Humic acids precipitation

Humic acid precipitates at salt concentrations >0.05 M and pH <4, while the fulvic acids are still soluble. Thus, humic acid is hardly ever found in salt water ( O.S M). Titration experiments with alkali indicate that there are consecutive steps of deprotoni-zation. A typical average pka-value would be 7 (26), and for a selected humic acid values of 4 and 9 are given for pk and pk, (27, 28). 31 aZ... 

The insoluble material was removed by centrifuging at 5000 rpm for 15 min in a Beckman Model No J-6B centrifuge. This extraction with alkali was repeated twice more and the solutions of sodium salts of acids were collected. Humic acids were precipitated from the combined NaOH solutions by adjusting the pH to 1 with 2N HCl slowly with stirring and the mixture was left overnight. The precipitated humic acids were collected by filtration through Whatman IMM paper and washed with O.IN HCl. The filtrates were extracted three times with ethyl acetate and the extracts dried over sodium sulfate and evaporated, the residue constituting the fulvic acids. Buth fulvic and humic acids (precipitates) were air-dried, and then dried in a vacuum desiccator over phosphorus pentoxide at room temperature. 

Humic and flilvic acids are traditionally extracted from soils and sediment samples as the sodium salts by using sodium hydroxide solution. The material that remains contains the insoluble humin fraction (Figure 3). The alkaline supernatant is acidified to pH 2 with HCl. The humic acid precipitates and the fulvic acid remains in solution with other small molecules such as simple sugars and amino acids. These molecules can be separated by passing the solution through a hydrophobic resin, such as the methacrylate cross-linked polymer, XAD-8. The fulvic acids will sorb to the resin while the more hydrophilic molecules pass through the column. The fulvic acid can be removed with dilute base. 

The insolubility of humic substances in nonpolar organic solvents has limited the use of solvent extraction as a method of isolating humic substances from water. The most effective method for solvent extraction was reported by Eberle and Schweer (1974). Humic acid was efficiently extracted with trioc-tylamine/chloroform at pH 5 and was recovered by back-extracting with water at pH 10 or above. Butanol has been used to extract freeze-concentrated humic substances however, not all the material was extracted (Shapiro, 1957). Another method involves acidification of a sample with acetic acid, followed by extraction with isoamyl alcohol. Humic acid precipitates at the interface (Martin and Pierce, 1975). This method is slow 5 hours were required to extract 100 mL of sample. No data on the behavior of fulvic acid in this solvent extraction were presented. 

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... 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

In another example, a multiresidue method using HPLC/ESI-MS was developed to determine six imidazolinone herbicides in five different soil types. Good recoveries (80-120%) and adequate sensitivity at the 2.0 ngg level were obtained for the compounds investigated. In the method, a 50-g soil sample was extracted for 1 h in 0.5N NaOH solution. A portion of the extract was acidified, to precipitate the humic acids, and the supernatant was then loaded on to a preconditioned trifunctional Cig SPE cartridge and eluted with ethyl acetate. Further cleanup was achieved using a tandem strong anion-exchange (SAX)-SCX SPE combination. Analytes were eluted... 

The aqueous fraction was acidified to pH 1 with 6N HC1, and the small amount of humic acids which precipitated was removed by filtration. The filtrate was extracted three times with 100-ml portions of ethyl acetate. The organic extracts were combined, dried over anhydrous sodium sulfate, and filtered. The solvent was removed by rotary evaporation and the residue contained the freed byproducts from the hydrolyzed esterified and insoluble-bound compounds. 

In a recent study of the complexation of technetium with humic acid (HA) Sekine et al. [34,35] obtained interesting results which show competition between Tc,v-0(0H) i precipitate formation and Tcin-HA precipitate formation during a reduction process of pertechnetate with Sn2+. A weighable amount of... 

Many researchers have attempted to unravel the mystery of the structure of humus. One approach has been to isolate fractions by extracting humus using various extraction procedures. These procedures result in the isolation of three or more fractions humic acid, fulvic acid, and humin. Humic material is isolated from soil by treating it with alkali. The insoluble material remaining after this treatment is called humin. The alkali solution is acidified to a pH of 1.0 and the precipitate is called humic acid, while the soluble... 

A fractionation procedure has been established and widely applied to studies of humic materials [42-44]. The procedure begins with natural OM (i.e., humus) and uses an aqueous basic solution (e.g., 0.1-0.5 mol/1 NaOH and Na2C03) to solubilize a fraction of the OM. The basic extract is then acidified which causes a precipitate to form, i.e., humic acids (HA). The fraction, which remains in solution, is called fulvic acids (FA). Humin is the name given to the insoluble organic fraction that remains after extraction of humic and fulvic acids. At nearneutral pH (pH 5 - 8), which is characteristic of most natural water, the FA are the most water soluble of these three fractions. HA are somewhat less soluble, with their solubility increasing as the pH increases. Humin is insoluble at all pH values. 

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 toxin is also likely to be adsorbed or complexed by soil humic acids. If the reaction is a simple adsorption reaction, all or part of the toxin might later become available for absorption by a receiver plant. If the toxin is complexed or precipitated by its reaction with soil humic substances, then it would be deactivated. 

If the iron is present as humic acid complexes,11 these can be coagulated with alum (Section 14.2). Instead of trying to precipitate the iron, it may be better to keep it in solution, in which case it can be complexed with a chelating agent such as NTA3- or EDTA4. As a last resort, Fe2+ or Fe3+ can be removed by cation exchange, but the absorption on the zeolite or resin is usually irreversible. 

Step 5. Adjust pH to 1.0 with HC1 to precipitate humic acid. Separate humic and fulvic acids by centrifugation. Rinse humic-acid fraction with distilled water until AgN03 test shows no Cl . Dissolve humic acid in 0.1 N NaOH and hydrogen saturate by passing solution through cation-exchange resin in H-form. 

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