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Over the past few decades, in order to meet the need for fresh water with the significant growth of human population and industrial activities, desalination plants have been built The development of desalination plants has caused concerns around the world about the pollution load of their desalination plants effluent and the methods to reduce this pollution load. Numerous studies have been conducted on solutions to reduce the negative effects of desalination plants effluent . One of these methods is the biological method. This study aimed to investigate the feasibility of reducing heavy metals pollution in desalination plants effluent of desalination plants in the west of Bandar Abbas city using biological methods. It was conducted in the summer of 2024 on a laboratory scale in the aquaculture hall of the Persian Gulf and Oman Sea Ecology Research Institute. This research was carried out in two stages The first stage, investigated the growth trend of the microalgae Dunaliella salina and the feasibility of reducing heavy metals by Dunaliella over a 14-day period in 3 treatments: microalgae cultivation in seawater (1), desalination plants effluent (2), and in sea water that their salinity had been increased to desalination plants effluent salinity amount (3) with 3 replications. The second stage, feasibility of growth and reduction of heavy metals by feeding Artemia fransiscana with Dunaliella. The second stage was carried out with 6 treatments over a 12-day period, with 2 densities of 50 and 100 Artemia nauplii per liter in Dunaliella culture tanks with the same initial density in seawater (4 and 5), desalination plants effluent (6 and 7), and sea water that their salinity had been increased to desalination plants effluent salinity amount (8 and 9). First, parameters such as salinity, pH, nitrate, ammonia, nitrite and heavy metals (nickel, lead, chromium, zinc, copper, cadmium, iron) were measured in the desalination plants effluent and seawater. In the first and second stage, cell density, carotenoid content (using spectrometry), growth trend, SGR and percentage of heavy metals removal (using ICP-AES) from the Dunaliella culture water and the percentage of heavy metals removal from the Artemia culture water was measured and calculated. The results showed that the primary physicochemical factors of the desalination plants effluent for temperature, pH and salinity were 32 c, 13.8 and 65 ppt. The measurements of nitrate, nitrite, ammonia, phosphate and sulfate in seawater and desalination plants effluent were within the permissible or near to permissible limit. The measurements of nitrate, nitrite, ammonia, phosphate and sulfate in seawater and desalination plants effluent were within the permissible range or near to permissible limit. Also, the initial heavy metals content of the desalination plants effluent in terms of iron, lead and cadmium was higher than the permissible level for culture water, but the rest of heavy metals were within the permissible range or near to permissible limit. The highest and lowest cell density in the first stage was obtained in treatment 1, 52.8, and treatment 3, 44.00 (× 106 cell/ml), respectively, that had a significant difference (P ≤ 0.05). The highest and lowest SGR was 16.56 and 16.39 in treatment 1 and 3, respectively, which had a significant difference (P ≤ 0.05). Also, highest and lowest content of carotenoid in treatment 1 and 3 was 0.466 and 0.390 μg/l, which had a significant difference (P ≤ 0.05). The percentage of heavy metals removal by Donalila in the 1, 2 and 3 treatments was (77/80, 51/77, 45/18), zinc (55/46, 63/42, 27/71), copper (41/48, 18/35, 86/16), nickel (41/26, 52/45, 28/12), lead (75/74, 62/76, 71/99), chromium (35/07, 37/42, 46/95) and cadmium (29/23, 24/01, 26/44). The results of data analysis showed a significant difference between treatments 1 and 2, as well as between treatments 2 and 3, except for lead (P≤0.05). In the second stage,were significant difference between all treatments in percentage of heavy metals removal (P≤0.05). There were significant difference in percentage of heavy metals removal between treatments 4, 6 and 8 (in Fe, Cu, Cr and Pb) and between treatments 5, 7 and 9 (in Fe, Ni, Cr and Pb) (P≤0.05). Therfore, the SGR of Dunalilla in treatment 2 as a salinity-resistant species was positive. Also the amount of carotenoid in seawater and desalination plants effluent had a slight difference. On the other hand, the results obtained regarding the percentage of heavy metals removal by Dunalilla were positive. This possible be due to the binding property of metals with protein and polysaccharide compounds of the cell wall of microalgae and the difference in the rate of reduction of each metals due to the different tendencies of each microalgae to absorb each metals. Also, considering that at higher densities of Artemia, the percentage of heavy metals removal was higher, It maybe due to the higher consumption of Artemia than Dunalilla and the removal of more algal cells from the water. Therefore, maybe that Dunalilla and Artemia, as organisms capable of removing heavy metals from water, can be proposed as a biological solution with higher economic efficiency to reduce heavy metals in desalination plants effluent and some wastewaters. Regarding the possibility of using the resulting biomass in various industries, there is a need to more accurately examine the accumulation and accumulation of these pollutants in their biomass.
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