Metal binding on resuspended sediment

Abstract

<P> Natural organic matter (NOM) on the surface of resuspended particles influences the partition of metal ions between free dissolved organic matter (DOM), and the particulate species. It also affects the size distribution of floes under agitation. Equilibrium between dissolved organic and particulate organic matter should, therefore, be an important indicator for predicting metal ion exchange. </p> <p> We examined the exchange of cadmium along a pH edge and compared aqueous, organic, and particulate forms of the metal ion in reactors containing either settled or resuspended sediments. A pH edge profile between 3 and 8 shows two regions of speciation: above and below pH 6.2, which corresponds to a 1 : 1, H : Cd exchange. At low pH, cadmium remains in the free form when sediments are settled, but, upon resuspension, about half of it transfers to the dissolved organic and particulate forms. On the other hand, at high pH, all cadmium is particulate in both cases. In both cases there is also a release of DOM centred at pH 6.2, followed by readsorption at higher pH values. </p> <p> The interesting finding is that cadmium binding on particles appears independent of the surge in DOM, as though cadmium is weakly bound to DOM and strongly bound to particles. We can assume that two types of organic matter exist, one dissolved, one particulate. </p> <p> To fix size distribution, the sediment/metal ion mixture was put in a Couette flocculator and subjected to a uniform shear stress. Then, the partitioning of the metal ions between the solution and sediment phases was determined. A distribution coefficient, KD was calculated for each metal ion. A correlation between log KD and log K (hydrolysis constant) was apparent, and our data was consistent with the reported metal ion binding to both sediments and artificial surfaces. We envision that organic matter accumulating on the particle creates a polyelectrolyte domain responsible for greater binding capacity. We propose that an increase in metal ion binding with increasing particle size is due to this polyelectrolyte effect, where binding is governed by particle volume, and not surface area. </p>