Photocatalytic reduction and cyclic adsorption of Cr(VI) by nanocellulose-polyethylenimine-polypyrrole composite aerogel
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摘要: 吸附与光催化还原相结合可以在光照条件下将 Cr(VI)还原成低毒性的 Cr(III)并固定在吸附材料中，是一种很有前景的铬污染治理方法。本文以纳米纤维素-聚乙烯亚胺气凝胶(CPA)为骨架，通过吡咯原位氧化聚合制得纳米纤维素-聚乙烯亚胺-聚吡咯(CPP-F)光敏感复合气凝胶。采用SEM、FTIR、UV-vis、XPS对气凝胶进行表征。通过黑暗及光照条件的对比，研究了CPP-F复合气凝胶对Cr(VI)的吸附、光催化原位还原及循环吸附性能，并对其循环吸附机制进行了分析。结果表明：CPP-F为黑色、结构均匀稳定的多孔气凝胶，在紫外、可见光和近红外区域均有较强的光吸收。负载Cr(VI)的CPP-F经光照处理后，Cr(III)含量由黑暗条件下29.76wt%上升至72.33wt%，其还原率是黑暗条件下的2.43倍，表明聚吡咯(PPy)具有优异的光催化性能，光照处理可促进Cr(VI)的原位还原。经过6次循环吸附，光照处理下CPP-F对Cr(VI)的循环吸附量比黑暗处理高64.97%，其原因可能是光催化还原产生的Cr(III)为Cr(VI)提供了新的吸附位点，促进了Cr(VI)的循环吸附。吸附-光催化还原-循环吸附相结合的方法比单一吸附法更具优势，在Cr(VI)污染治理方面有很大的应用潜力。Abstract: The combined method of adsorption-reduction is a promising method for chromium pollution control. In this paper, nanocellulose-polyethylenimine-polypyrrole (CPP-F) photosensitive composite aerogel was prepared by in-situ oxidation polymerization of pyrrole using nanocellulose-polyethylenimine aerogel (CPA) as skeleton. The aerogel was characterized by SEM, FTIR, UV-vis and XPS. The adsorption, in-situ reduction and cyclic adsorption properties of CPP-F composite aerogel on Cr(VI) were studied by comparing the dark and light conditions, and the cyclic adsorption mechanism was analyzed. The results show that CPP-F is a black porous aerogel with uniform and stable structure, and has strong light absorption effect in ultraviolet, visible and near infrared region. After light treatment, Cr(III) content in Cr(VI) loaded CPP-F increase from 29.76wt% under dark conditions to 72.33wt%, and its reduction rate is 2.43 times that of dark conditions, indicating that polypyrrole (PPy) has excellent photocatalytic performance, and light treatment can promote the in-situ reduction of Cr(VI). After 6 cycles of adsorption, the cyclic adsorption capacity of Cr(VI) by CPP-F under light treatment is 64.97% higher than that under dark treatment, indicating that photocatalytic reduction of Cr(III) provides a new adsorption site for Cr(VI) and realizes the cyclic adsorption of Cr(VI). The combination of adsorption-photocatalytic reduction-cyclic adsorption has more advantages than adsorption method, and has great application potential in Cr(VI) pollution control.
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Keywords:
- photocatalytic /
- nanocellulose /
- aerogel /
- PPy /
- Cr(VI) /
- photoreduction /
- cyclic adsorption
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图 1 (a) 纳米纤维素-聚乙烯亚胺-聚吡咯(CPP)合成机制(插图从左至右分别为纳米纤维素-聚乙烯亚胺气凝胶(CPA)及CPP数码照片；((b)~(d)) CPA、CPP-三氯化铁(CPP-F)、CPP-过硫酸铵(CPP-A)的SEM图像
Figure 1. (a) Synthesis mechanism of nanocellulose-polyethylenimine-polypyrrole (CPP) (Inset photos are nanocellulose-polyethylenimine aerogel (CPA) and CPP, respectively); ((b)-(d)) SEM images of CPA , CPP- ferric trichloride (CPP-F) and CPP-ammonium persulphate (CPP-A)
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图 2 聚吡咯(PPy)、CPA、CPP-F及CPP-A的FTIR图谱
Figure 2. FTIR spectra of polypyrrole (PPy), CPA, CPP-F and CPP-A
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图 4 (a) pH对Cr(VI)去除率的影响；(b) pH对CPP-F Zeta电位的影响；(c) Cr(VI)初始浓度对去除率的影响；(d)吸附剂用量对去除率的影响
Figure 4. (a) Effect of pH on removal efficiency of Cr(VI) ; (b) Effect of pH onZeta potential of CPP-F; (c) Effect of Cr(VI) initial concentration on removal efficiency of Cr(VI); (d) Effect of adsorbent dosage on removal efficiency of Cr(VI)
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图 6 (a) 负载Cr(VI)的CPP-F经黑暗和光照处理及酸-碱洗脱分析；(b) Cr(VI)的还原率在光照及黑暗处理下随时间的变化
Figure 6. (a) Analysis of Cr(VI) loaded CPP-F by dark and light treatment and acid-base elution; (b) Variation of Cr(VI) reduction rate with time under light and dark treatment
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图 7 CPP-F的XPS图谱： (a) 总谱；(b) 吸附Cr(VI)后；(c) 吸附后黑暗处理60 min；(d)吸附后光处理60 min
Figure 7. XPS spectra of CPP-F: (a) Full survy; (b) After Cr(VI) adsorption; (c) Dark treatment for 60 min after adsorption; (d) Light treatment for 60 min after adsorption
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目的
铬是一种常见的金属污染物，其中Cr(VI) 具有致癌性、致畸性和致突变性，对人类健康及生态环境构成严重威胁。吸附与光催化还原相结合可以在光照条件下将 Cr(VI)还原成低毒性的 Cr(III)并固定在吸附材料中，与单纯的吸附法相比是一种很有前景的铬污染治理方法。
方法本文以麦秆基纳米纤维素/聚乙烯亚胺复合气凝胶(CPA)为骨架，以FeCl为氧化剂通过吡咯原位氧化聚合制得纳米纤维素/聚乙烯亚胺/聚吡咯(CPP-F)光敏感复合气凝胶。分别采用扫描电镜(SEM)、红外光谱仪(FT-IR)、紫外-可见分光光度仪(UV-vis)及X-射线光电子能谱(XPS)对复合气凝胶氧化前后的微观形貌、官能团变化、光吸收性能及吸附与原位还原性能进行表征。研究了pH，Cr(VI)初始浓度、吸附剂用量及共存离子对CPP-F静态吸附性能的影响。分别采用酸-碱洗脱法及XPS法研究了黑暗及光照条件下，PPy对负载Cr(VI)的原位还原率的影响；对比研究了黑暗及光照条件下光催化还原对复合气凝胶循环吸附性能的影响。结合气凝胶对Cr(VI)的吸附及还原特性对复合气凝胶的循环吸附机理进行了分析。
结果以CPA为骨架，FeCl为氧化剂通过吡咯原位氧化聚合制得了结构稳定均匀的黑色纳米纤维素/聚乙烯亚胺/聚吡咯光敏感复合气凝胶(CPP-F)。SEM结果表明CPP-F 为大孔网络结构，说明引入PPy后未改变CPA的多孔结构。FT-IR结果表明在CPP-F均观察到吡咯环的特征峰，1554 cm处C=C伸缩振动的吸收峰及1312 cm处=C-N的拉伸振动峰，表明PPy被成功引入到气凝胶骨架中。UV-vis结果表明CPP-F在紫外、可见光和近红外区域光吸收范围及强度均有明显的增加，表明引入PPy后，其优异的光敏特性使复合材料的光吸收显著增强。静态吸附结果表明，pH=2时，Cr(VI)初始浓度为50 mg/L，吸附剂用量为100 mg/L为最优吸附条件，此时，CPP-F对Cr(VI)的去除率最大；当溶液中存在Na、Ca和Cu共存阳离子时，吸附效率略有下降；而在共存阴离子中，S的影响较大。负载Cr(VI)的CPP-F经光照及黑暗处理后，经酸碱洗脱后计算其原位还原率，经过60 min的光照处理后，Cr(VI)的还原率为76.92%，而在相同时间黑暗条件下，Cr(VI)的还原率仅为25.87%。进一步采用XPS对负载Cr(VI)的CPP-F进行了黑暗和光照条件下的对比分析，结果表明，负载Cr(VI)的CPP-F经光照处理后，Cr(III)含量由黑暗条件下29.76%上升至72.33%，其还原率是黑暗条件下的2.43倍，表明PPy具有优异的光催化性能，光照处理可促进Cr(VI)的原位还原。经过6次循环吸附，光照处理下CPP-F对Cr(VI)的循环吸附量比黑暗处理高64.97%，其原因可能是光催化还原产生的Cr(III)为Cr(VI)提供了新的吸附位点，促进了Cr(VI)的循环吸附。CPP-F对Cr(VI)的吸附-还原-再吸附机理分为吸附-还原(化学还原+光催化还原)-循环吸附3个主要阶段。
结论吸附-光催化还原-循环吸附相结合的方法比单一吸附法更具优势，在Cr(VI)污染治理方面有很大的应用潜力。
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吸附与光催化还原相结合可以在光照条件下将Cr(VI)还原成低毒性的Cr(Ⅲ)并固定在吸附材料中。但目前常用的光催化材料以纳米贵金属及纳米金属氧化物为主，存在自身吸附性能差、成本高，还原产生的Cr(Ⅲ)不能固定等缺陷。在生物质凝胶中引入兼具吸附和光催化性能的聚吡咯，可以使Cr(VI)的高效吸附、光催化原位还原及循环吸附协同进行，是一种有应用前景的铬污染治理方法。
本文以麦秆基纳米纤维素/聚乙烯亚胺气凝胶(CPA)为骨架，通过吡咯原位氧化聚合制得纳米纤维素-聚乙烯亚胺-聚吡咯(CPP-F)光敏感复合气凝胶。CPP-F 为黑色、结构均匀稳定的多孔气凝胶，在紫外、可见光和近红外区域均有较强的光吸收。负载Cr(VI)的CPP-F经光照处理后，Cr(Ⅲ)含量由黑暗条件下29.76%提高至72.33%，还原率是黑暗处理的2.43倍，表明聚吡咯(PPy)在光照处理下可促进Cr(VI)的原位还原。经过6次循环吸附，光照处理下CPP-F对Cr(VI)的循环吸附量比黑暗处理高64.97%，光催化还原形成的Cr(Ⅲ)为Cr(VI)提供了新的吸附位点，促进了Cr(VI)的循环吸附。吸附-光催化还原-循环吸附相结合的方法比单一吸附法更具优势，在Cr(VI)污染治理方面有很大的应用潜力。
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