Preparation, Characterization, Adsorption Equilibrium, and Kinetics for

Article pubs.acs.org/IECR

Preparation, Characterization, Adsorption Equilibrium, and Kinetics for Gold-Ion Adsorption of Spent Buckwheat Hulls Modified by Organodiphosphonic Acid Mingyu Xu, Ping Yin,* Xiguang Liu, Xiaoqi Dong, Qiang Xu, and Rongjun Qu* School of Chemistry and Materials Science, Ludong University, Yantai 264025, P. R. China S Supporting Information *

ABSTRACT: The unique chemical and physical properties of gold are increasingly being sought for use in a growing number of industrial and medical applications. The agricultural residue spent buckwheat hulls (60 mesh) modified with organodiphosphonic acid (denoted as ODPA-BH) was successfully developed and characterized, and cellulose with an organodiphosphonic acid group was calculated at the B3LYP/6-31G(d) level. ODPA-BH was employed to adsorb Au(III) ions in both spiked samples and industrial wastewater samples, and the relevant adsorption kinetics and isotherms were investigated. A better interpretation for the experimental data was obtained with the Langmuir isotherm equation, and the maximum adsorption capacity for Au(III) was found to be 465.16 mg/g at 35 °C. The adsorption kinetics were modeled by a pseudo-second-order rate equation, and the thermodynamic parameters ΔG, ΔH, and ΔS were determined to be −8.89 kJ·mol−1, 70.93 kJ·mol−1, and 257.25 J·K−1·mol−1, respectively.

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INTRODUCTION Buckwheat is considered to be a health food with high nutritional value, because its seeds contain many biologically active compounds. Buckwheat’s original habitats were located in north-central Asia, and China is one of its original habitats. Buckwheat hull (BH) is probably the major residue from the handling and processing of buckwheat. As one type of agricultural residue, buckwheat hull is rich in cellulose and has carboxyl and hydroxyl functional groups. Spent buckwheat hull could be applied as an absorbent material because of its granular structure, insolubility in water, well-functionalized surface property, high mechanical strength, and local availability at almost no cost. However, many of naturally available adsorbents have low metal removals and slow process kinetics. Thus, it is necessary to develop innovative inexpensive adsorbents with good affinity toward targeted metal ions, for which surface modification technology has proven to be effective.1 On the other hand, gold is a precious metal used as a global currency. The unique chemical and physical properties of gold are increasingly being sought for use in a growing number of industrial and medical applications, and it is necessary to recover gold from the mining, electronics, and electroplating industries. This provides a compelling reason for developing more efficient and environmentally friendly methods for the uptake of gold from mineral ores and waste materials.2−5 Wastewater containing gold is of particular concern, and adsorption is a highly effective, economical, promising, and widely applied method.6 Therefore, effective adsorbents with strong affinities and high loading capacities for the targeted precious-metal ions have been studied extensively. Chemical modification of the surface of agricultural residues through the covalent thermochemical reaction of an organic moiety is a promising approach to obtaining novel biomassbased adsorbents, and functional groups grafted in the © 2013 American Chemical Society

biopolymer structure can provide binding sites to remove metal ions from aqueous solutions and improve the adsorption properties.7 In related investigations, researchers have found that phosphonic acid groups have the ability to exchange ions. The presence of oxygen atoms in PO and PO groups enables them to coordinate with a variety of metal ions, and organophosphonic acids are usually used for the separation of lanthanides and actinides.8,9 If the organophosphonic acid groups are grafted on a solid matrix, this type of chemical modification can overcome their shortcoming of being soluble in water and allow them to be used in the adsorption of metal ions from aqueous solutions.10−12 The objective of the present work was to explore the adsorbent buckwheat hull (60 mesh) modified with organodiphosphonic acid (ODPA-BH) and investigate its thermodynamics, isotherm, and kinetics for Au(III) adsorption from aqueous solutions. Buckwheat hull is an agriculture residue that is nontoxic and biodegradable. ODPA-BH was successfully prepared in the present work simply by the thermochemical reaction of spent buckwheat hulls obtained from agricultural residues with organodiphosphonic acid. Therefore, ODPA-BH, which can be used as a lowcost adsorbent with high efficiency for water purification, would help industries improve the economic value and reduce the cost of waste disposal and provide facile processing of gold uptake.

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EXPERIMENTAL SECTION Materials and Instruments. Spent buckwheat hulls were collected from a buckwheat production site. They were washed with distilled water, dried at 50 °C, and then ground and passed Received: Revised: Accepted: Published: 8114

January 10, 2013 May 1, 2013 May 27, 2013 May 27, 2013 dx.doi.org/10.1021/ie4001162 | Ind. Eng. Chem. Res. 2013, 52, 8114−8124

Industrial & Engineering Chemistry Research

Article

to ascertain the nature of stationary points, and harmonic vibrational wavenumbers were calculated using the analytical second derivatives to confirm the convergence to a minimum of the potential surface. Moreover, Mulliken atomic charges of cellulose with an organodiphosphonic acid group were also obtained at the B3LYP/6-31G(d) level. Biosorption Experiments. Batch adsorption experiments were carried out by shaking a certain amount of the biomassbased adsorbent ODPA-BH with 10 mL of metal-ion solution in a series of flasks at pH 1.0−4.0 and 15−35 °C. At different time intervals, the adsorbent was filtered, and the concentrations of metal ion in solution were determined using an atomic adsorption spectrophotometer (GBC-932A). The adsorption amount was calculated according to the equation

through a 60 mesh sieve to obtain the uniform particle size. The powdered biomass (10.0 g) was agitated at 60 °C for 24 h in 50 mL of 20.0% 1-hydroxylethylidenediphosphonic acid (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) solution for the surface modification reaction. The resulting biomasses were filtered and dried, and then the treated sample was thermochemically reacted at 120 °C for 4 h. The products obtained were mixed in distilled water for 30 min, filtered, and washed with distilled water. Finally, ODPA-BH was dried in a vacuum oven at 45 °C for 48 h. The thermally treated sample was cooled to room temperature and then stored for subsequent adsorption experiments. All other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China; they were of analytical grade and were used without any further purification. The esterification reaction efficiency is the percentage amount of organophosphonic acid that is attached to the structure of BH, as reported by Altun and Pehlivan.7 The ODPA-BH product was soaked in pure water for 30 min, the pH of the system was adjusted, and the treated samples were filtered and washed with pure water. Then, the ODPA-BH was filtered, and 0.1 M NaOH solution was added. The NaOH remaining in the solution phase was titrated with 0.1 M HCl. Stock solutions containing various metal ions at a certain concentration were prepared by dissolving their relative metal salts (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) in distilled water. The pH value (1.0−4.0) of the solution was adjusted with hydrochloric acid aqueous solution (1 mol/L) and sodium hydroxide aqueous solution (1 mol/L). Au(III) solution (pH 2.5) was obtained using phosphate buffer. Distilled water was used to prepare all solutions. Fourier transform infrared (FTIR) spectra of the samples were recorded in the range of 4000−400 cm−1 with a resolution of 4 cm−1, by accumulating 32 scans using a Nicolet MAGNAIR 550 (series II) spectrophotometer (Nicolet Instrument Corporation, Madison, WI). The morphology of the compounds was examined on a JSF5600LV scanning electron microscope (JEOL Co., Tokyo, Japan). Energy-dispersive X-ray absorption spectroscopy (EDXAS) was performed on a NORAN Level-2 EDX analytical instrument (Electronics Co., Japan). Before observation, the sample was placed on a specimen stub covered with a conductive adhesive tab and provided with a sputtered 15-nm platinum coating. Porous structure parameters were characterized using an ASAP 2020 automatic physisorption analyzer (Micromeritics Instruments Co., Norcross, GA) through N2 adsorption at 77 K. Powder Xray diffraction (XRD) data were obtained using a Rigaku Max2500VPC diffractometer (Rigaku Co., Tokyo, Japan) with Cu Kα1 radiation (λ = 1.54056 Å). Thermogravimetric analysis (TGA) was recorded on a Netzsch STA 409 instrument (Netzsch Corporation, Selb, Germany), Test conditions: type of crucible, Al2O3 differential thermal analysis (DTA)/TGA crucible; nitrogen flow rate, 30 mL/min; heating rate, 10 K/ min. Atomic absorption analysis of transition-metal ions was performed with a GBC-932A flame atomic absorption spectrophotometer (GBC Scientific Equipment, Braeside, Victoria, Australia). Computational Details. Theoretical calculations of cellulose with an organodiphosphonic acid group were performed with the Gaussian 03 program13 using the B3LYP/6-31G(d) basis set to obtain the optimized molecular structure and vibrational wavenumbers. The frequencies of the required structure were evaluated at the B3LYP/6-31G(d) level

q=

(C0 − Ce)V W

(1)

where q is the adsorption capacity (mmol/g); C0 and Ce are the initial and equilibrium concentrations (mmol/mL), respectively, of metal ion in solution; V is the volume of the solution (mL); and W is the weight of ODPA-BH (g). Each batch experiment was carried out in triplicate, and the mean values and error ranges are presented in the related figures and tables. Desorption and Recycling Studies. After adsorption, the Au(III)-ion-loaded adsorbents were separated and lightly washed with deionized water to remove unadsorbed Au(III) ions on the surface of the adsorbent. They were then stirred at 25 °C for 24 h in different solutions of 0.1 mmol/L HCl, 0.1 mmol/L HCl + 1.0% thiourea, 0.1 mmol/L HCl + 2.0% thiourea, 0.1 mmol/L HCl + 3.0% thiourea, 0.1 mmol/L HCl + 4.0% thiourea, and 0.1 mmol/L HCl + 5.0% thiourea, which were employed as the desorption medium. The desorption ratio of Au(III) ions was then calculated as the ratio of the amount of desorbed Au(III) ions to the amount of initially absorbed Au(III) ions. Adsorption of Gold Ions and Copper Ions from Industrial Wastewater. The feed solutions for samples 1 and 2 were industrial wastewaters collected from manufacturing lines of gold plating. Adsorption experiments were conducted by adding 30 mg of ODPA-BH adsorbent to 20 mL of metalion solution at ambient temperature. At a certain time interval, the adsorbent was filtered, and the concentration of Au(III) ion in solution was determined by atomic absorption spectrometry.

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RESULTS AND DISCUSSION

Theoretical Calculations of Cellulose with Modified Organodiphosphonic Groups. Diphosphonic acids have been shown to be good candidates for coordinating with metal ions, where the organic part plays the role of a controllable spacer and the two PO3 groups can chelate with metal ions to form one-, two-, or three-dimensional structures.14 In the present work, the modified organic groups on functionalized spent buckwheat hulls that can chelate metal ions include two PO3 groups and one OH group. The introduction of the organodiphosphonic acid groups onto spent buckwheat hulls can make the agricultural residues form stable chelating compounds with many heavy-metal ions. In addition, the phosphonic acid groups can provide several oxygen atoms to coordinate metal ions.15 The aim of chemical modification with the designed organodiphosphonic acid, which has both an O donor atom in the hydroxyl functional group and O donor 8115

dx.doi.org/10.1021/ie4001162 | Ind. Eng. Chem. Res. 2013, 52, 8114−8124


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atoms in the PO3 group, is to make the material have excellent coordination properties with metal ions and to give a biosorbent with a high loading capacity for metal ions. ODPA-BH was developed as described in the Experimental Section; the thermochemical reaction of ODPA-BH is shown in Figure 1. To design the title biomass-based material, we

corresponding values for phosphonic acid in ref 16 and are slightly longer than the experimental value (1.47 Å). Moreover, the P24O15, P24O26, P3537, and P3539 bond lengths are in the range of 1.6196−1.6366 Å, comparable to those in phosphonic acid, 1.59−1.63 Å.16 Moreover, the bond angles C28P24O15/O25/O26 and C28P35O36/ O37/O39 are produced with reasonable accuracy as well. Table S2 (Supporting Information) presents the Mulliken atomic charges of the modified diphosphonic acid group and shows that the oxygen atoms in phosphonic acid groups have more negative charges. The Mulliken electronic populations of O15, O25, O26, O33, O36, O37, and O39 were fuond to be −0.552, −0.554, −0.752, −0.606, −0.520, −0.662, and −0.681, respectively, so that these oxygen atoms can chelate metal ions more easily. Therefore, the designed organic groups can provide a good adsorbent for use in adsorbing metal ions from aqueous solutions. Characterization of ODPA-BH. Panels a and b of Figure 3 show SEM images of BH and ODPA-BH, respectively, at 2000× magnification. Apparently, the surface of BH became rougher after the modification reaction, and more cavities on the surface of ODPA-BH might increase the contact area and facilitate diffusion during the adsorption process, thereby improving its adsorption ability for heavy-metal ions from aqueous media.17 Moreover, the particle appearances of these samples were similar, demonstrating that the particles of ODPA-BH had good mechanical stability and were destroyed during the overall reaction. Figure 3c displays the EDXAS spectrum of ODPA-BH, and the related data are reported in Table S3 (Supporting Information). It is obvious that there are the peaks for C, O, Mg, Si, P, K, and Ca, and their weight percentages were 53.17%, 40.62%, 0.55%, 0.65%, 4.44%, 0.39%, and 0.19%, respectively. The N2 adsorption isotherm data of ODPA-BH are shown in Figure 3d; adsorption measurements were followed by desorption measurements under the same conditions. These isotherms are of type III based on the IUPAC classification, suggesting that there is a weak interaction between ODPA-BH and N2. No hysteresis loop formed because of the microporous structure. 18 From the N 2 adsorption−desorption isotherm, it is clear that the adsorbed volume for ODPA-BH increased with increasing relative pressure, P/P0, indicating a wider pore size distribution. The inset in Figure 3d shows the pore distribution of ODPA-BH,

Figure 1. Thermochemical reaction of the biomass-based adsorbent ODPA-BH.

theoretically calculated the structure of cellulose with an organodiphosphonic acid group at the B3LYP/6-31G(d) level in advance. The optimized structure is displayed in Figure 2, and the corresponding selected bond lengths and bond angles are presented in Table S1 (available in the Supporting Information). The P24O25 and P35O36 bond lengths are 1.4867 and 1.4806 Å, respectively, which agree well with the

Figure 2. Optimized geometry of cellulose with an organodiphosphonic acid group. 8116



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Figure 3. (a,b) SEM images of (a) BH and (b) ODPA-BH. (c) EDXAS data for ODPA-BH. (d) Nitrogen adsorption−desorption isotherms of ODPA-BH. (e) XRD patterns of BH and ODPA-BH. (f) TGA curve of ODPA-BH.

m2·g−1 and 0.034 cm3·g−1, respectively, and the corresponding values for ODPA (120 mesh) were 31.2 m2·g−1 and 0.057 cm3·g−1, respectively. Figure 3e presents all of the diffraction peaks of BH and ODPA-BH, and their XRD patterns indicate the amorphous nature of the material lacking any crystallinity. The results also showed that no essential change occurred in the topological structure of BH before and after the

and the pore size distribution seems to provide useful information on porous solids. The porous structure parameters of BH and ODPA-BH were obtained from the nitrogen adsorption data. The pore structure development of BH was nearly negligible. The Brunauer−Emmett−Teller (BET) surface area and the Barrett−Joyner−Halenda (BJH) desorption cumulative volume of pores of ODPA-BH (60 mesh) were 17.6 8117



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Figure 4. (a) Static adsorption capacities of ODPA-BH for metal ions at room temperature. (b) Effect of pH on the adsorption of Au(III) by ODPABH. (c) Effect of ODPA-BH dosage on the adsorption of Au(III).

modification reaction, which implies that BH was stable enough to experience chemical modification reactions. No new diffraction peak appeared after the thermochemical reaction, meaning that the modified functional groups on the surface of BH existed in a noncrystalline state. The thermal stability of ODPA-BH was determined by thermal analysis, and the results are shown in Figure 3f. The thermal analysis represents several steps of decomposition in the temperature range of 25−800 °C. A weight loss of 10.7% in the temperature range of 25−200 °C corresponds to the release of physically adsorbed water. Further weight loss above 200 °C is due to the decomposition of the organic functional groups. It is noted that almost no weight was loss for ODPA-BH in the 25−50 °C range, and adsorbents are usually utilized at temperatures below 50 °C. Therefore, these data indicate that the resulting product ODPA-BH has good thermal stability and should be applied at temperatures below 50 °C. Moreover, IR spectroscopic analysis of both BH and ODPA-BH was performed to determine whether the organodiphosphonic acid esterification reaction had been carried out with BH. The broad peak at about 3400 cm−1 was the characteristic peak of BH corresponding to the presence of the hydroxyl groups of cellulose. The strong COC band at around 1036 cm−1 also confirms the cellulose structure. In contrast, in the IR spectrum of ODPA-BH, two major changes could be observed compared to that of BH: one is the appearance of peaks for both PO bonds at 1174 cm−1 and the (C)PO stretching vibration at 757 cm−1, and the other is a reduction in the hydroxyl (OH) stretching band at around 3400 cm−1. These facts reflect the result of the esterification reaction for organophosphonic acid. The esterification reaction efficiency for ODPA-BH reached 64.57%, and the organophosphonic acid groups played an important role in metal-ion adsorption. Because of the introduction of organodiphosphonic acid groups, it could be expected that ODPA-BH could present adequate chemical and physical characteristics to adsorb metal ions. Static Biosorption of ODPA-BH for Transition-Metal Ions. Figure 4a shows the saturation adsorption capacities of ODPA-BH for transition-metal ions, where the static adsorption capacities of ODPA-BH for Au(III), Hg(II), and Cu(II) were found to be 1.45, 0.61, and 0.17 mmol/g, respectively. However, those for Co(II), Cd(II), Cr(III), Zn(II), and Ni(II) metal ions were 0.028, 0.15, 0.21, 0.079, and 0.00 mmol/g, respectively. ODPA-BH had good adsorption capacities for Au(III), Hg(II), and Cu(II) metal

ions, especially for Au(III) ion. The adsorption capacities of BH, 60-mesh ODPA-BH, and 120-mesh ODPA-BH for Au(III) were also obtained, and they were found to be 0.68, 1.45, and 1.81 mmol/g, respectively. Although smaller ODPA-BH particles gave a higher adsorption capacity, considering the convenience of the adsorption operation, 60-mesh ODPA-BH was used in the subsequent experiments. Organodiphosphonic acid functional groups present in the BH biopolymer structure can further provide binding sites to remove metal ions from aqueous solution. As shown in Figure 1, the excess organodiphosphonic acid could be impregnated in the pores of buckwheat hull. The interactions of Au(III) ions with ODPABH might be dominated by adsorption, electrostatic attraction, chelation, and extraction. Effect of pH on the Adsorption of Au(III). The pH value of the metal-ion solution is one of the most important factors influencing the adsorption behavior of metal ions on adsorbents. Not only does it impact the surface structure of the adsorbents and the formation of metal ions, but it can also influence the interaction between adsorbents and transitionmetal ions. To investigate the effect of pH on the adsorption of gold ions, adsorption experiments were conducted in the pH range of 1.0−4.0, and the effect of pH on the adsorption for Au(III) on ODPA-BH is illustrated in Figure 4b. The results show that the adsorption capacities increased with increasing solution pH, and the good adsorption capacity of the adsorbent for Au(III) exhibited at about pH 2.5 can be explained by the surface charge of the adsorbents and the existing state of Au(III) ions in the aqueous solution. Then, the adsorption capacity decreased with a further increase in pH value. The pH values of real Au(III) samples/effluents from electroplating industrial wastewater are usually in the range of 2−3;19,20 therefore, all subsequent experiments were performed at pH 2.5. Effect of Adsorbent Dosage on the Adsorption of Au(III). The effect of the ODPA-BH dosage on the adsorption of Au(III) is shown in Figure 4c. It was observed that the Au(III) adsorption capacities increased initially and then decreased with increasing ODPA-BH dosage, and the maximum adsorption capacity was obtained with 30 mg of ODPA-BH. Although an increased adsorbent dosage provides an increased biomass surface area and the availability of more adsorption sites, the values of Au(III) uptake decreased with increasing adsorbent dosage. A higher biomass dosage could result in aggregates of functionalized adsorbent ODPA-BH and 8118



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might cause interference between binding sites at higher biomass dosage or insufficiency of metal ions in the solutions with respect to available binding sites. Adsorption Isotherms. The adsorption isotherms are characterized by definite parameters, whose values express the surface properties and affinity of biosorbent for metal ions. To investigate the adsorption capacity, Au(III) solutions with a wide range of concentrations were shaken for 24 h at different temperatures. The isotherm for the sorption of Au(III) onto ODPA-BH at pH 2.5 and 15−35 °C is shown in Figure 5. It

Table 1. Langmuir and Frendlich Isotherm Parameters and Coefficients for the Adsorption of Au(III) by ODPA-BH at Different Temperatures Langmuir model

can be seen from this figure that the adsorption capacity of the adsorbent increased with increasing temperature. At a certain temperature, it is clear that the adsorption capacity for Au(III) rises with an increase in the equilibrium concentration. In this study, two isotherm models were selected to fit the experimental data, namely, the Langmuir and Freundlich isotherm models. The Langmuir model, for homogeneous adsorption systems, is represented as (2)

where qe is the adsorption capacity (mg/g), Ce is the equilibrium concentration of Au(III) (mg/L), q is the saturated adsorption capacity (mg/g), and KL is the Langmuir adsorption constant (L/mg). The Freundlich isotherm model, which describes heterogeneous adsorption systems, can be expressed as ln qe = ln KF +

ln Ce n

T (°C)

q (mg/g)

KL (L/mg)

R2

KF (mg/g)

n

R2

15 25 35

257.73 390.11 465.16

0.0293 0.1288 1.0237

0.9898 0.9909 0.9723

64.68 124.60 219.69

4.6838 4.7259 2.5246

0.9933 0.9088 0.6516

equation than by the Freundlich equation, indicating that the adsorption of Au(III) by ODPA-BH obeys the Langmuir adsorption isotherm. It is well-known that the Langmuir equation is applicable to homogeneous adsorption, where the adsorption of each adsorbate molecule onto the surface has equal adsorption activation energy. The results show that the adsorption onto the adsorbent can be attributed to monolayer adsorption. The obtained maximum adsorption capacity of ODPA-BH for Au(III) was 257.73 mg/g at 15 °C, 390.11 mg/g at 25 °C, and 465.16 mg/g at 35 °C. Thus, this novel biomassbased adsorbent material has the best adsorption for Au(III). The adsorption capacity could reach to 465.16 mg/g with 30 mg of ODPA and an equilibrium gold concentration of 9.28 mg/L at pH 2.5 and 35 °C. The EDXAS results show that the weight percentage of P in ODPA-BH was 4.44%, so it is possible to calculate the amount of functional groups attached to the buckwheat hulls (F′, mmol/g) from the percentage of phosphorous in the functionalized buckwheat hull. The value of F′ for ODPA-BH was 1.25 mmol/g. The maximum capacity was 2.35 mmol/g (463.24 mg/g); therefore, the results show that other functional group such as hydroxyl groups also adsorbed gold ions from aqueous solution. As we compared the adsorption capacities of different types of adsorbents used for Au(III) adsorption in the literature, it was clear that the adsorption capacity of ODPA-BH was significantly higher than those of several other adsorbents such as L-lysine-modified cross-linked chitosan resin, thiol cotton fiber, and alfalfa biomass (Table S4, Supporting Information).4,21−24 The research results reported in this work show that ODPA-BH is favorable and useful for the removal of precious-metal ions, and its high adsorption capacity makes it a promising candidate material for Au(III) uptake. Adsorption Kinetics. Kinetic studies provide insight into both the adsorption rate and the mechanism of the adsorption process. To determine the uptake rate of Au(III) on ODPA-BH and the equilibrium time, studies of the adsorption kinetics were conducted. Figure 6a shows the kinetics of the adsorption of Au(III) onto ODPA-BH at 15−35 °C. It is clear that the adsorption capacity of ODPA-BH for Au(III) increased with increasing contact time. In the first 200 min, the adsorption was rapid, and it then slowed considerably. A possible reason for this behavior is that, in the initial fast adsorption step, Au(III) ions might easily enter accessible pore sites and bind with chelating ligands whereas, in the slow adsorption step, diffusion of some Au(III) ions into the deeper pores might be hampered. Figure 6a also shows the effect of temperature on the adsorption of Au(III) by the chelating resin ODPA-BH, which demonstrates that temperature generally has a positive effect on the adsorption capacity. From these experimental data, it is clear that about 90% of the equilibrium adsorption capacity could be achieved after 8 h and adsorption equilibrium

Figure 5. Isotherms of Au(III) adsorption onto ODPA-BH at different temperatures.

Ce C 1 = e + qe q qKL

Freundlich model

(3)

where qe is the adsorption capacity (mg/g), Ce is the equilibrium concentration of Au(III) (mg/L), n is the Freundlich constant, and KF is the binding energy constant reflecting the affinity of the adsorbent for Au(III) (mg/g). The isotherm parameters of the Langmuir and Freundlich models for the biosorption of Au(III) obtained using a leastsquares method are listed in Table 1. The fitting results show that the regression coefficients (R2) obtained for the Langmuir model were very close to 1 (0.9723−0.9909), suggesting that the Langmuir model can well interpret the studied adsorption procedure. From a comparison of correlation coefficients, it can be concluded that the data were fitted better by the Langmuir 8119



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Figure 6. (a) Kinetics of Au(III) adsorption onto ODPA-BH at different temperatures. (b) Bt versus time plots for Au(III) adsorption onto ODPABH at different temperatures (15 °C, R2 = 0.8717; 25 °C, R2 = 0.8760; 35 °C, R2 = 0.8845). (c) log Kc versus 1/T plot for Au(III) adsorption onto ODPA-BH. (d) Relationship between ln k and 1/T for ODPA-BH.

where qt is the amount of adsorbate taken up at time t and q0 is the maximum equilibrium uptake. Values of Bt can be obtained from corresponding values of F, and the plots of Bt versus time for Au(III) adsorption onto ODPA-BH at 15−35 °C were used to distinguish between filmdiffusion- and particle-diffusion-controlled adsorption. According to this analysis, plots that are straight lines passing through the origin reflect an adsorption process that is dominated by the particle diffusion mechanism; otherwise, the adsorption process is likely governed by film diffusion. As seen from Figure 6b, none of the Bt versus time plots passed through the origin in the cases studied. The linear equations of Bt−t for the lines in Figure 6b are Bt = 1.10t − 0.199 (15 °C), Bt = 0.890t − 0.157 (25 °C), and Bt = 0.631t − 0.109 (35 °C). Although none of the lines passed through the origin, the intercepts were near the origin, so the possibility of particle diffusion cannot be ruled out completely for the porous adsorbent ODPA-BH. Sun et al. investigated the Bt−t linear equation for Au(III) adsorption on polystyrene-supported bis-8-oxyquinoline-terminated open-chain crown ether and found that the rate-limiting adsorption process for Au(III) was particle diffusion.28 To investigate the mechanism of Au(III) adsorption onto ODPA-BH, pseudo-first- and pseudo-second-order equations were tested to fit the experimental kinetics data,29 and they can be expressed by the equations

was completely achieved within 14 h under the given test conditions. The adsorption of metal ions is generally considered to take place through the two mechanisms of film diffusion and particle diffusion. The kinetics experimental data are often analyzed by the Boyd and Reichenberg equations25−27 to distinguish filmdiffusion- from particle-diffusion-controlled adsorption. The relevant equations are F=1−

∞

6 π2

∑

6 π2

∑

n−1

2 2 1 ⎛ −Di tπ n ⎞ ⎜ ⎟ n2 ⎝ r0 2 ⎠

(4)

1 exp( −n2Bt ) 2 n

(5)

or F=1−

∞

n−1

with B=

π 2Di r0 2

= time constant (6)

where n is an integer that defines the infinite series solution; Di is the effective diffusion coefficient of metal ions in the adsorbent phase; r0 is the radius of the adsorbent particle, assumed to be spherical; and F is the fractional attainment of equilibrium at time t, which is expressed as q F= t q0 (7)

ln(qe − qt ) = ln qe − k1t 8120

(8)



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Table 2. Kinetic Parameters for the Adsorption of Au(III) by ODPA-BH at Different Temperatures pseudo-first-order kinetics

pseudo-second-order kinetics

T (°C)

qe,exp (mg/g)

k1 (min−1)

qe,cal (mg/)

R1 2

k2 (×103 g/mg·min)

qe,cal (mg/)

R22

15 25 35

144.22 ± 3.42 152.91 ± 1.29 171.75 ± 2.29

0.0041 0.0042 0.0050

74.44 81.45 182.14

0.8557 0.9746 0.9416

0.0297 0.0343 0.0417

144.45 153.14 171.99

0.9535 0.9562 0.9789

t 1 t = + 2 qt qe k 2qe

15, 25, and 35 °C, respectively) for Au(III) adsorption on ODPA-BH indicate that the adsorption process is spontaneous. The increase in ΔG with temperature shows that the adsorption is endothermic and more favorable at high temperature. The positive value of ΔH (70.93 kJ·mol−1) indicates that Au(III)-ion adsorption on ODPA-BH is endothermic. This result is consistent with the previously mentioned finding that the adsorption capacity of Au(III) ion increased with increasing temperature. The positive value of ΔS (257.25 J·K−1·mol−1) suggests an increase in the randomness at the solid/solution interface during the adsorption process. The magnitude of the activation energy (Ea) can provide information about whether the adsorption process is physical or chemical. The activation energy of the adsorption process was calculated by the Arrehenius equation

(9)

respectively, where qe is the amount of metal adsorbed at equilibrium per unit weight of adsorbent (mg/g); qt is the amount of metal ion adsorbed at time t; and k1 (min−1) and k2 (g/mg·min) are the rate constants of pseudo-first-order and pseudo-second-order adsorption, respectively. The experimental and calculated qe, k1, k2, and regression coefficient (R2) values are presented in Table 2. As can be seen from Table 2, the obtained coefficient values for the pseudo-second-order model (>0.9535) were better than those for the pseudo-firstorder model for the adsorbent (0.8557−0.9746), suggesting that the pseudo-second-order model is more suitable for describing the adsorption kinetics of Au(III) onto ODPA-BH. Moreover, the calculated qe values from the pseudo-secondorder model are much closer to the experimental values, qe(exp). Therefore, the adsorption kinetics can be wellapproximated by the pseudo-second-order kinetic model for Au(III) adsorption onto ODPA-BH. The pseudo-second-order kinetic model is based on the assumption that the ratecontrolling step in the adsorption process is the chemical interaction between superficial groups of the biosorbent and metal ions, and similar behavior has been reported for another adsorbent.29 The thermodynamic parameters for the adsorption process, such as the free energy of adsorption ΔG, enthalpy of adsorption ΔH, and entropy of adsorption ΔS, were calculated according to the equations30−34 Kc =

CAe Ce

log Kc =

ln k = −

(11)

ΔG = −RT ln Kc

(12)

where Ce and CAe are the equilibrium concentration in solution (mg/L) and the solid-phase concentration at equilibrium (mg/ L), respectively; Kc is the partition coefficient at each temperature; R is the gas constant (8.314 J/mol·K); and T is the temperature in kelvins. From the slope and y intercept of a linear plot of ln Kc versus 1/T (Figure 6c), the changes of enthalpy and entropy can be obtained. The thermodynamic parameters are listed in Table 3. The negative values of ΔG at all studied temperature (−3.59, −4.63, and −8.89 kJ·mol−1 at Table 3. Thermodynamic Parameters for the Adsorption of Au(III) by ODPA-BH T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/K·mol)

288 298 308

−3.59 −4.63 −8.89

70.93

257.25

(13)

where k is the pseudo-second-order rate constant of sorption (g/mg·min); A is the Arrhenius constant, which is a temperature-independent factor (g/mg·min); E a is the activation energy of sorption (kJ/mol); R is the gas constant (8.314 J/mol·K); and T is the absolute temperature (K). A straight line was obtained for a plot of ln k versus 1/T (Figure 6d), giving a slope of −Ea/R. The obtained activation energy value for ODPA-BH was 12.49 kJ/mol, which is smaller than that (16.29 kJ/mol) reported by Fujiwara et al.4 The activation energy for physical adsorption is relatively small, generally not more than 4.2 kJ/mol. However, chemical adsorption is specific, and the forces are much stronger than in physical adsorption.35 Therefore, the results suggest that the adsorption of Au(III) on ODPA-BH is chemical adsorption. Recycling Properties of ODPA-BH. To investigate the feasibility of reusing ODPA-BH, desorption experiments were conducted. Au(III)-ion-loaded ODPA-BH (60 mesh) was treated with 0.1 mol/L hydrochloric acid and different concentrations of thiourea at 25 °C for 24 h to remove the Au(III) ions and then neutralized, and a second round of metalion adsorption testing was conducted. The results of elution in Figure 7 show that the system of 5.0% thiourea + 0.1 mol/L HCl was very efficient and that the elution rate was 92.63%. The results for Au(III)-ion adsorption using the regenerated adsorbents are summarized in Table S5 (Supporting Information), and the Au(III) uptake capacities decreased gradually during successive uses. Adsorption of Gold and Copper Ions from GoldPlating Wastewater. The adsorption abilities of ODPA-BH from industrial wastewater systems (samples 1 and 2) were also investigated. The kinetics results for Au(III) and Cu(II) adsorption onto ODPA-BH from samples 1 and 2 at ambient temperature are shown in panels 1 and 4, respectively, of Figure 8. It is clear that the adsorption capacity of ODPA-BH for Au(III) increased with increasing contact time: the adsorption was rapid in the first 100 min and then slowed considerably. However, the adsorption capacity of ODPA-BH for Cu(II)

(10)

ΔS ΔH − 2.303R 2.303RT

Ea + ln A RT

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onto ODPA-BH. This reveals that the rate-limiting step might be chemical sorption involving valence forces through the sharing or exchange of electrons between ODPA-BH and metal ions.

■

CONCLUSIONS

ODPA-BH was successfully prepared simply by the thermochemical reaction of spent buckwheat hulls obtained from agricultural residues with organodiphosphonic acid in this work. The reported investigation demonstrates that ODPA-BH could be a potential adsorbent for removing Au(III) from aqueous solutions in both spiked samples and industrial wastewater samples. The modified organic groups were studied theoretically at the B3LYP/6-31G(d) level, showing that the oxygen atoms in the phosphonic acid groups and the hydroxyl group had more negative charges, providing advantages in chelating with metal ions. The thermodynamics and kinetics of the adsorption of Au(III) by ODPA-BH were studied in detail. The Langmuir and Freundlich isotherm models were applied to analyze the experimental data, and the best interpretation for the experimental data was given by the Langmuir isotherm equation. The maximum adsorption capacity for Au(III) was found to be 465.16 mg/g at 35 °C. Moreover, the results showed that the adsorption kinetics can be modeled by a pseudo-second-order rate equation. The thermodynamic parameters ΔG, ΔH, and ΔS were found to be −8.89 kJ·mol−1, 70.93 kJ·mol−1, and 257.25 J·K−1·mol−1, respectively. On the basis of the research results, it can be concluded that ODPABH can effectively be used for the uptake of Au(III) ions from aqueous solutions using the adsorption method.

Figure 7. Effects of thiourea concentration on the desorption rate.

remained low during the whole adsorption process. The adsorption capacity followed the order Au(III) > Cu(II), which is the same order of ionic radii for the metal ions studied. The greater the ionic radius, the smaller the hydrated ionic radius and the greater the affinity of the heavy-metal ions for the active sites of the adsorbent. The pseudo-first- and pseudo-secondorder kinetic plots and kinetic parameters for the adsorption of Au(III) and Cu(II) onto ODPA-BH at ambient temperature are shown in panels 2, 3, 5, and 6 of Figure 8, and the corresponding parameters are reported in Table 4. As can be seen from Table 4, the coefficient values obtained for the pseudo-second-order model (>0.9297) were better than those obtained for the pseudo-first-order model for the adsorbent (0.6648−0.9768), suggesting that the pseudo-second-order model is more suitable for describing the kinetics for the adsorption of Au(III) and Cu(II) in gold-plating wastewater

Figure 8. (1,4) Kinetics of Au(III) and Cu(II) adsorption onto BHJC from samples (1) 1 and (4) 2. (2,5) Pseudo-first-order kinetic plots for the adsorption of Au(III) and Cu(II) onto BHJC from samples (2) 1 and (5) 2. (3,6) Pseudo-second-order kinetic plots for the adsorption of Au(III) and Cu(II) onto BHJC from sample (3) 1 and (6) 2. 8122



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Table 4. Kinetic Parameters for the Adsorption of Au(III) and Cu(II) onto ODPA-BH from Samples 1 and 2 pseudo-first-order kinetics sample

ion

1

Au(III) Cu(II) Au(III) Cu(II)

2

■

qe(exp) (mg/g) 133.02 4.63 85.22 1.44

± ± ± ±

0.11 0.02 0.01 0.05

qe(cal) (mg/g)

R1 2

K2 (×10−3 g/mg·min)

qe(cal) (mg/g)

R22

0.0101 0.0046 0.0050 0.0045

171.54 3.47 71.12 1.48

0.9152 0.7800 0.9768 0.6648

0.0689 1.5634 0.0734 3.9690

151.52 5.48 99.90 1.59

0.9936 0.9875 0.9917 0.9297

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−Table S5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.Y.), [email protected] (R.Q.). Notes

The authors declare no competing financial interest. The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support provided by the National Natural Science Foundation of China (51102127 and 51073075), the Natural Science Foundation of Shandong Province (ZR2009BL014), and the Foundation of Innovation Team Building of Ludong University (08-CXB001) is greatly appreciated.

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pseudo-second-order kinetics

K1 (min−1)

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