Role of humic acid on benzo[a]anthracene: Insights from aging on adsorption, speciation distribution and bioavailability

As a crucial component of soil organic matter, humic acid (HA) persists in soil and exert a complex interaction with hydrophobic organic pollutants, yet its specific role still remains unclear. In this study, HA was obtained from weathered coal via alkaline dissolution and acidic precipitation for the adsorption of benzo[a]anthracene (BAA). Subsequently, an aging simulation was employed to assess its long-term performance. The results demonstrated a theoretical maximal adsorption capacity of 0.29 mg/g for BAA on HA, and aging led to a 32.7% decline in its adsorption performance. While the speciation distribution of sorbed BAA remained largely unchanged. Combined with the characterization results, it can be inferred that the surface interactions between HA and BAA, including partitioning, van der Waals forces, hydrophobic interactions and hydrogen bonds, could be identified as the mechanisms underlying the desorbing BAA fraction, whereas strong hydrogen bonds, n-π and π-π staking contribute to the formation of non-desorbing BAA. The bound residue fraction of BAA was ascribed to the internal sequestration by porous structure in HA. Upon inoculation with BAA-degrading bacteria, all three fractions of sorbed BAA were decreased, demonstrating their availability. Further applied HA into soil media, a significant passivation effect was obtained as a substantial amount of desorbing and non-desorbing BAA was converted into bound-residue and non-extractable fractions. Overall, this study highlighted the potential of HA as adsorbent for mitigating PAHs environmental activity and its synergistic remediation with microorganisms.