Hydrous FE and MN oxides — scavengers of heavy metals in the aquatic environment

The behavior of heavy metals in a natural aqueous system is attracting researchers because of environmental issues. Man's activities have increased the quantity and distribution of heavy metals in the atmosphere, on the land, and in rivers, lakes, and seas. The extent of this widespread contamination has caused concern about its possible effects on plants, animals and human beings. One of the first steps in any systematic study of the aquatic chemistry of a potential contaminant is an elucidation of the principal modes of transport and sinks for the contaminant. It appears that a large proportion of the metallic substances discharged into water bodies is ultimately incorporated into sediments. (Warren, L. J.,¹¹² and others). The sediment is a complex mixture with three main components: clays, organic matter, and oxides of iron and manganese. While the role of clays and biota in affecting the transport of heavy metals is commonly recognized, the significance of Fe and Mn among the factors that are involved is frequently overlooked. It has been shown by Goldberg¹⁸ and later by Subramanian⁸⁸ that the amounts of trace metals in seawater sediments are proportional to the iron or manganese content of the sediments. The observations by Mortimer,^27.28 Hutchinson²⁹ and others that oxidized sediments have much greater adsorptive powers than the reduced sediments led support to the idea that Fe and Mn play a significant role in influencing the distribution and the transport of a variety of metal ions of both the naturally occurring and polluted kinds. The ability of the metallic hydroxide coatings to act as sinks for various heavy metals is well known (Turekian and Scott,”; Martin et al.,¹⁹⁷ Carpenter et al.,^32,33 Robinson, G. D.³⁶ and many others); these sinks are comparable to the ferro‐manganese nodules in marine environment. These hydroxides and oxides readily sorb or coprecipitate cations and anions; even a low percentage of Fe (OH)₃ and MnO₂ has a controlling influence on the heavy metal distribution in aquatic system. That, Fe and Mn hydrons oxides are efficient scavengors of heavy metals, is shown by Goldberg;¹⁸ Jenne;¹⁴⁴ Gibbs;²² Filipek et al.;⁴³ Harrison et al. and others. Experimental studies on sorption/coprecipitation of heavy metals on/with hydrons iron and/or manganese oxides have been reported by many researchers (Morgam and Stumm;⁸¹ Posselt et al.;¹⁵¹ Guy et al.;¹⁵⁶ Longanathan et al.;⁷⁴ Davis and Leckie;⁷⁸ Irone and Munemori,^158,159 Singh and Subramanian; Oakley et al. 1981; and others). This incorporation of heavy metals into hydrons iron and manganese‐oxide can be due to adsorption (Specific or electrostatic), ion‐exchange or coprecipitation. Loganathan and Burean (1973), Subramanian (1976) and Burns (1976) proposed that the heavy metal ions are structurally bound in bydrous oxides of iron and manganese. The sorption of heavy metals by hydrous iron and manganese oxides is dependent on pH, ionic strength, metal concentrations etc. Benjamin and Leckie (1981) proposed an adsorption model for metals according to which adsorption mode is highly pH‐dependent, the complex species dominates metal adsorption at low pH and the free metal is dominant at high pH. The low solubility of hydrous Fe and Mn oxides together with their Iavege surface area approximately 300 m²/g, i.e., small grain size (of the order of 100 A) and favorable surface change in the pH range generally found in natural aquatic systems make these hydrous oxides an efficient scavenger of heavy metals. Practical application of the sorption ability of hydrous metal oxides has been made in water and wastewater treatment, particularly by the use of iron and aluminium salts for the removal of phosphorous compounds, organic contaminants and heavy metals. Coagulation as practiced in water treatment is brought about by metal ion hydrolysis species and not by free multivalent metal ions (Matijevic et al., 1961). Once the metal reaches the sediment it is not necessarily thereby fixed for all time, but may be recycled via biological and chemical agents, both within the sedimentary compartment and also back into the water column. The extent of recycling or remobilization has an important bearing on the environmental significance of heavy metals in sediments.