粉煤灰改良重金属污染农田的修复效果植物甄别

Remediation effects of heavy metals contaminated farmland using fly ash based on bioavailability test

为了评判粉煤灰作为重金属钝化剂在实际重金属污染土壤改良修复中的应用潜力,该试验以潼关县某村庄因散户冶炼黄金导致的重金属污染农田为研究对象,运用粉煤灰化学改良工程手段对污染土壤进行了原位修复,并通过种植植物甄别对修复效果进行了进一步分析评价。结果表明,试验区农田土壤中重金属Cd污染最为严重,其次为Hg和Pb,3种重金属的质量分数分别介于1.29~6.13,0.53~4.14和63.19~448.97 mg/kg之间,平均含量分别是《土壤环境质量标准》（GB15618-1995）二级标准值的6.17、1.64、0.33倍,其中Cd全部超出国家土壤环境质量标准二级标准,Cd、Hg、Pb污染超标率分别为100%、64.28%、7.14%。土壤Pb和Cd的生物有效性与总量呈显著正相关关系。经粉煤灰改良后,土壤Hg、Cd和Pb有效态质量分数平均值分别降低28.57%、24.36%、31.83%,粉煤灰对重金属Hg和Pb有一定的钝化作用。但植物甄别试验中,所有植物均吸收了一定的Cd,但植株中Cd含量没有达到Cd超富集植物的标准,其中曼陀罗对Cd的单株吸收量最高,为0.1498 mg/株。单位面积中繁穗苋对Cd的吸收量最高,为68.04 g/hm2,说明经粉煤灰化学改良的重金属污染农田土壤中重金属仍具有较高的生物可利用性。研究表明,采用粉煤灰进行重金属污染农田改良的手段仍需进一步研究,使修复后的土壤种植模式改变或种植低累积植物物种以阻隔Cd向作物可食部位的迁移。

Fly ash has been used as an immobilization agent over the years to stabilize the heavy metals in the contaminated soils. Fly ash is produced as by-product in coal-based power plants and available in huge quantity. For the assessment of heavy metal remediation in contaminated farmland soil by immobilization with fly ash, a field experiment in Tongguan County, Shaanxi, China was conducted using fly ash as in-situ immobilization agent, and the immobilization efficiencies of metals including cadmium （Cd）, mercury （Hg） and lead （Pb） were evaluated through phytoextraction test that planted beneficial local herbaceous plants such as Silphium perfoliatum, sunflower, trilobites ghost needle grass, Amaranthus paniculatus, ryegrass and mandala with poplar in mixed-cultivation pattern. The selected plants for bioavailability test were naturally planted on vast grasslands of Tongguan County. The study site was located in the north of the mining area of farmland. Long-time gold mining activities resulted in the huge quantity of heavy metals waste and tailing dump everywhere in the area, which contaminated local farmland. There were no adequate measures taken by local authorities to get rid of this waste, so 86.3% of the area experienced different levels of soil heavy metals pollution. The area of 1.33 hm2 fiat trapezoidal field was selected as study site to conduct the experiment. Fly ash was collected from coal-fired power plant near Tongguan County, and the pH value of fly ash was 10.58. All the studied plant seeds were collected from Yangling Agricultural Demonstration Zone. In the study site, the arable layer was fully blended with fly ash of 37.5 t/ha using a rotary cultivator. Then soil-fly ash flat land was irrigated with groundwater. The research area were divided into 22 test plots; after one month poplar trees were planted in 2 mx3 m pattern, and then when stable after 3 months, the grass seeds were planted in the spring of next year. Soil samples were collected from 0-20 cm depth and air dried at room temperature. Studied plants were randomly harvested and rinsed with tap water and then distilled water to remove traces of soil. Plants were dried in oven at 105 ℃ for 30 minutes. EPA （Environmental Protection Agency） 3050B and TCLP （toxicity characteristic leaching procedure） method were used to detect soil environmentally available and bioavailable heavy metals. Plant samples were digested in mixed acid using HNO3 - HC104 to assess heavy metal contents. The results showed that the concentration of Cd was the highest which ranged from 1.29 to 6.13 mg/kg, followed by Hg and Pb, from 0.53 to 4.14 and from 63.19 to 448.97 mg/kg respectively. The mean values of Cd, Hg and Pb were 6.17, 1.64 and 0.33 times those in the second standard value of Environmental Quality Standards for Soils in China, and the exceeding standard rates were 100%, 64.28% and 7.14%, respectively. And the biological effectiveness of Pb and Cd was positively correlated with the total concentration. Fly ash addition to soil reduced the average bioavailable concentrations of Cd, Hg and Pb, and the concentrations of Hg, Cd and Pb were reduced by 28.57%, 24.36% and 31.83%, respectively. In the phytoextraction experiment, all plants presented the ability of removing Cd, but the content of Cd within plants was less than the standard value of super enriched plants. Datura stramonium was 0.1498 mg/kg per plant for Cd, with the highest absorption capacity of Cd, and Amaranthus paniculatus was 68.04 g/hm2, with the highest absorption capacity per unit area. Our results indicate that in the heavy metals contaminated farmland after improved with fly ash, some of the heavy metals still have relatively high bioavailablity. And the planting patterns should be changed, or the crops accumulated with lower heavy metal should be selected in order to prevent heavy metals accumulation in the edible tissue of crops after planting. The fmdings of our study are providing a new exploration for the remediation of heavy metals contaminated farmland soils in various areas of China.