Chemical speciation and bioavailability of heavy metals in soil and surface water

Keywords:Metal speciation, Bioavailability, Lolium perenne , Free metal ion, Donnan Membrane Technique, Humic acid, Metal uptake, Speciation techniques, Competition, Soil (solution)The total metal content of the soil or total metal concentration in the soil solution or surface waters is not a good indicator for metal availability to organisms. Instead, the free ion activity model (FIAM) states that the free metal ion concentration is the determining factor in heavy-metal uptake. Even though several exceptions have been found to the FIAM,the free metal ion is often responsible for binding at an organism's surface. Competition between metal ions at this biotic surface is accounted for by the Biotic Ligand Model (BLM), which is an extension of the FIAM.One of the techniques that can be used to measure many different free metal ion concentrations simultaneously in a multicomponent sample is the Donnan Membrane Technique (DMT).With the DMT, the free metal ions equilibrate over a hydrophilic cation-exchange membrane between a sample compartment and an acceptor compartment, in which the free metal ion concentration is measured. The laboratory tests on the DMT showed that there is a high agreement between the calculated and observed free metal ion concentrations in solutions containing synthetic and natural ligands at various pH values. A new field DMT cell was developed to measure free metal ion concentrations in situ in surface waters. Because free metal ion concentrations in surface waters are commonly below the detection limit of theinductively coupled plasma - mass spectrometer(ICP-MS), the detection limit of the DMT was decreased by making use of a ligand in the DMT. The field DMT measured free metal ion concentrations in situ in the River Rhine in the Netherlands, the Furtbach Stream and Lake Greifen in Switzerland and in the River Wyre in the UK. The measured fractions of free metal ions ranged from 0.015 - 0.63 % for Cu to 4.3 - 13 % for Zn. The variability of the free metal fractions has a large influence on metal uptake and metal transport. The field DMT could detect small fluctuations in the free metal ion concentration due to changing water temperatures.Uptake studies with Lolium perenne in nutrient solutions showed that the metal content in the shoots of the plants depends on the metal content in the roots and that the metal content in the roots is linearly related to the adsorption of metals on the root surface. In a multicomponent system at metal concentrations of 1 μM, humic acid decreased Cu, Pb, and Fe adsorption at the root surface, but humic acid increased Cd, Zn, and Mn adsorption at the root surface. Apparently, complexation of cations with high affinity for (dissolved) organic matter may lead to increased uptake of cations with low affinity for organic matter because of competition between cations at the root surface. These plantstudies show that a ligand is able to do both, decrease and increase the bioavailability of heavy metals.In a pot experiment using ten different contaminated soils it was found that the metal adsorption at the root surface of Lolium perenne is pH dependently related to the free, labile or total metal concentration in CaCl 2 extractions of these soils. Free metal ion concentrations in the CaCl 2 extractions were measured by DMT, whereas Diffusive Gradients in Thin-films (DGT) and Scanning Chronopotentiometry (SCP) were used to measure the sum of free and labile metal fractions.Because of the pH dependent metal binding at the root surface, the pH of the soil has to be taken into account when CaCl 2 extractions are used as a basis for risk assessment towards plants.The rate-limiting steps in the metal transport from soil to plant, which can be controlled by metal diffusion in solution or metal uptake by the plant, can be determined by comparing the total, labile and free metal fractions.