Mutual Adsorption of Lead and Phosphorus onto Selected Soil Clay Minerals

Mutual adsorption of lead (Pb) and phosphorus (P) at pH 5 onto three soil clays materials (kaolinite, montmorillonite, and allophane) was studied to know interaction of the anion and the cation at surface of the clays. Adsorption of Pb was determined on montmorillonite, kaolinite and allophane with the following pretreatments; 1) untreated clay (control), 2) phosphate treated clay (P-clay) and 3) clay pre-treated with both P and Pb (P-Pb-clay). Adsorption of P was determined on montmorillonite, kaolinite and allophane with the following pretreatments; 1) control 2) Pb treated clay (Pb-clay) and 3) P-Pb-clay. The adsorption of Pb on the untreated clays was in the order: montmorillonite > allophane > kaolinite. On allophane and kaolinite Pb adsorption was in the order P-clay > P-Pb-clay > control. For montmorillonite, the trend was: P-Pb-clay = control > P-clay. Phosphorus adsorption was in the order Pb-clay = P-Pb-clay > control for montmorillonite and kaolinite, Pb-clay > control > P-Pb-clay for allophane. The findings suggested that pre-treatment with phosphate increases Pb adsorption on kaolinite and allophane, and decrease on montmorillonite, while pretreatment with Pb increases phosphate sorption on all clays, and both Pb and P increased adsorption on montmorillonite and kaolinite and decrease on allophane.

• Allophane
• Kaolinite
• Montmorillonite
• Mutual Adsorption
• Lead and Phosphorus

1. Introduction

Heavy metals are among the major contaminants of the environment, with serious effects on animal and human health [1] . Lead is among the most toxic heavy metals, even at low concentrations, to animals and human beings. Although Pb naturally occurs at low concentrations in the earth’s crust, volcanic activity, weathering and erosion of the soil materials [2] and anthropogenic activities such as coal burning, mine tailings, metalmelting and emission from car-exhaust cause serious environmental pollution with Pb [2] [3] . Addition to this, although Pb is not toxic to plants, its accumulation in tissue of plants growing on polluted soil could have serious consequences on animal and human health. These effects depend on the availability of Pb in the soil, which is affected by adsorption and desorption of the metal on the surfaces of soil colloids.

Montmorillonite is one of the most abundant clay minerals in soils, especially those that are not highly weathered and is potential binding agent for pollutants as a result of its high specific surface area and cation exchange capacity [4] [5] . On the other hand, kaolinite is a clay mineral that is abundant in highly weathered soils like Ultisols and Oxisols, in association with oxides of iron (Fe) and aluminum (Al) [4] [5] . Allophane is a unique clay mineral that is abundant in weathered volcanic ash soils, and is a principal material of clay fraction in andisols and podzols. The wall structure of as nano-ball allophane has been proposed as aluminm-nesosilicate structure composed of curved gibbsite sheet with monomeric SiO₄ tetrahedral attached to it [6] . The surface activity of these colloids contributes in the adsorption and immobilization of contaminants in soil.

The interaction between toxic metals and clay mineral colloids is important in surface chemistry, soil science, and pollution studies [7] [8] [9] . For example, the interaction between phosphate and soil minerals affects the surface activity of the colloid and mobility of P in the soil. Hence, adsorption of heavy metals on soil minerals could be affected by the presence of phosphate ions in the soil system. Extensive studies have been conducted to investigate the possibility of using phosphate to reduce mobility and bioavailability of heavy metals in soils and clay materials [10] [11] [12] . Based on work by [13] and [14] , the pretreatment of soils with phosphate is now a widely accepted technique to remediate soils and solid waste contaminated with Pb. Recently there have been studies conducted on the adsorption of Pb on kaolinite pretreated with phosphate [15] [16] [17] . However, there is a paucity of studies on mutual adsorption on Pb and phosphate on montmorillonite and allophane, which are among the most important colloids, is soils. Therefore, the objective of this study was to investigate the effect of mutual adsorption of Pb and phosphate by montmorillonite, kaolinite and allophane.

2. Material and Methods

2.1. Clay Minerals

Montmorillonite (JCSS-3101) and kaolinite (JCSS-1101) samples used in this study were supplied by the Clay Science Society of Japan. Na-montmorillonite and Na-kaolinite were prepared by saturating the clay samples with sodium (Na). The clay samples were washed three times with 1M NaCl followed by washing with 80% methanol until Cl⁻ free and finally with acetone and air-dried. Pumice grains containing nano-ball allophane were collected from a volcanic ash soil from Kakino, Kumamoto prefecture, Japan. In order to obtain the pure nano-ball allophane, free from contaminants such as imogolite, volcanic glass, and opaline silica, only the inner portion of the pumice grains was used [18] . The fraction with less than 0.2 μm equivalent diameter was separated by centrifugation after ultrasonification at 28 kHz and dispersion at pH 10. The collected sample was flocculated by saturation NaCl solution and washed with water, then stored as suspension at pH 6 in 10 mM NaCl. The prepared allophane was subjected to X-ray diffractometry, infrared spectroscopy, and thermal analysis, and was proven to be free from contaminants. The Si/Al ratio of allophane used was determined by the acid oxalate method [19] as 0.99, and contents of the other metals except for Na were negligible. The allophane was the high Si/Al type [18] .

2.2. Preparation of Clays

Phosphate pre-treatment of the clays was done by mixing the clays (0.5 g for kaolinite and 0.1 g for both montmorillonite and allophane), with 1.0 mM NaH₂PO₄ using 10 mM NaNO₃ as a background electrolyte solution and water to reach a final volume of 100 mL. As kaolinite was expected to be less reactive than montmorillonite and allophane, higher solid to solution ratio (0.5 g: 100 mL) was used. Solution pH was maintained at pH 5 by addition of 0.1 M NaOH or 0.1 M HNO₃ during the course of the experiment. The suspensions were shaken on a reciprocal shaker for 24 h followed by centrifuging at 8000 rpm for 25 min. The samples were washed with water to remove excess NaH₂PO₄. The phosphate-clays (P-montmorillonite, P-kaolinite and P-allophane) were immediately used in the form of wet paste in experiments with Pb. Clays pretreated with both P and Pb were prepared by simultaneous addition of P and Pb, by mixing clays (kaolinite (0.5 g) and montmorillonite and allophane (0.1 g), with 1.0 mM NaH₂PO₄ and 1.0 mM Pb(NO₃)₂ (equimolar solutions), with 10 mM NaNO₃ as the background electrolyte at pH 5. The suspensions were shaken on a reciprocal shaker for 24 h followed by centrifuging at 8000 rpm for 25 min. The samples were then washed with water to remove excess Pb(NO₃)₂. The treated clays (P-Pb-montmorillonite, P-Pb-kaolinite and P-Pb-allophane) were immediately used in the form of wet paste in the experiments.

Lead pre-treatment of the clays was done exactly the same as for phosphate pretreated clays, except that 1.0 mM NaH₂PO₄ was replaced with 1.0 mM Pb(NO₃)₂. The Pb-clays (Pb-montmorillonite, Pb-kaolinite and Pb-allophane) were immediately used in the form of wet paste in experiments with P. Control clays (montmorillonite, kaolinite and allophane) were prepared in the same way with the background electrolyte and water, but without added P or Pb. The treated and control clays were then used in batch adsorption experiments.

2.3. Lead Adsorption

Adsorption of Pb on treated montmorillonite was achieved by adding a series of initial Pb up to 1 mM to the wet pastes of clay 1) pre-treated with phosphate 2) pretreated with both phosphate and Pb and 3) with no phosphate nor Pb (control clay). The treatments were in duplicate. The suspensions were shaken on a reciprocal shaker for 24 h, at 20˚C ± 2˚C, before centrifugation at 8000 rpm for 25 min. The supernatant was carefully decanted and analyzed for Pb concentration by atomic absorption spectrophotometer. The amounts of Pb adsorbed were calculated from the difference between initial and final concentrations, and plotted against solution concentration at equilibrium. The same study was repeated with 1) kaolinite and 2) allophane with the same treatments.

2.4. Phosphate Adsorption

Adsorption of phosphate on treated montmorillonite was achieved by adding a series of initial phosphate up to 1 mM to the we pastes of clay 1) pre-treated with Pb 2) pretreated with both phosphate and Pb and 3) with no phosphate nor Pb (control clay). The suspensions were shaken on a reciprocal shaker for 24 h before centrifugation at 8000 rpm for 25 min. Phosphate in the supernatant was analyzed colorimetrically by the ascorbic molybdate method [20] . The amounts of phosphate adsorbed were calculated from the difference between initial and final concentrations, and plotted against solution concentration at equilibrium. The same study was repeated with kaolinite and allophane with the same treatments. The adsorption data were fitted to the Langmuir equation and sorption parameters calculated with the linearized equation (Equation (1)).

C / X = 1 / X m K + C / X m (1)

where X = amount of Pb adsorption (µmol∙g⁻¹), K = a constant related to binding energy (L∙µmol∙L⁻¹), X_m = maximum Pb adsorption (µmol∙g⁻¹), C = equilibrium Pb concentration (µmol∙L⁻¹).

3.1. Lead Sorption

The adsorption isotherms of Pb on three sorbents (montmorillonite, allophane, and kaolinite) at pH 5 are shown in Figures 1-3. The Pb adsorption increased with increasing Pb concentration in all cases. The curves for allophane were obtained with the same solid/solution ratio as that used for montmorillonite (0.1 g: 100 ml). Adsorption isotherms of Pb by three clays mineral demonstrate differences in adsorption capacity (Figures 1-3). The component could be classified according to their adsorption capacity: montmorillonite > allophane > kaolinite, as inferred by their CEC values. Similar results were obtained by other authors who study the adsorption of Pb by soil in different cation exchange capacity [21] and in cadmium [22] and nickel [23] and in zinc [24] . The results indicating that the montmorillonite and allophane are important sinks for heavy

metals in soils, due to the fact that they have a large specific surface area. However, kaolinite has a low CEC and, therefore, it is not expected to be an ion-exchanger of high order. The adsorption isotherms of Pb on all three minerals followed Langmuir type. The Langmuir equation parameters are summarized in Table 1. The trends of the Pb sorption isotherms and maximum adsorption on allophane were similar to those of kaolinite. Among all the three minerals, kaolinite had a lower Pb sorption than montmorillonite and allophane which had a similar order of magnitude. The maximum adsorption capacity of Pb in three minerals was in the order: montmorillonite > allophane > kaolinite. Montmorillonite has high Pb adsorption because it has high cation exchange capacity compare with allophane and kaolinite [25] [26] . Adsorption of Pb on kaolinite has been shown to involve both permanent and variable charge sites [27] . [28] and [29] showed significant inner sphere complexation of Pb on kaolinite. Using synchrotron X-ray absorption fine structure spectroscopy study demonstrates the Pb adsorption mostly the inner sphere, probably monodentate Pb complexation at aluminum-oxide surfaces [30] . Figure 3 shows that the Pb strongly adsorbed on nano ball allophane. Allophane was found to adsorb various cations and anions depending upon pH of the soil, cations being adsorbed mostly in high pH silanol groups (negative charges (Si-O⁻) at inner surface of the ball). Anions adsorbed at positive charge ( Al − OH 2 + ) at low pH at pore sites of the ball of allophane. The allophane strongly adsorbed Pb, this may be Pb ions that were strongly attracted to dissociated silanol group at the inner surface of hallow spherical of nano-ball allophane particle. Molecular orbital calculations with MOPAC AMI basis set indicated that could adsorbed not only on dissociated silanol groups but also with the undissociated silanol groups. The calculations also indicated that when metal ions interacted with undissociated silanol groups, dissociation reaction of the silanol groups accelerated [31] .

Table 1

Maximum adsorption (X m ) and binding energy (K) for lead on the three minerals as affected by pre-treatment

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