生物炭吸附水体中重金属机理与工艺研究进展

韩鲁佳 李彦霏 刘 贤 韩娅红

(中国农业大学工学院， 北京 100083)

生物炭吸附水体中重金属机理与工艺研究进展

韩鲁佳 李彦霏 刘 贤 韩娅红

(中国农业大学工学院， 北京 100083)

生物炭因其良好的表面特性和孔隙结构，广泛的原料来源和广阔的产业化发展前景，已成为当今环境、农业和能源等领域的研究热点。针对生物炭对水体重金属的吸附研究，本文基于生物炭原料和制备工艺的多样性，综合分析了国内外生物炭重金属吸附机理的研究成果，详细阐述、分析了5种吸附作用机制(物理吸附、静电作用、离子交换、络合反应和化学沉淀)及其相关表征手段；同时评述了吸附工艺条件和重金属种类对生物炭吸附重金属的影响；指出生物炭重金属吸附领域未来的研究中，应开展针对重金属吸附的生物炭原料特性及吸附产物的多维、微纳尺度表征方法研究。

生物炭； 水体重金属； 吸附机理； 表征

引言

重金属在环境中无处不在，危及人类健康[1]。由于人类各种生产活动：冶炼加工、矿山开采、化工生产等，将含有重金属的废水排放到水环境中，造成了严重的水体重金属污染。重金属污染与其他有机化合物污染不同，许多有机化合物可以通过自然界本身物理的、化学的或生物的方式净化，使有害性降低或解除。重金属具有持久性和富集性，很难在环境中降解，对人类及动植物危害极大[2]。环境中的铅、铜含量超标会显著抑制动植物生长，严重时甚至导致其死亡[3]; 同时，重金属还会通过食物链进入人体，干扰人体正常的生理功能进而对其健康造成危害[4-5]。因此，水体中重金属污染物的治理问题刻不容缓。

目前，移除和回收水体中重金属的方法主要有化学沉淀、氧化还原、离子交换、生物过滤、活性污泥法和吸附法等[6-11]。其中，吸附法具有净化效果好、操作简便、不会造成二次污染等优点，被视为一种有效且高效的处理方式[12-13]，被广泛应用于城市废水、工业污水中重金属的去除[14]。而决定其吸附效果和成本的核心因素是吸附剂[15-16]。因此，研制低廉高效的吸附剂是当前水体污染物吸附处理问题的研究热点。

生物炭(biochar)是生物质在一定温度和缺氧条件下热裂解形成的一类高度芳香难熔性固体物质[17-18]。通常生物炭具有一定的比表面积和丰富的极性官能团，例如羧基、酚羟基和氨基，且孔隙结构发达[19-21]，其作为一种潜在的新型重金属生物吸附材料已成为科技工作者的研究热点[22]。与目前水体污染物吸附常用的活性炭吸附剂[14]相比，生物炭未经活化处理，成本低，且来源更为广泛。生物炭的吸附应用是一个多赢策略，在治理水体中污染物的同时消除了焚烧、腐烂处理对环境造成的破坏[23]，并且生物炭生产过程中产生的生物油(bio-oil)和混合气(syngas)有助于缓解能源短缺[24]。

国内外研究表明生物炭可用于吸附水溶液中的Pb(Ⅱ)、Cu(Ⅱ)、Zn(Ⅱ)、Ni(Ⅱ)、U(Ⅵ)、Ag(Ⅱ)、Cr(Ⅴ)和Cd(Ⅱ)等金属离子[25-29]，但由于生物质原料特性迥异[30-32]，炭化工艺与转化过程复杂[33-35]，从而导致生物炭特性多变，可控性较弱。同时，生物炭的特性[36-37]和不同的吸附条件[38-39]以及目标重金属种类[40-41]均显著影响其吸附行为和效果。由此可见，生物炭吸附水中重金属的影响因素众多，并存在复杂的吸附机理。

1 生物炭原料种类、制备工艺及其特性

生物炭的来源非常广泛，主要包括农林业废弃物如秸秆、草本、果壳、果皮果渣等，工业和城市生活中产生的有机废弃物如生活垃圾、污泥等，养殖废弃物如畜禽粪便等。这些生物质都已被热转换为各种理化特性的生物炭并应用于水体、土壤等环境修复和改良。不同种类生物质制备的生物炭在元素组成、工业组成、矿质元素、比表面积、孔容、孔径、灰分含量、羧酸酯化、芳香化、脂肪族链状结构等理化性质上存在差异。

2 生物炭重金属吸附效果分析

生物炭原料和炭化温度显著影响其重金属吸附的能力，同一生物炭对不同重金属的吸附效果不同。研究显示，目前用于水体中重金属吸附的生物炭原料来源可分为秸秆类、畜禽粪便类、果壳类、果皮果渣类、木本类、草本类、泥类、水生植物类、骨类。用于吸附水体中重金属离子涉及Pb(Ⅱ)、Cu(Ⅱ)、Zn(Ⅱ)、Cr(Ⅲ)、Cr(Ⅵ)、Cd(Ⅱ)、As(Ⅴ)、Ni(Ⅱ)、Hg(Ⅱ)等[52-56]，其中对Pb(Ⅱ)、Cu(Ⅱ)、Zn(Ⅱ)、Cd(Ⅱ)的吸附量较大[57-59]。对Pb(Ⅱ)有较好吸附效果的是畜禽粪便类生物炭、秸秆类生物炭和骨类生物炭，例如400℃碳化的猪粪生物炭对Pb(Ⅱ)最大吸附量为230.7 mg/g[60](表1); 对Cu(Ⅱ)具有良好吸附效果的是畜禽粪便类生物炭、秸秆类生物炭和草本类生物炭，例如400℃炭化猪粪生物炭对Cu(Ⅱ)最大吸附量为88.23 mg/g[60]；对Zn(Ⅱ)具有良好吸附效果的是畜禽粪便类生物炭、果壳类生物炭、秸秆类生物炭和水生植物类生物炭，例如猪粪在400℃制备条件下对Zn(Ⅱ)的最大吸附量为79.62 mg/g[60]; 对Cd(Ⅱ)具有良好吸附效果的是畜禽粪便类生物炭、秸秆类生物炭、水生植物类生物炭和骨类生物炭，例如猪粪在400℃制备条件下对Cd(Ⅱ)的最大吸附量为117.01 mg/g[60]。综上所述，畜禽粪便类和秸秆类生物炭是极具潜力的重金属生物吸附剂。但值得注意的是最大吸附量不仅与生物炭原料、制备条件相关，同时也受到吸附条件(初始pH值、温度、吸附剂浓度、吸附质浓度)等因素的影响。表1中的吸附量均是在特定制备和吸附条件下所得，可比性较差，后续应将生物质特性与其吸附效果进行相关性分析。

3 生物炭重金属吸附机制

由于生物炭结构组成复杂，其对水体中重金属的吸附机理也十分复杂。如图1所示，生物炭重金属吸附机制主要表现为：物理吸附、静电作用、离子交换、络合反应和化学沉淀[52-53,85]。生物炭的理化性质差异，吸附条件和重金属离子类型的变化都会造成吸附机理的不同，具体表现为每种机理对吸附过程的贡献不同。

表1 生物炭原料、炭化温度及重金属最大吸附量Tab.1 Maximum adsorption of heavy metals in water bybiochar with different raw materials and pyrolysis temperature

图1 生物炭对水体中重金属的吸附机理示意图Fig.1 Conceptual illustration of heavy metal sorption mechanisms on biochar

3.1 物理吸附

物理吸附是由吸附质与吸附剂之间范德华力产生的，结合力较弱，属于可逆过程。生物炭粒子有着各种不同的比表面积和微孔，当制备温度在500～700℃时，更容易形成较高的比表面积和孔隙体积[86]。这些结构可以与重金属离子发生物理吸附[36,53,87-88]。

SHEN等[89]研究表明英国当地的硬木生物炭的粒径显著影响Pb(Ⅱ)的吸附，0.15 mm和2 mm生物炭对Pb(Ⅱ)的最大吸附量分别为47.66 mg/g和30.04 mg/g。生物炭在一定范围内，粒径减小，其比表面积会增加，进而提高对Pb(Ⅱ)的吸附能力。但是比表面积大小并不是决定吸附强弱的重要理化参数[10,61,90]。例如，畜禽粪便生物炭的比表面积随着炭化温度的升高而升高，但对重金属Pb(Ⅱ)的吸附却未随比表面积的升高而增加。水稻秸秆生物炭比表面积为73.40 m2/g，小于木屑生物炭的比表面积391.12 m2/g，但对Pb(Ⅱ)吸附量是水稻秸秆生物炭(126.58 mg/g)大于木屑生物炭(47.62 mg/g)，对Cd(Ⅱ)的吸附量也是水稻秸秆生物炭(60.61 mg/g)大于木屑生物炭(6.67 mg/g)。橡木生物炭的比表面积为1～3 m2/g，其对Cr(Ⅵ)的移除效果与比表面积约为1 000 m2/g的活性炭相当[10]。因此，生物炭表面的物理吸附占重金属吸附机理的比重较弱；同样的结果也被SHEN等[89]和ZHOU等[37]研究得到。

生物炭内部孔道也会发生物理吸附。LIU等[68]通过2种制备方式：300℃水热炭化和700℃热解分别制得2种松木生物炭，标记为H300和P700。测定了比表面积、平均孔直径和微孔体积差异。 H300和P700的平均孔径分别为0.86、1.48 nm。戴静等[61]分别测定了米糠、木屑、稻秆、玉米秸秆生物炭的平均孔径分别为2.15、2.03、3.56、2.33 nm。而金属离子Pb(Ⅱ)、Cu(Ⅱ)、Al(Ⅲ)、Cr(Ⅲ)、Cr(Ⅵ)、Ni(Ⅱ)、Cd(Ⅱ)、Zn(Ⅱ)的直径分别为0.238、0.146、0.107、0.123、0.088、0.138、0.19、0.148 nm。这些重金属离子直径均小于生物炭的平均孔径。因此，在生物炭与重金属离子接触时，会有部分重金属离子进入到生物炭的孔隙中，不断填充孔隙，形成生物炭对重金属离子的物理吸附。一般来说，重金属的直径越小，渗透到生物炭孔径中越多，从而提高吸附量[91-92]。

3.2 络合反应

除了物理吸附之外，其他化学吸附机制如表面络合反应也参与重金属离子的吸附[5,21,93-94]。与重金属离子相结合的主要官能团有羧基、磷酰基、羟基、硫酸酯基、氨基和酰胺基，其中氮、氧、磷、硫可作为配位原子与重金属离子配位络合。络合反应在秸秆类生物炭吸附重金属机制中有重要的作用。PAN等[95]发现4种秸秆生物炭(花生秸秆、大豆秸秆、油菜秸秆、水稻秸秆)对Cr(Ⅲ)的吸附量与其含氧官能团含量成正比，并且利用红外光谱分析吸附重金属前后的生物炭表面官能团，发现吸附前后出现了不同程度的官能团位点的迁移，说明这些含氧官能团均参与了生物炭对重金属离子的吸附过程。

值得注意的是，表面络合主要是含氧官能团尤其是羧基和酚羟基[37,82,96]，会与重金属形成络合物从而参与吸附反应[85,97]。在LU等[85]的研究中，通过FTIR对吸附前后的污泥生物炭进行光谱分析，发现羧基和羟基官能团含量发生变化，而羰基的量几乎没有变化。SHEN等[89]在研究中指出，硬木生物炭含有大量的羧基，会形成重金属络合物。以上研究结果说明羧基和酚羟基在与重金属的络合反应中起到了重要作用。

3.3 离子交换

离子交换反应需要在适宜的pH值条件下才能发生。酸性生物炭表面电离的质子与重金属离子发生离子交换反应[98-99]。可以与重金属离子发生交换的电离质子包括Ca2+、K+、Mg2+、Si+、Na+[61]，见图1。这些无机矿物成分参与了生物炭吸附重金属的反应。LU等[85]对Pb2+吸附完成后的溶液进行检测，发现有大量的Ca2+、K+、Mg2+、Na+离子被释放出来，并且释放的离子总量越高时，Pb(Ⅱ)的吸附量越大。这说明在生物炭与重金属接触时，溶液中的重金属离子会与生物炭表面电离的质子发生交换，造成电离的质子被释放到溶液中。但同时，若电离质子Ca2+、K+、Mg2+、Si+、Na+在溶液中存在量过大，则会对重金属的吸附造成抑制[65]，并且随离子浓度的增大，对吸附反应的抑制作用也越强。这是由于溶液中的这些离子会与重金属目标物竞争生物炭表面的离子交换位点[100]，直至达到吸附平衡。

生物炭的这些离子多存在于其表面的含氧官能团中，含氧官能团增多会增加生物炭与重金属离子发生离子交换的可能性。LIU等[68]采用300℃水热炭化和700℃热解两种方式制备松木生物炭，将其进行表征并应用于水体中Cu(Ⅱ)的吸附。结果显示，与松木原料相比，水热炭相比于原料提高了95%的含氧官能团，而热解炭降低了56%的含氧官能团。尽管水热炭的比表面积较低，但对Cu(Ⅱ)的吸附能力远高于热解炭，这是因为水热炭吸附机理中离子交换反应占据主导地位。

3.4 化学沉淀

值得注意的是，不可溶的P含量与吸附重金属无关，这是因为形成的P-Ca-Mg结构稳定的晶体物质(如磷钙矿(Ca,Mg)3(PO4)2)不会与重金属离子发生沉淀反应[90]。畜禽粪便生物炭的可溶性P含量在25～200℃时随着热解温度的升高而升高，原因可以归结为可溶性P质量几乎不变但生物炭总量降低；在200～500℃时随着热解温度的升高，可溶性P含量下降，是因为不可溶P-Ca-Mg晶体物质逐渐形成；在500℃可以观察到磷钙矿的存在。在200℃热解温度下得到的牛粪生物炭与Pb(Ⅱ)反应后会沉积β-Pb9(PO4)6晶体；在350℃热解温度下得到的牛粪生物炭与Pb(Ⅱ)反应后除了沉积β-Pb9(PO4)6晶体外还有Pb3(CO3)2(OH)2的形成，这是因为随着热解温度的升高，生物炭碱性和方解石(CaCO3)含量逐渐增强[90]。因此，化学沉淀作用是有关组分共同作用的结果。

3.5 静电吸附

静电吸附作用多发生在适宜的pH值条件下。当溶液pH值大于生物炭零电荷点(pHpzc)时，生物炭表面携带的负电荷能与带正电荷的重金属离子发生静电吸附作用[95,98,104-105]。当溶液pH值小于生物炭pHpzc时，生物炭表面会电离出质子，与带负电荷的重金属离子发生静电吸附作用。因此，静电吸附作用与重金属离子电动势，在水溶液中的存在形式以及生物炭表面的电荷量密切相关，而生物炭表面负电荷量又与生物炭表面含氧官能团，包括羧基(—COO-)、羰基(—CO-)和羟基(—OH)[106]、溶液pH值紧密相关[53]。

SHEN等[89]将制备的硬木生物炭用于水体中4种重金属的吸附，通过Langmuir吸附模型计算不同重金属离子吸附能力，由大到小分别为Pb(Ⅱ)、Cu(Ⅱ)、Ni(Ⅱ)、Zn(Ⅱ)，其中对Pb(Ⅱ)的吸附远高于其他3种重金属离子的原因是，Pb(Ⅱ)的电负性常数远高于其他3种(Pb(Ⅱ)、Cu(Ⅱ)、Ni(Ⅱ)和Zn(Ⅱ)的电负性常数分别为2.33、1.90、1.93、0.69)[93]，因此硬木生物炭对Pb(Ⅱ)有很强的静电吸附作用。果树枝生物炭[21]对Pb(Ⅱ)和Cr(Ⅲ)的吸附远大于Cu(Ⅱ)，这是因为Pb(Ⅱ)和Cr(Ⅲ)有着很高的电荷密度，远高于Cu(Ⅱ)，因此，Pb(Ⅱ)和Cr(Ⅲ)与生物炭之间的库伦引力更大[93]。

静电吸附在秸秆生物炭吸附重金属机理中占的比重小于化学沉淀。TONG等[63]通过慢速热解制备得到花生秸秆、大豆秸秆和油菜秸秆生物炭，虽然油菜秸秆生物炭表面负电荷最多，但是P含量小于花生和大豆生物炭，造成了油菜秸秆生物炭对Cu(Ⅱ)的吸附小于花生和大豆生物炭。

4 生物炭重金属吸附工艺条件

对于特定生物炭而言，影响生物炭重金属吸附效果的重要因素包括吸附溶液pH值、溶液温度、吸附剂用量、吸附平衡时间和吸附质初始浓度等。除生物炭原料、制备条件以外，吸附工艺条件对吸附效果的影响也不容忽视。

4.1 吸附溶液pH值

4.2 溶液温度

温度主要是通过影响生物炭吸附热力学过程和吸附热容等因素来进一步影响吸附效果。一般来说，吸附过程是吸热反应，可通过控制体系温度来提高吸附量[66,107,112-114]。MOHAN等[10]将橡木和橡树生物炭应用于水体中重金属离子吸附，研究结果表明，溶液的温度越高生物炭对重金属吸附能力越强。张双圣等[115]研究污泥生物炭吸附剂对Pb(Ⅱ)的吸附效果随溶液温度的升高而增强，当溶液温度超过30℃，吸附效果增强不明显。LIU等[66]发现重金属离子被吸附到生物炭表面需要足够的能量完成这一迁移过程，即生物炭吸附重金属的过程是吸热反应，因而溶液温度升高有利于吸附反应的发生。KOODYSKA等[60]研究结果也证明了这一点。

4.3 吸附时间

当生物炭吸附重金属时，只有保证吸附时间的充裕，才能使吸附达到平衡，充分利用生物炭吸附性能。通常生物炭重金属的吸附过程可以分为两个阶段[116-118]：第一阶段，在生物炭与重金属接触的前几分钟，吸附率和吸附量都快速增加，这是因为吸附位点几乎完全暴露；随着吸附位点被占据，吸附率变慢随即进入吸附的第二阶段，一般持续时间较长，随后达到吸附平衡，多为1～4 h[85,60,118]；当吸附时间超过24 h后，吸附量基本保持稳定。因此，在实际研究中，同时考虑时间效率和吸附效果，通常选用平衡时间作为吸附时间。

同一种材料吸附不同重金属时，吸附平衡时间也不尽相同。KOODYSKA等[60]在研究猪粪和牛粪对4种重金属(Pb、Cd、Zn、Cu)的吸附行为时，发现对Cu和Zn的吸附平衡时间是30～60 min，对Cd和Pb的吸附平衡时间为120～180 min。平衡时间除了与生物炭和目标重金属性质相关，还与它们的初始浓度密切相关[119]。

4.4 吸附剂用量与重金属的初始浓度

在吸附实验中，经常通过加大生物炭的量来増加与重金属离子接触的吸附点位，进而提高对重金属的去除效果[120]。然而,吸附剂增加过量后,由于吸附剂本身官能团对重金属的竞争吸附能力饱和或者吸附位点达到饱和，不仅造成浪费，还影响单位质量吸附剂的吸附能力。

PELLERA等[118]研究了稻壳、橄榄渣、橘子皮、粪便生物炭对Cu(Ⅱ)的吸附，4种生物炭的用量在2.5～25 g/L时，随着用量的增加，吸附量增加。生物炭用量从2.5 g/L增加到5 g/L时的吸附率远大于5 g/L增加到25 g/L，研究最终选择5 g/L的吸附剂用量。CHEN等[112]在研究玉米秸秆生物炭吸附重金属Cu(Ⅱ)时也发现相同规律，优化的生物炭用量也是5 g/L。

重金属初始浓度和吸附剂浓度对吸附量的影响有着相似的规律[118,120-121]。AMAN等[121]研究了马铃薯皮生物炭对Cu(Ⅱ)的吸附，当Cu(Ⅱ)的初始质量浓度在150～400 mg/L增加时，吸附速率逐渐降低。相同的规律也被PELLERA等[118]和EL-ASHTOUKHY等[120]得到。因此，实验中往往需要找到同时满足吸附率和吸附量的吸附剂和重金属浓度。

5 不同目标重金属吸附能力比较分析

特定种类生物炭在吸附不同种重金属时，对每种重金属的吸附能力也有差异[60]。INYANG等[5]采用消解甘蔗渣(DWSBC)通过热解方式制备生物炭应用于水体中Pb(Ⅱ)、Cu(Ⅱ)、Ni(Ⅱ)和Cd(Ⅱ)的吸附，DWSBC对4种重金属吸附能力由小到大依次为Pb、Cu、Cd、Ni。UCHIMIYA等[98]分别在350℃和700℃条件下慢速热解制备了鸡粪生物炭，结果显示，对Pb(Ⅱ)、Cu(Ⅱ)、Ni(Ⅱ)和Cd(Ⅱ) 4种重金属吸附能力由小到大依次为Pb、Cu、Cd、Ni。RAFATULLAH等[122]研究发现，柳桉木生物炭对不同重金属的吸附能力由小到大为Pb、Ni、Cu、Cr。PARK等[40]制备的芝麻秸秆生物炭对5种重金属的最大吸附量由小到大依次为Pb (102 mg/g)、Cd(86 mg/g)、Cr (65 mg/g)、Cu (55 mg/g)、Zn (34 mg/g)。MOHAN等[119]制备了橡木树皮生物炭，其单位表面积上对水中Pb(Ⅱ)、Cd(Ⅱ)的吸附量分别为0.515 7、0.213 mg/m2。根据目前已有研究表明，相较于其他重金属，生物炭对Pb(Ⅱ)表现出较高的吸附能力。

6 总结与展望

生物炭作为一种新型的吸附材料，具有绿色环保、价格低廉、来源广泛、吸附性能强等特点，使得其在重金属吸附方面表现出良好的潜力和前景。目前，虽然针对生物炭重金属的吸附研究取得了不少进展，但未来的研究工作还应侧重以下几方面。

(1)在生物炭特性及其吸附产物方面，针对最优吸附能力的物料选择和工艺参数缺乏研究[123]，因此，应开展针对重金属吸附的生物炭原料特性及吸附产物的多维、微纳尺度表征方法研究，以期为生物炭重金属吸附剂的定向生产奠定基础。

(2)在生物炭的吸附特性方面，由于生物炭的制备原料和制备条件不同，吸附也多在不同初始pH值、温度、吸附剂浓度、吸附质浓度等工艺条件下展开，所揭示的吸附效果均是在特定制备条件和吸附条件下，可比性较弱[124]。应将生物炭特性结合其吸附效果进行相关性分析。

(3)在吸附机理研究方面，已有研究工作尚处于定性描述阶段，对生物炭吸附重金属过程中，各个吸附机制对吸附过程的贡献以及生物炭表面官能团对吸附贡献的比重还不系统、详细。研究仅停留在实验室的模拟与分析上。应采用多维、微纳尺度表征手段分析吸附机制和吸附过程。

(4)已有研究多局限于生物炭对单一重金属溶液体系的吸附，而实际污染水体多为复合重金属污染[123]。当多种重金属同时被吸附时，是否产生相互作用，以及在重金属复合污染物体系中，吸附复杂机理研究应是未来的一个研究重点。

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ReviewofBiocharasAdsorbentforAqueousHeavyMetalRemoval

HAN Lujia LI Yanfei LIU Xian HAN Yahong

(CollegeofEngineering,ChinaAgriculturalUniversity,Beijing100083,China)

Biochar has a bright prospect due to its good surface properties, pore structure and broad raw materials of its production. It has already been a hotspot in the fields of environment, agriculture and energy. It can be produced from a variety of biomass feedstock, such as agricultural residues, manures, wood, bone and so on, under oxygen-limited conditions. It has great potentials for heavy metal remediation and waste water treatment due to its unique properties, low price, easy processing and wide range of preparation material. The existing literature was incorporated to understand the overall sorption research of heavy metals on biochar adsorbents. However, there was still lack of information on the roles of different sorption mechanisms for biochar. The effects of feedstock materials and pyrolysis temperature on biochar characteristics and metal adsorption capacity were discussed. The interaction mechanisms between biochar and heavy metals, such as ion exchange, complexation, physical sorption, precipitation and electrostatic interactions were analyzed in detail. The influence of adsorption conditions and heavy metal species on heavy metals adsorbed by biochar was also included. At the end, the future research directions on sorption of heavy metals by biochar were proposed. The review would help to build important theory and methodology foundation for directly controlling production and scientific utilization of biochar.

biochar; heavy metal; adsorption mechanism; characterization

10.6041/j.issn.1000-1298.2017.11.001

S216

A

1000-1298(2017)11-0001-11

2017-09-21

2017-10-09

国家自然科学基金项目(31271611)

韩鲁佳(1964—)，女，教授，博士生导师，主要从事生物质与生物处理工程研究，E-mail: hanlj@cau.edu.cn