Environmental aspects of acid sulphate soils - ROOT of content

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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
with a one unit decrease in pH. At pH < 4 the same relationship, approximately, holdsalso for the concentration of total dissolved Al. A roughly tenfold increase in theconcentration of dissolved AI per one unit pH decrease, indeed, follows from datapresented for acid sulphate soils by Satawathananont (1986) and by Hanhart andNi (1991) inVietnam.It remains to be seen whether [A13+][S0,2-][OH-] remains constant under evaporativeconditions, when highly soluble Al-sulphates (alum and pickeringite) precipitate, andin acidified surface waters. At least in open waters, Al3+ may simply behave conservatively,i.e. without regulation of its concentration by some solid phase, as shown byNordstrom and Ball (1986) for acid mine waters.Release of SO2Gaseous SO, is released during oxidation of sulphides and can be used in prospectingfor sulphide ores. The distinct sulphurous odour that can be noticed, sometimes, indrained mangrove marshes may also be attributed to SOz. There are indications fromacid-base budgets for whole soil profiles that a very large fraction of the potentialacidity present in pyritic sediments leaves the soil in unneutralized form as SO, (vanBreemen 1976). If this were true, oxidation of pyritic sediments is a source of air pollution,and could contribute to ‘acid rain’. In view of increasing SO, emissions fromindustrial sources e.g. in South East Asia (Rohde and Herrera 1988), a study of thebackground SO, concentrations and the possible contribution of acid sulphate soilsis relevant.Oxidation of Fe11The reactivity of Fe111 with pyrite (Equation 2) accounts for a high mobility of ironin zones of active pyrite oxidation, and for a failure of solid Fe111 to precipitate adjacentto pyrite, except at high pH or at exceptionally high rates of oxidation. WhereFeSO,,,, derived from pyrite (Equation 1) is transported before being oxidized to hydratedFe111 oxide, further acidification takes place at some distance from the pyriticzoneSuch ‘delayed’ acidification can be observed in shallower soil horizons, in pondedfields where FeSO,,,,, diffuses upward through cracks from the pyritic subsoil, in tubedrains and in open waters.Rapid oxidation of Fe2+, especially at low pH, is mainly due to autotrophic ironbacteria. In addition to acidification, clogging of drain tubes (Bloomfield and Coulter1973) and of fish gills (Singh et al. 1988) by FeIII(hydrous)oxide may be among theunwanted effects of reaction 4.In not-too-deeply-drained, highly organic acid sulphate soils in perhumid climates,oxidation of Fe2+ according to equation 4 seems to be hampered (Konsten et al., inpreparation). Restricted oxidation of Fe2+ may be due to insufficient aeration or toprotection of Fe2+ from O, by dissolved organic substances. Leaching of most of theiron derived from pyrite would explain the formation of acid sulphate soils with littleor no mottling from Fe111 oxide or jarosite. Such iron-poor acid sulphate soils395
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