Chapter 3 Theory
3.4. Water Quality
3.4.3. Sediment
Sediment is an essential component of any river system (Islam et al. 2015). Sediments are classified as sinks and carriers of pollutants in aquatic environments. The analysis of sediments is therefore employed to evaluate the health of an aquatic system (Bartoli et al.
2012). Natural and anthropogenic sources promote the metal content in sediment and also influence the form in which these elements behave in the environment. Changes including pH, temperature, redox potential, and ion exchange affects the fate of the metals in sediment (Filgueiras et al. 2002). Carbohydrates and minerals including iron and manganese oxides adsorb trace elements (Bartoli et al. 2012). Physical and chemical variations in the environment contribute to the release or binding of potentially toxic elements from anthropogenic sources to the fine particles in the sediment (Bartoli et al. 2012).
In addition, particle size distribution of the sediment is directly related to the heavy metal adsorption capacity by the sediment (Bartoli et al. 2012, Sadeghi et al. 2012). Metal ions bind to finer particles in sediment due to the higher surface area available. These ions partition between the organic matter, oxyhydroxides of Fe, Al and Mn, phyllosilicate materials, carbonates and sulfides (Filgueiras et al. 2002).
Heavy metals in sediment can be analysed in two ways viz. total and Bioavailable. Total metal content is the total amount of a metal in all fractions of sediment. The sediment is subjected to strong acids to decompose the sediment sample rendering metals aqueous. The total metal concentration in sediment is not sufficient for the assessment of environmental impact since the total concentration is not the driving factor to an elements toxicity (Islam et al. 2014). The major concern for heavy metals in sediment is the bioavailability and toxicity to organisms. The availability of metals for uptake by an organism is the determinant of toxicity. In order to determine the bioavailable metal content in sediment, fractionation of the sediment is utilised. Fractionation of sediment is a vital part of understanding the interactions of heavy metals and their concentrations and bioavailability within ecosystems (Islam et al.
2014). It is easier to monitor the soil from which plants uptake trace metals and introduce them into the food web indirectly (Ivezić et al. 2013).
20
The single extraction and sequential extraction procedures are the two main approaches utilised in the assessment of bioavailability. Single extractions utilise a single reagent to extract metals from a component in sediment while sequential extraction uses a series of reagents to extract metals from each of the components in sediment (Ivezić et al. 2013). The sequential extraction method is suitable for trace metal determination in soil however, it has been criticised for its lack of selectivity resulting from reagents dissolving compounds of little to no toxic effects, metals extracted in previous steps can be reabsorbed and redistributed, and speciation of the metal can change here are notable deviation of results between the single and sequential extraction procedures (Mossop and Davidson 2002, Ivezić et al. 2013). Sequential extraction is time consuming but provides vital information about the behaviour of metals in an ecosystem and their potential for ecotoxicity (Filgueiras et al.
2002).
Sequential extraction has become a widely used method for fractionation of trace metal concentration in sediment (Mossop and Davidson 2002). Different techniques have been developed using a range of reagents that have similar underlying principles. The Commission of the European Communities, Community Bureau of Reference (BCR) have produced a four step method to fractionate trace metals in sediment (Mossop and Davidson 2002). This sequential extraction method utilises four steps for extracting metals from different components of the sediment.
Fraction 1 (Exchangeable, water and acid-soluble): The first step of the BCR extraction utilises a weak acid e.g. acetic acid to release metals bound to carbonates and that are exchangeable with the extracting solution (Filgueiras et al. 2002, Mossop and Davidson 2002). These metals are bioavailable for plant uptake and are affected by the ionic strength of the extractant (Filgueiras et al. 2002).
Fraction 2 (Reducible iron and manganese oxides): Hydroxylamine hydrochloride adjusted to pH 2 is widely used in this step. The iron and manganese oxyhydroxides are well known structures that encage heavy metals (Filgueiras et al. 2002). The reduction of Fe(III) and Mn(IV) under anoxic conditions release adsorbed trace metals (Filgueiras et al. 2002).
21
Fraction 3 (Oxidisable organic matter and sulfides): This fraction of sediment requires oxidation of the organic and sulfide matter with hydrogen peroxide to release the metal ions and then extract the ions with a buffer e.g. ammonium acetate. This fraction is generally associated with humic substances and is not very mobile in sediment. This is important in polluted sediment as most pollution is composed of organic matter (Filgueiras et al. 2002).
Organic substances have a high affinity for divalent ions particularly Cu and Pb in aquatic environments and retain these elements for a longer period of time (Filgueiras et al. 2002, Giacalone et al. 2005).
Fraction 4 (Residual): The residual fraction is digested with strong acids including nitric and hydrochloric acids. This fraction consists of metals bound to silicates which are mainly clay type minerals, and potentially holds the highest concentration of metals which are not bioavailable for uptake by organisms.
Table 3.3: Summary of the sequential extraction method
Fraction Extractant Associated Metal Partitioning
1 Acetic acid Exchangeable, water and acid-
soluble 2 Hydroxylamine hydrochloride
(pH 2)
Reducible iron and manganese oxides
3 Hydrogen peroxide, Ammonium acetate
Oxidisable organic matter and sulfides
4 Nitric and hydrochloric acids Residual
Decreasing Availability
22