The impact of organic amendments on the fate and behaviour of phenanthrene in soil

Ejileugha, Chisom and Semple, Kirk (2025) The impact of organic amendments on the fate and behaviour of phenanthrene in soil. PhD thesis, Lancaster University.

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Abstract

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants that pose significant health risks to humans and to the environment. Due to their hydrophobic nature and strong affinity to organic matter, soils often act as long-term sinks for PAHs, leading to their persistence and negative impacts on soil health and function. Conventional physical and chemical remediation methods are often not environmentally friendly nor sustainable. Consequently, organic amendments offer a more sustainable, environmentally friendly, and cost-effective alternative for mitigating adverse impacts of phenanthrene in soil. This thesis investigated the effects of biochar, spent mushroom compost (SMC), SMC:biochar blends, particle size variations, and the interaction between amendment dose/type and contact time on phenanthrene extractability and/or mineralisation in soil. Additionally, it examined the influence of dimethyl sulfoxide (DMSO) on phenanthrene mineralisation. Findings revealed that both organic amendments and DMSO enhanced mineralisation kinetics by shortening lag phases, accelerating mineralisation rates, and increasing overall mineralisation extents. Among the factors studied, contact time and its interaction with amendment dose had a greater impact on mineralisation kinetics than amendment dose alone. Biochar application at concentrations ≥1% negatively affected mineralisation kinetics due to its strong sorption capacity. However, fine biochar particles, which possess higher surface area and shorter diffusion pathway, may facilitate faster desorption compared to coarse biochar. SMC, when applied in its unprocessed form containing a mixture of particle sizes, led to greater cumulative mineralisation than any single particle size fraction. Nevertheless, fine-particle SMC enhanced mineralisation rates and shortened lag phases more effectively than whole SMC, likely due to its faster decomposition and readily available nutrients. Blending SMC with biochar improved cumulative mineralisation compared to using either amendment alone, with the highest mineralisation observed in soils treated with a 5:1 SMC-to-biochar blend. DMSO treatment enhanced mineralisation kinetics, though higher concentrations (≥1.0%) could potentially pose toxicity risks, likely due to excessive PAH solubilisation and adverse effects on soil microbial communities. Additionally, available phosphorus, ammonium nitrogen, and total organic carbon were strongly correlated with mineralisation kinetics, highlighting their crucial role in PAH biodegradation. Overall, this thesis provides evidence-based insights into the efficacy of biochar, SMC, their blends, and particle size variations as effective amendments for mitigating PAH contamination in soil. It also underscores the potential of DMSO in enhancing PAH biodegradation.