Assessment of the effectiveness of biochar in reducing soil heavy metal mobility

Thesis Abstract

Soil contamination by heavy metals poses a significant environmental and public health challenge, especially in urban and industrial regions where anthropogenic activities have intensified metal accumulation in soil profiles. The mobility and bioavailability of heavy metals such as lead (Pb), cadmium (Cd), arsenic (As), and zinc (Zn) are critical factors influencing their environmental impact and potential uptake by plants and humans. This study aims to evaluate the effectiveness of biochar amendments in reducing the mobility of heavy metals in contaminated soils, with a focus on optimizing remediation strategies to mitigate environmental risks. The specific objectives are to quantify changes in heavy metal bioavailability following biochar application, explore the mechanisms of immobilization using spectroscopic and microscopic techniques, and develop predictive models of metal mobility under varying biochar application rates. The study employs an experimental research design comprising both laboratory and field components. A representative soil sample contaminated with heavy metals was collected from an industrially impacted site, with initial heavy metal concentrations serving as baseline data. A total of 100 soil samples were prepared and subjected to biochar amendments at different rates (0%, 2%, 5%, and 10% w/w). Laboratory incubation experiments were conducted over a period of 90 days to monitor changes in heavy metal mobility. For in-situ validation, a controlled field trial covering 0.5 hectare was established, with plots randomized for biochar application rates. Heavy metal speciation and mobility were assessed using atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), and sequential extraction procedures based on the Tessier method. Soil pH, cation exchange capacity (CEC), organic carbon content, and soil texture were also measured to elucidate influencing factors. Data analysis involved multivariate statistical techniques such as regression analysis and analysis of variance (ANOVA) to determine the significance of biochar amendments on heavy metal bioavailability. Structural equation modeling (SEM) was employed to explore relationships among soil physicochemical properties, biochar properties, and metal mobility. Expected results suggest that biochar significantly reduces the bioavailability and mobility of heavy metals, with rates of reduction correlating positively with biochar application rate. The immobilization mechanisms are hypothesized to involve sorption, complexation, and precipitation processes, potentially elucidated through surface characterization techniques such as scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The findings are expected to demonstrate improved soil physicochemical conditions, particularly increased pH and CEC, which contribute to metal immobilization. The study aims to fill existing gaps in understanding the quantitative effects of biochar on heavy metal mobility under field conditions, especially considering different application depths and biochar types. This research contributes to knowledge by providing empirical evidence on the efficacy of biochar as an in-situ remediation material for heavy metal-contaminated soils, aligning with the theoretical framework of sorption and geochemical immobilization theories. It advances the understanding of soil-metal interactions mediated by biochar and offers a predictive capacity for environmental managers. The main conclusion advocates for the strategic use of biochar in contaminated sites based on site-specific soil conditions and contamination levels, recommending optimal application rates to maximize remediation efficiency while maintaining soil health. Policy implications include promoting biochar as an eco-friendly, cost-effective soil amendment in heavy metal remediation programs, with further research needed to evaluate long-term stability and potential environmental trade-offs. Recommendations for future investigations include examining biochar modifications to enhance binding capacity, assessing impacts on soil microbial communities, and scaling pilot applications for broader validation. The study ultimately aims to inform sustainable land management practices and contribute to safer agricultural and urban environments.