CHEMICAL ASPECTS OF IRON-OXIDE COAGULATION IN WATER - LABORATORY STUDIES AND IMPLICATIONS FOR NATURAL SYSTEMS

Initial coagulation rates of colloidal hematite (α-Fe2O3) particles (diameter less than 0.1 μm) were measured experimentally in well-defined laboratory systems at constant temperature. The relative stability ratio, W, was obtained at various ionic strengths in NaCl medium and at pH values in the range from 3 to 12. Experimental W values ranged from 1 to 104 in various systems. The results delineate the roles of specific and generalized coagulation mechanisms for iron oxides. Among the specifically-interacting species (ΔGads0 >ΔGcoul0) studied were phosphate, monomeric organic acids of various structures, and polymeric organic acids. The critical coagulation-restabilization concentrations of specifically-interacting anions (from 10-7 to 10-4 molar) can be compared with the general effects of non-specific electrolyte coagulants (10-3 to 10-1 molar). The laboratory results are interpreted with the help of a Surface Complex Formation/Diffuse Layer Model (SCF/DLM) which describes variations of interfacial charge and potential resulting from variations of coagulating species in solution. Comparison of these laboratory experiments with observations on iron behavior in estuarine and lake waters aids in understanding iron removal mechanisms and coagulation time scales in natural systems. © 1990 Birkhäuser Verlag.