Fate and toxicity of polynuclear aromatic hydrocarbons (PAHs), substituted PAHs and heterocyclic PAHs in water

Abstract

Contamination by polynuclear aromatic hydrocarbons, also known as polycyclic aromatic hydrocarbons (PAHs), substituted PAHs (SPAHs) and heterocyclic PAHs (HPAHs) is widely reported in rivers, lakes, and estuaries. Substituted PAHs (SPAHs) are a subclass of PAHs characterized by the presence of various substituent groups, such as alkyl, nitro, and oxy groups. In contrast, heterocyclic PAHs (HPAHs) are in-ring substituted PAHs where nitrogen (N), sulfur (S), and oxygen (O) replace one of the carbon (C) in the aromatic ring. These compounds are primarily produced through anthropogenic activities such as the combustion of fossil fuels, biomass burning, and industrial processes. The enhanced physicochemical complexity of SPAHs and HPAHs caused by side chain or in-ring substitution, respectively, modifies key properties such as water solubility, hydrophobicity, and environmental persistence, ultimately influencing their behavior, transformation, and bioavailability. Alkyl substitution in SPAHs generally increases hydrophobicity, while polar substituents enhance water solubility and increase the toxicity. PAHs and their derivatives undergo transformations such as photolysis, microbial degradation, and chemical oxidation in aquatic systems. Toxicity of PAHs, SPAHs, and HPAHs varies with the molecular structure. Nitrated PAHs (NPAHs) and HPAHs containing nitrogen in the ring (PANHs) depict high acute toxicity, mutagenicity without metabolic activation, and adverse effects caused by generation of reactive oxygen species (ROS). Future research must focus on improving detection and quantification, assessing ecotoxicity and human health impacts, exploring environmental fate, and developing effective removal techniques, and regulatory strategies for the SPAHs and HPAHs.