Geospatial investigation on self-purification capacity of river Estuaries in the Caspian region: reducing heavy metals pollution

Abstract In today’s context, the adoption of sustainable wastewater treatment methods is crucial. River estuaries have the potential to offer an economically viable and environmentally friendly solution for wastewater treatment through the flocculation of pollutants. This study investigates the role of river estuaries flowing into the southern part of the Caspian Sea in the treatment of heavy metals. Two sets of experiments were designed for this purpose. The first set involved adjusting a series of discrete aquaria in various salinity regimes, while the second set utilized only one aquarium. The results from the first set indicate the capacity of the studied estuaries to remove heavy metals through the flocculation process in the following order: Zn (70%) > Mn (60%) > Cu (49%) > Pb (24%) > Ni (19%). However, the removal rates in the second set were reduced as follows: Zn (57%) > Mn (56%) > Cu (40%) > Pb (20%) > Ni (17%). It was observed that the flocculation process exhibits an unstable nature. Furthermore, the findings reveal that heavy metals flocculation primarily occurs upstream of the estuary. However, instability in the flocculation process occurs downstream, where water parameters undergo drastic changes. Statistical analyses indicate that an increase in pH plays a significant role in the destabilization of flocs. Conversely, the initial concentration of heavy metals, dissolved oxygen, and redox potential have a positive impact on the flocculation process