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煤矿开采利用过程中,天然放射性物质所产生的辐射是周边环境辐射的主要来源[1]。据统计,我国煤矸石年排放量达3.8亿t,目前已累计堆放煤矸石总量超过50亿t,规模较大的煤矸石山约1500座,其中有超过300座具有自燃危险[2]。由于矿区大量煤矸石的露天堆放,其中的放射性核素(如铀、钍、镭等)在经风化、雨淋、地表径流等外界作用后,向周边环境迁移、扩散,最后进入水环境和土壤环境中,通过各种途径被人体吸收,威胁人体健康,并对生态环境构成一定威胁[3]。近些年来,有关矿区煤矸石、土壤环境的放射性污染及其风险评价问题一直受到学者的广泛关注。许乃政等[4]对华东地区含煤岩系天然放射性水平进行评价。其中,普通煤田矿区的原煤、煤矸石等固体样品核素238U、232Th、226Ra、40K含量处于正常水平;而石煤矿区的固体样品的238U、226Ra核素富集明显;Zhou等[5]通过对淮南矿区煤矸石进行燃烧模拟实验,研究了煤矸石在燃烧过程中天然放射性核素的富集和挥发,并对其燃烧产物(炉渣)进行取样,结果表明,这些放射性核素主要存在于残渣中,且对周围环境没有造成直接的影响;Seref [6]测定土耳其某燃煤电厂附近土壤U和Th含量,采用土壤地累积指数、富集因子、内梅罗综合污染指数法进行风险评价,结果表明电厂周边土壤基本未受U和Th污染;Liu等[7]报道了华南某铀矿库周围土壤和沉积物中放射性核素的含量分布及来源解析,结果显示238U、226Ra和232Th的最高浓度出现在尾矿砂中,而40K的最高浓度出现在农田中;Belyaeva等[8]研究了埃里温市土壤的放射性污染特征,结果显示以火成岩为主的北部和中部,天然放射性核素水平最高,火力发电厂和金属精炼厂的运营并未影响邻近土壤中放射性核素浓度;张彬等[9]通过估算广东某铀废石堆周边土壤外源U输入通量,并结合放射性核素的逐级提取试验,发现铀废石堆对周边土壤产生了显著的放射性污染,且距废石堆愈近,土壤中外源U的输入通量愈大。
煤炭与很多天然放射性元素伴生,煤矿的开采利用使这些放射性元素随之迁移扩散,煤矸石的长时间堆积也造成这些元素进入周边环境并带来严重的环境污染问题[10]。为评估矸石堆对周边土壤的影响,以谢桥矿区矸石堆及周边土壤为研究对象,测试分析了所堆积煤矸石的物化特性,测定了煤矸石及土壤中放射性核素238U、232Th的活度浓度和赋存形态,并采用地累积指数法对采样点的土壤进行综合评价,研究可为矿区土壤修复提供理论支持。
小尺度矸石堆场及其周边土壤中放射性元素特征分析及风险评价
Characteristic and risk assessment of radioactive elements in small scale gangue dump and surrounding soil
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摘要: 煤矸石的露天堆放,不仅侵占大量的土地资源,还会带来严重的环境污染问题。为研究煤矸石中放射性核素对周边土壤环境的影响,采集煤矸石及矸石堆周边土壤样品,测试样品中放射性核素238U、232Th的活度浓度,分析煤矸石及周边土壤中放射性核素238U、232Th的分布特征,采用地累积指数法对土壤污染状况进行综合评价。结果表明,矿区煤矸石主要以粘土矿物为主,而粘土矿物是吸附铀、钍的关键性矿物;土壤中放射性元素238U、232Th活度范围分别为31.51—58.81、53.33—94.48 Bq·kg−1,远低于煤矸石,其活度浓度呈现出与矸石堆距离增加而减小的分布趋势;煤矸石中238U、232Th主要以残渣态为主,土壤样品中238U赋存形态以有机结合态和碳酸盐结合态最为突出,而232Th总体上以残渣态为主;地累积指数法评价结果表明,S1-S9号采样点处Igeo(238U)在不同深度段均小于0,表明周边土壤并未受到238U这种元素的污染,而靠近矸石堆的表层土壤(0—20 cm)受到232Th轻微污染,其污染范围限定在距矸石堆30 m以内。Abstract: The open-air stacking of coal gangue not only encroaches on a large amount of land resources, but also brings serious environmental pollution problems. In order to study the influence of radionuclides in coal gangue on the surrounding soil, coal gangue and soil samples were collected in the gangue piles and surrounding areas. The activity concentration and occurrence of radionuclides 238U and 232Th in coal gangue and soil samples were determined. By adopting the geoaccumulation index method, the pollution indices were calculated to assess soil pollution extend. The results showed that the coal gangue in mining area was mainly composed of clay minerals, which were the key minerals to adsorb uranium and thorium; The activity ranges of 238U and 232Th in soil were 31.51—58.81、53.33—94.48 Bq·kg−1, respectively, which were much lower than that of coal gangue. The concentrations of 238U and 232Th in soil showed a decrease trend, with the increase of distance from gangue pile; 238U and 232Th in the coal gangue were mainly residual states. In the soil samples, 238U occurred in organic and carbonate states, while 232Th was mainly in residual state; The geoaccumulation index showed that Igeo (238U) at sampling point S1-S9 was less than 0 in different depth, so the surrounding soil was not polluted by 238U, while the surface soil (0—20 cm) near the gangue pile was slightly polluted by 232Th, and its pollution range was limited to 30 m.
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Key words:
- gangue pile /
- soil /
- radionuclides /
- occurrence characteristics /
- risk assessment
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表 1 地累积指数评价分级标准
Table 1. Evaluation and classification standard of geological accumulation index method
污染等级 Pollution level 地累积指数 Geoaccumulation index 污染程度 Pollution degree 0 Igeo<0 无污染 1 0≤Igeo<1 轻微污染 2 1≤Igeo<2 轻度污染 3 2≤Igeo<3 中度污染 4 3≤Igeo<4 重度污染 5 4≤Igeo<5 严重污染 6 Igeo≥5 极严重污染 表 2 煤矸石主量元素含量(质量分数,%)
Table 2. Major element content of coal gangue
样品
Sample编号
NumberingSiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 煤矸石 gs-1 52.27 26.12 8.23 9.05 0.87 0.74 1.35 0.99 gs-2 52.4 25.8 7.27 10.22 0.81 0.84 1.23 1.04 gs-3 52.55 25.73 6.45 11.18 1.01 0.92 0.83 0.91 平均值 52.41 25.88 7.32 10.15 0.90 0.83 1.14 0.98 表 3 周边土壤样品中天然放射性核素比活度分析(Bq·kg−1)
Table 3. Analysis of natural radionuclide ratio activity in surrounding soil samples(Bq·kg−1)
核素
Radionuclid深度/cm
Depth平均值
Average中位数
Median最小值
Minimum最大值
Maximum标准差
Standard deviation238U 0—20 cm 50.89 52.97 39.41 58.81 6.66 20—40 cm 38.45 38.18 36.60 40.48 1.35 40—60 cm 32.00 32.13 31.51 32.39 0.34 232Th 0—20 cm 83.89 83.28 77.01 94.48 5.79 20—40 cm 73.79 74.00 72.04 75.57 1.11 40—60 cm 55.40 55.82 53.33 57.75 1.45 表 4 土壤剖面不同深度段238U、232Th各形态比例统计(%)
Table 4. Statistical fraction proportions of 238U and 232Th in different depth sections of soil profile
核素
Radionuclid深度/cm
DepthF1 F2 F3 F4 F5 238U 0—20 5.84 27.28 7.21 54.67 4.99 20—40 6.4 30.42 10.82 45.04 7.31 40—60 4.97 37.09 7.58 42.83 7.53 232Th 0—20 9.72 11.18 22.44 6.54 50.11 20—40 6.6 11.43 20.57 10.34 51.06 40—60 6.13 10.44 21.29 8.32 53.82 表 5 地累积指数法评价结果
Table 5. Evaluation results of geological accumulation index method
核素
Radionuclid采样点
Sampling point地累积指数
Geoaccumulation index采样深度0—20 cm
Sampling depth 0—20 cm采样深度20—40 cm
Sampling depth 20—40 cm采样深度40—60 cm
Sampling depth 40—60 cm238U S1 −0.08 −0.62 −0.58 S2 −0.10 −0.68 −0.58 S3 −0.23 −0.74 −0.58 S4 −0.15 −0.63 −0.59 S5 −0.30 −0.70 −0.59 S6 −0.18 −0.65 −0.60 S7 −0.43 −0.76 −0.61 S8 −0.58 −0.71 −0.62 S9 −0.66 −0.76 −0.62 S1 0.25 −0.11 −0.54 232Th S2 0.21 −0.08 −0.49 S3 0.06 −0.11 −0.51 S4 0.09 −0.12 −0.49 S5 0.11 −0.10 −0.46 S6 0.02 −0.08 −0.51 S7 −0.02 −0.14 −0.56 S8 −0.03 −0.12 −0.57 S9 −0.05 −0.14 −0.58 -
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