Heavy Metals Speciation, Mobility, and Bioavailability in Constructed Wetland Sediment Treating Polluted River Water: Implication for Environmental Safety

Abstract

Deshu Mamo Mekuria

The presence of toxic heavy metals in sediments beyond allowable limits are often posing a serious threat to local environment. Constructed wetland sediments contamined with heavy metals can affect sediment-dewlling biota such as benthic organisms, wetland plants, aquatic life including amphibian and reptiles. The objective of this study was to investigate heavy metals, mobility and bioavailability, level of environmental risks, and it implication for environmental safety. The study was conducted using the horizontal subsurface flow (HSSF) constructed wetland (CW) sediments designed for treating polluted Little Akaki River water. The sediment samples were collected from the CW and heavy metals speciation determined following the European Community Bureau References (BCR) sequential extraction procedures. The results showed that the highest percentage of metal fractions: Zn = 71.67%, Cr =82.20%, Cd = 76.47% and Pb = 74.96% were bound to residual fraction(R), implying that majority of the metals were stable, immobile and not bioavailable. The mobility factor (MF) for Zn (27.60%); Cr (14.24%) Cd (18.82% and Pb (21.53%); indicating low percentage of metals found in mobile chemical form. Furthermore, the environmental risk assessment code (RAC %) value for all heavy metals in the sediment samples were ranged: 5.37 % - 8.51%, falling in low risk category (RAC=1-10%). Low mobility, bioavailability and risk assessment code may be attributed to slight alkaline pH, low organic matter, and caly dominated sediment particles. High residual fraction occurred due to clay particles which can form strong bounding matrix with metals. Thus, metals in the CW sediments may pose low toxicity to wetland plants and sediment living biota surrounding environment.

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