Humic Substance Adsorptive Fractionation by Minerals and its Subsequent Effects on Pyrene Sorption Isotherms

Changes in the nonlinearity of pyrene sorption isotherms on humic substance (HS)-coated minerals (kaolinite and hematite) due to HS adsorptive fractionation processes were examined in model environmental systems at low mass fraction organic carbon (f_oc) levels (0.0001–0.0011) using purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). At a constant pH of 7, higher molecular weight (MW) fractions of PAHA were preferentially adsorbed on kaolinite whereas no adsorptive fractionation of PAHA occurred on hematite. At a constant f_oc level of 0.0005, preferential adsorption of higher MW PAHA fractions on kaolinite was enhanced with increasing pH. Nonlinear pyrene sorption isotherms were observed with the bulk PAHA-coated mineral systems, whereas more linear pyrene sorption isotherms were observed for the PAHA-mineral systems undergoing adsorptive fractionation. Although the degree of isotherm linearity may be affected by pH and/or structural rearrangement of the adsorbed HS fractions on minerals, this study suggests that HS adsorptive fractionation is more important than are changes in pH and f_oc levels with regard to the resulting pyrene sorption isotherms. Similar effects were not observed with SRFA, suggesting that the impacts of HS adsorptive fractionation on pyrene sorption isotherm nonlinearity are also influenced by the source and other biogeochemical characteristics of HS.

Key Words:

• Hydrophobic organic contaminant (HOC)
• Humic substances (HS)
• Pyrene
• Minerals
• Sorption
• Sorption isotherm
• Fractionation
• Dual-mode model.

Notes

^a Elemental composition data (ash- and moisture-free basis) for PAHA and SRFA from Ref. [ ²³ ].

^b Carbon distribution data determined by solid state ¹³C NMR integration. PAHA data from Ref. [ ⁴⁰ ]. SRFA data provided by the IHSS.

^{a a} Units in (μg/g)/(μg/L) ⁿ .

^b Units in (L/g).

^c The values represent standard errors.

^a Units in (μ g/g)/(μ g/L) ⁿ .

^b Units in (L/g).

^c The values represent standard errors.