Adsorbable organic halides
Adsorbable organic halides (AOX) is a measure of the organic halogen load at a sampling site such as soil from a land fill, water, or sewage waste.[1] The procedure measures chlorine, bromine, and iodine as equivalent halogens, but does not measure fluorine levels in the sample.[2]
Background
Utilization of halogen containing materials in processes such as water treatment, bleaching, or even general synthesis to create the final product, generates a number of organic halides. These organic halides are released in wastewater from the oil, chemical, and paper industries,
Determination
Persistent organic pollutants such as dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenols, dioxins, are all assessed in AOX analysis. Generally, the higher the amount of chlorine in an organic compound, the more toxic it is considered.[8] While there are several biochemical or electrochemical methods to remove organic halides, AOX has been preferred due to its low cost of operation and simplicity of design.[1]
In a lab, the determination of AOX parameter consists of adsorption of organic halides from the sample on to an
Where is the activated carbon and is any organic halide.
is the organic halide - activated carbon complex that can be filtered out.
Treatment
Physical separation
In water treatment plants, organic halides are adsorbed using GAC or PAC in agitated tanks.[6] The loaded carbon is separated using a membrane made out of materials like polypropylene [9] or cellulose nitrate.[1] Measuring the AOX levels into and out of the treatment zone shows a drop in organic halide concentrations. Some processes use a two-step GAC filtration to remove AOX precursors, and thus reduce the amount of AOX in treated waters.[11] A two step filtration process consists of two GAC filters in series. The first filter is loaded with exhausted GAC, while the second filter is loaded with fresh GAC. This set up is preferred for its increased efficiency and higher throughput capacity. The GAC is replaced cyclically and the extracted organic halide-carbon mixture is then sent for subsequent biological or chemical treatment such as ozonation to regenerate the GAC.[1][11] Often, these chemical treatments, while effective, pose economical challenges to the treatment plants.
Biological treatment
A more economically attractive option for treatment of the organic halides is through utilization of biological agents. Recently, bacteria (Ancylobacter aquaticus), fungi (Phanerochaete chrysosporium and Coiriolus versicolor), or synthetic enzymes have been used in the degradation of chlorinated organic compounds.[3] The microorganisms degrade halocompounds using either aerobic or anaerobic processes. The mechanisms of degradation include utilization of the compound as carbon source for energy, cometabolite, or as an electron acceptor.[3][8] Note that enzymatic or microbial action could be regulated through feedback inhibition-the final product in the series inhibits a reaction in the process. An example of a microbe that can degrade AOX is shown below in Figures 1[12] and 2.[13]
A sample dechlorination of chlorinated aliphatic hydrocarbons (CAHs) such as
In addition to dechlorination of CAHs, microbes have also been reported to act on chlorinated aromatic hydrocarbons. An example of a reaction where aromatic AOX content has been reduced is demonstrated in figure 2 above.
Related terms
Organic halides, extractable organic halides (EOX), and total organic halides (TOX) are related content for this topic. EOX provides information on how halides can be extracted using a solvent while TOX provides information about the total organic halide content in the sample. This value can be used to estimate biochemical oxygen demand (BOD) or chemical oxygen demand (COD), a key factor in estimating the required oxygen to burn the organic compounds to estimate the percentage of AOX’s and Extractable organic halides.
References
- ^ PMID 23524399.
- ISSN 0009-2347.
- ^ PMID 16551531.
- .
- ^ Hutchins, Floyd E (1979). Toxicity of Pulp and Paper Mill Effluent: a Literature Review. National Service Center for Environmental Publications. p. 2.
- ^ ISSN 1745-6592.
- ^ .
- ^ ISBN 9783540618683.
- ^ .
- ^ "ILIAS 3". cgi.tu-harburg.de. Retrieved 2016-10-11.
- ^ .
- ^ PMID 15672270.
- ^ PMID 18460800.
- ^ PMID 21377349.
- ISSN 1930-2126.