Bioremediation of Fluoride and Nitrate Contamination in Soil and Groundwater
In most of the Asian countries, groundwater is mainly used for drinking and irrigation purpose (Al-Hatim et al. 2015, Raj and Shaji 2017). As a result, groundwater demand is increasing continuously due to its over-exploitation (Gleeson et al. 2012, Gupta et al. 2013, Alhababy and Al-Rajab 2015). Fluoride (F~) is naturally found in soil and groundwater, but some anthropogenic activities such as iron, steel, glass and aluminum industries or agricultural practices by using phosphate fertilizers raise the concentration of F' in groundwater (Gupta et al. 2015,2019, Srivastav et al. 2018, Mauiya et al. 2019).
Fluoride is an all-encompassing, 13th most abundant electronegative element in the crust of the Earth (Ibrahim et al. 2011). Fluorine is an important trace element that maintains normal animal and human physiological function while excessive or inadequate intake may affect the health of living beings (Liu et al. 2014). Lack of fluoride may create teeth problems, whereas excessive fluorine has a detrimental effect on environmental quality and causes health disorders, including dental fluorosis, skeletal fluorosis, impaired thyroid function and decreased childhood intelligence. With rapid economic growth and widespread human activity, environmental pollution from nonpoint agricultural and point industrial sources is becoming more serious, resulting in air, water and soil contamination (Wu and Sun 2016). Fluoride (F) exhibits non-biodegradability by biological enrichment and food chain, with apparent toxicity to many species. Thus, a small amount of F pollution can pose harm to living beings (Szostek and Ciecko 2017).
The fluorosis epidemic is all embracing, and nearly 25 nations of the world are under its terrible fate. Many countries, notably India, Sri Lanka and China, the countries of the Rift Valley in East Africa, Turkey and parts of South Africa have reported the occurrence of high fluoride concentrations in groundwater (Singh and Gothalwal 2016b). In India, 15 states are endemic to fluorosis and about 62 million people are affected by fluorosis. Fluoride can act as double edged sword in drinking water (Biswas et al. 2016). While the concentration of fluoride is even less than 0.5 mg L' of drinking water, it has negative impact on human's health (Fawell et al. 2006). The optimal level of fluoride for drinking water is 0.6-1.2 mg L_1, whereas the discharge cap for industrial wastewater is 2 mg L- (BIS 2012).
Nitrogen, on the other hand, is an essential element for all living tilings. The atmosphere of Earth is rich in nitrogen, and it is naturally extracted from atmosphere. Nitrogen is a crucial building block in the synthesis of proteins, contributing 13% of the mass. Nitrogen acts as nutrient or bio-stimulant when it is present in water or soil. Nitrogen chemistry is complex as it occurs in eight different oxidation states, and it is noteworthy that living organisms can alter the state. Excess nitrogen may pose a serious environmental problem, including cultural eutrophication, methemoglobinemia, and other health problems. The pollution of nitrates in water and soil has thus become a growing concern for the environment. The maximum level of nitrate contamination, according to USEPA guidelines (1996), is 45 mg L-1, and BIS adopts the same standard. The adverse health and ecosystem effects of nitrate and concerns about decreasing water quality have increased interest in technologies for nitrate removal. The use of any mechanism in the nitrogen cycle that serves as a reservoir for nitrogen in groundwater is advantageous from a water quality point of view. Denitrification is the final step in the production of nitrogen. If oxygen is in shortage, bacteria in the soil and water use nitrate as an electron acceptor. Natural denitrification is a long term process and the rate of denitrification is increased in the biological denitrification technique by continuously supplying the carbon source and maintaining constant process parameters.
Chemical process such as precipitation, weathering, absorption and exchange of ions may influence concentrations of F' and nitrate in groundwater. Various modem methods of defluoridation and nitrate removal are already being used but these traditional methods have certain drawbacks, including high cost and energy consumption, secondary contamination after treatment and lower efficiency. In this context, bioremediation could be a cost-effective and environmentally friendly way to remove fluoride and nitrate compounds from soil and groundwater.