Silver Ion: Its role in the stability of Huwa-San
The production of Huwa-San involves the addition of low level silver ion (300 ppb) as silver nitrate to hydrogen peroxide. Silver nitrate salt is toxic and will be handled correctly, using the appropriate PPE during the manufacture of the biocide. The addition of silver nitrate is necessary as it is the most accessible of the silver salts that dissolve in water. It is important to recognise that after silver nitrate is added to the 50% hydrogen peroxide / water mix it is no longer present as silver nitrate but as silver and nitrate ions.
When the toxicity of the after- effects of a Huwa -San products is considered, we must consider the fate of the individual ions.
Silver Ion: How it behaves and the environmental consequence’s.
The functionality of Huwa-San relies on the stability of silver ion so other low level additions are made to ensure that the silver in Huwa-San is protected from reacting with other anions particularly the chloride ion in solution. Any reaction between silver and chloride would create the insoluble silver chloride meaning that the important silver ion is lost, and Huwa-San would lose efficacy.
After carrying out a disinfection – 99% of Huwa-San sold around the world will be used as a water disinfectant – the silver ion and nitrate ion will be discharged in the aqueous effluent from the system. The impact of stabilising chemicals will be lost, and the already low silver ion content say 60 ppb will be precipitated as silver chloride. In a sewage works this will be removed in the sludge and might eventually end up as fertiliser. On the odd occasion when the discharge is to a receiving stream or water some silver may eventually enter as a precipitated salt- we are now talking sub ppb levels of silver and bound silver ion is not toxic1.
1 All that is silver is not toxic: silver ion and particle kinetics reveals the role of silver ion aging and dosimetry on the toxicity of silver nanoparticles Smith JN et al. Particle and Fibre Technology Vol 15, Article no 47 (2018)
The World Health Organisation’s view on silver in potable water is:
On the basis of present epidemiological and pharmacokinetic knowledge, a total lifetime oral intake of about 10 g of silver can be considered as the human NOAEL (No observed adverse effect level). As the contribution of drinking-water to this NOAEL will normally be negligible, the establishment of a healthbased guideline value is not deemed necessary. On the other hand, special situations may exist where silver salts are used to maintain the bacteriological quality of drinking-water. Higher levels of silver, up to 0.1 mg/litre (a concentration that gives a total dose over 70 years of half the human NOAEL of 10 g), could then be tolerated without risk to health. Silver may be considered a heavy metal because its density is more than 5 times that of water. It is not toxic and at the correct dosage may improve health. One of the main areas of use for Huwa-San for disinfecting drinking water for poultry and pigs.
Silver ion present in an aqueous discharge following a Huwa- San disinfection is very low risk to the environment.
A diagram of the role of silver ion in Huwa-San
The Fate of Nitrate Ions Following a Disinfection
Nitrates are a form of nitrogen, which is found in several different forms in terrestrial and aquatic ecosystems. These forms of nitrogen include ammonia (NH3), nitrates (NO3), and nitrites (NO2). Nitrates are essential plant nutrients, but in excess amounts they can cause significant water quality problems. Together with phosphorus, nitrates in excess amounts can accelerate eutrophication, causing dramatic increases in aquatic plant growth and changes in the types of plants and animals that live in the stream. This, in turn, affects dissolved oxygen, temperature, and other indicators. Excess nitrates can cause hypoxia (low levels of dissolved oxygen) and can become toxic to warm-blooded animals at higher concentrations (10 mg/L) or higher) under certain conditions. The natural level of ammonia or nitrate in surface water is typically low (less than 1 mg/L); in the effluent of wastewater treatment plants, it can range up to 30 mg/L.
In the UK nitrate2 The 1991 Nitrates Directive (91/676/EEC) means that all water bodies are subject to a maximum nitrate (NO3) concentration of 50 mg l−1 (equivalent to 11.3 mg NO3−N L−1) regardless of whether or not they are used for drinking water supply; the Directive explicitly mentioned eutrophication for the first time. The subsequent Water Framework Directive (60/2000/EC) aims to achieve “good ecological status” for fresh and marine waters by 2015 relying, in large part, on existing legislation such as the Nitrates Directive and its designation of Nitrate Vulnerable Zones to help achieve this. It is likely that in the future nitrate levels will be restricted to 10 mg l-1 in line with the United States.
2: Nitrate in United Kingdom Rivers: Policy and Its Outcomes Since 1970† Burt TP Environ. Sci. Technol. 2011, 45, 1, 175–181 Publication Date: August 3, 2010 https://doi.org/10.1021/es101395s
It is obvious therefore that the silver and nitrate ions in Huwa-San present an insignificant risk to the environment.
