Chemical Speciation and Environmental Risk of Cd in Soil Stabilized with Alkali-modified Attapulgite
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摘要:
凹凸棒石进行碱改性后性能的提高，为其钝化修复重金属污染土壤提供重要基础。本文采用不同比例的氢氧化钠对凹凸棒石进行改性，利用扫描电镜（SEM）、X射线衍射（XRD）和傅里叶变换红外光谱法（FTIR）分析改性前后凹凸棒石理化特性的变化，并在人工配制的重金属Cd污染土壤上进行钝化实验，研究氢氧化钠改性凹凸棒石对污染土壤中Cd的化学形态变化以及环境风险的影响。结果表明：与对照相比，添加氢氧化钠与凹凸棒石质量比为1：2的改性材料，土壤pH值显著升高0.85个单位。酸溶态Cd含量显著降低46.25%，残渣态Cd含量显著增加1.99倍；土壤中Cd的风险评价指数和潜在风险指数分别由36.70%和207.90降至20.08%和86.40，有效降低了土壤中Cd的迁移能力和环境风险。SEM、XRD和FTIR分析表明，凹凸棒石经过改性后表面粗糙程度增加，Si-O-Si键等化学键打开，用于吸附重金属的活性位点增加。碱改性凹凸棒石主要通过吸附作用，硅羟基和氢氧根与Cd2+反应生成沉淀来固定土壤Cd，从而达到钝化修复Cd污染土壤的效果。因此碱改性凹凸棒石可对土壤中Cd进行有效钝化，在重金属污染土壤修复中具有较显著的应用前景。
Abstract:BACKGROUND Heavy metal pollution in soil has been a serious threat to human health and ecological environmental safety. Stabilization remediation has become an important means of remediation of heavy metal contaminated soil due to the high efficiency and low cost. Attapulgite modified by alkali with an improved performance, provides an important basis for its stabilization and remediation of heavy metal contaminated soil.
OBJECTIVES To analyze the changes in physical and chemical properties of attapulgite before and after modification, and to study the effects of attapulgite modified by NaOH on the chemical speciation changes and environmental risks of Cd in contaminated soil, and to explore the stabilization effects of attapulgite modified by NaOH on Cd in the soil.
METHODS Different proportions of NaOH were used to modify attapulgite. The surface characteristics, crystal structure and functional groups of the materials were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Stabilization experiments were carried out on artificially prepared heavy metal Cd contaminated soil to study the effects of NaOH-modified attapulgite on the changes of chemical speciation of Cd and environmental risks in contaminated soil.
RESULTS By adding the modified material with the mass ratio of NaOH to attapulgite of 1:2, the pH value of the soil was significantly increased by 0.85 units. The exchangeable Cd content decreased by 46.28% and the residual Cd content increased by 1.98 times. The risk assessment code (RAC) and potential risk index (PRI) of Cd in soil decreased the most from 36.70% and 207.90 to 20.08% and 86.40, respectively, which effectively reduced the transfer capacity and environmental risk of Cd in soil. According to SEM, XRD and FTIR analyses, after modification, the surface roughness of attapulgite increased, chemical bonds such as Si-O-Si bonds were opened, so active sites for adsorption of heavy metals increased. Attapulgite modified by alkali immobilized Cd mainly through adsorption, and the reaction of silanol and hydroxide with Cd2+ generated precipitate, so as to achieve the effect of stabilizing and repairing Cd contaminated soil.
CONCLUSIONS Alkali-modified attapulgite can effectively stabilize Cd in soil, which has a significant application prospect in remediation of heavy metal contaminated soil.
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表 1 氢氧化钠改性凹凸棒石钝化Cd污染土壤的pH值、阳离子交换容量(CEC)和电导率(EC)
Table 1. The pH, cation exchange capacity (CEC), electrical conductivity (EC) of Cd polluted soil stabilized with attapulgite modified by NaOH
钝化剂 pH 阳离子交换量(cmol/kg) 电导率(μS/cm) CK 7.29±0.03e 22.79±0.34f 328.67±5.51f NP01 7.29±0.03e 24.32±0.30e 317.33±9.29f NP12 8.14±0.01a 34.19±0.28a 805.67±3.51a NP14 8.10±0.00b 29.22±0.22b 763.33±6.81b NP16 8.03±0.00c 26.76±0.84bc 622.00±15.10c NP18 7.99±0.01d 25.24±0.56c 559.00±5.29d NP110 7.98±0.01d 25.06±0.07d 456.67±9.50e F值 2665.64*** 108.09*** 1529.53*** 注：同列不同小写字母(a、b、c、d)本身没有具体的含义，是通过相互之间的比较来体现差异是否显著(P < 0.05)，字母不同表示两者差异显著，差异性显著表明数据变化较为明显(本文其他表格和图内的a、b、c、d含义同此)。F值后的“***”表示在0.001水平上差异显著。 
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表 2 氢氧化钠改性凹凸棒石钝化土壤中Cd的BCR连续萃取态含量和回收率
Table 2. BCR sequential extractions and percentage recovery of Cd in soil stabilized with attapulgite modified by NaOH
钝化剂 酸溶态Cd (mg/kg) 还原态Cd (mg/kg) 氧化态Cd (mg/kg) 残渣态Cd (mg/kg) Cd回收率(%) CK 5.73±0.31a 7.03±0.20a 0.88±0.14e 1.97±0.14d 100.82 NP01 5.27±0.14ab 6.63±0.00b 1.24±0.20d 2.04±0.36d 98.04 NP12 3.08±0.57d 6.06±0.11cd 2.19±0.00c 3.93±0.11a 98.55 NP14 4.30±0.52c 6.16±0.11c 2.11±0.27c 2.69±0.08b 98.57 NP16 4.59±0.21bc 5.63±0.07e 2.29±0.09bc 2.43±0.07bc 96.52 NP18 4.53±0.32c 5.79±0.26de 2.59±0.30ab 2.14±0.13cd 97.27 NP110 4.96±0.29bc 5.89±0.26cde 2.70±0.17a 2.06±0.05d 100.84 F值 15.29*** 24.07*** 36.91*** 50.74*** 注：同列不同小写字母(a、b、c、d)表示处理之间差异显著(P < 0.05)。F值后的“***”表示在0.001水平上差异显著。
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表 3 钝化土壤的理化性质与Cd化学形态和环境风险指数之间的相关系数
Table 3. Correlation coefficients among physiochemical traits of the stabilized soil, chemical speciation and environmental risk factors of Cd in soil
指标 酸溶态Cd 还原态Cd 氧化态Cd 残渣态Cd RAC PRI pH -0.767** -0.844** 0.827** 0.579** -0.795** -0.659** EC -0.847** -0.602** 0.499* 0.819** -0.879** -0.876** CEC -0.839** -0.496* 0.397 0.867** -0.878** -0.897** 注：“*”表示相关系数达到显著水平(0.01 < P < 0.05)；“**”表示相关系数达到极显著水平(0.001 < P < 0.01)。
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