Ion exchange represents an efficient technique for removing of heavy metals from wastewater effluents. Limited number of studies focuses on fixed bed performance for metal removal. In this study, the removal of Ni+2 from synthetic wastewater using a strong acid and strong base ion exchange resins in fixed bed columns was investigated. The experiments were performed under different pH values (3-7) and initial heavy metal concentrations (1.8-3.8 g Ni/L). Investigation of the effect of regeneration on the resin was conducted. The results of the removal efficiency and rate of removal are shown on the breakthrough curves and the kinetic study of the process was determined. The treated wastewater was examined using the atomic absorption spectrophotometer to ensure the Ni2+ concentration complies with environmental regulations limits. The Thomas model was applied to follow the exchange dynamics, the results from experimental work compared to those using Thomas model. It is shown that the calculated breakthrough curves agreed well with the measured ones.
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Keywords: Nickel ion, Ion exchange, removal, fixed bed, regeneration.
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Based on the Malaysia Environmental Quality Report 2005, 47.5% of the water pollution source is generated by industrial sector. One of the most water polluting industries in Malaysia is the metal plating industry. The discharges from metal plating industry mainly consist of many heavy metals such as copper, lead, chromium, nickel, iron and zinc . heavy metals has many disadvantages to both human and environment as it can affects human's health as well as causing environmental pollution, nickel for example can cause cancer and anosmia which is lost of ability to smell . Malaysia government has set in the environmetal standard a limit for Nickel discharge in the wastewater which is 1 mg/L . Therefore, a suitable heavy metal removal method should be used to comply with the stricter regulation limit. A number of technologies for the removal of metal ions from aqueous solutions have been developed and used in the industry. The methods include coagulation, chemical precipitation, floatation, ion exchange, adsorption and reverse osmosis [4, 5, 6]. Ion exchange technology has many advantages as it can treat a large volume of effluent at one time and more efficient in removal of ion from the wastewater . In this method, ion exchange removes unwanted ions by transferring them to a solid material namely resins. Resins will accept ions from the solutions and giving back an equivalent number of desirable species stored on the resins. In this process, cation which is Nickel (II) ion is exchanged with hydrogen ion. In addition, anion such as sulphates is exchanged with hydroxyl ion. This process can remove almost 97% of Nickel in the solutions, which is acceptable to the Malaysia standards and producing pure water that save to be discharge to the drainage [8, 9]. Ion exchange method is widely applied in many other industry sectors, including petroleum and chemical industries, water softening process, and separation and purification in the food industry . In this study, the removal of heavy metal by ion exchange was studied to identify the optimum operating parameter for the project. The interest in heavy metal removal especially Nickel (Ni2+) from industrial wastewater has grown enormously due to stricter effluent limits in Malaysia. Malaysian environment discharge standards is known as "Malaysia A" and "Malaysia B" are exist based on WHO water standards . Therefore, an efficient separation system such as ion exchange is used for Nickel (Ni2+) removal from industrial wastewater discharge. Although ion exchange method has been applied in the industry for many years, there are limited studies reported for the removal of Nickel ions (Ni2+) from industrial waste water employing ion exchange method [11,12]. Hence, an exploratory research is planned to be conducted to identify and quantify the optimum conditions for Nickel ions (Ni2+) removal. Some previous studies review for the probable mechanism of the process [13-16]. Therefore, this study aims to identify the optimum operating conditions for Nickel ions (Ni2+) removal process. For the removal of nickel from wastewater, the parameters are varied according to acquire the effects of pH and the effects of initial concentration of Nickel [17-20]. Enhancement of the removal of nickel ions (Ni2+) from wastewater by using ion exchange method is one of the objectives of this study. Specifically, this study aims to determine the effect of different operating parameters in terms of pollutant's concentration, pH, and resin dosage on the rate of nickel ion removal from the wastewater, to determine the effect of operating parameters on the performance of ion exchange resin, to investigate the effect of regeneration to the exchange capacity of the resin and to calculate and identify the kinetics of the process.
2. Model description
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Several complete, microscopic models that take into account exchange equilibrium, film diffusion, and homogeneous particle diffusion model have been used to describe the dynamics of fixed bed ion exchange processes . However, the mathematical complexity and/or the need of knowing multi-parameters from separate experiments in these models make them rather inconvenient for practical use. Hence, a simple but accurate model is highly desired. Successful design of a fixed-bed process generally requires prediction of concentration-time profiles for the effluents (i.e., breakthrough curves). The maximum exchange capacity of a resin bed is also needed. Traditionally, the Thomas model can meet these requirements [22, 23], which assumes that at a constant flow rate and no axial dispersion, the behavior matches with the Langmuir isotherm and the second-order reversible reaction kinetics. Unlike ion exchange, this assumption leads some error when the Thomas model is used to predict the adsorption-desorption process because adsorption is usually not limited by chemical reaction kinetics . In an isothermal fixed bed packed randomly with the chelating resins, this macroscopic model can be represented by [22, 23].
Where kTh is the Thomas rate constant (m3/(mol min)), qo is the maximum amount of exchange equivalent to an equilibrium aqueous-phase concentration of Co (mol/kg), u is the volumetric flow rate (m3/min), W is the amount of resin in the bed (kg), and Veff is the effluent volume (m3). Hence, Eq. (1) can be rewritten as the following linear equation:
In eq. (3), at a constant flow rate, a plot of ln((Co/C) - 1) vs. t, then the kinetic parameter kTh and fixed-bed exchange qo in the model can be calculated from the obtained slope and intercept, and parameter t can be obtained at C = Co/2. Then, breakthrough curves can be obtained and be plotted as shown in Figs. 1-3. In this study, the predicted results match with the experimental data very well. For continuous flow ion exchange column, the first order reversible adsorption model such as Thomas's Model is selected as the model for kinetic study.
3.1. Material in experiments
3.1.1. Resin material
Synthetic resin was used in experimental work. Cation resin used in this experiment is Lewatit S1467, produced by Lanxess. The resin is in Na+ ionic form and was from sulphuric acid functional group. The resin is light brown, gel type beads that have cross linked polystyrene matric. Table 1 shows the physical and chemical properties of the resin material.
Physical and Chemical Properties.
3.1.2. Chemical reagents
Nickel sulphate (NiSO4.6H20) made by S&M Chemical Corporation used to prepare the synthetic wastewater. In order to regenerate cation resin, Sulphuric Acid (H2SO4) manufactured by Merck chemical was used. Meanwhile, Sodium Hydroxide (NaOH) manufactured by Merck chemical was employed to regenerate the anion resins.
3.2. Experimental studies
The experiments were conducted in continuous mode using synthesized waste water which was prepared initially, nickel sulfate (NiSO4.6H2O) was dissolved in distilled water to obtain the required nickel concentration. The experiment work was divided into two main parts which are study of removal of Nickel ion from wastewater solution based on different parameters and study of sequence mode of the ion exchange. All experiments were conducted at similar temperature and flow rate, which are at 25oC and 35 cm3/min. The diameter of the column used for the experiments is 2 cm. The pH of solution is manipulated for pH=3, pH=5 and pH=10.The pH is adjusted accordingly using 5% NaOH and 97% H2SO4. For the second parameter, which is Nickel concentration, the experiments are carried out by manipulating the initial concentration at 1.8 g Ni/L, 2.8 g Ni/L and 3.8 g Ni/L. In the second part of the study, the effectiveness of the resin regeneration was studied. The experiments were conducted by using 1.8 g Ni/L initial nickel (II) concentration and the wastewater was allowed to flow through the column for around one hour until the conductivity of the effluent similar with the conductivity of the influent. The parameter of the effluent was tested by using pH meter, and absorption atomic spectrophotometer (AAS).
4. Results and discussion
4.1. Effect of initial concentration
Nickel ion concentration is an important parameter affecting the ion exchange process. In order to study the effect, three different initial concentrations were used. The experiments were conducted at 1.8 g Ni/L, 2.8 g Ni/L and 3.8 g Ni/L of Nickel (II) Sulphate Solution. In order to minimize the effect of other parameters to the removal of Nickel (II) ion, the wastewater solution is prepared at pH=5.6 and flows through the column at constant flow rate 35 cm3/min.
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Fig. 1. Breakthrough curve of Ni (II) through the resins column at 1.8 g Ni/L.
Fig. 2. Breakthrough curve of Ni (II) through the resin column at 2.8 g Ni/L.
Fig. 3. Breakthrough Curve of Ni (II) through the resin column at 3.8 g Ni/L.
Breakthrough time for different initial concentration
Breakthrough curves at different initial nickel concentration (Co) are shown in Figures above. Breakthrough time in this experiment is defined as time taken for the 1ppm of Nickel ion concentration to be detected at the outlet stream. After the breakthrough time exceeded, the resin cannot be used and regeneration is needed in order to put the resin in service cycle again. At the chosen breakpoint concentration of Ce =1ppm, the time taken for breakthrough decreases with an increase in Co values. Figure 1 shows that the breakthrough time for initial concentration at 1.8 g Ni/L is at 60 min, whereas Figure 3 shows the breakthrough time for initial concentration at 3.8 g Ni/L is at 15 min. This situation due to the fact that for a given flow rate and quantity of resin, the exchange sites of the resin are exhausted earlier when a higher initial nickel concentration influent is encountered. Therefore, the operation period until the breakthrough point is less at the higher initial concentration, Table 2 shows the breakthrough time for different initial concentration. . Based on A.H. Norzilah et. al, the mass transfer between the ion in the wastewater solution and ion attached in the resin will increases as the initial concentration of the solution increasing [24-28]. The higher concentration gradient is the driving force for the mass transfer in the column. Thus, the ion in the solution will exchange faster with ion that attached in the resin and this situation will lead the resin exhausted fast. A comparable result is achieved by L.Lv et. al for their study on the removal of Pb2+ by microporous titanosilicate ETS-10 in a fixed-bed column, the trend for the breakthrough curve is similar. They concluded that a rise in the feed concentration reduced the volume treated and the breakthrough point for the system . Figure 4, shows the breakthrough curve calculation Thomas's Model.
Fig. 4. Graph for determination of Thomas parameters and exchange capacity for different inlet concentration.
Properties of Ion Exchange for Different Initial Concentration
Figure 4. Table 3 clearly show that as the initial concentration increases from 1.8 g Ni/L to 3.8 g Ni/L, the exchange capacity increases from 0.0454 mol/kg to 0.0464 mol/kg. Besides, from the Thomas Model, it can be concluded that the exchange capacity of the resin is inversely proportional with kTh. The differences between experimental values and calculated value for Thomas's Model are small (below 5%). The colour of the wastewater has significant changes from before treatment and after treatment. The typical wastewater from industry which contains Nickel (II) ions is usually in green colour. After the treatment, the colour for the effluent changed to colourless and it has pH=7 which is complying with the Malaysia's environmental regulations. Besides, from table 3, it clearly stated that the removal of nickel ion is around 97% for each experiment. Thus, ion exchange appeared as the best technique for heavy metal removal.
4.2. Effect of pH
The effect of pH on nickel removal is conducted with resin usage 31.41 cm3 at room temperature, and initial nickel concentration is 1.8 g Ni/L. Sulphuric Acid (H2S04) was used to reduced the pH to 3 and Sodium Hydroxide (NaOH) was used to regulate the pH to be 7. The range of the pH is below pH 8 to avoid chemical precipitation, Kumar S. , reported that the participation at high pH values will affected the exchange capacity due to domination of the chemical precipitation. Figures 5, 6 show the breakthrough curve for Ni (II) flows through the bed at pH 3, pH 7 respectively.
Fig. 5. Breakthrough Curve of Ni (II) through the resin column at pH=3.
Fig. 6. Breakthrough Curve of Ni (II) through the resin column at pH=7.
Breakthrough time for different pH
Results in table 4, shows that the breakthrough time is highly dependent with the pH of the solution. Figure 6 demonstrates that the higher the pH, the longer the breakthrough time but it is only applicable in range between 3 to 7. At pH= 3, the amount of hydrogen ion present in the mixture is higher. Thus, the Nickel ion needs to compete with the hydrogen ion during ion exchange process. Kumar addressed that the excessive protonation of the ions at the exchange surface will refuses the formation of links between Ni2+ ion and the active ion in the resin . For higher pH which is pH 7, the breakthrough time is the longest because of the tendency of ion exchange process to exchange ion with ion that have greater affinity makes Sodium ion (Na+) is more favourable ion in the process. Thus, moderate pH value (pH=5) is proposed to optimize the Nickel (II) removal.
Fig. 7. Graph for determination of Thomas parameters and exchange capacity for different pH.
Properties of Ion Exchange for Different pH
The value for exchange capacity , qo and Thomas coefficient is determined based on graph ln((Co/C)-1 versus time as illustrated in the Figure 7. Table 5 clearly shows that as the pH increasing from 3 to 7, the exchange capacity is increase from 0.02407 mol/kg to 0.0823 mol/kg.
4.3 Effect of Regeneration
For removal of Nickel (II) ion from waste water in plant, the regeneration of exhausted ion exchange resin is important in order to use the resin back into the service cycle. The repeated usage of resin can minimize the cost of the operation. The regeneration of Lewatit S1467 resin is accomplished by using 5% Sulphuric Acid as proposed by the manufacturer. The H+ in the acid solution will exchanged with the Nickel (II) Ni2+ in the immobile particle due to the selectivity. For this project, the Sulphuric acid is allowed to flows into the column for about 1 hour with flow rate of 35 cm3/min. Figure 8 , shows the effect of regeneration to the resin. The breakthrough time for virgin resin is 60 minutes and the time for regenerated resin to breakthrough is 50 minutes. The result indicated that the regeneration resin become exhausted first before virgin resin because of the presence of other ions in the unregenerate resin. By using Thomas Model, the exchange capacity for the resin is predicted. Figure 9, shows the determination of Thomas parameters and exchange capacity for virgin resin and regeneration resin.
Fig. 8. Breakthrough curve of virgin resin and regenerated resin for 1.8 g Ni/L.
Fig. 9. Graph for determination of Thomas parameters and exchange capacity for virgin resin and regeneration resin.
Properties of ion Exchange for regenerated resin
The test result indicated that the ion exchange capacities of both virgin and regenerated resins are almost similar with 4% percentages different. Hence, the regenerated resin can be used back in the process cycle without affecting on the capabilities of ion exchange resin; table 6 shows the properties of ion exchange for regenerated resin
The percentage removal of Nickel (II) ion from the solution is around 97 %. The initial concentration affected the breakthrough time of the system but the rate of removal still at the same level. The breakthrough time is decreasing with increasing of initial nickel (II) concentration, besides, the exchange capacity of the resin is increasing as the initial concentration keep on increasing. It is favourable to keep initial concentration at lower level as it will delay the need for regeneration, which reduces the cost of the operation. The effluent after treatment is colourless and has pH=7 which is complying with the regulation. The optimum pH for the removal is in the range pH 5 to 6. The effect of the resin regeneration is studied and it can be concluded that regeneration did not affect the exchange capacity; life time of the resin can be extended. Thomas Model was used to follow the fixed be dynamics, the calculated breakthrough curves agreed well with the measured results. The contact time of 1 hr can help to maintain around zero Ni (II) outlet concentration.