Kinetic Behavior and Rate-Controlling Mechanisms Governing the Simultaneous Sequestration of Polycyclic Aromatic Hydrocarbons (PAHs) and Heavy Metals in Contaminated Soil Using Bone Biochar over Extended Contact Time

Abstract

The introduction of crude oil into soil causes the toxic, persistent, and bio-accumulative polycyclic aromatic hydrocarbons (PAHs) and heavy metals to be present in the soil, raising serious environmental and health concerns. The kinetic behaviour and rate-controlling mechanisms in sequestration of PAHs and selected heavy metals in crude oil contaminated soil with the application of bone biochar in a long contact time were investigated. Sixteen priority PAHs and selected metals (cadmium (Cd), chromium (Cr), nickel (Ni), and lead (Pb)) were monitored throughout the remediation process, including at 21, 42, 63, 84, and 105 days. The results indicated progressive reduction of the contaminants with time of contact, which suggested that the sequestration was better as the time of contact with the bone biochar increased. After 105 days, the total PAHs were reduced from 47.11 mg/kg in the untreated soil to 3.83 mg/kg, which corresponds to a removal efficiency of 91.87%. Significant reductions were also observed for Cd (75.14%), Cr (76.05%), Ni (93.38%), and Pb (89.88%). Kinetic studies confirmed that the pseudo-second-order model fits well in the adsorption process for all the contaminants, with high correlation coefficients for total PAHs (R² = 0.9950), Ni (R² = 0.9970) and Pb (R² = 0.9876), indicating the leading role of chemisorption mechanisms. Mechanistic evaluation showed that the mechanisms of contaminant sequestration were through surface adsorption, intraparticle diffusion, pore filling, ion exchange, electrostatic attraction, surface complexation, and mineral precipitation. The porous structure and mineral content of the biochar had a positive effect on the stabilization of contaminants in the biochar matrix. The adsorption process exhibited a multi-stage behavior: a fast initial adsorption layer, a slower diffusion-controlled adsorption and then stabilization at equilibrium. This study demonstrates that bone biochar is an efficient, sustainable, and low-cost material for soil remediation of PAHs in combination with heavy metals (HMs) for long-term use.