Abstract
The influence of air-drying treatment on changes in the form of Mn, Co, Zn and Cu was investigated using air-dried and field moisture soils which included Fluvaquent (paddy), Dystrochrept (paddy) and Melanudand (upland). The sequential extraction method was used to evaluate the different chemical forms of the elements in the soils. 1) Compared with the field moisture. soils, the exchangeable forms of Mn, Co and Zn in the air-dried soils increased 9-25 times, 6 times and 1.4-1.7 times, respectively. Acid soluble forms of Mn and Co increased approximately 2 times, but that of Cu decreased. The Mn oxide occluded forms of Mn and Co decreased approximately 10% and 9%, respectively. The elements in organically bound, Fe oxide-occluded and residual forms were unchanged. 2) Changes in the chemical forms subjected to the air-drying treatment differed according to soil and element. The effects of drying were more strongly exhibited in the paddy soils than in the upland soil, and more noticeably in regards to Mn and Co as compared to Zn and Cu. 3) These results indicate that the increase in certain heavy metals with the air-drying treatment was caused by the dissolution of Mn oxides, which occluded those heavy metals. From a comparison between the redox potential of soils and MnO_2 calculated from the standard free energies of formation, it can be seen that MnO_2 in air-dried soil could be dissolved below pH2-3. Though surface or organic acidity may have occurred, it is believed that soil pH could not drop to 2-3 due to the buffering action of the soils. It is believed that organic matter derived from humus or microorganisms could dissolve Mn oxides.
