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随着人类生产生活的发展,越来越多的镉(cadmium,Cd)通过不同的途径进入到水环境中,对环境和生物产生潜在的危险。Cd2+在环境中的污染具有持久性,并且对生物器官有毒害作用甚至具有致癌风险[1]。因此,去除水溶液中的Cd2+十分重要。
目前已有多种方法可以去除水溶液中的Cd2+,其中吸附法是一种成熟有效的技术[2]。在生产生活中产生的大量生物质废弃物往往没有得到有效利用,近年来一些研究报道生物质可以直接作为吸附剂使用。贾郁菲等[3]利用甲壳生物质去除酸性矿坑水中的Cd2+,最大吸附量为235.45 mg·g−1。LEE等[4]利用柿子树叶生物质作为吸附材料,能够有效吸附水溶液中的Cu2+、Cd2+和Pb2+,其中对Cd2+的吸附量为18.26 mg·g−1 。刘栖萍等[5]通过10种树皮类吸附材料对Pb2+、Cd2+污染废水进行吸附,均有不错的吸附效果,对实际工业区废水中的Pb2+和Cd2+的吸附率最高可达98.21%。生物质材料对比生物炭可以有效降低材料制备时的能耗和污染,具有可观的利用前景。
番石榴是一种常见的亚热带水果,其果实和枝叶含有丰富的有机类物质。有研究表明番石榴树叶(guava leaf,Gl)可以吸附水中的Cd、Cr等重金属元素,ABDELWAHAB等[6]研究了Gl活化前后对Cd2+的吸附性能,研究表明Gl和AGl通过有机络合反应吸附溶液中的Cd2+,最大吸附量分别为10.53 mg·g−1和19.20 mg·g−1。MITRA等[7]使用Gl作为反应柱填料来吸附Cr6+,吸附量最高为8.72 mg·g−1。BEHERA等[8]利用Gl吸附30 mg·L−1的As3+溶液,去除率达到90.88%。上述研究可以看出Gl直接作为吸附剂使用效果并不出色,因此可以通过改性或与其他材料复合来提高吸附性能。类水滑石(layered double hydroxide, LDH)是一种具有层状结构的无机黏土矿物材料,其具有金属氢氧化物组成的阳离子层板和层间阴离子夹层,利用其组成可调控性等特性可以有效的去除Cd2+[9]。SHAN等[10]通过共沉淀法合成的镁铝类水滑石(MgAl-Layered double hydroxide, MgAl-LDH)可以有效吸附溶液中的Cd2+,最大吸附量能达到47.30 mg·g−1。ZHONG等[11]利用合成的MgAl-LDH和MgAlFe-LDH修复重金属污染的土壤,加入材料后土壤中的水提态Cd含量均降低了90%以上。通过将LDH和其他材料如生物炭等进行复合可以提升对重金属的去除能力,TAN等[12]将MgFe-LDH与猕猴桃枝生物炭复合用来吸附Cd2+,复合之后吸附量由1.78 mg·g−1提升到25.60 mg·g−1。
本研究通过将Gl与MgAl-LDH进行复合,合成了一种新型环保的复合材料Gl/MgAl-LDH,通过吸附动力学、等温吸附等实验探究了该复合材料对Cd2+的吸附特性,同时通过XRD、FT-IR和XPS等表征手段探究其性质特征和吸附机理,为生物质作为吸附剂的资源化利用提供了参考。
番石榴树叶/镁铝类水滑石复合材料对镉的吸附性能及机制
Adsorption performance and mechanism of cadmium by guava leaf/MgAl-LDH composites
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摘要: 采用共沉淀法将番石榴树叶(guava leaf,Gl)和镁铝类水滑石(MgAl-layered double hydroxide, MgAl-LDH)复合得到一种环保的新型复合吸附剂Gl/MgAl-LDH,并将其用于吸附溶液中的镉(cadmium,Cd)。通过吸附动力学、等温吸附探究了Gl/MgAl-LDH对Cd2+的吸附性能,并使用各种表征手段探究了材料的理化性质及吸附机理。结果表明,Gl/MgAl-LDH在60 min时即可达到吸附平衡,吸附量为37.50 mg·g−1,高于MgAl-LDH和Gl。Gl/MgAl-LDH对Cd2+的吸附过程符合拟二级动力学模型及Freundlich模型,最大吸附量为68.43 mg·g−1。表征结果表明LDH通过金属离子与Gl上的有机官能团发生络合反应,成功构筑到Gl表面。Gl/MgAl-LDH吸附后的XPS图谱中出现了Cd元素的谱峰,证实了Cd2+被有效吸附到材料表面。Gl/MgAl-LDH去除Cd2+的主要机理为吸附沉淀和有机络合。以上研究结果可为生物质作为吸附剂的资源化利用提供参考。Abstract: An environmentally friendly and novel composite adsorbent, Gl/MgAl-LDH, was obtained by Guava leaf (Gl) and MgAl-Layered double hydroxide (MgAl-LDH) composite with a co-precipitation method, and Gl/MgAl-LDH was used to adsorb cadmium (Cd) in solution. The adsorption performance of Gl/MgAl-LDH towards Cd2+ was investigated by adsorption kinetics and isotherm, its properties and adsorption mechanism were studied by different characterization instruments. The experimental results showed that Gl/MgAl-LDH could reach the adsorption equilibrium at 60 min, and the adsorption amount was 37.50 mg·g−1, which was higher than that of MgAl-LDH or Gl. Adsorption process of Cd2+ onto Gl/MgAl-LDH conformed to the proposed secondary kinetic and Freundlich models with a maximum adsorption amount of 68.43 mg·g−1. Characterization results showed that the complex reaction occurred between metal ions on LDH and organic functional groups on Gl, and LAD was successfully anchored on Gl surface. The appearance of the spectral peaks of elemental Cd in the XPS spectra of the adsorbed Gl/MgAl-LDH confirmed the effective adsorption of Cd2+ onto the material. The main mechanisms of Cd2+ removal by Gl/MgAl-LDH were adsorption precipitation and organic complexation. The results of this study provide a reference for the resource utilization of biomass as an adsorbent.
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Key words:
- adsorption /
- biomass /
- MgAl-LDH /
- composites /
- cadmium /
- resource utilization
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表 1 Gl/ MgAl-LDH、MgAl-LDH及Gl的表面性质
Table 1. Surface properties of Gl/ MgAl-LDH, MgAl-LDH and Gl
吸附剂 比表面积/(m2·g−1) 孔容/(cm3·g−1) 孔径/nm Gl/MgAl-LDH 42.41 0.23 21.57 MgAl-LDH 92.10 0.54 23.37 Gl 1.68 0.01 17.39 表 2 Gl/ MgAl-LDH、MgAl-LDH以及Gl吸附Cd2+的动力学拟合
Table 2. Kinetic fitting of Cd2+ adsorption by Gl/MgAl-LDH, MgAl-LDH and Gl
吸附剂 拟一级动力学 拟二级动力学 Elovich k1/(min−1) qe/(mg·g−1) R2 k2/(g·(mg·min)−1) qe/(mg·g−1) R2 A k3 R2 Gl/MgAl-LDHs 0.67 37.10 0.894 0.04 37.95 0.997 29.11 1.87 0.794 MgAl-LDHs 0.10 27.07 0.878 0.01 28.85 0.973 7.55 4.00 0.989 Gl 0.17 13.78 0.985 0.02 14.37 0.970 7.67 1.24 0.766 表 3 Gl/ MgAl-LDH、MgAl-LDH及Gl吸附Cd2+的等温吸附拟合
Table 3. Isothermal adsorption fitting of Cd2+ adsorption by Gl/MgAl-LDH, MgAl-LDH and Gl
吸附剂 Langmuir Freundlich qm/(mg·g−1) kL/(L·mg−1) R 2 kF/(mg·g−1) 1/n R 2 Gl/MgAl-LDH 64.14 1.49 0.843 35.63 0.16 0.983 MgAl-LDH 81.34 0.04 0.911 9.33 0.46 0.985 Gl 27.42 0.08 0.931 0.33 0.12 0.997 -
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