Current World Environment

• Several amounts from Tripoli were taken (0.5, 1, 2, 5, 1 and 10 mg) with 100 ml of metal ion solution and placed in the 250 ml of the plastic bottle flask (special design).
• Two different concentrations of each metal ions (100 and 10 mg.L^-1) were considered.
• The plastic bottle flask were capped and shaken in a fixed magnetic agitation speed at 400 rpm for 2.5 hours.
• After complete the agitation time, the sample was filtered through Whatman 540 mm filter paper.
• The exact concentration of metal ions and filterable metal concentrations were determined by AAS without dilution (Perkin Elmer AAS Analyst 300, with graphite furnace HGA 800, and auto-sampler AS 72). The average standard deviation of the measurement was within 0.01 to 0.1 %.
• Qe was determined and Ce vs. Ce/Qe and ln_Ce vs. ln_Qe were plotted.

The results were expressed as the removal efficiency (% sorption) and adsorption capacity of the adsorbent:

The removal efficiency (% sorption) can be defined as



The adsorption capacity of Tripoli (mg metal/g Tripoli) was calculated by the following equation:

Where

q : metal ions adsorbed by Tripoli (mg/g)
C⁰ : initial metal concentration (mg/L)
Ce : metal concentration at equilibrium (mg/L)
S: dosage= m/V
V : volume of metal solution (L)
m : mass of Tripoli (g)

Results and Discussion
 

Figure 5 e: The Effect of pH for the adsorption of Zn2+ by variant concentration of Tripoli: remaining concentration (mg.L-1) after the adsorption vs. Tripoli dosage g/100 ml model solution), pH = n, 4, 7 and 9, (where n = the original pH value for model solution before adsorption), 2.5 hours, v = 100 ml metal solution, 25°C, initial pH = 7, rpm = 400, and constant initial concentration (10 mg.L-1) Click here to view figure

Adsorption and Analysis of Natural Tripoli

Tripoli is considers a common name for the natural rocky crystalline silica that has sub-micrometer crystals (which is formed from amorphous, often biogenic). This silica undergoes the compacting over geologic time and then produces the Microcryptocrystalline Silica (MCCS). The physico-chimical properties of Tripoli were published.⁵² This kind of silica has so fine pores that are saturated with water, unless they are dried. Also, these pores have a certain cavities with different sizes for adsorbing the suitable metal ions. On the surface, it is ending with Si-O-H, which might bind metal ions via the coordination aspects. Therefore, the heavy metals can be stabilized by these pores and then the toxicity is somewhat removed. This is due to in situ encapsulated of the metal ions inside silica cavities and pores. In this case, the metal ions remain inert in the soil and environment. Basing on these fundamental aspects, the adsorption process via Tripoli is considering the potential alternative for removing heavy metals especially for these countries of limiting resources. This is due to the several reasons, for example, it has easy handling, low cost, naturally occurring, huge quantity, and safe for the environment. In the same time, the results reveal that all metal ions exhibit strong affinity for adsorbing heavy metals on Tripoli.
 

Figure 6: Langmuir isotherm model plot of qe/Ce of Zn2+, Ni2+ and Cu2+ on tripoli vs. Ce (ppm) of Tripoli: and Cu2+: 2.5 hours, v = 50 ml metal solution, 30°C, initial pH = 7, 00 rpm, and constant initial concentration (10 ppm). Click here to view figure

Diffractograms of the treated samples (Fig. 1) showed only quartz as main mineral phase by the percentage 100% as a microcryptocrystalline (MCCS) and other forms of silica. Whereas, the montmorillonite and dolomite were destroyed. The characteristic calcite peak at 34.2 (2ε) was reduced completely. This indicates the complete removing of the impurities through the chemical treatments by acids and bases. Calcinations did not loss of the original silica. It was used due to the decomposition of last concentrations of carbonates and organic matter as well as dehydration of the structural and adsorbed water.

The geochemical data have been shown the contents of non-treated Tripoli in the percentage of greater than 92.45 wt % of MCCS (SiO₂). The specific gravity of Tripoli was found in the range of 2.4-2.6, with white creamy color.^50,52 Table 1 presents the main chemical composition of the natural Tripoli. The determination description of these parameters can be found in.⁵³
 

Figure 7: Langmuir Isotherm Model Plot of qe/Ce of Zn2+, Ni2+ and Cu2+ on Tripoli vs. Ce (ppm) of Tripoli: and Cu2+: 2.5 hours, v = 50 ml Metal Solution, 30°C, Initial pH = 7, 00 rpm, and Constant Initial Concentration (100 ppm) Click here to view figure

The removing of original metals inside the natural Tripoli can be tested using EDTA. The test was performed by adding several drops of 1.00 x 10^-3 M EDTA solution and 2 mL of NH₃/NH₄Cl buffer to an approximately 5 mL sample from the filtrate, the negative test for the presence of metal ions is indicated by appearing the blue color of EDTA. Tripoli was then washed with excessive amounts of double distilled water until the filtrate gave a negative test for chloride ion upon addition of several drops of 0.1 M AgNO₃ solution to a sample from the filtrate.

Effect of Tripoli Dosage

The effect of Tripoli dosage is discussed by using different amounts of adsorbent (viz. 0.5, 1.0, 2.0, 5.0, and 10 mg), while the other parameters (T = 25°C. initial metal concentration = 10 or 100 mg.L^-1, pH = 7, rpm = 400, and the contact time = 2.5 hours) are keeping constant (see Figs. 2 & 3). The results reveal the strong adsorbing affinity for all metal ions via Tripoli at around pH = 7. These adsorbed metal ions reach the maximum percentage at about 100 mg Tripoli × L^-1 (see Figs. 2 and 3). This adsorbed percentage increases by increasing Tripoli dosage. Figure 2 clarifies the adsorption of metal ions in the diluted level (10 mg metal ion × L^-1) vs. adsorbent dosage, where attain the maximum percentage at about 95-99 % for all adsorbed metal ions. Fig. 3 exhibits the low adsorption percentage of the metal ions in the high level concentration (100 mg metal ion × L^-1), this is just when compares it with the diluted level. In this case, the percentages have been calculated at about 60, 85, and 90 % for Zn²⁺, Cu²⁺ and Ni²⁺, respectively.
 

Figure 8: Freundlich Isotherm plot of ln(qe) vs. ln(Ce), where Ce is the Equilibrium Concentration Zn2+, Ni2+ and Cu2+ Concentration on Tripoli: 2.5 hours, v = 50 ml Metal Solution, 30°C, Initial pH = 7, 00 rpm, and Constant Initial Concentration (10 ppm) Click here to view figure

The adsorption percentages of Zn²⁺ and Cu²⁺ ions increase sharply by increasing adsorbent doses in the range of 0.5 g to 5 g (approx.) and then it slightly increases in the range of 5 g to 10 g of Tripoli, while the percentage removal of Ni²⁺ increases sharply in the whole range of 0.5 to 10 g of Tripoli. In this level, the maximum adsorption might be attained. This means, the amount of ions bounded to the adsorbent are in the maximum numbers, while the amount of the free ions remains constant for any further addition from adsorbent. Generally, the removal percentages increase by increasing the Tripoli dosage for both diluted and concentrated levels. This is due to the greater availability of the surface area.

Variation of Initial Metal Ion Concentration

The effect of the Cu²⁺, Ni²⁺ and Zn²⁺ initial concentration of vs. the adsorption percentages on Tripoli is studied by using both diluted and concentrated solutions (10 to 100 mg × L^-1), while keeping all other parameters constant (T = 25°C, pH = 7, rpm = 400, and the contact time = 2.5 hours). As clearly appear in the Fig. 4, the removal percentages increase by increasing the initial metal concentrations (10 vs. 100 mg metal ion × L^-1). Such observations are quite common,^{52, 54} while it is in opposite of using natural zeolite.²⁰ This behavior is connected with the diffusion process of the metal ions through the micro-channel and pores on the silica, where the metal ions are in competitive to diffuse through these micro-pores. This competitive will lock the inlet of channel on the surface, in the same time it prevents the metal ions from passing deeply inside the silica, i.e. the adsorption occurs on surface only. Therefore, the removal percentages increases for each case of the diluted metal ions (10 mg × L^-1). While the adsorption is concentration independent at the level of the concentrated metal ions, (above 100 mg × L^-1). This may return to the reason of metal ions ratio to the available adsorption sites are modicum. That means the competition for the adsorption sites become fierce at high level concentrations.
 

Figure 9: Freundlich Isotherm Plot of ln(qe) vs. ln(Ce), where Ce is the Equilibrium Concentration Zn2+, Ni2+ and Cu2+ Concentration on Tripoli: 2.5 hours, v = 50 ml Metal Solution, 30°C, initial pH = 7, 00 rpm, and Constant Initial Concentration (100 ppm). Click here to view figure

In general, the removal percentages are high (Approx. above 92%), where the diluted Zn²⁺ is characterized as the highest removal efficiency (Figure 4). The order of the percentage removal via Tripoli is found as Zn²⁺ > Cu²⁺ > Ni²⁺. In our suggestion, the passing path of Ni²⁺ interrupts on the Tripoli surface through the adsorption process more than Cu²⁺ and then Cu²⁺ > Zn²⁺. This behavior relates to the metal-silica coordination aspect. It is known that the silica has Si-O-Si backbone. The oxygen in the silica backbone has two lone pairs, and they are available to interact with the empty or partially empty d-orbital of the metal ion.

The electronic states of the Zn²⁺, Cu²⁺ and Ni²⁺ are d¹⁰, d⁹ and d⁸, respectively. Zn²⁺ has full orbital and diffuses easily inside Tripoli structure without any strong interruption. The passing path of Ni²⁺ slightly interrupts through the interaction of Ni²⁺ with oxygen atoms on the silica surface, i.e. locking the surface pores. This interaction may prevent other ions to move inside the pores and then increase the removal percentage. Therefore, it is expected that Ni²⁺ will be adsorbed on the surface and near the surface more than Zn²⁺. The copper has the moderate behavior on the surface between Zn²⁺ and Ni²⁺.
 

Table 1: Langmuir Constants Click here to view table

This trend was compared with treated clay (Sh2 adsorbent).⁵⁵ The order of the removal capacity in the batch adsorption via Sh2 adsorbent has recently reported as Cu²⁺>Ni²⁺>Zn²⁺ for both natural and treated clay adsorbent. In our obtained trend is found easier for the explanation more than the reported study. This is due to the chemical composition of Sh2 clay where contains several adsorbent components, and then the adsorption is carrying out on several adsorbents, for example, silica, 34% calcite and chlorite, while Tripoli has one pure quartz component. In addition, the adsorption for these metals on Sh2 has a complicated story, where depends on the techniques and conditions. For example, the uptake capacity of metals in column experiments was reported as Ni²⁺>Zn²⁺>Cu²⁺, and differed from that established in the batch uptake.⁵⁵

Effect of pH

Figure 5(a-b) shows the effect of pH vs. the sorbet (Zn²⁺, Cu²⁺, Ni²⁺) behavior in the model solutions before adsorbing. The model solution (10 and 100 mg.L^-1) was prepared in the deionised water, and their pH values were changed by the buffer solutions. After forming and filtrating supernatant, the metal ion concentrations were detected by the AAS and then the remaining concentrations were calculated. Figure 5a shows the effect pH on metal ions in case of diluted model solution (10 mg.L^-1). This figure clarifies the precipitating of each metal ion in the basic medium (pH greater than 7), which were found about 70% for the Ni²⁺ and 100% for both metals of Zn²⁺ and Cu²⁺. It was found so complicated behaviors for the same metals in the concentrated model solutions (100 mg.L^-1) in both acidic and basic medium, which is clearly shown in the Figure 5b. Basing on these results, this study denoted the adsorption on Tripoli in the neutral medium.
 

Table 2: Freundlich constants Click here to view table

The values of pH are considering one of the important parameters for the adsorption metal ions on Tripoli. The initial pH value for the solutions were varied, which was started from the original value for the model solution (pH = n) to the pH = 4, 7 and 9.2. In this group of experiments, the amount of adsorbent (0.5, 1, 2, 5 and 10 mg of Tripoli/100 ml of model solution), the concentration of adsorbate (10 or 100 mg.L^-1), contact time (2.5 hours), Temperature (25 ºC) and the agitation speed (400 rpm) are kept constant. Figures 5(c-e) show the removal of heavy metals (Cu²⁺, Ni²⁺, Zn²⁺) on Tripoli where completes at about pH = 7 for all metals. Around this value, the percentage removal increases by increasing Tripoli dosage. The adsorption actualizes the maximum percentage for the Zn²⁺, Cu²⁺ and Ni²⁺ at 10 and 5 mg Tripoli/ 100 ml model solution, respectively.
 

Table 3: Chemical composition of the natural Tripoli Click here to view table

The metal ions were not adsorbed in acidic medium (0 %) as well as they were precipitated in basic medium (at about 95%). The adsorption can not be carried out in the basic medium due to the precipitating metal ions (sorbet: Zn²⁺, Cu²⁺ and Ni²⁺) as metal hydroxides M(OH)₂ (see Figs. 5a- 5b). Furthermore, the silica backbone structure (sorbent: Tripoli) hydrolyzes by increasing the number of protons in the acidic medium. Therefore, the best pH value must be at about 7 to actualize and achieve the maximum removal, which are clearly explicit in the Figures 5(c-e). For this reason, there are no removal incase of acidic medium as appeared in theses figures. These results regarding the pH effect are matching with the reported data when applied in wastewater. Where the extent of adsorption for these metals was largely independent of pH over a wide pH range, but was significantly reduced at very low (pH< 4) and to a lesser extent at high pH (pH> 10)⁵⁶ Therefore, good adsorption efficiency via Tripoli is expected at intermediate pH values likely to be encountered with many waste waters.
 

Table 4: Adsorption potential of various sorbents Click here to view table

Isotherm Models

To quantify adsorption capacity of Tripoli for the removal of Zn²⁺, Cu²⁺ and Ni²⁺ from the solution model of wastewater, the data were fitted into the Langmuir and Freundlich adsorption isotherm models.^54-57 The Langmuir model supposes a monolayer sorption with a homogeneous distribution of sorption sites and sorption energies, without interactions between the sorbet molecules. It can be represented in a linear form as follows 

Whereq_e is the equilibrium amount of metal ions sorbet per unit mass of Tripoli (mg/g), q_max is the maximum metal uptake per unit mass of Tripoli (mg/g), C_e is the equilibrium concentration of metal ions in solution (mg/dm³) and b is Langmuir constant (dm³/mol). b is related to energy of sorption which reflects quantitatively the affinity between the Tripoli and metal ions.

Therefore, a plot of C _e q _e versusC_e , gives a straight line of slope 1q _max and intercept1( q _max b ) as shown in Figs (6 and 7) for 10 and 100 ppm initial solution, respectively for each metal ions of Cu, Zn, and Ni.

The very useful relationship between standard free energy change, G ⁰ , and the Langmuir constant, b, is given by the following equation.^{43, 57}

G ⁰ = RT ln b

where R is universal gas constant (8.314 J/mol K), and T is the absolute temperature in K. The values of standard free energy change are listed in the Tables 2 and 3 for the three heavy metals at 10 ppm and 100 ppm. The negative value of _G⁰ confirms the feasibility of the process and the spontaneous nature of adsorption with a high preference for metal ions (Zn²⁺, Cu²⁺ and Ni²⁺, respectively) to adsorb onto Tripoli.

In contrast to the Langmuir model, the Freundlich isotherm is more widely used but it provides no information on the monolayer adsorption capacity. The linear form of this model is given as follow:



Where K _f and n are Freundlich constants, indicating the adsorption capacity and the adsorption intensity respectively. Theses values of and n are, respectively, determined from the intercept and slope of plotting versus.

The linear form of Freundlich adsorption isotherms of each metal ion (Zn²⁺, Cu²⁺ and Ni²⁺) at 10 ppm and 100 ppm are shown in the Figs. 8 and 9.

The goodness fit of the experimental data is measured by the regression coefficients, R² as shown in the Tables 2 and 3. These values indicate to the applicability of the Langmiur model to Ni⁺² and the applicability of Freundlich to Zn²⁺ and Cu²⁺ ions.

The Langmuir constant q_max (maximum adsorption capacity) and the Freundlich constant K_f were obtained from the linear equations. The values are summarized in the Tables 2 and 3.

It can be observed that the maximum adsorption capacity of Tripoli is the highest for Zn²⁺ followed by Cu²⁺ and Ni²⁺ for both initial concentrations 10 ppm and 100 ppm. The Freundlich constant K_f for Zn²⁺ is greater than the other heavy metals for initial concentration of 10 ppm and 100 ppm. The values of n for all three heavy metals were obtained as greater than one, which indicates good adsorption of these metals onto Tripoli.

Table 4 summarized the various adsorbent that used for adsorbing of Zn²⁺, Cu²⁺ and Ni²⁺. The comparison has been made in terms of (mg/g). Although a direct comparison of Tripoli with other reported adsorbents is difficult due to varying experimental conditions employed in those studies, but in general the adsorption capacity of Tripoli is considerably higher than many of those adsorbents.

Conclusion

Tripoli can be used for the removal of valuable and toxic Cu²⁺, Ni²⁺ and Zn²⁺ as single phase metal ions from the aqueous medium, which could be used as novel inorganic adsorbent material in the poor countries. This is due to the several reasons, for example, it has easy handling, low cost, naturally occurring, huge quantity, and safe for the environment. In the same time, the results reveal that all metal ions exhibit strong affinity for adsorbing via Tripoli, where attain the maximum percentage at about 95-99 % for all adsorbed metal ions. The suitable adsorption conditions to actualize the maximum sorption must be at about pH = 7, at about 50 to 100 mg Tripoli ×L^-1 and using the diluted solution. The calculated values by Langmuir and Freundlich models confirms the experimental data, which has the spontaneous nature as Zn²⁺>Cu²⁺>Ni²⁺. This is in consistence with Langmuir and Freundlich models that gives the same order. The equilibrium adsorption could be described by Langmuir for Ni⁺² and by Freundlich adsorption isotherms for Zn⁺² and Cu⁺² ions. The Tripoli adsorption capacities of Zn⁺², Cu⁺² and Ni⁺² ions are 14.66, 4.46 and 2.33 mg g^”1 respectively. This study is based on a simple component with single metal ions; the competition among ions should be including in future studies in order to simulate waters in the environment.

Acknowledgements

The XRD analysis was done at the Physics Department/ Mutah University (Jordan). The authors wish to thank both chemists of Samah Ramadneh and Nancy Okkeh (Department of chemical Science/ Mutah University) for their experimental contributions.

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