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.
Heavy metals are among the major contaminants of the environment, with serious effects on animal and human health [
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 [
The interaction between toxic metals and clay mineral colloids is important in surface chemistry, soil science, and pollution studies [
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 [
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.
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.
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 [
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), Xm = maximum Pb adsorption (µmol∙g−1), C = equilibrium Pb concentration (µmol∙L−1).
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 [
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