Four agricultural soils (0-15 cm) from different levels of As contaminated area, which received As contaminated irrigation water for several years were collected. The soils are designated as soil 1, soil 2, soil 3 and soil 4 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 300C. The bulk soils samples were mixed thoroughly by hand stored for analysis. Soils’ physical and chemical properties were determined using standard laboratory procedure. Total As content of the soil 1, soil 2, soil 3 and soil 4 were 20.3, 26.3, 14.1 and 24.3 mg/kg, respectively. Clay content of soil 1, soil 2, soil 3 and soil 4 were 22.2%, 28.9%, 30.5% and 18.5%, respectively. To study the adsorption characteristics of the soils, one equilibration time was used: the adsorption isotherm was calculated after 48 hours of adding sorption solution. For equilibration, 20 ml solution of 0-250 µg As/ml as Na2HAsO4.7H2O was added to 1 g of soils in 50 ml centrifuge tubes at room temperature (20 0C). The solution was shaken for 48 hours by an end to end shaker. After equilibration the suspension was centrifuged at 10,000 rpm for 10 min and the supernatant solution was filtered through Whatman No. 42 filter paper. The filtrates were analyzed for As using a FI-HG-AAS using matix-matched standards. Arsenic that had been lost from solution was considered to be sorbed. 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 = aCb ………………………(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.