Browsing by Subject "environmental Sciences/Ecology"

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  • Sun, Yan (Helsingin yliopisto, 2021)
    Soil pollution is one of the world’s biggest challenges. The most common soil pollutants include heavy metals and petroleum hydrocarbons. A great number of studies have developed strategies for pollutant remediation. For the removal of heavy metals including cadmium (Cd) in the soil, electrokinetic remediation has been rapidly developed in the recent decades as a both remediation and metal resource recovery method. However, field and full-scale applications of electrokinetic remediation in the removal of soil heavy metals are rare. As for organic pollutants including petroleum hydrocarbons, biostimulation with fertilization has been widely done and highly appreciated over physical and chemical methods ecologically and economically. Due to different soil physico-chemical properties and climates, fertilizers should be chosen carefully for the best outcome. Remediation strategies are often initially validated in the laboratory, which can still fail in the field tests or application usually due to soil heterogeneity that exists even within millimeters. A false conclusion may result from an improper experimental design in the laboratory, such as insufficient true replication (pseudoreplication) that fails to represent the sampling site, and composite sampling or randomized complete block design (RCBD) that only presents the average performance of the sampling site and can result in false positive or negative results for the component samples. This doctoral thesis presents three strategies in soil remediation, i.e. electrokinetic remediation of soil heavy metal Cd (paper I), biodegradation of soil gasoline aromatics stimulated by fertilization (paper II), and optimization of experimental design (paper III). Electrokinetic remediation of Cd was monitored for 14 days, in a pilot (4 m2) test and a full-scale (200 m2) application in paddy agricultural soil, at soil layers 0-10 cm, 10-20 cm, and 40-50 cm. A voltage of 20 V was applied at both scales. In the biostimulation study, three nitrogen fertilizers, i.e. inorganic NPK, urea, and methylene urea were compared in the biostimulation of gasoline aromatics removal in the soil collected from the high Arctic permafrost active layer and experimented in the laboratory at 10 °C for 28 days. Finally, the experimental design study that was conducted in the laboratory at 4 °C for 28 days illustrated how pseudoreplication and sole use of plot averages, would affect the conclusions drawn in biodegradation studies. In the experimental design study, three ecologically independent plots of soil were collected and processed independently in parallel. The soil was spiked with gasoline and divided for natural attenuation and biostimulation with methylene urea fertilization. Biodegradation results were analyzed individually within each plot of soil (pseudoreplication) as well as in a between-plot comparison by using the mean values of the three plots (plot averages). In 14 days, electrokinetic remediation showed a successful application at both scales, especially for the soil total Cd in the surface 0-10 cm layer, with a higher removal efficiency in the pilot test (87%) than the full-scale application (74%). The final concentration of soil total Cd was below the hazard threshold set for paddy agricultural soils in China. The higher removal efficiency in the pilot test can be due to the higher voltage gradient between the electrodes. The removal efficiency of the plant available Cd was lower than the soil total Cd, which can be due to the enhanced desorption of Cd cations through cation exchange and/or dissolution by lactic acid during electrokinetic remediation. In 28 days, biostimulation with urea fertilization managed to lower the concentration of the total gasoline aromatics below the initial level (by 47%). On day 7, the observed concentration of the total gasoline aromatics was higher than the initial level in the NPK and urea fertilized treatments, which indicated an enhanced desorption and extractability of soil gasoline aromatics. The enhanced desorption and thereafter bioaccessibility of gasoline aromatics in the NPK and urea fertilized treatments could be related to the enhanced biodegradation activity compared to natural attenuation and methylene urea fertilized treatment, mainly shown between day 7 and day 28. In contrast to the successful application of electrokinetic remediation and biostimulation with urea fertilization, the effect of methylene urea in biostimulation showed positive, negative, and negligible results. The random results of natural attenuation and biostimulation with methylene urea fertilization indicated the patchiness of biodegraders even within one plot of soil. Additionally, the results based on the between-plot comparison showed a dilution effect and could pose false positive and negative results for the component soil. Although electrokinetic remediation of heavy metal Cd and biodegradation of gasoline aromatics stimulated by urea fertilization showed success in the cases studied herein, the success of these remediation strategies cannot be generalized to other soil pollution cases due to soil heterogeneity at least, according to the third study. The results suggest that ecologically independent replication be adopted in the between-plot comparison and that parallel experiments and statistical analysis with within-plot replication for each replicate plot be performed to guide a more successful practical application of remediation strategies.