Role of salinity and aeration on flocculation and remobilization of metals during estuarine mixing

Multiple parameters may affect the fate of metals between solid and liquid phases in estuarine zones. The present investigation brings out the roles of salinity and dissolved oxygen (DO) in flocculation of colloidal metals and their subsequent remobilization during estuarine mixing. For this purpose, a series of aquaria containing river water with different salinity regimes under both natural and aerated conditions were set up. The same process was repeated in the second round of the experiments, while fine suspended sediments were added to the aquaria. Except for Cu and Zn, a remarkable increase in flocculation rate of metals was observed under aerated conditions (around 20% increase in DO value) in comparison with natural conditions. The flocculation pattern under natural conditions can be summarized as Zn (49.3%) > Cu (34.1%) > Ni (27.5%) > Mn (3.37%) > Pb (1.3%), while under aerated conditions, the pattern changes to Ni (62.5%) > Zn (53.4%) > Cu (34.1%) > Pb (20.7%) > Mn (18.7%). Most of flocculated metal contents were achieved in lower salinities (0.8 to 1.9 Psu). Mobility potential of all metals in sediments showed a decreasing trend at higher salinities from 5.9% for Zn to 30% for Pb. The reason behind the decreasing trend can be attributed to the lower metal concentrations reaching higher salinity regimes. In other words, much of metal contents floc at lower salinity regimes and very minute amount is left over for higher salinities. On the contrary, DO concentration has a remarkable effect on increasing the mobility potential of most of metals. The proportional increase order shows a decreasing pattern of Pb (80.1%) > Cu (54%) > Zn (23.3%) > Ni (14.1%). However, it should be noted that Mn due to its different affinity in fractionation schemes revealed relatively lower mobility under aerated conditions.