**3. Results and discussion**

### **3.1. Selection of detection technique**

To select the technique to quantify mercury was realized calibration curves with both mercury compounds by ASV, DSPV and SWSV. Figure 5 shows the comparison of mercury detection using different electrochemical techniques. Mercury chloride was not showed but resembled same behavior. Table 3 shows comparison of electrochemical techniques of both mercury compounds, where it shows that any technique can be used to quantify mercury by its low detection and quantification limits, but the use of ASV shows the best fit with the lowest DL and QL. In consequence, ASV was selected to quantify mercury in solution, which was removed from polluted bentonite and quartz.


**Table 3.** Parameters comparison of stripping voltammetry to quantify mercury.R2 is the correlation coefficient, m represents the slope of linear regression; DL means Detection Limits, QL represents Quantification Limits.

**Figure 4.** Scheme of a 3-electrode cell used in voltammetry techniques, where gas inlet is used for bubbling electrolyte solution with an inert gas and with controlled temperature.

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**8** 10 ASTM-D1193-99.Chitosan ASTM-D1193-99. Chitosan

**8** 22 mV/s. mV s-1.

**8** 29 (25±1ºC) (25 ± 1 C)

**8** 35 Figure 6 Figure 5

**9** 12 (Figure 7). (Figure 6).

**9** Table 3 **mM-1** mM-1

**Figure 5. Figure 5.** ASV-DSPV-SWSV comparison to select technique to quantify mercury removed. ASV-DSPV-SWSV comparison to select technique to quantify mercury removed.

#### **3.2. Electrochemical responses of removing and complexing agents**

After obtaining encouraging calibration results for mercury detection using ASV with sequential addition on removing agents proposed. Electrochemical answer of removing and complexing agents in the presence of mercury compounds was obtained (Figure 6).

3

Table 4 shows corresponding equations of different removing agents, including fitting (R2 ), sensibility (obtained from the slope m), detection and quantification limits (DL and QL respectively) for both mercury compounds. DL and QL represent fundamental performance characteristics of measurement processes, where DL or Limit of Detection (LOD) is defined as an indicator of the minimum detectable analyte net signal, amount or concentration. His term is widely understood and quoted by most chemists as a measure of the inherent detection capability. In general, the LOD is taken as the lowest concentration of an analyte in a sample that can be detected, but not necessarily quantified, under the stated conditions of the test. In another hand, QL or Limit of Quantification (LOQ) is the lowest concentration of an analyte in a sample that can be determined with acceptable precision and accuracy under the stated conditions of test. The general equation to determine detection and quantification limit is:

$$\frac{LOD}{LOQ} = \frac{F\left(SD\right)}{b}$$

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technique, calibration curves were created for all different removing agents for the addition

Experimental conditions for ASV were as follows: pre-concentration potential –0.6 V vs. Ag/ AgCl, deposition time 6 min, quiet time 30 s, scan rate 20 mV s-1. An increase in signal due to increasing mercury was monitored and recorded along with the increment in current associ‐ ated with the concentration addition. For SWSV were used an initial potential of -0.2 mV, a deposition potential of -0.6 V for a deposition time of 10 s; a quiet time of 5 s, a SW frequency of 50 Hz, a potential step of 0.005 V. For DSPV were used an initial potential of -0.2 mV, a deposition potential of -0.6 V for a deposition time of 10 s; a quiet time of 5 s, a potential step of 4 mV, a pulse width of 50 ms, a pulse period of 200 ms, pulse amplitude of 50 mV. All experiments were carried out at room temperature (25±1°C) (Anastasiadou et al, 2010). Calibration curves for mercuric quantification were done using electrochemical techniques to

To select the technique to quantify mercury was realized calibration curves with both mercury compounds by ASV, DSPV and SWSV. Figure 5 shows the comparison of mercury detection using different electrochemical techniques. Mercury chloride was not showed but resembled same behavior. Table 3 shows comparison of electrochemical techniques of both mercury compounds, where it shows that any technique can be used to quantify mercury by its low detection and quantification limits, but the use of ASV shows the best fit with the lowest DL and QL. In consequence, ASV was selected to quantify mercury in solution, which was

**(µA mM-1)**

ASV 2549.30x - 0.0355 0.993 2549.30 112.043 0.373 DPSV 299.43x - 0.5035 0.978 299.43 422.226 1.407 SWSV 467.50x - 1.3763 0.981 467.50 479.046 1.597

ASV 6793.30x - 5.4037 0.998 6793.30 42.046 14.015 DPSV 1313.70x - 1.0952 0.968 1313.70 96.237 32.079 SWSV 2347.90x - 1.6533 0.971 2347.90 95.385 31.795

**Table 3.** Parameters comparison of stripping voltammetry to quantify mercury.R2 is the correlation coefficient, m represents the slope of linear regression; DL means Detection Limits, QL represents Quantification Limits.

**DL (pM)**

**QL (nM)**

of both HgCl2 and HgO.

386 Environmental Risk Assessment of Soil Contamination

select the best.

**Mercury**

HgCl2

HgO

**3. Results and discussion**

**3.1. Selection of detection technique**

removed from polluted bentonite and quartz.

**Compound Technique Linear Equation R2 <sup>m</sup>**

Where F is a factor of 33 and 10 for LOD and LOQ, respectively. SD represent the standard deviation of the ordinate intercept, or residual standard deviation of the linear regression; and b the slope of the regression line. For a linear calibration curve, it is assumed that the instrument response y is linearly related to the standard concentration x for a limited range of concentra‐ **10** 4 Figure 8 Figure 7

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**Figure 6.** Electrochemical behavior of mercury compounds to removing agents: (A) HCl, (B) KCl, (C) KI, (D) KOH, (E) H2O, (F) EDTA, (G) chitosan, (H) HPCD and (I) EDTA + Cys + NaCl. **Figure 6.** Electrochemical behavior of mercury compounds to removing agents: (A) HCl, (B)KCl, (C)KI, (D) KOH, (E) H2O, (F) EDTA, (G) chitosan, (H) HPCD and (I) EDTA+Cys+NaCl.White the ASV showed before, calibration curves of each of one mercury compounds added to removing agents was obtained as Figure 7 shows with the ASV response for chitosan.

**Figure 7.** ASV detection of HgO addition to chitosan in 0.1 M HCl using vitreous carbon, platinum wire and Ag|AgCl as work, counter and reference electrode with a scan speed of 70 mV s-1 (A), and linear fit of HgO addition to chitosan (B). **Figure 7.** ASV detection of HgO addition to chitosan in 0.1 M HCl using vitreous carbon, platinum wire and Ag|AgCl as work, counter and reference electrode with a scan speed of 70 mV s-1 (A), and linear fit of HgO addition to chitosan (B).

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**Figure 8.** Removal percentages of mercury (HgCl2 and HgO) in bentonite using different removing agents with 10 mg L-1 (A) and 25 mg L-1 (B) mercury concentration.

**A B** 

**11** 32 Figures 9 and 10. Figures 8 and 9.

**11** 33 (Figure 11), (Figure 10), **11** 44 (Figure 11B). (Figure 10B). **11** 45 (Figure 11A). (Figure 10A).

**12** 7 (Figure 9, (Figure 8,

**13** Figure 8 Could you please change this figure by the annexed figure please?

4

5

Electrochemical Detection of Mercury Removal from Polluted Bentonite and Quartz using Different Removing Agents http://dx.doi.org/10.5772/57446 389


**Table 4.** Calibration curves corresponding to each removing agents.

tion. This model is used to compute the sensitivity b and the LOD and LOQ. Therefore, the LOD and LOQ can be expressed as

$$LOD = \frac{3S\_x}{b}; \; LOD = \frac{10S\_x}{b}$$

4

5

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**10** Figure 6 Could you please change this figure by the annexed figure increasing its size 50 % please?


A B C

D E F

G H I


Potential (V)

A


Potential (V)


Potential (V)




Current (uA)

Current (uA)

 KI KI + HgO KI + HgCl2

 EDTA EDTA + HgO EDTA + HgCl2

 (EDTA+Cys+NaCl) (EDTA+Cys+NaCl) + HgO (EDTA+Cys+NaCl) + HgCl2

Current (uA)

Potential (V)


Potenial (V)


Potential (V)

**Figure 6.** Electrochemical behavior of mercury compounds to removing agents: (A) HCl, (B) KCl, (C) KI, (D) KOH, (E) H2O, (F) EDTA, (G) chitosan, (H) HPCD and (I) EDTA + Cys + NaCl.

**Figure 6.** Electrochemical behavior of mercury compounds to removing agents: (A) HCl, (B)KCl, (C)KI, (D) KOH, (E) H2O, (F) EDTA, (G) chitosan, (H) HPCD and (I) EDTA+Cys+NaCl.White the ASV showed before, calibration curves of each of one mercury compounds added to removing agents was obtained as Figure 7 shows with the ASV response for chitosan.

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**A B**

**Figure 7.** ASV detection of HgO addition to chitosan in 0.1 M HCl using vitreous carbon, platinum wire and Ag|AgCl as work, counter and reference electrode with a scan speed of 70 mV s-1 (A), and linear fit of HgO addition to chitosan (B).

**Figure 7.** ASV detection of HgO addition to chitosan in 0.1 M HCl using vitreous carbon, platinum wire and Ag|AgCl as work, counter and reference electrode with a scan speed of 70 mV s-1 (A), and linear fit of HgO addition to chitosan (B).

0.0 0.1 0.2 0.3 0.4 0.5

Potential (V)

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**Figure 8.** Removal percentages of mercury (HgCl2 and HgO) in bentonite using different removing agents with 10 mg L-1 (A) and 25 mg L-1 (B) mercury concentration.

**A B** 

**11** 32 Figures 9 and 10. Figures 8 and 9.

**11** 33 (Figure 11), (Figure 10), **11** 44 (Figure 11B). (Figure 10B). **11** 45 (Figure 11A). (Figure 10A).

**12** 7 (Figure 9, (Figure 8,

**13** Figure 8 Could you please change this figure by the annexed figure please?




**10** Figure 7 Could you please change this figure by the annexed figure please?

Current (uA)

Current (uA)

 KCl KCl + HgO KCl + HgCl2

 H2O H2O + HgO H2O + HgCl2

 HPCD HPCD + HgO HPCD + HgCl2

Current (uA)

**10** 4 Figure 8 Figure 7


388 Environmental Risk Assessment of Soil Contamination

Potential (V)


Potential (V)


Potential (V)

0.00000 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0.00007 0.00008 0.00009 0.00010 0.00011 0.00012

Current (A)




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Current (uA)

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Current (uA)

 HCl HCl + HgO HCl + HgCl2

 KOH KOH + HgO KOH + HgCl2

 Chitosan Chitosan + HgO Chitosan + HgCl2

Current (uA)

Where Sa is the standard deviation of the response and b is the slope of the calibration curve. The standard deviation of the response can be estimated by the standard deviation of either y-residuals, or y-intercepts, of regression lines. This method can be applied in all cases, and it is most applicable when the analysis method does not involve background noise. It uses a range of low values close to zero for calibration curve, and with a more homogeneous distribution will result in a more relevant assessment (Currie 1995, 1999; Guidance 2000).

As can be seen in Table 4, we obtained a good QL and DL of mercury compounds in the presence of removing and complexing agents. So ASV is a good technique to determine mercury concentration in presence of removing agent too. In this way, removal efficiencies of mercury were analyzed in the next section.

#### **3.3. Mercury removal efficiencies**

Removal percentages were calculated in base of fitting equations for all the different removing agents, they are show in Figures 8 and 9. Due to structural differences in bentonite and quartz (Figure 10), mercury removal behaved differently for the two.

**Figure 8.** Removal percentages of mercury (HgCl2 and HgO) in bentonite using different removing agents with 10 mg L-1 (A) and 25 mg L-1 (B) mercury concentration.

**Figure 9.** Removal percentages of mercury (HgCl2 and HgO) in quartz using different removing agents with 10 mg L-1 (A) and 25 mg L-1 (B) mercury concentration.

Electrochemical Detection of Mercury Removal from Polluted Bentonite and Quartz using Different Removing Agents http://dx.doi.org/10.5772/57446 391

**3.3. Mercury removal efficiencies**

390 Environmental Risk Assessment of Soil Contamination

L-1 (A) and 25 mg L-1 (B) mercury concentration.

(A) and 25 mg L-1 (B) mercury concentration.

(Figure 10), mercury removal behaved differently for the two.

Removal percentages were calculated in base of fitting equations for all the different removing agents, they are show in Figures 8 and 9. Due to structural differences in bentonite and quartz

**Figure 8.** Removal percentages of mercury (HgCl2 and HgO) in bentonite using different removing agents with 10 mg

**Figure 9.** Removal percentages of mercury (HgCl2 and HgO) in quartz using different removing agents with 10 mg L-1

**Figure 10.** Representation of the structure of bentonite (A) and quartz (B) (Bruker AXS Advanced X-Ray Solutions Soft‐ ware).

Quartz is ordered as strong matrix of oxygen-silicon bonds, which prevent metal ions from penetrating its structure (Figure 10B). On the other hand, metal ions can easily slip into bentonite, which is formed of stacked layers, due to the gaps between the layers (Figure 10A). Because of the strong organized bonds in quartz, mercury is more easily removed from quartz than bentonite. Moreover, the particle size of bentonite is lower than quartz, and therefore has a bigger surface area and as a synthetic colloid, in addition, it has negative charges which attract positive ions. Once bentonite and quartz samples were contaminated with HgO or HgCl2, removing agents were added. After 24 h the sample was collected and analyzed by ASV.

In order of the results obtained, bentonite adsorbed chitosan easily, swelling up dramatically which made it difficult to remove the supernatant. Therefore, the removal of mercury from bentonite using chitosan was inefficient (Figure 8, less than 5 %). This effect was consequence of the affinity of chitosan to the layers of bentonite and it was retained inside of this clay. In contrast, when the chitosan was used to remove Hg2+ from quartz (Figure 9), this got a removal close to 30 % when it was 10 mg L-1, and the double of removal was when quartz had 25 mg L-1 mercury; this increase was proportional to the concentration of mercury ions over quartz, where chitosan took off the pollutant from particle surface to the solution. Anyway, all the different extracting agents used to remove mercury remove metals by electrostatic forces, forming ionic bonds. Thus, magnitude depends on ionic charge.

On the other hand, complexing agents act differently with metallic ions. Metals tend to lose electrons during chemical reactions, creating metallic ions. The positive charge of these cations attracts negative ions to form complexes held together by covalent bonds. Donat‐ ing species (ligands) needs to have a lone pair of electrons which can be donated to form a bond. Water, ammonia and halides are common inorganic ligands (Buffle, 1990; Ma‐ lone, 1999; Montuenga, 1979).

Stronger compounds as EDTA and chitosan tend to remove more mercury contamination present in the bentonite and quartz samples, than 0.1 M KI, 0.1 M KCl, 0.1 M KOH, 0.1 M HCl, 0.1 M EDTA, 10 % HPCD, 0.01 M chitosan and a mixture of 275 mg L-1 EDTA, 1.15 % cysteine and 0.5 % NaCl by the different arrangement of mercury ions in these chemical solutions, with less than 10 % removal from bentonite (Figure 8), and less than 20 % from quartz (Figure 9) in both concentrations of Hg2+: 10 and 25 mg L-1.

With ASV has been possible study the thermodynamic and kinetic of adsorption of Hg (II) on Ca-Bentonite, detecting the pollutant in solution to confirm that Ca-Bentonite has a good adsorption capacity of Hg2+ adjusting the results to Freundlich isotherm as a mathematical model, with a pseudo second order of reaction. Using the standard potential (E°) obtained after the ASV, we obtained different thermodynamic parameters as equilibrium constant (Keq), free standard energy (ΔG°) and entropy (ΔS°) of Hg2+ – Ca-Bentonite. These results indicate us that the process of adsorption is spontaneous, endothermic and irreversible by a possible inclusion and interchange of Hg2+ with Ca2+ between the Bentonite slides.

In addition, we have used the ASV to evaluate the electrorremediation of mercury polluted soil using complexing agents like EDTA removing up to 75 % of metal contaminants in mercury polluted soil samples by wetting them with 0.1M EDTA, placing them in an experimental cell equipped with Ti electrodes, and then applying a 5 V electric field for 6 hours in a batch reactor; Hg2+ was removed around 87 % in a time of 9 hours close to the anode side by the presence this complexing agent (Robles et al, 2012).
