Dissolved and colloidal substances

Figure 13 (after Buffle and VanLeeuwen, 1992) shows an example of the distribution of mineral and organic colloids as a function of size from angstrom to micrometer scales. This diagram shows that there is no clear boundary between dissolved and colloidal substances or between colloids and particulates. 

Lindstrom T, Soremark C, Westman L (1977) The influence on paper strength of dissolved and colloidal substances in the white water. Svensk Papperstid 80 341-345... 

The variety of tacky materials present in papermaking systems have different names for instance, the accumulated pollutants in the water recycling system are called dissolved and colloidal substances (DCS). The composition of DCS, which mainly come from pulp, filler, recycled water, and the chemicals added during the papermaking process, is very complex. 

It is operationally difficult to distinguish between dissolved and colloidally dispersed substances. For example, colloidal metal-ion precipitates occasionally have particle sizes smaller than 100 A, sufficiently small to pass through a membrane filter, and organic substances can exist as a stable colloidal suspension. Information on the types of species encountered under different chemical conditions (type of complexes, their stabilities, rate of formation) is a prerequisite to better understanding of the transformation in properties of toxic chemicals in a water body. 

Analytical Chemical Data for Natural Waters. While elemental compositions of various natural waters usually can be determined with good reliability, analytical methods to distinguish between free and complex-bound species, oxidized and reduced forms, simple and polynuclear metal ion forms, and even between dissolved and colloidal or suspended phases are often lacking. Data on the nature and amounts of the individual substances which make up the total concentrations of organic material found in different natural waters are not yet extensive. These analytical deficiencies relate almost solely to the highly reactive, non-conservative elements—e.g., iron, manganese, phosphorus, carbon, nitrogen, aluminum, and other metal ions. 

The term species refers to the actual form in which a molecule or ion is present in solution. For example, iodine in aqueous solution may conceivably exist as one or more of the species I2, I, la" HIO, IO , lOJ, or as an ion pair or complex, or in the form of organic iodo compounds. Figure 6.1 shows the various forms in which metals are thought to occur in natural waters. It is operationally difficult to distinguish between dissolved and colloidally dispersed substances. Colloidal metal-ion precipitates, such as Fe(OH)3(s) or FeOOH(s) may occasionally have particle sizes smaller than 100 A—sufficiently small to pass through a membrane filter. Organic substances can assist markedly in the formation of stable colloidal dispersions. Information on the types of species encountered under different chemical conditions (types of complexes, their stabilities, and rates of formation) is a prerequisite to a better understanding of the distribution and functions of trace elements in natural waters. 

Esthwaite Water (Tipping and Woof, 1983, a and b) is a softwater lake in the English Lake District. In this lake, humic substances extracted by butan-l-ol were present almost exclusively in the dissolved and colloidal size classes. The small fraction of humic substances in the particulate form should not be discounted, since it may explain a hypolimnetic accumulation of these substances. Humic carbon comprised 60-70% of the DOC in winter and early spring, but only 30-40% in summer. The higher proportion of nonhumic DOC during summer months presumably resulted from increased biological production of DOC. 

Kogel-Knabner, I. Totsche, K. U. (1998) Influence of dissolved and colloidal phase humic substances on the transport of hydrophobic organic contaminants in soils. Phys. Chem. Earth 23(2), 179-185. 

In the production and processing of chemical, semichemical and mechanical pulps and recovered paper, various inorganic and organic substances are accumulated in dissolved or colloidal dissolved form. Other water-soluble substances enter with the fresh water, fillers, recycled uncoated and coated paper broke and also by chemical additives. As the process water circuits are increasingly closed, the concentration of these water-soluble and colloidal substances and finely dispersed particles increases considerably and an additional contaminant load is thus imposed on the waste water. These substances interfere with the production process by increasing build-ups and deposits, they reduce the efficiency of the chemical additives, and impair the quality of the produced paper. Therefore these substances are also classed as detrimental substances (Fig. 3.18). 

The fluid portion of the blood, the plasma, accounts for 55 to 60% of total blood volume and is about 90% water. The remaining 10% contains proteins (8%) and other substances (2%) including hormones, enzymes, nutrient molecules, gases, electrolytes, and excretory products. All of these substances are dissolved in the plasma (e.g., oxygen) or are colloidal materials (dispersed solute materials that do not precipitate out, e.g., proteins). The three major plasma proteins include ... 

SALTING OUT. Reduction in the water solubility of an orgamc solid or liquid by adding a salt (usually sodium chloride) to an aqueous solution of the substance. Ions of the dissolved salt attract and hold water molecules, tli us making them less free to react with the solute. The result of this is to decrease ihe solubility of the solute molecules with consequent separation or precipitation. Colloidal suspensions of proteins, soaps, and similar substances are precipitated in tins way. 

Colloidal solutions (also known as sols) can be prepared by different methods depending on the nature of the substances. Many substances., e.g., gelatin, starch etc. form colloidal solutions by merely dissolving them in water. Metals and inorganic substances are brought into the colloidal state by special methods. Two types of methods are mainly used ... 

Wastewater treatment systems can be classified, in addition to pretreatment, as preliminary, primary, secondary, and tertiary (advanced) treatments. Pretreatment of industrial wastewater is required to prevent adverse effects on the municipal wastewater treatment plants. Preliminary treatment is considered as any physical or chemical process that precedes primary treatment. The preliminary treatment processes may consist of influent screening and grit removal. Its function is mainly to protect subsequent treatment units and to minimize operational problems. Primary treatment is defined as the physical or chemical treatment for the removal of settleable and floatable materials. The screened, degritted raw wastewater from preliminary treatment flows to the primary clarification tanks, which are part of the primary treatment facilities. Secondary wastewater treatment is the process that uses biological and chemical treatment to accomplish substantial removal of dissolved organics and colloidal materials. The secondary treatment facilities may be comprised of biological reactor and secondary clarification basins. Tertiary (advanced) wastewater treatment is used to achieve pollutant reductions by methods other than those used in primary and secondary treatments. The objective of tertiary wastewater treatment is to improve the overall removal of suspended solids, organic matter, dissolved solids, toxic substances, and nutrients. 

Humic substances occur in every natural water sample which has been analyzed for their presence. The amount and composition of humic substances vary considerably from soils, surface waters, and groundwaters, but their ubiquity in water is without question. Humic substances in water occur as a size continuum ranging from dissolved through colloidal, to particulate phases. The dissolved phase is a predominant phase in most streams and is the phase emphasized in this chapter. The geochemical activity and reactivity of dissolved and particulate organic phases are thought to be of sufficient difference in magnitude to merit the separation based on size at 0.45 /xm. 

Any attempt to design an effective procedure for the isolation of humic substances from soil should take account of the properties of the solvents or extractants to be used, of the solutes or material to be extracted, and of the types of associations that can exist between these solutes and other soil colloidal constituents. This chapter outlines some aspects of the composition, properties, and associations of soil humic substances which are relevant to their extraction, and it considers some of the properties of solvents that are used or might be considered for use as extractants of humic substances, or of molecules associated with these substances. It then compares the effectiveness and outlines some of the limitations of a selected number of solvents and procedures for dissolving and extracting humic substances. 

The 0.45 /am cutoff between dissolved and particulate fractions is arbitrary (Danielsson, 1982), and many researchers have commented on the inadequacy of this standard for removal of colloidal species. Sharp (1973) points out that colloids are of the approximate size range of 0.001-1.0 fim, and Kennedy et al. (1974) report that clay minerals of the types found in stream sediments can be much smaller than 0.45 jum. The concern of Kennedy et al. (1974) is that such material can pass the 0.45 /xm filter in sufficient quantities to seriously influence the analysis of dissolved inorganic species. These authors recommend a 0.1 xm filter pore size to remove clay colloids more effectively. The presence of these clay minerals is also of concern in the study of aquatiq humic substances however, decrease in pore size from 0.45 to 0.1 )um is accompanied by a decrease in flow rate through the filter, which is a major disadvantage when hundreds of liters of water are to be processed. Use of 0.45 /am filters represents a compromise between flow rate and rejection of clay minerals. 

Filtration. Sample should be filtered (sO.45 /tm) to separate dissolved humic substances from particular organic carbon and colloidal clays. Concentration. Humic substances should be concentrated by an efficient method, such as sorption on XAD-8 or Duolite A-7. 

The speciation, concentrations and residence times of dissolved substances in natural waters are dependent on many factors and processes. Important factors Include temperature, pH, redox potential, ionic strength and the concentrations of other dissolved species such as organic and Inorganic ligands as well as the presence of suspended particulate and colloidal matter. Important processes in addition to rate of input, and biochemical cycling include precipitation, complexatlon, coagulation and adsorption onto suspended particulate matter. 

Marc and Freundlich have put forward the view that the velocity of crystallisation of supercooled liquids is influenced by adsorption phenomena, the dissolved substance being adsorbed on the surface of the crystals. In this connection, Freundlich and Oppenheimer have shown that the crystallisation velocity of supercooled water is frequently, if not always, increased by colloidal substances the particles of which are non-spherical, whereas particles which are spherical, and also the truly dissolved substances, lower the velocity. 

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