Studying and determining simultaneously Zn2+, Cd2+, Pb2+ and Cu2+ in mud and water of the Dai Tu fish reservoir, Hoang Mai district, Hanoi

JOURNAL OF SCIENCE OF HNUE Natural Sci., 2008, Vol. 53, N ◦ . 5, pp. 95-103 STUDYING AND DETERMINING SIMULTANEOUSLY Zn 2+ , Cd 2+ , Pb 2+ AND Cu 2+ IN MUD AND WATER OF THE DAI TU FISH RESERVOIR, HOANG MAI DISTRICT, HANOI Duong Quang Phung and Nguyen Phuong Thao Hanoi National University of Education Abstract.Hoang Mai, a district in the south of Hanoi, has a lot of reservoirs, supplying fish to the whole city. Due to the development of the industry, many heavy metals from waste pollute this water source. In this research, the content of heavy metals in mud and water in Dai Tu fish reservoir, Hoang Mai, Hanoi, was determined to evaluate the pollution levels. According to the Vietnamese Standards, the result of research shows that the content of four ions: Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ is less than the acceptable level. Thus, it can be concluded that Dai Tu fish reservoir has not been polluted by these heavy metals. 1. Introduction In recent years, protection of environment is cared about all over the world. Pollutants, existing in nature or formed by human activities, have been destroying the environment. Almost the rivers, lakes and reservoirs in Hanoi are seriously pol- luted. They contain heavy metals such as: Zn, Cd, Pb, Cu, As, etc... and from many different sources. Through food, Zn, Cd, Pb and Cu come into human contact and cause many diseases. For example: Zn can damage stomach and curb the growth of the body, Cd can reduce kidney activity, Pb can influence the synthetic process of blood. Cu can cause anemia and influence genital ability. In [4], the background solution and optimal conditions for simultaneous de- termination of four heavy metal (Zn, Cd, Pb, Cu) were studied by dissolved anodic Volt-Ampere. Water and mud samples collected at the Dai Tu fish reservoir, Hoang Mai, Hanoi in Autumn 2007, were investigated. In this study, the content of Zn, Cd, Pb, Cu was continuely to determined. The studied samples collected in Dai Tu fish reservoir, Hoang Mai, Hanoi, in Spring 2008, were investigated to compare, to monitor and to evaluate. 95 Duong Quang Phung and Nguyen Phuong Thao 2. Content 2.1. Experiments 2.1.1. Chemicals and Equipment All chemicals used for the analyses were PA. Two-time distilled water was produced from a distillation device Hamilton (England). All standard solutions of cation Cu 2+ , Cd 2+ and Pb 2+ were purchased from Metrohm, Switzerland. The content of Cu, Cd, Pb was determined by the multi-purpose polarograph 797 VA Computrace (Metrohm, Switzerland), using three electrodes: - Working electrode: hanging mercury drop electrode. - Reference electrode: silver electrode. - Counter electrode: Pt electrode. 2.1.2. Experiments a. Researching for optimal conditions In order to receive the optimal process with high sensitivity, interactivity, accuracy, selectivity for all metals Cu, Pb, Cd, Zn by stripping voltammetry, the optimal conditions were investigated during the electrochemical process. The optimal condition of each element was studied, concerning its influence on analytic purposes during other fixed factors. * Optimality of buffer solution condition: - Optimality of buffer solution: Solutions of 0.1M KCl; 0.1M NaCl; 0.1M (NaAc + HAc); 0.1M (NH4Ac + HAc) at pH = 3.6 and 4.6, were investigated to find out the buffer solution with the highest adsorption peak. The ammonium acetate solution was chosen. - Optimality of pH: Different pH values of buffer ammonium acetate solution at 0.93; 3.1; 3.6; 4.0; 4.5; 5.0; 5.9; 6.5 were studied. The optimal pH of 3.6 was discovered and chosen. * Optimality of technical conditions: - Pulse amplitude is studied to distinguish the potential of metals, shift the potential of four metals to negative potential to make peak wider. - Pulse time is studied to get the most accurate and sensitive result of analysis. - Sweep rate is researched to achieve the balance and high peaks. - Mercury drop affects the concentration of metals depositing. The drop size was optimized. - Electrolytic time is studied to get the most stable peak of ions. - Optimal deposition potential is studied to get the most stable peak of ions. 96 Studying and determining simultaneously Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ ... - Optimal equilibration time is researched for achieving the most stable peak. - Dissolved oxygen and optimal initial purge time: affects the height of peak, therefore their impact must be studied. - The influence of Fe 3+ , Mn 2+ concentration: they are the most abundant in the earth's crust, especially in mud and water. On the other hand, they behave as electrochemical property in the analytic condition. They also reduced and concen- trated in amalgams. Thus, their influence should be studied. - The reciprocal influence between 4 analytic ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ): one advantage of the mercury drop electrode is that deposited metals amalgamate with mercury; metal-metal compounds thus aren't formed. Hence, it is better if metals are measured simultaneously. However, if the concentration of a metal ion in a solution is high, its concentration in mercury drop is also high. It is possi- ble that metal-metal compound exists. Additionally, due to the high concentration of one metal, its extended peak covers the whole of other ions peaks. It leads to an inaccurate result. For this reason, the reciprocal influence of analytic ions was studied. b. Building and evaluating calibration curves, and defining limit of detection, limit of quantifiable base on calibration curve Concentration of electrolytic base solution was used to be 0.04 mg/l. Ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) were added into a 25 ml volume flask. Two-time distilled water was added into the calibration mark of flask. Each measurement was carried out three times to get the average result. The Calibration curve is applied in defining limit of detection and limit of quantification. c. Applying the theoretical curve on practical samples Based on researched optimal conditions, some practical samples were analyzed. Samples, taken in the Dai Tu fish reservoir, Hoang Mai district, Hanoi for 3 weeks in April, 2008, were studied to determine simultaneously ions: Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ by generalized standard addition methods. d. Processing practical results All results that are achieved by searching optimal conditions, building and evaluating calibration curve, measuring metals concentration are processed by Mi- crosoft, mathematical statistics and coded Turbo Pascal [2]. 2.2. Results and discussion 2.2.1. Studying the optimal conditions and the influences of factors The results show that buffer solution of ammonium acetate at pH of 3.6 could be used for determining simultaneously four ions but Zn was not loosened in the analytic process. This buffer solution was better than the mixture of (KCl + HCl) 97 Duong Quang Phung and Nguyen Phuong Thao solution with pH = 2.0 [4]. In the analysis, the error of measurement was less than 5%. The optimal conditions of 797 VA Computrace is present in Table 1. The buffer solution was ammonium acetate. The optimal condition is in agree- ment with [4]. Table 1. The optimal conditions Working electrode HMDE Deposition time 180 s Mode DP Equilibration time 20 s Drop size 4 Pulse time 0.06 s Stirrer 2000 Pulse amplitude 0.05 V Initial time 180s Sweep rate 0.02 V/s Deposition potential -1.3 V Start to end potential -1.3 V → 0.2 V pH 3.60 2.2.2. Building the calibration curve The measurement of the analytical ions was based on: Ip = K.C2+M Based on the optimal conditions, base line of ions ( Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) was built in the concentration gap between 10 −9 M to 10 −6 M. The measurement was carried out using the optimal conditions. Concentration of electrolytic base solution was 0.04 mg/l. Ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) were added into 25 ml volume flasks. Two-time distilled water was added into calibration mark of the flask with [6]. Each measurement was carried out three times to get an average result. The relationship between the Pb concentration and the height of peak is showed in Table 2 and Figure 1. Table 2. The dependence of Ip (Pb 2+ ) on the concentration (M) Pb2+.108M 0.3864 1.159 3.092 11.594 30.92 57.97 77.29 Ip(nA) 19.2 27.3 54.4 130 270 479 610 Figure 1. The calibration curve of Pb determination 98 Studying and determining simultaneously Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ ... As showed in Figure 1, the concentration of Pb 2+ varies linearly with the height of the peak. After statistical processing, the calibration curve of Pb 2+ by the linear regression method on Turbo - Pascal, this regression equation was given: Ip = (77±3).CPb2+ + (27±13) In the concentration of Pb in the range of (0.3864÷77.29).10−8 M or (0.0008÷2.4) mg/l, the peak of Pb is linear with concentration. Similarly, it achieves: - The calibration curve of determining zinc. Ip = (40±1).CZn2+ + (321±21) In the concentration of Zn in the range of (1.23÷3690).10−8 M or (0.0008÷2.4) mg/l, the peak of lead is linear with concentration. - The calibration curve of determining cadmium. Ip = (68±2).CCd2+ + (13±2) In the concentration of Cd in the range of (71.4÷2140).10−8 M or (0.0008÷2.4) mg/l, the peak of lead is linear with concentration. - The calibration curve of determining copper. Ip = (47±2).CCu2+ + (67±21). In the concentration of Cu in the range of (1.25÷3750).10−8 M or (0.0008÷2.4) mg/l, the peak of lead is linear with concentration. 2.2.3. Evaluating the accuracy of the calibration curve of metal ion de- termination Apply these calibration curves to measurement metal ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) in water and mud of fish reservoir in Dai Tu village, Dai Kim precinct, Hoang Mai district, the content of ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) in 3 self-made samples (known concentration) were analyzed to evaluate the calibration curve [7]. According to these results obtained by measuring the content ions in 3 sam- ples, errors are quite small (around 5%) in the permissible limit. Errors between theory and experiment is random. It can be concluded that the technique is fairly accurate. It is possible to apply the calibration curve for determining simultaneously the content of 4 ions in mud and water from the fish reservoir. 2.2.4. The limit of detection (LOD), limit of quantifiable (LOQ) The detection limit is the lowest concentration of analytic substance, creating a signal which can be distinguished from the white signal (or base signal). Quantitative limit is the lowest concentration of analytic substance, creating a signal which can be measured. After studying, the LOD and LOQ of method were given: LODZn = 1.42.10 −9 (M) and LOQ = 4.73.10 −9 (M) LODCd = 4.43.10 −10 (M) and LOQ = 1.48.10 −9 (M) 99 Duong Quang Phung and Nguyen Phuong Thao LODPb = 7.1.10 −10 (M) and LOQ = 2.37.10 −9 (M) LODCu = 1.28.10 −9 (M) and LOQ = 4.27.10 −9 (M) 2.2.5. Determination of the content of ions Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ in water and mud [1] After finding out the optimal conditions and using the linear concentration gap, some water and mud samples from the fish reservoir in Dai Tu village, Hoang Mai district, Hanoi was analyzed by generalized additions of standard methods. The advantage of this method is the more exact results that can be found because it can eliminate the influences from other strange ions. a. Treating samples * Mud sample: after taking, mud is preliminary dry, and put into the drier at 150 0 C, then broken finely by grinder sift equally. Mud samples are weighed accu- rately, wetted by V ml of 18% HCl, then boiled for 45 minutes. Filtering solution (solution 1). The filter cake is added solution of 35% HCl and 98% H2SO4, then heated until occurring white smoke and nearly dry. The filter cake is dissolved by 18% HCl solution, and then warmed. This solution is added into solution 1 and then heated as humid salt. This salt is dissolved by H2O and then evaporated into humid salt (3÷4 times). Then the solution is added into 30% H2O2 solution and heated until the solution is clear and becomes humid salt. The Salt is dissolved and filled into the volumetric flask by 2% HCl solution. This is the analytic solution. * Water sample: after taking and determining the pH, mud is filtered as sus- pended matter. V ml of water sample is taken accurately, then the sample is acidified by concentrated HNO3 acid, added 30% H2O2 solution. Then the solution is boiled almost dry. The solution is added continuously to distilled water and boiled almost dry (4 or 5 times). This solution is determined the pH and is filled to the volume flask by water. This is the analytic solution. After tackling water and mud samples, we measure and evaluate the concentra- tion of ions in the samples. The measurement is carried out by 797 VA Computrace on the optimal conditions. - We build the generalized addition calibration curve of 4 ions of each sample. * The mud sample 1. - We build the generalized addition calibration curve by Microsoft Excel: - We also build the addition calibration curve by statistical processing: The addition calibration curve of Zn 2+ : Ip = (47.500±2.729).CZn2+ + (6.900±0.107) The concentration of Zn 2+ in measuring vessel: CZn2+ = (0.145±0.009) (mg/l) 100 Studying and determining simultaneously Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ ... a) The addition calibration curve of Zinc b) The addition calibration curve of Copper Figure 2. The addition calibration curve of ions in mud sample 1 The content of Zn 2+ in mud is: HZn2+ = (0.145± 0.009).25.250.1000 10.006.1000 = 90± 5(mg/kg) We carry out other ions (Cd 2+ , Pb 2+ , Cu 2+ ) of sample 1 and 4 ions of other samples similarly to the Zinc of mud sample 1. The results of determining the content of metal ions in mud and water from the Dai Tu fish reservoir for 3 weeks in April, 2008 is showed in the Table 3 and Table 4: Table 3. The content of 4 ions in mud samples from the Dai Tu fish reservoir Sample The content of metals Zn 2+ Cd 2+ Pb 2+ Cu 2+ week - 1 90±5 0.24±0.04 0.32±0.03 4.9±0.3 Mud week - 2 93±15 0.19±0.03 0.308±0.006 5.5±0.0 Sample week - 3 77±2 0.21±0.02 0.24±0.02 6.0±0.2 (mg/kg) Average value 87±16 0.21±0.05 0.29±0.04 5.5±0.4 Table 4. The content of 4 ions in water samples from the Dai Tu fish reservoir Sample The content of metals Zn 2+ Cd 2+ Pb 2+ Cu 2+ week-1 0.0400±0.0004 0.00032±0.00004 0.0022±0.0001 0.00990±0.00009 Water -2 0.0339±0.0004 0.00021±0.00001 0.0017±0.0001 0.00720±0.00006 Sample week-3 0.036±0.001 0.00029 ±0.00004 0.00143±0.00009 0.0075±0.0001 (mg/l) Average value 0.037±0.001 0.00027±0.00006 0.0018±0.0002 0.0082±0.0001 101 Duong Quang Phung and Nguyen Phuong Thao Comment: According to the results of determining heavy metals in mud and water by anodic stripping voltammetry on hanging mercury drop electrode, it was thought that: - The content of heavy metals in mud hardly changed in 3 weeks in April, 2008. - The content of metal ions in mud was hundreds of times higher than in water for 3 weeks in April. - The content of metal ions in the fish reservoir for 3 weeks in April, 2008 raised in comparison with 2007 (Table 5). In mud, the content of ions this year is about 15% and is higher than last year. In water, the content of ions this year is about 18% higher than last year. The difference can be explained by the higher pollution in the reservoir. The waste water containing a lot of heavy metals is flowing into the reservoir increasingly. Table 5. The content of ions in 2007 Sample The content of ions in 2007 Zn 2+ Cd 2+ Pb 2+ Cu 2+ mud 77±8 0.11±0.02 0.22±0.05 5±1 water 0.032±0.004 0.00018±0.00003 0.0015±0.0003 0.007±0.001 3. Conclusion 1- The optimal buffer solution, deposition time, deposition potential, start to end potential, drop size, initial time, pulse time, pulse amplitude,. . . have already been achieved. 2- Anodic stripping voltammetry on hanging mercury drop electrode with buffer solution of ammonium acetate was applied successfully to determine simul- taneously 4 metal ions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) in mud and water. Zinc was not loosened so much in the analysis and the error of measurement is in the permitable error. 3- This technique is highly accurate. Based on the statistic results, the relative errors are less than 5%; the iteration of results is high. The limit of detection and the limit of quantifications were studied. 4- The generalized addition standard method can be used. 102 Studying and determining simultaneously Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ ... 5- The content of heavy metal ions ( Zn 2+ , Cd 2+ , Pb 2+ , and Cu 2+ ) was studied, determined, observed, evaluated simultaneously in water and mud from the Dai Tu fish reservoir, Hoang Mai district, Hanoi for 3 weeks in April, 2008. This result shows that the fish reservoir is not polluted heavily by heavy metal. 6- The result of heavy metals determination in mud and water in fish reservoirs in the Dai Tu village - Hoang Mai district, Hanoi in April, 2008 is in agreement with previous research works on a seasonal and yearly basis [4]. REFERENCES [1] Pham Luan, 1998. Book on fundamentals of techniques for treated analytical samples. 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