Waterguide - Ammonium and ammonia
What are ammonium and ammonia?
Ammonia is a pungent, water-soluble poisonous gas with the molecular formula NH3 (atomic mass 17 g/mol). When ammonia is added to watery solutions, there is an acid-based reaction, resulting in a cation called ammonium NH4+ (atomic mass 18 g/mol).
The equilibrium between ammonia and ammonium depends on the pH value of the watery solution, as illustrated by the following figure.
How is ammonia made?
Ammonia is made using the Haber-Bosch method, named after the developers Fritz Haber and Carl Bosch.
The following are essential to its production:
- Nitrogen and hydrogen – ratio of 1:3
- Pressure 250 – 350 bar
- Temperature 450 – 550 °C
- α – iron as the catalyst
N2 + 3 H2 → 2 NH3
How do ammonia and ammonium occur in nature?
In nature, ammonia (or ammonium) can be synthesised from the nitrogen in the air. This nitrogen fixing process is catalysed using enzymes, as it is a very energy intensive process. Several types of bacteria have the ability to fix nitrogen from the air, for example cyanobacteria, clostridium and rhizobium in symbiosis with plants in the Faboideae family.
Ammonium also occurs as an end product during the decomposition of proteins from dead biomass (plant or animal). This decomposition process is carried out by the composers (bacterial or fungi).
In bodies of water, bacteria use oxygen to first oxidise the ammonium into nitrite and then into nitrate. This “nitrification” is an important self-cleaning property of bodies of water. However, if a high amount of ammonium enters the water (e.g. excessive fertilisation), nitrification can be fatal for fish, due to either the accumulation of the toxic intermediate product nitrate or due to the high oxygen consumption.
Ammonia by itself is already toxic for fish – even in a low concentration. Depending on the pH value of the water, an ammonium concentration of 0.5 to 1 mg/l is considered the critical threshold.
What problems can this lead to in technology?
The issue of ensuring a balance between ammonium and ammonia can cause a lot of problems in membrane-based processes. Membranes have the ability to retain charged particles, but they are permeable for gases. If water entering the water treatment process contains ammonium and the pH value is in the base range, the gaseous ammonia will pass through the membrane and be present in the permeate. This will result in the cleaned water having too high a conductivity.
Solution: Lower the pH value for the reverse osmosis process, and intercept and remove ammonium as salt at the membrane. Then increase the pH value again to achieve the desired conductivity.