Solid phase speciation of iron and manganese in alum shale soils studied by parallel and sequential extraction

The kind of association of heavy metals with soil components determines the mobility and availability of metals in soils. It is, therefore, necessary to understand and differentiate the different physicochemical forms of metals. A laboratory study was conducted to investigate the solubility of iron (Fe) and manganese (Mn) and their association with soil components by the newly developed sequential extraction procedure. Two soils, the alum shale (clay loam) and moraine (loam), developed on alum shale minerals were used in this study. Soils were adjusted to pH 6.2 and 7.2 in a previous experiment and used in this study. The extractants for the parallel and successive extractions were H2O, 0.1 M NH4OAc (soil pH), 0.3 M NH4OAc (soil pH), 1 M NH4OAc (soil pH), and 1 M NH4OAc (pH 5.0). The highest amount of Fe was extracted by 1 M NH4OAc (pH 5.0) followed by H2O and the amounts recovered by these two extractants differed significantly (P<0.05) with those by the other three extractants. The result revealed that in the clay loam soil, H2O extracted higher amounts of Fe than did 0.1 M or 0.3 M or 1.0 M NH4OAc (soil pH), and the amounts increased in the successive extractions (1 to 10). For Mn, the amounts extracted with 1 M NH4OAc (pH 5.0) in both soils and at both pH levels were significantly higher (P<0.05) than all other extractants. Both pH levels and soil types affected the extraction of Fe and Mn and thus significantly higher extraction at pH 6.2 than at pH 7.2 and from the loam than the clay loam was observed. The concentration of TOC in H2O extracts decreased in the successive extractions. In both soils, higher TOC was extracted at pH 7.2 than at pH 6.2 but the difference between pH levels was found significant (P<0.05) only in the loam soil. In a seven step sequential procedure soil samples were extracted sequentially with the reagents having increasing dissolution strengths, 1 M NH4OAc (soil pH), 1 M NH4OAc (pH 5.0), 1 M NH2OH.HCl (in 25% HOAc), 1 M NH2OH.HCl (in 0.1 M HNO3), 30% H2O2 (in 0.1 M HNO3), 30% H2O2 (in 1 M HNO3) and aqua regia. Ammonium acetate extracted appreciable amount of Mn (14%) from both soils, whereas this reagent could extract very small amount of Fe (<0.2%). The highest amounts (>80%) of both Fe and Mn were extracted by reducing agent (NH2OH.HCl). The higher values of Mn extracted by NH4OAc were associated with lower pH and the coarse textured soil (loam), whereas the higher values of Mn extracted with NH2OH.HCl were associated with higher pH and the fine textured soil (clay loam). Iron was not affected either by soil pH or by soil type. The order of extraction by different extractants was: NH2OH.HCl>aqua regia>H2O2>NH4OAc for Fe, and NH2OH.HCl>NH4OAc>aqua regia>H2O2 for Mn. In both soils, Mn was found in higher amounts in NH4OAc extracts than Fe, and thus Mn may be considered more mobile and bioavailable than Fe in similar type of soils.