Removal of arsenic from water using crumpled graphite oxide

2.1 Synthesis of GO

The synthesis of CGOs by cathodic plasma electrolysis, along with the characterization of their morphology and crystal structure, is detailed in our previous publication [8]. In this study, a DC voltage (Gitek GR 15H-30) of 80 V was applied for a cathode GR tip and a stainless-steel plate anode in a cathodic electrochemical discharge system, where the anode has a much larger surface (500 mm²) than the cathode (5 mm²). An electrolyte solution was prepared at pH 14 containing KOH (15%, 200 ml) and (NH₄)₂SO₄ (5%, 40 ml). After discharging, the synthesized CGO powder was collected from the solution via filtration and washed with DI water many times in order to remove the electrolyte contaminants. The as-synthesized CGOs were then dried at 100°C in a vacuum oven for 24 h and stored in a dryer cabinet at 50°C.

2.2 Characterization of adsorbent

The structural change in the RG and CGOs was investigated by a D2 X-ray diffractometer equipped with a Cu Ka tube and a Ni filter (λ=0.1542 nm). High-resolution transmission electron microscopy (HRTEM) images were recorded using a JEOL 2100F apparatus. Scanning electron microscopy (SEM) was done using a JEOL JSM-6500F scanning electron microscope. The measurements were characterized at Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan. The concentration changes of As(III) were determined by the inductively coupled plasma optical emission spectrometry (ICP-OES; OPTIMA DV7300) in Geological Test and Analyze Center, Hanoi, Vietnam.

2.3 Batch adsorption experiments

The As(III) removal tests were conducted in batch adsorption experiments at room temperature for As(III) solution prepared from stock solution (1000 mg/l, Sigma-Aldrich Dilutions). The effect of initial pH on the adsorption of As(III) was studied in a pH range of 2.0–12.0 using 40 ml of solutions with As(III) concentrations of 1.0 mg/l. The pH of the As(III) solution was adjusted to the required pH value by the addition of dilute HNO₃ or NaOH. The effect of contact time on the adsorption of As(III) was studied using 40 ml of solutions with As(III) concentrations of 1.0 mg/l in a range from 0 to 300 min. In order to determine the influence of initial concentration and equilibrium isotherms, experiments with different As(III) initial concentrations (0.5, 1.0, 1.5, 2.0, 2.5, 5, 10, 20, 25, 30, and 35 mg/l) were conducted at a contact time of 120 min and pH 4. To evaluate the removal efficiency of As(III) in real water, water samples taken from the Suoi Cat spring (Dai Tu District, Thai Nguyen Province, Vietnam) were also used in the adsorption experiment. The As(III) concentration and pH value of the samples were measured at 0.249 mg/l and 6.5, respectively. During the As(III) removal experiment, the suspension was magnetically stirred for a good dispersion of CGO (10 mg) in an As(III)-contaminated solution (40 ml, pH 6.5, 25°C). After the test periods, the solutions were centrifuged at a speed of 4000 rpm (3430×g) for 30 min, and the supernatant was taken for the determination of As(III) concentration by ICP-OES.

The adsorption capacity and removal efficiency were calculated using the following equations:

where V is the solution volume (l),

M is the amount of the adsorbent (g),

C₀ and C_e are the initial and equilibrium concentrations, respectively (mg/l),

q is the adsorption capacity (at equilibrium condition) (mg/g),

R is the removal efficiency of As(III).

The isoelectric point (pH_pzc) for the CGO was determined according to the method published in the literature [9]. In this study, the Freundlich and Langmuir isotherms [as expressed in Eqs. (3) and 4, respectively] were employed for the equilibrium data calculation.

where K_f is the coefficiency related to the sorption capacity, and n is related to the sorption intensity of the adsorbent for the Freundlich isotherm. q_e and C_e are the adsorption capacity and equilibrium concentration of As(III) at equilibrium conditions, respectively. q₀ and b are the monolayer adsorption capacity and Langmuir constant related to the free energy of adsorption, respectively.

3.1 Characterization of CGO

Figure 1 displays the X-ray diffraction (XRD) 2θ scans of the RG and CGO samples. It was obvious that the RG sample exhibited a strong peak at 2θ value of 26.5°, which was indexed to (002) crystal plane of the graphite structure. In the XRD, 2θ scans of the CGO sample showed only a broad diffraction peak at 2θ~9.8°, which were attributed to the (001) crystal plane of GO [10, 11]. These results indicated that the raw RG was almost oxidized and formed CGO during the plasma electrolysis process.

Figure 1:

XRD of CGO; inset is of RG.

Figure 2A shows the SEM images of RG and CGO, indicating their significant morphology difference. The RG contains thickened, dispersed flakes and an ordered layer structure; meanwhile, CGOs are crumpled ball-like structure particles, which are preferable for the adsorption process. This morphology can be further observed via transmission electron microscopy (TEM) image (Figure 2B). As shown in Figure 2B, the CGO is composed of many ridges with dimensions of 0.5–2 μm, consistent with the FESEM observation.

Figure 2:

(A) SEM image of CGO, (inset) RG. (B) TEM image of CGO.

3.2 As(III) adsorption

Figure 3A presents the pH_pzc of the CGO, revealing pH_pzc=7.2. The results indicated that As(III) was favored for the adsorption by CGO in a wide pH range (Figure 3B). However, the higher As(III) adsorption capacity was particularly performed in the pH range of 2–8 than in the pH below 4 (positively charged surface) or in the pH over 8 (negatively charged surface), suggesting that the adsorption mechanism of As(III) onto CGO is both electrostatic interaction and surface complexation, which is similar with previous results [12, 13]. Notably, about 98% of As(III) was adsorbed on CGO at pH=4; as a result, this pH is used for further experiments.

Figure 3:

(A) The pH_pzc of the CGO and (B) effect of pH on the adsorption of As(III).

Figure 4A shows the effect of adsorption time on As(III) removal efficiency using CGO. The removal efficiency of CGO for As(III) was rapid in the first 100 min, and then the removal efficiency curve became flat in about 120 min. The removal efficiency increased slowly, and the adsorption reached to its equilibrium after 180 min. Therefore, the optimum contact time for all further experiments was set at 120 min.

Figure 4:

Effect of (A) contact time and (B) initial concentration on the As(III) adsorption.

Figure 4B presents the effect of the initial As(III) concentration on the adsorption efficiency on the CGO (adsorbent dosage of 10 mg, pH 4, temperature of 25°C, contact time of 120 min). When the initial As(III) concentration increased, the removal efficiency decreased, but the adsorption capacity increased. This can be attributed to the existence of more As(III) ions in solution at a higher concentration; thus, more intense completion occurs between the adsorbate and available binding sites of the adsorbent, leading to higher adsorption. In addition, the adsorption rate follows the formula V_ad=k_ad·C·(1–θ); where θ is the coverage density. In low concentrations, 1–θ=const; therefore, if the concentration increases, V_ad will increase linearly. However, this stage is only true at a fixed boundary depending on the nature of the ion and adsorbent. Then, if the concentration continuously increases, V_ad will increase non-significantly to its peak, and the downhill trend of V_ad might appear.

Figure 5 displays the adsorption isotherms of As(III) on CGO using the Langmuir [Eq. (4)] and Freundlich [Eq. (3)] isotherms. Clearly, the Langmuir model (A) was fitted better than the Freundlich model (B) for expressing the adsorption of As(III) due to its higher regression coefficient. Additionally, the maximum adsorption capacity of As(III) was predicted as q_max=47.39, and the Langmuir constant is b=6.81, which compares favorably with previous results [7, 14]. It is also noted that previous works only focused on exfoliated GO nanosheets (flattened GO from the oxidized form of graphite via the above-mentioned method) for heavy metal adsorption [3]. The major advantages of these above-mentioned methods are the capability of preparing GO, which has a very high density of negative charge arising from the oxygen-containing functional groups, which can link with positively charged species such as metal ions. However, these methods need harsh reaction conditions. In the present study, the high removal efficiency of As(III) on CGO can be ascribed to both the electrostatic interaction between the oxygenated groups (as evidenced by the XRD results) with As(III) and other interactions, such as surface complexation, hydrogen bonding, and pore filling. Besides, CGO possesses crumpled ball-like structure particles, which increase the surface area for the adsorption sites. To verify this assumption, a structure-function relationship via spectroscopic analysis and characterization should be carried out in the future.

Figure 5:

(A) The dependence of C_e/q (g/l) on C_e (mg/l) of As(III) and (B) the dependence of logq on logC_e of As(III).

Table 1 represents the effective removal of As(III) of the water samples from Suoi Cat spring using the CGO adsorbent. Because of recent industrial pollution by Nui Phao Company (Dai Tu District, Thai Nguyen Province, Vietnam), this spring was seriously contaminated by metal ions, such as As(III). The results showed that the As(III) was effectively removed from 0.249 mg/l to 0.003 mg/l with an efficiency of 98.7% after 120 min at near-neutral pH environment (pH 6.5). The effluent quality in terms of As(III) concentration meets well the standard for As(III) concentration in the Vietnam National Technical Regulation on Surface Water Quality (Column A1, QCVN 08-MT:2015/BTNMT).