Competitive adsorption and photodegradation of salicylate and oxalate on goethite

This work deals with the competitive adsorption and photodegradation of salicylate and oxalate on α-FeOOH (goethite) which is a predominant crystalline modification of iron oxide present in sea sediment. pH is a crucial parameter in goethite photodissolution; therefore, by means of a pH stat titration, pH was kept constant during all experiments. For either acidic (pH 3) or neutral (pH 6) pH values oxalate is preferentially adsorbed on the goethite surface. Thus, in the presence of oxalate, adsorption of salicylate is suppressed and for oxalate concentration 2 mM is even negligible. Without oxalate the photodegradation of salicylate on goethite is negligible due to the short circuiting effect of photogenerated h+ and e−. When oxalate is present in the system photodissolution of goethite takes place, which results in the formation of OH radicals and photodegradation of salicylate. At pH 3 photodissolution of goethite is faster than at pH 6; oxalate disappears and salicylate photodegradation stops. On the other hand, due to the slower photodissolution of goethite at pH 6, the oxalate concentration decreases more slowly and salicylate photodegradation does not stop and also proceeds at longer irradiation times.