Aluminum geochemistry in peatland waters

Abstract

Abstract. The chemical speciation of aluminum was examined in surface water samples from Sphagnum peatlands in north-central Minnesota, from peatlands along the Canadian east coast, and from bogs in the Pennine Mountain area of England. In highly organic ([DOC] x 50 mg L-'), low pH waters, 80-90% of total dissolved Al was complexed with organic matter (OM), while in waters with low DOC ([DOC] x 5 mg L-') 5446% of total dissolved A1 existed as Al+' or other inorganic Al species. Batch titrations of OM with Al revealed a high Al binding capacity, 1.4-2.8 pmol (mg DOC)-', that generally was unsaturated with Al. Titrations of OM with A1 in conjunction with a continuous distribution model were used to determine Al-OM conditional stability constants. Binding capacity (pmol A1 (mg DOC)-') and strength (formation constant) increased from pH 3 to 5 but decreased above pH 5 due to formation of Al-hydroxy species including AI(OH),(s). The high binding capacity of OM in bog waters facilitates metal mobility, especially in low pH (< 5) wetlands where metal solubility is high and OM concentrations are highest. Results showed that the relative degree of organic matter saturation with metal ions was important in modeling A1 speciation in bog waters.

Transport and thermodynamics constrain belowground carbon turnover in a northern peatland

“…Elevated concentrations of Al coincided with the high H 2 concentrations around 100 cm (>300 lmol L À1 ) and 210 cm depth (150 lmol L À1 ) (electronic annex Fig. 3), which is much higher than generally observed in surface or pore-water in peatlands (Helmer et al, 1990) and could thus account for a limited production rate in these depths. Depending on its speciation, aluminum species are highly toxic to aquatic organisms (Gensemer and Playle, 1999), but information on their influence on methanogenic and fermentative bacteria does not seem available.…”

Hydrologic and biogeochemical controls on trace element export from northern Wisconsin wetlands

“…Sequestration by DOC reduces adsorptive losses to solid surfaces and enhances metal transport from wetland sediment porewaters to streams. Al and Pb are representative of metals with a high affinity for DOC (Christensen et al 1999;Helmer et al 1990;Schnitze and Skinner 1967;Sposito 1986) and their association with DOC is supported by metal-DOC Pearson correlation coefficients for the stream and rivulet dataset (r = 0.86 and r = 0.74, respectively) while La (r = 0.55), Ce (r = 0.52) and Zn (r = 0.55) are also correlated with DOC. Concentrations of all five metals were 2-3 times higher in the streams in the spring, when DOM and associated inorganic colloids are flushed from the wetland (Fig.…”

Distributions of pH and chemical components in Mizorogaike, a pond with a floating-mat bog

“…Helmer et al (1990) observed higher concentrations of dissolved aluminum in bogs than in fens (in North America and England), and Vitt et al (1995) also observed higher concentrations of dissolved aluminum and iron in a bog than in fens (in Canada). Helmer et al (1990) concluded that the concentrations of dissolved aluminum in bogs and fens are regulated by both the solubility of gibbsite and the concentration of DOC, because high concentrations (over 4 mM) of DOC facilitate formation of soluble aluminum-DOC complexes. Indeed, in Mizorogaike, when pond water sampled at station 2 in October 2001 (dissolved aluminum ϭ 0.44 µM, pH ϭ 6.5) was analyzed by the method of Sutheimer and Cabaniss (1995), 25% of the dissolved aluminum was present as organic complexes.…”

“…Monthly variations in a pH, b dissolved organic carbon (DOC), c dissolved organic phosphorus (DOP), d dissolved Al, e dissolved Si, f dissolved Fe, g dissolved Mn, h PO 4 -P, i particulate Al, j particulate Si, k particulate Fe, l particulate Mn, and m particulate P in Mizorogaike pond. Two data points (station 7 in July, dissolved Al ϭ 10 µM; station 4 in August, PO 4 -P ϭ 1.29 µM) were out of range and are not shown Helmer et al (1990), Thormann et al (2001), BendellYoung andPick (1995), and Gorham et al (1985), as shown in Table 2. The solid line represents a regression line derived from this study ([DOC, mM] ϭ Ϫ 0.80 pH ϩ 5.8; r ϭ 0.85), and the broken line represents a regression line derived from data for other bogs ([DOC, mM] ϭ Ϫ 3.4 pH ϩ 17; r ϭ 0.83).…”

“…Clear relationships between pH, alkalinity, and the concentrations of basic cations (Na ϩ , K ϩ , Ca 2ϩ , and Mg 2ϩ ) are observed in bogs and fens, and it is well established that the succession from fen to bog is accompanied by a decrease in alkalinity and in concentrations of basic cations (Gorham et al 1985;Wheeler and Proctor 2000). However, the distribution patterns of trace elements in bogs and fens have not been investigated thoroughly enough to establish general relationships between trace elements and pH (Urban et al 1987;Helmer et al 1990;Vitt et al 1995).…”