Classification of cyanides
Cyanide that may occur in water or wastewater is classified into three categories according to the strength of the metal cyanide bond:
1.) Free cyanide (CN-)
2.) Weak acid dissociable cyanides (WADs) (refer to free cyanide and cyanide complexes with metals such as cadmium, copper, nickel and zinc)
3.) Strong acid dissociables (SADs) (refer to cyanide complexes with metals such as cobalt, gold, iron and silver)
4.) Total cyanide (refer to all the categories mentioned above and the very strong complexes ferro and ferri cyanides and cobalt cyanides )
This classification is in accordance with various analysis parameters. WAD cyanide is the type primarily used to determine compliance with the environmental limit.
The values that are measured of it, nearly fully correspond to the ones yielded by the ISO/DIN-method (easily liberated cyanide). Additionally, the total cyanide is measured.
These parameters depend on the specified analysis methods used. This is because it is not possible to clearly classify the type of cyanide, since a mixture of different metals will typically be present and hence different kinds of cyanides.
Cyanide Oxidation with Hypochlorite Solution (chlorine bleach liquor)
The alkaline chlorination treatment, for which chlorine bleach is generally used, makes up the traditional process for cyanide destruction in waste water.
For a long time this process was its state-of-the-art, leading in the majority of cases to the desired cyanide concentrations with relatively low investment costs.
Cyanide detoxification by chlorine is becoming increasingly disputed and often needs to be substituted for following reasons:
1.) The reactions of chlorine with organic substances like tensides or complexing agents lead to the formation of chlorinated organic compounds, detected as AOX (absorbable organic halogen), and lead to exceeding the limits of AOX.
2.) Not only AOX is formed, but also other chlorinated, problematic substances may be produced: In the beginning of the chlorine treatment the tear gas cyanogen chloride and later the toxic chloramines.
3.) Depending on the composition of the water in terms of heavy metals, a successful treatment can be difficult and time-consuming or even impossible.
4.) After the detoxification of cyanide with hypochlorite, it may be very likely that organic complexing agents remain in the solution, which require huge amounts of sulfides or organ sulfides to keep within the heavy metal limits.
5.) In practice, the consumption of chlorine bleach liquor exceeds the stoichiometric expected consumption by four times of its value. This is caused by the chlorination of ammonia that is formed during the cyanide detoxification generating toxic chloramines like the forbidden trichloramin.
Meanwhile governmental institutions such as the „baden-württembergische Ministerium für Umwelt, Klima und Energiewirtschaft“ recommend the cyanide oxidation by the UV/H2O2 process being the more environmentally-friendly technology.
A number of reactions lead to the photochemical degradation of cyanides. Depending on the composition of the water and the present type of complexes, different pathways may occur for the reaction to take.
Investigations on exact mechanisms are rare and in the literature, one will find many contradictory descriptions of these possible pathways.
Even hydrogen peroxide, without UV irradiation, may on its own and under appropriate conditions lead to the oxidation of the free cyanide and part of the cyanide from cyanic complexes.
These processes might have quite long reaction times depending on the stability of the complexes.
The same holds for the highly reactive hydroxyl radical generated by photolysis of hydrogen peroxide (UV/H2O2-process) which reacts immediately with cyanide.
For the oxidation of stronger metal cyano complexes, the excitation of these compounds by UV irradiation plays an important role, since they can directly react with hydrogen peroxide when excited:
A well known photochemical reaction is the degradation of the very strong ferricyanides, which is investigated frequently in the field of solar photochemistry of surface water.
These reactions take place when ferricyanides are oxidized by the industrially applied enviolet® process with the aim to destroy the total cyanide. (e.g. Verichrome (UK) 1500 mg/L T-CN, limit: 1 mg/L T-CN)
According to the reactions mentioned above cyanide is oxidized to cyanate.
Adequate reactions take place when cobalt cyanide complexes are irradiated by UV light, even though it constitutes the most stable MeCN-complex.
The oxidation product cyanate reacts by hydrolysis to carbon dioxide and ammonia, but by oxidation as well nitrogen and carbon dioxide may be produced.
UV/H2O2 Oxidation of Organic Cyanides
Adjacent to inorganic cyanide, organic cyanides, i.e. nitriles, can be oxidized by enviolet® UV oxidation, for which reactions mainly take place that are described in the section “ COD/TOC degradation”.
Under specific conditions, nitriles release cyanide by hydrolysis, which is why an appropriate detoxification is required.
Normally during wastewater treatment or groundwater remediation, cyanide will not be the only contaminant since industrial processes additionally require other components like surfactants or organic complexing agents. Moreover, the wastewater typically consists of water from different production lines and diffuse sources.
For treating wastewater or groundwater, the cyanide degradation reactions mentioned above compete with the organic components of degradation. To minimize the undesired reactions, treatment conditions and the respective parameters need to be adapted to the typical matrix of the wastewater on site.
However, “side reactions” are sometimes desired and are enhanced if they lead e.g. to the degradation of complexing agents. The use of expensive organosulphides can be reduced or even completely avoided for achieving a sufficient precipitation of heavy metals.
These optimization possibilities of the enviolet® UV Oxidation guarantee an economic process with minimal investment and operating costs, especially when a complex composition of the wastewater is present. Therefore, wastewater which contains e.g. cyanide and EDTA at the same time can be treated easily (Industrial examples: Miba Gleitlager (Austria); DOW (Switzerland))
Elimination of cyanide in ground water is usually possible in a simple manner through our UV/H2O2 oxidation called enviolet®:
Current concentrations in the respective wells:
Source 1: T-CN (cyanide, total): 2.90 mg/L
Source 2: T-CN (cyanide, total): 0.18 mg/L
Aim of the UV/H2O2 process:
Reduce the amount to 50 µg/L
In industrial wastewaters, the oxidation of cyanide tends to be a preliminary stage of metal insolubilisation.
Example: Cyanide and wastewater containing EDTA
Typical wastewater values:
weak-acid dissociable cyanides: 550 mg/L
EDTA: 230 mg/L
TOC (total organic carbon): 1320 mg/L
Target of UV/H2O2 process:
adhering the limit of 0.2 mg/L weak-acid dissociable cyanide.
Destruction of the EDTA and further organic complexing agent by the UV-oxidation so that the consequent metal insolubilisation leads to a compliance with the metal safety values.
Many stripping solutions of surface engineering contain cyanide to remove the undesirable metal layer and they also have a reducing agent to protect the layer under it.
These strippers are very expensive to dispose and can be treated cost-effectively during our cyanomat process.
Example: Stripping solutions containing cyanide
Typical wastwater values:
Total cyanide: 50,000 mg/L
TOC (total organic carbon): 21,000 mg/L
Target of the UV/H2O2 process:
adeherence to the limit of 0.2 mg/L
weak-acid dissociable cyanides
Destruction of further organic substances (e.g. Nitrobenzolsufonate) by UV-oxidation so that the consequent metal insolubilisation leads to a compliance with metal safety values.