Humic material/substances aqueous

In addition to complexing metals in the aqueous phase, humic materials can also remove metals and radionuclides contained within the mineral matrix (18). The factors that control this behavior are not well understood however, it has direct implications on waste storage and containment strategies. The binding of organic and inorganic contaminants to humic substances is known to alter their bioavailability (41). Organic contaminants that are associated with humic substances are essentially unavailable for uptake by biota. In most cases studied, toxic metals associated with humic materials also have reduced uptake. With the new focus on bioremediation of polluted are as, the effect of the association of pollutants with humic materials on their phytotoxic properties must be considered, particularly for bound metals and radionuclides. 

Some of the first NMR spectroscopic applications in humic substance research involved the use of NMR for the examination of soluble humic substances.(5) The information obtained from NMR spectra of humic and fulvic acids dissolved in alkaline or neutral aqueous solutions has been quite useful in characterizing the aromatic and aliphatic structures of these complex materials. In as much as humic materials are inherently complex, the spectra show only broad signals with some fine structure which can be related to specific methyl, methylene, methine, and aromatic/olefinic hydrogens. Because NMR spectroscopy contains inherently more structural information for humic substances, the use of NMR has been limited since much of the work conducted prior to 1987. This is... 

During the last 15 years, interest in the study of aquatic humic substances has increased. The problems associated with isolating and concentrating this material from aqueous solution largely have been overcome, and humic substances can be easily extracted from any aquatic sample. Humic substances have been successfully isolated from waters with very low DOC values, such as seawater and groundwater, as well as more concentrated systems. 

Acidification of aqueous concentrates and extracts to pH near 1 is the standard procedure to precipitate humic from fulvic acid, and this procedure also has been applied to aquatic humic substances (Thurman and Malcolm, 1981). Aquatic humic substances that interact significantly with metal ions can be precipitated from water by addition of lead(Il) nitrate (Klocking and Mucke, 1969). Co-precipitation of aquatic humic materials with aluminum, copper, iron, and magnesium hydroxides has been used to recover aquatic humic substances from various types of water (Jeffrey and Hood, 1958 Williams and Zirino, 1964 Zeichmann, 1976). Humic acids can also be precipitated from an unconcentrated water sample by adding acetic acid and isoamyl alcohol to a sample contained in a separatory funnel, and after shaking, humic acid precipitates at the alcohol-water interface (Martin and Pierce, 1971). Precipitation methods are among the crudest of fractionation methods... 

Humins - the fraction of humic substances that is not soluble in water at any pH value. On this basis, humins include any humic-type material that is dissolved in non-aqueous solvents after the soil has been exhaustively extracted with basic aqueous solvents. Humins are often considered to consist mainly of humic materials strongly associated with the soil inorganic colloids. 

Laser flash photolysis studies of the well characterised soil humic substance, Armadale Fulvic Acid (P.E.I., Canada), have been carried out, with excitation at 355 nm on picosecond and nanosecond time scales. Three principal transient absorption sijtnals have been observed in aqueous solution a component with a maximum absorption at 675 nm and a lifetime of about J microsecond (at pH = 7.0), a second component with a maximum absorption at <175 nm and a lifetime of I -10 microseconds, and a third component with a broad, featureless transient absorption spectrum and a lifetime in excess of LUO microseconds. The 675 nm signal is believed to be a solvated electron on the basis of lifetime and quenchinjt data, and is observed 20 ps after excitation. The 475 nm signal is believed to he a radical cation on the basis of its concurrent appearance with the electron at 20 ps. The third featureless component emerges nanoseconds after excitation and is believed to correspond to the triplet states of the humic material. 

Organic matter extracted from earth materials usually is fractionated on the basis of solubility characteristics. The fractions commonly obtained include humic acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic acid), and humin (insoluble in alkaline solutions). This operational fractionation is based in part on the classical definition by Aiken et al. (1985). It should be noticed, however, that this fractionation of soil organic matter does not lead to a pure compound each named fraction consists of a very complicated, heterogeneous mixture of organic substances. Hayes and Malcom (2001) emphasize that biomolecules, which are not part of humic substances, also may precipitate at a pH of 1 or 2 with the humic acids. Furthermore, the more polar compounds may precipitate with fulvic acids. 

The classic definitions of soil humic and fulvic acids are based on solubility (Schnitzer and Khan, 1972). Thus, humic acid is the alkali-soluble material in soil, which is precipitated at pH 1. The material which remains soluble in the extract at pH 1 is fulvic acid. A more recent definition for aquatic humic substances is given by Thurman and Malcolm (1981). Here the material which adsorbs on an XAD column from an acid aqueous solution is defined as aquatic humus. That part of the adsorbed material which is soluble in acid and base is fulvic acid the portion insoluble in acid is humic acid. Another definition of an aquatic humic substance is based on adsorption by DEAE-cellulose columns (Miles etal., 1983). 

Natural organic matter from biosynthesis and biodegradation is ubiquitous in natural aqueous environments. Common examples of natural organic materials are extracellular polymeric matrices (biofilms) generated by bacteria, hydrocarbons from industrial effluents and fossil fuel products, and numerous humic substances. Because of their hydrophobicity, natural organic materials have low solubility in water and tend to accumulate at solid surfaces. As a result, in groundwater and soils, mineral surfaces are commonly coated with organic films. 

The aggregates of many surface-active compounds are capable of enhancing the solubility of nonpolar organic compounds by bringing them into association with the hydrophobic portion of the amphiphilic cluster (Kile and Chiou, 1989). Humic aggregates and mieelles, in particular, may incorporate nonpolar materials such as hydrocarbons, a property that gives them the ability to act as soaps and detergents and transport hydrophobic substances into the aqueous phase (Carter and Suffer,... 

The classical method for isolation of humic substances from solid-phase source materials such as soils, peat, and leonardite is alkaline extraction with aqueous NaOH, followed by precipitation of humic acid at low pH, and a series of desalting steps involving cation exchange, dialysis, etc., to obtain fulvic acid. Through this procedure, samples extracted from solid-phase materials include both... 

Knowledge of the complex chemistry of technetium is primarily based on laboratory experiments using the isotopic tracers Tc (Ti/2 = 6.02h), Tc (Ti/2 = 2.6 X 10 years), Tc (T1/2 = 90 days), and Tc (Ti/2 = 61 days). In aqueous solution, the physicochemical forms of technetium will depend on pH and redox conditions (Eh). From Eh-pH diagrams it can be deduced that TcOT is the only stable physicochemical form in aerobic natural waters. Under anaerobic conditions, however, Tc04T is reduced to Tc02, TcO(OH)2, or TcCle". Technetium-species in oxidation state IV may associate with organic material such as humic substances in anoxic sediments, and thereby behave as organic colloids. 

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