Microalgal-based technology for spent lubricant oil waste biodegradation.

Abstract

The consumption of petroleum-based products has led to several world's most devastating oil spillage, with more than 4 million tons of oil-based products spilt into the environment. This has resulted in the pollution of up to 320 km of the coastline and resulted in severe damages, killing thousands of marine organisms. A single clean-up effo1i could cost over 10 billion dollars which makes some oil spill incident an environmental catastrophe. The widespread use and improper disposal of petroleum-based products such as lubricant oil has increased the likelihood of unintended spills and, subsequent long-lasting, economic, environmental and health impacts. Hence, there is a need to develop a cost-effective, sustainable, and environmentally friendly clean-up approach to replace the limited conventional treatment methods. To this end, microalgae-based treatment has gained global attention as a promising alternative to these conventional techniques due to its viability, cost effectiveness, versatile metabolic capacity, sustainability, and eco-friendliness. Despite the excellent degradative capability of microalgae, their application in recalcitrant spent oil waste (SOW) pollutants treatment is scantily reported in literature. The SOW contains recalcitrant organic pollutants, and the present degradation effo1is for their removal are limited, necessitating novel removal strategies. To this end, the use of ultraviolet (UV) light radiation to enhance the microalgae Scenedesmus vacuolatus metabolic perfonnance and degradative abilities was evaluated in this study. In addition, the morphological, metabolic, enzymatic, and transcriptomic profile of the exposed S. vacuolatus were elucidated. Also, the key process parameters on SOW degradation were modelled and optimized. Firstly, the effects of UV radiation (wavelength 254 nm and intensity 1.4 m W/cm2) on S. vacuolatus at different time intervals (2, 4, 6, 12, 24 and 48 h) were established. The 24 h UV-exposure induced the most desirable degradative traits in S. vacuolatus and was selected for further studies. These desirable traits include improved chlorophyll ( chl a = 1.89-fold, chl b = 2.02-fold), carotenoid (1.24-fold), carbohydrates (4.62-fold), proteins (1.44-fold) and lipid accumulations (1.40-fold). Additionally, the 24 h UV-exposed S. vacuolatus exhibited an enhanced substrate affinity (1/Ks) (0.959) and specific growth rate (µ) (0.024 h-1) (1.50-fold, 2-fold, and 1.9-fold respectively). Preliminary assessment of spent coolant waste (SCW) biodegradation using the UV-exposed S. vacuolatus resulted in significant dehydrogenase activity (55%) and total petroleum hydrocarbon (TPH) degradation efficiency (100% after 5 weeks). Moreover, response surface methodology was used to investigate the effects of temperature (25 – 40℃), substrate concentration (5 – 25% v/v) and inoculum concentration (5 – 30% v/v) on TPH degradation of SCW. The optimized process showed 100% TPH degradation of SCW within 15 days of treatment lower than the period before optimization. Additionally, the sensitivity of the process parameters on the degradation process assessed using Artificial Neural Network (ANN) revealed high sensitivity of the degradation process to operational temperature. To further enhance the degradation process, nanomaterials with catalytic potentials was incorporated during the degradation process. This resulted in increased biomass concentration (2.48-fold), specific growth rate (1.62 times), growth constant (1.23 times), 1/Ks (14.29), and consequently, achieving 100% TPH degradation in 12 days. In addition, the biodegradation kinetics showed high biodegradation rate constant (K) and shorter half-life (T1/2) of 6.65 m1 t−1 and 0.08 days, respectively, especially in the degradation of monoaromatics, and PAHs which are important environmental pollutants. The degradation kinetics was best elucidated by the second-order reaction, suggesting SCW concentration is inversely proportional to the degradation half-life (T1/2). Using the obtained metabolic by-products via GC-MS analysis, enzymatic and transcriptomic assessment, a total of 116 metabolic pathways were strongly affirmed in UV_Sv+SCW. Of the 116 metabolic pathways linked to SCW HCs degradation, the UV-exposed Scenedesmus was observed to use the naphthalene degradation pathway for the oxidation of the PAHs fraction of SCW. Transcripts of biotechnological importance implicated in SCW degradation include six CoA-linked acetaldehyde dehydrogenase, three coatomer subunit alpha-3 and one arginine deiminase. Of these transcripts, the key transcript CoA-linked acetaldehyde dehydrogenase (gene name COO60DRAFT_1702088) encoding alcohol dehydrogenase enzyme was specifically used by UV-exposed microalgae to breakdown the aromatic compounds. This study further underscores the suitability of UV radiation as a physical agent to improve the degradative capability of S. vacuolatus through genetic modification and photosynthetic biomolecular apparatus enhancement. Moreover, the positive effects of metallic oxide nanoparticles on S. vacuolatus metabolism for an improved hydrocarbon waste degradation resulting from significant improvement in S. vacuolatus substrate affinity, growth rate, process kinetics and efficiency were elucidated. Hence, this study demonstrates a highly effective method for the degradation of TPH in SCW through the application of UV-exposed S. vacuolatus. The significant enhancement in SCW hydrocarbon biodegradation achieved in this study introduces an innovative approach for the treatment of recalcitrant hydrocarbons, with potential applicability to various environmental pollution contexts. Additionally, the microalgae-based treatment developed herein offers a cost-effective alternative for the degradation of environmental contaminants. The insights gained from this study will contribute to the advancement of environmentally friendly and efficient remediation strategies for recalcitrant organic pollutants, fostering the development of sustainable and green environmental practices.