IS SOLUBILITY THE ONLY CONTROL ON SOLUTE CONCENTRATIONS? The answer is NO! Solubility often controls the concentrations of major solutes such as Si, Ca, and Mg, and some minor or trace solutes such as Al and Fe. However, for many trace elements, sorption processes maintain concentrations below saturation with respect to minerals. In other words, sorption is a means to remove solutes even when the solution is undersaturated with any relevant solids.

Mineral Surfaces Minerals which are precipitated can also interact with other molecules and ions at the surface Attraction between a particular mineral surface and an ion or molecule due to: –Electrostatic interaction (unlike charges attract) –Hydrophobic/hydrophilic interactions –Specific bonding reactions at the surface

DEFINITIONS Sorption - removal of solutes from solution onto mineral surfaces. Sorbate - the species removed from solution. Sorbent - the solid onto which solution species are sorbed. Three types of sorption: –Adsorption - solutes held at the mineral surface as a hydrated species. –Absorption - solute incorporated into the mineral structure at the surface. –Ion exchange - when an ion becomes sorbed to a surface by changing places with a similarly charged ion previously residing on the sorbent.

Charged Surfaces Mineral surface has exposed ions that have an unsatisfied bond  in water, they bond to H 2 O, many of which rearrange and shed a H + ≡S- + H 2 O  ≡S—H 2 O  ≡S- OH + H + H+H+ OH H+H+ OH 2 OH

Surfaces as acid-base reactants The surface ‘SITE’ acts as an amphoteric substance  it can take on an extra H + or lose the one it has to develop charge ≡S-O- + H + ↔ ≡S-OH ↔ ≡S-OH 2 + The # of sites on a surface that are +, -, or 0 charge is a function of pH pH zpc is the pH where the + sites = - sites = 0 sites and the surface charge is nil OH 2 + OH O- O-O- OH 2 +

GOUY-CHAPMAN DOUBLE-LAYER MODEL STERN-GRAHAME TRIPLE-LAYER MODEL

Sorption to ≡S-OH sites ≡S-OH + M 2+  ≡S-OM + + H + ≡S-OH + L 2-  ≡S-L - + OH - In addition, can also have bi-dendate sorption reactions: ≡S-OH + M 2+ ≡S-O M+ 2 H +

pH zpc Zero Point of Charge, A.k.a: Zero Point of Net Proton Charge (pH ZPNPC ) or the Isoelectric Point (IEP) Measured by titration curves (pH zpc similar to pK a …) or electrophoretic mobility (tendency of the solids to migrate towards a positively charged plate) Below pH zpc  more sites are protonated  net + charge Above pH zpc  more sites are unprotonated  net - charge

POINT OF ZERO CHARGE CAUSED BY BINDING OR DISSOCIATION OF PROTONS

From Stumm and Morgan, Aquatic Chemistry

Anion-Cation sorption Equilibrium description for sorption of: ≡S-OH + M 2+  ≡S-OM + + H + Where  z is the stoichiometric net change in surface charge due to the sorption reaction (+1 here), F is Faraday’s constant (96485 Coulombs per mole),  is the electrical potential at the surface, R is the gas constant, and T is temperature in Kelvins, the whole right term is called the coulombic term

Inner Sphere and Outer Sphere Outer Sphere surface complex  ion remains bounded to the hydration shell so it does not bind directly to the surface, attraction is purely electrostatic Inner Sphere surface complex  ion bonds to a specific site on the surface, this ignores overall electrostatic interaction with bulk surface (i.e. a cation could bind to a mineral below the mineral pH zpc )

ADSORPTION OF METAL CATIONS - I In a natural solution, many metal cations compete for the available sorption sites. Experiments show some metals have greater adsorption affinities than others. What factors determine this selectivity? Ionic potential: defined as the charge over the radius (Z/r). Cations with low Z/r release their waters of hydration more easily and can form inner-sphere surface complexes.

ADSORPTION OF METAL CATIONS - II Many isovalent series cations exhibit decreasing sorption affinity with decreasing ionic radius: Cs + > Rb + > K + > Na + > Li + Ba 2+ > Sr 2+ > Ca 2+ > Mg 2+ Hg 2+ > Cd 2+ > Zn 2+ For transition metals, electron configuration becomes more important than ionic radius: Cu 2+ > Ni 2+ > Co 2+ > Fe 2+ > Mn 2+

ADSORPTION OF METAL CATIONS - III For variable-charge sorbents, the fraction of cations sorbed increases with increasing pH. For each individual ion, the degree of sorption increases rapidly over a narrow pH range (the adsorption edge).

SORPTION ISOTHERMS - I The capacity for a soil or mineral to adsorb a solute from solution can be determined by an experiment called a batch test. In a batch test, a known mass of solid (S m ) is mixed and allowed to equilibrate with a known volume of solution (V ) containing a known initial concentration of a solute (C i ). The solid and solution are then separated and the concentration (C ) of the solute remaining is measured. The difference C i - C is the concentration of solute adsorbed.

SORPTION ISOTHERMS - II The mass of solute adsorbed per mass of dry solid is given by where S m is the mass of the solid. The test is repeated at constant temperature but varying values of C i. A relationship between C and S can be graphed. Such a graph is known as an isotherm and is usually non-linear. Two common equations describing isotherms are the Freundlich and Langmuir isotherms.

FREUNDLICH ISOTHERM The Freundlich isotherm is described by where K is the partition coefficient and n  1. When n < 1, the plot is concave with respect to the C axis. When n = 1, the plot is linear. In this case, K is called the distribution coefficient (K d ).

LANGMUIR ISOTHERM The Langmuir isotherm describes the situation where the number of sorption sites is limited, so a maximum sorptive capacity (S max ) is reached. The governing equation for Langmuir isotherms is:

ION EXCHANGE REACTIONS Ions adsorbed by outer-sphere complexation and diffuse-ion adsorption are readily exchangeable with similar ions in solution. Cation exchange capacity: The concentration of ions, in meq/100 g soil, that can be displaced from the soil by ions in solution.

ION EXCHANGE REACTIONS Exchange reactions involving common, major cations are treated as equilibrium processes. The general form of a cation exchange reaction is: nA m+ + mBX  mB n+ + nAX The equilibrium constant for this reaction is given by:

Sorption of organic contaminants Organic contaminants in water are often sorbed to the solid organic fractions present in soils and sediments Natural dissolved organics (primarily humic and fulvic acids) are ionic and have a K oc close to zero Solubility is correlated to K oc for most organics

Measuring organic sorption properties K ow, the octanol-water partition coefficient is measured in batches with ½ water and ½ octanol – measures proportion of added organic which partitions to the hydrophobic organic material Empirical relation back to K oc : log K oc = log K ow