Sources of nitrate contamination

In the world, there are multiple sources that contribute to the overall nitrate content of natural waters such as geological characteristics, man-made sources, atmospheric nitrogen fixation and soil nitrogen. Wastewater in the upper soil layer can infiltrate the groundwater aquifer from the disposal ponds. The lack of a sewage system encourages these forms of nitrate pollution. Nitrate in groundwater and soil can be derived from natural or point sources, such as water distribution systems and livestock infrastructure by percolation induces contamination of surface water, groundwater and wells. One of the anthropogenic causes of groundwater nitrate pollution is waste materials. The use of nitrogen (N) fertilizer in agr iculture has significantly increased over the past 30 years to meet the food requirements of the speedily growing population. Therefore, the use of nitrate fertilizers causes the foremost predicament in groundwater contamination. Nitrate is a key contaminant present in effluent wastewater of various industries including fertilizer, metal ores, explosives, and paper mills. Generally, nitrate level in the contaminated water varies from 200 to 500 mgL"1 based on the nature of source as listed in Table 1. Low-level waste from nuclear industries contains as high as 50000 mg N03'L_1. Waste disposal site drainage and municipal waste are liable to get oxidized to nitrate that also augments pollution of groundwater with nitrate. Apart from chemical industries, nitrate waste is also foimed in electronic, mining, and petroleum industries. All these wastewaters, when disposed into the natural water bodies, cause severe health and environmental concerns.

Table 1. Nitrate levels of various wastewater sources as reported in the literature (Rajmolian et al. 2018).

Sr. No.

Wastewater source

Nitrate level (mg Г1)

Reference

1.

Domestic wastewaters

70-85

Oladoja and Ademoroti (2006), Wu et al. (2007)

2.

Fertilizer, diaries, metal finishing industries

200

Peyton et al. (2001)

3.

Brackish water

1000

Dorante et al. (2008)

4.

Tannery

222

Munz et al. (2008)

5.

Glasshouses waste

325

Park et al. (2009)

6

Explosives factoiy

3600

Shen et al. (2009)

7.

Nuclear plant

50,000

Francis and Hatcher (1980)

Remediation technologies available for fluoride and nitrate removal

Traditionally, various methods are being used for removal of fluoride (Figure 1) and nitrate from contaminated water, among them liming (Harrison 2005), ion-exchange or precipitation (Tressaud

2006), activated alumina (Ghorai and Pant 2005), alum sludge and calcium, reverse osmosis and electro-coagulation are popular in developing nations (Hu et al. 2003, Seim 2008). Nalgonda technique is most popular in our country but major drawback found in this technology is the presence of high residual aluminum concentration (0.2 mg L_1), even higher than WHO standards (Ayoob

Flow chart showing various technologies for remediation of fluoride

Figure 1. Flow chart showing various technologies for remediation of fluoride.

et al. 2008). Among other methods, adsorption is the reliable technique because it is easy to operate, and provides wider choice for adsorbents’ selection (Mohapatra et al. 2009).

Conventional methods of fluoride and nitrate removal from contaminated water (Piddennavar and Krishuappa 2013) have some limitations as they produce chemical waste in the environment system, have high cost and energy involved, and presence of secondary pollutants in wastewater (Geutili and Fick 2016). Therefore, microbial methods can be a viable alternative when they have the capability of developing resistance to different pollutants through bioaccumulation, biotransformation and biosorption (Chouhan et al. 2012). Cost-effectiveness, operational simplicity and less sludge production are major advantages of biological approach of remediation. However, in many studies phytoremediation and bioremediation are found as two of the most promising techniques for removal ofpollutants from groundwater and soil (Cho et al. 2011, Hu et al. 2000, Kushwaha et al. 2014, Sindelar et al. 2015).

Bioremediation processes of fluoride removal

A natural process of mineralization of organic and inorganic compounds through various microbes is called biodegradation. These microbes are bacteria, fungi and algae (Rutkowska et al. 2002). An essential step in the biodegradation is delialogenation of organohalogen compounds catalyzed by different delialogenases (haloaciddelialogenases, halohydrindehalogenases and haloalkanedehalogenase), which have the capability to dehalogenate the aliphatic and aromatic compounds (Janssen et al. 2005). The fluoroacetate degrading bacteria are Acmelobacter,

Arthrobacter, Aureobacterium, Bacillus, Pseudomonas, Streptomyces and Weeksella (Kumar and Haripriya 2013) with Synergistetes (a single ruminal bacterial phylum) which shows the ability to degrade sodium fluoroacetate (Leong et al. 2010, Davis et al. 2011) (Table 2).

The fluoroacetate is defluorinated by fluoroacetate dehalogenase and is found in Pseudomonas sp. The enzyme fluoroacetatedehalogenase is well known for its ability to break the highly stable carbon-fluorine bond (Donnelly and Murphy 2009) and is isolated and purified from various microbial species like Burkholderia sp. FA1, Fusariumsolani, Moraxella sp. B, Pseudomonas fluorescens DSM8341 and a soil Pseudomonas sp. (Leong et al. 2017). Fluorobenzeue is also used by microbial consortiums as the sole source of carbon and energy (Chen et al. 2015).

The monofluorobeuzoates are generally degradable under aerobic conditions and fluoroplieuols and fluorobenzoates under anaerobic conditions (Kuntze et al. 2011). Transformations of isomers like luorobenzoate have been studied already with the strict anaerobic Syntrophusaciditrophicus (Mouttaki et al. 2009). Under aerobic conditions, there are two possible pathways for the transformation of 2-fluorobenzoate (2-FB), both involve dioxygenation. The Pseudomonas aerugenosa and Acinetobacter RH5 are fluoride-resistant bacteria (Chouhan et al. 2012, Muklierjee et al. 2015). Such understanding will be usefi.il for designing strategies for removing micro pollutants from drinking water in engineered systems. The role of protozoans and phages in shaping prokaryotic degraders and manipulating in situ degradation is totally ignored. Only few smdies have confirmed the availability of bacteriophages in groundwater with lack of information to consider its influence on degrader (Meckenstock et al. 2015). However, some factors limit the bioremediation process, i.e., slow rate of degradation, adverse climatic conditions such as temperature, moisture, pH, ionic strength or redox status, limited supply of nutrients, limited amounts of electron acceptors and high concentrations of pollutants (Kumar et al. 2017).

Table 2. List of fiuonde compound degradmg microorganism (Smgh and Gothalwal 2016a).

Sr. No.

Fluoride compound

Microorganism

Reference

1.

3-Fluorobenzoate.

Sphmgomonas sp.

Boersma et al. (2004)

2.

Fluorobenzene

Sphingobactenum/Flavobactenum

andAlcaligenes

Carvalho et al. (2002)

3.

Fluoranthene

M. vanbaalenu PYR-1

Kweon et al. (2007)

4.

4-Fluorobenzyl

Alcohol Pseudomonas spp.

Maeda et al. (2007)

5.

Fluoranthene

Mycobacterium sp. JS14

Lee et al. (2007)

6.

2-Fluorophenol

Rhodococcus sp. FP1

Duque et al. (2012)

7.

a,a,a-Tn fluoroacetophenone

Gordonia sp. SH2

Hasan (2010)

8.

4-fiuorocmnamic acid

Consortium of Arthrobacter sp. strain

G1 and Ralstoma sp. stram HI

Hasan et al. (2011)

9.

Fluoroacetate

Synergistetes

Davis et al. (2012)

10.

Sodium fiuonde

Aspergillus sp. and Rhizopus sp.

Tamilvani et al. (2015)

11

Sodium fiuonde

Acinetobacter sp. RH5

Muklierjee et al. (2015)

 
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