Solids, humic particles

A two-dimensional numerical model was developed to describe the transport of dissolved organics originating from nonaqueous phase liquid pool dissolution in saturated porous media in the presence of dissolved humic substances. The model assumes that the dissolved contaminant (NAPL dissolved in the aqueous phase) may sorb onto the solid matrix as well as onto humic substances suspended in the aqueous phase, contaminant-humic particles and humic substances may sorb onto the solid matrix, the dissolved contaminant may undergo first-order decay, and humic substances are introduced into the aquifer from a line source. 

PROBABLE FATE photolysis, no direct photolysis, indirect photolysis is too slow to be important, the vapor is expected to react with photochemically produced hydroxyl radicals, with an estimated half-life of 22.2 hrs oxidation not an important process, photooxidation half-life in water 2.4-12.2 yrs, photooxidation half-life in air 21 hrs-8.8 days hydrolysis expected to be too slow to be important under natural conditions, first-order hydrolytic half-life 8.8 yrs volatilization not considered as important as sorption, however, there is very little data, volatilizes from dry soil surfaces, volatilization may be important in shallow rivers sorption adsorption onto solids and particles and complexation with humic material (flilvic acid) are the principal transport mechanisms biological processes bioaccumulation, biodegradation, and biotransformation by many organisms (including humans) are very significant fates... 

Hgure 2 (A) Calculated adsorption by a hydrous ferric oxide of several metal cations at a tolal added melal concentration of 10 mol I using the diffuse double layer surface complexation model. (From Dzombak DA and Morel FMM (1990) Surface Complexation Modeling Hydrous Ferric Oxide. New York Wiley.) (B) Experimentally measured cadmium ([Cd]total = 0.3 mmol I h adsorption by O (open circles) and B (filled circles) horizons (18.5gdm ) in 0.01 moll NaNOs. (Data from Lumsdon DG (2004) Partitioning of organic carbon, aluminium and cadmium between solid and solution in soils application of a mineral-humic particle additivity model. European Journal of Soil Science, in press.)... 

Kinetically, the adsorption of humic acids at a solid-water interface is controlled by convection or diffusion to the surface. Even at concentrations as low as 0.1 mg/e near-adsorption equilibrium is attained within 30 minutes. At high surface densities, a relatively slow rearrangement of the adsorbed molecules may cause a slow attainment of an ultimate equilibrium (Ochs, Cosovic and Stumm, in preparation). The humic acids adsorbed to the particles modify the chemical properties of their surfaces, especially their affinities for metal ions (Grauer, 1989). 

Humic acids (HA) are organic polyelectrolytes, which are most commonly identified with the organic material present in contemporary solid particles. HS are present in practically all soils and suspended and bottom sediments of rivers, lakes, estuaries, and shallow marine environments. 

It is commonly reported that dissolved humic substances (i. e., DHS) tend to coat mineral particles and thereby affect the surface chemistry of those materials. DHS coat the surfaces of solid particles even when they are present at very low concentrations. They furthermore impart a negative charge to the surfaces which they coat. The organic coating is expected to have a great significance on subsequent adsorption of various pollutants [88,91-93,287,288]. 

An alternative to sedimentation for removing suspended solids is flotation. This tends to be used for low-density particles that tend to float anyway during conventional sedimentation processes. Drinking water examples include algae and floes of humic and fulvic acids that result from the treatment of coloured waters [549]. Wastewater examples include fatty materials, pulp fibres, and oils that can be floated... 

The particle size distribution for the humic acid fraction is depicted in Figure 4. No material sedimented out until the most extreme conditions were applied (40,000 rpm for 24 hr), when some lightening of color at the top of the solution was observed. The sedimented particles had a Stokesian diameter of around 2 nm, which means that a particle size gap of three orders of magnitude exists between these and the next largest particles detected (5 xm). From the experimentally determined coal particle density of 1.43 g/cm, it was calculated that a solid sphere of diameter 2 nm would have a molecular mass of 4000. If the molecules were rod-shaped, even smaller molecular masses would be predicted. Literature values of the molecular mass of regenerated humic acids range between 800 and 20,000, with the values clustering around 1,000 and 10,000 (i5, 16, 17). 

To this purpose, isotopic data presented in this paper were obtained from several selected Gorleben groundwaters as part of the colloid characterisation programme. The contents of major and minor ions, light isotopes ( H, H, and and the U/Th isotopes were measured. Radiocarbon and were measured in dissolved inorganic carbon (DIG), ion the humic acid (HA-colloids) and fulvic acid (FA-solution) fractions of dissolved organic caibon (DOC). The and were also determined in dissolved sulphate phase. The U/Th isotope measurements were carried out on total and surface solid phases, colloid fraction (1-1000 nm particle size, HA) and solution (<1.5 nm, FA). 

Orlov et al. (1975) reported that the features of humic acids observed in electron photomicrographs were not representative of the shape or ordering of humic acid particles in solution. They reached this conclusion from two different experiments (1) They found by X-ray diffraction that humic acid particles are more ordered in an aqueous gel than in solid state, and (2) they found that the particles they observed in their electron photomicrographs were spherical, whereas their viscometry studies indicated that the particles were ellipsoidal, with an axial ratio of 1 6. 

Figure 13.34 Comparison of the adsorption of actinide cations of different oxidation states onto y-AljOj. The solid lines refer to Th(IV), Am(III), and Np(V). Open circles are adsorption data for Pu(V), with 2Pu(V) = 2 x lO" M. y-AliOj concentrations are 10 mg/L for Th and Am and 200 mg/L for Np and Pu. Modified after Bidoglio et al.. Interactions and transport of plutonium-humic acid particles in groundwater environments, Mat. Res, Soc. Symp. Proc. 1989, 127 823-30. Used by permission.

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