Adsorption of aqueous Cd 2+, Pb 2+, Cu 2+ ions by nano-hydroxyapatite: Single- and multi-metal competitive adsorption study

Geochemical Journal, Vol. 44, pp. 233 to 239, 10 Adsorption of aqueous Cd 2+, Pb 2+, Cu 2+ ions by nano-hydroxyapatite: Single- and multi-metal competitive adsorption study S. B. CHEN, 1,2 Y. B. MA, 1 * L. CHEN 3 and K. XIAN 3 1 Ministry of Agriculture Key Lab. of Plant Nutrition & Nutrient Cycling, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China 2 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China 3 Research Center of Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China (Received May 28, 09; Accepted October 6, 09) Being an inexpensive but efficient adsorbent, hydroxyapatite is extensively used for decontaminating wastewater and soils polluted by heavy metals. However, its solubility and grain size can affect its remediation effectiveness. This study investigated the ability of nano-hydroxyapatite (nano-hap) to adsorb aqueous Cd, Pb and Cu ions from single-metal and multi-metal ions reaction systems. Langmuir and Freundlich isotherm equations were employed to study the sorption constants. Based on the sum of squares errors (SSE), results showed that the Langmuir isotherm better fits sorption data than the Freundlich equation. The sorption affinity of nano-hap for Pb(II) is always higher than that for Cu(II) and for Cd(II); the sorption maxima for the Cd, Pb and Cu follow the order Pb 2+ > Cu 2+ > Cd 2+. This could be inversely proportional to the hydrated ionic radii as Pb 2+ (4.01 Å) > Cu 2+ (4.19 Å) > Cd 2+ (4.26 Å). The measured selectivity coefficients in multi-metal (Cd Pb Cu) reaction systems shows that Pb has the highest sorption selectivity on nano-hap among the metals investigated. This sorption selectivity coincided well with the sorption affinity order in mono-metal reaction systems. The ph of the solution is an important parameter in controlling Cd, Pb and Cu ions sorption on nano-hap. Indeed, the nano-hap sorption capacity increases with increasing ph up to a value of 6.25. This implies that the removal of metals from the solution is recommended for ph 6.25 or below, during remediation using nano-hap as a sorbent. Keywords: nano-hydroxyapatite (nano-hap), Cd, Pb, Cu, competitive adsorption, selectivity coefficients INTRODUCTION Toxic heavy metals present in wastewater, effluents and soils are a major environmental problem. Efforts have been made on remediation of soils and wastewater polluted by heavy metals, and several remediation practices have been proposed, such as physical and chemical extraction, chemical stabilization/solidification (S/S) and electrochemical methods (Khan et al., 04; Zhu et al., 04; Chen et al., 07; Zhou et al., 07). Among the in-situ remediation treatments, the use of phosphatic materials (e.g., phosphate clay, apatite, phosphorus fertilizer etc.) as a stabilizing agent to immobilize Pb in soils has received much attention in past years. Nriagu (1974) firstly suggested phosphate for removing soluble Pb from the aqueous phase. Since then, several studies on phosphate as an effective additive have been carried out, especially for Pb-contaminated media (Cheung et al., 02; *Corresponding author (e-mail: ybma@caas.ac.cn) Copyright 10 by The Geochemical Society of Japan. Zhu et al., 04; Chen et al., 06; Singh et al., 06; Agnieszka et al., 09). More recently, hydroxyapatite, has been proposed as a inexpensive but efficient adsorbent for removal/remediation of soils polluted by Pb (Hashimoto et al., 09; Slobodan, 09), Zn and Cd (Chen et al., 07; Zhu et al., 08) and Cu-, Znpolluted wastewater (Qian et al., 08; Wang et al., 09). Besides phosphates, activated carbons are considered also an efficient treatment for the removal of many organic compounds and metals from both liquid and gas phases (Cheung et al., 02; Danny et al., 04; Xiao and Thomas, 04; Chen et al., 06), but expensive. Though cheaper, little information is avalaible on the efficiency of hydroxyapatite as an additive to adsorb aqueous metals in single- and multi-metal reaction systems. The adsorption of aqueous metal ions is strongly influenced by the competition of those metal ions to occupy the limited number of sorption sites. This, in turn, will decrease the removal efficiency of adsorbent for the metals of interest. It is of critical importance to evaluate the overall performance of hydroxyapatite on adsorption of metals, especially competitive adsorption in multi-metal reaction systems. The main objective of this study is to examine 233

the potential use of nano-hydroxyapatite as an efficient adsorbent in the treatment of wastewater or remediation of multi-metal polluted soils. This will be made by 1) comparing quantitatively the efficiency of aqueous Cd, Pb and Cu adsorption on nano-hydroxyapatite and evaluate the impacts of ph on metals sorption in a single-metal reaction system; 2) evaluating the competitive sorption of Cd, Pb and Cu in multi-metal reaction systems with the developed selectivity coefficient measurement combined with isotherm equations. EXPERIMENTAL The nano-hydroxyapatite (nano-hap) used for this study was provided by the Xinjiang Center for Disease Control and Prevention of China. Surface area of the sample was measured by single point Brunauer, Emmett and Teller N 2 sorption procedure (BET-N 2 ), mineralogical analysis (X-ray diffraction, XRD) and grain size distribution (Laser diffraction analysis, LDA). Total metal concentration in nano-hap was determined by atomic absorption spectroscopy (AAS) following microwaveassisted aqua regia acid digestion following the method of Lu (1999). All digestions were conducted in triplicate and each batch included a standard reference material (GSS-6, China National Center for Standard Materials) and blank to validate the digestion procedure. The main composition of the nano-hydroxyapatite includes: SiO 2 = 23.1 wt%; CaO =.2 wt%; P 2 O 5 = 16.7 wt%, acid insoluble ash = 3.1 wt%. The measured average grain size is 97.3 nm, with a surface area of 71.8 m 2 g 1. The nano- HAP material did not contain toxic metals except 1.87 ppm of Pb. Adsorption study was conducted in batch experiments: cadmium, lead and copper stock solution of 5,000 mg L 1 was prepared from their nitrate salts Cd(NO 3 ) 2, Pb(NO 3 ) 2 and Cu(NO 3 ) 2. Deionized water was used for making metal solutions and washing, and the chemicals used were all of reagent grade. ml-aliquots of 0.05 M KNO 3 background electrolyte solution, containing a known amount of Cd, Pb and Cu concentration ranging from 0 to 180 mg L 1 were equilibrated with 0. g of nano-hap in ml polycarbonate centrifuge tubes. The slurries were shaken on an over-to-over shaker (~12 rpm) at room temperature (25 ± 2 C) for 24 hours according to our preliminary test because no significant adsorption was observed after 24-hours reaction. The supernatant and solid residues were separated by centrifugation at 4,000 rpm for min. The concentrations of Cd, Pb and Cu in the supernatant solutions, after reactions with nano-hap, were determined by inductively coupled plasma-oes (Agilent-70). The amount of adsorbed Cd, Pb and Cu was calculated as the difference between the amount added initially and that remaining in solution after equilibration. The effect of ph on the sorption of metals by nano- HAP was also investigated. A sample of 0. g nano-hap was equilibrated with ml of 0.05 M KNO 3 solution and the initial ph of the slurry was adjusted to different levels ranging from ph 3.52 to 7. by using 0.05 M HNO 3 /KOH. The slurries were shaken on an over-to-over shaker (~12 rpm) at room temperature (25 ± 2 C) for 24 hours. After stabilized the ph, metal concentration was raised equivalent from 0 to 180 mg L 1 by adding a concentrated stock solution. After addition of the stock solution of metals, the rest of experimental procedure including shaking, centrifugation and metal analysis remained similar to the previous sorption experiment. To study and compare the adsorption of aqueous Cd, Pb and Cu on nano-hap, the sorption data were fitted to the following Eqs. (1) and (2): Conventional Langmuir Ce 1 Ce = + () 1 x kb b m Freundlich equation Table 1. The percent of mole ratio of Cd, Pb, Cu added to solution in multimetal reaction system (X i *) Percent of mole ratio added in solution Cd(II) Pb(II) Cu(II) 0.05 0.70 0.25 0.60 0.35 0. 0.45 0. 0.55 0.10 0.60 0. 0. 0. 0. 0. 0. 0.60 0.15 0. 0.35 0. 0.45 0. 0.55 0. 0.65 *Total concentration of metal added to suspension was 0.0025 mol L 1. log x 1 log K f log C m n e = ( )+ ( ) ( 2) where C e is the concentration of metals (Cd 2+, Pb 2+ or Cu 2+ ) in equilibrium in the solution (mg L 1 ), x/m is the amount of the metal ions (mg kg 1 ) adsorbed on the HAP, b is adsorption maxima (mg kg 1 ) and k is the adsorption 234 S. B. Chen et al.

Table 2. Langmiur and Freundlich isotherm constants and the corresponding sum of squares errors (SSE) values for Cd(aq), Pb(aq), Cu(aq) at single-metal reaction systems Cd Pb Cu Langmuir isotherm b 66.7 80.6 73.5 k 0.46 2.12 1.37 SSE 2.36 10 3 1.86 10 2 1.76 10 2 Freundlich isotherm K f 64.5 43.0 57.46 1/n 0.73 1.85 1.46 SSE 4.61 10 3 2.96 10 2 2.56 10 2 Adsorption (mg.g 1 ) 100 90 80 70 60 10 0 (A) Pb Cu Cd 0 60 80 100 1 1 Final concentration (mg.l 1 ) affinity coefficient. The K f and 1/n denote the Freundlich empirical constants as adsorption maxima and bonding affinity coefficient, respectively. The data obtained in batch adsorption studies were also used to calculate the equilibrium metals adsorption capacity, the amounts of Cd 2+ (aq), Pb 2+ (aq) or Cu 2+ (aq) adsorbed by nano-hap was calculated as the following Eq. (3): Q e ( ) vc0 C = m e () 3 where Q e is the amount of metals sorbed by nano-hap in solution (mg g 1 ), C 0 and C e are the concentrations (mg L 1 ) of the metals in the initial solution and the equilibrium solution after the experiment, respectively, v (L) is the volume of the solution and m (g) the amount of HA used. The multi-metal ions reaction systems involved the Cd 2+, Pb 2+ and Cu 2+ competitive adsorption on nano-hap, in a 0.05 M KNO 3 background electrolyte solution. ml aliquots of 0.05 M KNO 3 background electrolyte solution containing a known amount of Cd, Pb and Cu composition were equilibrated with 0. g of nano-hap in ml polycarbonate centrifuge tubes. The metals were added to the suspension according to our preliminary result, the percent of mole ratio of individual metal was listed in Table 1, and the total mole of metals added in each suspension was 0.0025 mol/l. After addition of the stock solution of metals, the rest of experimental procedure remained similar to the previous adsorption experiment. Comparisons among different treatments including the analysis of variance were conducted using EXCEL (07) and SPSS Version 12.0 software (SPSS Inc., USA) and differences were reported at p < 0.05. Data were expressed as mean of three replicates. C/x/m (L.g 1 ) 2. 2.00 1.60 1. 0.80 0. 0.00 Cd Cu Pb 0 60 90 1 Final concentration (mg L 1 ) RESULTS AND DISCUSSION (B) Fig. 1. Langmuir sorption isotherms of Pb, Cd, Cu on nano- HAP (A) and their linear fittings (B) after the transformation by the Langmuir Eq. (1). The data are from 24 hrs adsorption experiment and nine final concentrations were used in this adsorption study. For sorption isotherms, several sorption equilibrium equations are available for data analysis. In this study, two isotherm equations, i.e., the Langmuir equation (Eq. (1)) and the Freundlich equation (Eq. (2)) were employed to study the adsorption process. The results showed that the Langmuir and Freundlich models apparently fit the isotherms for all the M 2+ (aq) species investigated with regression coefficients R 2 > 0.931(data not shown) and acceptable square errors. The isotherm constants derived from Eqs. (1) and (2) and the corresponding sum of squared errors (SSE) values are given in Table 2. The results of Fig. 1 and Table 2 showed that the sorption affinity and sorption maximum on nano-hap for the Pb(II) in single-metal sorption systems were always higher than Cu 2+ and Cd 2+ ions. The sorption maxima (m mol kg 1 ) Adsorption of aqueous Cd 2+, Pb 2+ and Cu 2+ by nano-hydroxyapatite 235

60 4.0 Cd adsorption (mg.g 1 ) 10 (Cd) ph6.25 ph5.45 ph7. ph4.60 ph3.52 logk T 12 3.0 2.0 1.0 0.0 2.0 3.0 XCd = 0.05 XCd = 0.10 XCd = 0.15 0. 0. 0.60 0.70 1.0 0 0 10 60 4.0 X Pb Final Cd concentration (mg.l 1 ) 4.5 4.0 3.5 XCd = 0.05 XCd = 0.10 XCd = 0.15 Pb adsorption (mg.g 1 ) 90 80 70 60 0 10 Final Pb concentration (mg.l 1 ) (Pb) ph6.25 ph5.45 ph7. ph4.60 ph3.52 logk T 13 3.0 2.5 2.0 1.5 1.0 5.0 4.5 4.0 0. 0. 0. 0.60 0.70 X Cu XCd = 0.05 XCd = 0.10 XCd = 0.15 Cu adsorption (mg.g 1 ) 80 70 60 (Cu) ph6.25 ph5.45 ph7. ph4.60 ph3.52 logk T 23 3.5 3.0 2.5 2.0 0. 0. 0. 0.60 0.70 X Cu Fig. 3. Selectivity coefficients K T ij plots with the varying mole ratio of metal (X i ) addition in multi-metal reaction systems (initial solution ph 6., X i = 0.0025 mol L 1 ). 0 10 60 Final Cu concentration (mg.l 1 ) Fig. 2. Effect of ph on aqueous Cd, Pb, Cu adsorption on nano- HAP in single-metal reaction systems. The data are from 24 hrs sorption experiment and nine final concentrations were used in this adsorption study. for the Cd, Pb and Cu ions followed the order Pb 2+ > Cu 2+ > Cd 2+, with a sorption maximum of 80.6, 73.5, 66.7 (m mol kg 1 ) for Pb 2+, Cu 2+ and Cd 2+ respectively. Danny et al. (04) investigated the ability of bone char clay and reported a similar result for the ability of calcium hydroxyapatite to adsorb Cu 2+ and Cd 2+ in solution. The sorption maxima seems to be inversely proportional to the hydrated ionic radii of the metals, being Pb 2+ (4.01 236 S. B. Chen et al.

Table 3. Langmiur parameters for the adsorption of Pb, Cd and Cu at various phs by nano-hap in single-metal reaction system ph Cd Pb Cu b k R 2 b k R 2 b k R 2 3.52 35.25 0.892 0.999 60.12 1.891 0.999 57.09 1.291 0.992 4.60 36.58 0.881 0.998 71.58 1.542 0.999 63.01 1.088 0.997 5.45 49.36 0.812 0.992 87.47 1.275 0.991 69.89 1.043 0.997 6.25 56.37 0.782 0.987 92. 1.155 0.968 87.09 1.060 0.883 7. 39.04 0.655 0.912 79.25 0.887 0.867 64.75 0.804 0.826 Å) > Cu 2+ (4.19 Å) > Cd 2+ (4.26 Å) (Lide, 1998). This is in agreement with results from Danny et al. (04) and Lee and Moon (01). Hillel (1998) explained that the smaller the ionic radius and the greater the valence, the more closely and strongly is the ion adsorbed. On the other hand, the greater the ion s hydration, the farther it is from the adsorbing surface and the weaker its adsorption. Figure 1 shows the adsorption isotherms of Cd 2+ (aq), Pb 2+ (aq) and Cu 2+ (aq) on nano-hap against the final metal concentration in solution in a single-metal reaction system. These isotherms reach a well-defined plateau and are of type L, based on the classification of Giles et al. (1960). They are characterized by decreasing slopes as equilibrium metals concentration increases, indicating high affinity of the sorbent for low concentrations of metals in solution (Wu et al., 1999; Singh et al., 06). During the sorption, the fast reactions were most likely the result of adsorption and initial co-precipitation of metals and phosphorus as a pyromorphite, during aqueous metal ions interaction with apatite compounds in nano-hap (Strawn and Sparks, 00; Singh et al., 06). The sorption data of metals on nano-hap at various phs were fitted into Langmuir equation due to its higher regression coefficients than that of Freundlich equation (data not shown). The plot of adsorption vs. the final concentration of metals in equilibrium solution shows that the adsorption of metals increases with increasing ph in solution except at ph greater than 6.25 (Fig. 2). This result indicated that solution ph is an important parameter in controlling metals sorption on nano-hap. The determined sorption maximum (b) and affinity parameter (k) of nano-hap for metal ions at various phs are listed in Table 3. The sorption maximum (b) and affinity parameter (k) derived from the Langmuir equation increases with increasing the ph in solution with a deviation for ph greater than 6.25. A meaningful relation between the adsorption potential (partition coefficient, K d ; K d = A i /C i ; where A i = amount of adsorbate on the solid at equilibrium and C i = total dissolved adsorbate remaining in solution at equilibrium) and ph in solution was found for all metals except for the treatments at ph 7.. This relation can be described for Cd, Pb and Cu as follows: logk d (Cd) = 7.068 + 0.046 ph, R 2 = 0.998, (p < 0.01); logk d (Pb) = 4.380 + 0.381 ph, R 2 = 0.992, (p < 0.01); logk d (Cu) = 4.705 + 0.185 ph, R 2 = 0.918, (p < 0.01). However, the determined affinity parameter (k) for all metals decreased with increasing ph in solution including the treatment of ph 7.. With increasing ph from 3.52 to 7. in solution, the sorption affinity parameter (k) decreased from 0.892 to 0.655, 1.891 to 0.887 and 1.291 to 0.804 for Cd, Pb and Cu respectively. In the case of multi-metal ions adsorption systems, the number of surface sites available for Cd 2+, Pb 2+ and Cu 2+ adsorption on nano-hap is constant. Consequently, the concentration of Cd 2+ (Pb 2+ or Cu 2+ ) in equilibration solution increases with increased other competitive metal ions. In multi-metal reaction systems, the competitive adsorption data of Cd 2+ and Cu 2+ cannot be fitted to the Langmuir isotherm with an acceptable square errors. However, the Langmuir equation can still be applied to fit the sorption data for Pb 2+ with an acceptable linear coefficient (R 2 = 0.907). On the other hand, the sorption maximum (b) derived from the equation is generally less than that in single Pb adsorption system, which implied that only high affinity sorption sites can adsorb Pb 2+ when some adsorption sites are occupied by Cd 2+ and Cu 2+ in solution. Analysis of competitive adsorption of Cd and Cu in the multi-metal ions adsorption systems shows that the equilibrium metals adsorption capacity (Q e ) significantly decreased in the multi-metal ions adsorption systems than that in single-metal sorption system, the amounts of Cd 2+ or Cu 2+ adsorbed were displaced with higher accessibility ion (i.e., Pb 2+ ) in equilibrium solution may be the main mechanism of this result. To investigate the overall competitive adsorption of metals in solution, a selectivity coefficient was developed. The more strongly adsorbed metal species (A), i.e., adsorbed to the larger maximum extent, can displace ions from all the sites including those available to the more weakly adsorbed species (B or C), whereas some of the surface sites will either not ion exchange or are not avail- Adsorption of aqueous Cd 2+, Pb 2+ and Cu 2+ by nano-hydroxyapatite 237

able to the more weakly adsorbed species (B or C). Selectivity coefficient (K T ij ) can be calculated as: K T ij = Xzi j (Ci)zj /X zj i (Cj)zi (i, j = 1, 2, 3) (4) where X is the mole ratios of metals (Cd 2+, Pb 2+ or Cu 2+ ) in equilibrium solution, Ci and Cj are the concentrations of metal (i) and metal (j) in solution (mol L 1 ), respectively, with i, j = 1 (Cd), 2 (Pb), 3 (Cu); z is the chemical valence of metal ion and T represents the ternary metal ions reaction system. The results (Fig. 3) indicate that the selectivity coefficients (logk T 12 ) for adsorption of Cd on nano-hap in multi-ion system decreased sharply with increasing Pb concentration (X Pb ) in solution. However, the selectivity coefficients (logk T 13 ) for adsorption of Cd on nano-hap only has a sharp decreasing when the addition of mole ratio of Cu (X Cu ) is less than 0., then it continued decreased with a very slow rate when the addition of mole ratio of Cu (X Cu ) is more than 0.. The difference between adsorption selectivity coefficients of Cd (logk T 12 ~logkt 13 ) shows the competitive impacts of Pb and Cu on Cd adsorption on nano-hap, in solutions. As indicated in single-metal adsorption system, Pb has the highest adsorption affinity on nano-hap among the metals investigated, followed by Cu and Cd. In the multiadsorption systems, competitive adsorption took place on the adsorbent surface sites that can be occupied by all species, that is, sites available to B and C, while noncompetitive adsorption occurs on the sites only available to A. The sharp decreased selectivity coefficients (logk T 12 ) showed that the Cd adsorbed on sites of nano- HAP was intensively displaced with increasing Pb 2+ concentration in solution. In case of selectivity coefficients (logk T 13 ), it sharply decreased with the addition of Cu in solution when Cu mole ratio less than 0. and then continued decrease with a slow rate with continuous addition of Cu in solution, this result indicated that Cu has a strong competitive impact on the Cd adsorption at low concentration (Fig. 3), however, the impact of Cu on Cd adsorption becomes insignificant when most of sites are not available for competitive adsorption after higher Cu 2+ addition in solution. The plot of selectivity coefficients (logk T 23 ) vs. X Cu showed also a competitive adsorption between Cu 2+ and Pb 2+ on nano-hap. Generally, the selectivity coefficients follow the order logk T 23 > logkt 13 > logk T 12. This result coincides well with the adsorption result found for mono-metal system which showed the Pb 2+ has the highest competitive adsorption capacity among the metals investigated. CONCLUSIONS The adsorption of aqueous Cd 2+, Pb 2+ and Cu 2+ on nano-hap was investigated in mono- and multi-metal reaction systems. Sorption behaviour was predicted with Langmuir and Freundlich isotherm equations. For monometal reaction systems, results showed that the sorption of metals on nano-hap are well fitted both using the Langmuir and Freundlich equations. The adsorption maximum and the sorption affinity of the metals on nano-hap follows the order Pb 2+ > Cd 2+ > Cu 2+, possibly related to the inverse of their hydrated ionic radii. The solution ph is an important parameter in controlling metals sorption onto the nano-hap. The sorption maximum (b) and sorption affinity parameter (k) derived from the Langmuir equation increased with increasing the ph in solution with a deviation for ph = 7.2. A meaningful positive relation between adsorption potential (K d ) and solution ph was observed except the treatments at ph = 7.2. The deviation of sorption maximum (b) at ph 7.2 implied that a possible mechanism for the metals adsorption, that is a solubility-controlled reaction in system, the decreased sorption maximum (b) at ph 7. for nano-hap probably related to its lower solubility at ph 7. than that at lower phs. The higher capability of nano-hap to adsorb aqueous Pb than other metals indicated its potential as promising way to remediate Pb-contaminated effluent, sediment and soils. In multi-metal ions reaction systems, the competitive adsorption of the metals on nano-hap was also investigated using the developed selectivity coefficient measurement combined with the isotherm equation. The result indicated that the competitive adsorption of multimixtures of Cd 2+, Pb 2+ and Cu 2+ on nano-hap decreased the adsorption capacity of nano-hap for individual metal ions. 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