Abstract
Earthworms are a major part of the total biomass of soil fauna and play a vital role in soil maintenance. They process large amounts of plant and soil material and can accumulate many pollutants that may be present in the soil. Earthworms have been explored as bioaccumulators for many heavy metal species such as Pb, Cu and Zn but limited information is available for mercury uptake and bioaccumulation in earthworms and very few report on the factors that influence the kinetics of Hg uptake by earthworms. It is known however that the uptake of Hg is strongly influenced by the presence of organic matter, hence the influence of ligands are a major factor contributing to the kinetics of mercury uptake in biosystems. In this work we have focused on the uptake of mercury by earthworms (Eisenia andrei) in the presence of humic acid (HA) under varying physical conditions of pH and temperature, done to assess the role of humic acid in the bioaccumulation of mercury by earthworms from soils. The study was conducted over a 5-day uptake period and all earthworm samples were analysed by direct mercury analysis. Mercury distribution profiles as a function of time, bioaccumulation factors (BAFs), first order rate constants and body burden constants for mercury uptake under selected conditions of temperature, pH as well as via the dermal and gut route were evaluated in one comprehensive approach. The results showed that the uptake of Hg was influenced by pH, temperature and the presence of HA. Uptake of Hg2+ was improved at low pH and temperature when the earthworms in soil were in contact with a saturating aqueous phase. The total amount of Hg2+ uptake decreased from 75 to 48 % as a function of pH. For earthworms in dry soil, the uptake was strongly influenced by the presence of the ligand. Calculated BAF values ranged from 0.1 to 0.8. Mercury uptake typically followed first order kinetics with rate constants determined as 0.2 to 1 h−1.
Highlights
Organic matter strongly affects the binding of mercury ions in the environment and affects the mobility and bioavailability of this metal in sediments (Gismera et al 2007).Mercury can be reintroduced into the aquatic systems if the solubility, mobility and bioavailability changes as a result of a change in different environmental factors such as pH, salt concentration, the presence of complexing agents, and temperature
The Hg2+ concentration in aqueous phase was observed to decrease at the same time as the concentration of Hg2+ in the soil and the earthworms increased
The distribution of mercury between soil, earthworms and aqueous phase in the control experiment, was clearly established by the trends observed from direct mercury analyser (DMA) analysis of the appropriate samples for mercury content (Fig. 3)
Summary
Organic matter strongly affects the binding of mercury ions in the environment and affects the mobility and bioavailability of this metal in sediments (Gismera et al 2007).Mercury (and other metals) can be reintroduced into the aquatic systems if the solubility, mobility and bioavailability changes as a result of a change in different environmental factors such as pH, salt concentration, the presence of complexing agents, and temperature. Organic matter strongly affects the binding of mercury ions in the environment and affects the mobility and bioavailability of this metal in sediments (Gismera et al 2007). Sediments are an important location as storage reservoirs for elemental mercury and facilitate Hg. Le Roux et al SpringerPlus (2016) 5:681 methylation, resulting in high concentrations of the more toxic monomethylmercury (CH3Hg+) in organisms (Inza et al 1997; Lawrence and Mason 2001; Burton et al 2006; Windmöller et al 2015). Earthworms in particular, have the ability to bioaccumulate toxins and can concentrate them internally to high levels. In turn they form the basis of many food chains, thereby passing these high levels on to the wildlife that feed on them. Because of the intricate relationship between earthworms and the soil and its contaminants, they can serve as useful biological indicators of soil contamination (Veiga et al 1999; Dai et al 2004)
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