Surface Complexation. Jean-François Gaillard, Notes for CE-367 OBJECTIVES To show how the presence of particles in natural and engineered systems controls the fate of many trace elements. The concepts on which the surface complexation model (SCM) is based will be briefly discussed. 1
INTRODUCTION The biogeochemical cycles of numerous trace elements, metals in particular, are controlled by sorption reactions at mineral surfaces. These reactions are interactions between the particles present in numerous aquatic systems - lakes, rivers, ocean - and dissolved species. For example, it has been shown that aquatic particles - either originating from atmospheric fall out, rivers or re-suspension of sediments - are actively scavenging trace metals (Cu, Cd, Pb) in the water column of Lake Michigan. Ultimately these particles, with their sorbed metal ions, settle and are incorporated into the sediments. The same processes are at work in the ocean, and in most aquatic systems since particles can constitute a large fraction of the matter that is transported. It is actually possible to explain the residence time of many elements in the ocean by considering primarily the partitioning of 2
these elements between the dissolved and particulate fraction and particle sedimentation rates. The conceptual framework for dealing with surface adsorption reactions has been established during the last 30 years. The two tenets of the recent theory of surface complexation are: 1. a physical theory of electrostatic interactions between charged surfaces and solutes, and 2. the description of chemical bonds between solutes and surface atoms
The term surface complexation stems for the description of the interactions of the various functional groups, that are present at mineral surfaces, with solutes using coordination chemistry principles. As a result, these reactions are complexation reactions that obey the mass law equation. But, because of electrostatic interactions between charged surfaces and the solution, equilibrium constants for surface complexation reactions contain an additional correction term: an electrostatic correction factor. This correction factor can take various forms depending on the electrostatic model that is selected for describing the water-solid interface. Adsorption Isotherms: The Langmuir Isotherm Before entering into some details of the SCM, let s review a simple adsorption isotherm (same temperature) that was originally
derived for the adsorption of gases at the surface of solids. Let s assume a simple 1:1 stoichiometry between a surface site S and C, the species that is adsorbed. S+C SC K ads = [ SC] [ S].[C] TOT S =[ S]+[ SC] K [ SC] = TOT S. ads.[c] 1+K ads.[c] An expression often used in the case of saturation surface coverage.
ADSORPTION OF METALS AT OXIDE SURFACES Numerous aquatic particles are oxides of: aluminium, iron, manganese, and silicon. The common view is that the surface groups of these oxides are hydroxyls that present acid/base properties: SOH H + + SO SOH + 2 SOH+H + As with a protonated ligand in solution, metal ions will compete with the proton in these acid/base equilibria so that one can also write: SOH + Me z+ SOMe (z 1)+ +H + 2 SOH + Me z+ ( SO) 2 Me (z 2)+ +2H + 3
If one knows the apparent constants for these equilibria, i.e., at a given ph an a given ionic strength or concentration of background electrolyte, one can calculate the fraction of metal adsorbed to the oxide surfaces (see book pages 521-525). Because of the competition between the proton and the metal ions in solution, the fraction of metal adsorbed increases sharply as the ph increases. These adsorption edges can occur at different phs depending on the affinity of the metal for the surface sites. In addition, the adsorption edge moves as a function of the relative concentrations of the metal in solution and of the available surface sites.
ADSORPTION OF ANIONS AT OXIDE SURFACES Because of the amphoteric nature of oxide surfaces, they can also be good candidates for the adsorption of anions, such as phosphate, but also oxy-anions such as AsO 3 4 ; SeO2 4 ; CrO2 4 ;VO3 The principle is the same, for example one can write: SOH + 2 +H 2PO 4 SOH 2 PO o 3 +H 2O SOH + 2 +H 2PO 4 SOHPO 3 +H+ +H 2 O This process is of particular importance in the case of the removal of phosphorus from the effluent of waste water treatment plants to control eutrophication. In tertiary treatment, phosphate ions are removed by precipitating hydrous iron oxides (HFO). The strong affinity of the phosphate ions for the surface 4 4.
hydroxyl groups allows to decrease significantly its dissolved concentration, the particles being removed by settling. In this case again, one can calculate, given the appropriate apparent equilibrium constants the load of HFO to lower the concentration of phosphate ions below a certain environmental threshold. ADSORPTION AT OTHER SURFACES These surface complexation principles have also been applied to other type of surface than oxides such as carbonates, sulfides, and organic matter. In every case these surface complexation reactions are in competition with complexation, acid/base, redox (if equilibrium is achieved), and precipitation reactions. The real challenges reside in the proper definition of the type of surface complex formed - stoichiometry and coordination chemistry of the surface species formed - and the determination of the relevant constants.
CHARGED SURFACES AND THEIR INTERACTIONS WITH SOLUTES To account for the interactions between surfaces and ions in solution the Gouy-Chapman theory is used quite often. The combination of this electrostatic model of the interface with specific surficial chemical reactions leads to the Stern theory. Here, we describe briefly the concepts upon which these two theories are based. The Gouy-Chapman theory treats the electrostatic interactions between a flat surface and the solution in the simple case of a single electrolyte. This theory is basically similar to the Debye- Huckel theory for electrostatic interactions between ions in solution, but due to the geometry of the problem (flat surface 5
compared to hard spheres) it presents an analytical solution -it actually predates the Debye-Huckel theory and served as basis for establishing it. It is a theory that has wide application in environmental sciences and engineering because it captures well the behavior of particles in solutions and allows to calculate critical parameters such as the Debye length. This parameter provides an idea of the thickness of the interface between the solid and the solution where electrostatic interactions are taking place. At long distance, i.e., in the bulk, the electrostatic effect due to th presence of the charged surface is negligible (it decays as 1 r 2, with being the distance away from the interface).