Application of Calcium Alginate Products for Seawater Desalination Process

. Clean water availability remains a persistent challenge for coastal communities to treat seawater. Despite abundant seawater, it should be treated to remove salt contents for daily needs. An effective method for reducing seawater's salt content involves absorption, utilizing a substance calcium alginate. We treated calcium from natural waste coral skeletons. The coral skeletons were collected from Prigi Bay, Trenggalek. The coral sample was analyzed for the Ca content of 90.8 and 93.41% prior and after calcination by XRF analysis for calcium alginate production. The determination of the optimal time required for calcium alginate to absorb NaCl efficiently. Synthesis of calcium alginate was achieved using the drop-wise method and characterized through FTIR and SEM instruments. NaCl absorption occurs within a 1 to 10-minute span to pinpoint the prime duration for calcium alginate to reduce NaCl levels. AAS instruments and argentometric titration were employed for Na + and Cl - ions analysis. Under optimized salt absorption conditions, calcium alginate reflected an ideal 8-minutes contact time, releasing in 88.17% and 50% for Na + and Cl - absorptions, respectively.


Introduction
Indonesia is an archipelagic country where most of its regions consist of the sea.However, there are still areas in Indonesia that lack clean water [1].The availability of clean water in coastal areas has been an ongoing issue, and it requires significant funds to provide clean water through wells or public water utilities [2].Coastal areas have abundant water resources, but seawater cannot be used directly for household purposes and needs to be treated.Seawater has a high salt content and total dissolved solids (TDS) should be removed and managed for clean and drinking water [3] The mineral-rich water is unsafe for drinking because it exceeds the body's mineral needs.Seawater has a TDS level above 3000 ppm correlated with high salinity and unsuitable for direct consumption.According to WHO standards, drinking water is considered safe if it has a TDS level of less than 100 ppm [4].Researchers have identified heavy metal content and total solids to investigate natural metal sources in marine waters as well as analytical methods to identify heavy metal isotope ratios [5]- [8].
We conducted using the desalination methods to provide clean water in the future.Abundant coral skeletons have the potential to provide calcium alginate membranes that can reduce the salt content of seawater.This research aims to understand the characteristics of calcium alginate membranes and their effectiveness and efficiency in reducing the salt content of seawater based on composition and required time.The desalination process utilized coral skeletons as calcium alginate membranes, which bound the salt from seawater.The abundant coral skeletons have not been fully utilized and have the potential to be used as a material for producing calcium alginate membranes.These coral skeletons contain calcium in the form of calcium carbonate (CaCO3).Calcium alginate membranes are thin layers used as separation tools based on their physical and chemical properties, synthesized through a crosslinking process between sodium alginate and calcium.

Materials and Method
The coral skeleton samples were collected from Prigi Bay, Trenggalek Regency.After dryness, the sample was conducted and analyzed at the analytical laboratory and the central laboratory for advanced mineral materials, Faculty of Mathematics and Natural Sciences, State University of Malang.The coral skeleton materials, 37% HCl (pa), distilled water, sodium alginate, solid NaCl, 2% K2CrO4 solution, and standard AgNO3 solution were used with the standard protocol.

Method
The synthesis of CaCO3 was carried out by grinding into a powder using a mortar and pestle until it reached a particle size of 200 mesh.This powder was then calcined for 4 hours at a temperature of 800 ⁰C and gradually cooled to room temperature.The result of the calcination was then analyzed using PANalytical XRF (X-Ray Fluorescence: Minipal 4) instrumentation to determine the percentage of Ca content in the sample [9].The process of producing calcium chloride involved the calcined calcium dissolving in a 1M HCl solution.The synthesis of calcium alginate was produced by sodium alginate in distilled water with a 1%(w/v) concentration, and assisted by mechanical stirring at 700 rpm.The sodium alginate solution was then reacted with the previously synthesized calcium chloride solution.The immersion time was 45 minutes.Afterward, the immersion result was filtered using a sieve.The calcium alginate particles were then washed with distilled water for 15 minutes.This washing process was repeated five times.The characterization of calcium alginate was performed by analyzing its functional groups using Fourier Transform Infrared (FTIR-Shimadzu IR Prestige 21) with KBr method [10].Furthermore, Scanning Electron Microscope (SEM-FEI Inspect-S50) was applied to determine the morphology of the membrane.The application of calcium alginate was carried out on a laboratory scale using NaCl solution.0.1 M NaCl solution was prepared and added to each of the 10 beakers.Subsequently, calcium alginate particles were added to each beaker.Stirring was performed using a shaker with different time variations, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 minutes.After stirring, the membrane and solution were separated for the analysis of Na + and Cl -content.The results of the application of calcium alginate were then analyzed for Na + Cl -contents using an AAS and the argentometric methods, respectively.

Results and Discussion
The results of the XRF analysis of the coral skeleton are presented in the listed in Table 1, as follows: Copper (Cu) 0.028 0.034 The results indicate that the coral skeleton contains 90.86% calcium, and after calcination, it increased to 93.41%.Due to its high calcium content, coral skeletons could be utilized as a source of calcium in the production of calcium alginate.The production of calcium alginate was carried out using the dropping method, where a sodium alginate solution was dropped into a calcium chloride solution using a simple custom-designed tool.A vacuum maintains pressure to ensure a stable dripping frequency.A syringe was used in the apparatus to reduce the size of the calcium alginate droplets.A column was employed as a container for the sodium alginate solution.An Erlenmeyer flask was filled with calcium chloride solution to bind the sodium alginate and form calcium alginate droplets.The residue NaCl was then removed by washing.In the optimum production of beads, it took 300 mL of sodium alginate solution and 150 mL of calcium chloride solution to release 100 grams of calcium alginate beads (Figure 2).

Fig. 2. Calcium alginate beads
The characterization of calcium alginate was performed by analyzing its functional groups using a FTIR spectrophotometer.Infrared spectroscopy is used to determine the functional groups present in a molecule based on the number of vibration frequencies of chemical bonds [11].

Fig. 3. FTIR spectra of calcium alginate
The FTIR analysis results show absorption peaks at specific wave numbers.Here are the absorption peaks and their corresponding functional groups identified in the FTIR analysis:  ions have a diameter of about 0.116 nm, while Cl -ions have a diameter of about 0.167 nm.Visually, the pores appear significantly larger than the diameters of Na + and Cl - ions.Since the pore size falls into the macro-pore range and is much larger than the diameter of Na + and Cl -ions, there is no physical interaction with Na + and Cl -ions, and blockage of the pores.
The absorption analysis of calcium alginate absorbent was performed by measuring the NaCl content in the filtrate after absorption.This process involved contacting the NaCl solution with calcium alginate beads and shaking them at 100 rpm at room temperature.Optimization was carried out during absorption to determine the ability of calcium alginate beads to absorb NaCl and to identify the optimum contact time for NaCl absorption.The NaCl absorption process using calcium alginate was conducted with 50 grams of calcium alginate and 50 mL of NaCl.Contact time varied from 1 to 10 minutes at room temperature.Subsequently, the calcium alginate beads were separated from NaCl, and the Na + and Cl -contents were analysed using an AAS and argentometric titration methods, respectively.Before absorption, the analysis of Na + using the AAS instrument showed a concentration of 1105.2 ppm.After the absorption process, the Na + concentration was determined as listed in Table 4, as follows: As listed in Table 4, the highest absorption value is achieved at 8-minute contact time, while the lowest absorption value is obtained at a 5-minute contact time.Based on this data, blank analyses were conducted at the 5th and 8th minutes.The blank used was distilled water on calcium alginate.Subsequently, Na + content analysis was performed using an AAS instrument, and Cl -content analysis was conducted through argentometric titration as follows: Based on the blank analysis, it was determined that the Na + concentration in the blank was 0.7196 ppm at the 5-minute contact time and 0.6921 ppm at the 8-minute contact time.The presence of Na + in the blank originated from the calcium alginate but still contains Na + .However, the Na + content in the calcium alginate accounted for only 0.071% of the total absorption, indicating a negligible impact and could be disregarded.The % absorption is presented in Figure 5. Figure 5 shows an increase in absorption from 2 to 3 minutes, followed by a decrease from 4 to 5 minutes.This is due to the competition between cations Ca 2+ and Sr 2+ .The highest decrease in Na + concentration occurs at an 8-minute contact time, with a decrease of 88.77%.At 9 and 10 minutes, the Na + concentration continues to decrease.The optimal absorption time for Na + is 8 minutes, and then decreases at 9 minutes.The fluctuation in Na + concentration is likely due to competitive cation interactions between Ca 2+ and Sr 2+ .The calcium source used is a natural material containing 3% Sr as an impurity.This competitive interaction can explain the variations observed in Na + absorption during the experiment.Ca 2+ and Sr 2+ ions have different solubilities in alginate.Calcium is generally more soluble in alginate than strontium.This means that calcium easily forms bonds with alginate and creates hydrogels.On the other hand, strontium has lower solubility in alginate.This implies that strontium doesn't efficiently form hydrogels.This is due to the size of ions and the strength of the electrostatic attraction between the ions and alginate molecules.Sr 2+ are slightly larger than Ca 2+ due to the weaker electrostatic attraction between strontium ions and alginate molecules, slowing down the rate of strontium dissolution in alginate compared to calcium.However, the difference in the dissolution rates of calcium and strontium may not be very significant.It can be influenced by various factors such as temperature, concentration, and pH of the solution.The presence of Sr 2+ cations can indeed interfere with the Na absorption process.This explains why the remaining Na + concentration is relatively high at the 5-minute contact time.The lowest remaining Na + concentration occurs at an 8-minute contact time.It can be concluded that the optimal absorption time for NaCl with calcium alginate is 8 minutes, and there is a decrease at 9 minutes.The Cl -content was analyzed using Mohr's argentometric titration method in the pH range of 7-10.A standard Silver Nitrate (AgNO3) solution is used as the titrant, and K2CrO4 is used as an indicator for the titration endpoint.The endpoint of the titration is marked by the appearance of a precipitate and the solution turning a reddish-yellow color, which indicates silver chromate (Ag2CrO4) compound.
The results of the Cl -analysis using argentometric titration are presented in the following table:  The figure shows an increase in Cl -absorption from 1 to 5 minutes, followed by stability from 5 to 7 minutes.The highest decrease in Cl -concentration occurs at an 8minute contact time, with a % decrease of 50%.Cl -concentration continues to decrease at minutes 9 and 10.This data confirms the effective absorption of Cl -by calcium alginate and highlights the optimal absorption time at 8 minutes.The optimal absorption time for NaCl was 8 minutes, with a decrease of 9 minute.The optimal absorption time for absorbing NaCl is indeed 8 minutes, resulting in an 88.17% decrease in Na + and a 50% decrease in Cl -.

Conclusion
The calcium content in coral skeleton was 90.86% before calcination and 93.41% after calcination, as analyzed using XRF instrumentation to identify the constituent elements of the coral skeleton.This indicates that coral has the highest calcium content and significant potential for producing calcium alginate membranes.The synthesis of calcium alginate was carried out using the dropping method.Calcium alginate was characterized using FTIR and SEM instruments.The absorption of NaCl was tested for a range of 1 to 10 minutes to determine the optimal contact time for calcium alginate in reducing NaCl concentration.Under optimal conditions for salt absorption by calcium alginate, the optimal contact time was found to be 8 minutes, with Na+ absorption achieving a rejection rate of 88.17%, and Cl-absorption resulting in a rejection rate of 50%.

Transmittance (%) 4 E3SFig. 4 .Figure 4
Fig. 4. SEM analysis of calcium alginate with magnifications of 300x (a), 500x (b), 1000x (c), and 5000x (d)Figure4reveals that the membrane has pores.At magnifications of 300x and 800x, there's no clear indication of pores in calcium alginate.Pores of calcium alginate became visible at 1000x magnification and were further clarified at 5000x magnification, which have a diameter of approximately 138.21782 nm.Identified Na + ions have a diameter of about 0.116 nm, while Cl -ions have a diameter of about 0.167 nm.Visually, the pores appear significantly larger than the diameters of Na + and Cl - ions.Since the pore size falls into the macro-pore range and is much larger than the diameter of Na + and Cl -ions, there is no physical interaction with Na + and Cl -ions, and blockage of the pores.The absorption analysis of calcium alginate absorbent was performed by measuring the NaCl content in the filtrate after absorption.This process involved contacting the NaCl solution with calcium alginate beads and shaking them at 100 rpm at room temperature.Optimization was carried out during absorption to determine the ability of calcium alginate beads to absorb NaCl and to identify the optimum contact time for NaCl absorption.The NaCl absorption process using calcium alginate was conducted with 50 grams of calcium alginate and 50 mL of NaCl.Contact time varied from 1 to 10 minutes at room temperature.Subsequently, the calcium alginate beads were separated from NaCl, and the Na + and Cl -contents were analysed using an AAS and argentometric titration methods, respectively.

Table 1
XRF analysis of elements in the coral skeleton.

Table 2 .
Functional groups in calcium alginate [12]functional groups that produce calcium alginate beads include the hydroxyl (-OH) group, carbonyl (C=O) group, and carboxyl (C-O) group.These results are consistent with the FTIR characterization findings byAyarza, et al. (2016)[12].The

Table 3 .
Na + concentration in the absorption study

Table 4 .
The result of blank analyses of Na +

Table 5 .
Results of Cl -Analysis with Argentometry Titration As listed in Tableresults, there is a significant decrease in Cl -concentration.The increase at 6 minutes suggests that there may be reduced interactions with calcium alginate, allowing Cl -ions to be released.The lowest remaining Cl -ions were observed at an 8-minute contact time, indicating that the optimal absorption time for NaCl with calcium alginate is 8 minutes.The Cl -absorption is shown in the following figure: