Binding to humic materials

Tatsumi K, A Freyer, RD Minard, J-M Bollag (1994) Enzyme-mediated coupling of 3,4-dichloroaniline and ferulic acid a model for pollutant binding to humic materials. Environ Sci Technol 28 210-215. 

Park JW, Dec J, Kim JE et al (2000) Dehalogenation of xenobiotics as a consequence of binding to humic materials. Arch Environ Contam Toxicol 38 405 110... 

De La Rosa, G., J. R. Peralta-Videa, J. G. Parsons, and J. L. Gardea-Torresdey. 2005. Using X-ray absorption spectroscopy to study the speciation and coordination of lead binding to humic materials. Spectroscopy 20, no. 7 24-30. 

Conditional stability constants have been determined for cadmium binding to humic acid in freshwater, log Kk 6.5 [27], which may be comparable to binding to humic acid coated particles. The experiments demonstrated the importance of cadmium uptake from particles rather than from the dissolved phase. The authors recognised that the overall conclusion was similar to previous studies [28], but there remain inconsistencies in the uptake levels which may be related to the heterogeneity of the systems. Uptake from the intestine into the mucosal cells was not investigated. It was presumed that the material was digested extracellularly by hydrolytic enzymes and the released metal was taken up by facilitated diffusion. 

Aqueous-phase bioreactors provide good process control, can be configured in several treatment trains to treat a variety of wastes, and potentially can achieve very low contaminant concentrations. A drawback of bioreactor treatment is that, unlike composting systems which bind contaminants to humic material, bioreactors accumulate the products of biotransformation. In addition, bioreactors have been shown to remediate explosives only at laboratory scale, so the cost of full-scale bioreactor treatment is unknown. Full-scale bioreactors will have to incorporate a variety of safety features that will add to their total cost. 

From the literature reviewed above it is clear that a number of authors have determined that certain compounds can and do bind to dissolved humic materials. Other authors have invoked this binding phenomenon to explain otherwise peculiar data. It would be desirable to incorporate this binding into environmental fate models, but there is not much data on the phenomena and there are few methods available to collect more of this data. 

Methods Available for Quantitatively Measuring the Extent of Binding of Organic Pollutants to Dissolved Humic Materials... 

Three methods were used in this research to measure the extent of binding of organic pollutants to dissolved humic materials. They were equilibrium dialysis, solubility measurements and changes in sorption behavior in the presence of humic materials. Other authors have used solubility measurements, ultrafiltration and volatilization measurements. The methods will be described in the following paragraphs. 

In a dialysis experiment, a dialysis bag containing the dissolved humic materials is placed in a solution of a pollutant (preferably radiolabeled). The dialysis tubing is chosen so the pollutant is free to diffuse through the bag while the humic materials are retained inside the bag. The solution is shaken at constant temperature until it comes to an equilibrium point. At equilibrium, the pollutant inside the dialysis bag consists of two fractions that truly dissolved and the bound to the humic materials. The concentration of pollutant on the outside of the dialysis bag consists only of the free, truly dissolved fraction. Any increase of the pollutant concentration on the inside of the dialysis bag is due to binding by dissolved humic materials. A series of dialysis experiments, therefore, can measure the bound fraction concentration as a function of the free concentration. 

In a solubility experiment the solubility of the compound of interest is measured in the presence and absence of dissolved humic materials. Two techniques were used to measure solubility a shake and filter method similar to that used by Yalkowsky, and a flow through column technique similar to that used by May et al. 9 The measured solubilities of a number of compounds in our experiments were always higher in the presence of humic materials. This increase in the solubility is due to the binding of the compound by humic materials. In the presence of humic materials the measured solubility consists of two fractions free and bound. The free concentration should be the same in the presence or absence of humic materials. The difference between the solubilities of the compound in the presence and absence of humic materials is therefore a measurement of the bound fraction. 

The difference between the concentration in the ultra-filtered water and the concentration inside the ultrafiltration cell is therefore a measure of the bound concentration. Griffin and Chian7-7-, Hassett 7-, and Diachenko have used volatilization measurements to determine the extent of binding of pesticides and pollutants to dissolved humic materials. In these experiments either the rate of gas stripping of a compound or its equilibrium vapor pressure is measured in the presence and absence of humic materials. The results obtained can be manipulated in such a way to determine the percentage of the pollutant bound. 

Using the above techniques we have measured binding constants for a number of compounds to dissolved humic materials. Some of the results will be summarized in the following paragraphs. 

Unfortunately there is also some bad news for modelers. Different humic materials bind compounds to dramatically different extents, and the reasons for this are unclear. Figure 6 shows the binding constants of DDT to seven different humic materials. Some of this data is from a factorial experiment which has been published elsewhere.7 Inspection of this data shows that the humic acids and the... 

With the data currently available, it is not possible to predict how strongly a particular sample of humic material will bind a pollutant without actually measuring the binding constant. 

There is a body of data in the literature which indicates that dissolved humic materials may play a significant and previously overlooked role in the behavior of organic water pollutants. It has been shown that dissolved humic materials can affect degradation rates and phase transfer rates for a number of compounds. A number of methods have been developed in this research and by other researchers which can make quantitative measurements of the extent of binding between organic water pollutants and dissolved humic materials. Hopefully these methods will be used by other researchers to gain more insight into this phenomenon. 

The data presented here indicates that the extent of binding for a particular compound is related to the octanol/water partition coefficient for that compound. This is very similar to the sorption of compounds from water to sediment. Compounds with log Kow values less than four (such as Lindane) will probably not be bound to an appreciable extent in the environment. Compounds with very high log Kow values (DDT and DEHP) may be bound to a significant extent. The extent of binding will depend on both the concentration of humic material and on the nature of the humic material. The humic materials used in this research showed dramatically different affinities for DDT. The reasons for this are poorly understood and deserve further study. 

DePaolis F, Kukkonen J (1997) Binding of organic pollutants to humic and fulvic acids Influence of pH and the structure of humic material. Chemosphere 34 1693-1704... 

Gauthier, T.D., Shane, E.C., Guerin, W.F., Seitz, W.R., Grant, C.L. (1986) Fluorescence quenching method for determining equilibrium constants for polycyclic aromatic hydrocarbons binding to dissolved humic materials. Environ. Sci. Technol. 20(11), 1162-1166. 

Calculations which consider binding of sorbing solutes to dissolved humic matter show that the previously reported inverse relationship between effect of sediment concentration and sorption equilibrium can be accounted for by desorption of humic material followed by binding to the DOC (46, 48). 

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