Studies on the transport of metal humic complexes through porous media

Humic substances are ubiquitous in nature, constituting the major fraction of organic materials present in soils. They are polymeric, polyelectrolytes and are known to form water soluble complexes with metals. Whilst their metal complexing ability has been widely studied, few studies have been conducted on their ability to transport metals through the terrestrial environment. Laboratory studies were undertaken to investigate those factors which may affect the transport of metal-humic complexes through columns of porous geological media. In the main, intact Clashach sandstone columns were used. The transport studies concentrated on europium (¹⁵²Eu) complexed to humic material radiolabelled with ¹²⁵I, so that the measurement of both species was facilitated at low concentrations. In order to measure the sorption of both the metal and humic species along the column and in the effluent, a computer operated dual detector instrument was designed and built. Initial studies comparing two methods of introducing the complexes onto packed sand columns viz. injection and flooding suggested that for tracer studies both methods were acceptable. However for metal complex migration studies insufficient activity was recovered from the injection technique to produce meaningful results. The effect of humic coating, and the presence of iron within the column, on the recovery of various metal humate complexes from both packed and intact columns were investigated. The presence of humic materials greatly increased the recovery and mobility of the europium, but was found to retard the mobility of cadmium and zinc. The presence of iron both naturally occurring in the column materials and added to the packed columns as goethite was shown to greatly retard the mobility of europium, cadmium and zinc. The effect of flow rate and column length on the recovery of europium humate from Clashach sandstone cores was also studied. Europium recovery was found to be related to the flow rate through the column, suggesting a kinetically controlled sorption step. Europium recovery was not found to be directly related to the column length as a high proportion of the metal is sorbed within the first few centimetres. The recoveries and mobilities of europium humate complexes were then modelled using a simple model. Batch equilibrium experiments were conducted to determine the extent of sorption of metal humate complexes on both natural sands and on crushed Clashach sandstone of differing surface areas.