CO-ADSORPTION OF THE LOW MOLECULAR CARBOXYLIC ACIDS AND CADMIUM IONS AT THE METAL OXIDE/ELECTROLYTE INTERFACE

摘要

Oxalate and citrate anions can form with Cd~(2+) cations two types of complexes, but the main complexed form in the measured pH range is CdC_2O_4 and CdHCit, respectively. The presence of oxalate or citrate anions (0.001 and 0.002 M) changes the cadmium adsorption on titania and goethite samples, causing an increase of Cd~(2+) adsorption at low pH and a decrease of Cd~(2+) adsorption at high pH. Results are explained with adsorption - desorption processes of oxalate (or citrate) anions on the metal oxide surfaces. Carboxylic ions adsorbing on titanium dioxide or goethite at low pH values change the surface affinity to metal cations. They have a higher affinity to Cd~(2+) than surface hydroxyl groups, and we observe a higher adsorption of cadmium. An increased pH leads to desorption of organic compounds, and their concentration in electrolyte solution increases. The organic compounds form complexes with cadmium cations and weaken Cd~(2+) adsorption at high pH. On the other hand, the presence of cadmium ions leads to increased citrate adsorption on silica, and oxalate adsorption on goethite and titania surfaces at high pH. This is caused by oxalate or citrate adsorption on the adsorbed cadmium cations.