Mobility of organic matter, iron oxide and lead in a model sand and disturbed natural soil as affected by surface properties under flow conditions

Abstract

During the past two decades, the transport of dissolved and colloidal lead (Pb) along with organic and inorganic colloids received attention. Dissolved organic matter (DOM) is the most mobile fraction of soil organic matter (SOM) and controls the mobility of metals and colloids in soil. In order to evaluate the mechanisms controlling the mobility of Pb2+, colloids and colloidal Pb, it is necessary to consider certain properties, such as physical and chemical heterogeneities on the solid matrix, the interaction of organic matter (OM) with colloids, and colloidal Pb, and flow conditions. The aim of the dissertation was to systematically evaluate (1) the effect of surface roughness and charge heterogeneity of complex model sand on the transport of organic matter-coated goethite (OMCG) colloids under flow conditions, classic DLVO (Derjaguin-Verwey-Landau-Overbeek) theory served as a tool to predict the general trend on OMCG colloids, (2) the impact of cycles drying and rewetting on OM retention in a disturbed natural soil material, and (3) the effect of OMCG colloids and DOM on Pb2+ transport under a heterogeneous structure of the solid matrix. For the first aim, the experiments conducted by assessing solid the surface properties of solid matrix with different complexity, from simple model sand, quartz sand (QS), to a complex matrix with varying percentages of goethite coated quartz sand (GCQS). The results showed that physical and chemical heterogeneities of the solid matrix significantly reduced colloid mobility under flow interruption phases. The results indicated that colloid retention on the solid matrix was strongly related to microscale surface roughness under continuous flow conditions. The additional colloid retention with increasing complexity on GCQS was caused by charge heterogeneity. Flow interruption increased retention due to physical heterogeneity. Mobility at different nanoscale roughness properties of colloid reduced colloid retention with lower nanoscale roughness height. Applying predictive model DLVO theory showed the shallower depth of secondary minimum in colloid with higher nanoscale roughness height led to release colloid into the effluent. For the second aim, OM retention on subsoil Cambisol was tested with extracted OM from an Oi-horizon. The cycle of drying and rewetting modified the magnitude of OM retention on the solid matrix. The results showed that drying increased soil water repellency and led to preferential retention of more hydrophobic components on the surface. Re-wetting increased mobility of hydrophilic components into the outflow. However, OM retention on the surface depended on the prior OM surface loading. The positive relation between hydrophilic components retention with higher OM surface loading during the second cycle of drying rewetting decreased the release of hydrophilic components into the effluent. It was due to the formation of a multi-layer structure of OM coating on the surface in the second cycle of drying rewetting. For the third aim, mobilization studies of dissolved Pb2+and colloidal Pb were conducted in the columns filled with model sands (QS, GCQS) and disturbed natural soil material (Cambisol). There was a particular focus on preconditioning the surface of GCQS and Cambisol with DOM from different origins (extracted from Oa-horizon and Oi-horizon samples). Based on these studies, the mobility of dissolved Pb2+ and colloidal Pb in the columns was highly controlled by OMCG colloids and OM of the solid matrix. The results showed that mobile colloids did not act as a co-transport for Pb in the column, and Pb mostly co-retained on the OM. In summary, the results suggest that mobility of colloidal Pb, OMCG colloid, and OM components cannot easily be assessed without considering the surface properties of the solid matrix and flow conditions. To obtain more insights into the mechanisms of mobility and retention of OMCG colloids, colloidal Pb and OM, upcoming results should be transferred to natural conditions