Effects of Spatial, temporal and pH changes on fractionated heavy metals in sediments of the Middleton River, Bayelsa State, Nigeria

Abstract

Any metallic element possessing relatively high density and toxicity even at lower concentrations is referred to as heavy metal.1 Heavy metals occur naturally as a composition of the earth’s crust and inundate the aquatic ecosystem via weathering and washing of bedrock and solid mineral materials, aerial downpour from volcanic discharge as well as anthropogenic inputs which include effluent discharges emanating from nearby industries, offshore equipment or installations and vehicular or boat transportation emissions). Heavy metals introduced in the marine ecosystem are mostly concentrated in coastal areas, near densely populated and industrialized regions. Heavy metals are usually associated to particles. These particles are often very small, and can therefore stay in solution for a very long time. Nevertheless they will end up in the sediments. Therefore concentrations in the sediments are often 10 to 100 times higher than those in solution. In the sediments, these particles may form important secondary source of contamination, even after the primary source has disappeared.2 Heavy metal fractionation is the identification of the volumetric amounts and specific species (or forms) which makes up an element within a given environment. Hence, heavy metal fractionation helps to identify the environmental toxicity potential of a metal on the basis of its biogeochemistry. The total metal concentration is usually measured to quantify the elemental burden in sediment and soils.3 Metal speciation includes the chemical form of the metal in the soil solution, either as a free ion or complexed to a ligand, in the gaseous phase and distributed amongst solid phases within the soil. However, the long-term bioavailability of metals to humans and other organisms is determined by the re-supply of the metal to the mobile pool (soil solution) from more stable phases (metals in and associated with solid species).4 It is extensively demonstrated that increasing heavy metal pollution from various industries constitutes environmental hazard for humans and other living things.5 A number of events affecting water quality have resulted in increased public concern about surface water quality.6 Such events as increased domestic wastes generation and indiscriminate disposal and discharge of untreated and poorly treated industrial wastes into surface water bodies impact negatively on water quality and lead to water quality deterioration.7,8 Heavy metals associated with crude oil include Pb, Cd, Cu, Zn, Ni, V and Cr amongst others.9 Diseases and pathological conditions related to heavy metal contaminants have been recently reviewed by Izah and Srivastav.6 The level of pH is a very important factor which determines how heavy metals are retained or released in an aquatic environment. Hydrogen ion possesses the tendency to be strongly attached to surface negative charges and possess the power to replace most other cations.10 Metal mobility tends to increase at lower pH and decreases at higher pH.11 Mobility of metals in soil with low pH has been reported to decrease in the order: Cd > Ni > Zn > Mn > Pb, even though the effect of pH on mobility of elements in soil is highly variable depending on the content and type of organic matter.12 The physicochemical properties of aquatic ecosystems are characterized by a number of independent parameters. Factors such as temperature, oxygen content and pH of a water body can alter the solubility of the salts in it, the forms of occurrence of particular species, as well as their bioavailability and toxicity. Thus, it is necessary to determine the various species of metals present in the different compartments of the aquatic ecosystem.13 pH is known to have significant effect on metal dynamics because it controls adsorption and precipitation, which are the main mechanism of metal retention in soils.14 Cd sorption by an acid sandy soil has been found to double for each 0.5 unit increase in