Low-molecular-weight organic acids impede the degradation of naphthol in iron oxides/persulfate systems: Implications for research experiments in pure conditions.

Naphthols are industrial contaminants occurring widely in soils and waters. Remediation of organic pollutants can be done by chemical oxidation using persulfate. However, most research experiments testing degradation of organic pollutants have been done in ideal conditions, e.g. using a pure compound in pure water, and thus are weakly representative of real natural conditions where pollutants occur in complex mixtures of numerous organic compounds. Therefore we tested here the effect of the presence of small organic acids, as typical compounds occurring in natural media, on the degradation of 1-napthol with persulfate and iron oxides. Results show that organic acids decreased naphthol transformation by 3.7% for malic acid, 53.2% for tartaric acid, 72.3% for citric acid and 77% for oxalic acids, in a magnetite/persulfate system during 10 h. Meanwhile, the dissolved Fe species increased gradually with the reaction time; the highest concentration of Fe ions reached to ∼18 μM L-1 in aqueous phase. Electron paramagnetic resonance technique was applied to determine reactive oxygen species (ROS). The spin density of ·OH, detected as the main ROS, decreased initially, followed by gradually increase, suggesting that organic acids might inhibit the degradation of 1-naphthol by competing with ·OH. These findings disclose the high inhibition of the transformation by organic acids, and thus, more generally, imply that studies using only pure contaminants are weakly representative for remediation of real, natural samples.