The influence of colloids on the geochemical behavior of metals in polluted water using as an example Yongdingxin River, Tianjin, China

The role of colloids in estuarine and marine systems has been studied extensively in recent years, whereas less is known about the polluted freshwater system. Yongdingxin River is one of the major recipients of industrial effluents in Tianjin. This article evaluates the role of colloids in controlling geochemical behavior of Cu, Zn, Fe, Mn, Hg and Cr at the confluences between Yongdingxin River and its major tributaries Beijing Drainage River, Jinzhong River and Beitang Drainage River. Based on the distribution of metal partitioning among particulate (>0.22 μm), colloidal (1 kDa to 0.22 μm) and truly dissolved (<1 kDa) fractions, the metals can be assigned to the following groups: Group 1 – organic colloidal pool-borne elements Cu and Cr; Group 2 – inorganic colloidal pool-borne metals Fe and Mn; Group 3 – Zn and Hg characterized by varying complexation patterns. The distribution of metal partitioning among particulate, colloidal and truly dissolved fractions was influenced by anthropogenic input. In addition, the theoretical concentrations of elements in case of conservative mixing between the waters of Yongdingxin River and the waters of its tributaries were compared with the measured values to evaluate the geochemical role of colloids. The result showed that all of the metals presented a non-conservative mixing behavior. Addition of colloids resulted in the removal of metals from the water column to bed sediment during river water mixing, which was furthermore confirmed by the similar partition coefficient of metal concentration between colloid and sediment.