Iron and manganese oxides in the soil-water environment

Abstract

The importance of iron and manganese oxides in soil-water environments and their roles in controlling the availability and mobility of contaminants and nutrients are determined. Sorption is simulated using the synthetic iron and manganese oxides: goethite (a-FeOOH), birnessite (Na4Mn14O27.9H20a) nd magnetite (Fe304) which are analogous to natural soil components. Goethite is investigated in the three possible modes of occurrences in soil: colloidal, aggregate, and as coatings on inert materials. The adsorption of cadmium and cobalt onto goethite occurs in a narrow pH range between 6.5 and 7.5. Metal retention increased with both temperature and contact time. A tenfold decrease in ionic strength has no effect on the adsorption pH. Goethite pellets have a different pattern of adsorption due to reduction in surface area and granulation. The sorbing capacity of goethite coated sand is lower than that of the colloidal goethite but has a similar adsorption curve. The coated material is shown to have potential in industrial applications and notably in effluent treatment. Cobalt and cadmium uptake on to hydrated suspensions of birnessite occur in a pH range (3.0-8.0) with sigmoidal shapes for the percent of adsorption curves. Birnessite uptake capacity increases with increasing pH with a maximum at about 6.4 pH. The pH, contact time and the surface area of the oxide are the main factors that control the uptake. The adsorption of coloured species and organic colloids on magnetite was investigated in which magnetite is used both as an adsorbent and as magnetic material. Colour species and organic colloids adsorbed in acid pH and the adsorption decreases with increasing alkalinity. Dissolutions of the magnetite itself increases in low and high pH conditions. The results of investigations of sorption of contaminants and nutrients onto iron and manganese oxides have been applied to shed light upon the behaviour of contaminants and nutrients in soil in the light of soil resource management. The importance of a detailed understanding of contaminant and nutrient transport behaviour in soil-water systems to achieve effective environmental management is demonstrated. Although soil pollution is of major environmental concern, it is probably the least understood source of pollution in terms of both transport of contaminants and remediation. The studies carried out in this work have indicate the types of information required to permit the development of soil management and remediation protocols that will assist in technical management of issues related to soil resources. Detailed knowledge from experimental work must form the scientific basis for the development of contaminated soil assessment and management in an integrated approach.