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近年来,土壤污染问题已引起广泛关注。据《全国土壤污染状况调查公报》[1]显示,全国土壤点位超标率为16.1%,主要存在重金属和有机物污染。石油生产、有机肥料和农药的生产及使用、电子垃圾的处理和火力发电厂燃料的燃烧等工农业生产活动造成有机污染物和重金属大量进入环境,造成土壤污染[2-3],且呈现多类型污染物复合存在的态势[4-5]。根据针对土壤与地下水修复行业的调查可知,2018年,复合污染修复项目占工业污染场地项目的37.5%、重金属污染占32.8%、有机污染占29.7%[6]。
土壤修复技术逐渐趋于多样化,其中,热处理技术由于具有良好的修复效果和较短的修复周期而被广泛应用于有机污染修复[7]。2007—2017年,热处理修复技术的应用比例为38.7%,2018年的应用比例为43.4%[6]。常用热处理技术包括工业炉窑协同处置、热解吸和原位热脱附等。早期的热处理修复研究主要集中在单一污染物。魏萌[8]的研究表明,热处理对焦化场地土壤多环芳烃具有良好的去除效果,而对土壤中的重金属全量则无明显影响。BONNARD等[9]发现,热脱附后,土壤中多环芳烃的去除率可达到94%,而重金属全量未发生改变,但对蚯蚓的生物毒性增大了,该处理通过改变重金属形态提高了重金属的生物有效性,进而增加了其遗传毒性。焦文涛等[10]发现,在高温条件下,土壤中稳定金属的存留率会增加,导致其生物毒性增大。焦化厂、电子垃圾拆解厂、火力发电厂以及污水灌溉等在生产使用过程中可能造成重金属和有机物的复合污染[5,11-14]。在热处理修复土壤有机污染时,往往忽略了土壤中重金属的赋存形态以及健康风险的变化。工业窑炉高温协同处置重金属污染土壤时,也应关注重金属风险的变化。
与大气环境、水环境管理思路不同,我国污染地块的管理思路主要是基于用地功能、环境和健康风险考虑。依据《建设用地土壤污染风险评估技术导则》(HJ 25.3-2019)[15] (以下简称《导则》),土壤重金属的健康风险采用全量进行评估,但土壤重金属的赋存形态是决定其在土壤中的迁移性、生物可利用性以及毒性的重要因素[16]。环境中的重金属不能完全被人体所吸收,因此,仅以重金属全量为评价依据可能会造成高估其环境风险[17]。XU等[18]发现,添加生物炭可降低土壤中重金属的可提取态,增加其残渣态,从而可降低白菜对重金属的可吸收性。XIA等[19]发现,添加羟基磷灰石会降低重金属的酸可提取态,增加其残渣态,从而降低了其浸出毒性。重金属可通过多种途径引起暴露及生物吸收,对生态环境和人体健康造成一定的威胁[20]。而热处理对土壤重金属赋存形态和生物可给性的研究较少。基于生物可给性的重金属风险评估更能反映土壤的实际风险,体外模拟实验能够反映土壤重金属在消化系统中的生物可利用性[21],其中基于生理学的提取实验(PBET)模型等被广泛认可和应用[22-23]。
本研究通过研究电镀企业场地土壤热处理对重金属赋存形态和生物可给性的变化,探究热处理对重金属人体健康风险的影响,以期为污染土壤热处理修复工程的健康风险评估和生态效应评估提供参考。
热处理对土壤重金属形态的影响及健康风险
Effects of thermal treatment on soil heavy metals speciation and health risks
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摘要: 为探究土壤热修复后的土壤重金属形态以及健康风险的变化,以退役电镀企业地块的污染土壤为研究目标,分别在200、400和600 ℃下处理土壤15 min,以分析热处理对土壤重金属Cu、Pb、Ni和Cd赋存形态的影响、生物可给性变化以及重金属人体健康风险的差异。结果表明,经热处理后,土壤Cu、Pb和Ni的酸可提取态增加,增加了胃肠阶段的生物可给性,而Cd酸可提取态减少,生物可给性降低。在基于生物可给性的风险评估中,热处理会增加Cu、Pb和Ni的健康风险,在600 ℃处理下,Pb对儿童的危害商从0.41增加至3.70,Cd的健康风险无显著性差异。本研究结果可为场地土壤热处理后的重金属健康风险及生态效应评估提供参考。Abstract: In order to study the variation of chemical speciation of soil heavy metals and health risks after thermal treatment, the temperature was set at 200, 400 and 600 ℃ respectively for 15 minutes to analyze chemical speciation of Cu, Pb, Ni and Cd, the variation of its bioavailability and the differences in human health risks in the contaminated soils of the decommissioning electroplating site. The results showed the acid soluble fraction of Cu, Pb and Ni and its bioavailability in the gastrointestinal stage were increased after thermal treatment, while the acid soluble fraction of Cd and its bioavailability were decreased. In the risk assessment based on bioavailability, thermal treatment increased the health risk of Cu, Pb and Ni. the hazard quotient of Pb for children increased from 0.41 to 3.70 at 600 °C, but there was no significant difference in hazard quotient of Cd. This work could provide a reference for the evaluation of the health risk and ecological effects of heavy metals in contaminated sites after thermal treatment.
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Key words:
- thermal treatment /
- soil heavy metals /
- speciation /
- health risk
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表 1 土壤重金属全量及形态分布特征
Table 1. Total concentration and speciation of heavy metals in soil
采样点
位编号pH 重金属 重金属全量/
(mg·kg−1)重金属形态占比% 酸可提
取态可还
原态可氧
化态残渣
态S1 7.86 Cu 3.49×102 10.3 36.91 17.84 34.88 Pb 5.62×103 0.47 79.85 11.17 8.36 Ni 8.32×103 2.71 33.13 26.55 37.61 Cd 6.06 17.48 21.11 28.08 33.36 S2 6.24 Cu 1.24×102 4.22 34.38 13.26 48.13 Pb 6.59×10 1.68 31.44 12.13 35.60 Ni 1.17×102 10.28 24.89 21.02 43.81 Cd 6.10 6.37 21.75 31.26 40.56 S3 8.00 Cu 2.54×102 5.42 49.39 11.71 33.49 Pb 6.44×10 0.68 41.11 17.11 47.12 Ni 8.57×10 8.84 27.44 18.88 44.84 Cd 6.26 14.67 19.41 22.67 43.18 S4 8.57 Cu 6.59×10 3.11 31.83 2.89 62.16 Pb 3.99×10 5.97 26.39 16.46 51.19 Ni 7.12×10 12.22 7.14 13.67 66.97 Cd 5.41 20.54 21.77 19.25 38.49 表 2 热处理前后基于PBET生物可给性
Table 2. Bioavailability of in PBET before and after thermal treatment
采样点位编号 处理温度/℃ Cu生物可给性/% Pb生物可给性/% Ni生物可给性/% Cd生物可给性/% GP IP GP IP GP IP GP IP S1 未处理 8.28 7.88 1.70 0.33 1.22 0.77 0.38 0.04 200 8.88 6.23 1.42 0.36 0.99 0.83 0.29 0.06 400 12.32 6.67 1.20 0.47 1.35 3.02 0.43 0.03 600 17.64 11.74 17.71 1.63 8.26 8.21 0.24 0.03 S2 未处理 0.00 6.61 3.21 12.21 3.15 5.18 0.57 0.07 200 1.56 5.96 11.11 13.39 8.56 4.83 0.92 0.09 400 2.18 3.25 2.30 28.93 9.51 5.35 1.08 0.13 600 2.59 7.53 19.48 45.14 11.84 26.55 0.47 0.12 S3 未处理 1.86 8.55 8.41 16.75 0.00 9.46 0.61 0.09 200 7.21 7.30 9.50 16.02 16.77 15.71 0.94 0.26 400 7.16 6.67 1.88 29.91 11.05 4.21 1.83 0.29 600 5.89 9.76 8.33 34.12 4.12 28.87 0.12 0.07 S4 未处理 0.00 4.54 11.27 30.51 0.00 1.75 0.73 0.00 200 1.83 5.05 0.00 24.79 11.04 1.96 1.02 0.00 400 0.65 2.82 11.34 39.77 7.42 9.37 0.96 0.00 600 0.00 3.23 14.81 50.96 1.32 32.05 0.06 0.00 注:GP为胃阶段,IP为肠阶段。 -
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