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Abstracts
Sulphide oxidation and arsenic speciation in tropical soils R.J. Bowell Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK The oxidation of arsenopyrite either through lateritic weathering or during mineral processing releases arsenic into the environment. At the Ashanti mine, in Ghana, West Africa, chemical weathering of gold-bearing sulphide lodes in-situ and sulphide mine tailings release arsenic and other potentially toxic elements into the environment. In the tropical environment, at Ashanti, high rainfall (1,250 mm yr-l), alternating wet and dry seasons, and high daily temperatures (20-30°C) lead to rapid rates of chemical weathering. Beneath the soil cover, a thick saprolite is developed in which a component of the liberated trace metals is generally retained, incorporated into the resistant iron oxyhydroxides and oxide minerals which form the gossans. In the saprolite, arsenopyrite becomes unstable due to increasing Eh and is replaced by limonite, amorphous Fe-Mn arsenates, pitticite, bukovskyite, kankite, scorodite, goethite, hematite, and arsenolite in box- and ladder-work textures. With more intensive weathering the 'arsenopyritedominated' gossans form a porous granular mass made up of loosely-aggregated but firmly joined iron oxide grains owing to the partial breakdown of scorodite. In the more common, 'mixed sulphide' gossans a complex cellular structure is formed and the gossan consists of a series of alteration products dominated by goethite. In the extremely weathered portion of both gossan types, goethite, lepidocrocite, and hematite are present pseudomorphing oxidised arse/aopyrite. The speciation of arsenic in soil porewaters both from uncontaminated soils and those overlain by mine waste has been assessed. In aerobic soils overlying, weathered goldarsenic mineralisation in the bedrock, arsenic was present in concentrations of 189 to 1,025 ~tg g-1 in soil and porewater concentrations ranged from 86.2 to 557 gg L -1 Arsenic in these waters was found to be largely present as arsenate (approximately 78 to 95 % of the total arsenic) with arsenite, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) also present. Where arsenite concentrations are greater than arsenate, no organoarsenical species were observed. In the uncontaminated soils overlying u n m i n e r a l i s e d bedrock, b a c k g r o u n d arsenic concentrations were in the range of 12.5 to 20.2 ~tg g-1 and porewater concentrations ranged from 11.2 to 20 ~tg L 4. In these porewaters arsenate was the
dominant species. No organoarsenical species were observed in these waters, suggesting that there is a p o s s i b l e t h r e s h o l d level of total arsenic concentration required, above which methylation of arsenic by soil microorganisms takes place. In the acidic soils contaminated by mine tailings, arsenite was the major species present (up to 45% of total arsenic in the aerobic soils and 79% of total arsenic in the anaerobic soils). Although the soils were underlain by unmineralised bedrock, soil arsenic concentration ranged from 40.5 to 1,290 ~tg g-I and arsenic concentration in extracted porewaters from anaerobic soils ranged from 70.2 to 110 ~tg L -I and in aerobic soils from 70.8 to 599 ~tg L -l. In the aerobic soils MMAA, DMAA, and arsenate were also present but only arsenate was found, along with arsenite, in the anaerobic soil porewaters. An experimental study on the effects of pH and organic acids on the sorption of As(V), As(III), and organoarsenical complexes has been carried out. The s o r p t i o n d e c r e a s e s in the order As(V)>DMAA=MMAA>As(III) below pH 7 and A s ( V ) > A s ( I I I ) > M M A A = D M A A above pH 7, sorption d e c r e a s e d in the order goethite > lepidocrocite > hematite. Increased fulvic acid concentration reduced arsenic sorption on the iron minerals. Therefore a drop in pH or increase in fulvic acid increases arsenic leaching while a reduction in leaching and greater sorption occurs at higher pH or in reducing environments at low pH. This has important consequences for the disposal of arsenic in mine tailings where reducing c o n d i t i o n s are c o m m o n since A s ( I I I ) will predominate over As (V) leading to leaching of arsenic. Similarly, where organic rich soils are used to cover mine tailings, fulvic acids leached by the groundwaters will prevent arsenic sorption assisting in arsenic leaching.
References
Bowell, R.J., Morley, N.H. and Din, V.K. 1994. Arsenic speciation in soil porewaters, Ashanti, Ghana. Applied Geochemistry, 9, 15-22. Bowell, R.J. 1994. Sorption of arsenic by iron oxides and oxyhydroxides in soils.Applied Geochemistry (in press).
Abstracts
Sulphide oxidation and arsenic speciation in tropical soils R.J. Bowell Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK The oxidation of arsenopyrite either through lateritic weathering or during mineral processing releases arsenic into the environment. At the Ashanti mine, in Ghana, West Africa, chemical weathering of gold-bearing sulphide lodes in-situ and sulphide mine tailings release arsenic and other potentially toxic elements into the environment. In the tropical environment, at Ashanti, high rainfall (1,250 mm yr-l), alternating wet and dry seasons, and high daily temperatures (20-30°C) lead to rapid rates of chemical weathering. Beneath the soil cover, a thick saprolite is developed in which a component of the liberated trace metals is generally retained, incorporated into the resistant iron oxyhydroxides and oxide minerals which form the gossans. In the saprolite, arsenopyrite becomes unstable due to increasing Eh and is replaced by limonite, amorphous Fe-Mn arsenates, pitticite, bukovskyite, kankite, scorodite, goethite, hematite, and arsenolite in box- and ladder-work textures. With more intensive weathering the 'arsenopyritedominated' gossans form a porous granular mass made up of loosely-aggregated but firmly joined iron oxide grains owing to the partial breakdown of scorodite. In the more common, 'mixed sulphide' gossans a complex cellular structure is formed and the gossan consists of a series of alteration products dominated by goethite. In the extremely weathered portion of both gossan types, goethite, lepidocrocite, and hematite are present pseudomorphing oxidised arse/aopyrite. The speciation of arsenic in soil porewaters both from uncontaminated soils and those overlain by mine waste has been assessed. In aerobic soils overlying, weathered goldarsenic mineralisation in the bedrock, arsenic was present in concentrations of 189 to 1,025 ~tg g-1 in soil and porewater concentrations ranged from 86.2 to 557 gg L -1 Arsenic in these waters was found to be largely present as arsenate (approximately 78 to 95 % of the total arsenic) with arsenite, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) also present. Where arsenite concentrations are greater than arsenate, no organoarsenical species were observed. In the uncontaminated soils overlying u n m i n e r a l i s e d bedrock, b a c k g r o u n d arsenic concentrations were in the range of 12.5 to 20.2 ~tg g-1 and porewater concentrations ranged from 11.2 to 20 ~tg L 4. In these porewaters arsenate was the
dominant species. No organoarsenical species were observed in these waters, suggesting that there is a p o s s i b l e t h r e s h o l d level of total arsenic concentration required, above which methylation of arsenic by soil microorganisms takes place. In the acidic soils contaminated by mine tailings, arsenite was the major species present (up to 45% of total arsenic in the aerobic soils and 79% of total arsenic in the anaerobic soils). Although the soils were underlain by unmineralised bedrock, soil arsenic concentration ranged from 40.5 to 1,290 ~tg g-I and arsenic concentration in extracted porewaters from anaerobic soils ranged from 70.2 to 110 ~tg L -I and in aerobic soils from 70.8 to 599 ~tg L -l. In the aerobic soils MMAA, DMAA, and arsenate were also present but only arsenate was found, along with arsenite, in the anaerobic soil porewaters. An experimental study on the effects of pH and organic acids on the sorption of As(V), As(III), and organoarsenical complexes has been carried out. The s o r p t i o n d e c r e a s e s in the order As(V)>DMAA=MMAA>As(III) below pH 7 and A s ( V ) > A s ( I I I ) > M M A A = D M A A above pH 7, sorption d e c r e a s e d in the order goethite > lepidocrocite > hematite. Increased fulvic acid concentration reduced arsenic sorption on the iron minerals. Therefore a drop in pH or increase in fulvic acid increases arsenic leaching while a reduction in leaching and greater sorption occurs at higher pH or in reducing environments at low pH. This has important consequences for the disposal of arsenic in mine tailings where reducing c o n d i t i o n s are c o m m o n since A s ( I I I ) will predominate over As (V) leading to leaching of arsenic. Similarly, where organic rich soils are used to cover mine tailings, fulvic acids leached by the groundwaters will prevent arsenic sorption assisting in arsenic leaching.
References
Bowell, R.J., Morley, N.H. and Din, V.K. 1994. Arsenic speciation in soil porewaters, Ashanti, Ghana. Applied Geochemistry, 9, 15-22. Bowell, R.J. 1994. Sorption of arsenic by iron oxides and oxyhydroxides in soils.Applied Geochemistry (in press).
