An investigation on flocculation, adsorption and desorption process during mixing of saline water with fresh water (Caspian Sea)

Introduction
Estuaries are very important aquatic systems as they preserve the coastal biota, are considered as a highly dynamic ecotones that include the mixing or transition zone between freshwater saline water and are fairly appropriate places for living a wide variety of fauna and flora. An estuary is a semi- enclosed coastal area where freshwater from rivers and stream mixes with saltwater from the ocean. Estuaries form a very reactive system through which heavy metals pass, as they are transported seawater.
Many animal species rely on estuaries for food and as a place to nest and breed. Estuaries are both chemically and physically dynamic ecosystems that, due to their location at the river-sea interface, act as buffer zones between the continent and sea. Thus, it is essential to study carefully and closely the overall geochemical cycle of trace elements during mixing of freshwater with saltwater from seas in estuaries.
 The chemistry of seawater and freshwater varies greatly. It is essential to find out estuarine process to predict the geochemical behavior of each individual element and its potential effect on different organism as well as the important role of these processes in the chemical mass balance between rivers and seas. It is generally known that, during estuarine mixing, the partitioning of metallic species between solution and suspended particles is governed by two important, contractive and non–biological mechanisms. These mechanisms are desorption of metals from re-suspension riverine particles matter and metal removal through flocculation of humic and fulvic acids-metals complexes. Flocculation process is one of the most important processes that occur in estuary, contributes to the concentration of heavy metals that has long lasting detrimental effects on environment to be on the decline remarkably and plays a vital roole in reducing the pollution load. In the present study, flocculation, adsorption and desorption process of such a wide variety of heavy metals as copper, zinc, nickel, lead and manganese during estuarine mixing of Karganrud River water with Caspian Sea water in relation to the diverse parameters such as pH, salinity, dissolved organic carbon, NO3 and Sodium hypochlorite that is a chemical compound with the formula NaClO is investigated.
Materials and Methods
 
The Caspian Sea covers an area of about 371,000 Km²that lies between the Caucasus Mountains andNorthern Iran, is the largest lake all across the world where a huge number of organisms are living and the salinity of Caspian Sea waters converts from 4 ppt in the northern parts to almost 13‰in the southern parts.The Karganrud River has a length of42.5 km with an average annual discharge of about 252×106 m3/year. The catchments area of the river is about 615.4 km²with an average precipitation of 1150 mm.River water and suspended matters were collected in pre-labelled and pre-cleaned 25L polyethylene bucket from the surface of Karganrud River at a point (Ca. 16 km upstream) where ensure no saline water can penetrate the fresh water. On the same day fresh water was filtered through 0.45μm Millipore AP and HA filters. Also, suspended matter sample was dried at 50o C for 29 hours. Approximately 1L of filtered fresh water was acidified with concentrated nitric acid (HNO₃) to a pH of 1.8 and kept in polyethylene bottles in a refrigerator prior to the analysis of dissolved trace metals. It should be noted that 5gr of riverine suspended matters was used for metal analysis. Similarly on the same day saline water sample from Caspian Sea was collected approximately 20 km away from the shore where no seawater diluted by river water (salinity =  0.21‰). The flocculation process was conducted by adding appropriate volume of filtered seawater to the constant volume of filtered river water at room temperature in eight proportions yielding salinity of 0.5-3‰. Also, to recognize the effect of NaClO on flocculation of metals, the increasing amounts of NaClO were added to each aquarium in the laboratory condition. The eight mixtures were kept for 24h with occasional stirring. The resulting flocculants were collected through 2.5 cm diameter Millipore membrane filter. Finally, after digestion of the filters, the concentration of metals was determined using inductively coupled plasma (ICP-AES). Also, to determine the capacity of adsorption and desorption of metals, the constant volume of fresh water sample was mixed with seawater sample to obtain a series of mixture with various salinity regimes (0.5-3‰). It should be pointed out that 5gr of suspended particle matters (SPM) are added to each aquarium. The eight mixtures were kept for 24 h with occasional stirring. Also, the physicochemical parameters (DOC, NO3, Salinity, pH) of each aquarium were measured prior to the metal analysis. Particulate samples were collected on 0.45μm Wattman filters and the concentration of Cu, Ni, Pb, Zn and Mn was determined using ICP-AES. . In addition, four-step chemical partition studies were done for any aquarium which adsorption process is observed.Of the existing clustering techniques the weighted pair group (WPG) method described by Davis (1973) was used in this study.
 
Results and discussion
 
Based on the results, the maximal removal of Cu and Zn occurs between salinities of 0.5-1.5‰. About 36% of Ni removes at the first step of mixing experiment (salinity = 0.21-0.5‰). Nickel shows the minimum flocculation tendency in comparison to copper, zinc, lead and manganese. It can be noticed that Mn undergoes maximum flocculation during mixing experiments. As is presented, the maximal adsorption capacity of all studied metals except for pb, occur between salinities of 0.5-1.5‰. The flocculation rate and maximal adsorption capacity of metals by suspended matters during estuarine mixing are in the following orders, respectively: Mn(94.8%) > Zn(60.04%) >Pb(36.63%) > Cu(30.32%) > Ni(14.84%) and Cu(13.68 mg/kg) > Zn(10.41 mg/kg) > Ni(6.58 mg/kg) >Mn(5.96 mg/kg) >Pb(0.146 mg/kg). In the present study, Cu shows maximum adsorption capacity between all studied metals. Based on results, the concentration of NO3 decrease with an increase in salinity in the area of study. The concentration of total dissolved organic carbon (DOC) in the fresh river water was about 1.92 mg/L that increased to 22.34 mg/L at a salinity of 3‰. Such an increase is indication of a marine origin in the estuarine zone. Cluster analysis shows that Mn, salinity, DOC and NaClO joined together with high similarity coefficient indicating flocculation of Mn is governed by NaClO, salinity and DOC. In the present study, pH doesn’t play any role on flocculation and adsorption processes of studied metals (Figure 1). Also, based on cluster, it can be inferred that adsorption rate of Mn, Zn and Cu is controlled by NO3. According to the chemical sequential exraction it can be noted that approximately 25% of the total heavy metals (Cu, Ni, Zn, Mn) contents were in the form of sulfide ions.
 
Figure 1- Dendrogram of cluster analysis for metals and other physic-chemical characteristics of Karganrud  River and  Caspian Sea water
 
Conclusion
 
In this study, flocculation, adsorption and desorption processes of copper, zinc, nickel, lead and manganese during mixing of Karganrud River water with Caspian Sea water at a wide variety of salinities  from 0.5 to 3 ppt were investigated. The highest percentage of flocculation observed for manganese in comparison with copper, zinc, nickel andlead. Also, Pb showed desorption behavior from suspended particulate matter during estuarine mixing. It can be clearly seen that the Maximum adsorption capacity belongs to Cu compared with other studied metals. Among studied physicochemical parameters of mixing samples, DOC shows a linearity increasing behavior toward salinity. Based on the cluster analysisthe flocculation process of Zn, Cu and to lower extent Ni is controlled by NO3. On the other hand, the flocculation process of Mn is mainly controlled by NaClO. According to the chemical partitioning study it should be noted that about 63% of concentration of adsorbed Cu found in carbonate fractions. Generally, the highest percent of metal contents found in sulfide and carbonate compounds. The flocculation and adsorption rate of studied metals showed that overall colloidal metal pollution loads can significantly be reduced by various percentiles at different salinity regimes.
 This not only states the importance of these processes in natural self-purification of estuarine ecosystems, but also shows the ecological importance of the estuarine process. Future investigations should focus on the role of seawater in the treatment of trace metals during industrial wastewater purification.