Article pubs.acs.org/IECR
Low-Cost Carbon Nanospheres for Efficient Removal of Organic Dyes from Aqueous Solutions Xianghua Song,† Yabo Wang,† Kean Wang,‡ and Rong Xu*,† †
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459 Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates
‡
ABSTRACT: Colloidal carbon nanospheres (CNS) with rich surface functional groups of −OH and −COO− were prepared from glucose solution via hydrothermal reaction and activated by NaOH solution. The nonporous CNS exhibited excellent adsorption performance toward basic dyes. High maximum adsorption capacities were obtained at 682 mg g−1 for Methylene blue, 395 mg g−1 for Methyl violet 2B, and 310 mg g−1 for Malachite green. Methylene blue with an initial concentration of 94 mg L−1 can be completely removed in 5 min at a dosage of 0.5 g L−1. This can be associated with the low mass transfer resistance due to the nonporous structure and the abundant surface active sites. The adsorption process is chemisorption in nature, while the kinetic data were well fitted to pseudosecond-order kinetic model. This material presented excellent adsorption capacities toward basic dyes with maximum adsorption capacity of 682 mg g−1 for Methylene blue B, 310 mg g−1 for Malachite green, and 395 mg g−1 for Methyl violet 2B. Furthermore, the dye saturated CNS was regenerated using an advanced oxidation method using Co2+ in aqueous solution as a homogeneous catalyst. After seven recycle runs, there was still 96% of adsorption capacity retained. The low-cost CNS nanomaterial has the potential to be applied as a new type of efficient adsorbent for water treatment.
1. INTRODUCTION Water contamination by dyes has become a serious environmental issue due to the rapid development of paper, battery, textile, and other industries. Most dyes are toxic and sometimes carcinogenic, which may significantly affect the photosynthetic activity in aquatic life.1 Dyes are generally resistant to aerobic digestion, and treating wastewater containing dyes is not a simple task.2 The commonly used methods for dye removal include coagulation, flocculation, adsorption, membrane filtration, oxidation, etc.3−5 Among these, liquid phase adsorption has been an attractive and promising option due to the advantages of flexibility and simplicity in design and operation.6−9 Activated carbons (AC) have been widely used as the efficient adsorbents in wastewater treatment due to their high surface areas and micro-/mesoporous structures.1,10−14 However the relatively high production cost, high synthesis temperature (600−900 °C), and the difficulty in regeneration remain the disadvantages of AC.15,16 Therefore, the development of high performance, low cost, and easily recyclable adsorbents has attracted considerable research attention. Colloidal carbon nanospheres (CNS) of controllable sizes (200−800 nm) can be synthesized from the abundant monosaccharide precursors (glucose, fructose, etc.) under mild hydrothermal conditions (140−180 °C).17−21 This process is simple and environmentally friendly involving no toxic reagents and solvents. The resultant nonporous CNS constitutes a hydrophilic surface enriched with functional groups of −OH and −CO.17,22 Up to now, CNS has been utilized as catalyst support for deposition of metal nanoparticles,23,24 template for fabrication of hollow metal/metal oxides nanostructures,19,25−27 and separation of water pollutants.28,29 Recently, our group reported that the surface of the as-prepared CNS can be readily enriched with −OH and −COO− groups by a simple activation process in aqueous © 2012 American Chemical Society
solution of NaOH. The resultant CNS with a large negative zeta potential (−42.7 mV) exhibited excellent performance for fast removal of Ag(I) ions from aqueous solutions.30 It is expected that such a nonporous adsorbent material with a negatively charged surface would be advantageous compared to conventional ACs with micro-/mesopores, in particular, for adsorption of bulky molecules like cationic organic dyes. In this work, the adsorption capacity and the kinetics of NaOH activated CNS (CNS/OH) were investigated for three basic dyes. The adsorption mechanism on CNS/OH was explored, while the effects of pH and ionic strength on adsorption were discussed. Furthermore, the dye exhausted CNS/OH was regenerated by the sulfate radical based advanced oxidation method.
2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis of the Adsorbent, CNS/ OH. Alpha-D(+)-glucose (99+%, anhydrous) was obtained from Acros Organics, NaOH (98.95%) from Fisher Scientific. Methylene blue (MB, C16H18N3SCl·3H2O, M.W. = 373.9), Methyl violet 2B (MV, C24H27ClN3, M.W. = 393.9), and Malachite green oxalate (MG, C52H54N4O12, M.W. = 927.02) were supplied by Sigma-Aldrich. Oxone (2KHSO5·KHSO4·K2SO4, 4.7% active oxygen) was purchased from Alfa Aesar. Co(NO3)2·6H2O (98%) was obtained from GCE. Extra pure charcoal activated carbon (AC) was supplied by Scharlau Chemie. The molecular structures of the dyes are shown in Figure 1. The experimental details for the synthesis of CNS and the subsequent activation with NaOH aqueous Received: Revised: Accepted: Published: 13438
April 7, 2012 September 19, 2012 September 21, 2012 September 21, 2012 dx.doi.org/10.1021/ie300914h | Ind. Eng. Chem. Res. 2012, 51, 13438−13444
Industrial & Engineering Chemistry Research
Article
NaOH aqueous solution, followed by washing with deionized water. It was found that there was a loss of about 30% CNS during regeneration mainly due to the washing process, as not all the CNS particles can be collected by centrifugation. To eliminate the effect of CNS loss, a few parallel batches of adsorption/regeneration were conducted at the same time and under the same conditions. After each run, the regenerated CNS from these parallel batches was mixed, and the same amount of CNS was taken out from this mixture for the next run, at the expense of decreasing the number of batches with recycling runs increased. To minimize the production of secondary wastewater, the Co(NO3)2 and NaOH solutions were reused during the regeneration and reactivation steps, without the addition of fresh solutions for each run.
3. RESULTS AND DISCUSSION 3.1. Properties of Carbon Nanospheres. As presented in our previous work, the as-prepared nonporous CNS has an average diameter of around 400 nm and a specific surface area of 12.7 m2 g−1. After NaOH activation, CNS/OH possessed abundant surface functional groups of −OH and −COO− as indicated by FTIR results and a negative zeta potential.30 As such, it was expected that the adsorption of cationic dyes could be enhanced through chemical bonding and electrostatic interaction. 3.2. Adsorption Isotherm. The CNS/OH samples presented excellent adsorption efficiencies for the three basic dyes. For example, the removal percentage of these dyes was all almost 100% when the initial concentrations were 303 mg L−1 (MB), 147 mg L−1 (MG), and 186 mg L−1 (MV), respectively. The adsorption isotherms of the three dyes on CNS/OH and native CNS were measured and shown in Figure 2 as symbols.
Figure 1. Molecular structures of basic dyes investigated.
solution have been described in our earlier report.30 In this work, the concentration of NaOH solution for CNS activation was kept at 0.5 M. As a control sample, AC was also treated with 0.5 M NaOH aqueous solution, and the as-obtained sample was denoted as AC/OH. 2.2. Dye Adsorption Study. The batch adsorber was used to study the adsorption capacities and kinetics of the three dyes on the native CNS and CNS/OH. All the experiments were carried out at the ambient temperature. For equilibrium experiments, 12.5 mg of the solid adsorbent was dispersed in 25 mL of aqueous dye solution with a certain initial concentration in the range of 150−460 mg L−1, 90−350 mg L−1, and 90−300 mg L−1 for MB, MV, and MG, respectively. The adsorption was carried out at room temperature for 24 h with orbital shaking at 300 rpm. During the kinetic study, 50 mg of CNS/OH was dispersed in 100 mL of solution with the initial dye concentration at 94 mg L−1 for MB and MG and 80 mg L−1 for MV. The mixture was shaken at 300 rpm at room temperature. At different time intervals, around 2 mL of the solution was withdrawn and filtered through a PTFE syringe filter for subsequent analysis. To study the effects of the pH value and ionic strength on dye adsorption, the pH value of the dye solutions was adjusted by adding 0.1 M NaOH or 0.1 M HCl solution, while the ionic strength of the dye solution was adjusted to 0.01 M, 0.1 M, 0.25 M, and 0.5 M using NaCl. The concentration of dye in the aqueous solution was analyzed using UV/vis spectroscopy (UV-2450, Shimadzu). For the adsorption isotherm and kinetic study of MB on AC and AC/ OH, the initial concentration of MB solutions were in the range of 37−260 mg L−1 and 83 mg L−1, respectively. Other conditions were kept the same as those for the CNS sample. 2.3. CNS/OH Regeneration and Recycling. The recyclability of CNS/OH was investigated by carrying out repeated adsorption, regeneration, and reactivation steps using MB as the model dye. During the adsorption step, 25 mg of CNS/OH was added into 50 mL of MB aqueous solution with an initial concentration of 150 mg L−1 in a conical flask. The suspension was stirred at 300 rpm for 6 h, after which the solid was separated via centrifugation and redispersed in 10 mL of 20 mM Co(NO3)2 aqueous solution. Then 0.1 mmol (60 mg) of the oxidant, oxone, was added into the above mixture followed by stirring at 300 rpm for 20 min. The treated CNS was separated via centrifugation and reactivated in 20 mL of 0.5 M
Figure 2. Adsorption isotherm of MB, MG, and MV on CNS/OH and native CNS. Symbols: experimental data; lines: Langmuir models.
We see that the isotherms are highly favorable. Similar to our previous findings for Ag(I) adsorption,30 the adsorption capacity of CNS/OH was much higher than that of native CNS. For example, the maximum adsorption capacity of MB was 682 mg g−1 on CNS/OH but only 56 mg g−1 on native CNS. The remarkably improved adsorption capacity is largely induced by the NaOH activation process, which graft enriched −COO− and −OH groups on the CNS surface. The maximum adsorption capacity of CNS/OH for MG and MV was found to be 310 mg g−1 and 395 mg g−1, respectively. The difference in 13439
dx.doi.org/10.1021/ie300914h | Ind. Eng. Chem. Res. 2012, 51, 13438−13444
Industrial & Engineering Chemistry Research
Article
Table 1. Adsorption Capacities and Kinetics of the Three Basic Dyes on Various Adsorbents dyes
Qmax (mg g−1)
equilibrium time, C0, adsorbent dosage
CNS/OH (this work)
Methylene blue Methyl violet 2B Malachite green
682 395 310
chitosan bead grass waste sunflower seed hull
Malachite green Methylene blue Methyl violet 2B
94 458 93
activated carbon from bamboo waste
Methylene blue
454
activated carbon by steam activation activated carbon from coconut husk
Methylene blue Methylene blue
588 435
activated carbon from lignite green macroalga montmorillonite clay activated carbon by sulfuric acid activation
Malachite green Methylene blue Methylene blue Methylene blue
125 417 349 16
5 min, 94 mg L−1, 20 min, 80 mg L−1, 20 min, 94 mg L−1, 0.5 g L−1 for all 5 h, 40 mg L−1, 2 g L−1 rapid adsorption in first 20 min; 70−380 mg L−1, 3.5 g L−1 30 min, for 25−50 mg L−1; 105 min, for 100−300 mg L−1, 1.5 g L−1 6 h, for 100−300 mg L−1; >24 h, for 400−500 mg L−1, 1 g L−1 2 h, for 50−300 mg L−1; 30 h, for 400−500 mg L−1, 1 g L−1 2−3 h, 500 mg L−1, 4 g L−1 30 min, 16.7 g L−1
