Efficiency evaluation of stable cyanide complexes conversion and its application

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0063 Natural Sci. 2016, Vol. 61, No. 9, pp. 113-122 This paper is available online at 113 EFFICIENCY EVALUATION OF STABLE CYANIDE COMPLEXES CONVERSION AND ITS APPLICATION Bui Thi Thu 1 , Trinh Kim Yen 1 and Dao Van Bay 2 1 Faculty of Environment, Hanoi University of Natural Resources and Environment 2 Faculty of Chemistry, Hanoi National University of Education Abstract. In this paper, we present the evaluation results of the conversion efficiency of stable cyanide complexes using a specialized distillation system. The experiments show that the optimal distillation time for complexes of [Fe(CN)6] 4- and [Fe(CN)6] 3- is 60 minutes, conversion efficiencies of [Fe(CN)6] 4- and [Fe(CN)6] 3- are 100 ± 0,08 % and 84 ± 0,02 %, respectively, and the average of recovery rate of complexes RTB is 90 ÷ 100%. We further use the above-derived optimal experimental conditions to determine the cyanide concentration in metal-plating waste-water in Dong Anh district, Hanoi city. Keywords: Stable cyanide complexes conversion, recovery rate, waste-water, metal plating. 1. Introduction During recent years, along with the industrial development, the environmental pollution has been increasing at an alarming rate. Due to typical characteristics of the industry of a developing country, without general planning, using obsolete processing technologies, slack environmental regulations, and other reasons, most of industrial wastes, without processing, are disposed directly to environment causing pollutions. Among these above-mentioned, water pollution is of great concern. Among the natural environmental destructions, polluted water resources have already been exceeding permissible limits as per various parameters, so that affecting directly to human being’s health and other livings’s. Among contaminated water, waste-water containing cyanide group draws particular caution because of its toxicity. Containing just only 50 mg of Acid HCN, the poluted watercan kill a person. Meanwhile, cyanide salts are used extensively in industries [1]. As highly toxic, cyanide quantity in waste-water should be limit to minimum. According to Vietnam’s national standards, the permissible limits of cyanide in surface of domestic water for irrigation, and waterways are 0.005 mg/l and 0.02 mg/L respectively [2, 3]. In water environment, cyanide exists as stable complex solutions with heavy metals and partly in free state. The cyanide determination in environmental samples requires a distillation to convert cyanide in complex solutions into free state [1, 4]. Received September 29, 2016. Accepted November 28, 2016. Contact Bui Thi Thu, e-mail address: btthu.mt@hunre.edu.vn Bui Thi Thu, Trinh Kim Yen and Dao Van Bay 114 Therefore, we evaluate the conversion efficiency of stable cyanide complexes to determine cyanide in metal-plating waste-water samples in Dong Anh district, Hanoi city. 2. Content 2.1. Experiment 2.1.1. Tools, equipment, chemicals * Tools, equipment Measure pH HM-25R meter produced by TOA Japan; Balance analytical Model GP 150 - 3P, Sartorius Germany, accuracy of ± 0.1 mg; Distill water-machine of British-made Bibby 2 times; Use the cyanide distillation equipment (KCM) from Behr Company (Germany). All glass equipments such as pipette flask types, etc. are soaked and cleaned carefully by a mixture of concentrated sulfuric acid and potassium dichromate. * Chemicals [4] - Standard cyanide solution with different concentrations: 1000 mg/L, 100 mg/L and 10 mg/L; - Analytical CN - solution, 2 mg/L concentration; - Chloramine T, 1% solution; - Pyridine-barbituric acid reagent: Add 15 g of C4H3N2O3 barbituric acid to a 250-mL flask; add distilled water (about 100 mL) to a coated cylinder; next, wet the barbituric acid; add 75 mL of pyridine C5H5N, then mix it up; add 15 mL of concentration of HCl, and shake the mixture of dissolved barbituric acid and the milky solution turns to yellow, then, add 250 mL of distilled water, and mix it up. This reagent should be stable for about 6 months in cool, dark conditions. - Complex solutions of [Fe(CN)6] 4- and [Fe(CN)6] 3- . - Acid and base solutions with different concentrations for pH adjustment. 2.1.2. Research on the conversion efficiency of CN - - contained complexes Ion CN - complexation with metals ion. For example: Fe 2+ + 6CN - → [Fe(CN)6] 4- β6 = 10 36,9 Fe 3+ + 6CN - → [Fe(CN)6] 3- β6 = 10 43,9 Cu + + 4CN - → [Cu(CN)4] 3- β4 = 10 30,3 Zn 2+ + 4CN - → [Zn(CN)4] 2- β4 = 10 19,62 Ag + + 4CN - → [Ag(CN)4] 3- β4 = 10 19,42 Au + + 2CN –  [Au(CN)2] - β2= 10 38,3 Efficiency evaluation of stable cyanide complexes conversion and its application 115 The complex of ion CN - with ion metals is usually sustainable. Thus, the study of bio- degradable complex to tranform it into HCN vapor distillation is very important. Since the samples of waste-water metal-plating include disproportionately-large iron complexes which are the most sustainable of all remaining ones (complexation constants β is the biggest). Therefore, we focus our experiment on the two sustainable complexes of the irons namely [Fe(CN)6] 4- and [Fe(CN)6] 3- . Figure 1 shows a flow chart of the conversion efficiency of CN - - containing complexes research process. The distillation time of stable cyanide complex solutions is set at = 10, 20, 30, 40, 60, 75 and 90 minutes. The distillation efficiency H is calculated as follows: exp. theo. m H .100% m  In the above equation, mexp. is the experimental distilled CN - and mtheo. is the theoretical CN - in the distillation flask. The optimal distillation time to achieve the highest distillation efficiency is determined based on its evaluation at different times. CN - is determined by photo-metric method based on the cyanide oxydate using chloramine T to generate CNCl at pH < 8 (in order to avoid the hydrolytic reaction). Then, CNCl combines with pyridine-barbituric reagent generating a pink complex and achieving the maximum absorption at 580nm-wavelength. The standard-line equation is A = (2.511 ± 0.072).CCN - + (0.018 ± 0.009). The correlation co-efficient R 2 = 0.99852 satisfies 0.99  R 2  1 [4-6]. To evaluate the efficiency of stable cyanide complexes, we re-determine the CN - concentration in [Fe(CN)6] 4- and [Fe(CN)6] 3- complexes (which are exactly known in the concentration) after distilling at a selected optimal time of t = 60 minutes. The recovery rate of complex solution is calculated according to the following equation: m+c m c C - C R% = .100 C In the above equation, R% is the recovery rate, Cm+c is the concentration of analytical substance in additional standard sample; while Cm is the concentration of analytical substance in sample, and Cc is the additional standard concentration. The general recovery rate is calculated by repeating the above experiments several times and taking the average of recovery rates [7, 8]. The metal-plating waste-water samplings in a private metal-plating operation in Dong Anh district, Hanoi city are realized based on national standards and technical regulations such as TCVN6663-1:2011 (ISO 5667-2: 1991), TCVN 6663-3:2008; TCVN 5999:1995 and QCVN 10: 2009/BTNMT. Sampling times are as follows: D1 - 2013: 01/04/2013, D2 - 2013: 06/09/2013, D1 - 2014: 15/03/2014, D2 - 2014: 12/06/2014, D1 - 2015: 12/03/2015, and D2 - 2015: 12/07/2015. Sampling locations are presented in Table 1. Bui Thi Thu, Trinh Kim Yen and Dao Van Bay 116 Table 1. Wastewater sampling locations in a private metal plating operation Z in Dong Anh district, Hanoi city Sample Notation Sampling Location Coordinates DA1 At the waste-water culvert gate of the Private Metal- Plating Operation Z, Hai Boi commune, Dong Anh district 21° 7'0.38"N 105°47'47.89"E DA2 At the waste-water canal of the Private Metal-Plating Operation Z, Hai Boi commune, Dong Anh district 21° 7'1.32"N 105°47'49.30"E DA3 At the 100-meter spot of wastewater culvert gate of the Private Metal Plating Pperation Z, Hai Boi commune, Dong Anh district 21° 7'2.82"N 105°47'50.80"E DA4 At the 200-meter spot of wastewater culvert gate of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 7'6.40"N 105°47'45.80"E DA5 At the disposal gate of zinc plating tank of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 6'59.89"N 105°47'47.82"E DA6 At the disposal gate of washing tank of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 6'59.71"N 105°47'47.79"E DA7 At the wastewater disposal gate of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 7'0.07"N 105°47'47.83"E DA8 At the disposal gate of gold plating tank of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 6'59.85"N 105°47'47.67"E DA9 At the gate of disposal washing tank of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 6'59.70"N 105°47'47.64"E DA10 At the wastewater disposal gate of the Private Metal Plating Operation Z, Hai Boi commune, Dong Anh district 21° 7'0.04"N 105°47'47.63"E Complex solutions and environmental samples are distilled through a specialized distillation system as shown in Figure 1. - Add 250 mL of complex solution (sample of environment) into a 500 mL-distilling flask. Add 5 mL of 1.25 M NaOH solution into a guaranted residual flask of absorption. - Set up equipment distillation as Figure 1. - Unlock K1 to add 25 mL of 9 M H2SO4 solution into a distilling flask to acidify sample, to release cyanide in the form of HCN vapor. Efficiency evaluation of stable cyanide complexes conversion and its application 117 - Slightly boil the solution in a distilling flask, and at the same time, open vacuum air (at a rate of 1.5 liters/min.), then distill and absorb HCN vapor in an absorption flask within a period of t min. - After the t-identified time, stop the distillation and transfer the solution into the absorption flask, together with the washed water into a volumetric flask of 50 mL, then add distilled water nearly up to its mark. Next, add HCl per drop to neutralize soda and adjust the pH on the flask to its optimum value. Finally, measure its norm by the mark (obtained DDX). This solution determine the optimum distillation time, re-defining the concentration of cyanide and recovery of complex solution. Figure 1. Cyanide distillation system 2.2. Results and Discussion 2.2.1. Evaluation results of the CN - concentration determination at different distillation times Considering the distillation time of complex solutions of [Fe(CN)6] 4- and [Fe(CN)6] 3- at different values (t = 10, 20, 30, 40, 60, 75 and 90 minutes), the results of determination of cyanide concentration in solution after the distillation are presented in Table 2. As shown in Table 2, when the distillation time varies from 10 to 60 minutes, distillated cyanide’s weights of both complex solutions increase. The distillated cyanide’s weight is stable and reach the maximum value according to the distillation time which varies from 60 to 90 minutes. Additionally, during this distillation time period, the efficiencies of complex solution of [Fe(CN)6] 4- and [Fe(CN)6] 4- reaches approximately 97.8% and 82.4%, respectively. Therefore, we select t = 60 minutes as the optimal distillation time. Bui Thi Thu, Trinh Kim Yen and Dao Van Bay 118 Table 2. Evaluation results of the distillation time for complexes [Fe(CN)6] 4- và [Fe(CN)6] 3- No. Distillatio n time (minutes) [Fe(CN)6] 4- [Fe(CN)6] 3- mlt CN - weight in distillation tank (mg) mtn Determine d CN - weight (mg) Efficienc y (%) mlt CN - weight in distillation tank (mg) mtn Determined CN - weight (mg) Efficiency (%) 1 10 0.125 0.064 51.20 0.125 0.051 40.80 2 20 0.077 61.60 0.064 51.20 3 30 0.095 75.60 0.081 64.80 4 40 0.107 85.60 0.091 72.80 5 60 0.122 97.80 0.103 82.40 6 75 0.122 97.80 0.103 82.40 7 90 0.122 97.80 0.103 82.40 2.2.2. Evaluation results of the CN - concentration-determination in complexes after distillation Table 3. The CN - concentration redetermination in [Fe(CN)6] 4- and [Fe(CN)6] 3- after distillation Samples Theoretical C o CN (mg/L) CCN- (mg/L) Average CCN- (mg/L) H (%) [Fe(CN)6] 4- (0.5 mg/L) 0.025 0.022 0.025 ± 0.002 100 ± 0.08 0.027 0.024 0.028 0.025 0.026 0.023 [Fe(CN)6] 3- (0.5 mg/L) 0.025 0.021 0.021± 0.001 84 ± 0.02 0.019 0.020 0.025 0.017 0.021 0.022 Efficiency evaluation of stable cyanide complexes conversion and its application 119 Table 3 shows results of the CN - concentration re-determination in complexes of [Fe(CN)6] 4- and [Fe(CN)6] 3- after distillation at its optimal time (with seven experiments). As shown in Table 3, the solution of [Fe(CN)6] 4- concentrations re-define approximately its initial concentration; For the solution of [Fe (CN)6] 3- , the concentrations re-defines by 0.021 mg/l (only 84%), perhaps due to larger stable constant β6 = 10 43.9 of the complex of [Fe(CN)6] 3- than that of the complex [Fe(CN)6] 4- (β6 = 10 31.9 ). 2.2.3. The recovery rate of cyanide in solution We repeat the distilling experiment six times for CN - solution samples with pre-defined concentrations to get the recovery rate of complex solutions of [Fe(CN)6] 4- and [Fe(CN)6] 3- as shown in Tables 4 and 5. Table 4. The recovery rate of complexsolution [Fe(CN)6] 4- Sample + 0.01 mg CN - /L R% Sample + 0.02 mg CN - /L R% Sample + 0.04 mg CN - /L R% 1 0.033 110 0.040 90 0.066 110 2 0.038 110 0.046 95 0.063 90 3 0.034 100 0.046 110 0.061 92.5 4 0.037 90 0.050 110 0.061 82.5 5 0.035 90 0.048 110 0.064 95 6 0.033 80 0.042 85 0.068 107.5 RTB% 96.7 100 96.3 Table 5. The recovery rate of complex solution of [Fe (CN)6] 3- Sample + 0.01 mg CN - /L R% Sample + 0.02 mg CN - /L R% Sample + 0.04 mg CN - /L R% 1 0.030 90 0.037 80 0.063 105.0 2 0.028 90 0.040 105 0.060 102.5 3 0.029 90 0.041 105 0.059 97.5 4 0.033 80 0.042 85 0.061 90.0 5 0.025 80 0.039 110 0.056 97.5 6 0.032 110 0.042 105 0.055 85.0 RTB% 90.0 98.3 96.3 Tables 4 and 5 indicate that the average recovery rates RTB varies from 90 to 100%, and totally guarantes the value as prescribed by AOAC (allowing 110% ÷ 80%) [3]. That confirms the withdrawal of the complex containing cyanide solution is reliable, i.e. in the stable complexation, CN - , after distillation, is liberated almost entirely in the form of HCN gas. This is fundamental to determine the cyanide concentration in the environmental samples. Bui Thi Thu, Trinh Kim Yen and Dao Van Bay 120 2.2.4. Applying to determine the cyanide concentration in metal-plating waste-water in Dong Anh district, Hanoi city We have conducted experiments in order to determine the cyanide concentration in ten waste-water samples of a private metal plating operation in Dong Anh district, Hanoi city. The cyanide concentration is compared with Cmax which is indicated in the National Technical Regulation on Industrial Waste-water QCVN 40:2009/BTNMT (Cmax = 0.108 mg/l). The results are presented in Table 6. As shown in Table 6, in 2015, the cyanide concentration in waste-water samples of the private metal plating operation in Dong Anh district, Hanoi city varies from 0 to 4.234 mg/L; 70 percent (7/10) of sampling locations surpasses the cyanide concentration permissible limit. The analytic results in ten locations also show that the average concentrations in the first, then the second analyses periods in 2015 are 1.279 mg/L and 1.239 mg/L, respectively. While the average concentration for the whole year is 1.259 mg/L. In Figure 2, we present the comparison results of the cyanide concentration in the waste- water in Dong Anh district, Hanoi city from 2013 to 2015. Figure 2. The cyanide concentration in metal plating wastewater in Dong Anh district, Hanoi city from 2013 to 2015 As shown in Figure 2, the cyanide concentration in waste-water in Dong Anh within three years (from 2013 to 2015) varies from 0 to 4.355 mg/L. Out of ten sampling locations, seven exceed the permissible limit from 1.04 to 4.32 times. The average concentration in three years is 1.299 mg/L exceeding the permissible limit by 13.22 times. The cyanide concentration decreases gradually from waste-water samples (taken near plating and washing tanks of the metal plating operation) nnamely DA5, DA6, DA8 to others (taken at the wastewater culvert gate). Especially, with samples at waste- water canals far from source namely DA3, DA3, DA4 and TO14, the cyanide concentrations are small since the waste-water is diluted. The analytic results in two seasons (dry and rainy seasons) are quite similar. However, the cyanide concentration in dry season is slightly greater than that in the rainy one. Since there are waste-water samples of a metal plating company, the cyanide concentrations are high. Efficiency evaluation of stable cyanide complexes conversion and its application 121 Table 6. Results of the CN - concentration determination in waste-water samples in Dong Anh district, Hanoi city (including additional analysis results in 2013 and 2014) L o c a ti o n D1-2013 D2-2013 D1-2014 D2-2014 D1-2015 D2-2015 C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) C C N tb ( m g /L ) S ta ti st ic C T B (m g /L ) D A 1 0 .1 2 8 0 .1 2 8 ± 0 .0 0 3 0 .2 5 2 0 .2 5 2 ± 0 .0 0 9 0 .1 8 2 0 .1 8 2 ± 0 .0 0 4 0 .1 1 5 0 .1 1 5 ± 0 .0 0 2 0 .1 1 2 0 .1 1 2 ± 0 .0 0 5 0 .1 0 0 0 .1 0 0 ± 0 .0 0 9 D A 2 0 .0 9 6 0 .0 9 6 ± 0 .0 0 7 0 .0 9 8 0 .0 9 8 ± 0 .0 0 5 0 .0 6 7 0 .0 6 7 ± 0 .0 0 4 0 .0 5 5 0 .0 5 5 ± 0 .0 0 3 0 .0 6 7 0 .0 6 7 ± 0 .0 0 4 0 .0 4 8 0 .0 4 8 ± 0 .0 0 4 D A 3 0 .0 5 2 0 .0 5 2 ± 0 .0 0 5 0 .0 4 5 0 .0 4 5 ± 0 .0 0 3 0 .0 4 2 0 .0 4 2 ± 0 .0 0 4 0 .0 3 7 0 .0 3 7 ± 0 .0 0 3 0 .0 4 5 0 .0 4 5 ± 0 .0 0 2 0 .0 4 0 0 .0 4 0 ± 0 .0 0 4 D A 4 0 .0 7 5 0 .0 7 5 ± 0 .0 0 4 0 .0 5 5 0 .0 5 5 ± 0 .0 0 3 0 .0 6 2 0 .0 6 2 ± 0 .0 0 4 0 .0 4 8 0 .0 4 8 ± 0 .0 0 4 0 .0 2 3 0 .0 2 3 ± 0 .0 0 3 0 .0 1 2 0 .0 1 2 ± 0 .0 0 3 D A 5 4 .0 6 2 4 .0 6 2 ± 0 .0 0 6 4 .3 5 5 4 .3 5 5 ± 0 .0 0 6 4 .2 3 1 4 .2 3 1 ± 0 .0 0 7 4 .1 9 2 4 .1 9 2 ± 0 .0 0 4 4 .2 3 4 4 .2 3 4 ± 0 .0 0 4 4 .1 2 6 4 .1 2 6 ± 0 .0 0 4 D A 6 2 .5 7 8 2 .5 7 8 ± 0 .0 0 8 2 .4 6 3 2 .4 6 3 ± 0 .0 0 6 2 .1 2 5 2 .1 2 5 ± 0 .0 0 5 2 .3 2 8 2 .3 2 8 ± 0 .0 0 5 2 .3 1 2 2 .3 1 2 ± 0 .0 0 8 2 .1 4 7 2 .1 4 7 ± 0 .0 0 7 D A 7 1 .0 2 5 1 .0