Geochemical Process of Arsenic-tained Site by Geothermal and Its Impact on Paddy Soils and Rice – A Case Study of Guandu Plain, Taiwan
Date Issued
2014
Date
2014
Author(s)
Lin, Sheng-Chi
Abstract
A study conducted in 2006 showed that over 100 hectares of rice paddies in Beitou, Taiwan, irrigated over the hundred years using water mixed with hot spring, was heavily polluted by arsenic. The objective of the present study was carried out to examine the intensively circulation of arsenic in the aqua system from geothermal (Thermal Valley) to paddy fields (Guandu Plain) and estimate the influence of arsenic-tained paddy soils on rice via analyzing arsenic-adsorbed forms in soil and arsenic species in soil solution. Meanwhile, the comparative analysis of arsenic fractionations by using sequential extraction procedure (SEP) between paddy soils in Guandu Plain and sediments in Huang Guang Creek (HGC) was executed in order to understand the existing arsenic behaviors and the potential hazards of arsenic in the surroundings. According to the analyzed datum of ion and anion in the Thermal Valley, supersaturated solution of lead (Pb2+) and arsenopyrite (H2AsO4-) leads to the process of beudantite precipitation via sulfide oxdation model. Furthermore, the XRD analysis of Guandu paddy soils indicated the beudantite particles are present in clay fractions. Soil samples and rice tissues were collected and analyzed separately, using Atomic Absorption Spectrophotometer (AAS) with automatic hydride generator (HFS-3); meanwhile, the SEPs of As-tainded paddy soils and sediments were also carried out and the arsenic speciation of soil solution in the rhizosphere was determined by the Inductively-Coupled Plasma Mass Spectrometer (ICP-MS) with high-pressure liquid chromatography system (HPLC). Arsenic contents of rice tissues depend on irrigation methods, quantity and speciation of arsenic in soils etc., however, the transformation mechanisms of arsenic from soils to rice don’t have the final conclusion until now. The studied results were showed that the mean arsenic contents in grains, straws and root were separately measured at 0.13 mg kg-1, 3.49 mg kg-1 and 98.7 mg kg-1, also indicated that rice tissues of arsenic has the decreasing trend following root-straw-grain. At the same time, the mean total arsenic concentration were estimated at 135 mg kg-1 (from 67 mg kg-1 to 438 mg kg-1) in paddy soils and 30.1 μg L-1 (from 1.5 μg L-1 to100.5 μg L-1) in paddy soil solution, respectively. The results of paddy soil solution indicated that the arsenite (AsIII) predominates (about 89%) under anaerobic conditions. Regression of soil arsenic levels with rice grains and straws were less significant compared to that with rice roots and so did soil solution arsenic content, including AsIII and AsV. Arsenic level in root strongly depended on arsenic concentration of soil suggesting that the high arsenic concentration may have the potential for translocation from root to grain which ultimately effects on the human health. The sequential extraction experiments resulted in the high portions of As remaining in the amorphous and poorly-crystalline hydrous oxides of Fe and Al (33-53 %), and residual (32-53 %) for paddy soils; mostly in the the amorphous and poorly-crystalline hydrous oxides of Fe and Al (32-38 %), well-crystalline hydrous oxides of Fe and Al (12-37 %), and specifically sorbed (18-29 %) for sediments. Comparing the distributions of arsenic fractionation between paddy soils and sediments, it was apparent that the percentages of amorphous or poorly crystalline Al and Fe hydrous oxides extracted and residual from paddy soils were greater than those removed from sediments, but the mobile of arsenic from the sediments in HGC was greater than from paddy soils in Guandu Plain, no matter what investigating total arsenic content or analyzing arsenic fractionation using SEP. The study suggests that more resources should be founded for the circulation of arsenic in Guandu Plain and its impact in the surroundings to make sure the safety and health of the local residents due to the going process of geochemical procedures that a large amount of arsenic is Continuously separated to surroundings from Thermal Valley in Beitou until now.
Subjects
砷物種
砷鉛鐵礬
水稻田
地熱溫泉
逐級萃取
SDGs
Type
thesis
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