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Influence of Selected Soil Properties on Arsenic Adsorption in Some Soils of Bangladesh

M. K. Uddin
Scientific Officer
Soil Science Division, BRRI Gazipur-1701

A. Islam
Scientific Officer
Soil Science Division, BRRI Gazipur-1701

M. A. Aziz
Scientific Officer
Soil Science Division, BRRI Gazipur-1701

M. R. Islam
Scientific Officer
Soil Science Division, BRRI Gazipur-1701

M. A. Saleque
Senior Scientific officer
Soil Science Division, BRRI Gazipur-1701

Abstract/ Executive Summary:

A laboratory experiment was conducted with ten young alluvial soil samples (0-15 cm) collected from five districts of Bangladesh such as Narayanganj (S₁, S₂), Satkhira (S₃, S₄), Jessore (S₅, S₆), Gazipur (S₇, S₈) and Moulovibazar (S₉, S₁₀) under different levels of As contaminated areas to determine the maximum adsorption capacity, energy of adsorption and buffering capacity of As. Arsenic was determined by a Perkin-Elmer Aanalyst100 AAS equipped with a FIAS-100 flow injection hydride generation system. The test soils showed a large capacity of As adsorption. Application of As linearly increased the equilibrium solution As and As adsorption by the soils. Relationship between the solution As concentration and adsorbed As was also linear. Conventional adsorption equations- Langmuir, Freundlich and Temkin were found to fit well to describe the As sorption characteristics of soils. The highest and the lowest adsorption maxima were 1339 µg g^-1 and 423 µg g^-1 in S₄ and S₃ soil, respectively. The highest and the lowest As buffering capacity of 122.8 and 48.2 were in S₄ and S₃ soil, respectively. Arsenic buffering capacity was positively correlated with the maximum adsorption capacity of the soils (r = 0.73). There was an apparent negative relationship between energy of adsorption and maximum adsorption capacity (r = -0.36). Among the As adsorption parameters, maximum adsorption capacity and As buffering capacity were highly correlated with only clay content of the soils (r = 0.84 and 0.83 respectively).

Keywords: Soil properties, Arsenic, Adsorption

Location: Soil Science Lab, BRRI, Gazipur

Start Date: 01-11-1998

End Date: 30-06-1999

Program Area: Crop-Soil-Water Management

Commodity: Contamination of soil

Objectives:

To determine the maximum adsorption capacity of arsenic (As), energy of adsorption and As buffering capacity of ten soils of different properties.

Methodology:

Ten soils samples (0-15 cm) of five districts of Bangladesh collected from different levels of As contaminated area. The soils are designated as S₁ through S₁₀ for convenience. Soil samples were air-dried, ground and passed through 2 mm sieve and stones, roots and plant debris were removed before the soils were air dried at 30 ⁰C. The bulk soils samples were mixed thoroughly by hand stored for analysis. Soils’ physical and chemical properties were determined using standard laboratory procedure. Test soils varied in total As concentration (1.0 to 29.2 µg g^-1), pH (4.5 to 7.9), organic carbon (0.45 to 1.86%), clay (10-64%) and dithionate extractable Fe (.48 to 2.40%) and Mn (0.71 to 4.65%). One gram of soil was shaken with 20 ml of 0.01M CaCl₂ solution containing 0, 0.25, 0.50, 1, 5, 10, 25, 50, 100 and 200 mg L^-1 As (as Na₂HAsO₄.7H₂O) for 24 hours in 50 ml glass tubes. After equilibration the suspension was filtered and the As from the filtrates were determined by a hydride generation atomic absorption spectrophotometer fitted with FIAS 100. The amount of As adsorbed was calculated as

Adsorbed As = (Applied As – As in solution) + As in control soil. The As adsorption data were interpreted from Langmuir, Freundlich and Temkin equations as described below:

Langmuir equation:

C/(x/m)= 1/kb + C/b ……………. (1)

Where, C is the equilibrium As concentration (mg/L), x/m is the mg As sorbed per kg soil, b is the adsorption maxima (the maximum adsorption capacity) and k is a constant related to the bonding energy of adsorption. A plot of C (x/m) versus C gives a straight line. The constant b and k were obtained from the slope and intercept.

Freundlich equation:

x/m = _aC^b ………………………(2)

By rearranging,

Log(x/m) = log a + b log C ……(3)

Where, x/m is mg As adsorbed per kg soil, C is the equilibrium As concentration (mg/L) and a and b are constants. A linear plot of Log x/m versus Log C leads to a and b from the intercepts and slope respectively.

Temkin equation:

x/m = a + blnC…………………..(4)

Where, x/m is the amount of As adsorbed on unit mass of adsorbent (mg/kg), C is the equilibrium As concentration (mg/L) and a and b are constants. A plot of x/m against lnC gives a straight line. The constant a and b are obtained from intercept and slope. The b of the equation (4) is considered as the As buffering capacity.

Source of Information: Bangladesh J. Prog. Sci. & Tech.,2(2):169-174. 2004

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Test soils showed a large capacity of arsenic adsorption. Application of As linearly increased the equilibrium solution As concentration as well as As adsorption. Solution As concentration is usually smaller compared to adsorbed As in any soil, but there is a close relationship between these two As forms. The relationships between equilibrium solution As and adsorbed As were tested to fit the Langmuir, Freundlich and Temkin adsorption isotherm to calculate adsorption maxima (b), energy of adsorption (k) and As buffering capacities of studied soils. All three adsorption isotherms were found to fit well in all soils. The highest As adsorption maxima (b) was observed in soil S₄ (1339 µg/g), and the lowest (423 µg/g) was found in soil S₃. The difference in b was attributed partly to the clay content of soil (r= 0.84). The organic carbon, pH, and dithionate extractable Fe and Mn showed a little influence on the As adsorption. The highest energy of adsorption (k) (0.34 ml/µg) was found in soil S₃ which showed the lowest b, but the lowest k (0.11 ml/µg) was obtained with the soil S₉ having the lowest clay. The As buffering capacity (BC)of the studied soils varied from 48.2 to 122.8 µg/ml. The BC was positively correlated with adsorption maxima (b). Another negative relationship between As adsorption maxima and energy of adsorption was observed.

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