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材料研究学报, 2023, 37(4): 291-300 DOI: 10.11901/1005.3093.2022.231

研究论文

N掺杂生物炭的制备及其对Co2+ 的吸附性能

余谟鑫1,2, 张书海1, 朱博文1, 张晨1, 王晓婷,1,3, 鲍佳敏1, 邬翔1

1.安徽工业大学化学与化工学院 马鞍山 243000

2.中钢天源股份有限公司 马鞍山 243000

3.马钢集团股份有限公司 马鞍山 243003

Preparation of Nitrogen-doped Biochar and its Adsorption Capacity for Co2+

YU Moxin1,2, ZHANG Shuhai1, ZHU Bowen1, ZHANG Chen1, WANG Xiaoting,1,3, BAO Jiamin1, WU Xiang1

1.School of Chemistry and Chemical Engineering, Anhui University of Technology, Ma'anshan 243000, China

2.Sinosteel New Materials Co. Ltd., Ma'anshan 243000, China

3.Magang (Group) Holding Co. Ltd., Ma'anshan 243003, China

通讯作者: 王晓婷，副教授，pingguo2911@sina.com，研究方向为纳米多孔炭材料的制备及应用

责任编辑: 吴岩

收稿日期: 2022-04-24   修回日期: 2022-05-31  

基金资助: 安徽省博士后科研项目(2021B547)
安徽省教育厅自然科学研究项目(KJ2021A0399)

Corresponding authors: WANG Xiaoting, Tel: 13855518820, E-mail:pingguo2911@sina.com

Received: 2022-04-24   Revised: 2022-05-31  

Fund supported: Anhui Province Postdoctoral Science Research Program(2021B547)
Natural Science Research Project of Anhui Province(KJ2021A0399)

作者简介 About authors

余谟鑫，男，1978 年生，博士

摘要

在芦荟叶皮中加入尿素用水热法制备炭前驱体，然后将其以不同的终温热解制备出N掺杂生物炭(NBC x )。使用扫描电子显微镜(SEM)、氮气吸附-脱附仪(BET)、X射线光电子能谱仪(XPS)、傅立叶红外光谱仪(FTIR)和动电位分析仪(Zeta)等手段进行表征，研究其对废水中Co2+的吸附性能。结果表明：NBC x 的表面有明显的层块堆积。NBC x 具有分级多孔结构，芦荟叶皮与尿素的质量比为2∶1、热解终温为800℃时制备的NBC800其比表面积为32 m2·g-1，总孔体积为0.04 cm3·g-1，其中非微孔比例高达75%。NBC800表面含有丰富的含氧和含氮官能团，N含量和O含量(摩尔分数)分别高达3.89%和46.35%，可与Co2+发生离子交换、静电吸附、络合作用和共沉淀等反应。用Langmuir等温线模型能很好地描述NBC800对Co2+的吸附过程，为单分子层吸附，最大理论吸附量高达228.31 mg·g-1。拟二级吸附动力学模型表明，吸附进行得较快。

关键词： 无机非金属材料; 芦荟叶皮; 生物质炭; N掺杂; Co2+; 化学吸附

Abstract

The nitrogen-doped biochar (NBC x ) was prepared from aloe vera leaf rind with adding urea as nitrogen source, namely the biochar (NBC x ) was first prepared by hydrothermal method, and then the mixture of NBC x and urea was pyrolyzed at different temperatures. The as-made NBC x was characterized by SEM、BET、XPS、FTIR and Zeta, and its application for adsorption of Co2+ in waste water was investigated. The results show that the NBC x has a hierarchical porous structure with a lamellar-like surface with many small flakes. The NBC800, prepared with the mass ratio 2∶1 for aloe vera leaf rind to urea by final pyrolysis temperature of 800℃, presents a specific surface area of 32 m2·g-1, and a total pore volume of 0.04 cm3·g-1 with the non-microporous ratio of up to 75%. The surface of NBC800 is rich in oxygen and nitrogen functional groups, and the content of N and O (mole fraction) is up to 3.89% and 46.35% respectively, which can react with Co2+ through ion exchange, electrostatic adsorption, complexation and co-precipitation. The Langmuir model fits well with the adsorption isotherm of Co2+ on NBC x, which demonstrated that the adsorption is monolayered. The maximum adsorption capacity of Co2+ on NBC800 is up to 228.31 mg·g-1. The quasi-second-order kinetic model can describe the adsorption process better. The adsorption rate is mainly controlled by chemical adsorption.

Keywords： inorganic non-metallic materials; aloe vera leaf rind; biochar; N-doped; Co2+; chemical adsorption

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本文引用格式

余谟鑫, 张书海, 朱博文, 张晨, 王晓婷, 鲍佳敏, 邬翔. N掺杂生物炭的制备及其对Co2+ 的吸附性能[J]. 材料研究学报, 2023, 37(4): 291-300 DOI:10.11901/1005.3093.2022.231

YU Moxin, ZHANG Shuhai, ZHU Bowen, ZHANG Chen, WANG Xiaoting, BAO Jiamin, WU Xiang. Preparation of Nitrogen-doped Biochar and its Adsorption Capacity for Co2+[J]. Chinese Journal of Materials Research, 2023, 37(4): 291-300 DOI:10.11901/1005.3093.2022.231

钴是一种剧毒重金属[1~3]。目前去除废水中Co2+的方法，有化学沉淀、混凝、离子交换、吸附、电化学和膜分离等[4]。吸附法操作简易、成本低且效率高[5]。吸附法使用的吸附剂，最常见的有沸石[6]、离子交换树脂[7]、壳聚糖[8]、活性炭[9]等。活性炭的孔隙率高、孔容大、官能团丰富，且易改性和再生[10]。传统的活性炭是用煤沥青等工业原料制造的，成本高且性能不高[11]。生物质原料的来源广泛，适合用作制造生物炭的原料[12]。Qiang和Yang等[13]研究了稻草在不同温度下热解吸附Ni2+、Co2+的性能和机理，发现稻草生物炭对Co2+和Ni2+的最高吸附量分别高达39.92和33.12 mg·g-1。余建星等[14]以核桃青皮为原料用水热法制备炭前驱体，然后以终温800℃活化制备出生物质炭(HBC800)。这种生物质炭对Ni2+的最大理论吸附量高达 127.39 mg·g-1。Elham Aboli等[15]以柑橘叶为原料制备的生物炭，对Pb2+、Co2+和Ni2+的最大吸附容量分别为69.82、60.60和58.14 mg·g-1。Arnelli等[16]使用稻壳为原料、以KOH为活性剂，高温热解制备活性炭再使用十二烷基硫酸钠(SLS)为表面活性剂对其进一步改性，在300 min内对40 mg·L-1的Ni2+的吸附效率为95.97%。

但是上述方法复杂且使用的化学试剂过多，容易造成污染。芦荟叶是制药、化妆品和食品工业的一种废弃物[17]。可使用芦荟叶皮作为炭源制造活性炭，用于处理水中重金属污染物[18]。

掺N炭材料具有独特的络合结构、高孔容、大的吸附容量、快速的吸附动力学、机械和热稳定性且成本低，受到极大的关注[19, 20]。尿素的N含量高、价格低廉，且其热解产物无毒[21]。本文使用芦荟叶皮为生物质原料、尿素为氮源，用水热法制备炭的前驱体，再对其高温热解制备N掺杂生物炭，研究其对Co2+的吸附性能并阐明吸附机理。

1 实验方法

1.1 实验用试剂和仪器设备

CH4N2O，Co(NO3)3·6H2O(分析纯)；芦荟叶皮；纯水。

GSL-1600X型高温真空管式炉；Nanosem430型扫描电子显微镜(SEM)；3H-2000PS1型自动氮气吸附仪(BET)；Nicolet is 50型傅立叶红外光谱仪(FTIR)；ESCALAB25型X-射线光电子能谱(XPS)；Zetasizer Nano ZS90型Zeta电位仪；ICPS-7510 PLUS电感耦合等离子体原子发射光谱仪。

1.2 N掺杂生物炭的制备

将用纯水洗净的芦荟叶皮在105℃干燥24 h，然后将其磨粉并过80 目筛。在一定质量的芦荟叶皮粉中按2∶1的质量比加入尿素，再加入去离子水(10 mL·3 g-1芦荟叶皮)混合均匀后放入高压反应釜。将高压反应釜置于鼓风干燥箱中(200℃，16 h)进行反应，制备出生物炭前驱体。将生物炭前驱体置于高温真空管式炉中在惰性气体氛围下(N2，60 mL·min-1)程序升温(5℃·min-1)至不同终温(600、700、800℃)热解3 h，然后自然降温至室温得到炭材料。将炭材料研磨至通过80目筛，得到N掺杂生物炭(NBC x，其中x代表热解终温)。

1.3 性能表征

1.3.1 吸附等温线的测定

在50 mL不同初始浓度的Co2+溶液(25、50、100、200、300、400和600 mg·L-1)中分别加入50 mg 的NBC x，在25℃下静态吸附1440 min后，测试溶液中Co2+剩余的浓度，绘制NBC x 吸附等温线。NBC x 对Co2+的吸附量qt (mg·g-1)[22]为

qt=(C0-Ct)Vm

式中qt (mg·g-1)为t时吸附剂已吸附的吸附质的量；C0 (mg·L-1)为吸附质的初始浓度；Ct (mg·L-1)为t时吸附质的浓度；V(L)为吸附质溶液的体积；m(g)为吸附剂的质量。

用Langmuir吸附等温线模型分析实验数据。Langmuir等温线模型[23,24]为

Ceqe=1QKL+CeQ

式中qe (mg·g-1)为平衡吸附量；Q (mg·g-1)为吸附到达平衡时单层最大吸附量；Ce (mg·L-1)为吸附质在平衡时的浓度；KL (L·mg-1)为表面吸附亲和性常数。

1.3.2 吸附动力学的测定

在8份50 mL的 300 mg·L-1 Co2+溶液中各加入50 mg的NBC x，放入恒温(25℃)振荡摇床(120 r/min)中。分别在振荡时间为10、30、60、120、240、480、840和1440 min时取样，以检测溶液中Co2+浓度。用准二级动力学模型模拟实验数据，准二级动力学模型可表示为[25, 26]

tqt=1K2qe2+tqe

式中t(min)为吸附剂与溶液的接触时间，qt (mg/g)为t时刻的吸附量，qe (mg/g)为平衡时的吸附量K2 (g·mg‒1·min‒1)，准二级动力学吸附速率常数。

1.3.3 动态吸附

在25℃测试NBC x 在流速1 mL·min-1条件下的动态吸附过程，为模拟固定床吸附。在玻璃圆管(长10 cm，内径0.8 cm)中加入200 mg 的NBC x 模拟吸附柱，用蠕动泵将Co2+溶液(50 mg·L-1)泵入吸附柱，自吸附柱下进上出流入收集装置，定时(10、30、60、90、120、180、240和300 min)取样测定溶液中Co2+的浓度。Ct达到C0的10%的t时刻为吸附剂NBC x 的穿透时间，Ct达到C0的90%的t时刻为吸附剂NBC x 的饱和时间。

1.3.4 吸附剂质量对吸附的影响

在8份50 mL浓度为1500 mg·L-1的Co2+溶液中分别加入50、100、200、300、400、500、600 mg的NBC x，放入恒温(25℃)振荡摇床(120 r/min)1440 min取样，以检测溶液中Co2+浓度并画出去除率曲线。去除率η(%)[27]为

η=(C0-Ct)C0×100%

式中C0 (mg·L-1)为吸附质的初始浓度；Ct (mg·L-1)为t时刻吸附质的浓度。

2 结果和讨论

2.1 NBC x 吸附Co2+ 前后的形貌

图1给出了NBC x 吸附Co2+前后的扫描电镜照片。图1a、b、c分别给出了活化终温为600、700、800℃时NBC x 的SEM照片。可以看出，这种生物炭材料具有明显的层块状堆积结构。与NBC600和NBC700相比，NBC800的表面层块堆积最复杂，层块最粗糙和细小，暴露出更多的活性官能团。其原因是，随着热解温度的提高NBC x 中的尿素和活性官能团热解生成气体，这些气体膨胀挥发产生更多的孔道，使得NBC x 表面更为粗糙。这有助于提高生物炭材料的比表面积和孔道的丰富性，为Co2+提供大量的吸附位着点[28]。图1d给出了NBC800吸附Co2+后的SEM照片，可见吸附Co2+后NBC800表面的层块上附着了大量的霜状物。发生这种现象的原因是，大量Co2+进入NBC800的孔道与炭材料表面的官能团发生络合作用或共沉淀作用，使吸附后的Co2+均匀分布在NBC800的表面。

图1

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图1   NBC x 吸附Co2+前后的SEM照片

Fig.1   SEM images of the samples NBC600 (a), NBC700 (b), NBC800 (c) and NBC800⁃Co2+ (d)


2.2 BET分析

图2给出了NBCx吸附Co2+前后的氮气吸脱附曲线。可以看出，相对分压为0~0.1时NBCx的吸附量增加较快；相对分压为0.1~0.4时出现吸脱附曲线平台，NBC x 的吸附量处于平衡；在高相对压力区(>0.4)曲线出现滞后环，兼有I型和IV型特征，表明NBCx表面有微孔和中孔[29,30]。图3给出了NBC x 吸附Co2+前后的孔径分布(由BJH法计算)，可见NBC x 的孔径分布较宽(0.5~30 nm)，表明NBC x 具有分层多孔结构。

图2

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图2   NBC x 吸附Co2+前后的氮气吸脱附曲线

Fig.2   N2 adsorption-desorption isotherm of NBC x


图3

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图3   NBC x 吸附Co2+前后的孔径分布

Fig.3   Pore size distribution of NBC x


表1列出了不同的NBCx比表面积和孔隙结构参数。从表1可见，随着热解终温由600℃提高到700℃，NBC x 的比表面积和总孔容明显增大，但是平均孔径减小。其原因是，随着温度的升高NBC x 中的活性官能团以有机小分子气体的形式脱除，在气体膨胀挥发的过程中制造了更多的微孔[31]。随着热解终温由700℃提高到800℃，NBC x 的比表面积下降而非微孔的比例明显提高。其原因是，随着温度的提高NBC x 中的活性官能团继续脱除，在气体膨胀挥发的过程中将一部分微孔扩大为中孔。吸附Co2+后NBC800的比表面积和孔结构参数都明显增大，非微孔的比例明显降低。其原因是，Co2+在NBC800表面活性官能团的作用下附着在其表面，使NBC800的孔道大量增加、表面结构更为复杂。这种现象与SEM观察的结果符合。

表1   NBC x 比表面积和孔隙结构参数

Table 1  Specific surface area and pore structure parameters of NBC x

Samples

Dap

/nm

SBET

/m2·g-1

Smic

/m2·g-1

Vt

/cm3·g-1

Vmic

/cm3·g-1

Non-Vmic/Vt
NBC6004.5833180.030.020.33
NBC7003.4269480.050.030.40
NBC8005.6832150.040.010.75
NBC800-Co2+4.003252490.300.150.50

Note:Da—average pore diameter; SBET—specific surface area obtained by the BET method; Smic—micropore specific surface area; Vt—total pore volume; Vmic—micropore volume

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2.3 XPS分析

图4给出了 NBC800吸附Co2+前后的XPS全谱。由图4a、d可见，NBC800富含O和N元素，C1s、O1s和N1s的宽谱峰分别出现在285、528和398 eV附近，由O和N组成的活性官能团是化学吸附的重要基础。吸附Co2+后，NBC800的全谱出现了明显的Co2p(790 eV左右)宽谱峰。从表2可知，随着热解终温的提高NBC800表面C元素(49.76%)的含量降低，O元素(46.35%)和N元素(3.89%)含量提高，吸附后各元素的含量都出现明显的变化，NBC800-Co2+表面Co元素的含量为13.93%，表明NBC800成功地吸附了Co2+。

图4

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图4   NBC800吸附Co2+前后的XPS谱图

Fig.4   XPS spectra of the samples (a) survey spectra of NBC800; (b) O1s spectrum of NBC800; (c) N1s spectrum of NBC800; (d) survey spectra of NBC800-Co2+; (e) O1s spectrum of NBC800-Co2+; (f) N1s spectrum of NBC800-Co2+; (g) Co2p spectrum of NBC800-Co2+


表2   XPS分析元素摩尔分数

Table 2  Elements content of XPS analysis (mole fraction, %)

SamplesC1sO1sN1sCo2p
NBC60082.416.41.2-
NBC70080.917.41.7-
NBC80049.7646.353.89-
NBC800-Co2+40.5441.613.9213.93

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由图4b、e可见，NBC800吸附Co2+前后O谱可分为3个特征吸收峰-OH、C=O和C-O，结合能分别约为533.8、532.9和531.8 eV，吸附Co2+前后3种形式的O元素相对含量明显变化。结合表3可知，在吸附前NBC800中-OH、C=O和C-O的含量高于NBC600和NBC700，分别为14.20%、21.69%和10.46%。NBC800吸附Co2+后C=O(14.28%)含量下降较明显，是相关活性官能团与Co2+发生络合反应引起的[32]。

表3   XPS分析中O1s、N1s和Co2p谱图的官能团摩尔分数

Table 3  Functional group content of O1s spectrum, N1s spectrum and Co2p spectrum in XPS analysis (mole fraction, %)

Samples-OHC=OC-OOxide-NGraphitic-NPyrolic-NPyridinic-NCo3+Co2+
NBC6004.636.375.400.190.240.380.39--
NBC7004.677.854.880.160.170.750.62--
NBC80014.2021.6910.460.360.412.051.07--
NBC800-Co2+13.2514.2814.080.421.121.510.874.978.96

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由图4c、f可知，NBC800吸附Co2+前后的N谱可分为4个特征吸收峰Oxide-N、Graphitic-N、Pyrolic-N和Pyridinic-N，其结合能分别约为402.1、400.7、399.8和398.8 eV，吸附Co2+前后4种形式的N元素相对含量变化明显。由表3可知，在吸附前的NBC800中Oxide-N、Graphitic-N、Pyrolic-N和Pyridinic-N的含量分别为0.36%、0.41%、2.05%和1.07%，都明显高于NBC600和NBC700。N元素主要以Pyrolic-N和Pyridinic-N的形式存在。NBC800吸附Co2+后Pyrolic-N(1.51%)和Pyridinic-N (0.87%)的含量明显降低，表明吡咯和吡啶基团参与了吸附过程，Co2+与活性基团的结合影响了Pyrolic-N和Pyridinic-N结合能的信号[33]。由图4g可知，NBC800吸附Co2+后Co2p的谱图可分为两个特征吸收峰Co2+和Co3+,结合能分别约为781.2和779.96 eV。由表3可知，Co2+和Co3+的占比分别为8.96%和4.97%，且部分Co2+被羧基等含氧官能团氧化成Co3+，表明Co2+成功地吸附在NBC800表面[34]。

2.4 FTIR分析

图5给出了NBC800吸附Co2+前后的FTIR图，图中3430 cm-1附近的FTIR强吸收带可能是自由氨基和羟基键(-NH2和-OH)伸缩振动引起的。NBC800吸附Co2+后吸收带强度出现明显减弱，其原因是Co2+与-NH2和-OH发生离子交换和络合作用等化学吸附行为削弱了相关基团的吸收峰[35,36]。在1640 cm-1附近的FTIR吸收峰可能是羧基、醛基和酮基中的-C=O键的伸缩振动引起的，NBC800吸附Co2+后吸收带强度出现明显的减弱和偏移，是Co2+与-COOH发生离子交换和络合等化学吸附行为引起的[37]。在1420 cm-1附近的吸收峰可能是芳香族化合物基团的伸缩振动引起的，NBC800吸附Co2+后吸收峰发生了减弱和偏移，因为芳香族化合物的π电子与Co2+的稳定结合降低了键能[38]。870 cm-1附近的吸收峰是吡啶和杂环化合物中C-H键的面外弯曲振动引起的，NBC800吸附Co2+后吸收峰发生了减弱和偏移。NBC800吸附Co2+后在405 cm-1附近出现了明显的吸收峰增强和偏移，表明NBC800成功地吸附了Co2+，吸附后吸收峰出现偏移现象是形成Co-O引起的。

图5

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图5   NBC800吸附Co2+前后的FTIR图

Fig.5   FTIR diagram of NBC800 before and after Co2+ adsorption


2.5 Zeta分析

图6给出了NBC x 在不同pH值下的电位。可以看出，pH值低于3时NBC x 都带正电荷。随着pH值的增大NBC x 的表面电荷迅速由正变负。-COOH基团不带正电荷且-OH基团的极性非常弱，因此NBC x 基团仅在低pH值下呈弱正电荷，与Zeta电位的结果一致。随着热解终温的提高NBC x 中的活性官能团进一步以有机小分子气体的形式脱除，NBC x 的零电荷点(pHPZC)应该增大。但是，NBC x 的pHPZC随热解终温升高呈现降低的趋势，NBC600的pHPZC=4.72，NBC700的pHPZC=4.11，NBC800的pHPZC=3.49。随着热解终温的提高NBC x 中的活性基团虽然脱除了一部分，但是这些小分子气体膨胀挥发的同时产生了更多的孔道结构，使更多的活性基团得以暴露，因此NBC x 的pHPZC呈现减小的趋势。NBC800的pHPZC=3.49，表明在pH>3.49时NBC800表面带负电，有利于吸引水体中Co2+等重金属阳离子，也有利于NBC800通过包括静电作用在内的物化作用吸附这些阳离子。

图6

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图6   NBC x 在不同pH值下的电位

Fig.6   Zeta potential diagram of NBC x


2.6 吸附等温线

图7a给出了NBC x 吸附Co2+的吸附等温曲线，可见NBC x 对Co2+的吸附能力较好。当Co2+溶液的C0较低时Co2+在NBC x 上的平衡吸附量较低，但是对Co2+的去除率较高。随着Co2+溶液的C0的提高固液间金属的离子浓度梯度增大，有利于克服NBC x 与溶液间的界面传质阻力，使Co2+在NBC x 上的吸附量很快增加，NBC x 孔道以外表面的活性基团吸附Co2+达到饱和后Co2+开始进入孔道内，孔道内壁的活性基团开始参与吸附使Co2+的去除率呈现减小的趋势。随着Co2+溶液中的C0继续增加NBC x 对Co2+的吸附达到平衡，对Co2+的去除率最小。NBC800吸附等温线的起始斜率最高，平衡吸附量受金属离子溶液的C0的影响较小。其原因是，NBC800表面丰富的中孔可大量吸附Co2+且孔道不易阻塞。

图7

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图7   NBC x 吸附Co2+的吸附等温曲线和Langmuir拟合

Fig.7   Adsorption isotherm of Co2+ by NBC x and Langmuir model fitting (a) adsorption isotherm curve; (b) Langmuir model fitting diagram


由图7b可见，Langmuir模型(R2>0.9923)的拟合效果更接近于NBC x 对Co2+的吸附行为，表明NBC x 吸附Co2+具有单分子层吸附特征，即吸附过程符合化学吸附(静电吸引、离子交换、金属离子和官能团的络合作用等)。根据模型的计算结果表明，NBC800对Co2+的吸附能力最强，最大理论吸附量为228.31 mg·g-1。

2.7 动力学分析

图8a给出了NBC x 对Co2+的吸附动力学曲线。可以看出，NBC x 对Co2+的吸附过程进行较快，在吸附开始后约300 min趋于平衡。在吸附过程的前期(0~200 min)溶液中Co2+的浓度较高，NBC x 表面仍有较多的活性基团未吸附，此时Co2+在NBC x 上的吸附量较低。随着吸附时间的增加，NBC x 对于Co2+的吸附量逐渐增加。随着NBC x 对Co2+的进一步吸附(200~300 min)溶液中的Co2+浓度降低，NBCx表面的活性基团被Co2+占据并饱和，NBC x 对Co2+的吸附达到平衡，对Co2+的去除率达到最大。

图8

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图8   NBC x 对Co2+的吸附动力学曲线和NBC x 的吸附动力学拟合

Fig.8   Adsorption kinetics curve of Co2+ by NBC x and fitting of pseudo-second-order kinetics model (a) adsorption kinetics curve; (b) fitting diagram of pseudo-second-order dynamics model


图8b给出了NBCx的吸附动力学拟合。可以看出，拟二级动力学模型(R2>0.9957)更好地描述了Co2+在NBC x 上的吸附动力学。根据该模型计算的平衡吸附容量NBC800(qe=222.72 mg·g-1)>NBC700(qe=117.51 mg·g-1)>NBC600(qe=60.27 mg·g-1)，与实验结果非常接近，表明NBC x 吸附Co2+的方式主要是Co2+与NBC x 表面官能团之间的化学相互作用。NBC x 对Co2+的吸附达到平衡的时间基本相同，但是NBC800具有最大的平衡吸附容量，表明NBC800可能比NBC600和NBC700具有更多的表面活性官能团。这一实验结果与XPS分析的结果一致，符合SEM和BET的预测。

2.8 固定床吸附

图9给出了NBC800的吸附穿透曲线。可见，在模拟固定床中NBC800作为吸附剂吸附Co2+的穿透时间为89 min，饱和时间为237 min。穿透时间和饱和时间较长，表明NBC800对Co2+有较大的吸附容量，与吸附等温线和吸附动力学的结果相同。

图9

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图9   NBC800对Co2+的吸附穿透曲线

Fig.9   Breakthrough curve for Co2+ dynamic adsorption on NBC800


2.9 吸附剂质量对吸附的影响

如图10所示，随着NBC800加入量的增加Co2+溶液的平衡浓度越来越低，去除率递增。NBC800的质量为500和600 mg时吸附接近饱和，去除率曲线逐渐平衡，并且接近于1。实验结果表明，NBC800对Co2+有较大的的吸附容量。

图10

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图10   吸附剂质量对吸附的影响曲线

Fig.10   Curve of effect of NBC800 mass on Co2+ adsorption


2.10 吸附机理

图11给出了NBCx的吸附机理图，其中M代表Co2+。由表征分析结果和吸附实验数据可知，NBC x 具有高度芳香化和杂环化的结构，表面有丰富的活性官能团。Co2+在NBC x 上的吸附机制可能是离子交换、络合作用、共沉淀和静电吸附等。参与反应的主要活性官能团包括羧基(-COOH)、酯(-COOR)、和氨基(-NH2)基团等。其中羟基和羧酸参与离子交换、静电作用、络合作用等，羧基参与离子交换和络合作用等。在-OH的作用下形成氢氧化物沉淀在NBC x 表面，是可能的吸附方式。FTIR分析结果表明，也可能存在芳香族基团与金属离子的络合；XPS分析结果表明，吡啶和吡咯基团也可能参与了吸附过程。

图11

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图11   NBC x 的吸附机理

Fig.11   Adsorption mechanism of Co2+ on NBC x


3 结论

N掺杂生物炭(NBC x )的表面有明显的层块堆积，具有分级多孔结构。在芦荟叶皮与尿素的质量比为2∶1、热解终温为800℃条件下制备的NBC800其比表面积为32 m2·g-1，总孔体积为0.04 cm3·g-1，其中非微孔的比例高达75%。NBC800表面含有丰富的含氧和含氮官能团，N含量和O含量(摩尔分数)高达3.89%和46.35%，可与Co2+发生离子交换、静电吸附、络合作用和共沉淀等反应。NBC800对Co2+的吸附为单分子层吸附，最大理论吸附量高达228.31 mg·g-1，Langmuir等温线模型能很好地描述。拟二级吸附动力学模型表明，这种吸附进行得较快，吸附速率主要由化学吸附控制。

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Six types of biochar (BSB, CSB, FSB, CFSB, MSB, and TSB) were prepared from different raw materials by loading magnesium ions (Mg) via an impregnation process. The adsorption kinetics and thermodynamics of heavy metals at high concentrations were analyzed. The adsorption mechanisms were investigated by zeta potential, scanning electron microscopy-energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma-atomic absorption spectroscopy analyses. The adsorption of heavy metals by BSB, CSB, FSB, CFSB, MSB, and TSB conformed to the Langmuir model and PS-order. The maximum theoretical saturation adsorption capacities for Cd(II), Cu(II), and Pb(II) were 333.33, 238.10, 75.19, 96.15, 66.23, and 185.19 mg·g; 370.37, 294.12, 111.11, 169.49, 84.75, and 217.39 mg·g; and 302.58, 200.00, 61.73, 90.91, 54.47, and 166.67 mg·g, respectively. According to the analysis of the contribution of adsorption, the adsorption process was mainly controlled by cation-π interactions, ion exchange, mineral precipitation, and functional group interactions. Biochars contain ash, functional groups and load a large number of Mg, which can form complexes with metal ions and perform strong ion exchange; therefore, mineral precipitation and cation exchange played dominant roles in the adsorption process. The prepared Mg-loaded biochars presented in this research showed excellent adsorption properties for heavy metals and have great potential for practical application; in particular, BSB had the strongest adsorption capacity for the three heavy metal ions.

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