Effect of Oxidation-Reduction Fluctuations on Metal Mobility of Speciated Metals and Arsenic in Bottom Sediments of Middleton River, Bayelsa State, Nigeria

Ao Aigberua,Jt Tarawou,Cy Abasi

doi:10.4314/jasem.v22i9.25

Abstract

Speciation and chemical mobility of heavy metals were assessed with a view to provide information on bioavailability of metals in a sediment matrix in order to account for the vulnerability of sediment dwelling fauna within the Middleton river ecosystem. Sediment was collected in triplicate from seven locations (five reflecting activities of oil installations in the study area and two controls). Sediments were collected in the months of March and August 2017 to reflect the dry and wet seasons respectively. Concentration of metals in each fraction was determined using a GBC Avanta Atomic Absorption Spectrophotometer. The residual fraction was the most important for Cr with average levels of (49.4%) and (41.1%) for the dry and wet seasons respectively. Also, the most prevalent fractions of Zn were the residual fractions which revealed (31.7%) and (42.6%) during the dry and wet seasons respectively. The metalloid (As) had (14.7%) and (13.8%) of its total concentration bound to the residual and crystalline Fe-oxide fractions during the dry and wet seasons respectively. The oxidation-reduction potential (ORP) was measured electrometrically using a redox potential meter; ORP ranged from (+288.67 to +351.33 mV) and (+331.00 to +465.67 mV) for the dry and wet seasons respectively, hence, depicting a fairly oxidizing sediment environment. Mobility factor of metals depicted (As > Cu > Cr) and (As > Cr > Cu) for the dry and wet seasons respectively. The effect of oxidation-reduction potential on metal mobility revealed the transportability of chromium with increasing redox potential while arsenic was barely mobile with increasing oxidation level. Also, zinc mobility diminished strongly with increasing oxidation-reduction potential. Keywords : Speciation, chemical mobility, bioavailability, Middleton River

Highlights

Heavy metals can be seen as essential and non-essential elements
Chemical speciation is the process of identifying and quantifying different species, forms of phases of elements that are present in a material (Ure et al, 1993)
Environmental conditions such as pH, redox potential or the presence of organic chelators can lead to the release of heavy metals into a water column (Forstner, 1985)

Summary

Introduction

Heavy metals can be seen as essential (macro and micro) and non-essential (toxic) elements. The sediments existing at the bottom of the water column play a major role in the pollution scheme of the river systems by heavy metals Environmental conditions such as pH, redox potential or the presence of organic chelators can lead to the release of heavy metals into a water column (Forstner, 1985). Bjerre and Schierup (1985) found conflicting reports regarding the effect of redox conditions on metal solubility: some studies reported that Zn availability increases in poorly drained water logged soils whilst others arrive at the opposite conclusion It was not specified if the opposing trend was due to varying circumstances such as different environmental conditions or soil types. The present study was driven by the need to investigate the role that oxidation-reduction fluctuations play in the available metal species that are found in bottom sediments of the Middleton River across the dry and rainy seasons

Methods

Results

Conclusion