Pollution, adsorption,, and mobility of mercury in Tanzania Soils, its uptake by plants, and effects on microbial activity in soils

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University of Dar es Salaam
Studies were conducted to evaluate mercury (Hg) pollution of effluent, air, and soil near a battery factory in Dar es Salaam, to study adsorption and mobility of Hg, its uptake by wheat and beans, and its effects on micro-organisms in Morogoro, Arusha, and Dar es Salaam soils. Effects of sample pretreatment on Hg losses from plant materials were also studied to establish optimal conditions. In adsorption, pollution and part of plant uptake studies Hg measurements were made using flameless atomic absorption spectrophotometry. In mobility and plant uptake studies Hg was determined by gamma scintillation counting. Total microbial populations were determined using the plate count method. Nitrogen fixation (nitrogenase activity) was assayed using the acetylene reduction technique. Nitrification was assessed by monitoring nitrate produced. Effluent, air, and soil near the battery factory were found to contain Hg levels of up to 5.2 mg/L, 4.0 *g/cubic metre, and 472 mg/kg, respectively. Soil Hg content decreased to 1.0 mg/kg two km away in the direction of winds, indicating the influence of winds on Hg dispersal. The soils studied displayed large capacity to adsorb applied Hg, but varied in their adsorption capacities. Mercury adsorption depended both on soil: solution ratio and on the initial Hg concentration in solution, indicating that results obtained using different ratios and concentrations should not be compared. Removal of organic matter from soil resulted in large reductions in Hg adsorption, as much as 95% from mercuric chloride and 31% from 2-methoxyethylmercury chloride (Aretan), suggesting that organic matter played a major role in the adsorption of inorganic Hg while soils' mineral matter was involved more in the adsorption of the organic Hg compound. For all soils, adsorption of mercuric chloride increased with increasing pH, between pH 5 and 8, when it was artificially manipulated. Adsorption of Aretan showed little or no change with this pH increase. In all soils, there were wide differences in the adsorption of both compounds among profile horizons. In the Morogoro soil, Aretan adsorption was correlated well (r=0.821*) with the natural horizonwise variation in pH. Similar correlations were obtained with organic matter and CEC distribution for both Hg compounds. No such correlations were observed with the Arusha and Dar es Salaam soils. All soils showed greater capacity for Aretan than for mercuric chloride adsorption, but the bonding energy was larger for the latter. The Freundlich isotherm described the adsorption of mercuric chloride better than that of Aretan. There was little mobility of Hg from mercuric chloride when leached with water or dilute salt solution, and it moved to only 8 cm depth even when leached with 0.5m calcium chloride or 0.1 M EDTA, indicating that these soils' ability to adsorb Hg may preclude any groundwater pollution by Hg. Crop plants generally absorbed very little Hg from soil up to 5 mg Hg/kg soil, although the absorption increased with iincreasign soil Hg levels. But at 50 mg Hg/kg soil, wheat straw and grain absorbed 261 and 236 µg Hg/kg, respectively, from mercuric chloride, but much less from Aretan. Bean straw from 5 mg Hg/kg soil contained 45 and 23 µg Hg/kg from mercuric chloride and Aretan, respectively, but only 1 µg/kg was translocated to grain fromboth compounds. This confirms that generally translocation to grain is very little and, suggests that such grain may not pose health risks under the conditions of these studies. But lettuce grown on peat with 50 mg Hg/L (as mercuric chloride) contained 370 *g Hg/kg of dry matter and this could be hazardous. Total microbial populations of the Morogoro and Arusha soils were not affected much by up to 100 mg Hg/kg in soil relative to the control, showing 10 7/g up to this Hg level. Non-symbiotic nitrogen fixation, and nitrification, wete very sensitive at less than 50 and 5 mg Hg/kg soil, respectively. This shows that availability of specific plant nutrients may be impaired by Hg pollution
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS WRE TD365.J67)
Soil pollution, Soil research, Mercury, Physiological effect, Tanzania
Semu, E. (1985) Pollution, adsorption,, and mobility of mercury in Tanzania Soils, its uptake by plants, and effects on microbial activity in soils, Masters dissertation, University of Dar es Salaam. Available at (