AQUATIC PHOTOLYSIS - PHOTOLYTIC REDOX REACTIONS BETWEEN GOETHITE AND ADSORBED ORGANIC-ACIDS IN AQUEOUS-SOLUTIONS

Photolysis of mono and di-carboxylic acids that are adsorbed onto the surface of the iron oxyhydroxide (goethite) results in an oxidation of the organic material and a reduction from Fe(III) to Fe(II) in the ir on complex. There is a subsequent release of Fe2+ ions into solution. At constant light flux and constant solution light absorption, the fac tors responsible for the degree of photolytic reaction include: the nu mber of lattice sites that are bonded by the organic acid; the rate of acid readsorption to the surface during photolysis; the conformation and structure of the organic acid; the degree of oxidation of the orga nic acid; the presence or absence of an alpha-hydroxy group on the aci d, the number of carbons in the di-acid chain and the conformation of the di-acid. The ability to liberate Fe(III) at pH 6.5 from the geothi te lattice is described by the lyotropic series: tartrate > citrate > oxalate > glycolate > maleate > succinate > formate > fumarate > malon ate > glutarate > benzoate = butanoate = control. Although a larger am ount of iron is liberated, the series is almost the same at pH 5.5 exc ept that oxalate > citrate and succinate > maleate. A set of rate equa tions are given that describe the release of iron from the goethite la ttice. It was observed that the pH of the solution increases during ph otolysis if the solutions are not buffered. There is evidence to sugge st the primary mechanism for all these reactions is an electron transf er from the organic ligand to the Fe(III) in the complex. Of all the i ron-oxyhydroxide materials, crystalline goethite is the least soluble in water; yet, this study indicates that in an aqueous suspension, iro n can be liberated from the goethite lattice. Further, it has been sho wn that photolysis can occur in a multiphase system at the sediment-wa ter interface which results in an oxidation of the organic species and release of Fe2+ to solution where it becomes available for further re action.