The differential responses of humic acid (HA) to the combined pollution of polycyclic aromatic hydrocarbons (PAHs) and cadmium (Cd) in soil:A review

Soil, as a vital carrier of ecosystems, is currently facing the challenge of combined pollution by contaminants such as polycyclic aromatic hydrocarbons (PAHs) and cadmium (Cd). Humic acid (HA), an important component of soil organic matter, possesses differentiated adsorption, complexation, and redox regulation capabilities toward PAHs and Cd, making it a potential preferred material for the remediation of soil combined pollution. HA achieves remediation of single pollution mainly through three pathways: adsorption, microbial action, and abiotic action. For the PAHs-Cd combined pollution system, it realizes efficient remediation via multi-mechanism synergistic regulation, whose core mechanisms are adsorption-redox linkage, enhanced complexation capacity through structural modification, and disruption of the mutual constraints between pollutants by complexation competition. HA can undergo structural modification induced by microbial degradation or photocatalysis, increasing the density of functional groups such as carboxyl and phenolic hydroxyl groups as well as hydroxylation sites on aromatic rings; combined with the molecular weight fractionation effect, it optimizes the targeted action on different pollutants, further improving the efficiency of synergistic remediation. Meanwhile, relying on its strong complexation capacity, HA breaks the structure of PAHs-Cd cation-π complexes, and the released PAHs are more easily adsorbed by HA or degraded by microorganisms; it also alleviates the toxic inhibition of PAHs on Cd-reducing microorganisms, relieving the mutual constraints between pollutants. This multi-dimensional synergistic regulation mechanism enables HA to effectively address the complexity of PAHs-Cd combined pollution and break through the limitations of single remediation mechanisms. This paper summarizes the key roles of HA in the remediation of single and combined pollution, discusses the shortcomings of current research in terms of mechanism quantification, application technologies, and long-term ecological effects, aiming to lay a scientific foundation for the future application research of HA in soil pollution remediation. Fig. Mechanism of HA-mediated remediation for PAHs-Cd co-contaminated soil.