Introduction
Nitrate removal is a complex process because nitrate is a stable and highly soluble ion with a low potential for precipitation or adsorption.
Nitrate is seldom present in geological formations and therefore contamination due to nitrate is mainly attributed to anthropogenic sources such as sewage and fertilizers.
Microbial nitrification is the natural origin of nitrate.
Through this process, ammonia is converted into nitrite and then nitrite to nitrate by the Nitrosomonas genus and Nitrobacter genus bacteria, respectively.
Since nitrate has a more stable oxidation state than nitrite, it is less absorbed by the aquifer matrix.
Due to its mobility, it can travel long distances and pollute the groundwater easily.
Pollution of water resources by nitrate occurs due to industrial wastewater containing nitrate, domestic wastewater, fertilizers in agriculture, discharges from animal operations, wastewater treatment facilities, septic systems and commercial activities.
Other sources include atmospheric deposition and the spreading of sewage sludge to land and seepage from landfills.
The main factors that affect the potential for nitrate contamination of groundwater include land-use practices, well-depth, and soil type.
Nitrate concentrations in groundwater can change over months and years due to changes in land use such as increased agricultural activities, deforestation, and installation of septic systems.
Groundwater concentrations exceeding an arbitrary threshold of 3 mg/l may be indicative of contamination of natural groundwater as a result of human activities.
A variety of analytical techniques and sensory methods have been employed to detect and identify the source of nitrate contamination.
Techniques like voltammetry, Spectroscopy, Ion-selective electrodes, etc. are used extensively in water quality assessment.
Both nitrate and nitrite are significant public health concerns since they can cause methemoglobinemia or “blue-baby syndrome.”
Nitrite interferes with the ability of infant hemoglobin to carry oxygen. Left untreated, this condition may lead to brain damage and death.
Excess levels of nitrate affect health by forming hypertension, thyroid disability, and carcinogenicity hazard of nitrosamine.
Studies on animals in the laboratory have not indicated that nitrate or nitrite is directly carcinogenic, but it may react in the stomach with food containing secondary amines to produce N-nitroso compounds (NOC) which are known to be carcinogenic in animals.
Elevated concentrations of nitrate in groundwater represent environmental health risks, nitrate export into adjacent surface water bodies may induce an increased level of nutrients (eutrophication) affecting adversely biodiversity, mammals, birds and fish population by producing toxins and reducing oxygen levels and can cause poisoning in animals.
Besides, denitrification processes contribute to the emission of greenhouse gases due to the production of N2O.
Nitrate is a stable and highly soluble ion with a low potential for precipitation or adsorption.
These properties make it difficult to remove from water using conventional processes such as filtration or activated carbon adsorption. As a result, more complex treatment processes must be considered.
These treatment processes– ion exchange, reverse osmosis, electrodialysis, biological denitrification, catalytic denitrification, hybrid systems based on fly ash adsorption, membrane filtration and electrocoagulation.
Chemical denitrification can be accomplished with the reduction of nitrate by metals.
Various metals have been investigated for use in nitrate reduction including aluminum and iron (both FeO and Fe2+).
The advantage of chemical denitrification over the removal technologies is that nitrate is converted to other nitrogen species rather than simply displaced to a concentrated waste stream that requires disposal.
Problems with the chemical denitrification of potable water are the reduction of nitrate beyond nitrogen gas to ammonia, partial denitrification and insufficient nitrate removal (nitrite can be converted to nitrate with the use of chlorine in disinfection).
No full-scale chemical denitrification systems have been installed in the United States for the removal of nitrate in potable water treatment.
A significant body of research has explored the use of zero-valent iron (ZVI) in denitrification.
Several patented granular media options are also emerging, including SMI-III® (Sulfur Modified Iron), Micro-Nose™ Technology, and Cleanit ®-LC.
The hydrogenation via catalytic method is one of the promising techniques for the removal of nitrate from water.
It needs very active catalysts because the reaction is performed preferably at an ambient/low temperature, the reaction scheme shows that nitrate is reduced to the desired products involving NO2 -, NO, N2O and N2.
The undesired byproduct NH4 is also formed by a side reaction due to over-hydrogenation.
This research aims to investigate the potential of removing nitrate from drinking water using sodium thiosulfate and its impact on human health.
Research results of nitrate removal
Sodium thiosulphate (Na2S2O3, STS) is an industrial compound that is typically available as the pentahydrate, Na2S2O3 · 5H2O.
It has medical uses in the treatment of some rare medical conditions.
These include Calciphylaxis in hemodialysis patients with end-stage kidney disease as well as cyanide poisoning.
It also functions as a preservative in table salt (less than 0.1 %) and alcoholic beverages (less than 0.0005 %).
Table 1 shows the effect of adding different doses of sodium thiosulfate on different parameters of raw water.
You can notice that there is an increase in the level of electrical conductivity and total dissolved substances by increasing the dose of 1.09% solution of sodium thiosulfate, but there is a decrease in the level of nitrate.
Table 1 The effect of adding different doses of sodium thiosulfate to different parameters of raw water.
| Sodium thiosulfate dose | 0 | 0.1 | 0.3 | 0.5 | 1 | 2 | 3 | 5 |
| Nitrate | 41.5 | 41 | 31 | 20.5 | 11.5 | 10 | 2.2 | 1.5 |
| Ph | 7.18 | 8.04 | 8.00 | 7.99 | 7.94 | 8.08 | 7.76 | 7.85 |
| Turbidity | 0.33 | 0.17 | 0.21 | 0.26 | 0.27 | 0.23 | 0.17 | 0.34 |
| EC | 925 | 931 | 954 | 989 | 1056 | 1209 | 1346 | 1611 |
| TDS | 427 | 437 | 448 | 465 | 497 | 572 | 639 | 767 |
Table 2 shows the percentage of reduction in nitrate level and percentage increase in EC and TDS levels at different doses of 1.09 % of sodium thiosulfate solution.
72% reduction in nitrate level was noticed when 1ml of 1.09% solution of sodium thiosulfate while the level of EC and TDS were still at the acceptable level in drinking water.
The lowest dose of sodium thiosulfate that gives a high percentage of nitrate removal (72%) and a lower increase in TDS (16%, the level of TDS still below 500mg/l) was 1ml of 1.09% solution (table 2).
Table 2 The percentage of reduction in nitrate level and percentage increase in EC and TDS level at different doses of 1.09 % of sodium thiosulfate solution.
| Sodium thiosulfate | 0.1 | 0.3 | 0.5 | 1 | 2 | 3 | 5 |
| % Reduction in nitrate concentration | 1% | 25% | 51% | 72% | 76% | 95% | 96% |
| % increase in EC | 0.6% | 3% | 7% | 14% | 31% | 45% | 74% |
| % increase in TDS | 2% | 9% | 9% | 16% | 34% | 50% | 80% |
Table 3 shows the effect of contact time on the percentage of removal of nitrate by Sodium thiosulfate.
More than two-thirds of the nitrate level was removed after 30 minutes of contact time between Sodium thiosulfate and raw water.
Table 3 The effect of contact time on the percentage of removal of nitrate by Sodium thiosulfate.
| Time | % of nitrate removal |
| 10 minutes | 56% |
| 30 minutes | 69% |
| 60 minutes | 94% |
Table 4 shows the effect of 1ml of 1.09% solution of sodium thiosulfate addition to raw water on the level of total hardness, alkalinity and sulfate after one hour of contact time.
Table 4 The effect of 1ml of 1.09% solution of sodium thiosulfate addition on different parameters of raw water.
| Test | Before addition | After addition |
| Total Hardness | 490 | 500 |
| Sulfate | 53 | 56 |
| Alkalinity | 259 | 235 |
Sodium thiosulfate health effect
Sodium thiosulfate (STS) is an industrial chemical that has also been approved for the treatment of certain rare medical conditions.
These include cyanide poisoning and calciphylaxis in hemodialysis patients with end-stage kidney disease.
Sodium thiosulfate is used as a therapeutic drug for the treatment of cyanide poisoning at a dose of 1ml/hr./kg of 10% solution at a total dose of 12g, treatment of nephrogenic systemic fibrosis at a dose of 12.5 g three times a week for three months.
Concurrent injections of sodium thiosulfate intraperitoneally or intravenously at a dose of 400 mg/kg once weekly for 3 consecutive weeks prevented the hypomagnesemia and the nephrotoxic effects of cisplatin and can be of clinical significance.
Compared with saline control solution, sodium thiosulfate alone also inhibited tumor growth significantly (p < .005).
The development of embryos exposed to 0.1~1 mol/L STS was severely retarded and was accompanied by malformation of multiple organs; embryos exposed to 10 micro-mol /L~10 mmol/L STS had circulatory, nervous and maxillofacial malformations.
Pioneering studies suggest that chemicals can have in many cases very similar toxicological and teratological effects in zebrafish embryos and humans.
Sodium Nitrate health effect
Larsen et al. tested for the first time in a double-blind crossover study the effects of sodium nitrate on blood pressure(BP) in healthy volunteers and reported a significant reduction in diastolic BP (23.7 mm Hg).
The BP-lowering effects of inorganic nitrate may derive from the increased generation of nitric oxide (NO), a pleiotropic molecule involved in the vasodilation of large arteries and resistance vessels.
The endothelial isoform of the NO synthase uses arginine and molecular oxygen as precursors to tonically release NO in the endothelium, which is important for the control of vascular tone, smooth muscle growth, platelet aggregation and inflammation.
Reduced NO bioavailability has been associated with impairment of endothelial function and increased risk of hypertension and cardiovascular diseases.
Numerous studies now show that administration of nitrate or nitrite has NO-like bioactivity in animals and humans including a reduction of blood pressure, protection against ischemia-reperfusion injury, and modulation of mitochondrial function.
Nitrate supplementation, either as a sodium salt (NaNO3) or as a natural resource (e.g., beetroot juice), reduces the oxygen cost of exercise and enhances exercise p performance.
Conclusion
Removal of nitrate from drinking water by adding 1.09 % of sodium thiosulfate is an easy, cost-effective method and removes a high percentage of nitrate.
Further research is needed to confirm that the addition of sodium thiosulfate to drinking water is a safe process.
Further studies are needed to know the chemical compounds produced from the reaction between Sodium thiosulfate and nitrate.
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