Transport of Humic-Coated Iron Oxide Colloids in a Sandy Soil: Influence of Ca²⁺ and Trace Metals

Abstract

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 (α-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 (α). At pH 5.7, pure hematite was positively charged and deposited rapidly (α ≈ 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 (α ≈ 0.01). Deposition rates of humic-coated hematite colloids strongly increased with increasing Ca2+ concentration. A transition from the slow (α < 1) to the fast (α = 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.