Arsenic Is A Well Known Toxic Metal Biology Essay

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Arsenic is a well-known toxic metal and is present mainly as oxyanion compounds in groundwater. The World Health Organizations current provisional guideline for arsenic in drinking water is 10 µg/L, but all developing countries affected with contaminated groundwater are still struggling to keep up with the previous WHO guideline value of 50 µg/L (2).

Arsenic compounds are widespread contaminants in the environment. Because of arsenic toxicity and induced carcinogenity (Eblin et al., 2006; Hughes, 2002), higher arsenic concentration in the environment represents serious problems for human health, especially for populations in Bangladesh, Pakistan, Western Bengal, Vietnam, China, Mexico and Chile. Arsenic occurs in ground water mostly in the form As (III) which is called as arsenite and As (V) which is called as arsenate. Under oxidizing conditions (aerated surface water), the arsenite can be converted into arsenate and under reducing conditions (anaerobic ground water) the arsenate can be converted into arsenite. (Malana, et al., 2011).

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In Pakistan, especially in the province of Punjab and Sindh, the presence of arsenic in drinking water systems and subsurface aquifers is potentially dangerous human health hazards. Various subsurface aquifers and tube wells are polluted with intolerable limits of arsenic. The tolerable limit of arsenic is 10 ppb as recommended by WHO. It is reported by PCRWR and UNICEF that arsenic contamination level lies in the range between 10 to 200 μg/Litre especially in ground water of different regions of Punjab province which poses serious threats to the people of the Punjab.

In Sindh, the arsenic contaminated water is reported in various districts including Sukkur, Khairpur, Jacobabad, Dadu, Thatta, Sanghar, Tando Allahyar, and Jamshoro. The concentration of arsenic in these districts is more than the prescribed value.

Arsenic, fluoride and heavy metals occur as minor constituents of ground water in all categories of hydro-geological settings in India. The high concentration of these minor constituents including iron and nitrate is of concern as large amount of ground water is abstracted by drilling water-wells both in rural and urban areas for drinking and irrigation purposes.

Fluoride is the first element of the halogen family in the periodic table that does not occur in the element state in the environment due to its high reactivity [M. Kanbur, 2009]. The presence of fluoride in drinking water in acceptable concentrations is known as an essential constituent for human health, especially in children below 8 years of age [M. Mohapatra, 2009]. However, when fluoride concentration exceed the acceptable level (1.5 mg L−1), it leads to serious health problems such as skeletal fluorosis, mottling of teeth and lesions of endocrine glands, thyroid, liver and some other organs. In some provinces of Pakistan, the level of fluoride in drinking water is greater than 1.5 mg L−1, which can lead to endemic fluorosis. This problem is visible in several countries, including China, the United States, Tanzania, Mexico, Kenya, Poland and Iran. [M. Mohapatra, 2009].

Several methods, such as reverse osmosis, ion exchange/adsorption, coagulation, precipitation, and electro coagulation have been used for the removal of excess fluoride from drinking water [7-9]. Among these methods, adsorption is the most extensively used and is a promising technique for the removal of fluoride. A large number of materials such as activated alumina, red mud, quartz, and fly ash have been suggested for the adsorption of fluoride from water [2,10-12].

In humans, increasing NO3 − concentrations in drinking water causes two adverse health effects: induction of "blue-baby syndrome" (methemoglobinemia), especially in infants, and the potential formation of carcinogenic nitrosamines [D. Majumdar, and N. Gupta, 2000. C.H. Tate, 1990].

The aim of our study was to evaluate the adsorption capacity

Materials and methods

Study was carried out at center for environmental protection studies, PCSIR (Pakistan council of scientific and industrial research) Labs complex Lahore by preparing samples in the laboratory. Samples solution contains 100 ppm fluoride, 100 ppm nitrate and 50 ppb arsenic. All the solutions were prepared in double distilled water having conductivity less than 1 µS/cm. Four glass columns having one inch internal diameter were used in the experiment. The first and second column contains anion exchange resin (IRA-400) and mixed bed ion exchange resins (i.e cation and anion exchange in the form of sodium and chloride). The resins become exhausted due to continuous adsorption then resins will be regenerated or recharge by specific regenerant (Faustand and Aley, 1983). The third column contains activated alumina (activation was done with 0.1 N HCl solution). The fourth column contains 150 g of activated carbon. The activated carbon was done with 1 M HCl and then with plenty of double distilled water until neutralization. The regeneration of resins after exhaustion was carried out by 10 % aqueous sodium chloride solution where as activated alumina was regenerated by 0.1 N sodium hydroxide.

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Operation conditions

Parameters

Ion exchange resins

Activated carbon

Activated alumina

Effective depth(m)

0.5

0.5

0.5

Effective size(mm)

0.4-0.6

2.0-2.5

2.0-2.5

Bed mass(g)

100

150

150

Flow rate (ml min-1)

10

10

10

pH

6.5± 0.15

7.0± 0.2

7.5± 0.2

Temperature (oC)

23.0 ± 2

22.0 ± 2

22.0 ± 2

Preparation of reagents

Stock solution of arsenic was prepared by dissolving the arsenic salt As2O3 in double distilled water. Arsenic solution used in the experiments was prepared by diluting stock solution to the desired concentration with double distilled water. Before the experiments, the pH value of the arsenic solutions was adjusted by adding 0.1 M HCl or 0.1 M NaOH. The stock fluoride solution was prepared by NaF where as nitrate solution was prepared by KNO3. All chemicals were of analytical grade and all the working solutions were prepared in freshly prepared double distilled water and the pH of the distilled water was around 5.6.

Analysis of water Quality

The changes in water quality before and after treatment were fallowed. The concentration of arsenic was determined on Inductive couple plasma (PerkinElmer optimum-5300). The concentration of fluoride and nitrate were determined by using ion chromatograph Shimadzu C-R4A Chromatopac.

Result and Discussion

Adsorption phenomena are operative in most natural physical, biological, and chemical systems. Adsorption operations employing solids such as activated carbon, metal hydrides and synthetic resins are widely used in industrial applications for purification of waters and wastewaters. Activated carbon is also commonly used as the material in arsenic treatment (Wennrich and Weiss, 2004, Gimbel and Hobby, 2000). Activated alumina and mixed bed resin efficiently remove nitrate from aqueous solution. The removal of nitrate was 98% and mixed bed resin was 97% as shown in table 1. Nitrate removal in case of IRA-400 resin was less (90 %) as compared to other adsorbent shown in figure 1. Activated carbon removal percentage of nitrate was 94 %. Activated carbon is generally considered as a universal adsorbent for the removal of diverse types of aquatic pollutants especially organic pollutants. However, it shows poor adsorption towards anionic pollutants. Only a few studies are available reporting the sorption of nitrate by activated carbon. Afkhami et al. [A. Afkhami, 2007]

Table 2: Summary of technologies for Nitrate removal

Technology

Water loss (%)

Removal efficiency

Optimum conditions

Operator skill

Ion exchange IRA-400 resin

1-3

90

pH 6.5-9.0(decreased efficiency at high pH)

High

Activated aluminia

1-2

98

pH 5.5-8.3(decreased efficiency at high pH)

Low

Activated carbon

1-2

94

Silica must less than 30 %

Medium

Mixed bed resin

1-3

97

pH 6.5-9.0(decreased efficiency at high pH)

High

Ion exchange resins are widely used to remove different contaminants from water especially dissolved solids (Veressinina et al, 2000). Ion exchange is a physical or chemical process in which ions held electrostatically on the surface of a solid phase are exchanged for ions of similar charge in a solution. It is a reversible interchange where there is no permanent change in the structure of the solid. The solid is typically a synthetic anion exchange resin, which is used to remove particular contaminants of concern. Ion exchange is commonly used in drinking water treatment for softening (that is removal of calcium, magnesium etc. in exchange of sodium) as well as removing nitrate, arsenate, selenate etc. from municipal water (Clifford, 1999).

For fluoride removal the ion exchange technologies were not as much efficient as activated alumina shown in table 2. Activated alumina efficiently absorbs the fluoride concentration up-to 99 % as compared to the other adsorbing materials used in the experiment. Activated alumina is porous, granular materials that contain ion exchange properties to removal of pollutant from water bodies (Driheus et al, 1998). The activated alumina show optimum removal at pH (5.5 to 6.5), thus source water source water required to pretreatment win hydrochloric acid.(Kowalski, 1999)

Table 2: Summary of technologies for fluoride removal

Technology

Water loss (%)

Removal efficiency

Optimum conditions

Operator skill

Ion exchange IRA-400 resin

1-3

85

pH 6.5-9.0(decreased efficiency at high pH)

High

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Activated alumina

1-2

99

pH 5.5-8.3(decreased efficiency at high pH)

Low

Activated carbon

1-2

90

Process efficiency not effected by silica, most economical for TDS 3000-5000 mg/L

Medium

Mixed bed resin

1-3

89

pH 6.5-9.0(decreased efficiency at high pH)

High

Table 2: Summary of technologies for arsenic removal

Technology

Water loss (%)

Removal efficiency

Observation and interference

Ion exchange IRA-400 resin

1-3

92

Pilot scale is used in industrialized as well as household system. Adsorption capacity is very high but needs continuously regeneration that increases its cost. The regeneration produces arsenic rich brine.

Activated alumina

1-2

96

In developing country used in household and industrial system. Long term of performance of regenerated media needs documentation.

Activated carbon

1-2

92

Process efficiency not effected by silica, most economical for TDS 3000-5000 mg/L

Mixed bed resin

1-3

94

Very effective for laboratories to obtained high efficiency but need continuously regeneration that increases its cost. Arsenic rich solution obtained in the form of regeneration waste.

Figure 1: Fluoride and nitrate removal by using different adsorbing media

Figure 1: Arsenic removal by using different adsorbing media

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