Determining the Optimal Enhancement Pattern for Heavy Metal Contaminant Removal in the Electrokinetic Method Based on Contaminant Retention Phases in Soil and Surface Potential Changes of Clay

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Determining the Optimal Enhancement Pattern for Heavy Metal Contaminant Removal in the Electrokinetic Method Based on Contaminant Retention Phases in Soil and Surface Potential Changes of Clay

ارائه دهنده: Provider: Mohammadamin Farahpour

اساتید راهنما: Supervisors: Vahidreza Ouhadi

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Mohammad Maleki, Reza Ghiasi, Alireza Bagherie

زمان و تاریخ ارائه: Time and date of presentation: 2024

مکان ارائه: Place of presentation: دپارتمان عمران

چکیده: Abstract: In recent years, various remediation techniques have been employed by researchers to clean up contaminants from clayey soils. One of the eco-friendly methods used is electrokinetic remediation. However, due to the poor performance of this technique in carbonate-rich clays, researchers have sought to enhance its overall efficiency. A review of past studies reveals that there has been little focus on applying theoretical and foundational principles before conducting experiments, such as determining the physical and geo-environmental characteristics of the clay, identifying contaminant-retention phases, and understanding the changes in the surface potential of clay particles. To overcome these challenges, researchers have often relied on trial and error, introducing random variables to improve efficiency. However, this approach has led to inconsistent contaminant removal, accumulation near the cathode, excessive use of enhancement agents, and a lack of understanding of the mechanisms involved in contaminant removal. In electrokinetic experiments, various mechanisms such as electrolysis, electroosmosis, and ionic migration affect the system. These mechanisms alter the surface potential of clay particles, the contaminant-retention phases in the clay, and functional groups, which in turn influence the remediation process. Therefore, a better understanding of the interactions between these parameters can help improve the efficiency of electrokinetic remediation. For instance, understanding the key components of clay particles in their interaction with heavy metal contaminants can guide the selection of the most suitable and optimally concentrated enhancement agents. This understanding can reduce costs in real-world projects and improve the overall efficiency of remediation methods. The main objectives of this research are: • To develop a framework for optimizing the enhancement of heavy metal removal in electrokinetic remediation based on contaminant-holding phases and surface potential changes in the clay. • To design and develop laboratory equipment for electrokinetic experiments based on previous studies and the proposed theoretical model. • To provide theoretical and practical foundations for the interaction process between heavy metals and kaolinite to optimize the removal of heavy metal contaminants using electrokinetic methods. To achieve these objectives, the interaction process between soil, heavy metal contaminants, and enhancement agents was first studied and determined through microstructural experiments. Then, an optimized enhancement model was designed to increase the efficiency of heavy metal removal in electrokinetic remediation. Microstructural analyses were conducted using X-ray diffraction, scanning electron microscopy, zeta potential measurements, sequential selective extraction, buffering, infrared spectroscopy, and chemical washing. Kaolinite soil with varying carbonate contents, lead as the heavy metal contaminant, and different enhancement agents such as acetic acid, hydrochloric acid, and EDTA in concentrations ranging from 0 to 200 cmol/kg-soil were used. To validate the results, similar experiments were also conducted on bentonite samples. The results showed that increasing the concentration of heavy metal contaminants and the carbonate content of the soil reduced the magnitude of the clay particle surface potential, leading to decreased electroosmotic flow and lower efficiency in the electrokinetic experiments. Sequential extraction experiments demonstrated that EDTA, even at low concentrations such as 0.01 M, could form stable complexes with lead contaminants and remove over 90% of lead from carbonate phases across alkaline, neutral, and acidic environments. Additionally, chemical washing experiments revealed that EDTA, even at low concentrations (e.g., 1 cmol/kg-soil), was significantly more effective than acetic acid and hydrochloric acid in desorbing lead contaminants. Summarizing the results, it was shown that the electroosmotic mechanism from the anode to the cathode could be enhanced using low concentrations of EDTA, while the ionic migration mechanism from the cathode to the anode could be strengthened with higher concentrations of EDTA. After understanding the interactions between the influencing components, a set of enhancement patterns for electrokinetic experiments was designed, using 0.05 M acetic acid and EDTA in the cathode reservoir and concentrations ranging from 0.5 to 30 cmol/kg-soil in the soil cell to further enhance the electroosmotic and ionic migration mechanisms. In general, the results indicate that for maximum efficiency in electrokinetic experiments, appropriate enhancement patterns should be applied at different points along the sample. Based on this study, a suitable enhancement pattern can be determined by understanding how to select, type, and concentration of enhancement agents based on clayey soil behavior before applying electrokinetic remediation. The results demonstrated that the contaminant removal efficiency in carbonate-containing kaolinite could be improved from 26.5% (baseline without enhancement agents) to around 55% (over 100% improvement), with more uniform contaminant removal along the sample length. In conclusion, the main innovation of this research is the demonstration that, based on the theoretical behavior of clay minerals, and by determining the relationship between changes in surface potential, functional groups, contaminant retention phases in the soil, and the impact of enhancement agents on these processes, it is possible to design optimal enhancement patterns. This can significantly increase the efficiency of heavy metal removal from soils using electrokinetic methods.

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