Environmental aspects of acid sulphate soils - ROOT of content

Release of trace metalsSeveral trace elements may accumulate together with Fe in sedimentary pyrite, eithersubstituting for Fe in pyrite (notably Ni and Co), or in related sulfides (Cu, Zn, Pband As) (Deer, Howie and Zussman 1965). These elements will be released duringpyrite oxidation. Extreme acidification due to pyrite oxidation should also lead torelease of trace metals from other minerals. Research on the release of potentiallytoxic heavy metals during acid sulphate soil formation is rare. Satawathananont (1986)observed concentrations of water-soluble Cu, Zn, Mo, Cd, Pb, Ni and As that wereabout an order of magnitude higher in an aerated pyritic soil (pH 2.9) than in threewell-developed acid sulphate soils (pH 3.9-4.5) and one non-acid marine soil (pH 4.9)from the Bangkok Plain. Also, after incubation under controlled Eh-pH in acid oxidizedconditions for two weeks, water-soluble trace metals were higher in the pyriticsoil than in the older leached soils (Table 1). Palko and Yli-Halla (1988,1990) observedsimilar contents of total Cr, Cu, Zn, Co, and Ni in acid sulphate soils and non-acidsoils from Finland, suggesting that pyrite formation did not cause accumulation ofheavy trace metals. Levels of soluble trace metals, however, were highest in the acidsulphate soils. These data indeed suggest that pyrite oxidation and associated acidificationmay release significant quantities of soluble trace metals.Release of + Al3Interest in the concentration of dissolved Al3+ stems from its potential toxicity toplant roots and to fish at concentrations exceeding 0.02 mg I-' to 10-50 mg I-', dependingon variety or species and growth stage (Thawornwong and van Diest 1974, Singhet al. 1988). Virtually concurrent with the decrease in pH during acidification frompyrite oxidation, dissolved AI is released, presumably by dissolution of clay minerals.Over a wide range of conditions in Thai acid sulphate soils, from pH below 2 in oxidizingpyritic soil in the laboratory, to pH close to 5 in old leached acid sulphate soilsin the field, activities of Al3+ obeyed the relationship [A13+][S0,2-][OH-] = 17.3, suggestingcontrol of dissolved Al by equilibrium with a basic sulphate AIOHSO, (vanBreemen 1973). This relationship was confirmed for Thai acid sulphate soils (Mooreand Patrick 1991) and for Indonesian acid sulphate soils (Groenenberg 1990). It couldbe explained by equilibrium with the mineral jurbanite, A10HS0,.5H20 (Nordstrom1982). Jurbanite, however, has never been identified in acid sulphate soils. Nevertheless,the empirical relationship appears to be useful in predicting AI concentrations.Sulphate activities vary relatively little in most acid sulphate soils, so constant valuesof [AI3+][S0,2-][OH-] imply that the activity of Al3+ increases about tenfoldTable 1 Contents of water-soluble trace elements (mg I-') in stirred soil suspensions (200 g of soil, 800ml of water) maintained at Eh= 400 mV and pH=3.5 for two weeks. Bang Pakong is a pyriticsoil, the others are surface soils of well developed acid sulphate soils. Data from Satawathananont(I 986)Soil Zn Cu Cd Pb Cr Ni AsRangsit very acid 16 0.55 0.29 2. I 0.35 3.0 2.8Rangsit 19 0.34 0.15 I .2 0.30 4. I 2.4Mahapot 18 0.89 0.23 I .9 0.46 7.3 2. IBangPakong 42 1.12 0.75 6.6 0.69 10.5 9.9394