Estimation of Mercury in Soil, Water and Plants Spectrophotometrically

Introduction

Mercury is one of the most toxic elements and is associated with many health problems, include, instantaneous neurological damages particularly irritability, paralysis, insanity, blindness, chromosome damage and birth defects [1].

It is found in different forms, as elemental mercury vapor in atmosphere, although, as organic and inorganic form. The main natural sources of mercury are volcanoes, forest fires and the weathering of mercury-bearing rocks [2]. Anyhow, industry produce huge amounts of mercury  [3].

The most important common sources of mercury in this field are, paper, cellulose and plastic industries, paint, pharmaceutical industries and pesticides. [4, 5].

Also, mercury assumes to be the most dangerous of all contaminants which may be consumed in our daily foods like fish, cereals and other food stuffs have resulted in numerous poisoning [6].

The most common methods for measuring total mercury include absorption spectrometry, neutron activation analysis, and cold vapor atomic absorption spectrometry [7, 8] and colorimetric solvent extractive method [9].

The goal of this study was to evaluate the mercury content in soil, water and plant samples at Koya area – Kurdistan region – Iraq, by simple soxhelt extraction, and then, spectrophotometric measurement.

Experimental

Apparatus

A CECIL (CE7200) double beam UV-visible spectrophotometer were used for measurements of absorbance

Reagents and Solutions

• A series of standard solutions (1, 5, 20, 25,100,180, and 200) ppm of Mercury (II) chloride (Romil, 99%) prepared to construct standard graph.
• 1 M HCl (Scharlau, 37%) solution.
• Oxalic acid (Fluka, 99.8%) 4% solution.
• Stannous chloride (Fluka, 95%) 2% solution.

λmax was determined using stock solution and the results show a maximum absorbance at 300 nm.

Procedure

2 gm of tested sample weighed to the soxhlet extraction apparatus, 50 ml of oxalic acid solution, 50 ml stannous chloride solution and 10 ml of HCl solution added. The mixture refluxed for about 2.5h (filter if need), after that the contents diluted to 250 ml distilled water. From the final solution, 5 ml diluted to 25 ml with distilled water.

Absorbance measured at 300 nm against a corresponding reagent blank and mercury content of unknown samples was determined using a concurrently prepared calibration graph.

Discussion and results

Mercury content in soil, agricultural products and water sample is convenient to geologist exploring and metals accumulation inspection [10].

The optimize operation conditions to get an effective extraction and obtain maximum absorbance have been investigated.

Sample digestion

The pretreatment step in quantitative determination of total mercury is an exceptive step, because of some mercury specifications .The accuracy of results depends on the efficiency of extraction and digestion processes [11, 12]. Where, the incomplete of ineffective extraction, loss of mercury vapor and faulty mercury oxidation – reduction reaction may influence in descend the accuracy of process. The optimum period for soxhlet extraction was 2.5h, as shown in figure 1.

Absorption spectra

For absorption measurements, a set of mercury standard solutions prepared,

absorbance was measured at λ_max of 300 nm. And then, calibration graph with   absorbance versus concentration plotted [13]. The concentration of unknown samples was directly obtained from this graph, as shown in figures 1 and 2.

Effect of acidity

Using of hydrochloric acid for digestion of samples was found to be the most suitable acidic media for determining of mercury in all samples under study.

The optimum acidity range for all consecutive measurements 10 ml of 0.1M HCl was used, as in figure 4.

Effect of extraction environment

The oxidizing environment bring into existence by addition of oxalic acid at the time of extraction. Mercury exists as Hg (II) in the acid digestion solution was reduced to its elemental form with SnCl₂. Hence, SnCl₂ is a convenient mercury reductant [14], according to the results, as in figure 5.

The optimized analytical parameters of all experiments are summarized in table 1.

Figure 1: Effect of time on absorbance Click here to View Figure

 

Figure 2: Determination of λmax Click here to View Figure

 

Figure 3: Calibration graph Click here to View Figure

 

Figure 4: Effect of acidity on absorbance Click here to View Figure

 

Figure 5: Effect of extraction conditions Click here to View Figure

 

Table 1: Optimized analytical parameters

Parameter	Studied range	Selected range
λmax (nm)	200 – 600	300
Acidity /HCl (M)	0.01 – 0.6	0.1
Time of extraction ( min.)	30 – 240	150
Temperature (ºC)	80	80
Oxalic acid ( %)	1 – 6	4
Stannous Chloride (%)	1 – 6	2

 

Table 2: Mercury content in some soil samples

No.	Soil Samples	Hg (ppm)
1	Direct waste	150
2	Direct surface (0 m distance)	147
3	Surface Fossa Soil (1-2 m distance)	143
4	South west (1Km distance)	124
5	Ground {Kelaspi} (3Km distance)	78
6	Auto industry	78
7	West (1Km distance)	68
8	East (1Km distance)	28
9	South (1Km distance)	25
10	North (1Km distance)	24
11	Well Soil (3Km distance)	20
12	Farming place at Tobzawa	5
13	Oxidizer	53

 

Table 3: Mercury content in some water samples

NO	Water Samples	Hg (ppm)
1	Fossa`s water	215
2	Lake (1Km distance)	200
3	Tobzawa	25
4	Chicken poultry (2Km East distance)	21
5	Chicken poultry (2Km South distance)	19
6	Kelaspy Well (3Km distance)	3

 

Table 4: Mercury content of some agricultural products

NO	Plant Sample	Hg (ppm)
1	Cucumber	80
2	Pepper	23

 

Acknowledgements

Insincerely thanks for the Erbil polytechnic university –  Koya technical institute, Oil technology department, for providing the necessary infrastructural facilities during my research.

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