Table 132

Examples for Research Topics and Applications Involving Organic Acids in the Environment

Air—Including Atmospheric Precipitation3-9

Contribution to acidic rain

Role in depletion of ozone layer

Identification of biogenic and anthropogenic sources of organic acids Photochemical transformation of organic compounds to organic acids

Water10-18

Formed as ozonation by-product in drinking water treatment from natural organic matter

Intermediates of anaerobic degradation of hydrocarbons (naturally occurring or at contamination sites) in groundwater

Metabolic pathways and organic acid turnover rates in deep seawater and sediment

Corrosion control of ultrapure water in industrial processes

Soil19-23

Sources: Excretion from plant roots and microorganisms. Anaerobic degradation of organic litter Metal mobilization. Complexation of Al (toxic for plants at certain concentrations)

Biological availability of nutrients (e.g., P, Fe)

Mineralization processes leading to podsolization of soil

Waste24-32

Control and monitoring of anaerobic digestion of organic material in wastewater treatment processes Monitoring of VFA in landfill leachates as indicators for onset of methane production, metal mobilization, and leaching of other contaminants Intermediates in remediation of organic pollutants with advanced oxidation processes Fermentation processes in biological materials analytical techniques in details, the motivation behind organic acid analysis is briefly summarized by matrix.

A. Air Including Atmospheric Precipitation

Organic acids in rain were first detected in the 1970s.5 Since then they have been found in tropospheric gas and aqueous phases, and also adsorbed onto tropospheric particulate matter. More specifically, organic acids have been measured in fog water, cloud water, rain, snow, ice, the gas phase, and on particles in varying locations including highly urbanized areas and remote regions. Formic and acetic acid are usually present in higher concentrations than any other organic acids, such as dicarboxylic and/or ketoacids. Organic acids contribute significantly to the total organic carbon content and to the acidity in fog, cloud water, and wet precipitation. It is reported that in North American rain up to one third of the total free acidity is caused by organic acids.33'34 However, it is postulated that organic acids do not contribute substantially to the long-term acidification of the environment since these are easily biodegradable.33 Major sources for organic acids in the troposphere are direct biogenic and anthropogenic emissions but also there are indirect contributions through photochemical formation from organic precursors.3 Biogenic sources include emissions from soil and especially vegetation,9 whereas anthropogenic emissions can be traced back to biomass combustion (e.g., forest fires, agricultural burnings)6-8 and incomplete combustion of fuel and fuel additives (e.g., motor exhausts).35,36 Photochemical processes in the gaseous phase and to some extent in the aqueous phase lead to organic acid formation via radical reactions involving, for example, oxidation of hydrocarbons and aldehydes by free radicals (e.g., -OH, HO2') and other oxidants (e.g., ozone).3'4'6'34'37'38 The role of particulate matter and therefore of heterogeneous interactions in these processes remains unclear at this time. With the exception of ketoacids which are susceptible to photolysis, main losses of organic acids from the troposphere occur through wet or dry deposition. A more detailed overview of organic acids in the troposphere can be found elsewhere.3 Current research continues to further catalogue possible emission sources of organic acids, documents their occurrences and residence times, elucidates reaction mechanisms leading to organic acid formation, and investigates their role in the depletion of the ozone layer.4 Ice-core samples can put current results into a historical context.6 It should also be noted that organic acids are not regulated as air pollutants.

B. Water

1. Drinking Water

Short-chain organic acids such as formic, acetic, oxalic, glyoxylic, pyruvic, and ketomalonic acid are formed as by-products in drinking water treatment plants employing ozone.10'39-44 Reasons for ozone applications include destruction of taste and odor compounds, color removal, and pretreatment.11 Moreover, ozone is often used as a disinfectant and as such it can at least partially substitute chlorine, thus lowering formation of chlorinated disinfection by-product, which is a major issue for the drinking water industry in North America. During ozonation natural organic matter in raw or partially treated water is oxidized, leading to the formation of a range of byproducts dominated by aldehydes and organic acids with the latter formed in higher concentrations.45 Ozone contactor effluents may have concentrations as high as 250 /Ag/l for formate or 195 /Ag/l for acetate.10 Subsequent treatment steps such as biological filtration can remove aldehydes completely, whereas organic acids may only be partially removed.10 Smaller concentrations of organic acids can therefore pass into the finished drinking water and serve as nutrients for microorganisms, thus leading to bacterial regrowth in distribution systems which are used to transport the finished drinking water to the end user. Current research continues to assess the formation, removal, and impact of ozonation by-products such as organic acids. Organic acids are not regulated in drinking water.

2. Wastewater

Volatile fatty acids (VFA) are defined as "water soluble fatty acids which can be distilled at atmospheric pressure."46 These are comprised of aliphatic, short-chain organic acids with chain lengths of up to seven C-atoms and are formed during anaerobic fermentation of organic material. Removal of organic substrate through anaerobic digestion has been applied primarily to waste sludge, but it is also used as pretreatment for high organic waste streams and is even becoming common for the treatment of dilute waste streams.25 Organic substrate removal is accomplished by fermentation, i.e., break down of larger molecules (e.g., fats, proteins, hydrocarbons) followed by a two-step anaerobic degradation process of these break-down products leading to hydrogen and methane as end products.24,25 More specificially, acidogenic bacteria convert organic breakdown products into VFA and H2 with acetate being the major product. This is followed by metabolization of VFA by methanogenic bacteria into methane. These two processes have to be in balance to ensure successful treatment. The difficulty lies in the fact that methanogenic bacteria metabolize at a slower rate than the acidogenic bacteria. In addition, methanogenic bacteria are usually more susceptible to sudden changes in their environment than the acidogenic bacteria.24-26 Hence, it is possible that VFA may not be consumed at the same rate as they are produced with the consequence that VFA concentrations increase. This may disturb the balance between these processes even further. Changes in background concentration and VFA composition are therefore used as indicators in the operation of these anaerobic processes and are monitored for process control purposes.24 - 26 VFA concentrations in wastewater and diluted sludge are quite high (medium to high mg/l), which simplifies the analyses of this rather complex matrix (high concentrations of inorganic and organic compounds) requiring only moderate detection limits.24'25

3. Groundwater

Organic acids in groundwater can be found close to seeps of naturally occurring hydrocarbons, or in the proximity of sites contaminated with, for example, petroleum hydrocarbons. Biodegradation of hydrocarbons under the anaerobic conditions of the groundwater aquifer leads to a variety of metabolic intermediates, including organic acids.14'47'48 High concentrations of short-chain aliphatic organic acids are most commonly observed as intermediates, even when aromatic hydrocarbons are the original source. Localized concentration of formate, acetate, and isobutyrate combined may be as high as 9000 ^imol/l,13 but vary considerably depending on the rate of organic acid production and consumption. Hence, factors such as the proximity to the hydrocarbon source, hydrocarbon loading, microbial population, presence of microbial inhibitors, availability of nutrients, and availability of electron acceptors affect the organic acid concentration in groundwater.

High organic acid concentrations in groundwater may cause mineral dissolution. This can lead to changes in soil structure, and complexation of metals such as Fe or Al, which may subsequently be mobilized into the aquifer.13 Hence, it is important to take the organic acid production and its consequences into account when observing and projecting the transport of hydrocarbon contamination in groundwater,13 and when estimating changes in the porosity of oil reservoirs due to organic acid formation in oilfield water.15,49

4. Seawater

In marine environments such as deep seawater and marine sediment pore water, low-molecular weight (LMW) organic acids are formed as intermediate metabolic break-down products from larger organic molecules under anoxic conditions.16,17 Depending on environmental conditions, organic acids are often further degraded to CH4 and CO2. Research focuses on the elucidation of metabolic pathways and organic acid turnover rates.17 There are also specialized research interests such as measuring acrylic acid in seawater, algal cultures, and sediment pore water.50 Acrylic acid may be released from (dimethylsulfonio)propionate, a compound present in many marine algae.

5. Other Water

Organic acids have also been analyzed in various other aqueous matrices. LMW organic acids have been detected as intermediates during remediation of organic pollutants using advanced oxidation processes30 and upon UV irradiation of aqueous dissolved organic matter.12,51 Ultrapure water used in certain industries for cooling and production processes (e.g., power generating industry, electronics industry) is monitored for inorganic anions and organic acids, since their presence may lead to corrosion and other disturbances of the production process.18

C. Soil

Organic acids play an important role in soils although these typically comprise only 0.5 to 5% of the dissolved organic carbon (DOC) in soil solutions.21 Acids such as lactic, acetic, oxalic, succinic, fumaric, malic, citric, and aconitic acid have been detected frequently in soil solutions.19 Main sources are excretion from plant roots (e.g., root exudates), release by microorganisms, and degradation of organic matter (e.g., plant litter). Concentrations of organic acids in soil solutions are in the /¿molar range with significant spatial and temporal variations. The highest concentrations are usually found in organic-rich soil layers.19'21 Concentrations are influenced by a number of very complex processes. These processes are based on proton release by organic acids causing pH change in localized environments, and on the complexing ability of various organic acid anions. Effects associated with these processes include an increase in mineral nutrients (e.g., phosphate, iron) which are biologically available for plants, thus enhancing plant growth; reduction in Al concentrations by complexation which could otherwise be toxic for plants; metal mobilization either by direct interaction (e.g., desorption) or through complexes and subsequent metal transport into deeper soil layers; stimulation of bacterial growth by enhancing the carbon source; and mineral weathering leading to podzolization.19-21 Research into these very complex processes has expanded over the last decade since more sophisticated analytical techniques became available to measure organic acids in soil matrices.

D. Others

1. Landfill Leachates

The anaerobic degradation of organic waste in landfills passes through several stages with methane and carbon dioxide as end products.27,28 During the acidogenic phase, organic acids, mainly VFA, are formed in high concentrations and thus contribute significantly to the DOC fraction in landfill leachates, which is the solution collecting at the bottom of a landfill. One concern associated with VFA production is the possible mobilization of heavy metals from the landfill into the underlying aquifer if there is either an incomplete seal of the liner or if there is no liner at all, which is the case for many older landfill sites. Monitoring VFA concentration in landfill leachates allows for the determination of the degradation stage of the organic waste in the landfill and the prediction of the onset of methane production. It also serves as an indicator for the mobilization of organic pollutants and heavy metals.27,29 When VFA concentrations are rising, precautions can be taken to prevent leakage of pollutants into the groundwater through, for example, collection and treatment of leachates.28

2. Biological Material

Matrices covered in this section are very diverse. Examples listed in Table 13.3 include silage juice, fermentation products, hydrolysated biomass residue, cellulose polymer, and also biological

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