Exposure assessment and remediation options of mercury for small-scale gold mining : a case study in Phichit Province, Thailand | |
| Author | Soontree Umbangtalad |
| Call Number | AIT Diss. no.EV-08-4 |
| Subject(s) | Mercury--Toxicology--Thailand--Pichit Gold mines and mining--Thailand--Pichit |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering and Management |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. EV-08-4 |
| Abstract | The research was designed to study the transformation and distribution of mercury released from mining activities, to evaluate the potential health impacts in exposed groups (gold miners and schoolchildren) and to remediate mercury contamination from mining activities at Phanom Pha, a small scale gold mine in Phichit Province, in Thailand. Sediment, air and surface soil samples were collected from the area at the mining site and adjacent areas. The average concentration of Hg near the area of amalgamation process was in the range of 0.25-0.32 mg/kg dry weight (surface and subsurface layers) and did not exceed the acceptable level set by many countries. Mercury deposition from the air and concentration in surface soil were 128 ug/m²/day and 20.96 mg/kg dry weight, respectively. Average Hg concentration in mine tailing was as high as 2.72 mg/kg dry weight in comparison with the background concentration (0.1 mg/kg). Both surface soils and mine tailing are local 'hot spots'. Major form of mercury in the sediment along the main track flow regime was strongly bound residual fraction which is in the range of 5070%, and would not readily released back into the solution under natural disturbance. Exposure assessment was conducted in 2 groups of miners by means of personal and biological sampling. Two miner groups with a total of 79 miners identified as individuals working in the amalgamation process and the ore preparation. The highly exposed individuals in the amalgamation process distinguished by elevated Hg concentration in their urine at 32.02 ug/g creatinine. The workers in the ore preparation area had lower Hg at 20.04 ug/g creatinine. None of the workers had body burden of inorganic mercury to exceed the ACGIH standard (35 ug/g creatinine) for acceptable Hg in urine indicated that they were not at risk. However, their cross average concentration of Hg in urine of both groups (22.85 ug/g creatinine) was three times higher than the average of a controlled group (5.34 ug/g creatinine). Hair Hg level in all miners studied was in the average of 1.18 ug/g. Suggesting that Hg in the hair is not an appropriate indicator of exposure by their source. In terms of risk factors focused, gender and cooking behavior were the two significant variables affecting on the concentration of Hg found in urine of gold miners. The HQ values of high exposed individuals were one to two orders of magnitude higher than the hypothetical safety level set as 1 whereas low exposure individuals had very low Hg values ranging from 0.03-0.39. Eating and drinking habits of mining workers were some of the predisposing factors for body burden of mercury. Hg exposure assessment was also carried out in a group of 59 school children who attended Phanom Pha elementary school near the mining site. A slightly higher Hg urine concentration in children who involved in mining activity was observed at 15.82 ug/g creatinine while in children who were not involved in mining had 9.95 ug/g creatinine. Average Hg values in two groups were not higher than the safe limit set by ACGIH indicating no risk from Hg intake. Hair Hg in all school children was quite low (0.94 ug/g) which was similar to control group (0.94 ug/g). Hazard Quotient based on inorganic Hg exposure for school children were lower than 1 indicating no risk situation. Elevated Hg in school children compared to reference group suggests potential direct and indirect exposure which requires thorough monitoring and sur eillances due to low dose developmental effects. Soil washing (ex-situ) technique was evaluated as a potential remedial options by physical separation of soil into different particle sizes, coarse (>1.00-2.00 mm), medium (0.15-1.00 mm) and fine (<0.15 mm) size fraction before beginning of soil washing to compare Hg removal efficiency in the mine tailing. In a batch washing experiment, Hg extraction was performed under L/S ratio of 2.5:1, 5:1, 7.5:1, 10:1 and 15:1 with various retention times. High Hg extraction of 89.32% was achieved with the presence of 0.1 M Na₂EDTA at optimum L/S ratio of 5:1, while 0.2 M citric acid was also proven to be an alternative washing solution with high Hg removal rate (82.76%) at optimum L/S ratio of 7.5:1. Following common findings were observed; lower the L/S ratio, longer the retention time requires to achieve high removal efficiency; coarser the soil texture, easier the leachability, regardless of L/S ratio used; finer the soil texture, longer the retention time to remove Hg. Therefore, particle size separation in Hg polluted soil is recommended prior soil washing. An electrokinetic soil remediation option with three purging solutions (Na₂EDTA, KI and citric acid) with different voltage gradients was tested. KI had more effective extraction efficiency on Hg from soil over Na₂EDTA with corresponding values at 74.31 and 67.12%, respectively. The efficiency of citric acid was very low at 35.33%. An improved efficiency under voltage gradient over 1.5 V/cm through 3.0 V/cm in Hg removal was highlighted. Comparing both Hg remedial options, it was demonstrated that soil washing might be a recommended option for this contaminated mine tailing, as it can be done at the site and treated soil can be reused again within the mining area |
| Year | 2008 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. EV-08-4 |
| 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 ;Visvanathan, C. (Co-Chairperson) |
| Examination Committee(s) | Yuwaree Inna ;Srinetr, Vithet |
| Scholarship Donor(s) | Royal Thai Government |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2008 |