Study on destruction of cyanide treated with oxidized decontaminants

In this paper, hypochlorites and peroxides worked as decontaminants for destruction of cyanide ion. Main factors influencing the residual CN- concentration after decontamination were investigated. And the amount of gasificatied HCN and ClCN formed during the decontamination were measured, and their assessments on their damage were further discussed. Results indicated that degradation rate of KCN and gaseous by-products was closely related to the type of decontaminants, their concentration of active ingredients, KCN concentration, pH value and temperature etc. Generally, hypochlorites decontaminants reacted with cyanide ion more quickly than peroxides. When the amount of hypochlorite decontaminant was high above 2 times of its stoichiometric amount, the residual CN- concentration could be down to 0.5 mg/L after 5 minutes, while for peroxides, it would take much longer for complete destruction. However, the massive gasificatied ClCN produced intensely with hypochlorites decontaminant would cause relatively larger damage depth than that of HCN with peroxides.


Introduction
Simple cyanide salts (NaCN, KCN,etc.) are industrially made in large quantities and used in electroplating, metallurgy, electroplating, plastics, mining and organic synthesis etc. However, they are the most toxic and most likely to be in the environment [1] . There are many international, national and local regulations and guidelines regarding cyanide in air, water and other media [2] . As an example, the maximum contaminant level for cyanide set by the US EPA in drinking water is 0.2 mg/L [3] . while the European Union [4] and China [5] . have an even lower limit of 0.05mg/L. And the maximum concentration being allowed discharged from industrial wastewater waters is 0.5 mg/L in China [5] .
CNO-+ 2H2O HCO3 +NH3 Due to its strong corrosivity and massive toxic by-product ClCN, the use of hypochlorites has been limited. Researchers around the world are working to explore a new environmentally friendly decontaminant. The peroxide decontaminant has received more and more attention, especially the organic peroxyl acid decontaminant. A binary solid peroxyl acid was developed in our lab. It is based on the reaction between the solid inorganic per-salt (A) and solid activator (B) after dissolved in the water to produce organic peroxide acid to achieve the purpose of decontamination [11] . It has been proved high efficiency for degradation of CWAs with low corrosion. However, its application for destruction of cyanide is not studied deeply.
According to the mechanism of oxidation destruction reaction, the liquid residue after decontamination is affected by such factors, mainly including concentration of CNand active ingredients, pH value and environment temperature, etc. These factors also affect the production of ClCN/HCN which would form poisoned atmosphere, resulting damage to people without protection.
In this paper, three kinds of oxidizing deconmitaminant are adopted: hypochlorite, binary solid peroxide acid and hydrogen peroxide. They are used for decontaminating KCN solutions to investigate the influence of types of decontaminants and their dosage, KCN concentration, pH value and environment temperature on degradation of cyanide, and the formation and damage of gasificatied ClCN/HCN were also studied.

Apparatus
The residual cyanide ion concentration were detected by a cyanide ion selective meter (ISM) named HI 96714, bought from Italian Hanna Corporation. A detect tube (sensibility:1mg/L) were utilized for qualitative detection of gasificatied ClCN and HCN. A HI2221 Specialty Laboratory pH/ORP/T Tester purchased from Italian Hanna Corporation was used for indication the change of temperature and pH online. The temperature was controlled by 501A Ultrathermostat purchased from Shanghai Experiment Instrument Factory.

Chemicals and Solution
KCN was purchased from International Chemical Import & Export Trade Co.,Ltd(China), and it was diluted to certain concentration (1.0% and 10.0%, wt %) before destruction reaction. Decontaminants: Hypochlorite (Active Chlorine, AC>60.0%) and Binary solid peroxyl acid decontaminant (A and B contained) obtained from Institute of Chemical Defense (China). They were both diluted to two kinds of different solution as decontaminants, that is 7.

Pretreatment for analysis of residual concentration of CN -
Residual concentration of CNwas determined by HI96714. Before measurement, excess residual oxidant was reduced by Na 2 SO 3 solution (12.6g/L), whose amount was determined in advance through a starch iodine titration method [12] .

Collection and analysis of gasificatied HCN and ClCN
Absorption apparatus as the figure 1 shows.The gas phase product HCN was took in NaOH solution (10g/L) through two stage collection, while ClCN gas was absorbed by distilled cold water with five flasks. To ensure gas is completely collected, a detector tube was used for indication.  Figure.2 (a) shows that AC played an important role in the destruction of KCN. 1.0% AC was not enough for complete degradation. The residual CNwas about 12.9 mg/L after 5.0 minutes with the AC run out. And with AC value increasing to 2.0%, the residual CNwas sharply down to 0.1 mg/L. However, when the AC value was further improved, promotion for destruction was not obvious, oppositely, excessive AC value resulted in higher residual concentration of CN -, that is because AC decomposed too fast due to high temperature caused by intense reaction (Table 1).
Similar result was found in experiment under the other condition. Figure.2(b)shows relatively low AC value (below or at 2.0%) was favorable for complete decontamination. The residual CNconcentration increased when AC value raise to above 2.0%.      Table 2 shows that pH had remarkable effect on the reaction, especially on highest temperature, AC concentration after reaction and production of ClCN gas. A strong alkaline keeped in suspension of calcium hypochlorite and 84 solution, therefore, the reaction was relatively mild with AC remaineed more and obviously less production of ClCN. So maintaining a high pH was necessary through the destruction process.

Effect of concentration of KCN solution.
In the experiment, molar ratio of decontaminatant to KCN was fixed to 2.0:1.0 and suspension of calcium hypochlorite was used as decontaminatant. Result in Table 3 shows that concentration of KCN solution also has effect on the violence of reaction. Compared with 1.0 % KCN, the reaction between 10.0 % KCN and suspension of calcium hypochlorite was more fierce with much higher temperature.
In a word, hypochlorite can quickly destruct CN -. Relatively low AC value, strong alkalinity and low concentration of KCN are favorable for safe and complete destruction of cyanide ion.  Table 4 ), resulting in a relatively strong oxidation. Figure.3 shows that the decontamination speed of solid peroxy acid with a ratio of 1.0:1.0 is greater than that of 1.0:0.5 as well as 10 % H 2 O 2 . Figure.4 shows that pH is lower when the ratio of A to B is 1.0: 1.0, thus the oxidation ability of the decontaminant is stronger, and its degradation of KCN was also much faster. Therefore, pH may be another factor which affect the decontaminating rate.    Figure.6 shows the reaction rate of liqiud phase is greatly influenced by the environment temperature. When the temperature rised up to 20℃, the time of decontaminating 10.0% KCN to discharge standards dropped. The higher the temperature, the faster the react, and the actual maximum temperature of decontamination reaction was 37.5℃ while the environment temperature was 20℃. However, when the temperature was further increased to more than 30℃, the rate of the decontamination significantly slowed down, that was because the decomposition of active oxygen further speeded up.And, when the temperature was increased to 40 ℃,the actual maximum reaction temperature have reached as high as 57.6 ℃, the speed reduction caused by active oxygen reduction can not be ignored.The time of decontaminating CNto discharge standard at this point need 60.0 minutes. In a word, solid peroxy acid reacted with KCN more mildly compared with hypochlorite solution. Relatively high concentration of active oxygen and KCN, suitable temperature and pH value are favourable for the destruction.

Hazard assessment of HCN or ClCN
Generated gas phase was mainly ClCN when KCN was treated with hypochlorite solutions.The amount of ClCN producted from suspension was less than that from supernatant whether the active chlorine was 2.0 % or 7.0 % (Table 6), and the lower the concentration of AC, the less gas ClCN. When the active chlorine was 7.0 %, the formation rate of ClCN was as high as about 8.5 % and 9.9 % respectively.  The gas phase is mainly HCN when KCN is treated with peroxyl acid. When the proportion of A to B was not same, the generation rate and amount of the HCN was affected by the temperature differently. The temperature affect little when the ratio is 1.0:0.5:10.0, the rate was similar to that of KCN hydrolyzed itself to HCN at different temperatures. However,when the ratio was 1.0:1.0, the formation rate of HCN was obviously lager and increased with temperature, about 3.3% of CNwas translated into HCN per minute under the condition of 40℃.The damage depth caused by gas phase HCN with solid peroxy acid was less than ClCN generated with hypochlorite [12] . And the damage depth is effected by amount of KCN, ratio of decontaminant, work speed, temperature and wind speed, etc. The more KCN, the smaller the wind speed, the greater the damage depth caused by HCN or ClCN.