Effect of Humic Substances on the Treatment of Drinking Water

7 Effect of Humic Substances on the Treatment of Drinking Water Downloaded by GEORGETOWN UNIV on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch007

A. Bruchet, C. Anselme, J. P. Duguet, and J. Mallevialle Centre de Recherche Lyonnaise des Eaux, 38 rue du Président Wilson, 78230 Le Pecq, France

Nonspecific parameters such as dissolved organic carbon, UV absorbance, andfluorescenceare no longer sufficient to optimize new water-treatment processes. Specific determinations with gas or liquid chromatographic techniques identify only 5-15% of the dissolved organic carbon present in raw waters. The remaining high-molecular-weight compounds are still poorly characterized. This chapter presents a pyrolysis-gas chromatographic-mass spectrometric technique used to study the background organic matrix of natural waters. Application to various soil and fresh-water samples indicates wide variations in the relative contributions of carbohydrates, polyhydroxyaromatic substances, amino sugars, and proteinaceous materials to the high-molecular-weight fraction. Possible consequences for water-treatment processes are examined. The change of concentration of these biopolymers during a clarification process is also reported.

THE INFLUENT OF A DRN IKN IGW - ATERT -REATMENT PLANT

usually contains between 2 and 6 mg of dissolved organic carbon (DOC) per liter. Specific organic chemicals detected by gas or liquid chromatographic techniques associated with mass spectrometry ( G C - M S or L C - M S ) typically represent only 5-15% of the D O C (I). The remaining high-molecular-weight fraction (85-95%) is usually referred to as "humic substances". These so-called humic substances, which have been operationally defined on the basis of their water solubility at acidic or basic p H (2), are not amenable to analysis with current analytical techniques and hence are poorly characterized.

0065-2393/89/0219-0093$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

Downloaded by GEORGETOWN UNIV on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch007

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AQUATC I HUMIC SUBSTANCES

The various models of humic substances advanced in the literature are derived from the study of their oxidation or hydrolysis byproducts or based on examination of their elemental composition, spectroscopic behavior, and other physicochemical properties. The Schnitzer-Kahn model (3), which involves a polyhydroxyaromatic core made up of phenolic and aromatic acids, has contributed greatly to the consensus that humic substances are highly aromatic and phenolic in nature. Though the predominantly aliphatic char acter of humic substances has been demonstrated in more recent studies (2), the aromatic-phenolic concept will probably prevail for a long time in the water-treatment industry. Early studies on the behavior of humic substances during water-treat ment processes often involved a molecular-weight separation followed by the evaluation of nonspecific parameters such as U V absorbance, fluores cence, or D O C for each fraction generated (4-6). More recently, gel per meation chromatography combined with pyrolysis-gas chromatography-mass spectrometry ( P y - G C - M S ) has provided better insight into the structure of the organic matrix, as well as the relationship of struc tures to molecular size and the treatment process (7). This chapter reports on careful evaluation of the use of P y - G C - M S to characterize humic substances from various sources. Two soil fulvic fractions and various natural waters are investigated to determine the structural var iability of humic substances and its possible consequences on water-treat ment processes. A n example of the behavior of aquatic humic substances during a clarification process is also described.

Experimental Materials and Methods Samples. Water samples were collected from the River Houlle near Dunkirk (France). This natural water usually contains a high background organic concentration, with typical DOC values rangingfrom4 to 12 ppm. Seasonal algal blooms are followed by increases in DOC and trihalomethane formation potential (THMFP). The Houlle raw water is treated at the Moulle treatment plant with breakpoint prechlorination followed by coagulation with FeS0 Cl,flotation,and activated-carbonfiltration.Various surface or ground-water samples collectedfromdifferent locations throughout France were characterized. Two different fulvic acids were used. The so-called "INRA" (Institut National Recherche Agronomique) fulvic acid was extracted from a podzol soil according to the method described by Holtzclaw and Sposito (8). The commercially available sample (Contech, ETC Limited-Ottawa, Canada) has been studied and some of its properties can be found in the literature (9, 10). 4

Sample Preparation, Py-GC-MS. The methods used for water-sample concentration, molecular-weight separation, and Py-GC-MS have been reported elsewhere (7). Briefly, water samples are concentrated by vacuum rotary evaporation at 40 °C in order to obtain a DOC between 100 and 200 ppm. The concentrates (10-



7. BRUCHET ET AL.Treatment of Drinking Water

95

mL aliquots) are injected into a column (Sephadex, Sephadex Pharmacia, Uppsala, Sweden) and eluted with ultrapure water at a rate of 100 mL/h. Detection of the fractions is by UV absorbance at 260 nm and total organic carbon (TOC) measurement on a TOC meter (Dohrman DC 80, Envirotech Corp., Santa Clara, CA). The Seph adexfractionsare concentrated by rotary evaporation down to a few milliliters and further reduced under a nitrogen stream to 100 μL·. Replicates (50-μί.) are then deposited into quartz tubes and allowed to dry slowly at room temperature. The quartz sample holders are inserted into a filament pyrolyzer (Pyroprobe 100, Chemical Data Systems) and heated to 750 °C at a rate of 20 °C/ms. The resulting final temperature inside the quartz tube is controlled with a type Κ thermocouple minithermometer (Cole Parmer Instrument Company) at 625 ± 5 °C. After pyrolysis, thefragmentsare separated on a 30-m fused silica capillary column (DB WAX) programmedfrom25 to 220 °C at a rate of 3 °C/min and identified by mass spectrometer (R 10-10 C, Ribermag, Rueil-Malmaison, France) operated at 70 eV and scannedfrom20 to 400 mlz.

Data Interpretation To simplify comparison of pyrolysis data, pyrochromatograms were coded and presented as pie charts. To draw the pie charts, a wide set of standards* representative of the various types of biological macromolecules (bo vine serum albumin, chitin, starch, cellulose, cellulose acetate, cellobiose, n-aeetylmuramyl L-alanyl D-isoglutamine, purified lignin) was submitted to flash pyrolysis. Pyrolysis fragments characteristic of each type of biopolymer (12) (e.g., acetamide for N-acetylamino sugars; pyrrole, 4-methylphenol, and phenol for proteins; furfural, methylfurfural, levoglucosenone, and carbonyl compounds for carbohydrates; phenols and methoxyphenols for polyhydroxyaromatic compounds) were selected. Intensities of these selected fragments weighed against the amount of material pyrolyzed (usually be tween 300 and 500 μg) were then used to determine average relative re sponse factors and to correct for differences in sensitivity between the four types of biopolymers. The percentages indicated in the pie charts are not absolute, but only relative to the set of standards used. To discriminate between the phenolic peaks arising from polyhydroxyaromatic substances and tyrosine-containing proteins, the following correc tion was performed. Because tyrosine leads to similar quantities of phenol and 4-methylphenol (in contrast to polyhydroxyaromatic compounds, which yield predominantly phenol), the response for 4-methylphenol was sub tracted from the area obtained for phenol and the difference was used to calculate the proportion of polyhydroxyaromatic compounds. Contact us for the list of selected standards, along with the individual and average relative response factors of their characteristicfragmentsused for these calculations. The percentages indicated in the pie charts are not absolute, but only relative to the set of standards used. *The list of selected standards, along with the individual and relative response factors of their characteristic fragments used for these calculations, can be obtained from us.


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AQUATC I HUMIC SUBSTANCES


Results and Discussion Comparison of Fulvic Fractions. The pyrochromatograms ob tained for the I N R A and Contech fulvic samples are shown in Figures 1 and 2. Their elemental analysis and T H M F P are reported in Table I. The main difference between the two fractions lies in the much higher N / C , O / C , and H / C ratios observed for the I N R A sample. The I N R A sample shows a higher nitrogen content (N = 3.7%). The low H / C ratio of the Contech sample indicates a more aromatic nature. In good agreement with the un usually high O / C ratio observed (0.95), most of the fragments detected in the pyrochromatogram of I N R A fulvic acid (furfural derivatives, anhydro sugars, and carbonyl compounds such as hydroxypropanone, cyclopenten1-one, acetic acid, and a lactone) are typical decomposition products of carbohydrates (12). The lactone detected (3-hydroxy-2-penteno-l,5-lactone) is characteristic of nonreducing xylopyranosyl residues (13). The nitrogen content of this fulvic fraction (3.7%) is found under the form of both N-acetylamino sugars (indicated by the acetamide peak) and peptidic material (presence of pyrrole). The low intensity of the phenolic peaks indicates that the polyhydroxyaromatic content is extremely low. By contrast, the dominant phenolic peak observed on the Contech pyrochromatogram points to a highly purified structure from which the soil

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Effect of Humic Substances on the Treatment of Drinking Water

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