Inferences from TPHs removal kinetics during phyto- and myco-remediation of a soil heavily contaminated with crude oil

Despite existing knowledge gaps such as the specific concentrations to which soils heavily contaminated with total petroleum hydrocarbons (TPHs) should be diluted for effective application of phyto and –myco-remediation agents and how enhancing agents influence TPHs removal rates at specific time intervals; kinetic modelling of TPHs removal during phyto- and myco-remediation can provide insightful inferences with practical implications. Therefore, in this study, soils heavily contaminated with crude oil were collected, and phyto- and mycoremediation carried out, both with and without the addition of Tween 80, over a period of 90 days, accompanied by concurrent soil analysis. The results revealed up to 445 grams of total petroleum hydrocarbons per kilogram of dry soil (g/kg dry soil), representing approximately 50 % TPHs contamination in the soil; and remediation efficiencies of 19, 68, 74, 87, and 88 %, for natural attenuation, sunflower, ferns, fermented palm wine, and the white rot fungus (Pleurotus ostreatus), respectively. Optimisation with Tween 80 increased the respective efficiencies to 31, 96, 93, 98 and 95 %. Kinetic modelling of the data revealed that natural attenuation of the highly contaminated soil proceeded predominantly by zero-order kinetics, which explains why natural attenuation is often ineffective for such soils. The phyto- and myco-remediation treatments shifted the removal kinetics from zero- order towards pseudo-first-order (PFO), and pseudo-second-order (PSO). The kinetic modelling, combined with tolerance limits, has been used to project the ideal initial (starting) concentrations for each agent. Thus, over the 90 days, the optimal initial concentrations are as follows: natural attenuation < 5 % of TPHs in soil; sunflower (Helianthus annuus) and ferns (Dryopteris affinis) < 9 %; fermented palm wine < 18 %; and Pleurotus ostreatus < 23 %. Finally, while appropriate dilution is necessary for optimal progression of natural attenuation and most phyto- and myco-remediation treatments, a thorough understanding of tolerance limits and removal kinetics will facilitate better decision-making during phyto- and myco-remediation.