1. Introduction

Heavy metal ions dissolved in water turn up a huge hazard by accumulating in a living organism[1]. When waste water which contains toxic heavy metal ions is discharged into an environment without any purification process, the toxicity will be accumulated in the inside of plants, marine organisms[2]. Thus, several purification systems have to be conducted before discharging of the waste water. In the purification systems, there are chemical precipitation, ion exchange, biosorption, reverse osmosis, membrane adsorption[3-7].

Cu²⁺ ions are known as one of the most existed heavy metals in the waste water[8]. Likewise other heavy metals such as mercury or lead, copper metals cause a big problem when they flow into the inside of a living organism. For example, when organism inhales spray containing copper metal, the possibility for having cancer increases[9].

Iron oxide nanoparticles such as Fe₃O₄ are used for arsenic adsorption in many years, it confirmed that they have a good affinity into heavy metals[10,11]. From their small sizes, they have a high specific surface area, relatively small resistance as well as their magnetic characteristic which enables fast separation using external magnetic field[12]. However, there are few drawbacks such as an aggregation during adsorption process, dissolved in acidic condition and lose their ability[13].

Porous carbon has a high specific surface area, a good chemical, mechanical property and high electrical conductivity[14]. If Fe₃O₄ nanoparticles uniformly embedded in porous carbon matrix, it efficiently prevent the aggregation of Fe₃O₄ nanoparticles during adsorption process. In addition, it improves the chemical, mechanical properties of composite. In this study, we synthesized porous carbon and Fe₃O₄ via their optimal composition and reaction conditions and identified the possibility for selective adsorption for Cu²⁺ ions as membrane.

2. Experimental

2.1. Materials

Dextrose and Fe(NO₃)₃⋅9H₂O were used as precursors for carbon and iron oxide each. Poly acrylic acid was used for synthesizing porous carbon.

2.2. Synthesis of Fe₃O₄/porous carbon composite

2 g PAA were mixed in 50 mL DI water and sonication process were carried out for 15 mins. Adding 2.49 g Fe(NO₃)₃⋅9H₂O, 2 mL HNO₃, additional sonication process were conducted. The solution was transferred into a beaker, 3.332 g dextrose were added and stirred for 1 h.

Synthesis of Fe₃O₄/porous carbon composite is as follows: the solution was transferred into a teflon-lined autoclave and the hydrothermal reaction was carried out at 180°C for 18 h. The solution was cooled down at room temperature and washed several times with ethanol, DI water. The sample was obtained by centrifugation and dried in a convection oven at 50°C for 12 h. The Fe₃O₄/porous carbon composite were obtained by the carbonization and oxidation process. The sample were heated to 900°C for 2 h in a electrical furnace at the heating rate of 5°C min^-1. Then, the sample were heated to 250°C for 2 h at the same heating rate.

2.3. Characterization

The obtained products were characterized by scanning electron microscope (SEM) using a SNE-3000M, X-ray diffractometer using a Rigaku RAD-3C for their morphology and structure. XRD analysis were carried in 2θ ranging from 10° to 70° with Cu Kα radiation at room temperature. For measuring a specific surface area and pore size distribution, Brunauere-Emmett-Teller measurement using Quantachrome NOVAtouch^TM was used.

2.4. Adsorption experiments

Cu²⁺ ions adsorption experiment for were carried out using thermostat shaker at 30°C. 50 mg Fe₃O₄, Fe₃O₄/ porous carbon composite were transferred in 50 mL, 150 ppm Cu²⁺ solution each. The adsorption was performed with 120 rpm rotating speed for 1 h. The pH value of the solution were fixed at 5.5 (± 0.1) using 0.1M NaOH. After the adsorption test, the residual concentration of Cu²⁺ ions were analyzed using the UV-vis spectra (Shimadzu, UV1800) with a maximum absorbance wavelength at 810 nm.

3. Results and Discussion

The synthetic process of Fe₃O₄/porous carbon composite is described in Scheme 1. For avoiding aggregation among Fe₃O₄ nanoparticles, porous structured carbon acts as matrix. Nitric acid was used for dissolving Fe(NO₃)₃⋅9H₂O, and dextrose was added additionally. The homogeneous solution was transferred into teflon- lined reactor for polymerization process. After that, following carbonization, oxidation were carried out to synthesize Fe₃O₄/porous carbon composite.

The Fe₃O₄/porous carbon composite was synthesized with 3 : 1 ratio of dextrose : Fe(NO₃)₃⋅9H₂O, 2.0 g PAA was used for its porous structure. XRD analysis were performed to identify structure of the composite. Fig. 1 shows that the composite included Fe₃O₄ was successfully synthesized from the XRD peaks. The peaks of Fe₃O₄/porous carbon composite at 26°, which shows broad carbon peak and Fe₃O₄ peaks at the same time. In comparing the Fe₃O₄ peaks (JCPDS 75-0449), the composite confirms that it is constructed with carbon and Fe₃O₄.

As shown in Fig. 2(a), (b), the composite has about 3~5 μm size with spherical shape. They show the composite consists of porous carbon matrix. From these porous structures, we can expect that the increased surface area will allow to facilitate the adsorption for Cu²⁺ ions. In Fig. 2(c), the SEM image of reference Fe₃O₄ particles shows that about 500 nm size of Fe₃O₄ nanoparticles compose nano-micro particles.

BET measurement were performed to analyze the specific surface area and pore size distributions of Fe₃O₄, Fe₃O₄/porous carbon composite. As shown in Fig. 3(a), the Fe₃O₄/porous carbon composite represents IV types isotherm which demonstrates a capillary condensation at mesoporous region[15]. In other words, hysteresis loop occurs between 0.4~0.95 relative pressure and the composite is mainly composed of mesoporous. In addition, the adsorption of Cu²⁺ ions can be expected to be more effective based on the specific surface area and the total pore volume of the Fe₃O₄/porous carbon composite than the Fe₃O₄ particles (Table 1). On the other hand, in the case of Fe₃O₄ particles, the hystersis loop is formed relatively small, which indicates that mesoporous are significantly less than the Fe₃O₄ particles/porous composite.

Adsorption experiments were conducted to identify that the Fe₃O₄/porous carbon selectively adsorb Cu²⁺ ions, one of the heavy metal ions. The adsorption experiments were performed in the condition that the Cu²⁺ ions are dissolved in the liquid phase. After 1 hour adsorption, the residual solution was analyzed by UV-visible spectra. As shown in Fig. 4(a), Cu²⁺ ions shows the maximum peak near the wavelength at 810 nm. Since Cu²⁺ ions tend to precipitate in the form of Cu(OH)2 in a solution having a pH value of about 6.5 or higher[16], the adsorption experiment was conducted after adjusting the pH of the initial solution about 5.5. In comparison to the adsorption capacity of Cu²⁺ ions at the wavelength of 810 nm, the adsorption capacity of the Fe₃O₄ particles shows 9.38 mg/g after 50 mins, whereas the adsorption capacity of the Fe₃O₄/porous carbon composite shows 17.14 mg/g [Fig. 4(a) inset]. In this reason, the Fe₃O₄/porous carbon composite has greater adsorption capacity, about 1.8 times higher than Fe₃O₄ particles. On the basis of the experimental results, the adsorption capacity is increased from less aggregated during adsorption process and its higher specific surface area of Fe₃O₄/porous carbon composite. That is, the interaction between positively charged Cu²⁺ ions and negatively charged Fe₃O₄ particles, the synergetic effect of preventing the aggregation of the Fe₃O₄ nanoparticles and increasing the contacting area enable its higher adsorption capacity [Fig. 4(b)][14].

In order to confirm the magnetic characteristic of the Fe₃O₄/porous carbon composite, it was dispersed in a solution in which Cu²⁺ ions contained, and the external magnetic force was applied. The Fe₃O₄/porous carbon composite is easily dispersed in DI water containing Cu²⁺ ions [Fig. 5(a)], and the solution becomes black due to the Fe₃O₄/porous carbon composite. As soon as the neodymium magnet was placed near the vial, the Fe₃O₄/porous carbon composite interacted rapidly and gathered near the magnet. After 60 seconds, most of the Fe₃O₄/porous carbon composite was collected [Fig. 5(b)]. On the basis of this, when the Fe₃O₄/porous carbon composite is used as a membrane, handy separation using external magnetic force will be possible in the solution where the heavy metals are dissolved.

4. Conclusions

Hydrothermal synthesis was used to synthesize the Fe₃O₄/porous carbon composite for the effective adsorption of Cu²⁺ ions. The composite consists of a porous carbon matrix in which Fe₃O₄ nanoparticles are embedded in. It shows an improved adsorption capacity of Cu²⁺ ions by increasing the contacting area based on the relatively high specific surface area. Aggregation during the adsorption process, which is a drawback of the Fe₃O₄ nanoparticles for Cu²⁺ ions adsorption, can be alleviated through the porous carbon matrix of the composite. In addition, the composite can be easily separated from a heavy meal ions containing solution by using an external magnetic force, suggesting the possibility of using it as a membrane for selective adsorption of Cu²⁺ ions.