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受人类活动如金属采矿冶炼、污水灌溉、农业生产等影响,农田土壤重金属污染问题日益突出。2014年国家环境保护部与国土资源部联合发布的《全国土壤污染状况调查公报》[1]显示,我国土壤中镉(Cd)污染物点位超标率为7.0%,位于无机污染物之首;砷(As)污染物点位超标率为2.7%,位于无机污染物第3;二者属于耕地土壤的主要污染物。土壤中As、Cd的累积不仅会导致粮食减产,带来经济损失[2],还会通过食物链富集并进入人体,从而对人群健康造成威胁[3]。然而,由于As、Cd的元素性质和赋存形态不同,在土壤pH-Eh的影响下,二者具有不同的迁移特性和生物有效性,致使Cd污染土壤的修复材料和方法普遍不适用于As污染土壤[4],因此,As、Cd复合污染土壤的联合修复是目前的研究难点。
一般的土壤修复手段有物理、化学和生物技术。其中,化学钝化具有简单、快速、相对成本低等优点[5],被广泛应用于重金属污染土壤的修复中。常见的化学钝化材料有金属及其氧化物、含硫物质、含磷物质、硅钙物质、黏土矿物、有机物料、生物炭等[6]。而金属及其氧化物、含硫、含磷材料及硅钙物质会造成土壤中金属、磷、钙的积累,易引发土壤二次污染的问题[7-10];有机物料、生物炭对As、Cd复合污染土壤的修复效果欠佳[11-12],对二者同时吸附的能力和亲和力不足,若要大规模应用就会导致较高的修复成本。层状双金属氢氧化物(layered double hydroxides,LDHs)是一种典型的阴离子层状黏土矿物,通常被称为类水滑石化合物,一般是由2种金属的氢氧化物层构成主体,层间填充带负电荷的阴离子和水,结构通式为[M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O[13]。该材料具有良好的热稳定性、亲水性、记忆效应和较高的阴离子交换能力,受到了国内外学者的广泛关注[14-17]。铁锰类LDHs具有治理土壤As、Cd复合污染的可能性。这是因为,铁氧化物对As(V)具有亲和力和高选择性,而锰氧化物具有将As (III)氧化为As(V)的作用,进而可以在较宽的pH范围内提高对As的吸附能力[18-21];且LDHs属于碱性材料,可促使土壤中Cd形成氢氧化物或碳酸盐沉淀,降低Cd的移动性[22-23]。此外,铁、锰都是环境友好型的金属元素,以铁、锰元素制成的LDHs材料对环境污染小,而且制备成本低。
然而,锰的氢氧化物在碱性条件下极易被氧化,这会破坏晶体生长[24],难以制备出完整层状结构的铁锰类LDHs,故需引入第3种元素保持其稳定性。周宏光[24]采用Mg2+介入的共沉淀法制备出具有良好层状结构且具有极高吸附容量和酸碱双功能性的FeMnMg-LDHs材料,对Cd2+的最大吸附量为59.99 mg·g−1,远高于其他同类型的LDHs材料,但此材料在使用过程中的Mg2+溶出率较大。廖玉梅等[25]对FeMnMg-LDHs进行了改性,引入Ni2+制备出具有良好层状结构和较高稳定性的FeMnNi-LDHs材料,对As(Ⅲ)的最大吸附量为240.86 mg·g−1,明显高于其他层状双金属氢氧化物,但该研究仅停留在重金属废水处理领域,还未应用于土壤重金属修复领域。
考虑到Ca2+的离子半径(0.098 nm)与Mg2+(0.065 nm)相差不大,且与Fe2+(0.076 nm)、Mn2+(0.080 nm)接近,故预期Ca2+能和Fe、Mn良好结合,提高材料的稳定性[26-27];另外,Ca2+还会和As发生沉淀作用,形成Ca3As2O8,从而降低As的有效性[28]。故本研究尝试引入Ca2+,采用共沉淀法制备FeMnCa-LDHs材料,通过XRD、SEM、FT-IR手段对材料的微观结构进行表征和分析;并分别开展土壤培养实验和小白菜盆栽试验,以考察FeMnCa-LDHs材料对土壤-作物体系中As和Cd的钝化效果,为铁锰类LDHs材料用于修复土壤砷镉复合污染的实际应用提供参考。
FeMnCa-LDHs材料对不同程度砷镉复合污染土壤的钝化修复
Passivation remediation of arsenic-cadmium contaminated soils with different pollution levels by FeMnCa-LDHs materials
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摘要: 针对土壤砷镉复合污染同时修复较为困难的问题,制备了新型FeMnCa-LDHs材料以实现对土壤中As、Cd的同时钝化。通过土壤培养实验和小白菜盆栽实验,研究了在不同污染水平下FeMnCa-LDHs材料施用量对土壤As、Cd的形态转化及对小白菜各部位中As、Cd质量分数的影响,并分析了两者的相关性。结果表明,在高污染水平下施用1.0%的FeMnCa-LDHs材料,可使弱酸提取态As和弱酸提取态Cd的质量分数分别下降12.1%和28.9%,As和Cd由弱酸提取态向更稳定的形态转化;材料对As的吸附作用及对土壤pH的提高是其同时钝化As和Cd的主要原因。在高污染水平下,0.5%的材料施用量可使小白菜地上部分中As和Cd的质量分数分别减少61.2%和53.0%。相关性分析结果表明,小白菜各部位中As和Cd的质量分数与土壤中弱酸提取态As和弱酸提取态Cd的质量分数呈显著正相关,与残渣态As和残渣态Cd的质量分数呈显著负相关,这说明FeMnCa-LDHs材料通过改变土壤中As、Cd的形态分布降低了As、Cd的生物有效性。本研究可为土壤As、Cd复合污染提供参考。Abstract: In view of the difficulty in simultaneous remediation of arsenic and cadmium pollution in soil, a novel material FeMnCa-LDHs was prepared to achieve simultaneous immobilization of arsenic and cadmium in the soil. Soil culture experiments and a pot-planting experiment of Brassica chinensis were carried out to explore the effects of different addition rates of FeMnCa-LDHs on the inactivation dynamics of arsenic and cadmium species in soil, as well as the mass fraction of arsenic and cadmium in Brassica chinensis under different heavy metal pollution levels, and the correlation between the two was analyzed. The results showed that 1.0% addition rate reduced the mass fraction of weak acid extractable arsenic and cadmium by 12.1% and 28.9%, respectively, for the highly polluted soil and both arsenic and cadmium transformed from weak acid-extractable form to residual state. The material's adsorption of arsenic and the increase of soil pH were the main reasons for its simultaneous immobilization of arsenic and cadmium. 0.5% addition rate could reduce the mass fractions of arsenic and cadmium in the aboveground part of Brassica chinensis by 61.2% and 53.0% for the highly polluted soil. The correlation analysis showed that the mass fractions of arsenic and cadmium in various parts of Brassica chinensis significantly and positively correlated with those extracted by weak acid in the soil, while negatively correlated with the residual forms. It showed that the FeMnCa-LDHs material reduced the bioavailability of arsenic and cadmium by changing the morphological distribution of arsenic and cadmium in soil. This study could provide a new solution to this issue in simultaneous remediation of arsenic and cadmium pollution in soil.
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Key words:
- arsenic /
- cadmium /
- layered double hydroxides /
- bioavailability
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表 1 3种污染水平土壤As和Cd的质量分数
Table 1. As and Cd mass content of soil at three pollution levels
mg·kg−1 污染水平 As Cd 低污染 7.93 0.45 中污染 150 0.6 高污染 230 2.0 注:低污染为不加外源重金属的原土实测值。 表 2 土壤各形态As、Cd与小白菜各部位中As、Cd的相关性分析
Table 2. Correlation analysis of As and Cd in different soil forms and in different parts of Brassica chinensis
供试指标 地上部分As质量分数 根部As质量分数 供试指标 地上部分Cd质量分数 根部Cd质量分数 弱酸提取态As质量分数 0.709**1) 0.741** 弱酸提取态Cd质量分数 0.964** 0.962** 可还原态As质量分数 0.505 0.574 可还原态Cd质量分数 −0.536 −0.519 可氧化态As质量分数 −0.284 −0.293 可氧化态Cd质量分数 −0.491 −0.466 残渣态As质量分数 −0.853** −0.892** 残渣态Cd质量分数 −0.851** −0.782** 注:1)**表示在0.01水平下显著相关(P<0.01)。 -
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