Humic substance adsorption mechanisms

Keywords. Sorption, Interaction mechanisms, Organic pollutants, Solid phases, Adsorption, Partitioning, Humic substances, Humus, Organic matter... 

Adsorption mechanisms represent probably the most important interaction phenomena exerted by solid surfaces on the environmental fate of organic pollutants [65, 127-130]. Adsorption controls the quantity of free organic components in solution and thus determines their persistence, mobility, and bioavailability. The extent of adsorption depends on the amount and properties of both solid phase-humic substances (SPHS) and organic pollutants. Once adsorbed on an SPHs >an organic pollutant may be easily desorbed, desorbed with difficulty, or not at all. Thus sorption phenomena may vary from complete reversibility to total irreversibility. 

Humic substances account for 40-70% of the DOC in rivers and 5-25% of the DOC in the ocean (Table I). It is important to note that recoveries of adsorbed humic substances from XAD resins are not quantitative, so the chemical characteristics of the recovered humic substances are not necessarily representative of all the humic substances retained by the resin. Tangential-flow ultrafiltration retains 45-80% of the DOC in rivers and 25-40% of the DOC in the surface ocean (Table I). Essentially all of the DOC retained during ultrafiltration is recovered for chemical characterization. In general, ultrafiltration recovers a larger fraction of the DOM from these systems. These methods also isolate DOM based on different mechanisms. Adsorption onto XAD resins at low pH chemically fractionates the DOM and isolates the more hydrophobic components, whereas ultrafiltration principally separates components of DOM on the basis of size and shape. 

The preceding discussion relies on an analogy between complexation in solution and complexation at the mineral surface, a fundamental tenet of the surface complexation model (27). Strong complexation of metals in solution by humic substances is well-documented (16, 42-44). Thus surface complex formation is a likely mechanism for the adsorption of humic substances on oxide surfaces. 

There is some, at least partly contradictory, evidence that different bind-ing/adsorption mechanisms exist for microbially significant substrates (e.g., carbohydrates and proteinaceous matter), resulting in different degrees of microbial availability of these substance classes. But more unequivocal data seem to be necessary. From soil science, it is known that humic substances bind/adsorb enzymes, especially hydrolases, and reduce their activity (Bums, 1982 Ladd and Butler, 1975). Except for a single paper by Baxter and Carey (1982), equivalent reports on the interactions of dissolved humic substances in lakes are lacking. 

Examples of successful fractionations of aquatic humic substances where only one fractionation mechanism was operative include utilization of the hydrophobic properties of XAD resins (Mantoura and Riley, 1975 Aiken et al., 1979), hydrogen bonding of weak-acid functionalities of humic constituents to weak-base anion-exchange resins (Kim et al., 1976), and use of ion-exchange celluloses for ion-exchange fractionation of aquatic humic substances without hydrophobic matrix adsorption (Sirotkina et al., 1974). These examples of successful fractionations demonstrate the potential for chromatography of aquatic humic substances when fractionations are designed carefully to avoid undesirable interactions. 

The influence of the pore size distribution of carbon on NOM uptake has been recognized by several researchers [57, 63]. Likewise, Karanfil and coworken [64] and Kilduffand coworkers [65] concluded that the adsorption of humic substances was largely governed by molecular size distribution in relation to pore size. Moreover, a good linear relationship (Fig. 25.6) was foundbetween the amount adsorbed by different carbons and their pore volume between 0.8 and 50 nm [61, 66] when the adsorption was carried out at pH 3. This is because electrostatic effects are minimized under these experimental conditions and nonelectrostatic interactions predominate. The adsorption mechanism would be due to hydrophobic and/or TT-TT-electron interactions, and in this case as with other electrolytes (see above). 

PROBABLE FATE photolysis no direct photolysis, half-life from surface waters 3500 hr, indirect photolysis is too slow to be important, photodegradation by hydroxyl radicals will occur with a half-life of 23.8 hrs oxidation not an important process, photooxidation half-life in air 4.7 days-46.6 days hydrolysis too slow to be important under natural conditions, first-order hydrolytic half-life 1163 days volatilization possible, but not important sorption sorption onto particles and biota and complexation with humic substances principal transport mechanism, little adsorption to soil or sediment is expected to occur biological processes bioaccumulation, biodegradation, and biotransformation by many organisms (including humans) are very significant fates... 

Adsorption is the physico-chemical interaction between solutes and the membrane. The adsorption of organics, or more specifically humic substances, is considered a major fouling mechanism in water treatment. NOM can either adsorb in the structure of the cake and give the cake cohesion, or in the bulk of the membrane. These interactions are strongly influenced by membrane solute affinities and the... 

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