Cyanides and their Attenuation in wastewaters using green chemistry

Cyanides, though naturally occurring, are environmental pollutants when not treated properly. Some methods used to attenuate cyanides in waste waters from industrial processes are based simply on changing the physico-chemical properties of the waste water such as the pH and temperature. The effectiveness of these methods are based on hydrolysis of the cyanide and volatilization of the hydrogen cyanide formed. Another reaction which takes place simultaneously is ultraviolet-catalysed oxidation which converts the cyanide to bicarbonates and carbonates. The changes in the cyanide degradation rate approaches a minimum faster if the cyanide solution is maintained at a higher than ambient constant temperature.


Introduction
The term cyanide is used to refer to a wide array of cyanide containing chemicals. A cyanide is any chemical compound that contains the monovalent combining group CN. This group, known as the cyano group, consists of a carbon atom triple bonded to a nitrogen atom 1 .

Occurrence
Cyanides are produced by certain bacteria, fungi, and algae and are found in a number of plants. Cyanides are found in substantial amounts in certain seeds and fruit stones, e.g., those of apricots , apples , and peaches 2 . In plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and defend the plant against herbivores. Cassava roots (also called manioc), an important potato -like food grown in tropical countries (and the base from which tapioca is made), also contain cyanogenic glycosides. 3 , 4 . A few species of centipedes, millipedes, insects, beetles, moths and butterflies secrete cyanide for defensive purposes in repelling predators such as toads and birds 5 .
The cyanide radical CN· has been identified in interstellar space 6 . The cyanide radical (called cyanogen ) is used to measure the temperature of interstellar gas clouds 7 .
Production Cyanide is manufactured from ammonia and natural gas, and is a by-product of the manufacture of acrylic fibres and plastics.

Types of cyanide
Cyanide can occur in many forms. It can exist as a free ion or can combine with many different elements. Table 1 below gives the various categories of cyanides. Cyanide is also used in the manufacture of nylon, plastic glass, perfume, soaps, fertilizers, paints, animal food supplements, and pharmaceuticals, including heart medication and medical test kits. Cyanide compounds are used in surgical dressings that promote healing and reduce scarring, in anti-cancer preparations, and in drugs to reduce high-blood pressure 5 , 1 .
1 weak acid dissociable cyanide 2 strong-acid dissociable cyanide Due to the high stability of their complexation with iron, ferrocyanides (Sodium ferrocyanide E535, Potassium ferrocyanide E536, and Calcium ferrocyanide E538) 8 do not decompose to lethal levels in the human body and are used in the food industry as, e.g., an anticaking agent in table salt 9 .
Cyanide is used in electroplating where it 'plates' one type of metal onto another. Examples are silver and gold plated dinnerware and jewellery, and brass plated bathroom fixtures 5 .
Due to its toxic nature, it has been used as a fumigant and poison since early times. Cyanide has been used to kill people in judicial executions, genocide, and mass suicide, and has been used as a chemical warfare agent 5 , 1 .

Destruction/Removal of Cyanides
The small amounts of cyanide entering our bodies from our daily intake of the above foods, is removed by the liver to prevent harmful levels building up in our systems 5 .
There are several methods to destroy, degrade and detoxify cyanide in the environment which involve physical, biological and chemical processes.

Environmental impacts of Cyanide
Cyanide has low persistence in the environment and is not accumulated or stored in any mammals that have been studied.
Cyanide biomagnification in food webs has not been reported possibly due to rapid detoxification at sub-lethal doses, and death at higher doses. Low levels of cyanide may be harmless under seasonal or other variations that allow organisms to recover and detoxify.
Cyanide is seldom persistent in surface waters and soils due to complexion or sedimentation, microbial metabolism and loss from volatilization, but may persist in groundwater for extended periods of time.
Long term adverse effects associated with mining sites are related primarily to metals rather than cyanide. Cyanide can naturally degrade rapidly in the environment by volatilizing to hydrogen cyanide gas (HCN) Cyanide can mix with air and water to produce ammonia and bicarbonate. Ammonia can be toxic in the aquatic system, but is an important component of the nitrogen cycle, vital for plant growth.
Consequently, low concentrations of ammonia are rapidly assimilated. Cyanide is toxic to most species in varying amounts.
Aquatic organisms are very sensitive to cyanide. Fish have been shown to be the most sensitive aquatic organism, followed by invertebrates. Algae and aquatic plants have been shown to be comparatively tolerant to cyanide. Low concentrations can sometimes enhance germination and growth. Elevated concentrations inhibit respiration and can lead to death 5 .
Birds seem to have varying sensitivity to cyanide. They may absorb cyanide through their skin when wading or swimming, or ingest it through drinking.

Water and Wastewater Quality
In Ghana, the discharge limits are 1.0 mg/L of total cyanide, 0.6 mg/L of WAD cyanide and 0.2 mg/L for free cyanide 10 .

Experimental
Factors affecting natural attenuation of Cyanides in the environment Dodge and Zabbon 11 and Chester Engineers 12 identified pH, temperature, interfacial surface area, pressure, concentration and degree of agitation as factors affecting cyanide volatilisation. Palaty and Horokova-Jakubu 13 cited in Simovic et al. 14 also studied volatilisation of cyanide from simple cyanide solutions. They identified the same variables that affected cyanide volatilisation as Dodge and Zabbon 11 . They also determined solution depth to be an important variable.
Simovic et al 14 . and Zaidi et al 15 have also attempted to quantify cyanide losses in process ponds.
This study is therefore to determine how pH and temperature influence the rate of attenuation of free cyanide in solution by simulating a cyanide-containing wastewater in the laboratory.
The experiments were designed to investigate how pH, temperature and light irradiation affect the degradation process of cyanide in water.

Preparation of Solutions
Solutions of sodium cyanide (NaCN) were prepared and exposed to the following conditions The rate and level of cyanide degradation together with the corresponding pH and temperature were monitored.

Equipment Used
The main equipment used was the Cyberscan pH meter which is equipped with a temperature probe.
The incandescent bulbs were suspended 16 cm above the beakers containing the cyanide solutions and placed in a fume chamber.

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The dark room was a cupboard with a door which excludes all light when shut.
Light Irradiation effect on Temperature and pH A mass of 0.027g of NaCN was dissolved in 1000cm 3 of distilled water. 200cm 3 each of this solution were put in three separate 500 ml glass beakers. Two of the beakers and their content were exposed to a 60 W incandescent bulb and a 100 W incandescent bulb respectively. The third beaker and contents was kept in a dark room.
The temperature and pH of the cyanide solutions were then monitored daily for four days. The results are shown in Figures 2 and 3.

Light Irradiation effect on Cyanide Degradation
A mass of 0.135g of NaCN were dissolved in 5000cm 3 distilled water. The solution was then divided equally into three parts. Two of these were exposed to a 60W incandescent bulb and a 100W incandescent bulb respectively. The third part was kept in a dark room. The concentration of free cyanide was monitored over 24 hours. The results are shown on Figure 4.

Solar radiation effect on Carbonate degradation and Bicarbonate Formation
A mass of 0.054g of NaCN were dissolved in 2000cm3 of distilled water and the solution divided equally into two portions. One portion was placed in the Darkroom while the other was placed in the open where it was exposed to sunlight for four days. The concentration of carbonate and bicarbonate were monitored daily for the period. Figure 5 shows the pH and the level of carbonate and bicarbonate in the solutions.
Influence of Solar radiation on Cyanide Degradation 0.054gm of NaCN were dissolved in 2000cm3 of distilled water. The solution was then divided into two portions. One portion was exposed to sunlight with the other portion kept in the dark room. The pH and the level of free cyanide were monitored daily for four days. Figure 6 shows the results of the level of cyanide in both solutions.

DISCUSSIONS OF RESULTS
The temperature of the solutions when exposed to the light bulbs increased with time to a high of about 40 o C, while the temperature of the solution in the dark room remained relatively stable at about 30 o C (Figure 2).
The pH of all solutions decreased with time. The pH of the solutions exposed to the 60 W and 100 W bulbs decreased initially and started increasing only after the third day (Figure 3), indicating that some chemical reactions might be taking place during the period of exposure.
This phenomenon could be explained by a close look at the reactions that may take place.
Two types of reaction could be proposed for the loss of cyanide in this study 16  Furthermore, the CO 2 in the atmosphere may also dissolve in the solutions according to the following reactions: which will also change the pH of the solutions. Consequently, the pHs of the solutions were found to decrease with time.
At this reduced pH, the cyanide hydrolyses to HCN which volatilises. The oxidation mechanism is slower and may not contribute much to the attenuation at this stage. This is confirmed by Simovic et al 14 who found out that ninety percent of the free cyanide was removed by hydrolysis and volatilization of HCN and an additional ten percent was removed by chemical oxidation 18 in surface waters of process ponds in mines.
With the exposure of the cyanide solution to sunlight, the oxidative reaction seemed to enhance the loss of cyanide from solution. It seemed the cyanate ion might have decomposed slowly to give the ammonium and bicarbonate ions as shown in equations 4.1 and 6.0. This causes a decrease in the pH of the solution.
The CO 3 2-(in equation 9.0) reacts with the CO 2 from the atmosphere CO 2 + CO 3 2-+ H 2 O → 2HCO 3 -(K diss = 2.08 x 10 10 )……………………….10.0 This explains the increase in the level of bicarbonate shown in Figure 4 below, implying that most of the cyanide in the sample exposed to sunlight degraded through the hydrolysis reaction.
The slower oxidative reactions which convert CNto CNOand HCN to HCNO may also take place with the sunlight being the catalyst as follows The CNOalso hydrolyses to give bicarbonates In effect, the concentrations of CNand CO 3 2will decrease while that of HCO 3will increase as confirmed by Figure 5.
This supports Miles 19 theory that the ultraviolet light acts as a catalyst to convert dissolved oxygen in solution to ozone, (see figure 5) which promptly reacts with the cyanide to cyanate as confirmed by the work of Frank and Bard 20 .

Rate of Degradation
The rate of degradation is not constant as shown in figure 6 below, indicating there are more than one parameter influencing it. This can be explained by the fact that though both hydrolysis (a chemical reaction) and volatilization are enhanced by high temperature, the individual contributing effects vary depending on the conditions.
The data in study indicates that under constant temperature as created by the 100W and 60W lamps, unlike under ambient conditions, the rates of degradation will attenuate to a constant figure with time.