Fate of mercury in sediments of the Bangpakong river estuary and its toxicity as influenced by salinity | |
| Author | Waewtaa Thongra-ar |
| Call Number | AIT Diss. no. EV-01-4 |
| Subject(s) | River sediments--Bang Pa Kong River Mercury |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Technical Science, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Abstract | Information on Hg in aquatic environments in Thailand is still limited, especially the fate of Hg in sediments. Therefore, this study attempts to develop a deep understanding of the fate and behavior of Hg in the sediments collected from the Bangpakong River estuary. The research framework was designed to cover a field survey for sampling of both sediment and water from the river following a series of laboratory experiments on Hg distribution and adsorption-desorption phenomena as well as lab scale toxicity tests of Hg to a larval fish. Sediment cores collected in the dry season (March 1999) were sectioned at every 2 cm intervals up to a depth of 10 cm and 5 cm intervals each for the rest of lower part. All individual sediment subsamples were analyzed for total Hg by cold vapour atomic absorption spectrophotometry. High enrichment of Hg was mainly distributed near the surface of the sediments (0-10 cm) except at the river mouth where a decrease of Hg in the surface sediments was observed comparing to the subsurface layers. The average Hg concentrations in the surface sediments (0-10 cm) for all stations ranged from 0.12 to 0.48 µ,g g-1 dry weight which exceeded the acceptable levels set in some countries. However, in Thailand there is no sediment quality standard developed yet. To assess the degree of Hg pollution in this area, the geoaccumulation index and enrichment factor were employed. Both indicators show Hg was more contaminated in the river downstream than in the upstream. Mercury in the sediments was found to be more associated with clay minerals than with organic matter. To investigate further the source of Hg contamination in this area, a sequential extraction procedure was used to fractionate the geochemical forms of Hg distributed in the sediment profiles. Five different fractions of Hg in sediments by Tessier's method were investigated include: (1) exchangeable; (2) bound to carbonates; (3) bound to Fe and Mn oxides; (4) bound to organic matter and sulfide; and (5) residual. Results indicated that over 90 % of Hg remained in the residual fraction which is within the crystalline lattices of minerals. Therefore, high enrichment of Hg found in the sediments was largely due to lithogenic rather than anthropogenic sources which agreed with the results obtained by direct analysis of bioavailable Hg fractions using the two different chemical extractants: ammonium acetateEDTA pH 4.65 and 0.5 N HCl. Both extractions gave very low bioavailable forms of Hg (<10 % of the total Hg). Seasonal variations of particulate Hg distribution were obviously distinguished between the dry and wet seasons in which the concentrations ranged from 1.25 to 2.20 µ,g g-1 dry weight and 0.54 to 0.69 µ,g g-1 dry weight, respectively. The particulate Hg concentrations increased with increasing salinities. This was probably due to the grain size distribution of particulate matter during the low and high water discharged periods. The relative high particulate Hg concentrations in the dry season may result from the finer particles that predominate during the low water discharge period, whereas the inverse was found in the wet season. Compilation with sequential extraction data, it could be presumed that the incorporation of Hg into the sediments resulted from the deposition of particulate matter containing high levels of Hg originating from weathering of rock minerals including some natural and anthropogenic atmospheric fallout. The particulate Hg concentrations in the dry season were about 2-12 times higher than those found in the surficial sediments (0-2 cm) at the same location. In order to understand Hg adsorption capacity and its remobilization in the sediments, Hg adsorption and desorption experiments were conducted at salinity levels of 0, 5, 15 and 30 %0 using a batch equilibration technique. Freundlich adsorption and desorption isotherms were used and experimental data fitted quite well by a nonlinear equation. Almost all portions (90iii 99 %) of added inorganic Hg (2-10 mg L-1) was sorbed by the sediments. Mercury adsorption isotherms or percentage adsorbed were similar among the four salinities tested. In the absence of salinity, Hg adsorption was highly influenced by pH. Whereas in the presence of salinity, Hg(II)-organic complexes predominated over Hg(II)-Cl complexes, neutralizing effect of chloride concentrations and pH on the adsorption phenomena. Organic matter and clay contents of the sediments acted as adsorbents and were responsible for Hg adsorbed. Increase in Hg desorbed from the sediments occun-ed only at the 30 %0 salinity level, but the amount was very low (<10 % of total adsorbed Hg). The Hg adsorption-desorption characteristics of the sediments studied indicated a very strong in-eversible sorption of added inorganic Hg. Based upon this study, sediment containing substantial amounts of organic matter and clay have the potential to retain Hg making it less bioavailable and mobile thus reducing potential toxicity to aquatic organisms in the environment. In addition, a short-term chronic toxicity test was can-ied out to investigate the toxic effects of Hg on growth and survival of larval seabass, Lates calcarifer as influenced by salinity. The static renewal tests were can-ied out for a 7-d exposure period at four salinity levels: 2, 10, 20 and 30 %0 using natural filtered seawater. Results indicated that salinity had no significant effects on survival and growth of larval seabass. This is possibly because Hg preferentially forms very strong complexes with sulfhydryl groups (-SH) in proteins rather than with chloride. Based on the actual Hg concentrations, the mean NOEC, LOEC and LC5o values for survival effect were 30.81, 52.52 and 46.20 µ,g L-1, respectively, while the mean NOEC, LOEC, IC25 and IC50 values for growth effect were 5.16, 12.56, 8.49 and 19.24 µ,g L-1 respectively. These values are much higher than the concentrations found in natural estuarine and marine waters as well as the recommended interim ASEAN marine water quality criteria for the protection of aquatic life (0.16 µ,g L-1). In conclusion, the Bangpakong River estuary has not been polluted with Hg or only minimally polluted with Hg. The sediments are a good sink for Hg. The natural salinity variation could not influence the Hg adsorption-desorption phenomena in the sediments including the toxic effects of Hg on larval fish. |
| Year | 2001 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Environmental Engineering and Management (EV) |
| Chairperson(s) | Preeda Parkpian, ; |
| Examination Committee(s) | Thanh, Nguyen Cong;Fukushi, Kensuke;Apisit Euimnoh;Ozaki, Hiroaki;Gambrell, Robert P.; |
| Scholarship Donor(s) | Royal Thai Government ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2001 |