**2. Materials and methods**

Field studies were carried out at Nilkanda Union under P. S. Sonargaon, District Narayanganj, lying between 23°30′ and 23°46′ N and 90°31′ and 90°41′ E (**Figure 1**) [29]. A good number of submerged and exposed soil (rice, wheat, and vegetable fields) and water (STW, HTW, and pond) samples were collected in four replications at a gap of 0.5–1.0 km in the winter and spring seasons and analyzed to determine the amount of As absorbed by different crops and its presence in soils and water sources. Water samples were collected from DTW and STW one from each at village Kachua, under P.S. Kachua, district Chandpur, about 100 km southeast of Sonargaon.

### **2.1 Selection of plants and cultivation for bioremediation of arsenic**

Six strains under three species of *Azolla* (*A. caroliniana* Dh 103; *A. filiculoides* Dh 104; *A. pinnata* var. *pinnata* Dh 111; *A. pinnata* var. *pinnata* Dh 112; *A. pinnata* var. pinnata Dh 113; and *A. filiculoides* Dh 115) and two species of *Spirodela* (*S. polyrhiza* Dh 116 and *S. punctata* Dh 117, Lemnaceae) were considered to select efficient As absorbing floating plants [9] by growing them in the growth room under controlled conditions: continuous light flux of 120 μE/m2 /s from daylight fluorescent tubes and 70–90% relative humidity at 30 ± 1°C. Liquid inorganic nutrient medium Chu 10D-N [30], a modified version of [31], with double the strength of P (3.56 mg/l) and K (9.45 mg/l) for *Azolla* strains, and IRRI medium with NH4NO3 as nitrogen source following [32] for species of *Spirodela* were grown in conical flasks of 150 ml capacity containing 50 ml medium adding arsenic trioxide as inorganic As (iAs) source before autoclaving. *S. polyrhiza* and *S. punctata* were grown at much higher arsenic concentrations as the plant is often found to grow in polluted waters and stressed environment [33]. The plants were grown at pH 6.00 [34, 35]. All plant biomasses after 3 days growth were oven dried at 90°C for 24 hour for determining growth and As absorbed [9].
