phase reduction. The effects of different preparation conditions on the synthesis of ZVI-BC were studied, and the composites synthesized under the optimal preparation conditions were characterized by X-ray diffraction (XRD), electron scanning microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Under the strictly anaerobic environment condition, the effects of initial pH, initial nitrate concentration, composite dosage and reaction temperature on the removal efficiency of nitrate in water were investigated. The ways of reducing nitrate by zero-valent iron-biochar composites were preliminarily discussed from the aspects of kinetics and nitrogen balance. The results show that the preferred preparation conditions of the composites were following: polyvinylpyrrolidone K30 (PVP) as solvent and ZVI∶BC=1∶0.1. The SEM images of the composites obtained under the optimized preparation condition showed that the zero-valent iron particles with chain-like connection were loaded on the surface of biochar with lamellar distribution structure, having a good removal performance for nitrate. The higher the initial concentration, the slower the nitrate removal rate at the initial stage of the reaction, the lower the nitrate removal rate. The removal rate of nitrate increased with the increase of composites dosage. The pH of the reaction system had slight effect on nitrate removal by the composites, and the higher the temperature of the reaction system, the faster the nitrate removal rate. The nitrate removal process could not be simply expressed by the use of reaction series, and ammonia nitrogen was the main final product of nitrate reaction. Biochar loaded with zero-valent iron could effectively reduce the agglomeration of zero-valent iron and further promote the removal of nitrate, which has a good application prospect in the removal of nitrate from groundwater.
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Key words:
- zero-valent iron /
- biochar /
- nitrate /
- groundwater
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表 1 不同反应动力学模型拟合参数
Table 1. Fitting parameters of various reaction kinetic models
硝酸盐初始
浓度/(mg·L−1)准一级动力学方程 准二级动力学方程 k1/
(mg·(L·min)−1)R2 k2/
(mg·(L·min)−1)R2 20 0.061 57 0.993 0.006 53 0.991 40 0.049 57 0.989 0.002 79 0.992 60 0.010 47 0.800 0.000 61 0.985 80 0.014 78 0.910 0.000 66 0.983 100 0.016 28 0.866 0.000 57 0.974 -
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