Photo-oxidation of cyanide in aqueous solution by the UV/H 2 O 2 process

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1 Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 8:13 19 (25) DOI: 1.12/jctb.1127 Photo-oxidation of cyanide in aqueous solution by the UV/H 2 O 2 process Sarla Malhotra, 1 M Pandit, 1 JC Kapoor 1 and DK Tyagi 2 1 Centre for Fire Explosive and Environment Safety, Metcalfe House, Delhi-1154, India 2 Multanimal Modi College, Modi Nagar, Uttar Pradesh, India Abstract: Photo-oxidation of cyanide was studied in aqueous solution using a low-pressure ultra-violet (UV) lamp along with H 2 O 2 as an oxidant. It was observed that by UV alone, cyanide degradation was slow but when H 2 O 2 was used with UV, the degradation rate became faster and complete degradation occurred in 4 min. The rate of degradation increased as the lamp wattage was increased. It was also observed that cyanide oxidation is dependent on initial H 2 O 2 concentration and the optimum dose of H 2 O 2 was found to be 35.3 mmol dm 3. Photo-oxidation reactions were carried out at alkaline ph values (1 11) as at acidic ph values, cyanide ions form highly toxic HCN gas which is volatile and difficult to oxidise. By the UV/H 2 O 2 process, using a 25 W low-pressure UV lamp and at alkaline ph of 1.5 with an H 2 O 2 dose of 35.3 mmol dm 3, cyanide (1 mg dm 3 ) was completely degraded in 4 min when air was bubbled through the reactor, but when pure oxygen was bubbled the time reduced to 25 min. The cyanide degradation reaction pathway has been established. It was found that cyanide was first oxidised to cyanate and later the cyanate was oxidised to carbon dioxide and nitrogen. The kinetics of cyanide oxidation were found to be pseudo-first order and the rate constant estimated to be min 1 at 4 C. The power required for complete degradation of 1 kg of cyanide was found to be 167 kwh (kilowatt hour). 24 Society of Chemical Industry Keywords: photo-oxidation; cyanide; hydrogen peroxide; UV radiation INTRODUCTION Cyanide is a highly toxic compound. The lethal dose for humans is 5 mg kg 1 of body weight. 1 Cyanide effluents are generated from various industries such as metal ore processing, smelting, chemical and petrochemical, gold mining, photography, heat treatment and plating. Different types of cyanide complexes are used in electroplating processes and the concentration in the bath is 2 5 g dm 3. Gold-mining operations employ sodium cyanide solution to extract gold from polarised ore (cynuration) followed by zinc precipitation. CN ion complexes with the Fe(III) ion and copper present in the cytochromeoxidase enzyme. Cyanide fixation leads to the formation of Fe 3+ cyancytochrome oxidase which blocks the electron transfer to oxygen needed to produce the activated form required for reaction with protons to form water. This affinity for heavy metal complexation is responsible for the CN ions capacity to inhibit numerous enzymes having iron, copper or cobalt active centres and therefore is responsible for its toxicity. CN ions also complex with the Fe(III) present in methaemoglobin to form cyanomethaemoglobin. Conventional treatment methods such as biotreatment, adsorption on activated carbon, chemical oxidation using caustic chlorine, etc have certain limitations. The most widely used caustic chlorine method produces secondary by-products such as trihalomethanes and cyanogen chloride that are highly toxic and carcinogenic. The destruction of the intermediate product cyanate requires huge amounts of chlorine and in treated water the dissolved chlorine concentration becomes very high, which is highly harmful to the aquatic life. 2 Advanced photo-oxidation process using UV radiation in combination with various oxidants such as H 2 O 2, Fenton s reagent, TiO 2,ozone,isa highly promising technology. In the photo-oxidation process, highly oxidising hydroxyl radicals are produced (E = 2.8 V) which take part in the oxidation process. In this process no toxic by-products are produced. The end products are carbon dioxide, water and other gaseous products. If treated properly the discharged water can be recycled or reused. The photo-oxidation process has been used by many investigators to detoxify different organic compounds. Nicole and colleagues 3,4 investigated the degradation of trihalomethanes by UV irradiation at nm and used model substrates such as CHCl 3,CHCl 2 Br, Correspondence to: M Pandit, Centre for Fire Explosive and Environment Safety, Metcalfe House, Delhi-1154, India panditdr@yahoo.co.in (Received 11 August 23; revised version received 7 May 24; accepted 3 June 24) Published online 12 October Society of Chemical Industry. J Chem Technol Biotechnol /24/$3. 13

2 SMalhotraet al CHBr 2 Cl and CHBr 3 in dilute aqueous solution (<1 6 mol dm 3 )at2 C. Results showed that only brominated trihalomethanes were photolysed and the organic halogens present in CHCl 2 Br, CHBr 2 Cl and CHBr 3 were completely converted into chloride and bromide ions during the time of irradiation. Sundstrom and colleagues 5,6 studied the photolysis of a number of halogenated aliphatics often present in water at low concentration levels. For example, 8% of 58 ppm trichloroethylene contained in a water sample could be removed within 4 min of irradiation using a low-pressure Hg lamp. The authors studied other halogenated aliphatics such as tetrachloroethylene, 1,1,2,2-tetrachloroethylene, dichloromethane, chloroform, carbon tetrachloride and ethylene bromide. Photolysis of a 53 ppm dichloromethane sample yielded only a 3% removal of contaminant after 3 h of irradiation. Zeff and Leitis 7 published results of the photolysis of 1 ppm methylene chloride in distilled water (1.8dm 3 photo-reactor using a low-pressure Hg lamp of 15 W). Removal of 6% of the initial substrate was found to occur in 25 min of irradiation time. Guittonneau et al 8 studied the oxidation of a number of volatile polychlorinated hydrocarbons such as chloromethanes (CHCl 3, CCl 4 ) and chloroethanes (C 2 H 2 Cl 4,C 2 HCl 5,C 2 Cl 6 ) in different combinations in dilute aqueous solutions. They conducted experiments in a 4 dm 3 batch reactor equipped with a 4 W low-pressure Hg lamp at 16 C and ph 7.5. They concluded that low concentrations of volatile organic chlorides are rather difficult to degrade, as rather large portions of it may be stripped by air or oxygen added in a photochemical reactor. Domenech and Peral 9 have reported that treatment of cyanide-containing wastewater by photo-oxidation using a titanium(iv) oxide-coated zeolite resulted in CN removal of 9% after 2 h using an initial CN concentration of 2.6 ppm. Mitsubishi Electric Corp 1 has reported that cyanide-containing wastewater is treated in stages by ozonisation and UV irradiation. In the first stage the wastewater is irradiated with UV and ozonised. In the second stage the effluent from the first stage reactor is ozonised and irradiated with a light of 31 nm wavelength. The waste gas is decomposed and discharged. Cifuentes et al 11 have reported that ZnO and TiO 2 semiconductors promote photo-oxidation of cyanides to cyanates in mining and ore-processing wastewaters, but the ZnO was more efficient and cost-effective. Marsman and Applelman 12 have carried out pilot-scale experiments with UV irradiation followed by bio-film reactor treatment of ground water from a former gas-works containing complexed cyanides and free cyanide and showed that the UV treatment dissociated the complex cyanides and oxidised some of the free cyanide but mineralisation of the free cyanide in the biofilm reactor is probably not possible without a carbon source. Wada and Naoi 13 have carried out treatment of industrial wastewater containing CN by ozonation, UV, and an ion exchange system. Theis et al 14 have carried out destruction of hexacyanoferrate(ii) and (III) ions by advanced oxidation through the use of immobilised titanium dioxide as a photo-catalyst and O 3 or H 2 O 2 as oxidising agents. Results show a sequential progression from Fe(CN) 6 CN CNO which is completed in a single photochemical reactor over a period of 2 4 h. Cost analysis suggests the process is competitive with other methods. Abe et al 15 have carried out treatment of industrial waste waters containing ferricyanide or ferrocyanide complex compounds by dosing with H 2 SO 4 or water-soluble sulfate salts then with an oxidising agent (ie O 3,H 2 O 2, NaClO) under UV or visible light irradiation for decomposing the ferricyanide or ferrocyanide complex compounds to form hydroxide. Yuan et al 16 have carried out treatment of cyanide-containing wastewater using a solid catalyst, ie hydroquine redox resin. The results showed that this oxidation catalyst has a fairly high stability. The present study was undertaken to investigate detailed photo-oxidation of CN in the aqueous phase and to optimise the parameters for complete degradation of cyanide and establish the kinetics of photo-oxidation. EXPERIMENTAL Materials NaCN (AR grade) was obtained from M/s SD Fine Chemicals and hydrogen peroxide (3% w/v) was supplied by E Merck India Ltd. The strength of H 2 O 2 was checked each time. All other reagents, such as 4-dimethylaminobenzylidinerhodanine, silver nitrate, potassium titanium oxalate, etc, were of high purity and purchased from E Merck India Ltd. UV lamps Three low-pressure lamps of different wattage, ie 8, 15 and 25 W, were used. In a low-pressure lamp, 81.7% UV radiation is emitted at nm. The spectrum data of the low-pressure lamp is shown in Fig 1. These % Emission Wavelength (nm) Figure 1. Emission spectrum of low-pressure Hg lamp. 14 J Chem Technol Biotechnol 8:13 19 (25)

3 Photo-oxidation of cyanide in aqueous solution lamps were obtained from GE Electric Company, USA. Experimental procedure Experiments were carried out in an annular type batch photo-reactor made up of boro-silicate glass. The reactor assembly is shown in Fig 2. The effective volume of the photo-reactor was around 1 cm 3. The length and O.D. of the reactor were 55 mm and 95 mm with a wall thickness of 2.5 mm. A double walled immersion well (O.D. of 7 mm) made of high purity quartz was placed inside the glass reactor fitted with a standard joint at the top. The UV lamp was kept inside the immersion well. Water was circulated through the annular space of the immersion well to remove heat generated by the UV lamp. Cyanide solution was taken inside the glass reactor for photo-oxidation studies. Air was bubbled into the reactor solution through the bottom inlet in order to mix the liquid uniformly. A sintered disc (Grade 2 Borosil, pore dia 4 9 µm) was provided for producing small bubbles. There was a sampling port at the middle of the reactor, so that periodic samples can be withdrawn for analysis. A Tefloncoated thermocouple was introduced into the reactor solution and it was fitted with a temperature indicator outside. The reactor was covered with a photo-reactor safety hood, so the person working was not affected by harmful UV radiation. Moreover, safety goggles and proper safety clothes were used during experiments. Analytical procedure Cyanide concentration was determined by titrimetric method for high cyanide concentration (>1 ppm) using silver nitrate and 4-dimethylaminobenzylidinerhodanine as indicator and by colorimetric method for low concentration (<1 ppm). 17 Cyanate concentration was determined by hydrolysing it to ammonia at acidic ph (1.5 2.) and measuring ammonia by the Nesslerisation method. 17 The H 2 O 2 concentration was determined colorimetrically using potassium titanium oxalate solution at nm. 18 RESULTS AND DISCUSSION Initial experiments were carried out with an 8 W lowpressure Hg lamp. It was found that UV alone was not effective for the degradation of cyanide. In the case of UV alone, after irradiation of 9 min, 1 mg dm 3 of cyanide ion was reduced to 98 mg dm 3 only. When H 2 O 2 (35.3 mmol dm 3 ) was used along with UV, the rate of degradation was much faster and it took 65 min for complete degradation of cyanide with an initial concentration of 1 mg dm 3. Light itself can degrade many compounds by initiating bond cleavage of the organic compounds, though the rate of degradation is much slower. H 2 O 2 is also a very strong oxidising agent. The combination of H 2 O 2 and UV can create a very fast and efficient process for water treatment by producing hydroxyl radicals, according to eqn (1): hv H 2 O 2 2OH (1) The decomposition takes place at nm. In a lowpressure Hg lamp 81.7% UV radiation is emitted at nm. The hydroxyl radical is a very reactive free radical and one of the most powerful oxidising agents (E = 2.8 V), second after fluorine (E = 3.6 V). 19 These radicals have one electron deficiency and due to their excited state they tend to react very rapidly with other molecules. Initially partial oxidation products are produced which are subsequently oxidised to carbon dioxide, water and other gaseous products. Cooling water inlet Effluent inlet UV lamp Quartz immersion well Figure 2. Photo-reactor. Air inlet Power Supply Cooling water outlet Air outlet Effluent outlet Sampling port Borosilicate glass reactor (L=55mm Id=9mm od=95mm) Effect of lamp wattage Experiments were carried out with low-pressure Hg lamps of different wattage, ie 8 W, 15 W and 25 W. The quantity of H 2 O 2 addedineachexperimentwas 35.3mmoldm 3 and the ph was 1.5. It was observed that with increasing wattage the rate of degradation also increased. The results are shown graphically in Fig 3. It is seen that in the case of the 25 W lamp, the time required for degradation of 1 mg dm 3 cyanide was only 4 min and for the 15 W lamp it took 45 min and for the 8 W lamp it took 65 min. As the wattage increases the radiation intensity also increases and more photons are absorbed which facilitates the degradation process. All further experiments were carried out with the 25 W lamp only. Effect of dissolved oxygen It was found that when oxygen was bubbled into the reactor, the rate of degradation increased. Three separate experiments were carried out with the 25 W low-pressure Hg lamp along with a H 2 O 2 dose of J Chem Technol Biotechnol 8:13 19 (25) 15

4 SMalhotraet al Concentration of CN - (mg dm -3 ) H 2 O 2 dose = 35.3 mmol dm -3 8 W 15 W 25 W Figure 3. Effect of lamp wattage on photo-oxidation of cyanide. Concentration of CN - (mg dm -3 ) H 2 O 2 dose = 35.3 mmol dm Air Nitrogen Oxygen Figure 4. Role of dissolved oxygen on photo-oxidation of cyanide using 25 W low-pressure Hg lamp mmol dm 3. Air, oxygen and inert nitrogen gas were bubbled through the reactor at the rate of 1dm 3 min 1. When pure oxygen was bubbled through the reactor solution then the time taken for complete degradation of cyanide was only 25 min whereas with air the degradation took 4 min. The results are shown in Fig 4. It is seen that dissolved oxygen enhances the degradation process. Some oxygen molecules absorb the photons and active oxygen molecules are formed, which may take part in the oxidation process. The photolytic reaction of oxidants to produce free radicals has been described by Shimoda et al. 2 hv O 2 ( 3 g ) O 2 ( 3 u + ) 2 O( 3 P) (2) Singlet oxygen plays a role in conversion of cyanide to cyanate: CN + O CNO (3) Optimisation of H 2 O 2 dose Experimental studies were carried out with the 25 W lamp along with H 2 O 2 of different doses. The results show that the cyanide oxidation depends on the 8 9 Concentration of CN - (mg dm -3 ) mmol dm mmol dm mmol dm mmol dm Figure 5. Effect of H 2 O 2 concentration on photo-oxidation of cyanide with 25 W low-pressure Hg lamp. initial concentration of H 2 O 2. With increasing H 2 O 2 concentration the rate of cyanide oxidation increases, but beyond a value of 35.3 mmol dm 3, the increase in the initial H 2 O 2 concentration retards the oxidation rate. The results are shown graphically in Fig 5. It was inferred that 35.3mmoldm 3 H 2 O 2 was the optimum dose in this process. If more than this dose of H 2 O 2 is used then less reactive hydroperoxyl radicals are produced (eqn (4)) and moreover OH will readily dimerise to H 2 O 2 according to eqn (5). 21,22 These hydroperoxyl radicals are less reactive and do not appear to contribute to the oxidation process. It is also reported that these hydroperoxyl radicals undergo a chain termination reaction 23 (eqn (6)) and in aqueous solution, H 2 O 2 dissociates to form the HO 2 anion and O 2 in a chain reaction: 24 H 2 O 2 + OH HO 2 + H 2 O (4) OH + OH H 2 O 2 (5) HO 2 + OH H 2 O + O 2 (6) OH + H 2 O 2 O 2 + H + + H 2 O (7) HO 2 + H 2 O 2 O 2 + OH + H 2 O (8) The existence of an optimum dose of H 2 O 2 has been described by several investigators earlier while working on photodegradation of other organic compounds. Galbriath et al 25 pointed out that there is an upper limit to the amount of H 2 O 2 that can be added above which peroxide scavenges the hydroxyl radicals. Shu et al 26 investigated the decolorisation of Acid Red 1 and Acid Yellow 23 by the UV/H 2 O 2 process. They found an optimum dose of 9.8mmoldm 3 H 2 O 2 for a mol dm 3 of Acid Red aqueous solution. Similar observations on optimal doses of H 2 O 2 have 22,26 31 been reported by various research groups. Reaction pathways of cyanide oxidation Cyanate was found as the intermediate product of cyanide oxidation by the UV/H 2 O 2 process. Cyanide is 16 J Chem Technol Biotechnol 8:13 19 (25)

5 Photo-oxidation of cyanide in aqueous solution first oxidised to cyanate by hydroxyl radicals according to the following equation: CN + OH CNO + H 2 O (9) The presence of the intermediate product cyanate was confirmed by analysing the samples at different time intervals for cyanate along with cyanide. A similar result was also reported by other investigators. In the second step cyanate is further oxidised to carbon dioxide (which ultimately forms bicarbonate) and nitrogen gas: CNO + 3OH HCO 3 + 1/2N 2 (g) + H 2 O (1) It has been reported 33 that during cyanate oxidation, formation of end products depends on the amount of excess H 2 O 2 present in the reaction medium. It may form carbon dioxide and nitrogen gas, as given in eqn (1). The two possibilities are carbon dioxide and either nitrite or nitrate according to the following equations: OCN + 6OH HCO 3 + NO 2 + H + + 2H 2 O (11) OCN + 8OH HCO 3 + NO 3 + H + + 3H 2 O (12) Moreover cyanate can undergo natural hydrolysis and will produce ammonium and bicarbonate ions at acidic ph values: 32,33 OCN + 3H 2 O NH HCO 3 + OH (13) In the present study, no nitrate, nitrite or ammonium ions were detected. This may be because of low H 2 O 2 concentration in the reaction medium when cyanate oxidation started (see Fig 6). Natural hydrolysis did not take place because the photo-oxidation was carried out at alkaline ph values. So the final end products are carbon dioxide and nitrogen. Concentration of CN -, CNO - (mg dm -3 ) CN - (mg dm -3 ) CNO - (mg dm -3 ) H 2 O 2 (mmol dm -3 ) 9 12 Figure 6. Concentration profile of CN, CNO and H 2 O 2 with time during photo-oxidation of cyanide with 25 W low-pressure Hg lamp Concentration of H 2 O 2 (mmol dm -3 ) When 1 mg dm 3 of cyanide was photo-oxidised with the 25 W Hg lamp, 35.3mmoldm 3 of H 2 O 2 and pure oxygen bubbling through the reactor, the time taken for conversion of cyanide to cyanate was 25 min and complete oxidation of cyanate to CO 2 and N 2 occurred in 2 h 15 min. The concentration profiles of cyanide, cyanate and H 2 O 2 with time are shown in Fig 6. It was found that initially the CN concentration was decreased and the cyanate concentration was increased. At the beginning, the cyanate oxidation rate was slow. It was observed that large amounts of H 2 O 2 were consumed at the beginning. Kinetics of cyanide oxidation The main oxidation of cyanide occurs through the reaction of OH radicals. The reaction can be expressed by the following equation r A = dc A /dt = kc A C OH (14) By the pseudo-stationary hypothesis (ie the concentration of hydroxyl radicals can be considered as constant in the presence of excess H 2 O 2 ) then the rate expression may be simplified into a pseudo-first order kinetic model and its final form is: ln C t /C = k 1 t (15) where C t is the concentration of cyanide at time t and C is the concentration at time, and k 1 is the pseudo-first order rate constant (min 1 ). When ln C t /C was plotted against t, a straight line relationship was obtained for the UV/H 2 O 2 process, which is shown in Fig 7. The rate constants were estimated to be min 1 and min 1 for H 2 O 2 concentrations of 35.5mmoldm 3 and 52.9mmoldm 3, respectively. The above rate expression is applicable to a specified reactor system. A more useful rate expression can be -ln (C t /C ) H 2 O 2 dose= 35.3 mmol dm -3 H 2 O 2 dose= 52.9 mmol dm Time (min) Figure 7. Determination of pseudo-first order rate constant of cyanide during photo-oxidation of cyanide using t as independent variable. J Chem Technol Biotechnol 8:13 19 (25) 17

6 SMalhotraet al -ln (C t /C ) The energy consumption for the UV/H 2 O 2 process was found to be 92% less than for UV alone. For the UV/H 2 O 2 process it was 167 kwh kg 1 CN and for UV alone it was 2127 kwh kg 1 CN ; 58.76% degradation occurred in 5 h by UV alone whereas 1% degradation of CN was observed in 4 min by the UV/H 2 O 2 process Pt/V (kwh m -3 ) Figure 8. Determination of pseudo-first order rate constant of cyanide during photo-oxidation of cyanide using Pt/V as independent variable. derived by considering UV treatment as a function of batch volume (V ), power output (P) andtime(t): 35 C CN = f (V, P, t) (16) This can be arranged into a form of energy required per unit volume (1 m 3 ) of wastewater (Pt/V ), with the familiar units of kwh. So using Pt/V as independent variable in place of t, eqn (15) becomes: ln(c /C t ) = k 2 (Pt/V ) (17) To verify this relationship, the pseudo-first order relation ship was plotted (see Fig 8) and found to follow straight line behaviour and the rate constant was found to be.343 m 3 (kwh) 1. Equation (17) can be useful for determining treatment time or power for any batch size of cyanide waste. Energy consumption Fig 9 depicts results of energy required to remove 1kg of CN by a UV and a UV/H 2 O 2 process. kwh kg -1 CN UV UV/H 2 O 2 Figure 9. Energy consumption during photo-oxidation of cyanide. CONCLUSION In the present study the oxidation of cyanide was studied by the UV/H 2 O 2 process. Use of H 2 O 2 as an oxidant has a number of advantages in comparison to other chemicals or photochemical water treatment. H 2 O 2 is commercially available and there is no problem associated with the thermal stability and storage on-site. It has infinite solubility in water, and as the UV/H 2 O 2 system is a homogeneous one, there is no mass transfer problem associated with this process. Two hydroxyl radicals are formed from each molecule of H 2 O 2 photolysed. This system is a very cost-effective source of hydroxyl radicals and is a simple operational procedure. In this process no toxic by products are produced. The intermediate product cyanate is also oxidised to carbon dioxide and nitrogen. The results of the study showed that the oxidation rate of cyanide is strongly accelerated by the UV/H 2 O 2 process and with increasing wattage of the UV lamp the rate also increases. It was also observed that if pure oxygen was bubbled through the reactor the rate of oxidation also increased compared with bubbling air under the same experimental conditions. The photooxidation process is influenced by the initial H 2 O 2 present and the optimum H 2 O 2 dose was found to be 35.3mmoldm 3 (3% w/v). The reaction pathways for cyanide oxidation by the UV/H 2 O 2 process have been established. It was found that cyanide is first oxidised to cyanate and later cyanate is oxidised to carbon dioxide and nitrogen gas. But the rate of cyanate oxidation is slower than that of cyanide. Using a 25 W low-pressure Hg lamp along with 35.3 mmol of H 2 O 2 and bubbling pure O 2 at ph 1.5, 1 mg dm 3 of cyanide was completely degraded in 25 min and the intermediate product, cyanate, is also oxidised to carbon dioxide and nitrogen in a total time of 2 h 15 min. When air was bubbled through the reactor cyanide was degraded in 4 min. The photo-oxidation of cyanide should be carried out at alkaline ph because at acidic ph, toxic volatile hydrogen cyanide gas will form and it is difficult to oxidise hydrogen cyanide. The kinetics of the cyanide oxidation was modelled in a pseudo-first order rate process and it was verified by the experimental results. The rate constant found at 4 Cwas min 1. The power required for complete degradation of 1 kg of cyanide was found to be 167 kwh. REFERENCES 1 Extremely Hazardous Substances. Superfund Chemical Profiles, Vol 1. USEPA, Noyes Data Corporation, New Jersey (1989). 18 J Chem Technol Biotechnol 8:13 19 (25)

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