Distribution and physicochemical properties of aquatic colloids in the Yangtze estuarine and coastal ecosystem

Cross-flow ultrafiltration (CFUF) technique was applied for isolating colloids from waters in the Yangtze estuarine and coastal ecosystem. The stability and physicochemical properties of colloids were analyzed quantitatively. Colloidal size and Zeta-potential at XP and WSK station decreased with the increasing pH, and reached the point of zero charge at pH<2 and pH 3.8, respectively. The colloidal organic carbon (COC) concentration in the collected samples was 7.7-35.7 μmol·L^-1, occupying 7.1%-41.7% of the total dissolved organic carbon (DOC). Three-dimensional excitation/emission matrix fluorescence spectroscopy (3DEEM) was used to analyze the samples and the results showed tryptophan-like fluorescence peaks, UV fulvic-like fluorescence peaks and humic-like fluorescence peaks. Concentrations of colloidal Na, Mg, K, Ca, Co, Cr, Cu, Fe, Li, Mn and Ni accounted for 0.33%, 5.7%, 0.975%, 1%, 7.2%, 7%, 11.9%, 15.7%, 5.5%, 10.5% and 11.3% of the dissolved phase in average, and the binding capacity of Co, Cr, Cu, Fe, Li, Mn, Ni with COC was generally higher than that of Na, Mg, K, Ca. There were significant correlations between colloidal size and salinity, between DOC and salinity, and between UOC and salinity, but no linear relationship between COC and salinity was found. The total concentration of trace metals (Co, Cr, Cu, Fe, Li, Mn, Ni) and COC showed a linear relationship.