N掺杂生物炭的制备及其对Co2+ 的吸附性能

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

(1)

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

(2)

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

(3)

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%

(4)

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



图1NBC x 吸附Co2+前后的SEM照片

Fig.1SEM 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



图2NBC x 吸附Co2+前后的氮气吸脱附曲线

Fig.2N2 adsorption-desorption isotherm of NBC x

图3



图3NBC x 吸附Co2+前后的孔径分布

Fig.3Pore 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

Table 1

表1

表1NBC x 比表面积和孔隙结构参数

Table 1Specific 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
NBC600	4.58	33	18	0.03	0.02	0.33
NBC700	3.42	69	48	0.05	0.03	0.40
NBC800	5.68	32	15	0.04	0.01	0.75
NBC800-Co2+	4.00	325	249	0.30	0.15	0.50

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



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



图4NBC800吸附Co2+前后的XPS谱图

Fig.4XPS 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+

Table 2

表2

表2XPS分析元素摩尔分数

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

Samples	C1s	O1s	N1s	Co2p
NBC600	82.4	16.4	1.2	-
NBC700	80.9	17.4	1.7	-
NBC800	49.76	46.35	3.89	-
NBC800-Co2+	40.54	41.61	3.92	13.93

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]

Table 3

表3

表3XPS分析中O1s、N1s和Co2p谱图的官能团摩尔分数

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

Samples	-OH	C=O	C-O	Oxide-N	Graphitic-N	Pyrolic-N	Pyridinic-N	Co3+	Co2+
NBC600	4.63	6.37	5.40	0.19	0.24	0.38	0.39	-	-
NBC700	4.67	7.85	4.88	0.16	0.17	0.75	0.62	-	-
NBC800	14.20	21.69	10.46	0.36	0.41	2.05	1.07	-	-
NBC800-Co2+	13.25	14.28	14.08	0.42	1.12	1.51	0.87	4.97	8.96

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



图5NBC800吸附Co2+前后的FTIR图

Fig.5FTIR 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



图6NBC x 在不同pH值下的电位

Fig.6Zeta 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



图7NBC x 吸附Co2+的吸附等温曲线和Langmuir拟合

Fig.7Adsorption 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



图8NBC x 对Co2+的吸附动力学曲线和NBC x 的吸附动力学拟合

Fig.8Adsorption 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



图9NBC800对Co2+的吸附穿透曲线

Fig.9Breakthrough 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



图10吸附剂质量对吸附的影响曲线

Fig.10Curve 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



图11NBC x 的吸附机理

Fig.11Adsorption 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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