Application of biochar derived Sewage Sludge on fractionation of Copper and Zinc in the presence of earthworms in calcareous contaminated soils

Human activities, including the use of sewage sludge as fertilizer, cause excessive accumulation of heavy metals in the soil. The conversion of sewage sludge to biochar is a potential way its disposal and is a cost-effective technology for the remediation of soils contaminated with heavy metals and the environment due to the reduction of bioavailability of heavy metals. Also, using biological methods such as the use of soil organisms, including earthworms, is a new and promising way to improve contaminated soils. Several studies have been done about the effects of biochar and earthworms on fractionation of heavy metal at the world, but a report has not yet been presented about the effect of integrated application of biochar and earthworms on fractionation of copper and zinc elements. Therefore, the present study was carried out to investigate the effect of pyrolysis temperature change and application rate of biochar on the Copper (Cu) and Zinc (Zn) fractions and their uptake by E. fetida earthworms in a contaminated calcareous soil from the area surrounding the old Ahangaran lead-zinc mine.

The experiment was carried out as a factorial experiment based on completely randomized design with three replications under environmental conditions of the laboratory of Soil Sciences Department of Bu-Ali Sina University in Hamedan. Experimental factors included earthworms Eisenia fetida species in 2 levels (with and without earthworms) and biochar produced from sewage sludge at two different temperatures of 300 and 600 ° C in 4 levels (0, 2, 4 and 8%) were added to contaminated calcareous soil. 12 earthworms were introduced into each container, and the containers were stored in a climatic chamber with 16 hours of light and 8 hours of darkness at 25 ° C for 42 days. The method of sequential extraction was used to investigate the fractionation of Cu and Zn. Statistical analysis of the data using SPSS and MSTATC software and means comparison was performed by the Duncan test at 5% probability level.

According to the results of the analysis, activity of the earthworm in the soil treated with biochar produced at 300 °C had no significant effect on the amount of copper and zinc in the exchangeable fraction. While reduced the amount of Cu and Zn in the exchangeable fraction and increased the amount of Cu and Zn in the residual fraction of the soil treated with biochar produced at a temperature of 600 °C. Increasing the application rate of biochar significantly reduced the amount of copper and zinc in the exchange sector, as this decrease was evident in the biohazard produced at a temperature of 600 °C. So, the activity of the earthworm at a level of 8% biochar produced at a temperature of 600 °C resulted in a decrease of the amount of exchangeable copper from 0.391 to 0.256 in comparison to the absence of earthworms. The concentration of exchangeable zinc decreased from 242.1 mgkg-1 in control treatment to 0.579 and 0.283 mgkg-1 in 8% biochar produced at 300 and 600 ° C treatments, respectively. Due to low motility of heavy metals in soils treated with biochar, the concentration of Cu and Zn in the body of the earthworm has decreased and this trend was higher in the biochar produced at 600 ° C compared to the biochar produced at 300 ° C . Therefore, heavy metals fractionation could be changed as a result of the activity of earthworms in soils treated with biochar, this trend indicates that the distribution of the metals in biochar-amended soil is gradually shifting from the more labile fractions to the more stable fraction.