Transport of humic-coated iron oxide colloids in a sandy soil: Influence of Ca2+ and trace metals

Understanding colloid release, transport, and deposition in natural heterogeneous porous media is a prerequisite for evaluating the potential role of colloids in subsurface contaminant transport. In this study, we investigate the influence of adsorbed humic acid, solution Ca2+ concentration, and adsorbed trace metals (Cu2+, Pb2+) on the transport and deposition kinetics of colloidal hematite particles (alpha-Fe2O3; 122 nm diameter) in a sandy soil matrix. A short-pulse chromatographic technique was used to measure colloid deposition rate coefficients and collision efficiencies (alpha). At pH 5.7, pure hematite was positively charged and deposited rapidly (alpha approximate to 1) even at low electrolyte concentrations (10(-4) M CaCl2). Adsorption of humic acid to the hematite caused reversal of surface charge from positive to negative. As a result, colloid deposition rates were decreased by approximately 2 orders of magnitude (alpha approximate to 0.01). Deposition rates of humic-coated hematite colloids strongly increased with increasing Ca2+ concentration. A transition from the slow (alpha < 1) to the fast (alpha = 1) deposition regime was observed at approximately 10(-3) M CaCl2. Substituting Ca2+ with Cu2+ or Pb2+ decreased electrophoretic mobility and colloid mobility, but the effects were small compared with Ca2+ concentration effects. The results of this study demonstrate that adsorbed natural organic matter and solution ionic strength play a key role in controlling colloid mobility in soils and surface near groundwater aquifers.