Environmental Water Treatment Knowledge: The Meaning and Role of Ammonia Nitrogen
In natural surface and groundwater bodies, nitrogen is mainly present as nitrate nitrogen (NO3-). The ammonia nitrogen in polluted water bodies, present as free ammonia (NH3) and ammonium ions (NH4+), is also called aqueous ammonia, or un-ionized ammonia. Un-ionized ammonia is the main factor causing toxicity to aquatic organisms, while ammonium ions are relatively essentially non-toxic. For Class III surface water in the national standard, the concentration of un-ionized ammonia nitrogen is <=1 mg/L.
Ammonia nitrogen is a nutrient in water bodies that can cause eutrophication and is a major oxygen-consuming pollutant in water; it is toxic to fish and some aquatic organisms.
In an environment with pH greater than 11, ammonium ions convert to ammonia; the ammonia transfers through the hydrophobic membrane of the ammonia-sensitive electrode, causing a change in the electromotive force of the electrode, and the instrument measures the ammonia nitrogen concentration based on the change in electromotive force.
Rinse the sample container, reagent-volume container and electrode mounting tube with the new water sample.
Use a peristaltic pump for sample intake. The water sample does not directly contact the peristaltic pump tube-there is an air buffer. The intake volume is controlled by a visual measurement system.
As with sampling, the auxiliary reagents are also dosed by the peristaltic pump and the dosing volume is controlled by the visual measurement system.
Mix the water sample and reagents by bubbling.
The reaction time is automatically controlled by the measurement system.
Within a user-defined measurement cycle, the analyzer automatically performs calibration and cleaning using built-in calibration standard solution and cleaning solution.
3 How to judge the performance of an ammonia-gas-sensing-electrode-method instrument
1. Range: the ammonia nitrogen range specifications of the electrode method are divided into 0-1200; 0-2000; 0-3000; 0-10000, etc. The range can be switched freely; the larger the range, the stronger the adaptability of the electrode used by the instrument.
2. Minimum detection limit: the lower the minimum detection limit of the instrument, the better the quality of the electrode, generally 0.05 mg/L.
An alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light reddish-brown colloidal compound whose color intensity is proportional to the ammonia nitrogen content; its absorbance is usually measured in the wavelength range of 410-425 nm to calculate the content.
The minimum detection concentration of this method is 0.025 mg/L (photometric method), and the upper limit of determination is 2 mg/L. Using the visual colorimetric method, the minimum detection concentration is 0.02 mg/L. After appropriate pretreatment of the water sample, this method can be used for the determination of ammonia nitrogen in surface water, groundwater, industrial wastewater and domestic sewage.
2.1 Nitrogen-ball Kjeldahl distillation apparatus: 500 mL Kjeldahl flask, nitrogen bulb, straight condenser tube and conduit.
All water used for preparing reagents shall be ammonia-free water.
3.1 Ammonia-free water may be prepared by one of the following methods:
Distillation method: add 0.1 mL of sulfuric acid to each litre of distilled water, redistill in an all-glass distiller, discard the first 50 mL of distillate, collect the rest of the distillate in a ground-glass-stoppered bottle and store tightly stoppered.
Ion exchange method: pass distilled water through a strongly acidic cation exchange resin column.
3.2 1 mol/L hydrochloric acid solution.
3.3 1 mol/L sodium hydroxide solution.
3.4 Light magnesium oxide (MgO): heat magnesium oxide at 500 degrees C to remove carbonates.
3.5 0.05% bromothymol blue indicator solution: pH 6.0-7.6.
3.6 Anti-foaming agent, such as paraffin flakes.
Boric acid solution: weigh 20 g of boric acid, dissolve in water and dilute to 1 L.
0.01 mol/L sulfuric acid solution.
3.8 Nessler's reagent: may be prepared by one of the following methods:
Weigh 20 g of potassium iodide and dissolve in about 100 mL of water; while stirring, add mercuric chloride (HgCl2) crystalline powder (about 10 g) in small portions several times until a vermilion precipitate that is difficult to dissolve appears, then switch to dropwise addition of saturated mercuric chloride solution and stir thoroughly; stop adding the mercuric chloride solution when a trace of vermilion precipitate no longer dissolves.
Separately weigh 60 g of potassium hydroxide, dissolve in water and dilute to 250 mL; after cooling to room temperature, slowly pour the above solution into the potassium hydroxide solution, dilute to 400 mL with water and mix well. Let stand overnight, then transfer the supernatant to a polyethylene bottle and store tightly stoppered.
Weigh 16 g of sodium hydroxide, dissolve in 50 mL of water and cool thoroughly to room temperature.
Separately weigh 7 g of potassium iodide and mercuric iodide (HgI2) and dissolve in water, then slowly pour this solution into the sodium hydroxide solution while stirring, dilute to 100 mL with water, store in a polyethylene bottle and keep tightly stoppered.
3.9 Potassium sodium tartrate solution: weigh 50 g of potassium sodium tartrate (KNaC4H4O6.4H2O), dissolve in 100 mL of water, heat to boiling to remove ammonia, cool and make up to 100 mL.
3.10 Ammonium standard stock solution: weigh 3.819 g of superior-grade ammonium chloride (NH4Cl) dried at 100 degrees C, dissolve in water, transfer to a 1000 mL volumetric flask and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per mL.
3.11 Ammonium standard working solution: pipette 5.00 mL of the ammonium standard stock solution into a 500 mL volumetric flask and dilute to the mark with water. This solution contains 0.010 mg of ammonia nitrogen per mL.
4.1 Water sample pretreatment: take 250 mL of water sample (if the ammonia nitrogen content is high, take an appropriate amount and add water to 250 mL so that the ammonia nitrogen content does not exceed 2.5 mg), transfer to a Kjeldahl flask, add a few drops of bromothymol blue indicator solution, and adjust to about pH 7 with sodium hydroxide or hydrochloric acid solution. Add 0.25 g of light magnesium oxide and a few glass beads, immediately connect the nitrogen bulb and condenser, and insert the lower end of the conduit below the surface of the absorption liquid. Heat and distill until 200 mL of distillate is obtained, then stop distillation and make up to 250 mL.
When using the acid titration method or Nessler colorimetric method, use 50 mL of boric acid solution as the absorption liquid; when using the salicylic acid-hypochlorite colorimetric method, use 50 mL of 0.01 mol/L sulfuric acid solution instead as the absorption liquid.
4.2 Preparation of the standard curve: pipette 0, 0.50, 1.00, 3.00, 7.00 and 10.0 mL of the ammonium standard working solution into 50 mL colorimetric tubes respectively, add water to the mark, add 1.0 mL of potassium tartrate solution and mix well. Add 1.5 mL of Nessler's reagent and mix well. After standing for 10 min, measure the absorbance at a wavelength of 420 nm using a 20 mm path-length cuvette with water as the reference. Subtract the absorbance of the zero-concentration blank tube from the measured absorbance to obtain the corrected absorbance, and plot a standard curve of ammonia nitrogen content (mg) versus corrected absorbance.
4.3 Determination of the water sample:
Take an appropriate amount of the water sample after flocculation-precipitation pretreatment (so that the ammonia nitrogen content does not exceed 0.1 mg), add to a 50 mL colorimetric tube, dilute to the mark and add 0.1 mL of potassium sodium tartrate solution. The rest is the same as for preparation of the standard curve.
Take an appropriate amount of the distillate after distillation pretreatment, add to a 50 mL colorimetric tube, add a certain amount of 1 mol/L sodium hydroxide solution to neutralize the boric acid and dilute to the mark. Add 1.5 mL of Nessler's reagent and mix well. After standing for 10 min, measure the absorbance following the same steps as for the standard curve.
4.4 Blank experiment: use ammonia-free water instead of the water sample and carry out a blank determination of the whole procedure.
Process flow diagram of a high-concentration ammonia nitrogen wastewater system
Ammonia nitrogen (N, mg/L) = m/V x 1000
where: m - the amount of ammonia nitrogen found from the standard curve, mg;
V - the volume of the water sample, mL.
6.1 The ratio of mercuric iodide to potassium iodide in Nessler's reagent has a great influence on the sensitivity of the color-developing reaction. The precipitate formed after standing should be removed.
6.2 Filter paper often contains trace ammonium salts; when used, wash it with ammonia-free water. The glassware used should be protected from contamination by ammonia in the laboratory air.
Ammonia nitrogen in wastewater mainly consists of two categories: ammonia nitrogen formed from ammonia water, and ammonia nitrogen formed from inorganic ammonia, mainly ammonium sulfate, ammonium chloride, etc. In total there are four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-).
In natural surface and groundwater bodies, nitrogen is mainly present as nitrate nitrogen (NO3-).
High-ammonia-nitrogen wastewater is generally formed by the co-existence of ammonia water and inorganic ammonia.
Generally, for wastewater with pH above neutral, the main source of ammonia nitrogen is the combined effect of inorganic ammonia and ammonia water.
Under acidic pH conditions, the ammonia nitrogen in wastewater is mainly caused by inorganic ammonia.
Ammonia nitrogen in water can be converted into nitrite under certain conditions; if drunk over a long period, the nitrite in the water will combine with protein to form nitrosamines, which are strong carcinogens and extremely harmful to human health.
After subtracting the absorbance of the blank experiment from the absorbance measured for the water sample and finding the ammonia nitrogen amount (mg) from the standard curve,
The main form of ammonia nitrogen that harms aquatic organisms is free ammonia, whose toxicity is tens of times greater than that of ammonium salts and increases with greater alkalinity. The toxicity of ammonia nitrogen is closely related to the pH and water temperature of the pool water; generally, the higher the pH and water temperature, the stronger the toxicity, and its harm to fish is similar to that of nitrite.
The harm of ammonia nitrogen to aquatic organisms is divided into acute and chronic. Chronic ammonia nitrogen poisoning harms by: reduced feeding, slowed growth, tissue damage and reduced oxygen transport between tissues. Fish are relatively sensitive to ammonia nitrogen in water, and high ammonia nitrogen content can cause fish death. Acute ammonia nitrogen poisoning harms by: aquatic organisms becoming excited, losing balance in the water and convulsing, and in severe cases even dying.
