Fe-Al-organic colloids control of trace elements in peat soil solutions: Results of ultrafiltration and dialysis

Size fractionation of similar to 40 major and trace elements (TE) in peat soil solutions from the Tverskaya region (Russia) has been studied using frontal filtration and ultrafiltration through a progressively decreasing pore size (5, 2.5, 0.22 mu m, 100, 10, 5, and 1 kD) and in situ dialysis through 6-8 and 1 kD membranes with subsequent analysis by ICP-MS. In (ultra) filter-passed permeates and dialysates of soil solutions, Fe, Al, and organic carbon (OC) are well correlated, indicating the presence of mixed organo-mineral colloids. All major anions and silica are present in dissolved forms passed through 1 kD membrane. According to their behavior during filtration and dialysis and association with mineral or organic components, three groups of elements can be distinguished: (i) species that are weakly affected by size separation operations and largely (> 50-80%) present in the form of dissolved inorganic species (Ca, Mg, Li, Na, K, Sr, Ba, Rb, Cs, As, Mn) with some proportion of small (1-10 kD) organic complexes (Ca, Mg, Sr, Ba), (ii) biologically essential elements (Co, Ni, Zn, Cu, Cd) mainly present in the fraction smaller than 1 kD and known to form strong organic complexes with fulvic acids, and, (iii) elements strongly associated with aluminum, iron and OC in all ultrafiltrates and dialysates with 30-50% being concentrated in large (> 10 kD) colloids (Ga, Y, REEs, Pb, Cd, V, Nb, Sn, Ti, Zr, Hf, Th, U). For most trace metals, the proportion in the colloidal fraction correlates with their first hydrolysis constant. This implies a strong control of negatively charged oxygen donors present in inorganic/organic colloids on TE distribution between aqueous solution and colloid particles. It is suggested that these colloids are formed during plant uptake of Al, Fe, and TE from mineral matrix of deep soil horizons and their subsequent release in surface horizons after litter degradation and oxygenation on redox or acid/base fronts. Dissolved organic matter stabilizes Al/Fe colloids and thus enhances trace elements transport in soil solutions.