Chemical compositions of natural zeolites, porcelanite (opal-CT) and local sands were determined by X-ray fluorescence (XRF) and correlated with their Pb(II) removal efficiencies. Zeolites and porcelanite were from the Mikawer, Aritain and Hannon areas in Jordan. Sands (white, red and yellow) were from the United Arab Emirates (UAE). The effect of Pb(II) concentration and zeolite dosage on removal efficiency was investigated at 25.0°C using the batch equilibrium method. Commercial kaolinite, silica and alumina were also studied for comparison. Removal efficiencies, in mg Pb(II)/g adsorbent, were: 76.9, 52.7 and 42.1 for Hannon, Mikawer and Aritain zeolites, respectively; 58.2 for porcelanite; 29.7, 11.0 and 8.5 for yellow, red and white sand, respectively; 7.2, 3.3 and 1.3 for kaolinite, silica and alumina, respectively. XRF data indicate that adsorbents with intermediate molar ratios of Si/Al, in the range 2.70 - 2.93, are most efficient in Pb(II) removal. Scanning electron microscope (SEM) images of adsorbents suggest that morphology, in addition to chemical composition, plays a key role. In particular, a combination of factors, including shapes and sizes of crystals, channels in zeolites and pores in porcelanite, appear to favor removal of Pb(II).
Pb(II) in the environment has a high level of toxicity and poses a serious hazard to human and animal health. Effluents from battery and electronics factories augment the concentration of Pb(II) in reservoirs and wastewaters. Column filtration and batch equilibrium by low cost adsorbents are the most common methods used in removing Pb(II) and other toxic heavy metals from wastewaters. UAE sands were shown to be efficient adsorbents for the removal and recovery of Pb(II) and other heavy metals from aqueous solutions [
Natural zeolites are considered low cost resources. They are crystalline hydrated aluminosilicates with a framework structure containing pores occupied by water, alkali and alkaline earth cations. Their tetrahedral sieve-like structure carries a negative charge due to replacement of some of aluminum by silicon. The negative charge is balanced by cations, such as sodium, calcium, potassium, or magnesium. Zeolites can lose most of their loosely bound water molecules without affecting the integrity of their molecular structure. They have high cation exchange capacity and display good selectivity for heavy metal cations that enable them to be used for the purification of industrial wastewater. Volcaniclastic rocks in Jordan are developed from cinder stratovolcanoes and are characterized by the presence of rich zeolitized beds. The zeolitization of these rocks is a function of high porosity, permeability and arid climate [
White, yellow and red sand samples were collected from several locations in the UAE. White sand is most commonly found close to the seashore (composed mainly of rhombic calcite) whereas yellow and red sands are in the inland desert. The yellow and red colors of the particles are related to the presence of iron oxides coating. Mikawer, Aritain and Hannon zeolites were obtained from areas with the same names in Jordan. Porcelanite was obtained from the Azraq area in Jordan. Kaolin, a hydrated aluminum silicate, was lot #120M0110V, from Sigma Aldrich (USA). Alumina was from Riedel-de-Haen (Germany) and silica was from Sharlau (Spain). Zeolites were pulverized to a fine powder. Sand and zeolite samples were washed repeatedly with deionized distilled water, dried to a constant weight at 110˚C then passed through a 300 μm sieve.
All primary chemicals used were of analytical reagent grade. Pb(NO3)2, NaOH and HNO3 were from Panreac (Spain).
Pb(II) concentrations were determined using a Varian Liberty axial sequential inductively coupled plasma- atomic emission spectrometer “ICP-AES” (Australia). pH was measured on a 550A Thermo Orion pH meter (USA) fitted with a combined glass electrode. Solutions were shaken at 25.0˚C using an Edmund Buhler KS-15/ TH-15 shaker (Germany). Sand and zeolite samples were sieved using impact test sieves from Standard Sieve (USA), mesh model BS410, 1986 ST. Three brass frames with sieve sizes 300, 150 and 75 µm were employed. Fraction selected for experiments were less than 300 µm in diameter.
XRF images and spectra were obtained using a Horiba, XGT-7200V X-ray analytical microscope with silicon drift detector (SDD). Intensity measurements of compressed powdered samples were conducted using a tube voltage of 50 kV and an automatic tube current of 1 mA max with acquisition time of 60 s. Detected elements were Al, Si, K, Ca, Ti, Sr, S, Cl, Mg, Cr, V, Mn, Br, and Fe. Adsorbent images were taken using a charge- coupled device (CCD) as the X-ray detector. The CCD had a 100-fold magnifying scale.
SEM micrographs on sands were obtained using a high resolution TESCAN Vega 3-LMU, equipped with a 4-lens electron optics system and has a magnification range 2.5 - 1,000,000. Micrographs on platinum coated samples of zeolites and porcelanite were obtained using an FEI-INSPECT-F50, equipped with high resolution Schottky field emission (FEG), with magnification up to nano size.
Batch equilibration studies on sand were performed using 0.100 dm3 Pb(II) solutions. Initial Pb(II) concentrations were 300 ppm for all adsorbents. Additional removal experiments were carried out using 30 ppm solutions on alumina and silica, and 1000 ppm solutions on zeolites. Runs were conducted in triplicates. A known mass of adsorbent was added to Pb(II) solutions, the pH adjusted to the optimal removal value of 4.0 using 1.0 mol dm−3 HNO3, then equilibrated at 25.0˚C for 2 h at 200 rpm. Final Pb(II) concentrations were determined by ICP. Prolonging the contact time between adsorbent and solution beyond 2 h did not result in further removal of Pb(II).
The dependence of Pb(II) removal efficiency for 300 ppm solutions on adsorbent dosage is shown in
White and red sands are also capable of completely removing Pb(II), but only at significantly higher adsorbent dosages (2). For kaolinite, silica and alumina, 100% removal efficiencies could only be attained with low (30 ppm) Pb(II) concentrations and at adsorbent dosages higher than 3.0 g/0.100 dm3.
High removal efficiencies with zeolites was observed even for 1000 ppm Pb(II) solutions. At this concentration, 100% removal was attained with zeolite dosages above 2.5 g/0.100 dm3. Maximum adsorbent capacities for
Pb(II) are shown in
Zeolite tuff locations are given in
Adsorbent | mg Pb/g adsorbent |
---|---|
Hannon | 76.9 ± 2.3 |
Mikawer | 52.7 ± 1.6 |
Aritain | 42.1 ± 1.3 |
Porcelanite | 58.2 ± 1.6 |
Yellow sand | 29.7 ± 0.9 |
Red sand | 11.0 ± 0.4 |
White sand | 8.5 ± 0.3 |
Kaolinite | 7.2 ± 0.2 |
Silica | 3.3 ± 0.1 |
Alumina | 1.3 ± 0.1 |
Locality | Latitude (E) | Longitude (N) |
---|---|---|
Mikawer | 35˚42'17" | 31˚36'06" |
Aritain | 36˚51'23" | 32˚04'44" |
Hannon | 37˚37'44" | 32˚23'15" |
UAE Sands | Zeolites | Kaolinite | |||||
---|---|---|---|---|---|---|---|
Yellow | White | Red | Hannon | Mikawer | Aritain | ||
SiO2 | 28.8 | 18.6 | 49.6 | 45.0 | 46.5 | 46.7 | 61.0 |
K2O | 0.91 | 0.81 | 0.97 | 1.01 | 1.29 | 1.45 | 0 |
CaO | 46.7 | 70.4 | 22.7 | 8.26 | 8.60 | 8.43 | 0 |
TiO2 | 0.18 | 0.12 | 0.12 | 3.06 | 3.54 | 2.55 | 1.35 |
Fe2O3 | 1.25 | 0.80 | 1.19 | 17.92 | 17.3 | 14.4 | 0.67 |
SrO | 0.20 | 1.09 | 0.05 | 0.11 | 0.14 | 0.10 | 0 |
Al2O3 | 8.36 | 5.22 | 9.00 | 13.8 | 14.6 | 14.9 | 37.0 |
MgO | 12.9 | 0 | 16.0 | 10.5 | 7.58 | 10.6 | 0 |
Si/Al | 2.93 | 3.02 | 4.68 | 2.77 | 2.70 | 2.66 | 1.40 |
ples contain phillipsite which occurs as colorless radiating aggregates in the cement and grows at the expense of the glassy matrix. Phillipsite is also found as single prismatic crystals of spherulitic texture. Chabazite occurs in Mikawer and Hannon as isolated crystals or aggregates [
Thick deposits of porcelanite (active silica) are found in northeastern Jordan (Badia region) and samples for this study were collected from the Azraq area, located within this region. Porcelanite occurs in two layers: the lower (0.6 - 1.3 m thick) is white, gray, and pink and the upper (0.4 - 0.6 m) is white-gray. The layers are separated by dark brown-black chert (5 - 15 cm) and chalk (0.6 - 1.8 m). The mineralogical composition of porcelanite is dominated by opal-CT (cristoballite-tridymite) and quartz. The SiO2 content is >90%. The high purity of the porcelanite layers and the low iron content (average 0.06%) result in a high average whiteness index (87%) [
Chemical compositions of zeolites, sands and kaolinite, as determined by XRF, are given in
SEM micrographs for the three zeolites, porcelanite and the three types of sand are presented at various resolutions in Figures 2-4, respectively. The micrographs show that sands have significantly larger grain size (lower porosity) than zeolites or porcelanite, with portions of the sand surfaces being very smooth. The lower porosity of sand in comparison with zeolites and porcelanite is related to the absence of adsorptive surfaces. Zeolites and porcelanite have a typical microporous texture that results from their fine crystal size (high surface areas) (
latter within its structure but also to form strong bonds with it.
It is worthy of note that zeolites and porcelanite are significantly more efficient removers of Pb(II) than sands and this is notwithstanding the comparable Si/Al ratios of zeolites to those for white and yellow sands. The efficiency difference suggests that morphology, in addition to chemical composition, has an important role. In particular, a combination of factors, including shapes and sizes of the crystals, channels in the zeolites and pores in porcelanite investigated appear to favor accommodation of Pb(II).
In addition to investigating and comparing Pb(II) removal efficiencies by zeolites, sands and other common adsorbents, this work is a preliminary attempt to correlate these efficiencies with chemical compositions of adsorbents. Removal efficiencies, in mg Pb(II)/g adsorbent, were: 76.9, 52.7 and 42.1 for Hannon, Mikawer and Aritain zeolites, respectively; 58.2 for porcelanite; 29.7, 11.0 and 8.5 for yellow, red and white sand, respectively; and 7.2, 3.3 and 1.3 for kaolinite, silica and alumina, respectively. The bulk molar ratio of Si/Al in an adsorbent, as obtained by XRF, appears to be an efficiency indicator for sands, zeolites and other aluminum silicates.
The high removal efficiencies of zeolites and porcelanite allow for their use in municipal and industrial wastewater treatment plants. The high adsorption capacity of zeolitic tuff and porcelanite, in comparison with sand, enables their use as filters.
This work was supported by the American University of Sharjah grant FRG12-2-10. The authors wish to thank Mr. Nasser Abdo and Mr. Thomas Job for performing the measurements.