Abstract
Natural zeolite obtained from Sukabumi, Bandung, Indonesia was used as sorbent material for reducing salinity of seawater. The sorption ability of zeolite was enhanced through thermal activation by using furnace. In this study we obtained the optimum condition of activation is 225 ˚C for 3 h. The sorption effectiveness was determined through measurement the reduction of salinity (Rs) and efficiency of reduction (ð°). The highest value of Rs and ð° are 3.2 ppt and 9.14%, respectively at 7.5 g of sorbent dose. Effectiveness of desalination process can be enhanced through the increase of zeolite dose. Natural zeolite from Sukabumi possess great potential as low cost sorbent material for seawater desalination.
1. Introduction
Fresh water is one of the most important human needs. Preservation and availability of fresh water are demanded to ensure the human life. To address the threat of water shortages and ensure the continuous availability of fresh water, it is needed the right strategy that should be tailored to the local potential and characteristics of each country. Indonesia possess abundant seawater resources. Two-thirds area of Indonesia is sea. Therefore, seawater desalination becomes one of potential method to overcome the water crisis in the future [1].
Seawater desalination refers to the removal of salts and minerals to produce fresh water [2, 3]. Various desalination technologies have been developed such as multi-stage flash distillation (MSF), multiple effect distillation (MED), vapor compression distillation (VCD), reverse osmosis (RO), and electro dialysis (ED) [4-6]. This technologies were able to produce fresh water with high level of pureness. However, the operation cost of this technologies are still considered expensive, so it is necessary to find a cheaper alternative methods.
Sorption method is supposed as prospective technique for use in seawater desalination. Sorption method is interesting due to its simplicity and high efficiency [7]. Furthermore, the availability of a wide range of sorbent materials make sorption method is potential developed as cost-effective model [8]. A number of sorbent material such as carbon active [7], fly ash [9], clay [10] and natural zeolites [11] have been used in sorption systems for wide range application. Natural zeolites are considered as low-cost sorbent material with abundant availability [12]. In addition, it gained a significant interest due to their valuable properties such as ion-exchange [13], high sorpsibility for inorganic and organic ions, ease of activation and regeneration as well as non-toxic material [14].
Indonesia has abundant natural zeolite, one of them is located in Sukabumi, West Java. Numerous authors have reported the use of natural zeolites for various applications. Al-Anber et al. have reported utilization of natural zeolite as ion-exchange and sorbent material in the removal of iron [15]. Removal of ammonium from grey water using natural zeolite was reported by Widiastuti et al. [16]. Adsorption of Cu(II) ions from solution by natural zeolite has been reported by Kyziol-Komosinska et al. [14]. Removal of phenol from aqueous solution by natural zeolite was reported by Yousef et al. [17]. Removal of Mn²⁺, Zn²⁺, and Cr³⁺ from solution by clinoptilolite was reported by Inglezakis et al. [13]. Doula reported the use of clinoptilolite for simultaneous removal of Cu, Mn and Zn from drinking water [18]. Nevertheless, the use of natural zeolite for seawater desalination has not been extensively studied. Wajima [19] investigated ion exchange properties of five different Japanese natural zeolites in seawater and found that sodium ions could be reduced by all zeolites, although anions, Cl- and SO₄²⁻, in seawater showed barely changes. Swison [20] reported desalination by natural zeolite membranes offers a robust alternative to the thin film membranes currently used in the desalination industry. The natural zeolite that was used in the study is clinoptilolite from British Columbia, Canada.
Continuing our work [12, 21] to find out cost-effective sorbent material for used in seawater desalination, in this study we investigate the possibility of natural zeolite from Sukabumi, Bandung for reducing the salinity (concentration of salt ion) of seawater. Effect of treatment parameters, particle size and zeolite dose to the effectiveness of desalination were investigated. The sorption kinetic of salt ions onto zeolite particles was also studied.
2. Methods
We used natural zeolite from Sukabumi, Bandung, West Java, Indonesia. The zeolites were prepared into a fine powder by using mechanical mortar to increase the surface area as well as to promote the activation process. The zeolite powders then put in alumina crucibles and heated by using furnace in atmospheric condition. The ranges of activation temperature are 100 to 600˚C with activation duration are 2 to 4 h. The morphology of zeolite powder was studied by Scanning Electron Microscopy (Jeol JSM-6510LA). The schematic diagram of activation process is shown in Fig.1. The detail of activation steps were reported in reference [12].
We used seawater from Java Sea with average salinity of 35.0 ppt. The desalination process was conducted by inserting powder zeolite into 100 ml of seawater. The zeolite dose was varied at 1.5, 3.5, 5.0 and 7.5 g. Each sample was shaken mechanically so that the powders zeolite can disperse homogeneously in the seawater. The seawater salinity after treated by zeolite powder was measured by Salinometer (Mettler Toledo SG7-FK2). Effectiveness of desalination was quantitatively expressed by reduction of salinity and efficiency of reduction values. The reduction of salinity (Rs) and efficiency of reduction (ð°) can be determined by Eq. 1 and Eq. 2, respectively.
Fig. 1. The schematic diagram of thermal activation process.
3. Results and Discussion
Fig. 2 shows SEM image of natural zeolites particles from Sukabumi, West Java, Indonesia. The type of zeolite from Sukabumi is clinoptilolite with Si/Al ratio lays 4.0 to 5.0. Based on SEM image we determined the average size and the surface porosity of zeolite particle. The average size and surface porosity are 17.66 ± 0.13 ðĩm and 65 %, respectively [21, 22]. Particle size and surface porosity are important parameters in the sorption system because small size of sorbent increases the specific surface area (m2/g) while high surface porosity increases the external surface area (m2). Combination of both parameters simultaneously enhance the effective surface area. Natural zeolite with high effective surface area would have high sorption ability and capacity. Therefore, it will effective for use as sorbent material in seawater desalination.
Fig. 2. SEM image of powder natural zeolite
Fig. 3 shows the change of seawater salinity due to the addition of activated zeolite at temperatures of 100 ˚C to 600 ˚C. We can see that efficiency of reduction (ð°) of un-activated zeolite is lowest (0.2%). ð° of natural zeolite increase (2.6% to 4.9%) by increasing temperature from 100 ˚C to 225 ˚C and decrease (4.9% to 1.7%) when the activated temperature was further increased until 600 ˚C. Natural zeolite activated at 225 ˚C possess the highest reduction of salinity with ð° of 4.9%. The increment ð° of natural zeolite when activation temperature increased from 100 ˚C to 225 ˚C was considered due to the increase of effective surface-area due to disposal of organic substance that clog the channels or contaminate the surface of zeolite framework [23]. Besides, increasing the activation temperature leads to increment dealumination therefore increase Si/Al ratio of zeolite. Zeolite with high Si/Al ratio has significant sorption and ion exchange ability [14, 24]. The increase of effective surface-area and dealumination were expected improving effectiveness the sorption of salt ions onto the zeolite framework. However, when activation temperature was further increased from 225 ˚C to 600 ˚C, the reduction efficiency of natural zeolite was decrease. It was probably due to solidification of zeolite as a result of high temperature, while the degree of solidification increase by the increase of temperature [24]. This solidification leads to the decrease effective surface area of zeolite and then decrease its sorpsibility [12].
Fig. 3. Efficiency of reduction as function of activation temperature for 3 h.
Fig. 4 shows the efficiency of reduction as function of activation time. We can see that when activation time was decreased from 3 h to 2 h the value of ð° decreases from 4.9 % to 2.8 %, while when it was increased to 4h, ð° decreases to 3.4%. It indicate that 3 h is the optimum duration for activation process. The decrease of ð° due to shortening the activation duration indicates insufficient time for activation process. In this condition, the surface of zeolite was not optimally exposed due to impurities or undesirable substances that covering the zeolite surface and block the zeolite channel have not been properly removed. On the other hand, the over duration of activation leads to the decrease of surface area due to solidification. In this condition, the available space that had been formed will be vanished, the adjacent site that had been estranged will be closed back. Consequently the sorpsibility of zeolite decreases then the salinity of seawater was small reduced. Similar phenomenon was also reported by Margeta et al. [25] on development of natural zeolites for their use in water-treatment systems.
Fig. 4. Efficiency of reduction as function of activation time at 225 ˚C.
Fig. 5 shows the sorption rate of salt ion onto zeolite. It can be seen that the sorption rate of salt ion onto zeolite is fast. It is an important property for sorbent material. A good sorbent material exhibits fast rate sorption, while low grade sorbent shows slow rate of sorption. Fast rate of sorption indicates that the cooperative interaction between salt ions and zeolite particles are quickly happened after contact time. We observed that the reduction rate of seawater salinity follow the exponential function. At the beginning, the reduction rate is fast then increase slowly until achieve the saturation condition. At saturation condition, the reduction of salinity was not farther occurred. Saturation condition occurs doe to the electric field of zeolite was not strong enough to attract another salt ions. Therefore, there are no more ions adsorbed on the zeolite. As a result, salinity of seawater was no farther reduced. Due to natural zeolite exhibits fast sorption to the salt ions, it becomes potential for use as sorbent material in seawater desalination. This result is similar to the adsorption characteristics of natural zeolites as solid adsorbents for phenol removal from aqueous solutions that has been reported by Yousef et al. [17].
Fig. 5. Reduction of salinity as a function of contact time
Fig. 6 confirmed that surface area possess significant contribution to the sorpsibility of zeolite. We used 3.5 g zeolite with variation of particle size. We can see that small particle zeolite leads to high efficiency of reduction (ð°), while bigger particle produces smaller efficiency of reduction. The ð° value of zeolites with particle size of 17.66, 51.43, 78.61 and 102.24 ðĩm are 1.7, 0.8, 0.3 and 0.1 respectively. This trend can be simplify explained as follow. Considered that shape of particles zeolite are cylindrical, so the surface area of a zeolite particle can be expressed as As = 4âžr² and the particle volume is V = (4/3)âžr³ with r refers to particle radius. If the size of particle is d = 2r, so that the surface-to-volume ratio of particle with diameter d can be roughly expressed as As /Vs = âžd² /(âžd³ / 6) = 6/d . For n particles with diameter of d, we obtained As /Vs = n6 / d . It indicates that As /Vs is proportional with the particle amount and conversely proportional with the particle size. Smaller particle possess higher As /Vs value, while bigger particle own smaller As /Vs value. In addition, due to we used similar dose (g) of zeolite, therefore small size of particle own high quantity (n) than bigger one. Hence, As /Vs highly decreases with the increase of particle size, and consequently reduce ð° of zeolite as sorbent material
Fig. 6. Efficiency of reduction as a function of zeolite particle size.
We showed that the effectiveness of desalination process can be adjusted by controlling the dose of zeolite. We used zeolite with variation dose of 1.5, 3.5, 5.0 and 7.5 g. The reduction of seawater salinity (Rs) after treated by zeolite at various dose is shown in Fig. 7. It can be seen that the value of Rs linearly increase with the increase of zeolite dose. At 1.5 g zeolite, the value of Rs is 0.4 ppt with ð° = 1.14%. At zeolite dose of 3.5, 5.0 and 7.5 g, the Rs values are 1.7, 2.3 and 3.2 ppt that are correlate with ð° value of 4.85, 6.57 and 9.14 %, respectively. This behavior can be simply explained as following. The increment of zeolite dose leads to the increase of zeolite particles amount. If each particle zeolite own x effective surface area, thus for ð° particle there will be nx effective surface area. We can see that the effective surface area is linearly proportional with the particle amounts, therefore the salinity of seawater is significantly reduced at high dose of sorbent. On the other word, efficiency reduction of desalination can be easily increased by the increase of sorbent dose. This behavior shows different trend with Kumar et al. [7] on the adsorption study of dye from aqueous solution by cashew nut shell. They found that the percent removal is increase exponentially rather than linearly with the increase of sorbent dose. Therefore, we notify that the characteristic of sorption depend on the sorbate-sorbent interaction. Besides, each sorption systems are likely possess different characteristic.
Fig. 7. Reduction of seawater salinity as a function of zeolite dose.
4. Conclusion
We have modified natural zeolite from Sukabumi, West Java, Indonesia as sorbent material for reducing salinity of seawater. The optimum condition of modification is 225 ˚C for 3 h. The highest value of Rs and ð° are 3.2 ppt and 9.14%. Zeolite with high specific surface area (m2/g) possess high sorpsibility. Effectiveness of salt reduction can be easily enhanced by the increase of zeolite dose. We found that natural zeolite from Buah Batu possess high potential as cost-effective sorbent material for seawater desalination.
Source: Edy Wibowoa,b, Sutisnaa, Mamat Rokhmata, Riri Murniatia, Khairurrijala and Mikrajuddin Abdullaha*
a Department of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology
The 10 largest coal producers and exporters in Indonesia:
- Bumi Resouces (BUMI)
- Adaro Energy (ADRO)
- Indo Tambangraya Megah (ITMG)
- Bukit Asam (PTBA)
- Baramulti Sukses Sarana (BSSR)
- Harum Energy (HRUM)
- Mitrabara Adiperdana (MBAP)
- Samindo Resources (MYOH)
- United Tractors (UNTR)
- Berau Coal
