| Abstract | Research was undertaken to assess the potential of Thai native plant for arsenic removal form soil. Screening for the new arsenic accumulation plant species was set as the first objective of the experiments. Pityrogramma calomelanos (silverback fern) showed the highest arsenic content in their leave, about 200 times higher than other fern species collected from the Ron Phibun district arsenic-contaminated sites in southern part of Thailand. Arsenic accumulation capacity of silverback fern was investigated as the second objective of this research. Arsenic content in full grown (20-month old) silverback fern leave reached the maximum accumulation of 4616 mg As/kg dried weight at 61h week after exposure to 10 mg/L arsenic contaminated nutrient media in hydroponic culture. After 6th week of exposure, the arsenic content in leave decreased to 3300 mg As/kg and become stable until lih week. Silverback fern accumulated more than 90% of arsenic in its above ground biomass while growing very well throughout the period of experiment as it continued increasing the average plant weight linearly. Analyses were conducted in 5 individual replicates and the mean of values was considered as the arsenic content level. Nutrient film technique was selected in this study as a growing unit for the hydroponic culture due to its simple operation and providing sufficient oxygen supply to the plant roots. EDTA (Ethylenediamine tetraacetic acid) and DMSA (Dimercaptosuccinic acid) organic chelating agents were selected to investigate the effect of chelating agent on arsenic phytoextraction or the accumulation of arsenic in the leave. Both chelating agents are organic compounds that may stabilize several metal ions in aqueous solution and were used as metal antidote in human and animal for over decades. EDTA generally forms a complex with +2 or +3 metal ions and cannot form any complex with metalloid such as arsenic or antimony, whereas DMSA was reported to form a strong complex with several metal ions including metalloid such as arsenic. This part of study is aimed to compare the effect of arsenic-DMSA complex and effect of EDTA on phytoextraction of arsenic by silverback fern in hydroponic culture, which is the third objective of this research. Addition of DMSA to growing media decreased arsenic content in plant down to 5-fold, while EDTA increased the arsenic content in plant up to 2-fold at 6th week after exposure to arsenic compared to control. It indicated that silverback fern tend to uptake arsenic in the form of oxyanion, rather than in the form of As-DMSA complex. The addition of EDTA increased the mobility of other 2+ or 3+ metal ions species, therefore an increase in arsenic accumulation of silverback fern may result from an interaction between several species of ion to plant rather than arsenic ion alone. Arsenic accumulation characteristic of the EDTA added plant was the same as control, which the maximum accumulation of 8290 mg As/kg dried weight was observed at 6th week after exposure. After that the accumulation dropped back to 5250 mg As/kg and become stable until 1 ih week after exposure. On the other hand, arsenic accumulation characteristic of DMSA added plant was completely different, as the arsenic content in leave increased continuously until lih week which was the end of the experiment. Arsenic content in DMSA added plant at 6th week and lih week were 752 mg As/kg and 1702 mg As/kg, respectively.Both chelating agents added plant were growing well in the hydroponic culture and accumulated arsenic in its leave more than 90% of overall arsenic accumulation. In the last objective of this study, phytoextraction of arsenic from contaminated soil was simulated for the comparison to hydroponic culture experiments. Hydroponically grown plant is generally exposed to emich nutrient environment, therefore plant will not develop the mechanism of nutrient uptake as found in plant growing in the soil. Arseniccontaminated soil collected from Ron Phibun in southern Thailand was obtained and characterized as acid soil, low organic matter and high minerals. The soil texture was sandy clay loam contained 57% of sand. Sequential extraction of arsenic-contaminated soil indicated that 0.1 % of arsenic is presented in exchangeable form, 1 % of arsenic bound with carbonate, 42% of arsenic bound to oxides of iron and manganese, 16% of arsenic bound to organic matter, and 41 % of the arsenic presented in the residue. The measurement of total arsenic in soil by atomic absorption spectrophotometer gave the concentration of 3506 mg As/kg dried weight, which can consider as the extremely high arsenic-contaminated soil. Although only small fraction of soil arsenic was in the plant available form (0 .1 % of total arsenic content). But due mainly to low pH together with high amount of arsenic associated with oxides of iron and manganese (42%), this acidic condition is in favor of remobilization of arsenic from the oxide bound. Therefore, it might induce toxicity to the environment through high soluble arsenic species. After transplanted silverback fem to the soil, plants were able to grow moderately in the highly arsenic-contaminated soil, and showed the potential to extract arsenic ranging from 4895, 6335, and 8462 mg As/kg after 6, 12, 18 weeks respectively. The increase in arsenic level in plant was proportional to duration of phytoextraction, and can be explained by the continuous increase in arsenic level from exchangeable fraction of soil. When compared to sand media, silverback fem grown in 300 mg As/kg sand media showed the arsenic content in leave of 4762, 2418, and 2092 mg As/kg after 6, 12, 18 weeks, respectively. The arsenic content in plant leave reduced continuously through the period of experiment due to the low arsenic retention capacity of sand. Hydroponic experiment indicated that EDTA had a potential to enhance arsenic accumulation in silverback fem whereas DMSA showed the reverse effect. EDTA was selected to add in the soil in order to recheck the potential of enhancing arsenic accumulation in the plant. After applied EDTA to the arsenic-contaminated soil, arsenic extraction efficiency by plant was unexpectedly lowered (stable at around 4300 mg As/kg) than the no-EDTA added soil. The distribution of arsenic changed during phytoextraction of EDTA added soil was almost similar to that of no-EDTA added soil except the arsenic content in the exchangeable fraction of soil when EDTA was added was 43.6 mg As/kg, lowered than 70.3 mg As/kg in no-EDTA added soil. These findings lead to conclude that the application of EDTA to soil might inhibit some of the arsenic mobilization processes of soil or microorganisms. |