Coastal environments are subjected to heavy metals pollution due to population and industrial activities. Coastal areas are considered as monitoring sites for environmental pollution worldwide, so many studies have been achieved to assess coastal pollution. Most of the previous studies used coastal sediments as Abrahim and Parker , Diaz-de Alba et al.  and Hahladak et al. . Almasoud et al.  concluded enrichment of zinc, copper, chromium and lead of anthropogenic sources along the Arabian Gulf Coast. Yousef et al.  reported high arsenic and mercury concentrations along Turut Island Coast due to landfilling and oil pollution. Nour and Elsorogy  concluded enrichment of Pb, Ni, Cd, Co, Cu and Zn on the Sabratha Coastline, Libya due to different anthropogenic activities.
Alharbi et al.  studied the distribution of heavy metals along Al khobar coast, Arabian Gulf of Saudi Arabia to identify the sources of pollution and compare with other neighboring worldwide coasts. This evaluation helps the coastal strategies for better management of the coastal activities where the Arabian Gulf is one of the important marine waterways that include a lot of commercial and industrial activities.
El Sorogy et al.  studied the distribution of heavy metals along Dammam coastal area, Saudi Arabian Gulf to evaluate the human activities and compare the rate of pollution in Dammam coastline with other neighboring worldwide coasts. The most recorded pollutants were sewage effluents, landfilling due to coastal infrastructural development, oil spills, petrochemical industries and desalination plants.
On the other hand, population and industry rapid growth have increased the demand for clean water, which necessitates the presence of many seawater desalination plants. The desalination cost will fall if additional income generated from seawater mining (recovery of valuable minerals). As well as the developing countries can afford fertilizers containing plant nutrients (Mg-K-Ca-S) through seawater mining. Moreover, seawater mining is advantageous as high grade minerals on land mining are depleting leaving low grade minerals, so increase the cost. Also, land mining results in environmental problems and hazards to miners Paripurnanda et al. , however, the advantage of seawater mining is that seawater is homogenous and there is no mineral grades difference as on land mining. There is a great demand for using these minerals in industry, agriculture, environmental remediation and medicine (Table 1).
Generally, there are four methods often used for mining from seawater; solar evaporation, electro-dialysis ED,u membrane distillation/crystallization MD/MDC and adsorption/desorption/crystallization. The latter technique will be applied in this paper where solvent is applied to recover copper, cobalt and nickel. Shinde and Dhadke , studied the solvent extraction separation of copper and nickel
Table 1. Seawater minerals and their uses.
by Cyanex 301. Shamsul Baharin Jamaludin , investigated the extraction of copper, nickel and zinc from their sulphate solutions using [bis(2,4,4-trimethyl pentyl) phosphinic acid] (Cyanex 272, HA) as an extractant. Moreover, Sadat et al., , studied the Separation of Cu from dilute Cu-Ni-Co bearing bioleach solutions using solvent extraction with Chemorex CP-150.
The present work aims to determine the heavy metals distribution along Jazan coastline, southwest Saudi Arabia. Moreover, a trial for mining minerals from seawater, solvent extraction of a spike solution containing copper, cobalt and nickel has been investigated to attain the optimum extraction conditions.
The study area, Jazan, is located along the red sea coast at southwest of Saudi Arabia Kingdom. In order to assess the heavy metal distribution along Jazan coastline, 15 seawater, 10 seaweeds and 15 sediments samples (at 5 to 30 cm depth) were collected and analyzed for its heavy metal concentration Li, Be, V, Mg, Mn, Co, Ni, Cu, Rb, As, Se, Sr, Cd, Pb and U using Inductively Coupled Plasma Mass Spectrometer (ICP-MS): Nex1ON 300 D (Perkin Elmer, USA) at Jazan University. Total dissolved solids (TDS), pH and electrical conductivity (EC) were directly measured. A closed microwave system, CEM MDS-2000 (USA) was used for wet digestion of sediments samples.
Seawater and sediment samples are divided into three main groups (from 3 localities); Group (1) includes samples 1 to 5, Group (2) includes samples 6 to 10 while Group (3) includes samples 11 to 15. Spike solution consists of copper, cobalt and nickel chlorides (Merck, AR grade) was prepared by dissolving the required amounts in double-distilled water. One ml of concentrated HCl was added to the solution to avoid further hydrolysis. The commercial extractant, Cyanex 923, obtained from Cytec Inc. of Canada, was used. Kerosene was used as organic phase diluent. All other reagents used were of analytical reagent grade.
2.3. Extraction Procedure
Solvent extraction of a spike solution containing copper, cobalt and nickel has been carried out from 1 M chloride solution using Cyanex 923 diluted with kerosene. Equal aliquots (10 ml) of the aqueous phase and the organic phase were shaken for 10 minutes in a separating funnel. The phases were allowed to settle for 10 minutes then separated. The concentration of elements in the aqueous phase before and after the extraction was determined using an atomic absorption spectrophotometer, then the percentage of extraction has been calculated. The relevant factors affecting the extraction as pH, extractant concentration, metal ions concentration, time, temperature as well as O/A phase ratio have been studied and optimized.
3. Results and Discussions
3.1. Heavy Metals Assessment at Jazan Coastline
Physical properties of seawater and sediments samples of Jazan coastline as well as heavy metal concentration of all samples are shown in Table 2.
From the resulted data, it is clear that pH ranges from 7.40 (sample no. 14) to 8.10 (sample no. 1, 8 and 16). Whereas EC of seawater ranges from 68.180 (sample no. 6) to 73.800 µs/cm (sample no. 1). Finally, TDS ranges from 46.450 to 55.680 mg/L (samples 14 and 4 respectively).
The data summarized in Table 3, Table 4 and Figure 1, Figure 2, showed that the distribution of Cd in seawater samples varied from 114 µg/L in samples no. 4 and 6 to 130 µg/L in samples no. 11 and 12 while in sediment samples ranged from 38 µg/g in sample no. 1 to 80 µg/g in sample no. 8. Moreover, Pb in seawater samples varied from 123 to 199 µg/L while in sediments samples ranged from 90 to 148 µg/g. In the meantime, Cu in seawater samples varied from 360 to 404 µg/L while in sediments samples ranged from 68 to 890 µg/L. As in seawater samples varied from 30 to 110 µg/L while in sediments samples ranged from 34 to 66 µg/L. Finally, Co and Ni in seawater samples varied from 48 and 52 µg/L to 92 and 120 µg/L respectively while in sediments samples ranged from 40 and 90 µg/g to 64 and 220 µg/g respectively.
In other words; seawater samples are divided into three main groups; Group 1 includes samples 1 to 5 that recorded the lowest values of V, Mn, Pb and highest values of Co and As, Group 2 includes samples 6 to 10 that recorded the lowest values of Be, Co and highest values of Ni, V while Group 3 includes samples 11 to 15 that recorded the lowest values of Co, As and highest values of Be, Mn, Cu, Cd, Se and Pb.
Table 2. Physical properties of seawater and sediments samples of Jazan coastline.
Table 3. Metal conc. in seawater samples of Jazan coastline (µg/L).
Table 4. Metal conc. in sediments samples of Jazan coastline (µg/g).
Figure 1. Metal conc. in seawater samples of Jazan coastline.
In the meantime, the average values of all heavy metals in seaweeds samples are recorded in Table 5 and Figure 3. Comparison of average values of Jazan sediments with other coasts worldwide is shown in Table 6, where the high As, Pb and Cd values may be attributed to industrial city of Jazan, desalination plants as well as fishing boats.
3.2. Solvent Extraction of Spike Seawater Solution
The extraction of copper, cobalt and nickel from spike 1 M chloride solution using Cyanex 923 as extractant diluted with kerosene was investigated. The effect of shaking time, acid concentration, extractant concentration, H+ ion concentration and temperature on the extraction was studied.
3.2.1. Effect of Acid Concentration
The extraction of Cu, Co and Ni with 0.1 M Cyanex 923 in kerosene was studied by varying the HCl concentration from 1 M to 8 M. The extraction increased till 7 M HCl and then decreased. Data are shown in Table 7, Figure 4. The decrease may be attributed to the competition of HCl with metal species for solvent extraction.
Figure 2. Metal conc. in sediments samples of Jazan coastline.
Table 5. Metal conc. in seaweeds samples of Jazan coastline (µg/g).
3.2.2. Effect of Time
The effect of time on the extraction of Cu, Co and Ni from 5 M HCl with 0.1 M Cyanex 923 in kerosene at 1:1 phase ratio was investigated. As the time increase, the extraction increased and then decreased. The equilibrium is achieved in only 3 min. So, in all experiments, three minutes shaking time was maintained. Data are in Table 8, plotted in Figure 5.
Figure 3. Metal conc. in seaweeds samples of Jazan coastline.
Table 6. Comparison between metals conc. of Jazan sediments and other worldwide localities (after El-Sorogy et al., 2018) (µg/g).
3.2.3. Effect of Solvent Concentration
The extraction of Cu, Co and Ni from 5 M HCl solution with 0.1 M to 1 M Cyanex 923 in kerosene was investigated. The extraction increased with increasing the solvent concentration as shown in Table 9, Figure 6.
Figure 4. Effect of acid conc. on the metal extraction from spike seawater solution.
Figure 5. Effect of time on the metal extraction from spike seawater solution.
Table 7. Effect of acid conc. on the metal extraction.
Table 8. Effect of time on the metal extraction.
3.2.4. Effect of Hydrogen Ion Concentration
The effect of hydrogen ion concentration on the extraction of Cu, Co and Ni with 0.1 M Cyanex 923 and 7 M chloride ion concentration was studied. The copper extraction increased from 84.5% to 92.5% with increasing the hydrogen ion concentration from 1 to 5 M. Other data are shown in Table 10, Figure 7.
Table 9. Effect of solvent conc. on the metal extraction.
Table 10. Effect of [H+] on the metal extraction.
Figure 6. Effect of solvent conc. on the metal extraction from spike seawater solution.
Figure 7. Effect of [H+] on the metal extraction from spike seawater solution.
3.2.5. Effect of Temperature
The effect of temperature on the extraction of Cu, Co and Ni from 5 M HCl, 7 M hydrogen ion conc. with 0.1 M Cyanex 923 at 1:1 phase ratio was studied from 20˚C to 40˚C. Extraction of all metal ions increased till 35˚C then decreased.
From the overgoing study of the heavy elements distribution along Jazan coastline, it is showed that no contamination except for arsenic, cadmium and lead that may be attributed to Jazan new industrial city, desalination plants, sewage effluent and fishing boats.
Moreover, solvent extraction of a spike seawater solution containing copper, cobalt and nickel has been investigated and the optimum extraction conditions are 7 M hydrochloric acid concentration, 5 M [H+] for 3 min time with Cyanex 923 solvent conc. of 0.1 M (1:1 Phase ratio) at temp. 25˚C. These conditions realized 92.5% copper extraction as well as 95.6% and 96.2% cobalt and nickel extraction respectively.
Thus in desert countries like Saudi Arabia, where fresh water is less potable, the desalination cost will fall down if additional income generated from seawater mining (recovery of valuable minerals). At the same time, mineral resources are preserved for the future generations.
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