Study of supporting electrolytes of KCl-Acetate buffer - NH4Ac for simultaneous determination of Zn2+, Cd2+, Cu2+, Pb2+ in acid mine drainage in Mao Khe, Quang Ninh by anodic stripping voltammetry at hanging mercury-drop electrode

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0061 Natural Sci. 2016, Vol. 61, No. 9, pp. 93-103 This paper is available online at 93 STUDY OF SUPPORTING ELECTROLYTES OF KCl-ACETATE BUFFER - NH4Ac FOR SIMULTANEOUS DETERMINATION OF Zn 2+ , Cd 2+ , Cu 2+ , Pb 2+ IN ACID MINE DRAINAGE IN MAO KHE, QUANG NINH BY ANODIC STRIPPING VOLTAMMETRY AT HANGING MERCURY-DROP ELECTRODE Nguyen Hoang Nam 1 , Tran Thi Hong Van 2 and Nguyen Thi Thanh Huong 3 1 Faculty of Environment, Hanoi University of Mining and Geology 2 Faculty of Chemistry, Hanoi National University of Education 3 Faculty of Chemistry, Military Medical University Abstract. The simultaneous determination of Zn 2+ , Cd 2+ , Cu 2+ , Pb 2+ in acid mine drainage (AMD) by differential pulse-anodic stripping voltammetrics (DPASV) with high sensitivity, selectivity, and accuracy is difficult because of many influential factors in the waste-water. This paper presents the results of studying supporting electrolyte for simultaneous determination of these ions in AMD in Mao Khe, Quang Ninh province to serve the waste- water treatment process. The working electrode as a stationary mercury electrode, and a platinum electrode and an Ag/AgCl/KCl (sat) electrode as the counter and reference electrode, respectively were used. In the electrolyte background of 0.12 M KCl, a buffer solution of 0.08 M NaAc, 0.08 M NH4Ac and 50 μL of 1 g/L Ga, and certain parameters including a deposition potential of -1.15 V, deposition time of 90 s, sweeping rate at 0.015 V/s, pulse amplitude of 0.05 V, pulse time of 0,04 s, stirring rate at 2000 V/min, equilibration time 10 s were applied. The method reached high repeatability. The limit of detection of Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ions reached 0.216; 0.071; 0.107 and 0.202 ppb, respectively. The limit of quantification reached 0.720; 0.237; 0.357 and 0.673 ppb, respectively. Keywords: Stripping methods, determination, heavy metal AMD, electrolyte, supporting. 1. Introduction Toxic and persistent substances in the environment continuously increase, owing to mining activities. In particular, the rapid diffusion of heavy metals as environmental contaminants has called for attention to their determination at trace and ultra-trace levels [1]. In fact, such elements tend to accumulate in all the aquatic matrices in the environment (suspended matter, sediment, and biota [2]), resulting in a definite presence in the aquatic food chain, and ground water which is dangerous for humans, as a consequence of the consumption of marine products. Received November 24, 2016. Accepted December 5, 2016. Contact Nguyen Hoang Nam, e-mail address: nguyenhoangnam@humg.edu.vn Nguyen Hoang Nam, Tran Thi Hong Van and Nguyen Thi Thanh Huong 94 Together with numerous simple treatment procedures [3, 4], several methods described for metal determination at low concentration, such as spectroscopy [5-7] and Voltam-metric [8] techniques have been the most frequently employed. In particular, anodic as well as cathodic stripping Voltammetry methods form a valid and effectual option for the multi-component analysis of metals, since the high sensitivity of the Voltammetry method [8] can be associated with considerable selectivity especially reached once the alternative electrolyte background selected is suitable. However, problems arise when determining directly and simultaneously various elements in environmental samples, sediment samples, etc. There are many factors hindering the determination process such as the voltammetric signals quite unrepeatedly and non-interferedly relevant to hindering elements, with very high concentration ratios. Most authors solve such problems by employing mathematical methods (chemo-metrics), which are often complicated and difficult to follow or apply, or only determine one or two elements simultaneously. It remains difficult to obtain the specification of 4 elements including: Zn, Cd, Pb, and Cu in environment samples simultaneously. The present work suggests, instead, a simple and quick procedure proving that voltammetric techniques with an appropriate electrolyte background are alternative to the standard additional method, allowing the repeatability, as well as density of highly voltammetric signals. This fact, especially once applied to samples of great interest, e.g. environmental matrices, may certainly encourage the employment of the same voltammetric techniques. The technique may also be applicable to cases of impossible simultaneous metal determination is, owing to high concentration ratios, and simultaneous small peak potential differences. 2. Content 2.1. Materials and methods 2.1.1. Apparatus Voltammetry measurements were carried out with a Metrohm (Switzerland) Model 757 VA Computrace, employing a stationary mercury electrode as the working one, while an Ag/AgCl/KCl (sat) electrode and a platinum wire used as the reference and counter electrode, respectively. The Voltammetry cell was kept at 20.0 ± 0.5 °C. The solutions were de-aerated with pure nitrogen for 15 minutes, prior to the measurements, then a nitrogen blanket was maintained above the solution during the analysis. The solutions were de-aerated for 2 minutes, after each standard addition. All solutions were prepared with de-ionized water (Millipore, Milli Q), and all reagents (Merck, Germany) were of ‘ultra-pure’ grade. KCl 3 M, acetic + sodium acetate solution buffer 1 M (pH = 4.75), ammonium acetate solution 1 M was prepared. Aqueous stock solutions of Cu 2+ , Pb 2+ , Cd 2+ and Zn 2+ and Ga 3+ were prepared by dilution of the respective standard of 1 g/L solutions (Merck, Germany). The Teflon Voltammetry cell was rinsed with ultra-pure concentrated nitric acid to minimize potential contamination every day. Study of supporting electrolytes of KCl-acetate buffer - NH4Ac for simultaneous determination 95 2.1.2. Sample preparation Before the Voltammetry determinations, an examination sample was prepared from standard solution of Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ 10 mg/L by: taking 50 µL of standard solutions (Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ ) 10 mg/L, next pouring a determined volume of electrolyte solution in a 25-mL flask, then adding ultrapure distilled water to the mark. The AMD sample was taken in August 2016, from the positions of S1 (input), (S2.), and S3 (output) of a constructed wetland treatment system in Mao Khe, Quang Ninh. Before analyses, samples were filtered by 0.45 µm membranes, then treated by micro-wave: take 100 μL of treated sample, and 1 mL of 3 M KCl + 2 mL of 1 M acetate buffer + 2 mL of 1M NH4Ac + 50 μL of 1 g/L Ga all in a 25-mL flask, next norm it two times by distilled water, then put in electrolyser. Proceed with recording differential pulse lines in conditions of table 1. Add a 20 μL standard solution containing Zn 2+ concentration of 10 mg/L, Pb 2+ , Cd 2+ , and Cu 2+ (of 10 mg/L/each) twice repeat measuring 3 times. 2.1.3. Optimal condition examination Supporting electrolyte is examined from a standard solution and added various electrolytes of various volumes to other optimal conditions such as: potential, sweep rate, Hg drop size, etc. as shown in Table 1. Afterward, find the optimal condition to apply to reality [9]. Table 1. The optimal parameters used to determine Cd 2+ Cu 2+ , Pb 2+ , Zn 2+ simultaneously Working electrode HMDE Deposition potential (V) -1.15 Measuring mode DP Deposition time (s) 90 Drop size 4 Equilibrium time (s) 10 Stirring rate 2000 (r/min) Pulse amplitude (V) 0.05 Blank purge time 100 s Pulse time (s) 0.04 Start Potential (V) -1.2 Voltage step time(s) 0.4 End potential (V) 0.0 Sweep rate (V/s) 0.015 2.2. Results and discussions 2.2.1. Examine choosing supporting electrolytes To find the optimal electrolyte background for analysis, the examination of dependence of maximal current intensity on different electrolyte backgrounds in fixed condition of parameters has been proceeded. Pour 50 μL of standard solution (Cu2+, Pb2+, Zn2+, Cd2+) 10 mg/L into a 25-mL flask, next add different electrolytes of various volumes of electrolyte solution (see Appendix 1), then proceed recording 3 times repeated measurements of dissolving current intensity in conditions stated in Table 1, take the average value and deviation of the repeated measurements. * Supporting electrolyte of KCl Result of measuring dissolving current intensity in various conditions of KCl supporting electrolyte indicates that: in different concentrations of KCl, stripping current intensity Ip of ions decreases in the follow of Ip(Zn) > Ip(Cd) > Ip(Pb) > Ip(Cu). When the concentration of KCl Nguyen Hoang Nam, Tran Thi Hong Van and Nguyen Thi Thanh Huong 96 increases from 0.06 M to 0.5 M, the Ip value of Cd 2+ and Cu 2+ decreases regularly from 120 to 62 nA, but slowly and insignificantly from 92 to 46 nA. At CKCl = 0.12 M, Ip of Zn 2+ reached maximum. Once the electrolyte potential enrichment is greater, the enrichment of heavy metal ions on the mercury drop has lower efficiency, resulting in a smaller dissolving current. When the concentration of KCl increases, it is very efficient to determine Pb, however the repeatability is low and in-appropriate for determining Cd 2+ and Cu 2+ . The study results show that, at CKCl = 0.12 M, Ip of ions did not reach its maximum, while Ip of Zn did, with the highest repeatability (Figure 1). Therefore, we can see that, in this condition of background solution, it is only suitable for determining ion Pb 2+ and Zn 2+ , as a result, CKCl = 0.12 M is selected to proceed the next study. Figure 1. Dissolving current intensity Ip of Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ (10 mg/L) in supporting electrolyte of KCl (0.12 M) * Supporting electrolyte of KCl + NaAC buffer The result indicated that when changing acetate buffer concentration, the dissolving current intensity changed as well. In general, in the same concentration of studied ions, the solution with a presence of acetated ion lowed the dissolved current intensity in comparison to using only KCl supporting electrolyte (see Appendix 1). However, the intensity of dissolved current of Zn changed without any pattern, while that of Cd increased regularly. The Ip of Pb had atrend to increase once the concentration of electrolyte increased, while that of Cu decreased with an almost unchanged intensity starting from a concentration of buffer solution of NaAc at 0.08 M. Besides, it had the higher repeatability of measurements and more balanced peaks than those with only KCl (see Figure 2). The results also indicated that at the acetate concentration of CNaAc = 0.08 M, the intensity of dissolved current of Zn and Pb was the biggest. Moreover, the measurements witnessed the highest repeatability in the examined concentrations. The results showed that the presence of acetate benefited the determination of Cu and Cd in the solution. Therefore, the concentration of acetate buffer chosen was 0.08 M, and we used this fixed concentration to examine the optimal one of NH4Ac. Study of supporting electrolytes of KCl-acetate buffer - NH4Ac for simultaneous determination 97 Figure 2. Intensity of dissolve current Ip of Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ (10 mg/L) in supporting electrolyte of KCl (0.12M) * Supporting electrolyte of KCl + NaAc buffer and NH4Ac The results of studying the effect of NH4Ac to dissolve current intensity of 4 aboved ions indicates that the concentration of NH4Ac changes relating to its peak changes. In general, with the presence of NH4Ac, the dissolving current Ip intensities of the 4 ions significantly increase and were all higher than both the KCl-background condition and the mixture of KCl electrolyte background with NaAc buffer, despite having the same concentration. Its value increase by 1.5 times compared to that of Ip using solution of KCl electrolyte background and NaAc buffer. However, when the concentration of electrolyte background increases, the dissolving current intensity has a different trend of decreasing insignificantly compared to other electrolytes having very high repeatabilities. When the concentration of NH4Ac increase from 0.02 M to 0.08 M, the dissolving current intensities of Zn, Cd, Pb, Cu (within the range of 0.06 M to 0.08 M) is very stable having balanced peaks and high repeatabilities, despite at no maximum value. The peaks have very high resemblances. Adding NH4Ac into the solution, on one hand, the concentration of CH3COO - increases, while on the other hand, ion NH 4+ forms a a heavy metal ammine complex which is discharged easier than any ones of Chloro, acetate, and aquo. It also strengthens ions conductivity for peak to become balanced with ahigher repeatability. Therefore, the mixture of electrolyte background of KCl (0.12 M) + NaAc buffer (0.08 M) and NH4Ac (0.08 M) is chosen to examine the optimal concentration of Ga 3+ . Figure 3. Dissolving current intensity Ip of Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ (10 mg/L) in supporting electrolyte of KCl (0.12 M), acetate buffer (0.08 M) and NH4Ac (0.08 M) Nguyen Hoang Nam, Tran Thi Hong Van and Nguyen Thi Thanh Huong 98 * Supporting electrolyte of KCl + NaAC buffer + NH4Ac and Ga 3+ In the presence of Ga 3+ , the peaks of all 4 metals are clear; intensities of stripping currents are greater than those without Ga3+. Especially when the intensity of stripping current of Cu increases tremendously, due to Ga 3+ hindering the forming institute of heavy metal on Hg drop. Besides, it distances the peaks of Cd and Pb simultaneously even at the bottom of heavy metal peaks, particularly in the cases of Cu and Cd. At an adequate content of Ga 3+ , the intensity of dissolving current of heavy metals changed insignificantly without affecting the current intensity itself. Also, the repeatability of the measurements is very high (Figure 4 and Appendix 1) the sample analysis process showed an exceeding amount of Ga could be added to achieve best results. Figure 4. Differential pulse anodic stripping voltammetry of 4 elements (10 mg/L) in supporting electrolyte of KCl (0.12 M) + acetate buffer (0.08 M) + NH4Ac (0.08 M) and 20 µL Ga 3+ (1 g/L) As shown from the results, the simultaneous determination of all 4 ions has good Ip values in all 4 supporting electrolytes, and the higheset one in electrolyte of KCl + acetate buffer + NH4Ac + Ga 3+ . The examination processing samples was done many times to determine the repeatability of the measurement. In this supporting electrolyte, the mixture gives the best repeatability result. Therefore, to analyze Cu 2+ , Pb 2+ , Zn 2+ , Cd 2+ , the background mixture of KCl + acetate buffer + NH4Ac + Ga is selected. 2.2.2. Assessment of repeatability, limit of detection and limit of quantification * Assessment of repeatability To assess the repeatability of the measurement, we proceeded to record the line of stripping Volt - Ampere of Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ in a fixed concentration (0.025 mg/L), then repeated multiple times in the same condition. The measurement conditions were stated in Table 1. The results are shown in Table 2. The results indicates that in the study condition, the error are very low, repeatability of measurement is very high, this shows the electrolyte background is highly reliable and could be applied very well to measure simultaneously 4 elements. Study of supporting electrolytes of KCl-acetate buffer - NH4Ac for simultaneous determination 99 3. .S C LOD X  10. .S C LOQ X  Table 2. Results of assessment of measurement repeatability Measure value of repeatability n IP (nA) Zn 2+ Cd 2+ Pb 2+ Cu 2+ 1 275.08 215.17 151.06 121.00 2 276.98 215.79 150.73 120.34 3 275.30 215.54 151.09 120.98 4 276.05 215.65 150.98 121.06 5 276.35 215.69 150.78 120.34 6 275.98 215.27 150.88 121.03 7 274.67 215.28 151.03 120.45 8 275.09 215.65 151.08 120.65 9 274. 45 215.35 150.46 121.02 10 275.31 215.45 150.64 120.35 Average value 275.53 215.48 150.87 120.72 Standard deviation S 0.792 0.205 0.215 0.325 Average standard deviation X S 0.250 0.065 0.068 0.103 * Limit of detection (LOD) and limit of quantification (LOQ) LOD as convention is considered to be the concentration of examining substance which gives signal 3 times higher than standard deviation of background line. If the concentration of the substance in sample is C then LOD is: Limit of quantification (LOQ) is considered to be the lowest concentration of the analyzing substance which the analysis system can quantify with the different analysis signal from quantify with signal of blank sample or signal of background line, conventionally calculated by formula: The study result indicates that the method Volt-ampere-stripping voltammetry with hanging mercury-drop electrode, using electrolyte background of KCl (0.12M) + NaAc buffer (0.08 M) + NH4Ac (0.08 M) and Ga (20 µg/L), has high sensitivity, repeatability and applicability for analyses of quantifying traces. The results of studying repeatability to calculate limit of detection of substances gives Table 3. Nguyen Hoang Nam, Tran Thi Hong Van and Nguyen Thi Thanh Huong 100 Table 3. Results of LOD and LOQ Zn 2+ Cd 2+ Pb 2+ Cu 2+ LOD (ppb) 0.216 0.071 0.107 0.202 LOQ (ppb) 0.720 0.237 0.357 0.673 2.2.3. Determine simultaneously 4 ions Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ in AMD in Mao Khe, Quang Ninh AMD sample has various metal ions depending on their mining types. In coal mines such as Mao Khe, Quang Ninh, the content of Fe and Mn in waste-water is very high. Waste-water sample after being micro-waved was measured directly without extracting hindering substances, enriching the number of measurement repeatability of 3, using the standard additional methods to determine the concentration of ions in sample. The results of direct simultaneous analyses of 4 ions Zn 2+ , Cd 2+ , Pb 2+ and Cu 2+ are indicated in Table 4 and presented in Figures 5 - 7. Table 4. Average value of Zn, Cd, Pb and Cu in samples Samples Adding time Ip (nA) Zn 2+ Cd 2+ Pb 2+ Cu 2+ S1 0 43.72 1.08 18.01 4.13 1 292.75 46.67 90.82 28.01 2 489.61 98.00 161.38 58.34 S2 0 33.7 0.5234 15.01 3.02 1 255.23 41.69 81.08 27.26 2 445.61 85.70 142.25 57.23 S3 0 21.67 KPHĐ 10.45 1.12 1 203.05 56.98 82.17 28.67 2 424.28 94.12 142.96 56.78 Figure 5. Peak DP-ASV determines Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ in sample S1 Study of supporting electrolytes of KCl-acetate buffer - NH4Ac for simultaneous determination 101 Figure 6. Peak DP-ASV determines Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ in sample S2 Figure 7. Peak DP-ASV determines Zn 2+ , Cd 2+ , Pb 2+ , Cu 2+ in sample S3 Table 5. Results of determining simultaneously contents of ion Zn, Cd, Pb and Cu in AMD samples Samples C (mg/L) Zn 2+ Cd 2+ (*10 3 ) Pb 2+ Cu 2+ S1 0.469 ± 0.003 2.538 ± 0.00 0.256 ± 0.001 0.113 ± 0.007 S2 0.354 ± 0.004 1.530 ± 0.00 0.211 ± 0.006 0.068 ± 0.001 S3 0.149 ± 0.001 0.00 ± 0.00 0.185 ± 0.001 0.036 ± 0.001 The results indicates that the measurement has high repeatability, sensitivity, and peak bottom narrowly-balanced despite samples containing heavy metal are measured directly on spots with many obstacles to repeatability, sensitivity, and reliability. Resulted with electrolyte background, it could be widely used to analyze directly and simultaneously 4 ions Zn 2+ , Cd