Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Nucleophilic aliphatic substitution reactions of propachlor, alachlor, and metolachlor with bisulfide (HS-) and polysulfides (Sn2-).

Reactions of bisulfide and polysulfides with alachlor, propachlor, and metolachlor were examined in aqueous solution to investigate the role reduced sulfur species could play in effecting abiotic transformations of chloroacetanilide herbicides. Experiments at 25 degrees C demonstrated that reactions were approximately first-order in HS- concentration and revealed that polysulfides are considerably more reactive than HS-. delta H not equal to values for reactions of the three chloroacetanilides with HS- are statistically indistinguishable at the 95% confidence level, as are delta S not equal to values, despite significant differences in second-order rate constants (kHS-). Transformation products were characterized by GC/MS (in some cases following methylation) and were found to be consistent with substitution of chlorine by the sulfur nucleophile. Products containing multiple sulfur atoms were observed for the reactions of chloroacetanilides with polysulfides, while products resulting from reaction with HS- only possessed a single sulfur atom. When second-order rate constants at 25 degrees C are multiplied by HS- and polysulfide concentrations reported in salt marsh porewaters, predicted half-lives range from minutes to hours. HS- and especially polysulfides could thus exert a substantial influence on the fate of chloroacetanilide herbicides in aquatic environments. Oxidation of the resulting sulfur-substituted products could generate ethane sulfonic acid derivatives, previously reported as prevalent chloroacetanilide degradates.

Effect of flue gas desulfurization (FGD) by-product on water quality at an underground coal mine.

In this paper, a field study was carried out to examine the effect of flue gas desulfurization (FGD) by-product on water quality at an underground coal mine in central-eastern Ohio. Flue gas desulfurizalion by-product was injected into the down-dip portions of the Robert-Dawson mine in an attempt to seal major seeps exiting the mine and to coat exposed pyritic surfaces. Immediately following grout injection, significant increases in acidity, iron, aluminum, sulfur, and calcium were observed at most surface and ground water locations near where grouting was carried out. Following this initial flush of elements, concentrations of most constituents have decreased to near pre-grouting levels. Data from the site and geochemical modeling suggest that an increase in water level or rerouting of drainage flow resulted in the dissolution of iron and aluminum sulfate salts and ferrihydrite. Dissolution of the FGD grout material resulted in increases in calcium and sulfate concentrations in the drainage waters. Water within the mine voids was saturated with respect to calcium sulfate and gypsum immediately following grout injection. Based on an analysis of core samples obtained from the site, acid mine drainage (AMD) was in contact with at least some portions of the grout and this resulted in grout weathering. Subsequent transport of calcium and sulfate to the underclay, perhaps by fracture flow, has resulted in the deposition of gypsum and calcium sulfate solids.

Effect of detector wavelength on the determination of the molecular weight of humic substances by high-pressure size exclusion chromatography.

High-pressure size exclusion chromatography (HPSEC) has proven to be an effective method for determining the molecular weights (MW) of humic substances (HS) from a variety of aquatic and terrestrial environments. HPSEC systems often use a variable wavelength UV-vis detector, which detects the analytes based upon their chromophoric composition. HS contain a range of moieties with chromophores having unique molar absorptivities (for a given wavelength), and the calculated MW may be dependent upon the wavelength chosen for the analysis. As a consequence, the choice of wavelength becomes an important parameter for the reliable determination of MW by HPSEC. The effect of UV-vis detector wavelength on the determination of the MW distribution of selected humic and fulvic acids by HPSEC is examined in this paper. For the HS examined, both the number (Mn) and weight average (Mw) MW increased with increasing wavelength. The relative increase in MW was most pronounced for Lake Fryxell fulvic acid, with a 63 and 21% increase in Mn and Mw, respectively, between 220 and 380 nm. The increases observed for Suwannee River humic and fulvic acids were less pronounced. Mn was more sensitive to changes in detector wavelength than Mw, and as a result the target HS appeared to be less polydisperse at higher wavelengths. Within the range of wavelengths commonly used for the determination of MW of HS by HPSEC (i.e., 220-280 nm), the magnitude of the increases in MW was not significant compared to variability in MW that results from changes to other operational parameters in HPSEC.