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Stability of organic matter in landfill leachates
Water Research Vol.11. pp. 225 to 232.PergamonPress lq". Printedin Great Britain.
STABILITY OF ORGANIC MATTER IN LANDFILL LEACHATES E. S. K. CHIAN Department of Civil En~neering. University of Illinois, Urbana. Illinois 61801, U.S.A. IReceived 7 September 1976~
Abstract--Samples of grossly polluted groundwater and of leachate collected from wells or underdrains near solid waste disposal sites were subjected to extensive organic analyses. The soluble organics were concentrated, separated and characterized by membrane ultrafiltration, gelpermeation chromatography and analysis for functional groups and specific organics. Free volatile fatty acids represented the largest group of organics, and this fraction showed a rapid decrease with increasing age of the fill The most stable group of organics with increasing age was a fulvic-like material with a relatively high carboxyl and aromatic hydroxyl group density. Increasing stability was further noted for carbohydrates. hydrolyzable amino acids and aromatic hydroxyl compounds in that order. Only leachate from a recently generating fill can be treated successfully by microbial processes because of its large biodegradable organic content; leachate from stabilized landfills is best treated by physical-chemical processes.
INTRODUCTION
The resulting higher moisture in the solid waste enhances acid fermentation of the organic matter, releasing large quantities of free volatile fatty acids, while the lowered pH solubilizes large amounts of heavy metals. The results of numerous leachate and polluted groundwater analyses show that chemical oxygen demand (COD) values as high as 89,520 mg/I can be encountered, while iron concentrations can reach 2820 mg/l and zinc 370 mg/1. Analyses of leachate samples and polluted groundwater samples collected from landfills of different ages show that the absolute concentration of most pollutants decreases the longer the fill is subject to leaching by infiltrating rainwater (Chian & DeWalle, 1976a). A gradual change in the relative composition was also noted, as indicated by a decrease in the biochemical oxygen demand (BOD) fraction of the COD with increasing age of the fill. The free volatile fatty acid carbon fraction of the total organic carbon (TOC) decreases parallel to the BOD/COD ratio with increasing age of the fill (Chian & DeWalle, 1976b). while an unidentified fraction, presumably consisting of refractory organics, shows a gradual increase. The purpose of the present study was to conduct a more detailed analysis of these refractory organics using membrane ultrafiltration, gelpermeation chromatography and specific organic anslyses, making it possible to assess the attenuation of those organics in groundwater and to predict how effectively they can be removed by microbial and physical-chemical treatment processes.
The composition of organic matter present in wastewater, surface water and groundwater will determine to a large extent its amenability to microbial degradation and to attenuation by physical-chemical processes in treatment units or in the natural environment. Degradation of organics by micro-organisms occurs sequentially through well-defined pathways (Mahler & Cordes, 1971), resulting in the formation of ATP. Organics having a high energy-yielding potential or electron equivalents (McCarty, 1971) are generally more readily removed than those with low energy-yielding potential (Chian & DeWalle, 1975). When organics are not able to enter certain pathways because of the compound's complex structure, microbial degradation may not occur at all (Ludzack & Ettinger. 1960). Removal of organics by physical-chemical processes is also determined by the character of the compounds. For example, removal by adsorption processes is dependent upon the molecular weight and chemical structure of the compound (DeWalle & Chian. 1974). In-depth knowledge of the composition of the organic matter present can therefore be used to predict how effective biological and physical-chemical processes will be in removing those organics. The degree of organic matter attenuation in surface waters will determine the extent of treatment to be employed by downstream users, while attenuation in groundwater will determine its potential for aquifer contamination. MATERIALS AND METHODS A substantial amount of groundwater pollution is caused by organics leaching from septic tanks, unTwelve representative leachate or heavily polluted lined surface impoundments, spills and solid waste groundwater samples were collected from a number of landfills. These pollutants have frequently caused solid waste lysimeters and from pilot-and field-scale solid waste landfills. The sites are situated in the United States aquifer contamination (Miller et al., 1974). Infiltrating at representative locations and cover a range of temperarainwater leaches contaminants from the solid waste, ture and precipitation regimens, The samples were coland these substances then enter underlying soil strata. lected anaerobically from underdrains or wells that 225 w.R, i I,,2--G
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E.S.K. CHIAN
reached to the polluted groundwater. Gross parameters were analyzed according to Standard Methods (APHA, 1971). After ultracentrifugation at 30.000rpm for 30min (L2-65B ultracentrifuge; Beckman, Palo Alto. CA) the supernatant was concentrated with a 500-MW ultratiltration (UF) membrane (UM 05; Amicon, Lexington, MA) while a separate aliquot was fractionated with a 10,000-MW UF membrane (UM 10). The 500-MW UF permeate was analyzed for free volatile fatty acids by gas chromatography (GC) using a Hewlett-Packard 5750 B Chromatovaphy (Palo Alto, CA) equipped with a flame ionization detector and a column consisting of 20% neopentyl glycol succinate and 2°0 phosphoric acid on Chromosorb PAW 60--80 mesh (Mateles & Chian, 1969). The 500-MW UF retentate was further fractionated using a G-75 Sephadex column (Pharmacia, Piscataway. N.J.}. All fractions were characterized by TOC, while the Sephadex fractions were further analyzed for carbohydrates as dextrose (anthrone test), amino acids as lysine (ninhydrin test after 24 h acid digestion with 6N HCI), carboxyl groups as acetic acid (hydroxylamine test), carbonyl groups as acetophone (2,4-dinitrophenylhydrazine test), and aromatic hydroxyl groups as tannic acid (Folin-Denis test), as described by Chian & DeWalle (1976c). RESULTS Analysis of the gross properties of the twelve leachate samples showed that the COD ranged from 81 to 71,680mg/1 and that lower values were generally observed the longer the fill was subjected to leaching. Most of the organics were present in the soluble form, since the suspended solids varied between 9 and 923 mg/l. When the supernatant of a sample collected from the underdrain of a University of Illinois lysimeter 2 months after placement of the solid wastes was subjected to ultrafiltration, it was found that 73% of the original TOC of 17,060mg/l permeated a 500-MW UF membrane. When a separate aliquot was ultrafiltered, 90% permeated a 10,000-MW U F membrane. Further analyses of the 500-MW permeate for free volatile fatty acids showed the presence of acetic acid (1748 rag/l), propionic acid (509 mg/l), isobutyric acid (312 rag/l), butyric acid (3075 rag/l), isova-
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leric acid (717 mg l). valeric acid (564 mg/l), and hexanoic acid (1488 mgl). The carbon from these free volatile fatty acids comprised 78% of the TOC of the 500-MW UF permeate corresponding to 490/0 of the initial TOC of the leachate sample. When the 500-MW UF retentate, representing 27% of the original TOC. was applied on a Sephadex G-75 column (Fig. 1), the TOC distribution in the eluate showed that 22°° of the retentate, or 6% of the original TOC. was excluded from the column as it eluted between 20 and 36 ml (desLm'tated as Fraction l(a)). This exclusion indicates an apparent molecular weight larger than approximately 30.000-50,000. The majority of the 500-MW UF retentate consisted of organics with an apparent molecular weight larger than 500 but smaller than 1000-3000 (Fraction 3(a); Fig. l). and this fraction represented 68% of the UF retentate or 18.5°o of the original TOC of the sample. Fraction 2(a), with a molecular weight between 3000 and 30,000, represented only 2.5% of the original TOC. Analyses of the Sephadex fractions showed that there were considerable differences in the specific organics present (Fig. 1), with relatively high concentrations of carbohydrates observed in the high molecular weight fraction and substantial quantities of aromatic hydroxyl and carboxylic compounds present in the low molecular weight fraction. Although the TOC data suggest that fraction 3(a) is rather homogeneous, colorimetric tests showed that this was not the case, since the maxima of the different organic compounds did not elute at the same volume; the specific compounds associated with organics of decreasing molecular weight were carbohydrates (maximum at 77 ml), carbonyl (81 ml), carboxyl (83 ml) and aromatic hydroxyl compounds (85 ml), respectively. Calculations of the amount of organic carbon contributed by functional groups or specific organics in the 500-MW UF retentate showed that 17% consisted of carbohydrates, 6% of carboxyl compounds, 3.3% of
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Fig. 2. Eluate of the 500-MW UF retentate of the Dupage County leachate as separated on a Sephadex G-75 column as characterized by total organic carbon, specific organics and functional groups. carbonyl compounds and 2.6~ of aromatic hydroxyl compounds. It was therefore assumed that the carbon in the model compounds used in the colorimetric tests reflected the actual structures and that a particular organic structure did not respond to more than one colorimetric test. Intrared analyses of Fraction l(a) showed that it was similar to a humic--carbohydrate like complex (DeWalle & Chian, 1974a) associated with lipids, while Fraction 3(a) corresponded closely to a fulvic-like material. Membrane ultrafiltration and geipermeation was then applied to a sample obtained from a 13-yr-oid fill in Dupage County in northern Illinois (Hughes et al., 1971) having TOC of only 644 mg/l. Concentration through a 10,000-MW U F membrane resulted in the retention of no material, while only 6 ~ of the TOC in the original sample was retained when a separate aliquot was ultrafiltered with a 500-MW membrane. Since the concentration of the fatty acids in this sample was below the detection limit of the gas chromatography, it was concluded that most of the organics were present as low molecular weight refractory molecules. The 500-MW UF retentate was further separated using a Sephadex G-75 column (Fig. 2). The TOC data show that only 7 ~ of the 500-MW UF retentate, corresponding to 0.5~ of the original TOC, is present as the high molecular weight humic carbohydrate fractions (fraction l(b)). Approximately 35~ of the organic compounds present in this molecular fraction are carbohydrates, which is comparable to the percentage in the high molecular weight fraction of the leachate obtained from the young fill (Fig. 1). Further comparison of fractions l(b)and l(a) showed that the percentage of TOC consisting of carbonyl compounds had decreased from 5 to 1~, while the aromatic hydroxyl contribution decreased from 4 to I~. The organics associated with the carboxyl group, however, showed a substantial increase from 5 to 36~. Evaluation of the absorption spectrum of
the 2,4-dinitrophenylhydrazine derivatized carbonyl group showed that all groups were present as aliphatic aldehydes and not as aliphatic ketones or aromatic aldehydes. Aldehydes, as opposed to ketones, can be oxidized and converted into carboxyl groups. The observed relative increase in carboxyl groups in fraction l(b) may therefore have occurred at the expense of aldehyde groups. The increased oxidation state of the sample was also reflected in the C O D / T O C ratio, which decreased from 2.9 in the first sample to 1.6 in the second sample. The molecular weight fraction included in the G-75 Sephadex (Fraction 3(b)) showed less noticeable changes than fraction 3(a). The contribution of the carboxyl group to the organic carbon in this fraction increased slightly, from 6 to 7~o, while that of the carbonyl group remained the same, and that of the aromatic hydroxyl group decreased from 5 to 3~. The contribution of the carbohydrate content was 35~ in fraction l(b) and only 9~o in fraction 3(b), whereas the contribution of aromatic hydroxyls was l ~ in fraction l(b) and 3 ~ in fraction 3(b). The absolute concentration of carbohydrates therefore tends to reflect the magnitude of the high molecular weight humic-carbohydrate like fraction, while the aromatic hydroxyls may be used as an indicator of the fulviclike material. Based on the results of these gelpermeation chromatography analyses, 500-MW and a 10,000-MW U F membranes were selected to fractionate additional leachate samples collected from landfills having ages between those of the first and second fractionated samples. Since some of these samples showed a substantial flux decrease, a 1000-MW instead of a 500-MW U F membrane was selected. The results in Fig. 3(a) and 3(b) show a gradual decrease of both the high molecular weight fulvic-like material with increasing age of the landfill. The results clearly indicate that the high molecular weight fraction in the
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membranes in leachate samples collected from landfills of different ages. 10,000-MW UF retentate decreases more rapidly with age than that in the 500-MW UF retentate, indicating that the latter fraction is less amenable to attenuation processes, such as microbial degradation or physicalchemical removal. Despite the considerable spread of the data, Fig. 3(a) and (b) also show that the magnitude of the UF retentates tend to increase during the first few years after installing the landfill and to decrease thereafter. Results of the membrane fractionation reflect the changes observed for carbohydrates, proteins and aromatic compounds as measured in the unfractionated samples (Fig. 4). It was noted that proteins are present in high concentrations in both the high molecular weight fraction (e.g., the 500-MW UF retentate) as hydrolyzable amino acids, and in the 500-MW UF permeate as free amino acids. Qualitative analyses of the 500-MW UF permeate with a Technicon Automated Amino Acid Analyzer (Tarrytown, NY) showed, in decreasing order, the presence of ornithine, lysine and valine. Changes in protein concentration in the unfractionated samples therefore reflect the magnitude of both fractions. The carbohydrate concentrations primarily reflect the size of the high molecular weight fraction. Figure 4 shows that the aromatic hydroxyl compounds associated with the fulvic-like material decrease the least with time, a finding which agrees with the results of the gelpermeation chromatography (Figs. 1 and 2). When the relative decrease of the fractions in the membrane retentate were compared with that in the membrane
permeate, i.e., the free volatile fatty acid fraction, it was found that the latter decreased more rapidly with time. These results, therefore, tend to indicate that with respect to attentuation processes, the free volatile fatty acids are less stable than amino acids, proteins, carbohydrates, and humic-like materials, and that the latter group is less stable than aromatic hydroxyl-, carboxyl- and fulvic-like substances. D I S C U S S I O N
Results of the organic analysis of the molecular weight fractions obtained by membrane fractionation and gelpermeation tend to confirm the investigations of soil and aquatic humic substances summarized previously (Chian & DeWhalle, 1976b). All showed significant concentrations of carbohydrates and hydrolyzable amino acids in the high molecular weight fractions, while carboxyl groups, aromatic hydroxyl groups, color and fluorescence are primarily contributed by fractions of lower molecular weight. Most of the previous studies used methods for functional group analyses developed in the soil sciences, primarily titration techniques (Schnitzer & Kahn, 1972). Many tests, however, give arbitrary results. The outcome of the carboxyl and aromatic hydroxyl test, for example, is dependent upon the generally unknown spectrum of dissociation constants (van Dyke, 1966). The colorimetric tests selected in the present study for functional group analyses may therefore be less arbitrary. The similarity of organics present in the
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Fig. 4. Carbohydrates (a), proteins (b) and aromatic hydroxyls (c) in leachate samples collected from landfills of different ages. different molecular weight fractions as observed in these studies would indicate that similar bacterial or physical--chemical processes govern the composition of organics in the natural environment. Several studies do indicate that bacterial processes are the most important. The relative composition of the individual sugars in the carbohydrate fraction tends to be similar to that of bacterial sugars (Forsyth, 1950). It was also noted that labeled glucose, added as a substrate to soil bacteria, was often recovered in the acid-hydrolyzable amino acids in the high molecular weight humic fraction (Broadbent, 1968), normally excreted by bacteria. The relatively high concentration of polar organics, such as free volatile fatty acids and dissolved free amino acids, confirms earlier studies of leachate samples. It was calculated that the fraction of the COD consisting of free volatile fatty acids can vary from 80% (Mao & Pohland, 1973) and 759/0 (Burrows & Rowe, 1975) to 40~ (County of Sonoma, 1973) and 25~ (Hughes et al., 1971), with COD values ranging from 3260 to 170,000mg/l. Such high concentrations are generally not observed during bacterial degradation of soil organic matter. It is well known, however, that free volatile fatty acids are intermediates during decomposition of complex organics in soil. Schwartz et al. (1954), for example, showed that the addition of glucose to aerobic soils rapidly increased the acetic acid content. Takeda & Furusaka
(1975) noted that glucose amendments to anaerobic soils produced mainly acetic and butyric acid, while additions of peptone or amino acids increased the amounts of branched fatty acids; branched acids were also noted in the present study. While bacteria produce free volatile fatty acids, fungi excrete primarily nonvolatile acids such as citric, oxalic, fumaric, succinic and malic acid (Stevenson, 1967). However, anaerobic conditions, along with high moisture content, generally favor production of bacterial acids as opposed to fungal acids (Wang et al.. 1967). Adamson et al. (1975) also noted that butanol, ethanol, acetone and smaller quantities of other alcohols and carbonyls were formed when glucose was added to anaerobic solids. They reported that 8~ of the organics remaining in the soil after the glucose additions could be accounted for by these compounds. This result is comparable to those of Burrows & Rowe (1975) who noted that 4?/o of leachate COD consisted of propanol, ethanol, acetone and other alcohols. The presence of dissolved free amino acids such as the ornithine, lysine and valine found in the present study has also been noted in other investigations. Crawford et al. (1974), for example, found that omithine, serine and glycine represented the majority of dissolved free amino acids in an estuary, a result of their very low bacterial uptake rates and of the low percentage respired. Similarly, low uptake rates were
230
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noted for valine, arginine and lysine. Glutamic and in filtrated seawater (500-MW UF permeate) aspartic acid. on the other hand. were found to have degraded more rapidly after incubation than did the highest bacterial uptake rates and highest percent- other fractions. A relative increase was sometimes age respiration; these acids also represent the major- noted for the 100.000-MW UF retentate fraction. ity of the acids in the intracellular amino acid pool DeWalle & Chian (1974a) similarly noted that low (Clark et al., 1972). Clark et al. (1972) and Degens molecular weight organics such as free volatile fatty et al. 11964) noted a high concentration of dissolved acids, amino acids and aldehydes were degraded preNycine. serine and omithine in seawater samples, ferentially, followed by an increase of a bacterially while Brown et al. (1972) noted that leucine, lysine excreted, high molecular weight humic-carbohydrate and valine were the main hydrolyzable amino acids like fraction. Only extended bacterial degradation in anaerobic sediments. These amino acids also de- caused a gradual decrease of this high molecular creased the least with increasing burial time or in- weight fraction (Chian & DeWalle, 1975). These creasing sediment depth. Glutamic and aspartic acid, studies therefore tend to support the results of the on the other hand, showed the most rapid decrease present study and may explain the small increase in with burial time. Attachment or amino acids to humic size of the 10,000-MW UF retentate during the first substances reduces their rate of decomposition, and few years (Fig. 3(a) and 3(b)); the more rapid decrease Piper and Posner (1968) have shown that the amino of the 10,000-MW UF retentate as compared with acids most likely to attach to humic substances are the 500-10,000-MW fraction thereafter could result those that have a free amino acid group such as from preferential bacterial degradation. The presence lysine, ornithine and arginine. In the present study, of significant amounts of fulvic-like organics in leathe detection of ornithine, lysine and valine as chate from older fills agrees with results from Benoit opposed to glutamic and aspartic acid may thus indi- & Starkey (1968), who found that incorporation of cate that the bacterial degradation of leachate occurs aromatic hydroxyl compounds makes the resulting organic matter complex less susceptible to microbial at a relatively slow rate. After release of the bacterially derived organics, degradation. subsequent degradation will occur. Degens et al. The preferential removal of the organics in the (1964), for example, noted a rapid decrease of carbo- 10,000-MW UF retentate could also be caused by hydrates in sediments with increasing burial time, coagulation processes. Gjessing (1970), for example, while a less rapid decrease was observed for amino noted that the high molecular weight fraction was acids. No reduction in the amount of humic sub- subject to greater reduction by minerals than were stances was noted, while aromatic compounds the lower molecular weight organics. Chian & Deshowed an increase with burial time. Both lshiwatari Walle (1976a) found that lime coagulation removed (1971) and Swain (1970) noted that the stability of primarily the high molecular weight fraction and did carbohydrates, hydrolyzable amino acids, humic acids not greatly affect the fulvic-like material. In the and aromatic compounds in sediments i~acrease in present study, however, no significant correlation that order. Although there is a considerable spread was observed between the magnitude of the high in the analytical results, Fig. 4 similarly tends to indi- molecular weight fraction and the concentration cate increasing stability for carbohydrates, hydrolyz- of Ca and Fe which could have caused such able amino acids and armatic hydroxyl compounds. coagulation. Adsorption processes definitely do not result in the In the present study it was found that the humicpreferential removal of high molecular weight carbohydrate like material in the 10,000-MW UF retentate decreased more rapidly with increasing age organics. Tan (1975), for example, reported that than the fulvic-like material present in the adsorption of humic acids onto kaolinite and mont500-10.000-MW membrane fraction. This difference moriUonite reduced primarily the low molecular could be a result of preferential microbial degrada- weight fraction included in Sephadex G-50 but did tion, enhanced coagulation and adsorption, and the not reduce the excluded and therefore larger than more rapid downward leaching of this high MW frac- 10,000-MW fraction. Similarly, Bloomfield et al. tion. Its decrease could also be caused by gradual (1975) found that clay particles removed observed conversion to the fulvic-like material by chemical pro- principally the 10,000--30,000-MW UF fraction, while cesses. Mathur & Paul (1967), in studying the fungal a smaller decrease was observed for the organics in degradation of humic acids separated by gelpermea- the 30,000-MW UF retentate. DeWalle & Chian tion chromatography, observed that the largest remo- (1974b) noted that activated carbon preferentially val occurred with the <6000 MW fraction included adsorbed the fulvic-like organics while it was less in G-25. Substantial losses were also noted for the effective in the removal of the high molecular weight > 35,000 MW fraction excluded from G-75, while the humic-carbonydrate like organics. Leaching prosmallest losses were observed with the intermediate cesses likewise do not preferentially affect the high molecular weight fraction. Degradation of the high molecular weight fraction. Gonzales & Hubert (1972) molecular weight fraction resulted in the formation and Leenheer & Malcolm (1973) actually found that of intermediate molecular weight organics. Ogura leaching would preferentially remove the fulvic-like (1975) observed that low molecular weight organics organics because of their greater mobility.
Stability of organic matter in landfill leachates A gradual conversion of high moiecular weight humic substances into low molecular weight fubic acids by microbial or chemical processes has been noted in several studies. Aleksandrova (1966), for example, reported that 40°~ of humic acid incubated in soil appeared to be converted to fulvic acids, while the condensation of fulvic acid into humic acids was observed to a much lesser extent. Environmental conditions can also influence the microbial or chemical process that determine the relative degradation of fractions having different molecular weights. Both Brown et al. (1972) and Berryhill et al. (1972) noted that humic acids were most abundant in reducing sediments, while fulvic acids constituted the largest fraction in oxidizing environments, and that these oxidizing environments also contained less organic barbon. Since a gradual increase of the ORP from - 2 0 0 to +200-MV with increasing age of the fills was found in the present study, such conversion could well have occurred. In addition, Wright & Schnitzer (1960) observed that the relative magnitude of the fulvic-acid fraction in podzols was positively correlated with increasing humification of soils, which also corresponded to an increasing content of carboxyl and carbonyl groups and a decrease in the amount of compounds having aliphatic structures. The results of these related studies tend to agree with the results obtained in the present study, in which increasing stability (and hence decreasing susceptability to attenuation processes) was observed for free volatile fatty acids, low molecular weight aldehydes and amino acids, carbohydrates, hydrolyzable amino acids, humic acids, aromatic hydroxyls, and fulvic acids in that order. The sequence reflects, most likely, increasing stability of the organic matter to microbial degradation. Some of the attenuation, however, may result from preferential coagulation processes. The analysis of organic matter in leachate and polluted groundwater collected below landfills thus shows that stability of the organics against bacterial degradation increases with increasing age of the landfill. This finding implies that in the case of a recently installed fill, biological treatment processes such as activated sludge, aerated lagoons or anaerobic filters will be effective in removing from the leachate those organics which are the result of high concentrations of biodegradable free volatile fatty acids. Organic matter from older fills is primarily present as refractory fulvic-like organics, which are better removed by physical--chemical processes such as adsorption onto activated carbon (DeWalle & Chian, 1974b) and reverse osmosis (Chian & DeWalle. 1976a). Organics in the leachate from a recently installed fill will also be largely attenuated by underlying soil strata if microbial methane fermentation occurs to remove the large concentrations of free volatile fatty acids. The attenuation of leachate from older landfills will be less significant because physical-chemical attenuation processes in soil are generally less effective than microbial processes (Genetelli & Cirello, 1976); the mol-
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ecular weight range as well as the functional groups of the fulvic-like organics, however, are well suited for adsorption processes. CONCLUSION It was concluded that concentration, separation and identification of organics in grossly polluted groundwater and leachate can be achieved by membrane ultrafiltration and gelpermeation chromatography followed by specific organic analyses with colorimetric tests. Analyses of a sample collected from the underdrain of a recently installed solid waste lysimeter showed that the majority of the organics consisted of free volatile fatty acids. The next largest group is a fulvic-like material of intermediate molecular weight, also characterized by a relatively high carboxyl and aromatic hydroxyl group density. A small percentage of the organics consisted of a high molecular weight humic-carbohydrate-like complex. Analysis of samples collected below landfills of different ages showed that the free volatile fatty acid fraction decreased considerably with increasing age of the fill. The high molecular weight fraction present in the 10,000-MW UF retentate decreased more rapidly than the 500-10,000-MW fulvic-like fraction, reflecting the greater stability of the latter fraction. The results of the organic analysis tended to agree with those of membrane separation; increasing stability was noted for carbohydrates, hydrolyzable amino acids and aromatic hydroxyl compounds with increasing age of the fill. The results of organic analyses are necessary to select optimal treatment processes for the removal of organic matter from polluted groundwater or leachate. Leachate from a recently generating fill is therefore best treated by microbial processes, while organics in stabilized leachate are preferably removed by physical-chemical processes such as activated carbon adsorption and reverse osmosis.
REFERENCES Alecksandrova, L. N. (1966) Int. Soc. Sci. Trans.,--II, IV Committee, Aberdeen, p. 73. American Public Health Association, (1971) Standard Methods for the Examination of Water and Wastewater,
13th Edit., Washington, DC. Benoit, R. E. & Starkey, E. L. (1968) Inhibition of decomposition of cellulose and some other carbohydrates by tannin, Soil Sci. 105, 291. Berryhill. H. L. et al. (1972) Organic and trace element content of halocene sediments in two estuarine bays, Pamlico sound area, North Carolina, Geol. Survey Bull. 1314-E, Washington, DC. p. 32. Bloomfield, C. et al. (1975) A comparison of the composition and properties of natural and laboratory prepared humified organic matter. Soil Biol. Biochem. 7, 313. Broadbent. F. G. (1968) Nitrogen immobilization and relation to n containing fraction of organic matter. Isotopes and Radiation in Soil Organic Matter Studies. Atom. Energy Agency, Vienna, p. 131.
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Brown. F. S. et al. 119721 Early diagenesis in a reducing fjord, Saanich inlet, British Columbia--Ill. Changes of organic constituents of sediment. Geochim. cosmochim. Acta 36. 1185. Burrows, W. D. and Rowe. R. S. 11975} Ether soluble constituents of landfill leachate J. Water Pollut. Control Fedn. 47. 921. Chian. E. S. K. and DeWalle. F. B. (19751 Sequential substrate removal in activated sludge systems fed with naturally occurring wastewater. Pro 9. Water Technol. 7, 235. Chian, E. S. K. and DeWalle, F. B. (1976a) Santitary landfill leachates and their treatment J. Environ. Engng. Die. ASCE. t02. 411. Chian, E. S. K. and DeWalle. F. B. (1976b). The use of membrane ultrafiltration and specific organic analysis for characterization of soluble organic matter. Enciron. Sci. & Technol. To be published. Chian, E. S. K. and DeWalle, F. B. (1976c). Characterization of leachate generated from landfills, final report, U.S. EPA contract 68-03-0162. September. Clark. M. E. et al. (1972) Dissolved free amino acids in Southern California coastal waters. Limnol. Ocea,. 17, 749. Clark, V. L. et al. (1972) Changes in free amino acid production and intracellular amino acid pools of Bacillus licheniformis as a function of culture age and growth medium. J. Bacteriol. 112, 715. County of Sonoma (1973). Sonoma County Refuse Stabilization Study. Second Annual Report Dept. of Public Works. Santa Rosa, CA. Crawford, C. C. et al. {1974) The utilization of free amino acids by estuarine microorganisms. Ecology, 55. 551. Degcns, E. T. et al. (1964) Biochemical compounds in offshore California sediments and seawaters. GeochinL cosmochim. Acta 28. 45. DeWallc, F. B, and Chian, E. S. K. (1974a) The kinetics of formation of humic substances in activated sludge systems and their effect on flocculation. Biotech. & Bioen9. 14. 739. DeWalle, F. B. and Chian. E. S. K. (1974b) Removal of organic matter by activated carbon columns. J. Environ. Engng. Dic. ASCE 100, 1089. Forsyth, U. G. C. (1963) Studies on the more soluble complexes of soil organic matter--2. The composition of the soluble polysaccharide fractions. Biochem. J. 46, 141. Gaudy, A. F. et al. (1963) Sequential substrate removal in heterogenous populations. J. Water Pollut. Control Fedn. 35. 903. Genetelli, E. J. and Cirello, J. (1976), Gas and leachate from Landfills; formation, collection and treatment. U.S. Environmental Protection Agency, Report EPA-600/ 9-76-004, 190 p. Gjessing, E. T. (1970) Reduction of aquatic humus in streams. Vattern, 1, 14. Gonzales. A. and Hubert, G. (1972) Chemical and physical properties of non dialysable organic material of four podzols from Quebec. Can. J. Soil Sci. 52, 1. Hughes, G. M. et al. (1971) Hydrogeology of Solid Waste Disposal Sites in Northeastern Illinois, Report SW-12d, U.S. Environmental Protection Agency, Washington, DC.
Ishiwatari. R. (1971) Molecular weight distribution of humic acids from lake and marine sediments. Geochem. J. 5, 121. Leenheer. J. A. and Malcolm. R. L. (1973) Fractionation and characterization of Natural Organic Matter from Certain Rivers and Soils by Free Flow Electrophoresis, U.S. Geolo~cal Survey, Water Supply Paper 1817-E, Washington. D.C.. 14 p. Ludzack. F. J. and Ettinger, M. B. (1960) Chemical structures resistant to aerobic biochemical stabilization. J. Water Poll,a. Control Fedn. 32, 1173. Mahler, H. R. and Cordes. E. H. (1969) Biological Chemistry Harper & Row. New York, p. 1009. Mateles, R. I. and Chian. E. S. K. (1969) Kinetics of substrate uptake in pure and mixed cultures. Environ. Sci. & Technol. 3. 569. Mathur. S. P. and Paul, E. A. (1967) Microbial utilization of soil humic acids. Can. J. Microbiol. 13. 573. Mao, M. C. M. and Pohland, F. G. (1973) Continuing investigations of landfill stabilization with leachate recirculation, Special Research Report, Dept. of Civil Enginering. Georgia Inst. Technol, Atlanta. GA. McCarty. P. L. (1971) Energetics and bacterial growth. Organic Compounds in Aquatic Environments (Edited by Faust S. D. and Hunter, J. V.). Marcel Dekker, New York 495 p. Miller, D. W. et al. (1974) Groundwater Contamination in the Northeast States, U.S. Environmental Protection Agency Report EPA-660/2-74-056, Washington, D.C., 338 p. Ogura, N. (1975) Further studies on the decomposition of dissolved organic matter in coastal seawater. Marine Biol. 31. 101. Piper, T. J. and Posner, A. M. (1972) Humic acid nitrogen. Plant & Soil 36, 595. Schnitzer, M. and Kahn, S. U. (1972) H,mic Substances in the Environment. Marcel Dekker. New York. Schwartz, S. M. et al. (1954) Separation of organic acids from several dormant and incubated Ohio soils. Soil Sci. Am. Proc. 18. 174. Stevenson, F. J. (1967) Organic Acids in Soil, in Soil Biochemisto,, Vol. I, (Edited by McLaren, A. D. and Peterson. G. H.). Marcel Dekker, New York p. 119. Swain. F. M. 0970) Non-Marine Organic Geochemistry. Cambridge University Press, Cambridge, 445 p. Takeda, K. and Furusaka, C. (1975) Studies on the bacteria isolated anaerobically from paddy field soil. Soil Sci. Plant Nutr. 21, 119. Tan, K. H. (1975) The catalytic decomposition of clay minerals by complex reaction with humic and fulvic acid. Soil Sci. 120. 188. van Dyke, H. (1966} Some physico-chemical aspects of the investigation of humus, The use of isotopes in soil organic matter studies. Proc. Syrup. FAO. IAEA, Brunswick, Volkenrode. FRG, 1963, Pergamon Press, Oxford, p. 129. Wang. T. S. C. et al. (1967) Dynamics of soil organic acids. Soils Sci. 104, 138. Wright, R. T. and Schnitzer, M. (1960) Oxygen containing functional groups in the organic matter of the Ao and Bh horizons of a podzol. Trans. Seventh Int. Cong. Soil Sci. 2. 120.
STABILITY OF ORGANIC MATTER IN LANDFILL LEACHATES E. S. K. CHIAN Department of Civil En~neering. University of Illinois, Urbana. Illinois 61801, U.S.A. IReceived 7 September 1976~
Abstract--Samples of grossly polluted groundwater and of leachate collected from wells or underdrains near solid waste disposal sites were subjected to extensive organic analyses. The soluble organics were concentrated, separated and characterized by membrane ultrafiltration, gelpermeation chromatography and analysis for functional groups and specific organics. Free volatile fatty acids represented the largest group of organics, and this fraction showed a rapid decrease with increasing age of the fill The most stable group of organics with increasing age was a fulvic-like material with a relatively high carboxyl and aromatic hydroxyl group density. Increasing stability was further noted for carbohydrates. hydrolyzable amino acids and aromatic hydroxyl compounds in that order. Only leachate from a recently generating fill can be treated successfully by microbial processes because of its large biodegradable organic content; leachate from stabilized landfills is best treated by physical-chemical processes.
INTRODUCTION
The resulting higher moisture in the solid waste enhances acid fermentation of the organic matter, releasing large quantities of free volatile fatty acids, while the lowered pH solubilizes large amounts of heavy metals. The results of numerous leachate and polluted groundwater analyses show that chemical oxygen demand (COD) values as high as 89,520 mg/I can be encountered, while iron concentrations can reach 2820 mg/l and zinc 370 mg/1. Analyses of leachate samples and polluted groundwater samples collected from landfills of different ages show that the absolute concentration of most pollutants decreases the longer the fill is subject to leaching by infiltrating rainwater (Chian & DeWalle, 1976a). A gradual change in the relative composition was also noted, as indicated by a decrease in the biochemical oxygen demand (BOD) fraction of the COD with increasing age of the fill. The free volatile fatty acid carbon fraction of the total organic carbon (TOC) decreases parallel to the BOD/COD ratio with increasing age of the fill (Chian & DeWalle, 1976b). while an unidentified fraction, presumably consisting of refractory organics, shows a gradual increase. The purpose of the present study was to conduct a more detailed analysis of these refractory organics using membrane ultrafiltration, gelpermeation chromatography and specific organic anslyses, making it possible to assess the attenuation of those organics in groundwater and to predict how effectively they can be removed by microbial and physical-chemical treatment processes.
The composition of organic matter present in wastewater, surface water and groundwater will determine to a large extent its amenability to microbial degradation and to attenuation by physical-chemical processes in treatment units or in the natural environment. Degradation of organics by micro-organisms occurs sequentially through well-defined pathways (Mahler & Cordes, 1971), resulting in the formation of ATP. Organics having a high energy-yielding potential or electron equivalents (McCarty, 1971) are generally more readily removed than those with low energy-yielding potential (Chian & DeWalle, 1975). When organics are not able to enter certain pathways because of the compound's complex structure, microbial degradation may not occur at all (Ludzack & Ettinger. 1960). Removal of organics by physical-chemical processes is also determined by the character of the compounds. For example, removal by adsorption processes is dependent upon the molecular weight and chemical structure of the compound (DeWalle & Chian. 1974). In-depth knowledge of the composition of the organic matter present can therefore be used to predict how effective biological and physical-chemical processes will be in removing those organics. The degree of organic matter attenuation in surface waters will determine the extent of treatment to be employed by downstream users, while attenuation in groundwater will determine its potential for aquifer contamination. MATERIALS AND METHODS A substantial amount of groundwater pollution is caused by organics leaching from septic tanks, unTwelve representative leachate or heavily polluted lined surface impoundments, spills and solid waste groundwater samples were collected from a number of landfills. These pollutants have frequently caused solid waste lysimeters and from pilot-and field-scale solid waste landfills. The sites are situated in the United States aquifer contamination (Miller et al., 1974). Infiltrating at representative locations and cover a range of temperarainwater leaches contaminants from the solid waste, ture and precipitation regimens, The samples were coland these substances then enter underlying soil strata. lected anaerobically from underdrains or wells that 225 w.R, i I,,2--G
226
E.S.K. CHIAN
reached to the polluted groundwater. Gross parameters were analyzed according to Standard Methods (APHA, 1971). After ultracentrifugation at 30.000rpm for 30min (L2-65B ultracentrifuge; Beckman, Palo Alto. CA) the supernatant was concentrated with a 500-MW ultratiltration (UF) membrane (UM 05; Amicon, Lexington, MA) while a separate aliquot was fractionated with a 10,000-MW UF membrane (UM 10). The 500-MW UF permeate was analyzed for free volatile fatty acids by gas chromatography (GC) using a Hewlett-Packard 5750 B Chromatovaphy (Palo Alto, CA) equipped with a flame ionization detector and a column consisting of 20% neopentyl glycol succinate and 2°0 phosphoric acid on Chromosorb PAW 60--80 mesh (Mateles & Chian, 1969). The 500-MW UF retentate was further fractionated using a G-75 Sephadex column (Pharmacia, Piscataway. N.J.}. All fractions were characterized by TOC, while the Sephadex fractions were further analyzed for carbohydrates as dextrose (anthrone test), amino acids as lysine (ninhydrin test after 24 h acid digestion with 6N HCI), carboxyl groups as acetic acid (hydroxylamine test), carbonyl groups as acetophone (2,4-dinitrophenylhydrazine test), and aromatic hydroxyl groups as tannic acid (Folin-Denis test), as described by Chian & DeWalle (1976c). RESULTS Analysis of the gross properties of the twelve leachate samples showed that the COD ranged from 81 to 71,680mg/1 and that lower values were generally observed the longer the fill was subjected to leaching. Most of the organics were present in the soluble form, since the suspended solids varied between 9 and 923 mg/l. When the supernatant of a sample collected from the underdrain of a University of Illinois lysimeter 2 months after placement of the solid wastes was subjected to ultrafiltration, it was found that 73% of the original TOC of 17,060mg/l permeated a 500-MW UF membrane. When a separate aliquot was ultrafiltered, 90% permeated a 10,000-MW U F membrane. Further analyses of the 500-MW permeate for free volatile fatty acids showed the presence of acetic acid (1748 rag/l), propionic acid (509 mg/l), isobutyric acid (312 rag/l), butyric acid (3075 rag/l), isova-
8O0 28
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leric acid (717 mg l). valeric acid (564 mg/l), and hexanoic acid (1488 mgl). The carbon from these free volatile fatty acids comprised 78% of the TOC of the 500-MW UF permeate corresponding to 490/0 of the initial TOC of the leachate sample. When the 500-MW UF retentate, representing 27% of the original TOC. was applied on a Sephadex G-75 column (Fig. 1), the TOC distribution in the eluate showed that 22°° of the retentate, or 6% of the original TOC. was excluded from the column as it eluted between 20 and 36 ml (desLm'tated as Fraction l(a)). This exclusion indicates an apparent molecular weight larger than approximately 30.000-50,000. The majority of the 500-MW UF retentate consisted of organics with an apparent molecular weight larger than 500 but smaller than 1000-3000 (Fraction 3(a); Fig. l). and this fraction represented 68% of the UF retentate or 18.5°o of the original TOC of the sample. Fraction 2(a), with a molecular weight between 3000 and 30,000, represented only 2.5% of the original TOC. Analyses of the Sephadex fractions showed that there were considerable differences in the specific organics present (Fig. 1), with relatively high concentrations of carbohydrates observed in the high molecular weight fraction and substantial quantities of aromatic hydroxyl and carboxylic compounds present in the low molecular weight fraction. Although the TOC data suggest that fraction 3(a) is rather homogeneous, colorimetric tests showed that this was not the case, since the maxima of the different organic compounds did not elute at the same volume; the specific compounds associated with organics of decreasing molecular weight were carbohydrates (maximum at 77 ml), carbonyl (81 ml), carboxyl (83 ml) and aromatic hydroxyl compounds (85 ml), respectively. Calculations of the amount of organic carbon contributed by functional groups or specific organics in the 500-MW UF retentate showed that 17% consisted of carbohydrates, 6% of carboxyl compounds, 3.3% of
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Fig. 2. Eluate of the 500-MW UF retentate of the Dupage County leachate as separated on a Sephadex G-75 column as characterized by total organic carbon, specific organics and functional groups. carbonyl compounds and 2.6~ of aromatic hydroxyl compounds. It was therefore assumed that the carbon in the model compounds used in the colorimetric tests reflected the actual structures and that a particular organic structure did not respond to more than one colorimetric test. Intrared analyses of Fraction l(a) showed that it was similar to a humic--carbohydrate like complex (DeWalle & Chian, 1974a) associated with lipids, while Fraction 3(a) corresponded closely to a fulvic-like material. Membrane ultrafiltration and geipermeation was then applied to a sample obtained from a 13-yr-oid fill in Dupage County in northern Illinois (Hughes et al., 1971) having TOC of only 644 mg/l. Concentration through a 10,000-MW U F membrane resulted in the retention of no material, while only 6 ~ of the TOC in the original sample was retained when a separate aliquot was ultrafiltered with a 500-MW membrane. Since the concentration of the fatty acids in this sample was below the detection limit of the gas chromatography, it was concluded that most of the organics were present as low molecular weight refractory molecules. The 500-MW UF retentate was further separated using a Sephadex G-75 column (Fig. 2). The TOC data show that only 7 ~ of the 500-MW UF retentate, corresponding to 0.5~ of the original TOC, is present as the high molecular weight humic carbohydrate fractions (fraction l(b)). Approximately 35~ of the organic compounds present in this molecular fraction are carbohydrates, which is comparable to the percentage in the high molecular weight fraction of the leachate obtained from the young fill (Fig. 1). Further comparison of fractions l(b)and l(a) showed that the percentage of TOC consisting of carbonyl compounds had decreased from 5 to 1~, while the aromatic hydroxyl contribution decreased from 4 to I~. The organics associated with the carboxyl group, however, showed a substantial increase from 5 to 36~. Evaluation of the absorption spectrum of
the 2,4-dinitrophenylhydrazine derivatized carbonyl group showed that all groups were present as aliphatic aldehydes and not as aliphatic ketones or aromatic aldehydes. Aldehydes, as opposed to ketones, can be oxidized and converted into carboxyl groups. The observed relative increase in carboxyl groups in fraction l(b) may therefore have occurred at the expense of aldehyde groups. The increased oxidation state of the sample was also reflected in the C O D / T O C ratio, which decreased from 2.9 in the first sample to 1.6 in the second sample. The molecular weight fraction included in the G-75 Sephadex (Fraction 3(b)) showed less noticeable changes than fraction 3(a). The contribution of the carboxyl group to the organic carbon in this fraction increased slightly, from 6 to 7~o, while that of the carbonyl group remained the same, and that of the aromatic hydroxyl group decreased from 5 to 3~. The contribution of the carbohydrate content was 35~ in fraction l(b) and only 9~o in fraction 3(b), whereas the contribution of aromatic hydroxyls was l ~ in fraction l(b) and 3 ~ in fraction 3(b). The absolute concentration of carbohydrates therefore tends to reflect the magnitude of the high molecular weight humic-carbohydrate like fraction, while the aromatic hydroxyls may be used as an indicator of the fulviclike material. Based on the results of these gelpermeation chromatography analyses, 500-MW and a 10,000-MW U F membranes were selected to fractionate additional leachate samples collected from landfills having ages between those of the first and second fractionated samples. Since some of these samples showed a substantial flux decrease, a 1000-MW instead of a 500-MW U F membrane was selected. The results in Fig. 3(a) and 3(b) show a gradual decrease of both the high molecular weight fulvic-like material with increasing age of the landfill. The results clearly indicate that the high molecular weight fraction in the
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membranes in leachate samples collected from landfills of different ages. 10,000-MW UF retentate decreases more rapidly with age than that in the 500-MW UF retentate, indicating that the latter fraction is less amenable to attenuation processes, such as microbial degradation or physicalchemical removal. Despite the considerable spread of the data, Fig. 3(a) and (b) also show that the magnitude of the UF retentates tend to increase during the first few years after installing the landfill and to decrease thereafter. Results of the membrane fractionation reflect the changes observed for carbohydrates, proteins and aromatic compounds as measured in the unfractionated samples (Fig. 4). It was noted that proteins are present in high concentrations in both the high molecular weight fraction (e.g., the 500-MW UF retentate) as hydrolyzable amino acids, and in the 500-MW UF permeate as free amino acids. Qualitative analyses of the 500-MW UF permeate with a Technicon Automated Amino Acid Analyzer (Tarrytown, NY) showed, in decreasing order, the presence of ornithine, lysine and valine. Changes in protein concentration in the unfractionated samples therefore reflect the magnitude of both fractions. The carbohydrate concentrations primarily reflect the size of the high molecular weight fraction. Figure 4 shows that the aromatic hydroxyl compounds associated with the fulvic-like material decrease the least with time, a finding which agrees with the results of the gelpermeation chromatography (Figs. 1 and 2). When the relative decrease of the fractions in the membrane retentate were compared with that in the membrane
permeate, i.e., the free volatile fatty acid fraction, it was found that the latter decreased more rapidly with time. These results, therefore, tend to indicate that with respect to attentuation processes, the free volatile fatty acids are less stable than amino acids, proteins, carbohydrates, and humic-like materials, and that the latter group is less stable than aromatic hydroxyl-, carboxyl- and fulvic-like substances. D I S C U S S I O N
Results of the organic analysis of the molecular weight fractions obtained by membrane fractionation and gelpermeation tend to confirm the investigations of soil and aquatic humic substances summarized previously (Chian & DeWhalle, 1976b). All showed significant concentrations of carbohydrates and hydrolyzable amino acids in the high molecular weight fractions, while carboxyl groups, aromatic hydroxyl groups, color and fluorescence are primarily contributed by fractions of lower molecular weight. Most of the previous studies used methods for functional group analyses developed in the soil sciences, primarily titration techniques (Schnitzer & Kahn, 1972). Many tests, however, give arbitrary results. The outcome of the carboxyl and aromatic hydroxyl test, for example, is dependent upon the generally unknown spectrum of dissociation constants (van Dyke, 1966). The colorimetric tests selected in the present study for functional group analyses may therefore be less arbitrary. The similarity of organics present in the
Stability of organic matter in landfill leachates 6
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Fig. 4. Carbohydrates (a), proteins (b) and aromatic hydroxyls (c) in leachate samples collected from landfills of different ages. different molecular weight fractions as observed in these studies would indicate that similar bacterial or physical--chemical processes govern the composition of organics in the natural environment. Several studies do indicate that bacterial processes are the most important. The relative composition of the individual sugars in the carbohydrate fraction tends to be similar to that of bacterial sugars (Forsyth, 1950). It was also noted that labeled glucose, added as a substrate to soil bacteria, was often recovered in the acid-hydrolyzable amino acids in the high molecular weight humic fraction (Broadbent, 1968), normally excreted by bacteria. The relatively high concentration of polar organics, such as free volatile fatty acids and dissolved free amino acids, confirms earlier studies of leachate samples. It was calculated that the fraction of the COD consisting of free volatile fatty acids can vary from 80% (Mao & Pohland, 1973) and 759/0 (Burrows & Rowe, 1975) to 40~ (County of Sonoma, 1973) and 25~ (Hughes et al., 1971), with COD values ranging from 3260 to 170,000mg/l. Such high concentrations are generally not observed during bacterial degradation of soil organic matter. It is well known, however, that free volatile fatty acids are intermediates during decomposition of complex organics in soil. Schwartz et al. (1954), for example, showed that the addition of glucose to aerobic soils rapidly increased the acetic acid content. Takeda & Furusaka
(1975) noted that glucose amendments to anaerobic soils produced mainly acetic and butyric acid, while additions of peptone or amino acids increased the amounts of branched fatty acids; branched acids were also noted in the present study. While bacteria produce free volatile fatty acids, fungi excrete primarily nonvolatile acids such as citric, oxalic, fumaric, succinic and malic acid (Stevenson, 1967). However, anaerobic conditions, along with high moisture content, generally favor production of bacterial acids as opposed to fungal acids (Wang et al.. 1967). Adamson et al. (1975) also noted that butanol, ethanol, acetone and smaller quantities of other alcohols and carbonyls were formed when glucose was added to anaerobic solids. They reported that 8~ of the organics remaining in the soil after the glucose additions could be accounted for by these compounds. This result is comparable to those of Burrows & Rowe (1975) who noted that 4?/o of leachate COD consisted of propanol, ethanol, acetone and other alcohols. The presence of dissolved free amino acids such as the ornithine, lysine and valine found in the present study has also been noted in other investigations. Crawford et al. (1974), for example, found that omithine, serine and glycine represented the majority of dissolved free amino acids in an estuary, a result of their very low bacterial uptake rates and of the low percentage respired. Similarly, low uptake rates were
230
E.S.K. CHI,x~
noted for valine, arginine and lysine. Glutamic and in filtrated seawater (500-MW UF permeate) aspartic acid. on the other hand. were found to have degraded more rapidly after incubation than did the highest bacterial uptake rates and highest percent- other fractions. A relative increase was sometimes age respiration; these acids also represent the major- noted for the 100.000-MW UF retentate fraction. ity of the acids in the intracellular amino acid pool DeWalle & Chian (1974a) similarly noted that low (Clark et al., 1972). Clark et al. (1972) and Degens molecular weight organics such as free volatile fatty et al. 11964) noted a high concentration of dissolved acids, amino acids and aldehydes were degraded preNycine. serine and omithine in seawater samples, ferentially, followed by an increase of a bacterially while Brown et al. (1972) noted that leucine, lysine excreted, high molecular weight humic-carbohydrate and valine were the main hydrolyzable amino acids like fraction. Only extended bacterial degradation in anaerobic sediments. These amino acids also de- caused a gradual decrease of this high molecular creased the least with increasing burial time or in- weight fraction (Chian & DeWalle, 1975). These creasing sediment depth. Glutamic and aspartic acid, studies therefore tend to support the results of the on the other hand, showed the most rapid decrease present study and may explain the small increase in with burial time. Attachment or amino acids to humic size of the 10,000-MW UF retentate during the first substances reduces their rate of decomposition, and few years (Fig. 3(a) and 3(b)); the more rapid decrease Piper and Posner (1968) have shown that the amino of the 10,000-MW UF retentate as compared with acids most likely to attach to humic substances are the 500-10,000-MW fraction thereafter could result those that have a free amino acid group such as from preferential bacterial degradation. The presence lysine, ornithine and arginine. In the present study, of significant amounts of fulvic-like organics in leathe detection of ornithine, lysine and valine as chate from older fills agrees with results from Benoit opposed to glutamic and aspartic acid may thus indi- & Starkey (1968), who found that incorporation of cate that the bacterial degradation of leachate occurs aromatic hydroxyl compounds makes the resulting organic matter complex less susceptible to microbial at a relatively slow rate. After release of the bacterially derived organics, degradation. subsequent degradation will occur. Degens et al. The preferential removal of the organics in the (1964), for example, noted a rapid decrease of carbo- 10,000-MW UF retentate could also be caused by hydrates in sediments with increasing burial time, coagulation processes. Gjessing (1970), for example, while a less rapid decrease was observed for amino noted that the high molecular weight fraction was acids. No reduction in the amount of humic sub- subject to greater reduction by minerals than were stances was noted, while aromatic compounds the lower molecular weight organics. Chian & Deshowed an increase with burial time. Both lshiwatari Walle (1976a) found that lime coagulation removed (1971) and Swain (1970) noted that the stability of primarily the high molecular weight fraction and did carbohydrates, hydrolyzable amino acids, humic acids not greatly affect the fulvic-like material. In the and aromatic compounds in sediments i~acrease in present study, however, no significant correlation that order. Although there is a considerable spread was observed between the magnitude of the high in the analytical results, Fig. 4 similarly tends to indi- molecular weight fraction and the concentration cate increasing stability for carbohydrates, hydrolyz- of Ca and Fe which could have caused such able amino acids and armatic hydroxyl compounds. coagulation. Adsorption processes definitely do not result in the In the present study it was found that the humicpreferential removal of high molecular weight carbohydrate like material in the 10,000-MW UF retentate decreased more rapidly with increasing age organics. Tan (1975), for example, reported that than the fulvic-like material present in the adsorption of humic acids onto kaolinite and mont500-10.000-MW membrane fraction. This difference moriUonite reduced primarily the low molecular could be a result of preferential microbial degrada- weight fraction included in Sephadex G-50 but did tion, enhanced coagulation and adsorption, and the not reduce the excluded and therefore larger than more rapid downward leaching of this high MW frac- 10,000-MW fraction. Similarly, Bloomfield et al. tion. Its decrease could also be caused by gradual (1975) found that clay particles removed observed conversion to the fulvic-like material by chemical pro- principally the 10,000--30,000-MW UF fraction, while cesses. Mathur & Paul (1967), in studying the fungal a smaller decrease was observed for the organics in degradation of humic acids separated by gelpermea- the 30,000-MW UF retentate. DeWalle & Chian tion chromatography, observed that the largest remo- (1974b) noted that activated carbon preferentially val occurred with the <6000 MW fraction included adsorbed the fulvic-like organics while it was less in G-25. Substantial losses were also noted for the effective in the removal of the high molecular weight > 35,000 MW fraction excluded from G-75, while the humic-carbonydrate like organics. Leaching prosmallest losses were observed with the intermediate cesses likewise do not preferentially affect the high molecular weight fraction. Degradation of the high molecular weight fraction. Gonzales & Hubert (1972) molecular weight fraction resulted in the formation and Leenheer & Malcolm (1973) actually found that of intermediate molecular weight organics. Ogura leaching would preferentially remove the fulvic-like (1975) observed that low molecular weight organics organics because of their greater mobility.
Stability of organic matter in landfill leachates A gradual conversion of high moiecular weight humic substances into low molecular weight fubic acids by microbial or chemical processes has been noted in several studies. Aleksandrova (1966), for example, reported that 40°~ of humic acid incubated in soil appeared to be converted to fulvic acids, while the condensation of fulvic acid into humic acids was observed to a much lesser extent. Environmental conditions can also influence the microbial or chemical process that determine the relative degradation of fractions having different molecular weights. Both Brown et al. (1972) and Berryhill et al. (1972) noted that humic acids were most abundant in reducing sediments, while fulvic acids constituted the largest fraction in oxidizing environments, and that these oxidizing environments also contained less organic barbon. Since a gradual increase of the ORP from - 2 0 0 to +200-MV with increasing age of the fills was found in the present study, such conversion could well have occurred. In addition, Wright & Schnitzer (1960) observed that the relative magnitude of the fulvic-acid fraction in podzols was positively correlated with increasing humification of soils, which also corresponded to an increasing content of carboxyl and carbonyl groups and a decrease in the amount of compounds having aliphatic structures. The results of these related studies tend to agree with the results obtained in the present study, in which increasing stability (and hence decreasing susceptability to attenuation processes) was observed for free volatile fatty acids, low molecular weight aldehydes and amino acids, carbohydrates, hydrolyzable amino acids, humic acids, aromatic hydroxyls, and fulvic acids in that order. The sequence reflects, most likely, increasing stability of the organic matter to microbial degradation. Some of the attenuation, however, may result from preferential coagulation processes. The analysis of organic matter in leachate and polluted groundwater collected below landfills thus shows that stability of the organics against bacterial degradation increases with increasing age of the landfill. This finding implies that in the case of a recently installed fill, biological treatment processes such as activated sludge, aerated lagoons or anaerobic filters will be effective in removing from the leachate those organics which are the result of high concentrations of biodegradable free volatile fatty acids. Organic matter from older fills is primarily present as refractory fulvic-like organics, which are better removed by physical--chemical processes such as adsorption onto activated carbon (DeWalle & Chian, 1974b) and reverse osmosis (Chian & DeWalle. 1976a). Organics in the leachate from a recently installed fill will also be largely attenuated by underlying soil strata if microbial methane fermentation occurs to remove the large concentrations of free volatile fatty acids. The attenuation of leachate from older landfills will be less significant because physical-chemical attenuation processes in soil are generally less effective than microbial processes (Genetelli & Cirello, 1976); the mol-
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ecular weight range as well as the functional groups of the fulvic-like organics, however, are well suited for adsorption processes. CONCLUSION It was concluded that concentration, separation and identification of organics in grossly polluted groundwater and leachate can be achieved by membrane ultrafiltration and gelpermeation chromatography followed by specific organic analyses with colorimetric tests. Analyses of a sample collected from the underdrain of a recently installed solid waste lysimeter showed that the majority of the organics consisted of free volatile fatty acids. The next largest group is a fulvic-like material of intermediate molecular weight, also characterized by a relatively high carboxyl and aromatic hydroxyl group density. A small percentage of the organics consisted of a high molecular weight humic-carbohydrate-like complex. Analysis of samples collected below landfills of different ages showed that the free volatile fatty acid fraction decreased considerably with increasing age of the fill. The high molecular weight fraction present in the 10,000-MW UF retentate decreased more rapidly than the 500-10,000-MW fulvic-like fraction, reflecting the greater stability of the latter fraction. The results of the organic analysis tended to agree with those of membrane separation; increasing stability was noted for carbohydrates, hydrolyzable amino acids and aromatic hydroxyl compounds with increasing age of the fill. The results of organic analyses are necessary to select optimal treatment processes for the removal of organic matter from polluted groundwater or leachate. Leachate from a recently generating fill is therefore best treated by microbial processes, while organics in stabilized leachate are preferably removed by physical-chemical processes such as activated carbon adsorption and reverse osmosis.
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