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

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 flat 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 m×3 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 - HClO4 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 findings of our study are providing a new exploration for the remediation of heavy metals contaminated farmland soils in various areas of China.