Contrasting the sorption of Zn by oxyhydroxides of Mn and Fe, and organic matter along a mineral-poor to mineral-rich fen gradient

Abstract

The geochemistry of Mn and Fe in surface pools, pore-waters and surface peats and the sorption of Zn by the surface peats was contrasted among 15 peatlands sampled along a mineral-poor to mineral-rich fen gradient. Sorption of Zn by surficial peats was compared via distribution coefficients, both total (K DT) and partial (K DERMn, K DRFe and K DORG), where ER Mn, R Fe and ORG are amounts of Zn recovered from the easily reducible Mn oxides, reducible Fe oxides, and organic components of peat, respectively. Apparent stability constants (K As) for Zn sorption onto oxides of Fe recovered from the surface peats were also calculated and compared along the same gradient. Peat geochemistry was peatland dependent; mineral-poor fens had less easily reducible Mn and greater amounts of organic matter (%Loss on Ignition; LOI) versus mineral-rich fens (range of 0.66–8.6 mm kg −1 for ER Mn and 20–88% LOI for organic matter). Reducible Fe also varied among peatlands (range 51–315 mm kg −1) but was independent of the mineral-poor to mineral-rich fen gradient. Comparison of partial K Ds for amounts of Zn sorped onto the ER Mn, R Fe and ORG components of peat indicated that sorption was dominated by R Fe in all peatlands. K DTs ranged from 0.54–2.00. In contrast to other aquatic systems, however, the range in K DTs was not related to either surface or pore-water pH. K As ranged from 0.36 to 3.06 and were also independent of surface or pore-water pH. However, average K As (but not K DTs), were greater for mineral-poor fens ( P<0.02), suggesting greater Zn binding by surface peats of mineral-poor fens versus either the moderately poor or mineral-rich peatlands. Other water chemistry variables, such as pore-water base cation concentrations, weakly correlated to Zn partitioning onto R Fe ( r=−0.35, P=0.05), but did not fully explain differences in Zn partitioning among peatlands. Greater average K As for the mineral-poor peatlands may in part be due to the presence of strong metal-organic matter-Fe oxide complexes in the Sphagnum dominated peatlands as well as lower pore-water base cation concentrations that occur in the mineral-poor peatland as compared to the more mineral-rich fens.