PHYSICAL AND PHOTOCHEMICAL PROPERTIES OF SOIL AND AQUATIC NATURAL ORGANIC MATTER

Natural organic matter (NOM) comes from the degradation of plants and plankton. In inland waters, where most NOM comes from surrounding soils [1], its chemical properties may be similar to NOM from precursor soils. This hypothesis is examined here regarding NOM’s photochemical properties.

Much of NOM’s ecologically important chemistry is due to a fraction called the humic substances, HS, which are photoreactive. For example, upon absorbing light, HS produce singlet oxygen (¹O₂), a strong oxidant. Previous studies have shown relationships between molecular size, spectroscopic properties, and photoreactivity of HS [2,3]. Here, we report such relationships for whole soil NOM and its humic and fulvic acid fractions (HA and FA) in comparison to aquatic isolates from the nearby Rappahanock River.

Soil NOM (SNOM) was extracted with 0.1 M NaOH. River NOM (RNOM) was isolated by adsorption chromatography. NOM samples were acidified to precipitate HA, leaving the FA in solution. Each fraction (whole NOM, FA and HA) was concentrated by ultrafiltration, and concentrates were diluted in phosphate buffer for experiments.

Apparent molecular weights (M_w) were found by size exclusion chromatography. Absorbance spectroscopy was used to measure E₂/E₃ ratios (A₂₅₄ divided by A₃₆₅). Relative steady state ¹O₂ concentrations were measured using furfuryl alcohol (FFA) as a probe in optically matched samples irradiated at 365 nm. Here, FFA loss by reaction with ¹O₂ follows pseudo-first order kinetics, and rate constants (k_obs) are proportional to ¹O₂ concentrations.

Soil NOM results are generally consistent with other work, where an inverse correlation between M_w and E₂/E₃ is typical [3]. This was observed for the FA and HA but not unfractionated NOM (Table 1).

Table 1: Apparent M_w, E₂/E₃ ratios, and k_obs for FFA loss.

Sample	E2/E3	Apparent Mw (kDa)	kobs (min-1 x 103)
FA	4.65	48	5.7 ± 0.1
HA	2.90	70	5.1 ± 0.3
NOM	3.46	45	7.8 ± 0.1

Rate constants for FFA loss correlated inversely with M_w. This implies that lower molecular weight HS are more photoreactive. Ongoing work is investigating these relationships for fractionated and unfractionated RNOM.

References

[1] Stephens, B.M; Minor, E.C. Aquat. Sci., 2010, 72, 403

[2] Boyle, E.; Guerriero, N., Thiallet, A.; Del Vecchio, R.; Blough, N.V. Environ. Sci. Technol., 2009, 43, 2262

[3] Peuravuori, J; Pihlaja, K. Anal. Chim. Acta, 1997, 337, 133