Effects of humic acids on the sorption and bioavailability of pyrene and 1,2-dihydroxynaphthalene

Abstract

Sorption of organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) is an important process that controls their mobility and toxicity in soil. The relationship between the chemical properties of various types of humic acids (HAs) and their pyrene (4-ring PAH) sorption ability was investigated in batch experiments. Commercial HAs as well as those obtained from Umbric Andosol, Dystric Cambisol and cow manure compost were used. Additionally, fractionated Aldrich humic acid (AHA), which was obtained by fractional precipitation with ethanol, was evaluated to better understand how its chemical characteristics affect pyrene sorption. The value of the carbon-normalized partition coefficient (Kdoc ×105 L kg-C−1) for pyrene in all commercial HAs was within the expected range in the two soil HAs (0.43–2.69). Correlation analysis indicated that Kdoc values for fractionated AHA series tended to increase with increasing aromaticity (r = 0.82). Subsequently, the effects of various HAs on the toxicity of pyrene and 1,2-dihydroxynaphthalene (DHN), which is one of the major microbial metabolite of PAH such as pyrene, to a test microorganism were examined by spread plate counting by using a solid agar medium. While pyrene (a poorly water-soluble compound) did not inhibit bacterial growth, the inhibitory effect of the water-soluble DHN was apparent at 10 mg L−1. However, the addition of HA at 25 mg-C L−1 mitigated DHN toxicity. Furthermore, the detoxification effect tended to increase with increasing aromatic carbon content of HA (r = 0.94). The observed reduction in toxicity of DHN may be due to its sorption to the aromatic moiety of HA, by which the bioavailability is reduced.