Removal of zinc ions from aqueous solutions by sorptive-flotation using limestone as a lowcost sorbent and oleic acid as a surfactant

S.E. Ghazy^† and A.H. Ragab^†

^† Chemistry Department, Faculty of Science, Mansoura University, P.O. Box 66,
Mansoura, Egypt. ghazyse@mans.edu.eg; shabaneghazy@yahoo.com
^† New Mansoura Water Treatment Plant, Mansoura City, Egypt. Fidodido_ahr@yahoo.com

Abstract - Environmental pollution, mainly in the aquatic systems, due to developments in industry, is one of the most significant problems of this century. Many industrial wastewater streams (ca. the metal working, semiconductor, and copper industries, mine water, etc.) contain heavy metals, which are of great environmental concern and must be removed prior to water discharge or water recycling. The present study aims to develop a simple, rapid and economic procedure for Zn²⁺ ions removal under the optimum conditions. It is based on the sorption of Zn²⁺ ions from aqueous solutions onto limestone fines (LS), which is an inexpensive and widespread over the globe, followed by flotation with oleic acid (HOL) surfactant. The different parameters (namely: solution pH, sorbent, surfactant and zinc concentrations, shaking times, ionic strength, temperature and the presence of foreign ions) influencing the sorptive-flotation process were examined. About 100 % of Zn²⁺ ions were removed from aqueous solutions at pH 7 after shaking for 5 min and at room temperature (~25°C). The procedure was successfully applied to recover almost Zn²⁺ ions spiked to some natural water samples. A mechanism for sorption -flotation is suggested.

Keywords - Zinc; Sorptive-Flotation; Limestone; Low-Cost Sorbent; Oleic Acid.

I. INTRODUCTION

Mobilization of heavy metals in the environment due to industrial activities is of serious concern due to their toxicity for humans and other life organisms. Removal of toxic heavy metals from industrial waste waters is essential to control environmental pollution (Puranike and Pakniker, 1999; Guangy and Thiruvenkatachari, 2003). At least 20 metals are classified as toxic and half of these are emitted into the environment in a quantity that poses risks to human health (Nasir et al., 2007). The ability of water body to support aquatic life as well as its suitability for other uses, however, depends on trace elements.

Trace concentration of zinc are important for the physiological functions of living tissues and regulate many other biochemical processes. However, just like other heavy metals, when Zn is discharged into natural waters at increased concentration in sewage, industrial waste water or from mining operations it can have severe toxicological effects on humans and aquatic ecosystem (Kortonekamp et al., 1966).

The free zinc ion is a powerful Lewis acid up to the point of being corrosive. Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. Hence, it is essential to remove Zn from industrial waste waters before transport and cycling into the natural environment.

Many technologies that could eliminate and/or reduce the presence of heavy metals in industrial effluents have been developed. These include precipitation and co-agulation, cementation, membrane separation, solvent extraction, ion-exchange, adsorption and bio-sorption (Palterson, 1989; Ghazy et al., 2005). Flotation as a solid/liquid or liquid/liquid (or both) separation process has recently received a considerable interest owing to: simplicity, rapidity, economic, good separation yields (R > 95%) for small impurity concentrations (10^-6-10^-2 mol/L), a large possibility of application for species having different nature and structure, flexibility and friability of equipment and processing for recovery purpose (Stoica et al., 1998; Ghazy et al., 2003). It is believed that this process will be soon incorporated as a clean technology to treat water and wastewater (Rubio et al., 2002). For the aforementioned reasons a combination of adsorption and flotation into unified operation termed sportive-flotation could be considered as a vital process.

In recent years there has been a considerable interest in the development of new products which are abundant in nature, low in cost and have minimal environmental impact for restoration or remediation of natural resources (Gomez del-Rio et al., 2004). Limestone (LS), which is produced in large quantities in many countries (among them is Egypt), is a low-cost reactive medium that can be used for the subsequent clean up of industrial effluents, leachates and contaminated ground water (Gomez del-Rio et al., 2004; Komnitsas et al., 2004; Bailey et al., 1999). Hence, the present work aims to establish a rapid and simple sorptive-flotation procedure using LS (which is naturally occurring or readily available and alterative to existing commercial adsorbents) as a sorbent and oleic acid as a surfactant for removing zinc from aqueous solution and natural waters under the recommended conditions.

II. MATERIALS AND METHODS

A. Samples and Reagents Employed

The Limestone (LS), CaCO3, samples used in this study were obtained from the Al-Mokattam area in Cairo (Egypt) where some private and governorate quarries are located. The samples were crushed and pulverized in the laboratory and those with a mean size of ca. 12.5 µm were used in the experiments. The sample contained 92% calcite (CaCO3) and 3% dolomite (MgCO3), as found by chemical analysis (Ghazy et al., 2001), with the reminder being composed of common minor constituents such as silica, clays, feldspar, pyrite, and sedrite (Bates and Jackson, 1980). The samples were dried for 2 h in an oven at 125°C, packed into stopper bottles and stored in desiccators for future use. Functional groups of LS were characterized through infrared analysis. The LS spectrum coincided with pure CaCO3. The surface area and porosity of LS was measured using Brunauer, Emmett, Teller (BET) method. LS presented no BET porosity and its measured surface area was 3.88m²/g. The pH values of points of zero charge (pH PZC) were 9.1 (not aged), 6.2 (aged 60 min) and 8.3 (aged several days) and this agreed with the previously reported data (Somasundran and Ager, 1967).

All the solutions were prepared from certified reagent grad chemicals. A zinc sulfate heptahydrate (ZnSO₄.7H₂O) stock solution was prepared and the working solutions were made by diluting the former with doubly distilled water. An oleic acid (HOL) stock solution, 6.36×10^-2 mol/L, was prepared from food grade with sp. gr. 0.895 (provided by J.T. Baker Chemical Co.) by dispersing 20 mL of HOL in 1 L kerosene. Aqueous solutions of HNO₃ and NaOH were used for pH adjustments.

B. Apparatus

C. Procedure

To study the various parameters affecting the sorptive-flotation process, a 20 mL aliquot of a suspension containing 6.5 mg/L Zn²⁺ ions and 1000 mg/L sorbent (LS) of initial pH 5 was introduced into the flotation cell. The cell was shaken for 5 min (optimized time) to ensure complete adsorption of Zn²⁺ ions by LS. Then 3 mL of 1×10^-3 mol/L HOL was added. The cell was again inverted 20 times by hand and allowed to stand for 5 min to complete flotation.

The residual Zn²⁺ ions concentration in the mother liquor was analyzed using a Perkin-Elmer 2380 atomic absorption spectrophotometer at a wavelength of 213.9 nm. The floatability percentage of Zn²⁺ ions (%F) was calculated from the relationship:

%F = (C_i-C_r) / C_i × 100, (1)

where C_i and C_r denote the initial and residual Zn²⁺ ions concentrations.

To study the flotation of LS alone, the previous procedural steps were conducted in the absence of Zn²⁺ ions. After complete flotation, the LS-containing float was filtered through a G5 sintered-glass filter (porosity, 1-1.5 µm) and dried to constant weight in an oven at 125°C. The floatability percentage of LS (%F), was calculated from:

%F=C_f/C_i×100, (2)

where C_i and C_f denote the initial and float concentrations of the LS sorbent. All experiments were carried out at room temperature (~ 25 °C).

To asses the applicability of the procedure, another series of experiments were conducted on 1 L suspension of natural water samples (placed in a 2 L glass beaker) containing 5.0 or 8.0 mg of Zn²⁺ ions, 1000 mg/L of LS and 10 ml of 1×10^-3 mol/L of HOL with an initial pH of 7. The suspension was stirred magnetically for 10 min at 250 rpm where it was noted that the Zn-LS-HOL system was self-floatable without a stream of air bubbles being necessary.

A. Floatability of Powdered Limestone

A preliminary series of experiments (using a constant concentration of HOL equal to 1×10^-3 mol/L at pH 7) was undertaken in order to float various concentrations of LS. The results obtained are depicted in Fig. 1, from which it is clear that a maximum flotation (~100 %) for LS was attained over a wide concentration range up to 4000 mg/L. Hence, the concentration of powdered limestone favorable for Zn²⁺ ions removal was presumed to be 1000 mg/L.

Figure 1. Floatability of different concentrations of LS using 1× 10^-3 mol/L HOL at pH 7.

Another series of experiments were conducted to study the effect of pH on the flotation of 1000 mg/L of LS using 1×10^-3 mol/L HOL. The results are depicted in Fig. 2 and show that maximum flotation of LS (~100 %) was attained over the pH range 4-10. It should also be noted that at pH values less than 2, the process was not effective due to the partial dissolution of the LS sorbent. For this reason, a pH value of 7 was employed in all subsequent measurements.

Figure 2. Floatability of 1000 mg/L of LS at different pH values using 1×10^-3 mol/L HOL

Figure 3. Floatability of 1000 mg/Lof LS at different concentrations of HOL using different pH values

B. Floatability of Zn²⁺ Ions .

Effect of pH
Generally, adsorption and flotation are sensitive to variations in the pH of the medium and knowledge of how the pH value affects each system was a prerequisite to study sorptive-flotation method (Ghazy et al., 2001). From the speciation diagram reported by Bradl (2004), using the reactions and equilibrium constants, it was concluded that at acidic pH < 7, the predominant ionic species are Zn²⁺ ions, whereas, zinc is present mainly as Zn(OH)⁺ and Zn(OH)₂ near pH 9. By increasing the pH more than 9, the negative species Zn(OH)₃^- and Zn(OH)₄^2- appear (Baes and Messmer, 1976)..

The influence of pH on the combined process (sorption and flotation) for the removal of 6.5 mg/L of Zn²⁺ ions was investigated in the presence of 1000 mg/L of LS using 1×10^-3 mol/L of HOL with or without shaking for 5 min (Fig. 4). The pH of solutions was adjusted with HNO₃. At low pH values less than 3, the floatability is small due to the solubility of the LS (consists mainly from CaCO₃ and MgCO₃) at acidic medium; thereby hindering the sorption of Zn²⁺ ions. Around pH 5, the floatability increases to a maximum (~99 %) but with shaking for 5 min only, the flotation increase to about ~100 %, which maybe due to a good aggregation of the metal ions on the LS surface according to ion-exchange mechanism between Zn²⁺ ions and calcium containing LS in a similar manner to that reported (Mandjiny et al., 1995). Moreover, Zn²⁺ ions may be adsorbed electrostatically with the negatively charged surface of limestone (Ghazy et al., 2001). Therefore, pH 7 was recommended throughout all experiments.

]]> Figure 4. Floatability of 6.5 mg/L of Zn²⁺ ions at different pH values with or without shaking for 5 min using 1000 mg/L of LS and 1×10^-3 mol/L of HOL

Effect of Surfactant Concentration
In order to find the suitable concentration of HOL for removing 6.5 mg/L of Zn²⁺ ions from aqueous solutions, a series of experiments were conducted at pH 7 in the presence of 1000 mg/L LS using different concentrations of HOL. The results obtained are presented in Fig. 5. It can be seen that the removal of Zn²⁺ ions attained ~95%. An enhancement in the flotation efficiency of Zn²⁺ ions to ~ 100% was achieved after shaking for 5 min. These maximal values were attained over the HOL concentration range of 5.0 ×10^-4-2 ×10^-2 mol/L.

Figure 5. Floatability of 6.5 mg/L Zn2⁺ ions using different concentrations of HOL at pH 7 with and without shaking

Higher concentrations of the surfactant impaired flotation. This impairment has been discussed by some workers (Ghazy et al., 2001), who concluded that poor flotation at high surfactant concentration was caused by the formation of stable, hydrated envelope of surfactant on air bubbles surface or, perhaps, by the formation of a hydrated micelle coating on the solid surface. As a result, the hydrophobocity of the resulting surface was not satisfactory for flotation. Consequently, the concentration of HOL employed was fixed at 1×10^-3 mol/L throughout all other studies.

Effect of Sorbent and Metal Ion Concentrations
Two parallel series of experiments were conducted to study the influence of powdered limestone concentration (Fig. 6) and changing the zinc ion concentration (Fig.7) on the removal percentage of Zn²⁺ ions from aqueous solutions at pH 7 using 1×10^-3 mol/L of HOL without or with shaking for 5 min. As can be seen from Fig. 6, the removal percentage of Zn²⁺ ions increased as the LS dose increased, while it decreased as the concentration of the metal ion increased Fig. 7. The reason for attaining a maximum removal of Zn²⁺ ions (~ 100 %) at higher LS dose, after shaking for 5 min may be due to an increase in the number of binding sites on the sorbent available to Zn²⁺ ions. Hence, 1000 mg/L of LS may be a suitable dose for the removal of Zn²⁺ ions having a concentration of 6 mg/L after shaking for 5 min.

Figure 7. Floatability of different concentrations of Zn⁺² ions at pH 7 using 1000 mg/L of LS and 1×10^-3 mol/L of HOL

Effect of Temperature
Studies of the influence of temperature on the flotation efficiency of Zn²⁺ ions seemed to be important from a practical viewpoint, especially in the case of hot industrial effluents. For such studies, one solution containing 6.5 mg/L Zn²⁺ ions and 1000 mg/L of LS and a second solution containing 1×10^-3 mol/L of HOL were either heated or cooled to the same temperature using a water bath. The surfactant solution was quickly poured onto the Zn²⁺ ions solution contained within a flotation cell jacketed with 1-cm thick fiberglass insulation. The mixture was then floated using the previously described procedure. The results obtained (Table 1) indicated that the percentage removal of Zn²⁺ ions (~ 100%) was not markedly affected by raising the temperature from 3 to 80° C after shaking for 5 min. Therefore, the simple procedure presented here may find application in the removal of Zn²⁺ ions from hot industrial wastewater.

Table 1. Effect of temperatures on the floatability (%) of 6.5 mg/L of Zn⁺² ions at pH 7 using 1000 mg/L of LS and 1×10^-3 mol/L of HOL after 5 min shaking

Effect of Foreign Ions
Under the optimized conditions determined as above, the percentage removal of 6.5 mg/L of Zn²⁺ ions from a solution of pH 5 containing 1000 mg/L of LS and 1×10^-3 mol/L of HOL was studied (with shaking for 5 min) in the presence of high concentrations of various cations and anions, usually found in some natural water samples. All cations were used as their nitrates whereas the anions were used as their sodium or potassium salts. The tolerable amounts of each ion giving a maximum error of ± 2% in the flotation efficiency are summarized in Table 2. An inspection of the data indicates that the floatability of Zn²⁺ ions was quantitative (~ 100 %) in most cases. Moreover, shaking for 5 min enhances the process. However, it should be noted that higher concentrations of Al³⁺, Cr³⁺, Cu²⁺ or Cd²⁺ could have harmful effect on the removal process. This may be due to competition between these cations and Zn²⁺ ions for sorption onto the active sites of LS. This problem could be overcome by increasing the amount of sorbent. Thus, the recommended procedure could be fairly employed for the removal of Zn²⁺ ions from various complex water samples.

Table 2. Effect of some selected foreign ions on the floatability (%) of 6.5 mg/L of Zn²⁺ ions at pH 5 using 1000 mg/L of LS and 1×10^-3 mol/L of HOL after 5 min shaking

IV. APPLICATION

To investigate the applicability of the recommended procedure, a series of experiments was performed to recover 5.0, 8.0 and 15.0 mg/L of Zn²⁺ ions added to aqueous and some natural water samples. The sportive-flotation experiments were carried out using 1 L of clear, filtered, uncontaminated sample solutions after adjusting their pH values to 7. The results obtained are listed in Table 3 and show that the recovery was satisfactory (~ 100 %). Moreover, the data indicated that the method could be successfully applied for the separation of Zn²⁺ ions from natural water samples containing large amounts of salt matrix under the recommended conditions.

V. CONCLUSION

Limestone (LS) was used in this study as a good sorbent for the heavy meals as zinc. Flotation technique which has advantages (compared with other separation methods) of being simple, inexpensive, less time consuming and expected to be soon incorporated as a clean technology to treat water and wastewater has been applied in this investigation. The removal of Zn²⁺ ions at pH 7, attained ~100% in the presence of LS and oleic acid as a cheap surfactant. Moreover, the recommended procedure was successfully applied to some natural water samples and was nearly free from interferences of some selected foreign ions. Moreover, the zinc ions were essentially held up by LS sorbent and would not leached out by acids owing to the solubility of the sorbent. Therefore, the metal-loaded solid waste could be solidified to an environmentally safe form used in building constructions, thereby serving the double-fold aim of water treatment and solid waste disposal.

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Received: August 2, 2009.
Accepted: April 28, 2010.
Recommended by Subject Editor Ana L. Cukierman.