植物营养与肥料学报   2017, Vol. 23  Issue (6): 1676-1685 
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农田土壤重金属污染化学钝化修复研究进展
胡红青1, 黄益宗2, 黄巧云1, 刘永红1, 胡超1    
1. 华中农业大学资源与环境学院,武汉 430070;
2. 农业部环境保护科研监测所,天津 300191
摘要: 土壤重金属化学钝化修复是指向污染土壤中添加钝化剂,使重金属由活性向稳定化形态转化,以降低重金属的迁移和生物可利用性,从而修复重金属污染土壤的方法。本文综述了近些年国内外各类钝化材料修复重金属污染土壤的作用效果和机理、实例等方面的研究进展,并讨论了原位修复土壤重金属中亟待解决的问题,旨在为农田土壤重金属污染的化学钝化剂筛选与应用提供参考依据。
关键词: 重金属     土壤污染     化学钝化     钝化材料    
Research progress of heavy metals chemical immobilization in farm land
HU Hong-qing1, HUANG Yi-zong2, HUANG Qiao-yun1, LIU Yong-hong1, HU Chao1    
1. College of Resource and Environment, Huazhong Agricultural University, Wuhan 430070, China;
2. Agro-Environmental Protection Institute, Ministry of Agriculture, Tianjin 300191, China
Abstract: Soil heavy metal pollution remediation by chemical immobilization is a method that applies immobilization reagent to reduce the mobilization and availability of heavy metals. This paper summarized the research progress of efficiency and mechanism of various immobilization materials to agricultural land, and discussed the main problems for in situ remediation. The purpose of the paper is to provide base for choice and application of immobilization materials.
Key words: heavy metal     soil pollution     chemical immobilization     immobilization materials    

随着城镇化、工业化的发展和城市污泥、废弃物进入农业生态系统,土壤重金属污染态势日趋严峻。据国家环保部、国土资源部等的调查[1],我国土壤各种污染物超标点位占调查总点位的16.1%;而耕地土壤点位超标率高达19.4%,污染情形不容乐观。

由于我国人口压力大,优质耕地资源短缺与粮食生产需求的矛盾异常突出,不可能将污染土壤进行大规模休闲、种植非粮食作物或开展植物修复;工程措施则代价高昂难以实施,且污染土壤填埋并不去除重金属类污染物,所以对农田重金属污染土壤而言,切实可行且能保证作物安全生产的修复措施应是化学钝化,尤其是对中轻度污染的农田土壤。

化学钝化修复是向污染土壤中施入各种钝化剂,利用吸附、沉淀、氧化还原、络合等机制,改变污染物的形态与活性,使其转化成非活性、植物难吸收的组分,从而实现修复利用的技术。目前采用的钝化剂主要包括各类含磷物质、粘土矿物、生物炭、氧化物、有机物等,它们对不同污染物以及土壤类型、污染程度的修复效果有一定差异,相关综述论文也常见报道。本文就一些主要的化学钝化材料修复重金属污染研究进展作一概述,为进一步推动农田重金属污染土壤修复研究与应用提供参考。

1 含磷物质对污染土壤中重金属的钝化

含磷物质除提供植物磷营养外,对重金属的钝化修复是当前土壤重金属污染修复研究的热点领域之一,也是一种廉价、环境友好的修复材料[2],其可以通过释放磷来有效地固定土壤中的重金属。在实际应用中,常见的含磷材料有磷酸及可溶性磷酸盐,磷酸钙、磷灰石、磷矿粉、骨粉等难溶含磷材料[35],以及活化磷矿粉、溶磷菌–磷矿粉、动物粪便–磷矿粉堆肥等复合含磷材料[69]。含磷材料修复的对象主要包括Pb、Cd、Cu、Zn、Ni、Hg、Cr、Co以及As等[29]

磷酸盐可以直接参与土壤重金属的钝化,也常与其他矿物材料混合使用。采用磷酸酸化磷矿粉处理Pb污染的土壤,可将土壤中非残渣态Pb转化为残渣态,降低土壤中Pb的淋溶毒性[10]。Pb与磷形成了极稳定的磷氯铅矿[Pb5(PO4)3Cl],明显降低了植物对Pb的吸收[11]。另外,用磷酸化生物炭处理铅污染土壤,发现其有很好的修复效果[12]。与磷酸的钝化作用相比较,可溶性磷酸盐 (如磷酸铵、磷酸氢钾) 等也可直接参与重金属的钝化作用。用磷酸氢二铵处理土壤60天后,Cd的溶出量从306 mg/kg降低到34 mg/kg,磷含量增加会相应提高Cd的稳定效果[13]。雷鸣等[14]研究了磷酸氢二钠对污染土壤中重金属 (Pb、Cd、Zn) 向水稻迁移的影响,发现其显著提高了土壤pH值,降低了土壤中交换态Pb、Cd、Zn含量,同时明显降低了水稻各器官中Pb、Cd的含量。

过磷酸钙和重过磷酸钙等也被用于修复重金属污染的土壤。用过磷酸钙修复Pb、Cu污染的土壤,一段时间培养后,Pb和Cu大幅度转化为残渣态[15]。重过磷酸钙用于钝化修复Pb、Cu和Zn复合污染土壤,4周后发现可有效地降低提取态Pb和Cu,但对土壤中Zn的稳定化影响较小;磷处理可抑制Pb和Cu在土壤剖面中的径向迁移[16]。在Pb、Cd、Cu和Ni污染的土壤中施加重过磷酸钙处理后,Pb和Cd向残渣态转化[17],降低大白菜对重金属的吸收[18]。林笠等[19]采用盆栽试验研究了重金属Cd、Pb复合污染土壤中添加磷对草莓累积重金属的影响,结果表明,添磷后不仅能显著降低Cd、Pb对草莓产量和品质的影响,还能降低Cd、Pb在各组织中的累积。

含磷材料还包括磷酸钙、天然磷灰石、磷矿粉、骨粉等难溶磷酸盐矿物,它们是碱性矿物,有效磷远低于可溶性磷酸盐及磷肥。用磷矿粉处理重金属污染的土壤能增加植物对As的吸收,降低蕨类植物体内Pb、Cd含量[20]。羟基磷灰石可显著降低土壤中Pb、Zn、Cd、Co和Ni的生物有效性,增强它们的地球化学稳定性[2122]。纳米磷材料的性质有别于普通含磷矿物,用纳米Ca3(PO4)2处理射击场的重金属Pb、Cu、Zn污染后,土壤中可提取态重金属大幅度降低,部分Cu和Pb结合在纳米磷酸钙表面[23];而用负载纳米羟基磷灰石的生物炭原位修复Pb污染土壤,Pb的固定率达到74.8%,残渣态增加到66.6%,土壤中生物有效性Pb显著减少[24]

难溶磷矿物的磷有效性低,为提高有效磷的释放,溶磷菌–磷矿粉、有机酸活化磷矿粉、动物粪便–磷矿粉堆肥等也被用于处理不同污染程度的土壤。磷矿粉经处理后,有效磷含量提高,对重金属的钝化效率也高于原磷矿粉。Park等[6]利用溶磷菌处理磷矿粉后,固定污染土壤中的Pb效果更强。与溶磷菌相比,草酸处理磷矿粉后,能更好地钝化土壤中重金属Pb、Cu、Cd,毒性淋溶分析显示Pb含量低于美国EPA标准[9];砖红壤中施加磷矿粉和草酸活化磷矿粉后,交换态铅含量下降,稳定态Pb、Cu含量增加,且活化磷矿粉的效果更佳[7]。许学慧等[8]在Cd、Cu污染的矿区土壤中添加磷矿粉和活化磷矿粉,可降低土壤中交换态重金属的含量,减少莴苣对重金属Cd和Cu的吸收;施加活化磷矿粉后莴苣根和地上部重金属含量比对照最高可降低55%和59%。

含磷材料在土壤重金属原位修复中具有重要的实际意义。该方法对土壤环境的扰动少,除了提供磷素外,大部分磷材料可提高土壤的pH,影响重金属在土壤中的形态,加快重金属由可溶性向难溶性的转化,减少植物对重金属的吸收。现有研究表明,含磷材料主要对重金属Pb、Cd、Cu等有较好的钝化效果,其机理表现在以下方面[3, 2425]:提高土壤pH,使重金属离子生成氢氧化物沉淀;利用释放的磷酸根与重金属离子作用,生成溶解度更小的磷酸盐矿物 (磷氯铅矿等);土壤重金属离子与含磷矿物晶格中的阳离子发生同晶置换而被固定;金属阳离子在矿物表面发生静电吸附和共沉淀作用被固定 (图1),实际环境中这几种作用机理可能是共存的。

图1 含磷材料对土壤中重金属的钝化作用机理 Fig. 1 Immobilization of heavy metals in soil by phosphorous-containing materials
2 无机矿物对重金属的钝化

无机矿物也常用于土壤重金属的化学钝化,主要包括膨润土、凹凸棒石、海泡石、沸石等无机矿物,赤泥、飞灰、磷石膏和白云石残渣等工业副产物。此外,还有一些化学制品,如硫酸亚铁等。这些通常不提供植物营养成分,而且可以改良土壤性质。

2.1 粘土矿物钝化剂

用于土壤污染物钝化的粘土矿物主要包括海泡石、凹凸棒石、膨润土 (蒙脱石) 等,它们较大的比表面积决定了其良好的吸附性能,可通过吸附、离子交换、配位反应和共沉淀等反应钝化重金属[26]

凹凸棒石也称坡缕石,对Cd、Pb和Cu污染土壤具有良好的修复效果[27]。其对Zn的钝化以吸附和表面络合为主[28],对Cd以碳酸盐、氢氧化物或表面络合的形式固定[29]。谢晶晶等[30]认为,Zn2+ 在凹凸棒石表面先发生快速吸附,其后为慢速沉淀,表面快速水化时可提高悬浮液的pH值,诱导了Zn2+ 水解沉淀。

Zhang等[31]实验证明凹凸棒石添加量为红壤的 1%~4% (质量比) 时,土壤中可提取态重金属的浓度都有明显降低。殷飞等[28]发现添加20%凹凸棒石降低可提取态Pb、Cd、Cu、As的比例最高达35%~54%,植物易吸收的可交换态Pb显著减少,残渣态Pb显著增加。Liang等[29]也表明,凹凸棒石能降低水稻土中Cd的可交换态,增加碳酸盐结合态和残渣态,并降低糙米中23%~56%的Cd。

凹凸棒石对重金属的吸附能力可通过改性得到加强。将凹凸棒石改性成微纳米网加入污染土壤,能明显降低土壤Cr (Ⅵ) 的淋洗量,并能将Cr (Ⅵ) 还原成Cr (Ⅲ)[32]。添加10%富钙凹凸棒石可以分别降低土壤酸溶态Cd 56%和Pb 82%[33]。凹凸棒石–磁铁复合物在去除U (Ⅵ) 方面比单一组分更优越[34]

蒙脱石掺入沉积物后可固定Zn,但不能提高Cu的稳定性[35]。0.5%膨润土可明显降低Pb、Zn和Cd的水溶性[36]。Zhang等[31]发现蒙脱石对Cu吸附量可达3741 mg/kg,按2%施入土壤可降低对蚯蚓60%的重金属毒性。

相比较单一蒙脱石,其改性产物的环境应用正引起更多关注。蒙脱石–OR–SH (钙基蒙脱石酸活化后,在乙醇–水–巯基硅烷溶液中分散) 饱和吸附的Cd无毒性,在连续盆栽4季作物后,对土壤Cd仍保持显著的钝化效果[37]。施加巯基化改性膨润土能有效固定土壤Cd和Pb,显著降低土壤中重金属的活性态含量,并将其转化为稳定的铁锰结合态,有较好的钝化长效性。另外,蒙脱石与有机聚合物的复合研究也有大量报道[3839]。将壳聚糖加载到蒙脱石后,该复合物对Pb2+、Cu2+ 和Cd2+ 的最大吸附量分别为49.3、28.2和20.6 mg/g[4041]

海泡石有较好的重金属吸附能力[42],能降低水稻土中可交换态Cd并增加碳酸盐结合态和残渣态,使Cd以碳酸盐、氢氧化物或者表面络合的形式被固定[29]。添加0.5%~5%的海泡石可降低菠菜对Cd吸收量的28.0%~72.1%,当5%海泡石加入土壤,酶活性和微生物量也得以恢复[43]。海泡石的添加可使TCLP-Cd降低0.6%~49.6%,而植物吸收降低14.4%~84.1% [44]。将1%~5%海泡石加入土壤后,Cd、Zn和Pb的淋洗量降低60%~70%,而苜蓿茎秆中Zn的浓度最高降低45%。当添加量为5%时,土壤呼吸活性、脱氢酶和碱性磷酸酶活性分别增加了25%、138% 和42%[45]。Li等[46]的实验则表明,可交换态Cd降低14.3%~49.0%,而糙米中的Cd含量降低34.5%~44.4%。海泡石改性后有更好的钝化效果,如经过氧化氢改性后可极大地促进其对Pb的吸附,比天然海泡石提高43.5%[47]

2.2 工业废弃物的应用

粉煤灰颗粒呈多孔型蜂窝状结构,比表面积大,碱性,具有较高的吸附重金属能力[48]。可施入污染土壤以固定重金属[49]。实验表明,经粉煤灰改良后,土壤中Hg、Cd和Pb有效态含量平均降低 24.4%~31.8%,钝化作用明显[50]

自然沸石或改性沸石均可用于稳定土壤中重金属污染物[51]。其作用机理是通过增加碱度而促进表面对重金属的吸附;或重金属离子与沸石内阳离子的交换。通过在沸石的孔口附近交换阳离子来改变其孔道的尺寸,可赋予沸石新的吸附性能[52]。研究表明,2%沸石在土壤中培养一个月可导致Zn、Pb的生物有效性降低 15.9%和6.1%[43]。污染土壤中添加沸石可增加淋出液pH并降低Pb的生物有效性[53]。硝酸钾、氢氧化钠改性沸石比天然沸石能更显著地降低土壤酸提取态Zn的含量[52]

赤泥是铝土矿经强碱浸出氧化铝后产生的残渣。在含Pb 913 mg/kg的土壤中加入1%赤泥,可以使NH4NO3提取Pb降低90%[54]。添加5%赤泥可使土壤交换态Pb和Zn降低99%以上[55]。2个月赤泥处理使生物有效性Cu含量比对照降低 13.2%[56]。但也有研究表明,5%的赤泥添加使Cd、Ni、Pb和Zn的不稳定态降低22%~80%,而As和Cu的不稳定态却分别增加了24%和47%,当赤泥添加量为5%或更高时,Cd、Ni、Pb和Zn流动性的降低更甚于As、Cu、Cr和V[54]

2.3 其他材料的应用

一些铁基材料也用于土壤重金属的钝化。如钢渣具有较高的pH值,导致重金属形成化学沉淀。据殷飞等[28]报道,添加20%钢渣能显著降低土壤中可交换态Cd以及可交换态和碳酸盐结合态Zn含量,并显著增加残渣态Cu含量。据报道,硫酸亚铁加入土壤1个月后,土壤酸提取态As含量比对照处理降低86.6%,2个月后,土壤As的生物有效性含量比对照降低90.8%,优于骨炭、磷酸二氢钙和堆肥[57]。随硫酸铁用量增加,对有效态As的固定效果明显增加;当Fe3+/ PO43–摩尔比为7.2时,7 d后土壤有效态Pb、Cd、As去除率分别为99%、41%、69%。

Rinklebe等[58]比较了活性炭、膨润土、生物炭、壳聚糖、粉煤灰、有机粘土、沸石等对Cu污染土壤的修复能力。除有机粘土和沸石外,其他改良剂均明显增加土壤pH。Tica等[59]比较了磷灰石和Slovakite (白云岩、膨润土、沸石等的混合物) 的钝化效果,两者均能有效降低重金属Pb、Zn、Cu和Cd的毒性,而Slovakite效果更佳。

大量天然及废弃物材料因廉价易得吸引了许多研究者的关注。目前对这些材料的应用特性和效能已有许多试验,但以下方面尚需进一步加强研究:1) 单一矿物对重金属的微观稳定机制;2) 钝化剂加入后重金属的长期稳定性;3) 粘土矿物的改性及产物的效能。

3 生物炭对重金属的钝化 3.1 生物炭钝化土壤重金属的效果及机理

生物炭是土壤重金属修复研究中的一种重要材料。田间试验证明,小麦秸秆生物炭可有效固定土壤中的Cd和Pb[60]。将稻秆和稻壳生物炭施入土壤,短期内可以有效钝化重金属[6162]。生物炭对重金属生物有效性的影响源于改变土壤pH,增加土壤有机质含量,改变土壤氧化还原状况及微生物群落组成等多种机制的协同作用,而生物炭对重金属的吸附机理主要有静电作用、离子交换、阳离子π键、沉淀反应等[63]

生物炭对重金属的钝化效果受到多因素的影响,如生物炭的来源、制备条件 (温度、炭化时间等)、土壤性质、重金属种类及污染程度等。生物炭的表观性质在一定程度上决定了其对重金属的固定能力。不同原材料和热解温度会得到性质不同的生物炭,对土壤重金属的修复效果和机制也有差别。硬木在600℃时制得的生物炭对Cu和Zn的吸附量高于棉花秸秆450℃时制得的生物炭[64]。将竹炭和水稻秸秆生物炭按不同比例施加到Cu、Pb、Zn、Cd污染土壤中,发现后者钝化效果更好[65]

3.2 生物炭复合材料的研究

生物炭因其在高温裂解过程中部分基团损失、吸附后分离难等不足,已有学者开始研究将生物炭与其他材料复合或者进行化学改性,加强其吸附能力。主要有以下方法:1) 用KOH、H2O2、O3、H2SO4 /HNO3等改性生物炭,提高生物炭的比表面积,增加其表面官能团 (如羧基),提高对污染物的固定能力[6667];2) 与磁性吸附剂 (如纳米氧化铁、零价铁等) 复合,可以赋予生物炭磁性[68],利于回收;3) 结合纳米技术制备新型复合材料,提高生物炭的封存和处理能力;4) 用化学修饰法将锰或镁氧化物、过磷酸钙等与生物炭复合,在生物炭表面添加一些能与污染物相互作用的基团,提高吸附效果[6970]

Inyang等[67]对比了甘蔗渣生物炭与经厌氧消化的甘蔗渣制备的生物炭对水中Pb2+ 的去除效果,发现后者对Pb2+ 的最大吸附量是前者的20倍。Agrafioti等[71]分别将CaO溶液、FeO粉末、FeCl3溶液与稻壳、有机固体废弃物混合,用于As (V) 的去除,发现其对As的去除率显著高于原始生物炭。Zhao等[70]研究表明用生物炭与磷肥共热解后增加生物炭对重金属的固定率。

4 石灰对重金属的钝化 4.1 石灰对土壤重金属的钝化效果与机制

钙可与镉发生同晶替代作用。试验表明,施用生石灰处理在2年中可使糙米中镉含量降低至国家食品卫生标准限值 (0.2 mg/kg) 以下[72]。Pandit等[73]研究发现施石灰能降低菠菜中镉的浓度。Tan等[74]研究石灰钝化土壤后5种蔬菜 (莴苣、大白菜、花椰菜等) 体内含镉量的变化,发现其降低40%~50%。

施用石灰可降低土壤中有效态铜含量[75]。铅污染土壤经石灰处理后,玉米对铅的吸收明显下降,其籽粒含铅量可达到国家食品卫生标准[76]。吴烈善等[77]在人工污染的黄色黏土中添加石灰处理,土壤Pb、Cu、Cd、Zn的稳定率可达98.5%~99.8%。石灰对铬 (Cr6+) 和汞 (Hg2+) 的吸附很稳定[78],施用6%石灰后,土壤能固定69%的Cr6+ 和63%的Hg2+

石灰通过降低土壤中H+浓度,增加土壤颗粒表面负电荷,促进对重金属离子的吸附,降低重金属的迁移性。另外,石灰可改变重金属形态,促进金属碳酸盐形成,减少活性重金属的比例[79]

4.2 石灰与其他材料配施的效果

2%石灰–烧石膏–木炭 (质量比3∶1∶2) 施用在湖南衡阳一土壤中,镉固定率达58.9%[80]。2%天然腐熟牛粪 +2%石灰组合施用,Pb、Cu、Cd、Zn稳定效率达95.9%~99.4%[77]。石灰和有机肥复合施用使土壤中交换态Cd含量降低54.7%,远高于单独施用石灰的[81]。Wang等[82]在草甸土进行Cd的钝化实验,0.2%石灰 +5%蛇纹石复配的效果最好,处理60 d后有效态Cd含量降低29.1%。He等[83]研究施用石灰、矿渣和甘蔗渣在第四纪红黏土的钝化效果,发现复合施用效果最佳,镉含量降低58.3%~70.9%,结合种植低Cd积累的水稻品种,可使糙米中的Cd含量降至污染物限度。

5 其他钝化剂对重金属的钝化 5.1 有机钝化剂

有机物料不仅提供植物养分,改良土壤,同时也是有效的土壤重金属吸附、络合剂,被广泛应用于土壤重金属污染修复中。有机物通过提升土壤pH、增加土壤阳离子交换量、形成难溶性金属–有机络合物等方式来降低土壤重金属的生物可利用性[8486]。目前常用的有机钝化剂主要包括植物秸秆[8788]、畜禽粪便[8990]、城市污泥和有机堆肥等[84, 91]

紫云英施入农田中,土壤有效铜和镉的含量降低,同时降低了稻草和谷粒中铜和镉的含量[87]。水稻秸秆和磷肥混施可降低土壤中重金属的植物有效性[92]。水稻秸秆堆肥施用增加了农田土壤中重金属Zn、Cd和Pb的碳酸盐结合态、铁锰氧化物结合态、有机质结合态和残渣态重金属的比例,也降低了农田土壤中重金属的生物有效性[88]

家禽粪便、生物固体等可增加土壤中溶解性有机质含量,并与重金属形成较稳定的金属–有机络合物,降低重金属的生物可利用性,特别是腐熟度较高的有机质可通过形成粘土–金属–有机质三元复合物增加重金属吸附量[93]。施用猪粪后,稻麦两季表层土壤重金属Cu、Zn含量略有升高,静态环境容量均降低[94]。家禽粪便、生物固体等使用后,可强烈地与Hg结合而固定之[95]。在农田土壤中添加猪粪,可使土壤有效铜、镉显著降低,同时也极大降低稻草和谷粒中铜、镉的含量[87]

Hashimoto等[89]研究了畜禽粪便对Pb淋溶的影响,发现畜禽粪便能显著降低土壤水溶态及可交换态Pb含量,促使其向残留态转化,降低其迁移和生物可利用性。张亚丽等[90]向Cd污染土壤施加猪粪等有机物,也得到类似结果。施用15 g/kg的粪肥和压滤泥浆均降低了土壤外源Ni的植物有效性[86]

腐熟堆肥施入土壤后可减少重金属的生物有效性[96],不但可以显著降低污染土壤中As、Cd、Pb、Zn等的生物有效态含量,还可显著降低植物对重金属的吸收[96]。添加生物堆肥到铜污染土壤中,显著降低了CaCl2提取的铜含量,增加了土壤的pH值[97]

腐殖酸能与重金属结合,也是土壤重金属的钝化剂。用腐植酸与膨润土 (或过磷酸钙) 处理Pb污染土壤,发现分别投加20%腐植酸与20%膨润土、10%腐植酸与6%过磷酸钙,固定40 d后土壤中有效态铅含量均大幅降低[98]。添加主要成分为腐殖酸的褐煤到铜污染土壤中,显著降低了土壤中CaCl2提取的铜含量[97]

5.2 铁粉

纳米铁或含铁纳米材料在土壤重金属治理过程中也发挥着重要的作用。有研究者利用零价纳米铁降低污染土壤中Cd、Cr和Zn的有效性,发现其能明显提高金属的稳定性,对Cr的修复效果和稳定性很好[98]。研究证实,有机堆肥配合铁砂等在钝化重金属污染物时表现出加和作用,可显著降低重金属的生物有效性,并可能超过无机钝化剂的单独作用[91]

纳米零价铁粉施于砷污染土壤中,能使砷由水溶态和吸附态向非晶质铁铝氧化物结合态和晶质铁铝氧化物态转化,其中水溶态和吸附态砷可减少70%和18%,而非晶质铁铝氧化物结合态和晶质铁铝氧化物态砷分别最大增加42%和51%,并显著降低三七中的砷含量[99] 。磷酸铁纳米材料可以显著降低土壤中水溶态、可交换态和碳酸盐结合态Cu含量,促使Cu向残渣态转化;铁纳米材料可显著降低土壤淋洗液中Cr含量[100]

纳米零价铁配合低分子量有机酸施用可增加农田土壤中铅的去除,0.2 mol/L柠檬酸配合2.0 g/L零价铁对农田土壤铅的去除效率能增加83% [101]。生物炭负载纳米零价铁能有效固定土壤中铬,当施用8 g/kg生物炭负载纳米零价铁于土壤中15 d后,土壤中六价铬不可检出,进而降低铬在土壤–植物系统的转移[102]

6 结语

随着我国农田土壤重金属污染面积的增加,寻找切实可行的处置方法刻不容缓。从国内外的研究与实践来看,土壤重金属的化学钝化措施可以较好地固定重金属,降低重金属的活性和环境风险,但是该技术在实际应用中尚有一些亟待深入研究的问题。

1) 钝化与其他技术联用 钝化能使重金属的形态暂时改变,但并未从土壤中彻底根除。当外界条件改变时,固定的重金属还可能重新释放,导致二次污染。微生物修复技术利用微生物产生的硫化物等来固定土壤中重金属,具有持久性作用。此外,利用作物轮作–磷修复措施也可以较好地修复农田重金属污染。

2) 方案优选及钝化剂改性 污染土壤常是多种重金属共存的体系,同时地域、气候等环境因素对钝化剂的要求不完全相同。因此,必须结合每种重金属的性质来选择不同的钝化剂和修复措施。钝化剂改性可以根据不同重金属特性增强其钝化功能,形成广谱性多功能钝化材料。

3) 新型高效环保钝化剂研发 钝化剂包括人工合成的材料和天然材料,有些天然材料中含有重金属以及放射性物质,遗留在土壤环境中也会对环境造成一定的副作用,当它们累积到一定量时,这些材料的环境负效应就需要考虑了。因此在选用不同材料修复被重金属污染的土壤时,必须环境友好,同时要提高其修复效率。

4) 钝化机理与产物稳定性 钝化剂的性质是决定钝化重金属机理的主要因素。当前,宜对不同材料钝化重金属机制开展深入研究,为进一步的实践奠定理论基础。在所形成的重金属难溶物中,氢氧化物和碳酸盐的溶解度要大于磷酸盐沉淀物的溶解度,所以,利用重金属的溶解性选用不同的钝化剂和措施可以有效地降低重金属的生物活性,更多地将重金属离子转化为活性更低的难溶矿物,以达到更强的钝化效果。

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