Binding of methylated naphthalenes to concentrated aqueous humic acid

Abstract

In laboratory batches of 1 g/l Aldrich humic acid (HA), apparent binding coefficients (KOC,app) for 1-methylnaphthalene (1-MN) and 1,3-dimethylnaphthalene (1,3-DMN) were determined by apparent solubility (comparison of solubility in water and HA solution). For the HA batches with 1-MN, two trends were observed: delayed sorption (up to 12 days), and an increase in the apparent solubility of 1-MN as the amount of this hydrocarbon as a non-aqueous liquid was increased. These two trends suggest that formation of a colloidal phase may have interfered with our apparent solubility tests. Alternatively, the non-aqueous 1-MN may have caused a slow change in the conformation and/or aggregation of the humic acid and its binding properties. Other batch tests, using solid phase microextraction (SPME), did not have a non-aqueous hydrocarbon phase. Compared with the apparent solubility results, the SPME tests indicated similar but lower KOC,app values for binding of 1-MN and 1,3-DMN to Aldrich HA at 1 g/l HA. Also, KOC,app values for 2-MN, 1,7-DMN and 2,3,5-trimethylnaphthalene in 1 g/l HA were obtained by SPME. Other SPME tests indicated that the KOC,app values for 1-MN and 1,3-DMN in 3 g/l Aldrich HA were lower, 35–50% of those in 1 g/l HA. These significant differences are probably related to changes in the conformation and/or aggregation of the HA. With an increase in HA from 3 and 10 g/l, no significant change in KOC,app was observed for 1,3-DMN and 2,3,5-TMN, but the KOC,app value for 1-MN was slightly larger at 10 g/l. These data do not support an earlier report that HA micelles form above 7.4 g/l Aldrich HA.