Preparation and Ammonium Adsorption Properties ... - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 6034–6041

Preparation and Ammonium Adsorption Properties of Biotite-Based Hydrogel Composites Yian Zheng†,‡ and Aiqin Wang*,† Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, China

We present here a simple and effective strategy to obtain a series of hydrogel composites using biotite (BT) particles and poly(acrylic acid) (PAA) as the inorganic and organic components, respectively. These hydrogel composites were prepared at room temperature via a self-induced redox system based on ammonium persulfate and Fe(II) arisen from iron-rich BT. After NaOH activation, as-prepared hydrogel composites were used as the adsorbents to remove ammonium (NH4+) from its aqueous solution. The effects of preparation and adsorption conditions on NH4+ removal were investigated. The results show that the adsorption of NH4+ onto as-prepared PAA/BT (30%) is complete within 15 min, with the equilibrium adsorption capacity of 32.87 mg N/g at an initial NH4+ concentration of 100 mg N/L. In addition, the reusability of as-prepared PAA/BT for NH4+ removal was evaluated and the adsorption mechanism of NH4+ onto as-prepared hydrogel composite was proposed. 1. Introduction Today, large-scale wastewater production is an inevitable consequence of contemporary societies. Ammonium nitrogen (NH4+), an important member of the group of nitrogencontaining compounds that act as nutrients for algae and plants,1 is associated with both point and diffuse source pollution including effluent from sewage treatment works and surface runoff from agriculture practices such as fertilizer and manure application. Nitrogen pollution of watercourses can lead to eutrophication, and it is this that could be a major and global environmental problem and could pose a direct threat to public health.2,3 Therefore, total removal or at least a significant reduction of NH4+ in water is thus obligatory prior to disposal into streams, lakes, seas, and land surfaces. The traditional method for NH4+ removal from municipal and industrial wastewaters is biological treatments based on aerobic nitrification and anoxic denitrification,4,5 both of which may produce nitrous oxide (N2O).6 N2O formation can occur during the oxidation of ammonia to nitrite and N2O gas has an adverse impact on the environment.5 When toxic upsets occur the system receives a loading shock, resulting in lower treatment efficiency or process.7 If toxicants do hinder the activity of nitrifying bacteria, it will cause leakage of ammonia into the effluent, often resulting in breaches of discharge consents, by which unacceptable peaks may appear in the effluent of nitrogen-containing wastewater.8 Adsorption is a conventional but important separation process and due to its high efficiency, easy handling, and availability of different adsorbents, adsorption technology has been used widely in the chemical, biological, analytical, and environmental fields. For NH4+ removal, Bernal and co-workers used natural zeolites and sepiolite as the adsorbent materials and found that NH4+ adsorption capacities of zeolites were from 8.149 to 15.169 mg N/g; up to 10.3 times higher than that of sepiolite.9 The studies from Lebedynets et al. revealed that the equilibrium * To whom correspondence should be addressed. Tel.: +86-9314968118. Fax: +86-931-8277088. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.

adsorption time was 24 h and the maximum adsorption capacity was 11.6 mg/g at an initial NH4+ concentration of 1000 mg/L using clinoptilolite as the adsorbent.10 Vassileva et al. observed that the optimum time to establish adsorption equilibrium between activated carbons and the ammonium ions of different concentrations was 2 h with the monolayer adsorption capacity of 11.57 mg/g.11 Maranõń et al. used volcanic tuff as the adsorbent to remove NH4+ and found that the maximum adsorption capacity was 19 mg/g within a contact time of 3 h.12 In most cases, the adsorbents have large internal porosities or adequate surface area for adsorption, such as activated carbon,11 zeolite,10,13,14 and natural (or modified) clays.15 However, the diffusion limitations within the particles may decrease the adsorption rate and the available adsorption capacity. So, it is urgent to develop an adsorbent with a small diffusion resistance, and a high adsorption capacity for NH4+. Hydrogels are defined as water-swollen, three-dimensionalstructured polymer networks. They can absorb a large amount of water compared with other water-absorbing materials and have became ubiquitous and indispensable materials in many applications. In addition, these hydrogels can be prepared with some smart characters and then exhibit single or multiple stimuli-responsive properties to various external parameters such as temperature,16 pH,17,18 and solvent composition.19 Because of superhydrophilicity, these hydrogels can swell quickly in an aqueous solution, allowing thus lower mass transfer resistance, which is responsible for the faster adsorption rate.20 Besides, a large quantity of functional groups can be realized within the polymeric networks, making them exhibit some specific adsorption sites with affinity for heavy metals or dyes.20-22 By consideration of the limitations of pure polymeric hydrogels, such as poor gel strength and stability, some inorganic clay minerals such as attapulgite,23 montmorillonite,24 vermiculite,25 and sepiolite26 have been incorporated into hydrogel networks. Clays are natural, abundant, and inexpensive materials that have high mechanical strength and chemical resistance. So, the preparation of organic-inorganic hydrogel composite has attracted increasing attention, and some hydrogel composites have been used as the adsorbent to adsorb toxic pollutants in water. Li and co-workers prepared a hydrogel composite by

10.1021/ie9016336  2010 American Chemical Society Published on Web 06/15/2010

Ind. Eng. Chem. Res., Vol. 49, No. 13, 2010

introducing Laponite into poly (acrylamide) hydrogel and found that this type of hydrogel could be used as an excellent adsorbent for removing Basic Blue 12 (BB 12), Basic Blue 9 (BB 9), and Basic Violet 1 (BV 1).27 Wang et al. utilized chitosan-g-poly (acrylic acid)/montmorillonite as the adsorbent to investigate its efficacy for removing methylene blue and the maximum adsorption capacity was found to be 1859 mg/g even when 30 wt % montmorillonite was introduced.28 Kasg¸o¨z et al. reported that the clay incorporation into the hydrogel can not only increase the adsorption capacity but also speed the adsorption rate, leading the adsorption system to reach equilibrium within a few minutes.20 Biotite (BT, ideal formula [K(Mg,Fe)3(Si3Al)O10(OH)2]) is a 2:1 phyllosilicate mineral with tightly held, nonhydrated interlayer cations. The 2:1 layer has octahedrally coordinated cations sandwiched between two sheets of Si, Al tetrahedra. The main cations in the octahedral layer are Mg and divalent iron Fe(II). Structurally bound Fe on or beneath the surface of BT can control the redox state of associated solutes, and then BT can be used for the reduction of chromate.29,30 Because of the stronger reducing ability, it is expected that BT added into the solution and ammonium persulfate (APS) present in the solution would construct a self-induced redox system by which the hydrogel composite would be prepared at room temperature without additional heating and reagents. Reducing ability of BT is unique among the silicate minerals and has not attracted much attention. Consequently, the aim of this study is: (i) to prepare a series of hydrogel composites at room temperature using the reducing ability of BT; (ii) to evaluate the potential of as-prepared adsorbent for NH4+ removal; (iii) to investigate the adsorption kinetics, adsorption isotherms, regeneration, and reusable ability of as-prepared adsorbent; (4) to reveal the adsorption mechanism of this kind of adsorbent for NH4+ removal. 2. Experimental Section 2.1. Chemicals. Acrylic acid (AA, chemically pure, Shanghai Shanpu Chemical Factory, Shanghai, China) was distilled under reduced pressure before use. Ammonium persulfate (APS, analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and N,N’-methylene-bisacrylamide (MBA, chemically pure, Shanghai Yuanfan additives plant, Shanghai, China) were used as received. Biotite (BT, Longyan Colloidal Co., Ltd., Fujian, China) was milled through a 200 mesh sieve prior to use, with a chemical composition of analytical SiO2 40.30 wt %, MgO 21.82 wt %, Al2O3 18.88 wt %, Fe2O3 9.68 wt %, K2O 8.17 wt %, TiO2 0.42 wt %, CaO 0.29 wt %, and Na2O 0.21 wt %. Other reagents used were all analytical grade, and all solutions were prepared with distilled water. Acid activation of BT was performed according to the following procedure. Ten grams of BT powder was immersed in 100 mL of 2 mol/L HCl solution for 2 h at room temperature under stirring (1250 rpm) and then isolated from the solution by centrifugation, washed with distilled water for several times until no Cl- was detected. The product was dried and ground to 200-mesh size prior to use. After acid-activation, BT has a chemical composition of analytical SiO2 42.65 wt %, MgO 21.45 wt %, Al2O3 18.17 wt %, Fe2O3 9.13 wt %, K2O 7.84 wt %, TiO2 0.36 wt %, CaO 0.06 wt %, and Na2O 0.19 wt %. 2.2. Preparation of PAA/BT Composites. A series of hydrogel composites with different amount of BT were prepared by the following procedure. AA (7.2 g) and an appropriate amount of MBA were dissolved in 30 mL of distilled water in a four-neck flask equipped with a stirrer, a condenser, a

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thermometer, and a nitrogen line until MBA was dissolved completely. Then, a calculated amount of BT powder (weight ratio of BT to the total amount of material involved except for water) was dispersed. The mixture was stirred at room temperature 14 ( 2 °C for 30 min while radical initiator APS (80.2 mg) was added. Within a few seconds, the hydrogel composite was formed. After polymerization, the resulting product was dried at 70 °C to a constant weight. The dark product was milled through a 40-80 mesh sieve prior to use. 2.3. Measurement of Swelling Ratio. A series of accurately weighed dried samples (m1, 0.05 g) were immersed in 100-mL NaOH solutions (pH ) 12) at room temperature until swelling equilibrium was achieved. Swollen samples (m2) were then separated by filtering with 100-mesh stainless screen and hanging up for 10 min. The equilibrium swelling ratio (Q) was calculated according to the following equation Q ) (m2 - m1)/m1

(1)

where m1 and m2 are the weights of the dry sample and the swollen sample, respectively. Q was calculated as grams of liquid per gram of sample. 2.4. Adsorption and Analytical Procedure. Prior to adsorption, a series of dried hydrogels (0.05 g) were immersed in NaOH solutions with different pH values until swelling equilibrium was achieved. It should be mentioned here that an initial pH value of solution was adjusted according to the neutralization degree (ND, defined as the molar ratio of sodium hydroxide moiety to total AA moiety in the adsorbent) of AA. The swollen samples were then separated and transferred to a 50-mL conical flask while 25 mL of NH4+ solution was added. The mixture was shaken in a thermostatic shaker (THZ-98A) at 30 °C/120 rpm for a given time, and then the suspension was centrifuged at 4500 rpm for 10 min. The NH4+ concentration in the solution was measured according to Nessler’s reagent colorimetric method. The adsorption capacity of hydrogel composite for NH4+ was calculated from the following equation qe )

C0V1 - CeV2 m

(2)

where qe is the adsorption capacity of NH4+ onto as-prepared hydrogel adsorbent (mg N/g), C0 is the initial NH4+ concentration (mg N/L), Ce is the equilibrium NH4+ concentration (mg N/L), m is the mass of dried adsorbent used (mg), and V1 and V2 are the volumes of NH4+ solution before and after the adsorption (mL). According to the adsorption capacity, the optimum pH allowing the hydrogel composite to swell was chosen. Thereafter, the effect of contact time on the adsorption capacity was studied by agitating 25 mL 100 mg N/L NH4+ solution with swollen hydrogel composite (0.05 g dried weight). At the end of predetermined time intervals the adsorbent was separated and the NH4+ concentration remaining in the solution was analyzed. The effect of pH on NH4+ removal was studied by contacting 25 mL of 100 mg N/L NH4+ solution adjusted to initial pH 2.0-9.0 with swollen hydrogel composite (0.05 g dried weight) for 30 min. HCl or NaOH solutions (0.1 and 1.0 mol/L) were used to adjust the pH values required. To obtain the adsorption isotherms, 25 mL of 10-2000 mg N/L NH4+ solution was contacted with swollen hydrogel composite (0.05 g dried weight) for 30 min, and then the adsorbent was removed and the NH4+ concentration was measured on the supernatant liquid. 2.5. Evaluation of Reusability. Reusability was determined according to the following procedures: (i) Swollen hydrogel

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adsorbent (dried weight: 0.05 g) was contacted with 25 mL of NH4+ solution with the concentration of 100 mg N/L and shaken in a thermostatic shaker at 30 °C/120 rpm for 30 min, and then the suspension was centrifuged at 4500 rpm for 10 min. The analytical result was recorded as the first adsorption capacity; (ii) NH4+-loaded adsorbent was separated from the NH4+ solution and stirred in 25 mL of 0.1 mol/L NaOH solution for 10 min. After centrifugation, the adsorbent was eluted with 20 mL distilled water for several times. The obtained swollen adsorbent was used as the adsorbent to contact again with 25 mL 100 mg N/L NH4+ solution, and the suspension was shaken in a thermostatic shaker at 30 °C/120 rpm for 30 min to obtain the second adsorption capacity; (iii) a similar procedure was repeated and the adsorption capacity after several times was then achieved. 2.6. Characterization. The contents of elements in the BT powder were detected by a PANalytical Magix (PW2403) X-ray fluorescence analyzer. X-ray diffraction (XRD) patterns were obtained from an X’Pert PRO diffractometer (Cu-Ka radiation, 40 kV, 30 mA). Scanning electron microscopy (SEM) studies were carried out in a JSM-5600LV SEM instrument (JEOL) after coating the sample with gold film. Fourier-transform infrared (FTIR) spectra of the samples were recorded over the range of 400-4000 cm-1, in a Thermo Nicolet NEXUS TM FTIR spectrometer using KBr pellets. 3. Results and Discussion 3.1. XRD Patterns. Fe(II) is an important reductant that is found in many silicates, such as BT in this study. Under acidic conditions, BT dissolution would occur, leading thus the release or exposure of Fe(II) into the surrounding solution. This can be testified by the observed difference in the content of Fe2O3, one of the major components of BT. Before and after acid activation, Fe2O3 content is determined to be 9.68 and 9.13%, respectively. In this case, Fe(II) released into the solution and APS present in the solution would construct a self-induced redox system, and the polymerization reaction of AA would take place at room temperature without additional heat and reagents. As a consequence, inorganic BT particles are expected to be embedded within the polymeric networks, forming an organic-inorganic hydrogel composite. XRD patterns of BT and PAA/BT with different BT content were shown in Figure 1. It seems that secondary sheet silicate vermiculite25 (2θ ) 6.23, basal spacing 14.2 Å) is observed in the powder XRD pattern of natural BT. After polymerization, none of this phase is detected indicating that these vermiculite flakes are exfoliated. However, compared to natural BT, PAA/ BT shows a similar diffractogram except for a significant reduction in the reflection intensities, as a result of lower content of BT in the hydrogel composite. Therefore, it can be concluded that when the hydrogel composite is formed, the BT particles are present within the polymeric network without destroying the crystalline structure. 3.2. SEM Morphology. The surface morphologies of PAA/ BT hydrogel composites were shown in Figure 2. It can be observed that PAA/BT shows a tight surface. When more BT particles were introduced into the hydrogel composite, more protuberances appeared and spread on the surface, which is ascribed to the existence of a large number of BT particles embedded within the polymeric matrix. Generally, a coarse and undulant surface can benefit for the water permeation, making thus the adsorbate be easily accessible.

Figure 1. Powder XRD patterns of BT. (a) PAA/BT (5 wt %), (b) PAA/ BT (10 wt %), and (c) PAA/BT (30 wt %).

3.3. Effects of Preparation Conditions on Adsorption Capacity. As shown in Figure 3, with increasing ND of AA, a monotonic increase in adsorption capacity is observed until 100% ND is reached at that point the adsorption capacity seems to be highest. Furthermore, only a few milligrams of NH4+ are adsorbed by PAA/BT without neutralization with alkaline solution. This result is expected to be correlative with the number of -COO- groups distributed on the polymer chains. With increasing ND of AA, the number of negatively charged carboxylate groups attached to the polymer chains tends to increase, and accordingly, more positively charged NH4+ molecules will be captured within the hydrogel networks. During the whole adsorption process, the swelling ratio of as-prepared hydrogel composite keeps almost constant after 40% ND is achieved. On the basis of the above discussions, it can be deduced that the electrostatic attraction between adsorbent and adsorbate, rather than the expanded hydrogel networks, may dominate the whole adsorption process. Hydrogels are defined as water-swollen, three-dimensionalstructured polymer networks. When hydrogels are swollen in an aqueous solution, the size of polymeric network would be altered according to the cross-linking degree during the preparation process. Cross-linking degree has a direct effect on the level of swelling ratio of a hydrogel (Figure 4). But, it is clear here that the cross-linking degree has a neglectable effect on the adsorption capacity. When MBA content is increased from 0.2 to 1.5 wt %, the maximum difference in adsorption capacity is found to be only 0.8 mg N/g. That is, no significant increase or decrease in adsorption capacity is visible with changing MBA content in this study, implying that the constructed threedimensional networks can only “catch” a few NH4+ molecules, but has no important contribution to its final adsorption capacity. Biotite is considered as a cost-effective inorganic component, and the introduction of a large amount of BT particles into the hydrogel allows to obtain a novel type cost-effective adsorbent. The effects of BT content on the adsorption capacity were investigated, as shown in Figure 5. The adsorption capacity for NH4+ increases with increasing BT content up to a maximum value of 10 wt % and then decreases with further increase in BT content. This maybe attributed to the higher swelling ratio of hydrogel composite with 10 wt % BT. Higher swelling ratio can benefit for the higher adsorption capacity, and similar results have been reported by Kasg¸o¨z and co-workers.22 Compared with


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Figure 2. SEM images for (a) PAA/BT (5 wt %, 200×), (b) PAA/BT (5 wt %, 1000×), (c) PAA/BT (30 wt %, 200×), and (d) PAA/BT (30 wt %, 1000×).

Figure 3. Effects of ND of AA on adsorption capacity and swelling ratio of PAA/BT (10 wt %).

Figure 4. Effect of MBA content on adsorption capacity and swelling ratio of PAA/BT (10 wt %).

hydrogel composite with 5 wt % BT, the introduction of 30 wt % BT into the polymer networks can afford comparable adsorption capacity for NH4+ and lower application cost and thus is chosen to investigate its potential as a candidate for NH4+ removal from an aqueous solution. For these hydrogel composites, the swelling ratio is considered as the key property. Based on above discussions, it can be concluded that though swelling ratio can contribute to the final adsorption capacity, it is not the decisive one. 3.4. Effects of Solution Conditions on Adsorption Capacity. Contact time is an important variable in the adsorption process. Preliminary experiment showed that 80% NH4+ can be adsorbed by PAA/BT hydrogel composite within 1 min and 90% NH4+ can be adsorbed within 5 min. The adsorption equilibrium can be achieved with 15 min, meaning that it is a fast process for the adsorption of NH4+ onto as-prepared hydrogel composite. The result could be reasonably attributed to the decrease in diffusion resistance and the presence of functional groups distributed on polymer chains. When contacting with water, as-prepared hydrogel composite can be swollen

quickly as a result of anion-anion electrostatic repulsion forces, expanding the polymeric networks of as-prepared hydrogel composite. Then, the diffusion limitations within the hydrogel networks are decreased, leading the adsorption system to reach equilibrium quickly. The adsorption capacity of PAA/BT (30%) at the equilibrium condition is observed to be 32.87 mg N/g. The adsorbent used in this study is an ionic polymer networks and then the dissociation of functional groups on the active sites of the adsorbent would be considerably affected by changing initial pH values. As shown in Figure 6, the equilibrium adsorption capacity of PAA/BT (30%) for NH4+ keeps roughly constant in the pH range from 4.0 to 9.0, which is correlated with the transformation between -COOH and -COO- groups, buffer action of carboxyl and carboxylate groups. This is a typical characteristic of hydrogel composites consisting mainly of PAA, and similar results have been observed in our earlier studies.19,25 In a stronger acid medium, the adsorption capacity of PAA/BT (30%) exhibits a sudden decrease. This is due to some -COO- groups have transformed to -COOH groups, which cannot facilitate the electrostatic interaction between

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Figure 5. Effect of BT content on adsorption capacity and swelling ratio of PAA/BT.

Figure 6. Adsorption capacity and equilibrium pH values as a function of external pH.

PAA/BT and positively charged NH4+. Nevertheless, the equilibrium pH value may control the whole adsorption process as a result of faster achievement of adsorption equilibrium. It is measured that the equilibrium pH values lie within pH 6.0-7.5 except for 4.6 when an initial pH is fixed at 2.0. It is reported that the pka value of PAA is about 4.7,31 and then the carboxylic acid groups can be easily ionized above pH 4.7, which cannot be ignored to be the reason for lower adsorption capacity of PAA/BT for NH4+ at lower pH value. There is no doubt that as-prepared hydrogel adsorbent shows its potential to be applied in a wide pH range. Figure 7 shows the effects of NH4+ concentration on adsorption capacity. Increasing NH4+ concentration has a dramatic positive impact on NH4+ removal. Generally, higher

Figure 7. Adsorption isotherm data for NH4+ onto PAA/BT (30 wt %).

adsorbate concentration provides higher driving force, resulting in more collision between adsorbate ions and active adsorption sites on PAA/BT surface.32 This result is encouraging, as higher adsorption capacity is one of the important parameters for effective wastewater treatment. The equilibrium adsorption isotherm, usually the ratio between the quantity adsorbed and that remaining in solution at a fixed temperature at equilibrium, is fundamentally important since the equilibrium studies give the adsorption capacity of an adsorbent and describe the adsorption isotherm by constants whose values express the surface properties and affinity of an adsorbent for adsorbate. In addition, a reliable prediction of adsorption parameters and quantitative comparison of adsorption behavior for different adsorbents can be obtained and evaluated by an accurate description of equilibrium adsorption capacity. Among many isotherms, the two most frequently used equations applied in solid/liquid systems are the Langmuir and Freundlich models which describe respectively the adsorption behavior onto the monolayer homogeneous and heterogeneous sites, as described by qmbCe 1 + bCe

(3)

Freundlich equation qe ) KCe1/n

(4)

Langmuir equation qe )

where qe is the equilibrium adsorption capacity of NH4+ onto adsorbent (mg N/g), Ce is the equilibrium NH4+ concentration (mg N/L), qm is the monolayer adsorption capacity of the adsorbent (mg N/g), b is the Langmuir adsorption constant (L/ mg), K is a constant related to the adsorption capacity (L/g),

Scheme 1. Model Scheme for the Adsorption of NH4+ onto PAA/BT Hydrogel Composite


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Figure 8. FTIR spectra of PAA/BT (30 wt %) before swelling (a), preswollen (b), and after adsorption of NH4+ (c).

Figure 9. The amount adsorbed for NH4+ as a function of adsorption-desorption cycle. Adsorption conditions: contact time 30 min; 120 rpm; 30 °C; initial NH4+ concentration 100 mg N/L; pH 6-7. Desorption conditions: 25 mL 0.1 mol/L NaOH/10 min, 20 mL distilled water/5 min/twice.

and 1/n is an empirical parameter related to the adsorption intensity, which varies with the heterogeneity of the adsorption material. By nonlinear regression of the experimental data, the calculated qm, b, and correlation coefficient R2 are 344.47 mg N/g, 0.00256 L/mg, and 0.9756 for PAA/BT (30 wt %), respectively. Such high adsorption capacity of PAA/BT toward NH4+ is not comparable by other adsorbents. It is reported that the maximum amounts of exchanged NH4+, calculated from the Langmuir model, were 11.5, 8.121, 6.588, 9.479, 9.64, and 8.61 mg/g using transcarpathian clinoptilolite,10 natural Turkish clinoptilolite,33 New Zealand clinoptilolite,34 mordenite,34 natural Turkish (Yildizeli) zeolite,35 and zeolite 13X13 as the

adsorbents, respectively. The adsorption capacity of as-prepared hydrogel composite is calculated to be about 30 times those reported adsorbents. We had utilized chitosan-g-poly (acrylic acid)/rectorite (CTS-g-PAA/REC) as the adsorbent to investigate its adsorption capacity for NH4+ and found that the maximum adsorption capacity was 109.2, 123.8, and 61.95 mg N/g for CTS-g-PAA, CTS-g-PAA/REC (10 wt %), and CTS-g-PAA/ REC (30 wt %), respectively.36 Obviously, as-prepared PAA/ BT hydrogel composite possesses higher adsorption capacity for NH4+. In addition, it is observed that the linearity from Langmuir model is not very high, and then the Freundlich model is used to describe the adsorption behavior. The results indicate that by using the Freundlich model a better linearity is attainable with K, n, and R2 is 7.40 L/g, 1.94, and 0.9922, respectively. Then, Freundlich model can explain the adsorption data well. 3.5. Adsorption Mechanism. For positively charged NH4+, the predominant adsorption mechanism may be related to the ionized -COOH groups (in the form of -COO-) present within PAA/BT polymeric networks. When the hydrogel composite immerses in NaOH solution, the water molecules penetrate quickly into the hydrogel composite and dissociate -COOH groups to -COO- groups, resulting in a dimensional increase of polymeric networks as a result of the negative charge repulsion. Subsequently, NH4+ molecules in the solution are diffused and trapped quickly within the polymeric networks due to the ionic interaction between positively charged NH4+ and negative adsorption sites of the adsorbent (-COO-), as shown in Scheme 1. This can be testified by comparing the FTIR spectra before and after adsorption for NH4+ (Figure 8). It is observed that after swelling in NaOH solution the asymmetric vibration absorption (νasCOO) and symmetric vibration absorption (νsCOO) of carboxylic group are obviously enhanced. However, after the adsorption, all the main absorption bands of PAA/BT can still be observed only a slight shift of νasCOO from 1 575 to 1 571 cm-1. Previous results show that PAA/BT before swelling

Figure 10. The color in residual solution after adsorption with different adsorbents in Taihu Lake. (1) Reference, (2) attapulgite, (3) vermiculite, (4) activated carbon, 5-PAA/BT (30 wt %). Initial NH4+ concentration is determined to be 7.37 mg N/L.

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has a negligible adsorption capacity for NH4+, and then the significant improvement in NH4+ adsorption is attributed to the appearance of νasCOO and νsCOO-, confirming that the electrostatic attraction is the dominant adsorption mechanism during the whole experiments. 3.6. Reusability. A good adsorbent, in addition to its fast adsorption rate and high adsorption capacity, must also exhibit good regeneration ability for multiple uses. Preliminary experiment indicated that this hydrogel adsorbent can be easily regenerated under quite mild conditions, and the adsorbed NH4+ can be almost desorbed completely upon contacting with 25 mL 0.1 mol/L NaOH solution for 10 min. Then, the reusability of PAA/BT for adsorbing NH4+ is evaluated, as shown in Figure 9. Only a slight decrease in the adsorption capacity is observed after six cycles of adsorption-desorption process, confirming the reusability of as-prepared hydrogel composite for NH4+ removal. Experimental results clearly demonstrate that asprepared hydrogel composite is amenable to efficient regeneration and reusable for multiple cycles of adsorption. 3.7. Application. For the adsorbent technology, adsorption materials as the key parameter in adsorption process should be attracted much attention. In recent years, polymeric hydrogels have generated great interests for wastewater treatment due to their superior properties such as high adsorption capacity and fast adsorption rate. PAA/BT in this study can be easily synthesized at room temperature as a result of a well construction of self-induced redox system after the addition of ironrich BT. This hydrogel adsorbent shows fast adsorption kinetics and quite higher adsorption capacity for NH4+ within a wider pH range. To assess the efficacy of as-prepared hydrogel adsorbent in real water body, a natural water sample collected from Taihu Lake in Jiangsu was used. With the addition of Nessler’s reagent, the appearance of a yellow coloration indicates the presence of NH4+ in the residual solution and the intensity of color is directly proportional to NH4+ concentration. We have found that after the adsorption using PAA/BT as the adsorbent the color is the lightest and comparable with that of reference (Figure 10). However, a deep yellow color is observed by the treatment of some clays and activated carbon, meaning that a large amount of NH4+ is still present in the residual solution. The adsorption capacity of PAA/BT for NH4+ in Taihu Lake Water is about 10, 13, and 50 times than those of activated carbon, vermiculite, and attapulgite, suggesting that PAA/BT is a promising adsorbent for NH4+ removal in real water body. 4. Conclusions Adsorption is a robust and effective technique used in water and wastewater treatments. In comparison with conventional biological methods, adsorption technology offers the advantages of low operating costs, easy handing, and availability of different adsorbents. In this study, we prepared a series of hydrogel composites at room temperature using a self-induced redox system. The ND of AA and BT content in the hydrogel composite can produce significant effects on the adsorption capacity for NH4+, while the cross-linking degree shows no obvious contribution in spite of the significant dependence between swelling ratio and cross-linking degree. The adsorption equilibrium of NH4+ onto as-prepared hydrogel composite can be complete within 15 min. As-prepared hydrogel composites can be applied in a wide pH range (4.0-9.0) with the maximum adsorption capacity of 344.47 mg N/g and has better regeneration and reusable abilities. During the whole adsorption process, the electrostatic attraction between NH4+ and -COO- groups is the governed adsorption mechanism.

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ReceiVed for reView October 19, 2009 ReVised manuscript receiVed May 28, 2010 Accepted June 4, 2010 IE9016336

Preparation and Ammonium Adsorption Properties ... - ACS Publications

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