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冷冻法作为一种清洁、高效且具有广阔应用前景的水处理技术,在废水处理[1-3]和食品工业[4-5]等方面展现出众多优势,例如设备腐蚀低、没有二次污染和无需化学添加剂等[6]。冷冻过程中,水分子在凝固点结晶为冰晶,同时可溶性污染物则在液相中积聚[7]。因此,通过分离冰-液体混合物可以得到洁净水。根据冰晶的生长方式,冷冻法可分为渐进冷冻法和悬浮结晶法[8]。与渐进冷冻法相比,悬浮结晶法具有更大的固液界面,更高的能量传递效率和更快的冰晶生长速度[9]。因此,悬浮结晶法在实验和理论研究方面均受到了更加广泛的关注。
近年来,研究人员从多个方面对冷冻法进行研究,例如通过建立数学模型预测冰晶生长速度,将数字图像处理技术运用于研究冰晶的形状和大小[10-11]。有研究[12-13]表明,由于在悬浮结晶过程中,污染物会进入冰晶中,从而降低冰晶纯度,因此,冰晶纯度是冷冻法的研究核心。但是,从分子水平层面来看,对杂质进入冰晶的机理仍然缺乏相关的报道和研究。
本研究以4种不同官能团有机物污染物(即己烷、正己酸、正己醇和正己醛)为研究对象,利用悬浮结晶法进行污染物分离实验,探讨了不同有机污染物在冷冻浓缩过程中所形成的冰晶中的杂质浓度变化。此外,运用量子化学计算,通过几何优化和约化密度梯度(RDG)分析,从理论上研究了拥有不同官能团的有机物污染物与水分子之间形成的氢键强度,以探究官能团与水分子形成的氢键强度对冰晶纯度的影响。
有机物官能团与水分子间氢键强度对冷冻结晶过程中冰晶纯度的影响
Influence of hydrogen bond strength between functional groups of organic pollutant and water molecules on ice impurity during suspension crystallization
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摘要: 冷冻法在有机废水处理方面取得了良好的应用效果,但其分离机理缺乏深入的研究。为深入研究冷冻法对有机污染物的处理效果及分离机理,采用悬浮冷冻结晶法,对废水中己烷、正己酸、正己醇和正己醛4种具有不同官能团的有机物污染物进行分离实验,并采用量子化学方法计算了氢键结合能,进一步探讨了有机物官能团对冰晶杂质浓度的影响机理。悬浮结晶实验结果表明,废水中己烷、正己酸、正己醇和正己醛的去除率分别可达67.07%、87.75%、94.71%和95.32%。量子化学计算结果表明,极性有机物官能团和水分子间缔结的氢键结合能越大,其对应的冰晶杂质浓度越高,从而导致有机物去除率越低。正己醛3种极性有机物(正己酸、正己醇和正己醛)中和水分子间氢键结合能最小,对应的去除率最高。而己烷作为非极性有机物,无法和水分子间缔结产生氢键,因而极易在悬浮结晶时析出,与悬浮冰晶混合于溶液上层,故其去除率最低。以上研究结果从分子水平加深了对冷冻法在有机废水处理中作用机理的理解,可为该技术在水处理领域的深入研究提供参考。Abstract: The freezing method has been applied very well in treating organic wastewater, but its separation mechanism still needs in-depth investigation. In order to further study the treatment effect and separation mechanism of organic pollutants by freezing method. In this study, suspension crystallization method was used to separate four organic pollutants with different functional groups, including hexane, hexanoic acid, N-hexanol and N-hexanal. Quantum chemistry method was used to calculate the hydrogen bonding energy, and the influence mechanism of organic functional groups on the concentration of ice crystal impurities was further discussed. The results of suspension crystallization experiments showed that the removal rates of hexane, hexanoic acid, N-hexanol and N-hexanal in wastewater could reach 67.07%, 87.75%, 94.71% and 95.32%, respectively. Quantum chemical calculation results indicated that the greater the hydrogen bonding energy between the polar organic functional groups and water molecule, the higher the concentration of the corresponding ice crystal impurities, resulting in a lower organic removal rate. Among the three polar organic substances (hexanoic acid, N-hexanol and N-hexanal), N-hexanal had the smallest hydrogen bonding energy and the highest removal rate. As a non-polar organic substance, hexane cannot form hydrogen bonds with water molecules, so it was easily to precipitate during suspension crystallization and mixed with suspended ice crystals in the upper layer of the solution, so its removal rate was the lowest. The above research results have deepened the understanding of the mechanism of the freezing method in organic wastewater treatment in the view of the molecular level, and can provide a reference for the in-depth study of this technology in the field of water treatment.
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Key words:
- hydrogen bond /
- functional groups /
- ice impurity /
- quantum chemistry /
- removal rate
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表 1 悬浮结晶测试期间的实验条件
Table 1. Experimental conditions during the suspension freeze test
冷冻温度/℃ 转速/(r·min−1) 冷冻时间/min −0.4 200 30 −0.9 200 30 −1.9 200 30 −2.6 200 30 −0.9 100 30 −0.9 200 30 −0.9 300 30 −0.9 400 30 −0.9 200 10 −0.9 200 30 −0.9 200 60 −0.9 200 120 表 2 模型化合物和稳态能量
Table 2. Model compounds and steady state energy
计算化合物类型 化合物单点能/(kJ·mol−1) 分子间作用力/(kJ·mol−1) H2O −200 677.59 — 己烷 −622 531.83 — 正己醇 −820 010.87 — 正己醛 −816 830.71 — 正己酸 −1 014 428.12 — 己烷&H2O −823 211.35 −1.93 正己醇&H2O −1 020 714.94 −26.48 正己醛&H2O −1 017 532.61 −24.31 正己酸&H2O −1 215 149.81 −44.10 -
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