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四环素(tetracycline, TC)作为一种广谱抗生素,被广泛用于治疗和预防人类和动物的细菌性疾病[1],往往没有经过代谢,通过排泄物和肥料而排入环境中[2]。因此,TC在地表水中的浓度达到几十至几百微克每升,由于其生物毒性和易导致耐药细菌等原因,所以TC对生态环境和人类健康构成了潜在的风险[3]。另外,由于活性炭、聚合物树脂等多种用于去除TC的吸附剂多为粉末状,在实际净水过程中易造成损失,并且操作和重复使用非常困难。ZIF-8-壳聚糖复合材料[4]、MeOH-生物炭[5]、CC1.00-CB[6]和热活化膨润土[7],在其制备合成过程中具有高温、能耗较大、制作成本较高以及操作复杂等缺点。因此,需开发环境友好、可持续和具有经济性的吸附剂用于去除水中的TC。
天然木头是一种绿色环保、低成本的材料,并且具有独特的中空纤维结构,其已用于水处理吸附膜的研究中[8-10]。另外,由于TC具有的酰胺羰基(HN—C=O)结构,可以与羧基(—COOH)形成氢键而被吸附去除[11],因此,使用—COOH改性木膜吸附四环素具有可行性。
木膜中的纤维素含有大量易改性的羟基(—OH)[8],可与无毒柠檬酸进行酯化反应,从而将—COOH引入木质纤维素结构中[12]。但是,由于木膜中交联酯键等结构的存在,使得柠檬酸不易与纤维素上的—OH进行反应,因此,在改性过程中,需将木膜进行预处理。在预处理方法中,由于NaOH溶液的再利用和再循环性能,因此,常压下使用NaOH溶液对木膜进行碱预处理是一种经济环保的方法[11]。
椴木(basswood, BW)是一种纤维素含量非常高、具有规则的孔道结构、并且是最便宜的木材之一,有大量的剩余废料[12]。本研究报道了通过采用不同浓度的碱液和CA对椴木木膜进行改性,将—COOH引入至木膜的纤维素结构中,用于吸附水中的TC,通过对吸附前后的改性木膜进行红外表征,阐明了在不同pH下,—COOH与TC的相互作用机制,以期为设计具有环境友好、经济和有效的水处理膜材料提供有效的策略。
碱-柠檬酸改性木膜对四环素的吸附性能
Adsorption performance of alkali-citric acid modified wood membranes towards tetracycline
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摘要: 含羧基(—COOH)的无毒柠檬酸(citric acid, CA)改性木膜(wood membranes, WMs)在水处理领域具有广阔的应用前景。然而,由于木质纤维素的结构特点,—COOH很难引入木膜壁内。对木膜进行不同浓度NaOH的预处理,再与柠檬酸进行酯化反应,用于吸附水中的四环素(tetracycline, TC)。通过碱液预处理,有利于CA与木质纤维素的酯化反应,使得木膜的—COOH含量大幅地提高。TC吸附曲线表明,当碱浓度为6% 以及TC出水浓度为0.5 mg·L−1时,改性木膜的有效过滤体积为1 968 BV(bed volume, BV),比未改性木膜的吸附率提高了约48倍;在4次循环后改性木膜对TC的吸附率也可达97%。当出水浓度为0~0.5 mg·L−1时,水处理成本为0.036 0~0.083 25 元·m−3。因此,改性木膜在去除水中TC方面具有很好的应用前景。以上研究结果可为解决木膜应用中的关键局限性,设计具有环境友好、经济和有效的环境修复膜材料提供参考。Abstract: Innocuous citric acid (CA) modified wood membranes (WMs) with carboxylic groups (—COOH) have broad application prospects in water purification. However, due to the structural characteristics of lignocellulose, carboxylic groups are difficult to introduce into the wood membrane wall. Herein, the wood membranes were modified via the pretreatment with various NaOH concentrations and the subsequent esterification with CA for tetracycline (TC) adsorption in water. The pretreatment of alkali was beneficial to the esterification of CA and lignocellulose, and the content of carboxylic groups in wood membrane increased greatly. The TC adsorption curves showed that at the alkali concentration of 6% and TC effluent concentration of 0.5 mg·L−1, the modified WM had effective filtration volume of 1 968 BV (bed volume), which was about 48 times higher than unmodified WM. The TC adsorption efficiency by modified WM could reach up to 97% after four cycles. At effluent TC concentrations of 0~0.5 mg·L−1, the cost of water treatment is 0.036 0~0.083 25 yuan·m−3. The modified WM has a great potential in TC removal from water. The study provides effective strategies reference for overcoming critical limitations related with the application of WM and design an environmentally friendly, economical and effective membrane material for environmental remediation.
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Key words:
- citric acid /
- wood membrane /
- tetracycline /
- alkali /
- adsorption
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表 1 在pH为3和5时,6-BW和6-BW吸附TC表面的各种元素的原子百分比
Table 1. Atomic percentage of various elements on the surfaces of 6-BW and TC adsorbed 6-BW at pH=3 and 5
% 样品 C1s O1s N1s P2p 6-BW 60.41 37.81 1.41 0.42 6-BW TC (pH=3) 60.99 37.12 1.36 0.54 6-BWTC (pH=5) 60.95 37.32 1.35 0.38 表 2 用所用方法制备的6-BW的成本分析
Table 2. Cost analysis of typical 6-BW prepared using employed method
材料 使用量 单价/元 总价/元 柠檬酸(AR) 0.56 g 0.013 15 7.363×10−3 NaOH(AR) 0.084 g 0.013 15 1.104×10−3 HCl(AR) 6.5×10−7 m3 0.028 52 1.854×10−5 木材 0.140 g 0.065 74 9.203×10−3 6-BW — — 0.017 69 -
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