生物炭修复重金属污染农田土壤的机制及应用研究进展

2020-08-12 14:10李鸿博张昊楠王琳玲肖劲光武2
农业工程学报 2020年13期
关键词:官能团改性重金属

李鸿博,钟 怡,张昊楠,王 鑫,陈 静,王琳玲,肖劲光,肖 武2,,王 薇

·农业生物环境与能源工程·

生物炭修复重金属污染农田土壤的机制及应用研究进展

李鸿博1,钟 怡1,张昊楠1,王 鑫1,陈 静1,王琳玲1※,肖劲光3,肖 武2,3,王 薇3

(1. 华中科技大学环境科学与工程学院,武汉 430074;2. 中国电建集团中南勘测设计研究院有限公司,长沙 410014;3. 中电建环保科技有限公司,长沙 410014)

将生物质转化为生物炭并用于重金属污染农田土壤修复中,是有效利用生物质资源、保障粮食安全的有效途径之一。然而,生物炭的应用效率受其特性和土壤环境影响极大。该研究综述了生物炭特性,并探讨了生物质和热解温度对其影响规律,阐明了生物炭对重金属的直接固定作用,以及通过影响土壤pH值、阳离子交换量(Cation Exchange Capacity,CEC)、矿物组分和有机质等,进而间接固定重金属的作用机制。同时,该文系统总结了国内外生物炭在田间试验中的应用,从土壤重金属迁移性和生物有效性、作物累积重金属和作物产量等3个方面阐明了生物炭的应用效果和作用规律。针对田间试验条件区别于室内试验的特殊性,探讨了生物炭施撒方式及用量、施肥等田间管理和气候环境等现场条件对生物炭应用的影响,并对今后完善生物炭在土壤修复中作用机制、扩大研究尺度和长期土壤监测等方面研究进行了展望。

生物炭;农田;重金属;土壤修复;田间应用;进展

0 引 言

土壤是人类赖以生存和发展的重要基础,然而随着工农业迅速发展,土壤重金属污染日趋严重。《全国土壤污染状况调查公报》显示,中国农田土壤中重金属等污染物点位超标率达19.4%[1]。经调查,中国湖南、广东和广西等地,稻米超标率高达60%~88%[2]。因此,开展重金属污染农田土壤修复,保障农产品安全,已十分迫切[3]。生物炭由于具有较强重金属吸附性和对土壤的改良作用,故被视为优良的土壤修复材料[4],从而逐渐被学者关注。

生物炭是农林废弃物、畜禽粪便和污泥等生物质材料在无氧或限氧的气氛条件下,经低-中温热裂解得到的一种富碳产品[4]。一方面,生物炭具有孔隙度和比表面积大、化学官能团丰富、离子交换能力强等特性[5],对重金属具有较强的固定作用,能降低土壤中重金属的有效性,消减其对生态环境的危害[6]。另一方面,生物炭含有稳定态有机碳、可溶性有机物及矿物质灰分等多种组分[7],可提高土壤有机质含量、增加土壤持水量、改善土壤结构和微生物群落生境[8],能提供作物生长所需养分、减少化肥投入、提高作物产量等[5,9]。中国具有丰富的废弃生物质资源,每年仅作物秸秆就有8.27亿t[10],污泥产量每年超过625万t[11]。生物炭的利用可实现大量农林、城市固废的高效资源化,同时避免因秸秆、污泥焚烧处置产生的环境污染,对于维持农田生态系统平衡与稳定具有重要意义和广阔的应用前景[4]。

生物炭在降低重金属生物利用度和改善土壤质量方面虽然有效,但受其自身特性和土壤性质等条件影响较大[12-13]。同时,多数研究是在实验室中进行,较少在田间检验。与实验室条件下简化、均匀和精心管理相比,温度、降水和农艺管理等复杂的现场条件都将影响田间试验结果。因此,本文在总结近年生物炭性质及固定重金属机制的基础上,分析归纳了近年田间试验中生物炭在土壤中重金属有效性、作物重金属累积和作物产量等方面作用规律和田间条件影响效应,以期为重金属污染农田土壤修复和未来研究问题提供参考。

1 生物炭相关论文统计

本文通过Web of Science数据库对生物炭相关论文进行统计,利用主题词按照“biochar”,“biochar or HM”,“biochar or soil”和“biochar or HM or soil”等检索式进行检索,其中HM为“Cu or Zn or Pb or Ni or Cd or Cr or As or Hg or copper or zinc or lead or nickel or cadmium or chromium or arsenic or mercury or heavy metal”。统计了2008年1月1日—2019年12月31日的所有期刊论文文献,如图1所示,自2010年以来,生物炭相关的论文数量保持较高的年增长率,其中,生物炭与重金属关联的文献数占比达63%以上。“生物炭+土壤”主题的文献数亦协同增长,其中与重金属关联的文献数近3 a占比更是高达75%。由此表明,生物炭在重金属污染土壤的修复应用已逐渐成为当前环境科学领域的研究热点。

图1 2008-2019年生物炭相关论文发表量

2 生物炭的性质及改性

2.1 生物炭的性质及影响因素

生物炭对重金属的固定作用,与其表面特性密切相关。生物炭比表面积大且疏松多孔,其比表面积一般在几十到几百m2/g之间[5],而孔径可分为微孔(<2 nm)、介孔(2~50 nm)和大孔(>50 nm)。大孔可改善土壤孔隙结构,促进微生物附着生长,而微孔或介孔则与污染物、矿物元素等物质的吸附迁移有关[14]。生物炭表面丰富的官能团可有效络合重金属,主要包括羧基、羰基、(酚)羟基等含氧官能团,氨基、亚氨基等含氮官能团,以及巯基、硫代羰基等含硫官能团。其中,含氧官能团是生物炭吸附重金属最重要的官能团。

生物炭多呈碱性,pH值多为6~12,灰分和表面官能团是决定其pH值的主要因素。灰分是热解中有机成分逸失后残留的无机组分,通常以矿物质元素(K、Ca、Mg等)的氧化物、碳酸盐、硅酸盐或磷酸盐形式存在,可促进生物炭pH值升高。含氧官能团形成了生物炭良好的吸附特性、亲水或疏水的特点以及对酸碱的缓冲能力。此外,含氧官基团使生物炭表面带有负电荷,从而有较强电负性,使其与阳离子重金属具有较强的交换能力和静电吸附能力[15]。

生物炭制备过程中,不同生物质来源和热解温度等,都将造成生物炭比表面积、pH值、灰分、阳离子交换量(Cation Exchange Capacity, CEC)等特性的差异,从而影响生物炭固定重金属。此外,H/C可反映生物炭芳香化程度,O/C反映含氧官能团数目,O/C和N/C可反映生物炭极性,间接体现生物炭性质,因此本文择取近5年相关文献的生物炭特性数据,总结绘制了上述6种性质与不同生物质来源和热解温度的相关图,如图2所示。

注:基于文献[5,7,9,14,16-24]中数据绘制本图。图中的H/C、O/C和N/C均为元素原子比。

热解温度的影响:如图2a所示,随着热解温度升高,挥发性物质释放增多,产生更多孔隙[25],比表面积得以增大。但当热解温度过高时,孔隙内部结构被破坏,孔径增大,导致比表面积不增反降[26]。如图2b和2c所示,生物炭灰分含量和pH值随热解温度升高而增加,主要由于生物炭中有机酸等在更高热解温度下转化成更多灰分,碳酸盐总量提高,使得生物炭pH值增大[27]。然而,生物炭表面官能团含量和CEC与热解温度呈负相关[28]。随着热解温度升高,生物炭H/C降低(图2d),表明其芳香化增强,原因是高温会破坏一些官能团内部的分子键[29],使得烷烃基团、羟基、羧基和氨基等基团减少[30]。同时,图2e显示热解温度的升高引起O/C降低,表明生物炭的含氧官能团被逐渐去除,从而导致CEC的降低。

生物质原料的影响:生物质原料来源可大体分为秸秆、木屑等植物源生物质,畜禽粪便等动物源和污泥生物质。如图2b所示,植物源生物炭因含碳量高,故其灰分含量较低,多在18.1%~36.7%(质量分数,下同),小于动物源生物炭(42.7%~54.5%)和污泥生物炭(63.6%~86.4%)。更高的灰分产生更高的阳离子交换量(CEC),这可能是由于生物质中碱和碱金属促进了表面含氧官能团的形成[31]。如图2e,污泥源生物炭的O/C要高于植物源生物炭,而生物炭O/C与CEC呈正相关[25],故前者CEC一般高于后者。如Pariyar等[32]分别用稻壳、松木屑、餐厨垃圾和污泥为原料在550 ℃下热解,所得生物炭的CEC分别为29.59、47.43、17.35和48.43 cmol/kg。

不同来源生物炭的比表面积大小往往和灰分含量情况相反,如图2a,植物源生物炭(6.77~70 m2/g)普遍大于动物源生物炭(0.47~13.93 m2/g),这可能是由于后者含碳量较低和高H/C、O/C所致[33]。污泥生物炭(14.37~192.97 m2/g)受其原料组分影响,差异较大。对于生物炭pH值而言,如图2c所示,植物源生物炭普遍高于动物源和污泥,原因可能与纤维素、木质素含量和含氧官能团有关[25]。

2.2 生物炭改性方法

2.2.1 化学活化改性

化学活化改性是使用酸、碱和氧化剂等活化剂改性生物炭,可去除生物炭表面杂质,改变其表面化学结构,从而具有更多官能团、微孔结构和更大的比表面积,以增强其对重金属的吸附能力。盐酸、硫酸、硝酸和磷酸被广泛用于酸改性。使用硝酸(25%)改性生物炭可引入羧基和羰基,增加电负性[34]。氢氧化钾和氢氧化钠可有效增多含氧官能团[35],是常用的碱改性剂。将H2O2或KMnO4等氧化剂改性生物炭,可显著增加其比表面积和含氧官能团。KMnO4改性生物炭的比表面积从101增至205 m2/g,对Pb、Cu和Cd的吸附能力显著增强。

化学活化剂的浓度对其改性生物炭性能影响显著,如磷酸浓度(10%~50%)的升高使得生物炭的比表面积和pH值下降,但在30%下的酸性官能团更为丰富,对Pb的吸附量也最高[36]。Sheng等[37]研究发现,当KOH与生物炭质量比为3:1时,制得的生物炭性能最优。Zuo等[38]使用不同浓度(10%~30%,体积分数)H2O2处理的生物炭吸附Cu,其中,20%的H2O2浸渍的生物炭对Cu的吸附量最大。因此,活化剂量的优化在进行改性研究时需要特别考虑。

2.2.2 有机改性

羧基、氨基、巯基等对重金属有较强吸附能力的官能团,可以通过表1中所述海藻酸钠、聚乙烯亚胺、巯基乙醇等醇类或聚合物等向生物炭表面引入,可显著增强表面络合沉淀作用,提高改性生物炭对重金属的选择吸附能力和结合力[39]。草酸、柠檬酸和氨基磺酸[40]等有机酸可以通过酯化作用等将官能团转移至生物炭表面,使生物炭O/C增大,含氧官能团增多,重金属的吸附量增加[15]。但有机酸由于没有无机酸的强腐蚀性,故难以增大生物炭比表面积,甚至略有降低[41]。

尽管有机改性剂可定向引入有机官能团,但成本过高而限制了其使用,促进其循环利用或是降低成本的一种方法。此外,制备原始生物炭的热解温度也影响有机改性性能。使用丙烯腈改性不同热解温度(350、450、550 ℃)制得生物炭,发现仅350 ℃时生物炭被成功引入了氰基,提高了对Cd的吸附量(28.2增至85.7 mg/g),其余温度下,吸附量无显著提高甚至降低[42]。

2.2.3 金属盐或氧化物改性

金属盐或氧化物改性生物炭可改善孔结构,增加生物炭CEC、含氧官能团和吸附位点,增强与重金属的结合能力。如表1所示,此改性方法亦能提升对As、Cr等阴离子重金属的吸附能力。常使用的金属盐或氧化物主要有FeCl2、FeCl3、Fe(NO3)3、AlCl3、MgCl2、MnCl2、CaCl2、CaO和Fe2O3等。金属盐或氧化物改性可通过2种方式改性:1)金属盐或氧化物与生物质混合,然后共热解;2)生物质热解生成生物炭后,再与金属盐或氧化物通过浸渍或再热解等手段改性[43]。第1种方法简单、成本低,适于大规模生产,但金属粒子的大小、形状和组成类型不易控制。第2种方法可较易控制金属纳米颗粒,甚至可以在生物炭上合成多层金属纳米颗粒,但该方法相对复杂和昂贵。

除铁锰外的其他过渡金属元素改性亦有研究。ZnO改性生物炭的比表面积从2.39增至18.53 m2/g,表面官能团增多,对Cu的最大吸附量可达216.37 mg/g,同时可降低土壤中有效态Cu含量[44]。Zhu等[45]使用硝酸铋改性生物炭,改善了微孔结构,有效催化还原土壤中氧化铁,从而加快固定土壤中砷,降低其可生物利用率。使用金属盐改性生物炭的方法,操作复杂且成本较高。选用天然矿物等低成本原料参与改性,开始被大家关注。如使用蒙脱土改性生物炭,引入了硅醇基和镁、铝氧化物等,对Zn的吸附量增长了3.3倍[46]。Zou等[47]使用红泥改性生物炭,增加了比表面积,施用于污染土壤后,增强了土壤微生物活性,促使生物炭表面生成含铁次生矿物,从而使As得到有效固定。

表1 改性生物炭的制备及其对重金属污染土壤的修复效果

3 生物炭对重金属的固定机制

生物炭可利用其高比表面积、CEC、有机碳含量和活性官能团等特性,通过物理吸附、静电吸附、离子交换、络合、沉淀和氧化还原等作用直接吸附重金属离子[54,57],同时,生物炭特殊组分可通过影响土壤性质(如pH、CEC、有机碳、矿物组成等)间接影响土壤中重金属有效态或作物对重金属的累积,基于上述作用,本文绘制了相关机制示意图,如图3。

图3 生物炭在土壤中固定重金属的可能机制示意图

3.1 生物炭对重金属的直接固定作用

3.1.1 物理吸附

生物炭能将重金属吸附在其表面或扩散进孔道内,通过范德华力等作用固定重金属。通常较大比表面积和孔隙度,利于生物炭对重金属的物理吸附。Deng等[19]研究了水稻秸秆在400和700 ℃下制得生物炭对Cd和Ni的吸附能力,表明比表面积较大的700 ℃下制得生物炭对重金属具有更高吸附能力。

3.1.2 静电吸附

生物炭表面电荷通过静电作用吸附重金属,从而达到固定目的。生物炭与重金属的静电吸附取决于环境pH值和生物炭的零电荷点(pHPZC)。当介质pH值>pHPZC时,生物炭表面呈负电,可与阳离子重金属发生静电吸附;当介质pH值<pHPZC时,生物炭则与阴离子重金属发生静电吸附[56]。同时,pH值的升高可增加生物炭对重金属的吸附。静电吸附也会随着重金属初始浓度的增加而增强[58]。

3.1.3 离子交换

生物炭表面盐基离子可与重金属离子发生置换而固定重金属。同时,羧基等含氧官能团也可以通过离子交换途径吸附金属离子[15]。生物炭表面的强电负性使其具有较高的阳离子交换量,并能释放Ca2+和Mg2+等阳离子,从而可在生物炭表面与重金属离子交换[59]。动物源生物炭比植物源生物炭含有更高Ca2+,因此离子交换在动物源生物炭固定Cd和Cu占主导地位[60]。

3.1.4 络合作用

生物炭表面官能团提供重金属结合位点以形成络合物,从而固定重金属[19]。络合作用在矿物含量低的生物炭中体现尤为显著。例如,由作物残渣制得生物炭主要通过形成表面络合物来吸附重金属[61]。而相较于羟基而言,高硫生物炭表面的巯基对Hg、Cd等的络合能力更强[62]。利用巯基乙醇通过催化酯化法可制备巯基改性水稻秸秆生物炭,其对Cd吸附量提高3倍(45.13 mg/g),Cd可形成稳定的镉硫络合物而不易被解吸下来,而未改性生物炭吸附的Cd中,57%可被解吸下来[53]。

3.1.5 沉淀作用

生物炭灰分中的矿物成分可与重金属发生沉淀反应等,从而实现重金属的固定。稻秆生物炭中的C2O42-和CO32-可与Pb分别形成PbC2O4和Pb3(CO3)2(OH)2沉淀,是固定Pb的主要机制[9],此外Pb也可与生物炭含有的其他矿物形成氯化物、磷酸盐、硫酸盐沉淀等[63]。Xu等[64]研究了牛粪生物炭吸附Cd时沉淀作用的贡献,结果表明沉淀作用占总吸附量的88%,并且可溶性CO23−的贡献率要大于PO34−。Zhang等[65]使用磷酸钾改性生物炭,可有效促进土壤中Cu和Cd从酸溶态向更稳定态转化,重金属迁移率降低2~3倍,其中改性生物炭中的含磷化合物与Cu、Cd形成沉淀物在此起主要作用。

3.1.6 氧化还原作用

生物炭含有醌和酚羟基等多种官能团,使其具备存储和传递电子的能力[66],可以氧化As(III)或还原Cr(VI)从而有效固定重金属。生物炭表面芳香环含有的醌、酚羟基等多种分子结构上的某些原子发生电子离域,引起原子轨道出现未成对孤电子,即形成持久性自由基(Persistent Free Radicals,PFRs)[67]。Mohan等[68]研究发现,生物炭的酚羟基可充当Cr(VI)的电子供体,随后自身被氧化成醌基并与吸附态Cr(VI)发生络合反应。Zhong等[69]研究发现,生物炭PFRs在碱性条件下可直接氧化As(III),而在中性和酸性条件下,生物炭PFRs将O2还原为O2·−,进而转化为·OH和H2O2,实现对As(III)的氧化。

3.2 生物炭对土壤固定重金属的间接作用

3.2.1 生物炭对土壤pH值的影响

随着生物炭施用量的增加,可明显增加土壤pH值,特别是对酸性土壤[70-71],随之重金属的水解得以增加,从而提高土壤对其吸附,并加速土壤重金属形态向可氧化态和残渣态转化[13]。土壤pH值的增大也可能增加重金属的络合,减少重金属从土壤的解吸[72],故能降低重金属的生物有效性。

3.2.2 生物炭对土壤CEC的影响

生物炭施用于土壤可有效增加土壤CEC[73]。随着生物炭大量施用,土壤中溶解性和可迁移性重金属浓度明显降低,其中主要归因于生物炭表面大量的阳离子交换位点[74]。例如,Jiang等[72]的研究中加入生物炭30 d后,土壤CEC增加的同时,生物炭对Pb的固定显著增强。对于低CEC或酸性土壤,生物炭可明显提高其CEC,但在碱性土壤中,这种影响并不明显[75]。

3.2.3 生物炭对土壤矿物组分的影响

随着生物炭的加入,生物炭表面存在的大量矿物质可能在土壤中释放,释放的矿物质可能在生物炭表面形成矿物相并从土壤溶液中吸附金属[76]。如,土壤中磷的浓度随着生物炭投加量的增加而增高,从而与Pb形成稳定矿物相而留在土中[63]。Bian等[77]使用激光刻蚀技术对修复后的生物炭进行分析,发现Cu和Pb显著增加,而K、Mg和P显著减少,表明生物炭可能通过释放矿物到土壤中来增强其对重金属的固定。同样地,生物炭中的Ca、Si和Mn氧化物也可能部分溶解,从而为土壤中重金属离子提供活性吸附位点[76]。

3.2.4 生物炭对土壤有机质含量的影响

生物炭在土壤中可以释放溶解性有机质,进而增加土壤有机质含量。由于重金属和含氧官能团的络合作用,可以减小重金属迁移能力和生物有效性[3]。土壤有机质的增加会将不稳定态Pb转化为较稳定的有机结合态[78],从而减少植物对重金属的吸收。然而土壤有机质的增加对不同重金属可能有不同作用。在加入生物炭后,使溶解有机质和土壤pH值升高,孔隙水中的Cu和As浓度增加了30多倍,但孔隙水中的Zn和Cd却显著减少了[79-80]。

4 生物炭在田间修复中的应用研究

生物炭通过降低土壤中重金属迁移性和生物有效性,削弱对作物毒害,使作物保质增产。但囿于土壤环境的复杂性,使得生物炭在不同田间试验中的应用效果差异较大,生物炭的种类和田间管理对其修复效率也存在一定影响。

4.1 生物炭对土壤中重金属的影响

生物炭具有的高矿物含量(碳酸盐、磷酸盐、二氧化硅等)、pH和阳离子交换能力等,可以快速络合沉淀重金属,此外,具有较大孔隙和特定结合位点的生物炭可有效固定重金属[81]。多数试验表明生物炭的施用会使活性态重金属含量下降。Zhang等[54]在农田施用1.6%生物炭材料后,土壤中有效态Cd含量(质量分数)由0.88降至0.66 mg/kg。然而,重金属的种类、浓度和形态等会导致生物炭对土壤修复产生不同结果。Khan等[81]研究表明,生物炭可显著降低田间Pb和Cd的生物有效态浓度,但对Zn却没有影响,这可能是由于不同重金属的不同活性导致。在复合重金属污染土壤中,多种重金属的竞争吸附可能使某一重金属具有更强活性,如Pb在生物炭表面比Cd更易形成络合物,同时锰氧化物对Pb的吸附能力明显强于Cd[82]。

Zheng等[83]研究表明,生物炭的施用使得Cd、Zn和Pb的有效态含量大幅降低,而As却有所增加。生物炭会促进As(V)还原为As(III),导致As毒性和迁移性在污染田中增加[84]。因此,Cd等阳离子型重金属和As的不同地球化学行为使得他们难以被共同固定。Tang等[13]使用硫酸亚铁改性生物炭在田间施用2 a,有效态Cd和As分别下降了74%和14%,铁基生物炭通过提升土壤pH值,释放铁离子与As形成铁砷沉淀等,从而达到同时固定Cd和As的目的。

4.2 生物炭对作物累积重金属的影响

生物炭可以通过降低重金属在土壤中的迁移性和生物有效性,减少重金属在植物中的积累。在许多田间试验中,生物炭可有效降低农作物中重金属的积累(如表2),但也有例外。如Zheng等[83]的研究表明,施用豆杆和稻秆生物炭后,稻米中Cd含量明显降低,但Zn的含量差异却不显著。这可能是因为水稻植株中有大量的Zn转运体专门负责Zn的吸收和转运,而生物炭添加对元素转运的限制并不显著[85]。植物中重金属的积累也与污染状况有关。如Zhang等[73]发现Cd重度污染土壤经生物炭处理后,生菜组织中的浓度并未降低,可能是土壤有机质浓度过高,施用生物炭量不足以使有机质固定在土壤中。此外,针对中国南方稻田Cd和As共污染普遍的问题,于焕云等[86]将零价铁改性生物炭应用于稻田中,零价铁的溶蚀产物和生物炭分别对As和Cd具较强固定能力,在2.25 t/hm2零价铁改性生物炭处理下,大米中的Cd和As分别减少了48%和24%。

4.3 生物炭对作物产量的影响

生物炭可通过固定污染土壤中重金属,以减少植物中重金属含量,降低毒性以提高稻田的生产力[83]。Zhang等[87]研究表明,添加1.5和3.0 t/hm2污泥生物炭,粮食产量分别提高1.1和1.8倍。同时,生物炭可提高土壤持水能力,增加养分和微量营养素的含量[4]。杜彩艳等[88]研究发现生物炭可以显著提升土壤酸碱度和有机质含量,Cd、Cu和Zn含量分别显著降低37.46%、12.03%、21.63%,玉米产量显著提高18.92%~27.67%。

生物炭可改变土壤微生物和土壤酶活性,改善作物生长的根区环境,促进作物增产。王彩云等[89]研究发现,生物炭可抑制土壤真菌生长,保持土壤细菌群落结构,增加细菌和真菌含量比值,使黄瓜增产11.4%~26.8%。李新宇等[90]研究发现,生物炭可改善土壤酶结构,降低土壤重金属活性,从而增加番茄产量。

除了生物炭本身的影响外,土壤质地对作物产量也具有重要影响。生物炭效应的聚类分析表明,生物炭对作物产量的增长效应多发生在质地较重的土壤中,而不是在质地较轻和中等质地的土壤中[12]。Bian等[77]研究表明,生物炭在施用后的第1年,对作物产量没有影响,第2年和第3年方有显著增产效果。因此,需要对生物炭在重金属污染土壤中的应用进行长期的研究。

4.4 生物炭应用的田间管理

4.4.1 生物炭施撒方式的影响

生物炭在工程应用中一般通过表层撒施、机械点施等方式将生物炭翻耕于15~30 cm土层内。研究显示,增加施用深度可显著提高作物产量,但作物对重金属的积累量也有提高[91]。Düring等[92]发现翻耕后的菠菜幼苗中重金属浓度与未翻耕相比显著升高。这可能是由于深耕使生物炭改善了作物根际环境的pH值和养分等条件,促进了作物的生长,但深耕可能使得重金属下移,以及稀释生物炭而削弱对重金属的固定,从而增加作物对重金属的吸收。

4.4.2 生物炭用量和种类的影响

生物炭在田间试验施用比例通常为1.5~40 t/hm2。一般而言,随着生物炭施用量的提高,土壤有效态重金属和作物中重金属降低量增加,作物增产率提高[3]。Houben等[93]使用不同生物炭用量(1%、5%、10%)修复比利时某镉、锌和铅污染土壤,经56 d培育后,土壤中CaCl2提取态重金属随生物炭用量的增加而降低。然而,Bian等[71]使用较高用量生物炭(40 t/hm2)修复污染土壤后,小麦反而减产1.81%。有研究表明,生物炭用量较高时,与植物生长发育呈负相关[94],可能是因为生物炭较强的吸附能力,使得土壤中有效性养分降低[95]。如施用生物炭后,由于生物炭具有很高的C/N以及不稳定碳分解导致氮的固定,从而显著降低土壤碱解氮[96]。

生物炭来源不同对土壤修复效率的影响亦有不同。Moore等[97]在田间试验中使用鸡粪生物炭和燕麦壳生物炭修复Cu污染土壤,结果表明,5%的鸡粪生物炭减少了90%可交换态Cu含量,而燕麦壳生物炭仅降低了68%。

4.4.3 施用化肥的影响

为了保证农作物产量一般需要施用化肥,故统计了文中提及的田间试验中肥料施用情况,如表3所示,磷肥和钾肥在51~100和>100~150 kg/hm2的使用量较为频繁。而氮肥用量差别较大,但多在150 kg/hm2以上,用量较多。一方面,土壤有机碳含量增加,会使得有效氮降低[98];另一方面,生物炭与氮肥配施效果处理重金属污染土壤可显著提高水稻产量,降低稻米中重金属浓度[99]。

Moreno-Jimenez等[100]研究发现在仅加入生物炭或者矿质肥料时,土壤溶液中的Pb和Cd浓度均明显升高。而在同时使用生物炭和矿质肥料时,Pb和Cd在土壤中有效态和作物中的含量均有所降低。施用肥料后,土壤中Pb和Cd的生物有效性明显提高归因于铵化后土壤瞬间酸化。

施用生物炭后,因其石灰化效应可有效缓冲土壤酸化,从而减轻了对生物有效性的影响。过多地使用肥料,使得土壤酸化,有效态重金属含量升高,但生物炭可有效蓄水保肥。因此,田间试验条件下如何合理施用化肥,以平衡好生物炭和肥料的关系需要特别考虑。

表2 生物炭修复重金属污染农田田间试验总结

表3 各肥料在不同用量在文献中出现频次

注:由文献[3,6,70-71,73,77,83,88,96,99-103,105]中数据整理得到。

Note: This table is sorted out based on the data in literature [3,6,70-71,73,77,83,88,96,99-103,105].

4.4.4 气候环境的影响

温度和降水是影响田间试验重要因素,会影响生物炭改良后农作物的重金属积累。例如,Ge等[109]研究表明气候变暖可以逐渐降低土壤孔隙水的pH值,增强水溶性Cd和Cu的浓度,从而增加粮食中的重金属浓度。在淹水土壤中,生物炭降低植物体内Pb和Cd累积量的作用大于旱地土壤,这是因为水可以改变重金属组分的有效性[110]。Wagner等[108]通过2 a的田间试验研究了施用芒草生物炭后果园草的生长情况,发现在气温很低的冬季(−2~4 ℃),生物炭用量与作物产量呈负相关,而在夏、秋季时(6~24 ℃),作物产量与生物炭用量呈正相关。Sui等[3]对施用了20 t/hm2生物炭的农田开展了长达3 a的监测,结果表明土壤中Cd和Pb的迁移率和生物有效性仅在第1年和第3年有所下降,推测可能是气候导致,例如交替干旱和洪水等,影响了动态氧化还原过程,从而降低了生物炭在碱性土壤中对Cd和Pb迁移性影响[3]。值得注意的是,有很多生物炭田间试验设计未考虑区域土壤类型、水文等田间试验位置的影响因素,同时也缺乏在干旱和半干旱地区进行生物炭田间试验的数据。

5 结论与展望

生物炭作为重金属污染土壤修复材料具有价格低廉、环境友好、效果明显等优势。生物炭在进行土壤污染治理的同时,也有效实现了固废资源化,因此其技术潜力和应用前景巨大。通过文献分析和综合研究,得出如下结论:

1)生物炭具有较高pH值,较大比表面积和丰富表面官能团等特性,因而对重金属具有较强固定能力。通过酸碱、氧化剂、有机溶剂和金属盐或氧化物增强生物炭特性,而改性剂的种类或浓度会影响改性效果。

2)生物炭通过物理吸附、静电吸附、络合沉淀、氧化还原等直接固定重金属,对不同的重金属的主要吸附固定机理不同,可以据此确定生物炭的改性方向;生物炭通过对土壤pH值、阳离子交换量(Cation Exchange Capacity, CEC)、矿物组成和有机质含量影响,间接固定重金属。

3)在多数田间试验中,出于经济性考虑,水稻和小麦秸秆被广泛选用。生物炭的施用降低了污染土壤中重金属的迁移率和生物有效性,降低了植物对重金属的吸收,同时,可通过改善物化条件,提高土壤微生物和酶活性,从而提高作物产量。但不同的田间现场条件和气候环境影响,可能造成加入生物炭前后作物重金属积累量与作物产量无显著差异。

综上,考虑到生物炭特性和土壤介质的复杂性,在进行土壤修复前,应综合考虑生物炭的性质、施用量、土壤污染状况以及生物炭、重金属与土壤之间可能存在的相互作用等多种因素。故生物炭在实际修复应用中仍有一些待进一步解决的问题:

1)生物炭主要通过吸附、共沉淀等作用,达到降低土壤中重金属的生物有效性和移动性,其修复效果的长效性有待进一步研究;同时将土壤与生物炭分离很难,生物炭所吸附的重金属离子是否会因土壤环境的改变而重新释放到土壤环境中产生二次污染也值得进一步探讨。

2)生物炭对重金属污染土壤的修复机理缺乏全面系统的研究。生物炭不仅通过自身特性直接影响重金属和土壤性质,还通过影响土壤中的其他要素来间接影响其性质。生物炭对重金属的吸附固定由多种机理共同控制,目前学界对不同机理的贡献率的认识仍存在较大差异。同时,生物炭对土壤重金属作用机制的认识仍未到微纳尺度水平,尤其是生物炭施用条件下与土壤矿物和有机质等组分的相互作用的认识仍不深入。

3)通过寻找合适的化学或生物改性剂,以期增强生物炭固定重金属的能力。如生物炭结合微生物共同修复污染土壤,生物炭负载微生物,利用微生物固定重金属或促进重金属的氧化、还原作用;利用生物炭特有的层状晶形结构,选择合适的聚合物、单体等进行层间嵌套,制备纳米复合型生物炭材料。

4)生物炭的田间应用研究多集中在亚热带季风气候地区,针对不同气候类型下生物炭的修复效果需作进一步研究。同时当前田间试验规模均较小,需扩大生物炭应用于重金属污染土壤修复的研究尺度,长期定位监测生物修复效果评估,对生物炭的大规模工业应用需要深入评估。

5)预防生物炭在应用过程中的环境风险。生物炭在制备过程中,除了会使得它本身含有的重金属在环境中富集,同时也会产生新的污染物,这些污染物进入土壤环境中对土壤性质和土壤微生物群落等的影响,及其所引起的环境风险需要考虑。

[1] 中华人民共和国生态环境部和中华人民共和国自然资源部. 全国土壤污染状况调查公报[EB/OL]. 2014-04-17 [2020-07-01]. http://www.mee.gov.cn/gkml/sthjbgw/qt/ 201404/ t20140417_270670.htm.

[2] 汪鹏,王静,陈宏坪,等. 我国稻田系统镉污染风险与阻控[J]. 农业环境科学学报,2018,37(7):1409-1417.

Wang Peng, Wang Jing, Chen Hongping, et al. Cadmium risk and mitigation in paddy systems in China[J]. Journal of Agro-Environment Science, 2018, 37(7): 1409-1417. (in Chinese with English abstract )

[3] Sui F, Zuo J, Chen D, et al. Biochar effects on uptake of cadmium and lead by wheat in relation to annual precipitation: a 3-year field study[J]. Environmental Science and Pollution Research, 2018, 25(4): 3368-3377.

[4] 陈温福,张伟明,孟军. 生物炭与农业环境研究回顾与展望[J]. 农业环境科学学报,2014,33(5):821-828.

Chen Wenfu, Zhang Weiming, Meng Jun, et al. Biochar and agro-ecological environment: Review and prospect[J]. Journal of Agro-Environment Science, 2014, 33(5): 821-828. (in Chinese with English abstract)

[5] Kwak J H, Islam M S, Wang S, et al. Biochar properties and lead (II) adsorption capacity depend on feedstock type, pyrolysis temperature, and steam activation[J]. Chemosphere, 2019, 231: 393-404.

[6] Zhang C, Yu Z, Zeng G, et al. Phase transformation of crystalline iron oxides and their adsorption abilities for Pb and Cd[J]. Chemical Engineering Journal, 2016, 284: 247-259.

[7] Qin J, Qian S, Chen Q, et al. Cow manure-derived biochar: Its catalytic properties and influential factors[J]. Journal of Hazardous Materials, 2019, 371: 381-388.

[8] Zhang M, Shan S, Chen Y, et al. Biochar reduces cadmium accumulation in rice grains in a tungsten mining area-field experiment: effects of biochar type and dosage, rice variety, and pollution level[J]. Environmental Geochemistry and Health, 2019, 41(1): 43-52.

[9] Shen Z, Hou D, Jin F, et al. Effect of production temperature on lead removal mechanisms by rice straw biochars[J]. Science of the Total Environment, 2019, 655: 751-758.

[10]丛宏斌,姚宗路,赵立欣,等. 中国农作物秸秆资源分布及其产业体系与利用路径[J]. 农业工程学报,2019,35(22):132-140.

Cong Hongbin , Yao Zonglu , Zhao Lixin , et al. Distribution of crop straw resources and its industrial system and utilization path in China[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2019, 35(22): 132-140. (in Chinese with English abstract)

[11] Yang G, Zhang G, Wang H. Current state of sludge production, management, treatment and disposal in China[J]. Water Research, 2015, 78: 60-73.

[12] Awad Y M, Wang J, Igalavithana A D, et al. Biochar effects on rice paddy: Meta-analysis[J]. Advances in Agronomy, 2018, 148: 1-32.

[13]Tang X, Shen H, Chen M, et al. Achieving the safe use of Cd- and As-contaminated agricultural land with an Fe-based biochar: A field study[J]. Science of the Total Environment, 2020, 706: 135898.

[14]Yang X, Ng W, Wong B S E, et al. Characterization and ecotoxicological investigation of biochar produced via slow pyrolysis: Effect of feedstock composition and pyrolysis conditions[J]. Journal of Hazardous Materials, 2019, 365: 178-185.

[15]Nazari S, Rahimi G, Khademi Jolgeh Nezhad A. Effectiveness of native and citric acid-enriched biochar of Chickpea straw in Cd and Pb sorption in an acidic soil[J]. Journal of Environmental Chemical Engineering, 2019, 7(3): 103064.

[16]Wang X, Zhou W, Liang G, et al. Characteristics of maize biochar with different pyrolysis temperatures and its effects on organic carbon, nitrogen and enzymatic activities after addition to fluvo-aquic soil[J]. Science of the Total Environment, 2015, 538: 137-144.

[17]Jin J, Li Y, Zhang J, et al. Influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochars derived from municipal sewage sludge[J]. Journal of Hazardous Materials, 2016, 320: 417-426.

[18]Subedi R, Taupe N, Pelissetti S, et al. Greenhouse gas emissions and soil properties following amendment with manure-derived biochars: Influence of pyrolysis temperature and feedstock type[J]. Journal of Environmental Management, 2016, 166: 73-83.

[19]Deng Y, Huang S, Laird D A, et al. Adsorption behaviour and mechanisms of cadmium and nickel on rice straw biochars in single-and binary-metal systems[J]. Chemosphere, 2019, 218: 308-318.

[20] Wang X, Li C, Li Z, et al. Effect of pyrolysis temperature on characteristics, chemical speciation and risk evaluation of heavy metals in biochar derived from textile dyeing sludge[J]. Ecotoxicology and Environmental Safety, 2019, 168: 45-52.

[21]Huang H, Yao W, Li R, et al. Effect of pyrolysis temperature on chemical form, behavior and environmental risk of Zn, Pb and Cd in biochar produced from phytoremediation residue[J]. Bioresource Technology, 2018, 249: 487-493.

[22]Yuan H, Lu T, Huang H, et al. Influence of pyrolysis temperature on physical and chemical properties of biochar made from sewage sludge[J]. Journal of Analytical and Applied Pyrolysis, 2015, 112: 284-289.

[23] Jin J, Wang M, Cao Y, et al. Cumulative effects of bamboo sawdust addition on pyrolysis of sewage sludge: biochar properties and environmental risk from metals[J]. Bioresource Technology, 2017, 228: 218-226.

[24]Veksha A, Giannis A, Liang Y N, et al. Insights into the speciation of heavy metals during pyrolysis of industrial sludge[J]. Science of The Total Environment, 2019, 691: 232-242.

[25]Ronsse F, van Hecke S, Dickinson D, et al. Production and characterization of slow pyrolysis biochar: Influence of feedstock type and pyrolysis conditions[J]. GCB Bioenergy, 2013, 5(2): 104-115.

[26]Homagain K, Shahi C, Luckai N, et al. Biochar-based bioenergy and its environmental impact in Northwestern Ontario Canada: A review[J]. Journal of Forestry Research, 2014, 25(4): 737-748.

[27]兖少锋,陈瑾,王丽乔,等. 雷竹落叶生物炭对微囊藻毒素的吸附性能[J]. 环境化学,2014,33(4):617-623.

Yan Shaofeng, Chen Jin, Wang Liqiao, et al. Adsorption of microcystin-LR on the leaves-phyllostachys praecox-derived biochar[J]. Environmental Chemistry, 2014, 33(4): 617-623. (in Chinese with English abstract)

[28]Gul S, Whalen J K, Thomas B W, et al. Physico-chemical properties and microbial responses in biochar-amended soils: Mechanisms and future directions[J]. Agriculture, Ecosystems & Environment, 2015, 206: 46-59.

[29] Li H, Dong X, Da Silva E B, et al. Mechanisms of metal sorption by biochars: Biochar characteristics and modifications[J]. Chemosphere, 2017, 178: 466-478.

[30]Ahmad M, Rajapaksha A U, Lim J E, et al. Biochar as a sorbent for contaminant management in soil and water: A review[J]. Chemosphere, 2014, 99: 19-33.

[31]Tag A T, Duman G, Ucar S, et al. Effects of feedstock type and pyrolysis temperature on potential applications of biochar[J]. Journal of Analytical and Applied Pyrolysis, 2016, 120: 200-206.

[32]Pariyar P, Kumari K, Jain M K, et al. Evaluation of change in biochar properties derived from different feedstock and pyrolysis temperature for environmental and agricultural application[J]. Science of the Total Environment, 2020, 713: 136433.

[33]Lu H, Zhang W, Yang Y, et al. Relative distribution of Pb2+sorption mechanisms by sludge-derived biochar[J]. Water Research, 2012, 46(3): 854-862.

[34]Jin J, Li S, Peng X, et al. HNO3modified biochars for uranium(VI) removal from aqueous solution[J]. Bioresource Technology, 2018, 256: 247-253.

[35]Cazetta A L, Vargas A M M, Nogami E M, et al. NaOH-activated carbon of high surface area produced from coconut shell: Kinetics and equilibrium studies from the methylene blue adsorption[J]. Chemical Engineering Journal, 2011, 174(1): 117-125.

[36]Zhou N, Chen H, Feng Q, et al. Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels[J]. Journal of Cleaner Production, 2017, 165: 221-230.

[37]Shen Y, Zhang N. Facile synthesis of porous carbons from silica-rich rice husk char for volatile organic compounds (VOCs) sorption[J]. Bioresource Technology, 2019, 282: 294-300.

[38]Zuo X, Liu Z, Chen M. Effect of H2O2concentrations on copper removal using the modified hydrothermal biochar[J]. Bioresource Technology, 2016, 207: 262-267.

[39]Deng J, Li X, Liu Y, et al. Alginate-modified biochar derived from Ca(II)-impregnated biomass: Excellent anti-interference ability for Pb(II) removal[J]. Ecotoxicology and Environmental Safety, 2018, 165: 211-218.

[40]Deng J, Li X, Wei X, et al. Sulfamic acid modified hydrochar derived from sawdust for removal of benzotriazole and Cu(II) from aqueous solution: Adsorption behavior and mechanism[J]. Bioresource Technology, 2019, 290: 121765.

[41]Sun L, Chen D, Wan S, et al. Performance, kinetics, and equilibrium of methylene blue adsorption on biochar derived from eucalyptus saw dust modified with citric, tartaric, and acetic acids[J]. Bioresource Technology, 2015, 198: 300-308.

[42]Luo M, Lin H, Li B, et al. A novel modification of lignin on corncob-based biochar to enhance removal of cadmium from water[J]. Bioresource Technology, 2018, 259: 312-318.

[43]Tan X, Wei W, Xu C, et al. Manganese-modified biochar for highly efficient sorption of cadmium[J]. Environmental Science and Pollution Research, 2020, 27: 9126-9134.

[44]Wang Y, Wang L, Deng X, et al. A facile pyrolysis synthesis of biochar/ZnO passivator: Immobilization behavior and mechanisms for Cu (II) in soil[J]. Environmental Science and Pollution Research, 2020, 27: 1888-1897.

[45]Zhu N, Qiao J, Yan T. Arsenic immobilization through regulated ferrolysis in paddy field amendment with bismuth impregnated biochar[J]. Science of the Total Environment, 2019, 648: 993-1001.

[46]Song J, Zhang S, Li G, et al. Preparation of montmorillonite modified biochar with various temperatures and their mechanism for Zn ion removal[J]. Journal of Hazardous Materials, 2020, 391: 121692.

[47]Zou Q, An W, Wu C, et al. Red mud-modified biochar reduces soil arsenic availability and changes bacterial composition[J]. Environmental Chemistry Letters, 2018, 16(2): 615-622.

[48]Hemavathy R, Kumar P S, Kanmani K, et al. Adsorptive separation of Cu (II) ions from aqueous medium using thermally/chemically treated Cassia fistula based biochar[J]. Journal of Cleaner Production, 2020, 249: 119390.

[49]Jin H, Capareda S C, Chang Z, et al. Biochar pyrolytically produced from municipal solid wastes for aqueous As(V) removal: Adsorption property and its improvement with KOH activation[J]. Bioresource Technology, 2014, 169: 622-629.

[50]Wu W, Li J, Lan T, et al. Unraveling sorption of lead in aqueous solutions by chemically modified biochar derived from coconut fiber: A microscopic and spectroscopic investigation[J]. Science of the Total Environment, 2017, 576: 766-774.

[51]Wang H, Gao B, Wang S, et al. Removal of Pb(II), Cu(II), and Cd(II) from aqueous solutions by biochar derived from KMnO4treated hickory wood[J]. Bioresource Technology, 2015, 197: 356-362.

[52]Xia Y, Luo H, Li D, et al. Efficient immobilization of toxic heavy metals in multi-contaminated agricultural soils by amino-functionalized hydrochar: Performance, plant responses and immobilization mechanisms[J]. Environmental Pollution, 2020, 261: 114217.

[53]Fan J, Cai C, Chi H, et al. Remediation of cadmium and lead polluted soil using thiol-modified biochar[J]. Journal of Hazardous Materials, 2020, 388: 122037.

[54]Zhang J, Zhou H, Gu J, et al. Effects of nano-Fe3O4-modified biochar on iron plaque formation and Cd accumulation in rice (L.)[J]. Environmental Pollution, 2020, 260: 113970.

[55]Lin L, Li Z, Liu X, et al. Effects of Fe-Mn modified biochar composite treatment on the properties of As-polluted paddy soil[J]. Environmental Pollution, 2019, 244: 600-607.

[56]Dong H, Deng J, Xie Y, et al. Stabilization of nanoscale zero-valent iron (nZVI) with modified biochar for Cr(VI) removal from aqueous solution[J]. Journal of Hazardous Materials, 2017, 332: 79-86.

[57]Zhou N, Chen H, Feng Q, et al. Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels[J]. Journal of Cleaner Production, 2017, 165: 221-230.

[58]Tong X J, Li J Y, Yuan J H, et al. Adsorption of Cu (II) by biochars generated from three crop straws[J]. Chemical Engineering Journal, 2011, 172(2/3): 828-834.

[59]Li M, Lou Z, Wang Y, et al. Alkali and alkaline earth metallic (AAEM) species leaching and Cu (II) sorption by biochar[J]. Chemosphere, 2015, 119: 778-785.

[60]Lei S, Shi Y, Qiu Y, et al. Performance and mechanisms of emerging animal-derived biochars for immobilization of heavy metals[J]. Science of the Total Environment, 2019, 646: 1281-1289.

[61]Wu W, Li J, Lan T, et al. Unraveling sorption of lead in aqueous solutions by chemically modified biochar derived from coconut fiber: A microscopic and spectroscopic investigation[J]. Science of the Total Environment, 2017, 576: 766-774.

[62] Liu P, Ptacek C J, Elena K M, et al. Evaluation of mercury stabilization mechanisms by sulfurized biochars determined using X-ray absorption spectroscopy[J]. Journal of Hazardous Materials, 2018, 347: 114-122.

[63]Liang Y, Cao X, Zhao L, et al. Biochar- and phosphate- induced immobilization of heavy metals in contaminated soil and water: implication on simultaneous remediation of contaminated soil and groundwater[J]. Environmental Science and Pollution Research, 2014, 21(6): 4665-4674.

[64]Xu X, Cao X, Zhao L, et al. Removal of Cu, Zn, and Cd from aqueous solutions by the dairy manure-derived biochar[J]. Environmental Science and Pollution Research, 2013, 20(1): 358-368.

[65]Zhang H, Shao J, Zhang S, et al. Effect of phosphorus- modified biochars on immobilization of Cu (II), Cd (II), and As (V) in paddy soil[J]. Journal of Hazardous Materials, 2020, 390: 121349.

[66]Keiluweit M, Nico P S, Johnson M G, et al. Dynamic molecular structure of plant biomass-derived black carbon (biochar)[J]. Environmental Science & Technology, 2010, 44(4): 1247-1253.

[67]Yang J, Pignatello J J, Pan B, et al. Degradation of p-nitrophenol by lignin and cellulose chars: H2O2-mediated reaction and direct reaction with the char[J]. Environmental Science & Technology, 2017, 51(16): 8972-8980.

[68]Mohan D, Rajput S, Singh V K, et al. Modeling and evaluation of chromium remediation from water using low cost bio-char, a green adsorbent[J]. Journal of Hazardous Materials, 2011, 188(1/2/3): 319-333.

[69]Zhong D, Jiang Y, Zhao Z, et al. pH dependence of arsenic oxidation by rice-husk-derived biochar: Roles of redox-active moieties[J]. Environmental Science & Technology, 2019, 53(15): 9034-9044.

[70]Chen D, Guo H, Li R, et al. Low uptake affinity cultivars with biochar to tackle Cd-tainted rice: A field study over four rice seasons in Hunan, China[J]. Science of the Total Environment, 2016, 541: 1489-1498.

[71] Bian R, Li L, Bao D, et al. Cd immobilization in a contaminated rice paddy by inorganic stabilizers of calcium hydroxide and silicon slag and by organic stabilizer of biochar[J]. Environmental Science and Pollution Research, 2016, 23(10): 10028-10036.

[72]Jiang T Y, Jiang J, Xu R K, et al. Adsorption of Pb (II) on variable charge soils amended with rice-straw derived biochar[J]. Chemosphere, 2012, 89(3): 249-256.

[73]Zhang R H, Li Z G, Liu X D, et al. Immobilization and bioavailability of heavy metals in greenhouse soils amended with rice straw-derived biochar[J]. Ecological Engineering, 2017, 98: 183-188.

[74] Li Z, Qi X, Fan X, et al. Amending the seedling bed of eggplant with biochar can further immobilize Cd in contaminated soils[J]. Sci Total Environ, 2016, 572: 626-633.

[75]Hilber I, Wyss G S, Mäder P, et al. Influence of activated charcoal amendment to contaminated soil on dieldrin and nutrient uptake by cucumbers[J]. Environmental Pollution, 2009, 157(8/9): 2224-2230.

[76]Rees F, Simonnot M O, Morel J L. Short-term effects of biochar on soil heavy metal mobility are controlled by intra-particle diffusion and soil pH increase[J]. European Journal of Soil Science, 2014, 65(1): 149-161.

[77] Bian R, Joseph S, Cui L, et al. A three-year experiment confirms continuous immobilization of cadmium and lead in contaminated paddy field with biochar amendment[J]. Journal of Hazardous Materials, 2014, 272: 121-128.

[78]Abdelhafez A A, Li J, Abbas M H. Feasibility of biochar manufactured from organic wastes on the stabilization of heavy metals in a metal smelter contaminated soil[J]. Chemosphere, 2014, 117: 66-71.

[79]Beesley L, Dickinson N. Carbon and trace element fluxes in the pore water of an urban soil following greenwaste compost, woody and biochar amendments, inoculated with the earthworm Lumbricus terrestris[J]. Soil Biology and Biochemistry, 2011, 43(1): 188-196.

[80]Beesley L, Moreno-Jiménez E, Gomez-Eyles J L. Effects of biochar and greenwaste compost amendments on mobility, bioavailability and toxicity of inorganic and organic contaminants in a multi-element polluted soil[J]. Environmental Pollution, 2010, 158(6): 2282-2287.

[81]Khan S, Reid B J, Li G, et al. Application of biochar to soil reduces cancer risk via rice consumption: A case study in Miaoqian village, Longyan, China[J]. Environment International, 2014, 68: 154-161.

[82]Ming H, Naidu R, Sarkar B, et al. Competitive sorption of cadmium and zinc in contrasting soils[J]. Geoderma, 2016, 268: 60-68.

[83]Zheng R, Chen Z, Cai C, et al. Mitigating heavy metal accumulation into rice () using biochar amendment: A field experiment in Hunan, China[J]. Environmental Science and Pollution Research, 2015, 22(14): 11097-11108.

[84] Vithanage M, Herath I, Joseph S, et al. Interaction of arsenic with biochar in soil and water: A critical review[J]. Carbon, 2017, 113: 219-230.

[85]Ishimaru Y, Bashir K, Nishizawa N K. Zn uptake and translocation in rice plants[J]. Rice, 2011, 4(1): 21-27.

[86]于焕云,崔江虎,乔江涛,等. 稻田镉砷污染阻控原理与技术应用[J]. 农业环境科学学报,2018,37(7):1418-1426.

Yu Huanyun, Cui Jianghu, Qiao Jiangtao, et al. Principle and technique of arsenic and cadmium pollution control in paddy field[J]. Journal of Agro-Environment Science, 2018, 37(7): 1418-1426. (in Chinese with English abstract)

[87]Zhang Y, Chen T, Liao Y, et al. Modest amendment of sewage sludge biochar to reduce the accumulation of cadmium into rice (Oryza sativa L.): A field study[J]. Environmental Pollution, 2016, 216: 819-825.

[88]杜彩艳,王攀磊,杜建磊,等. 生物炭、沸石与膨润土混施对玉米生长和吸收Cd、Pb、Zn 的影响研究[J]. 生态环境学报,2019,28(1):190-198.

Du Caiyan, Wang Panlei, Du Jianlei, et al. Influence of fixed addition of biochar, zeolite and bentonite on growth and Cd, Pb, Zn uptake by maize[J]. Ecology and Environmental Sciences, 2019, 28(1): 190-198. (in Chinese with English abstract)

[89] 王彩云,武春成,曹霞,等. 生物炭对温室黄瓜不同连作年限土壤养分和微生物群落多样性的影响[J]. 应用生态学报,2019,30(4):1359-1366.

Wang Caiyun, Wu Chuncheng, Cao Xia, et al. Effects of biochar on soil nutrition and microbial community diversity under continuous cultivated cucumber soils in greenhouse[J]. The Journal of Applied Ecology, 2019, 30(4): 1359-1366. (in Chinese with English abstract)

[90] 李新宇,孟康,李小英,等. 生物炭对元谋燥红壤土壤肥力与番茄生长的影响[J]. 西部林业科学,2019,48(2):114-120.

Li Xinyu, Meng Kang, Li Xiaoying, et al. Effects of biochar on boil fertility and tomato growth in yuanmou dry red soil[J]. Journal of West China Forestry Science, 2019, 48(2): 114-120. (in Chinese with English abstract)

[91] Xu M, Wu J, Luo L, et al. The factors affecting biochar application in restoring heavy metal-polluted soil and its potential applications[J]. Chemistry and Ecology, 2018, 34(2): 177-197.

[92]Düring R A, Hoß T, Gäth S. Sorption and bioavailability of heavy metals in long-term differently tilled soils amended with organic wastes[J]. Science of the Total Environment, 2003, 313(1/2/3): 227-234.

[93] Houben D, Evrard L, Sonnet P. Mobility, bioavailability and pH-dependent leaching of cadmium, zinc and lead in a contaminated soil amended with biochar[J]. Chemosphere, 2013, 92(11): 1450-1457.

[94]Petelina E, Klyashtorin A, Yankovich T. 38th British Columbia Mine Reclamation Symposium, September 22-25, 2014[C]//British Columbia: University of British Columbia Press, 2014.

[95] Beesley L, Moreno-Jiménez E, Gomez-Eyles J L, et al. A review of biochars’ potential role in the remediation, revegetation and restoration of contaminated soils[J]. Environmental Pollution, 2011, 159(12): 3269-3282.

[96] 吕波,王宇函,夏浩,等. 不同改良剂对黄棕壤和红壤上白菜生长及土壤肥力影响的差异[J]. 中国农业科学,2018,51(22):4306-4315.

Lü Bo, Wang Yuhan, Xia Hao, et al. Effects of biochar and other amendments on the cabbage growth and soil fertility in yellow-brown soil and red soil[J]. Scientia Agricultura Sinica, 2018, 51(22): 4306-4315. (in Chinese with English abstract)

[97] Moore F, Gonzalez M E, Khan N, et al. Copper immobilization by biochar and microbial community abundance in metal-contaminated soils[J]. Science of the Total Environment, 2018, 616/617: 960-969.

[98]丛日环,张丽,鲁艳红,等. 长期秸秆还田下土壤铵态氮的吸附解吸特征[J]. 植物营养与肥料学报,2017,23(2):380-388.

Cong Rihuan, Zhang Li, Lu Yanhong, et al. Adsorption-desorption characteristics of soil ammonium under long-term straw returning condition[J]. Plant Nutrition and Fertilizer Science, 2017, 23(2): 380-388. (in Chinese with English abstract)

[99] 陈少毅,许超,张文静,等. 生物质炭与氮肥配施对污染稻田田面水中无机氮及Cu、Zn、Cd含量的影响[J]. 水土保持学报,2014,28(3):253-258.

Chen Shaoyi, Xu Chao, Zhang Wenjing, et al. Influences of combined application of biochar and nitrogen fertilizers on concentrations of inorganic nitrogen and Cu, Zn and Cd in surface water of paddy soils[J]. Journal of Soil and Water Conservation, 2014, 28(3): 253-258. (in Chinese with English abstract)

[100] Moreno-Jiménez E, Fernández J M, Puschenreiter M, et al. Availability and transfer to grain of As, Cd, Cu, Ni, Pb and Zn in a barley agri-system: Impact of biochar, organic and mineral fertilizers[J]. Agriculture, Ecosystems & Environment, 2016, 219: 171-178.

[101] Cui L, Pan G, Li L, et al. Continuous immobilization of cadmium and lead in biochar amended contaminated paddy soil: A five-year field experiment[J]. Ecological Engineering, 2016, 93: 1-8.

[102] Cui H, Fan Y, Fang G, et al. Leachability, availability and bioaccessibility of Cu and Cd in a contaminated soil treated with apatite, lime and charcoal: A five-year field experiment[J]. Ecotoxicology and Environmental Safety, 2016, 134: 148-155.

[103] Xing Y, Wang J, Shaheen S M, et al. Mitigation of mercury accumulation in rice using rice hull-derived biochar as soil amendment: A field investigation[J]. Journal of Hazardous Materials, 2019, 388: 121747.

[104] 李衍亮,黄玉芬,魏岚,等. 施用生物炭对重金属污染农田土壤改良及玉米生长的影响[J]. 农业环境科学学报,2017,36(11):2233-2239.

Li Yanliang, Huang Yufen, Wei Lan, et al. Impacts of biochar application on amelioration of heavy metal-polluted soil and maize growth[J]. Journal of Agro-Environment Science, 2017, 36(11): 2233-2239. (in Chinese with English abstract)

[105]Nie C, Yang X, Niazi N K, et al. Impact of sugarcane bagasse-derived biochar on heavy metal availability and microbial activity: a field study[J]. Chemosphere, 2018, 200: 274-282.

[106]Pan D, Liu C, Yu H, et al. A paddy field study of arsenic and cadmium pollution control by using iron-modified biochar and silica sol together[J]. Environmental Science and Pollution Research, 2019, 26(24): 24979-24987.

[107]Karer J, Zehetner F, Dunst G, et al. Immobilisation of metals in a contaminated soil with biochar-compost mixtures and inorganic additives: 2-year greenhouse and field experiments[J]. Environmental Science and Pollution Research, 2018, 25(3): 2506-2516.

[108]Wagner A, Kaupenjohann M. Biochar addition enhanced growth of D actylis glomerata L. and immobilized Zn and Cd but mobilized Cu and Pb on a former sewage field soil[J]. European Journal of Soil Science, 2015, 66(3): 505-515.

[109]Ge L, Cang L, Ata-Ul-Karim S T, et al. Effects of various warming patterns on Cd transfer in soil-rice systems under Free Air Temperature Increase (FATI) conditions[J]. Ecotoxicology and Environmental Safety, 2019, 168: 80-87.

[110]李富荣,李敏,朱娜,等. 水作和旱作施用改良剂对蕹菜-土壤系统中铅镉生物有效性的影响差异[J]. 农业环境科学学报,2017,36(8):1477-1483.

Li Furong, Li Min, Zhu Na, et al. Comparing the effects of soil amendments on Pb and Cd bioavailability in water spinach under water submer-sion cultivation and dry farming conditions[J]. Journal of Agro-Environment Science, 2017, 36(8): 1477-1483. (in Chinese with English abstract)

Mechanism for the application of biochar in remediation of heavy metal contaminated farmland and its research advances

Li Hongbo1, Zhong Yi1, Zhang Haonan1, Wang Xin1, Chen Jing1,Wang Linling1※, Xiao Jinguang3, Xiao Wu2,3, Wang Wei3

(1.430074;2.410014;3.410014)

Heavy metals contaminated soils pose a serious risk to human beings and animals via direct exposure and food chain. Biochar, a carbon-rich material, is used to remediate heavy metals contaminated farmland. Thisstrategy provides an effective method for utilizing biomass resources and ensuring food safety. With increasing attention, the number of published articles concerning biochar has been increasing in the recent ten years, therefore providing researchers with a large amount of evidence and insights. In this study, the latest studies of biochar in the remediation of heavy metals contaminated farmland were reviewed, with the focus on possible mechanisms of biochar-heavy-metal interactions, related impact factors, and in-situ application of biochar at the field scale. Biochar showed a strong sorption ability, attributed to its physiochemical properties such as large specific surface area, abundant functional groups and high cation exchange capacity. The application effect of biochar was greatly influenced by its characteristics. After summarizing biochar’s physiochemical property data in recent years, the study discussed the changing law of biochar’s properties with the alteration of feedstocks and pyrolysis temperature, respectively. To modulate the properties of biochar for soil remediation, various modifiers with different concentrations were adopted, including acids, bases, oxidizing agents, organic solvents and metal salts or oxidizing agents. In general, the purposes of modification were to enlarge the surface area, to change the functional groups, and to increase the adsorption performance and catalytic capacity. Furthermore, the immobilization mechanisms of heavy metals by biochar were illustrated. The direct immobilization could be achieved through physical absorption, electrostatic attraction, ion exchange, complexation, precipitation, and redox reaction. Besides, the indirect effects of biochar on heavy-metal mobility and bioavailability, which could be achieved via impacting soil characteristics and thus heavy-metal-soil complexation, were less understood and could be largely underestimated. Biochar addition could alter many soil properties including pH value, dissolved organic carbon, mineral composition, and cation exchange capacity. These changes would affect heavy-metal-soil interactions and thus heavy-metal mobility and bioavailability. Many laboratory studies had demonstrated biochar’s effectiveness in decreasing the bioavailability of heavy metals as well as improving soil quality. However, the value of biochar in the remediation of contaminated land had not been well tested in the field. In different field trials, distinct results (beneficial, neutral or adverse effects) had been reported due to wide variations in field conditions and biochar characteristics. To better understand whether biochar application could provide a promising direction for soil remediation, this review was undertaken to assess the published field trial. The results of most previous field trials indicated that biochar could potentially reduce heavy-metal bioavailability in the field. Meanwhile, a significant decrease in the heavy-metal enrichment of the crops was observed. It was found that the use of biochar may help increase crop yields on polluted farmland and reduce the amount of mineral fertilizer used in the field. The application of biochar could inactivate heavy metals through improving soil physicochemical properties (pH, cation exchange capacity, water retention capacity etc.). In addition, it also could be used to enhance the uptake of soil nutrients for plant growth. However, according to a majority of studies, biochar’s effectiveness in reducing the impacts of heavy metals depended on a myriad of factors in the field, including biocharapplying process (variety and dosage rate of the biochar, mixing depth), agronomic measure (nitrogen-phosphorus-potassium fertilizer application) and climatic conditions (air temperature and precipitation). In the last part, future research on the perfection of the mechanisms of soil remediation using biochar, the expansion of the scale, and the long-term monitoring on soil was prospected.

biochars; farmland; heavy metals; soil remediation; field application; advance

李鸿博,钟 怡,张昊楠,等. 生物炭修复重金属污染农田土壤的机制及应用研究进展[J]. 农业工程学报,2020,36(13):173-185.doi:10.11975/j.issn.1002-6819.2020.13.021 http://www.tcsae.org

Li Hongbo, Zhong Yi, Zhang Haonan, et al. Mechanism for the application of biochar in remediation of heavy metal contaminated farmland and its research advances[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2020, 36(13): 173-185. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2020.13.021 http://www.tcsae.org

2020-02-17

2020-06-21

湖南省重点研发计划项目(2016SK2057);水利部公益性行业科研专项(201501019);污染控制与资源化研究国家重点实验室开放课题(PCRRF18027)

李鸿博,博士生,主要从事农田土壤重金属污染修复技术研究。Email:hblee@hust.edu.cn

王琳玲,博士,副教授,主要从事固废资源化与污染土壤修复研究。Email:wanglinling@hust.edu.cn

10.11975/j.issn.1002-6819.2020.13.021

X53; X712

A

1002-6819(2020)-13-0173-13

猜你喜欢
官能团改性重金属
沉淀/吸附法在电镀废水重金属处理中的应用
有机物官能团的定量关系在解题中的应用
改性废旧岩棉处理污水的应用研究
改性复合聚乙烯醇食品包装膜研究进展
硫化氢下铈锰改性TiO2的煤气脱汞和再生研究
在对比整合中精准把握有机官能团的性质
鱼头中重金属含量真的很高?
吃蘑菇不会重金属中毒
同分异构之官能团位置异构书写规律
逆向合成分析法之切断技巧