Preparation of Biochar Composite Microspheres and Their Removal Ability for Oil Agents in Dyeing Wastewater

1. Introduction

With the continuous development of social economy and industry, freshwater resources are becoming increasingly scarce with extremely low per capita water availability, uneven spatial distribution and serious water pollution, which have become the main factors restricting the sustainable development [1,2]. With the increasing awareness of environmental protection, the national requirements for the discharge of wastewater from textile printing and dyeing industry are also getting higher. How to efficiently and cost-effectively treat printing and dyeing wastewater is a difficult problem faced by current textile printing and dyeing enterprises, among which oil dispersants are one of the typical pollutants in printing and dyeing wastewater [3]. The highest allowable discharge concentration of oily wastewater in China is only 1mg/L, and due to its complex composition, difficulty in demulsification, and easy introduction of new pollutants by demulsifies, it is difficult and costly to treat [4]. Biochar materials, due to their large surface area, high porosity, rich surface functional groups, and strong adsorption and removal ability for organic pollutants, have been widely used in the field of wastewater treatment [5]. In addition, biochar materials have a wide range of raw material sources and low cost, and their application in the treatment of printing and dyeing wastewater can effectively reduce the cost of wastewater treatment and realize the resource utilization of biomass [6].

Synthetic fibers are hydrophobic polymer materials that must use oil dispersants as smoothing agents, antistatic agents, emulsifiers, etc. in spinning and textile processing [7]. Before the dyeing processes, the oil dispersant needs to be removed in order to carry out normal dyeing, which results in a large amount of oil dispersant in printing and dyeing wastewater. Because the oil dispersants in synthetic fibers are rarely composed of a single chemical structure material, but rather require various substrates (surfactants, mineral oils, advanced alcohols, fatty acid esters, etc.) to be combined according to their respective characteristics and purposes, resulting in high COD value, strong biological toxicity, difficulty in demulsification, difficulty and cost of treatment, and easily causing large-scale environmental pollution [8]. However, the common degreasing methods were hard to break emulsion with the addition of surfactants. The high cost and low efficiency were the main problems during the oily wastewater treatment process [9,10]. Therefore, how to prepare a cheap oil removal adsorbent is a meaningful thing.

Herein, the biochar microspheres deriving from waste bamboo were prepared using hydrothermal method. After activation and modification, the magnetic lipophilic biochar microspheres (Fe₃O₄@L-ABM₅₀₀) with high degree of graphitization were obtained. The (Fe₃O₄@L-ABM₅₀₀) displayed excellent removal ability for DTY oil in dyeing and finishing wastewater. The high removal efficiency and low preparing cost of biochar microspheres may promote the large-scale utilization of waste biomass, and decrease the treatment cost of oily wastewater.

2. Experiments

2.1. Materials and Chemicals

Phloroglucinol (C₆H₆O₃, AR grade), Iron sulfate heptahydrate (FeSO₄•7H₂O, AR grade) and Ferric chloride hexahydrate (FeCl₃•6H₂O, AR grade) were purchased from National Drug Group Chemical Reagent Co., Ltd. (Shanghai, China). Lauric acid (C₁₂H₂₄O₂, AR grade) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Ammonium hydroxide (NH₃•H₂O, 28%) was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Polyester oil (DTY, Commercial grade) was purchased from Huiya Environmental Protection Technology Co., Ltd. (Hangzhou, China).

Simulated oily dyeing wastewater: 0.1 mL oil agent, 0.05 g disperse red dye and 0.05 g sodium dodecyl sulfate were added into 100 mL deionized water, and then stirred for 30 min. The simulated oily printing and dyeing wastewater with COD concentration of 3150 mg/L used in this study was obtained.

2.2. Preparation of Biochar Composite Microspheres

Firstly, 3.2g bamboo flakes and 0.8g phloroglucinol were added into 120 mL distilled water and stirred at 60℃ for 2 h. Then, the mixture was put into a hydrothermal reactor and pyrolyzed at 200℃ for 24 h with heating rate of 1.5 ℃/min in the muffle furnace. The mixture was ultrasonic treated for 30 min at 500 W to remove carbonized bamboo pieces and large particle aggregates through a 240-mesh screen. After centrifugation at 4000 r/min for 10 min, the biochar microsphere (BM) was obtained.

0.2g biochar microsphere (BM) was added into the KOH solution with mass fraction of 10%, and stirred for 10 min at 500 rpm. After dried at 60℃, the BM was pyrolyzed at different temperatures (300℃, 500℃ and 700℃) in the tube furnace with N₂ atmosphere for two hours, respectively. The activated biochar microspheres (ABM₃₀₀, ABM₅₀₀, ABM₇₀₀) were obtained after washing with distilled water for three times. Then, the prepared ABM₃₀₀, ABM₅₀₀ and ABM700 (0.2 g) were mixed with lauric acid (0.1 g), and pyrolyzed with above conditions. The hydrophobic modified biochars (L-ABM₃₀₀, L-ABM₅₀₀, L-ABM₇₀₀) were obtained.

Finally, 0.2 g L-ABM₃₀₀, L-ABM₅₀₀ and L-ABM₇₀₀ were dispersed into 80 mL iron ion solution (50℃) with 0.24 g FeSO₄•7H₂O and 0.19 g FeCl₃•6H₂O, respectively. The pH of the iron ion solution was adjusted to 8.5 with ammonium hydroxide, and cured for 5min. The magnetic biochar microspheres (Fe₃O₄@L-ABM₃₀₀, Fe₃O₄@L-ABM₅₀₀, Fe₃O₄@L-ABM₇₀₀) were prepared after washing with distilled water for 3 times.

2.3. Adsorption Tests

0.03 g of Fe₃O₄@L-ABM₃₀₀, Fe₃O₄@L-ABM₅₀₀, Fe₃O₄@L-ABM₇₀₀ were added into the simulated oily dyeing wastewater (10 mL), and stirred for 30 min, respectively. Then, the mixture was centrifuged at 4000 rpm for 10 min to remove the biochar microsphere. The COD concentration of residual solution was tested, and the removal rate and adsorption quantity were calculated.

Then, 0.01 g, 0.02 g, 0.03 g, 0.04 g and 0.05 g of Fe₃O₄@L-ABM₅₀₀ were added into 10 mL oily dyeing wastewater, and stirred for 30 min. Based on above results, the effects of different times (10 min, 20 min, 30 min, 40 min and 50 min) and initial pH values (5-9.5) on the adsorption capability of biochar microspheres under the same condition were also investigated. The COD concentration of residual solution was tested as aforementioned.

2.4. Adsorption Kinetics

The adsorption kinetics were fitted by the Pseudo-first-order and Pseudo-second-order models as follow [11,12]:

$$\mathrm{Pseudo}-\mathrm{first}-\mathrm{order} \mathrm{model}\text{:} \ln \left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{\mathrm{Q}}_{\mathrm{e}}-{\mathrm{K}}_1\mathrm{t}$$

(1)

$$\mathrm{Pseudo}-\sec \mathrm{ond}-\mathrm{order} \mathrm{model}\text{:} \frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{K}}_2{\mathrm{Q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{e}}}$$

(2)

In Eq. (1-2), Q_t (mg/g) is the adsorbance at t (min), Q_e (mg/g) is the saturated adsorption quantity, K₁ (min^-1) and K₂ (min^-1) are the equation constants, respectively. The t (min) is the adsorption time.

2.5. Analytical Methods

Scanning electron microscopies (SEM) of biochar microspheres were performed using the SEM (SU8010, Hitachi, Japan). Fourier transform infrared (FTIR) spectra were characterized by a FTIR apparatus (Nicolet iS50, Thermo, USA). XPS was performed on a photoelectron spectrometer (ESCALAB 250XI, Thermo, USA) with a monochromated Al- K_α source at a residual gas pressure of less than 10⁻⁸ Pa. All the binding energies were referenced to the C 1 s peak at 284.6 eV of the surface adventitious carbon. The X-Ray Diffractions (XRD) of the bio-char materials were conducted by an x-ray diffractometer (D8, Brook AXS Co., Ltd., Germany) in range of 15^o-80^o at the rate of 0.1^o/min). The residual COD concentration was measured using potassium dichromate method (Hach, USA). All of the other parameters were measured following standard methods [13,14].

3. Results and Discussion

3.1. Characterizations

The morphologies of BM, ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀ were characterized by SEM images (Figure 1).

As shown in Figure 1 (a), the spherical biochar (BM) with the size of 0.5-2 μm was successfully prepared by hydrothermal carbonization method from bamboo slices, which had smooth surface. The biochar microsphere (ABM₅₀₀) activated by KOH was shown in Figure 1 (b). It can be seen that a large number of structural defects are generated on the surface of biochar microsphere. The structural defects will increase the specific surface area of the microsphere, and improve its contact area for oil agent [15,16]. The high surface area can promote the grafting reaction with lauric acid, and improve its adsorption capacity. The SEM image of Fe₃O₄@L-ABM₅₀₀ was displayed in Figure 1 (c). More defects occurred and many nanoparticles were loaded on the microspheres. The nanoparticles around microspheres can be attributed to the Fe₃O₄ components.

Raman analyses for Fe₃O₄@L-ABM₃₀₀, Fe₃O₄@L-ABM₅₀₀ and Fe₃O₄@L-ABM₇₀₀ were conducted to explore the influences of pyrolysis temperature on the biochar microspheres (Figure 2). In Figure 2, all of the Fe₃O₄@L-ABM₃₀₀, Fe₃O₄@L-ABM₅₀₀ and Fe₃O₄@L-ABM₇₀₀ showed characteristic peaks at around 1350 cm^-1 and 1580 cm^-1, corresponding to the D band and G band, respectively. Herein, the D band represent the disordered graphite carbons, and the G band reflect the sp² hybrid carbon-carbon bond resonance of graphite crystalline. The higher intensity ratio of characteristic peaks (I_D/I_G) indicate the more crystal defects and disordered carbon components in the carbon material. In Figure 2 (a-c), the I_D/I_G values of Fe₃O₄@L-ABM₃₀₀, Fe₃O₄@L-ABM₅₀₀ and Fe₃O₄@L-ABM₇₀₀ were 1.31, 0.88 and 0.85, respectively. The significant decrease of I_D/I_G values indicated that the higher pyrolysis temperature improved the graphitization degree of biochar microspheres. The higher degree of graphitization, the biochar microspheres may have stronger affinity for oil agent [17,18].

To explore the crystal structures and functional groups of biochar microspheres prepared at different conditions, the X-ray diffractions and FTIR spectra analyses were conducted in Figure 3. In Figure 3 (a), all of the BM, Fe₃O₄@ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀ displayed obvious gentle peaks at 23.1°, which can be ascribed to the amorphous carbon in biochar microspheres. After the pyrolysis at 500℃, the peak intensity (23.1°) of Fe₃O₄@ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀ were weaker than that of BM, indicating that the pyrolysis process improved the graphitization degree of biochar. The result was consistent with above Raman analysis. Compared with BM, the Fe₃O₄@ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀ appeared new characteristic peaks at 30.1°, 35.5°, 43.1°,53.4°,57.0° and 62.6°, which can be attributed to the (220), (311), (400), (422), (511) and (440) planes of Fe₃O₄ (JCPDS No.79-0419), respectively [19]. These indicated that the Fe₃O₄ was successfully loaded on the activated biochar microspheres. The biochar composite materials with Fe₃O₄ in-situ loaded can be separated rapidly from the clean water in the presence of magnetic field, which may improve the adsorption efficiency.

The FTIR spectra of BM, ABM₅₀₀, L-ABM₃₀₀, L-ABM₅₀₀ and L-ABM₇₀₀ were listed in Figure 3 (b). The peaks at 3411cm^-1 can be ascribed to the stretching vibration (υ_O-H) of hydroxyl group (O-H), and the two peaks occurred in range of 2900 cm^-1-2670 cm^-1 correspond to the stretching vibration (υ_C-H) of methyl group (-CH₃). The peaks at 1628 cm^-1 and 1383 cm^-1 indicated the presence of stretching vibration (υ_C=C) of C=C and in-plane bending vibration (δ_C-H) of C-H, respectively [20]. The biochar microspheres showed similar functional groups before and after modification. Because the carboxylic groups and alkane structures involved in lauric acid also exist in the biochar microsphere. However, the intensity of the characteristic peaks was different. Compared with BM, the intensity of υ_C-H and δ_C-H in ABM₅₀₀ decreased significantly, indicating the reduction of -CH₃ and -CH₂ contents after pyrolysis. These also indicated that the pyrolysis process improved the graphitization degree of biochar microsphere. Besides, the intensity of υ_C-H and δ_C-H in L-ABM₅₀₀ was stronger than that of ABM₅₀₀, which can be attributed to the introduced alkane structures from lauric acid. These may evidence that the grafting reaction between lauric acid and biochar microsphere was successful.

To obtain the chemical composition of as-prepared biochar microspheres, the XPS spectra were performed as Figure 4. In Figure 4 (a), all of the three biochar microspheres (BM, Fe₃O₄@ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀) had characteristic peaks at 286.3 eV and 531.9 eV, corresponding to the binding energies of C1s and O1s, respectively [21]. These mean that the biochar microspheres had oxygen-containing functional groups expect the graphite structure. Considering the FTIR spectra analysis (Figure 3 (b)), the oxygen-containing functional groups involve hydroxyl group (O-H), carboxyl group (-COOH) and so on.

Compared with BM, the Fe₃O₄@ABM₅₀₀ and Fe₃O₄@L-ABM₅₀₀ show a new characteristic peak at about 712.3 eV, which corresponds to the binding energy of Fe 2p in loaded Fe₃O₄.

Besides, the high-resolution spectra of Fe 2p in Fe₃O₄@ABM500 and Fe3O4@L-ABM500 were also conducted (Figure 4 (b)). In Figure 4 (b), the characteristic peaks appear at 710.2eV and 723.5eV, corresponding to the binding energies of Fe 2p_3/2 and Fe 2p_1/2 in Fe₃O₄, respectively [22]. These indicated that the Fe₃O₄ has been successfully loaded onto the biochar microspheres. The result was consistent with the XRD analysis results.

3.2. Adsorption Process Analysis

The influences of pyrolysis time, biochar dosage, adsorption time and initial pH on the adsorption processes were shown as Figure 5. In Figure 5 (a), it can be seen that maximal COD removal rate of Fe₃O₄@L-ABM₅₀₀ for oily wastewater reached 85.5% with the adsorption quantity of 887.3 mg/g. The Fe₃O₄@L-ABM₅₀₀ pyrolyzed at 500℃ had higher COD removal rate than the Fe₃O₄@L-ABM₃₀₀ and Fe₃O₄@L-ABM₇₀₀ pyrolyzed at 300℃ and 700℃ respectively. This phenomenon can be ascribed to the graphitization degree and functional group content of biochar microspheres. Because the Fe₃O₄@L-ABM₃₀₀ was pyrolyzed at a relatively low temperature (300℃). The Fe₃O₄@L-ABM₃₀₀ has low graphitization degree, resulting in the poor lipophilicity and weak adsorption ability for oily wastewater. With the increase of pyrolysis temperature, the graphitization degree of biochar microspheres improved, which can contribute to the adsorption ability for oily wastewater. However, the higher pyrolysis temperature, the less functional groups reserved in the biochar microspheres. The high pyrolysis temperature (700℃) of Fe₃O₄@L-ABM₇₀₀ adverse to the grafting reaction of lauric acid onto biochar microspheres. Hence, the Fe₃O₄@L-ABM₇₀₀ had lower adsorption ability than Fe₃O₄@L-ABM₇₀₀.

The adsorption ability of Fe₃O₄@L-ABM₅₀₀ for oily wastewater with different dosage was displayed in Figure 5 (b). It can be seen that the COD removal rate increased with the increase of biochar dosage. When the dosage of Fe₃O₄@L-ABM₅₀₀ was 0.03 g, the COD removal rate reached 86.06% with the adsorption quantity of 1037.2 mg/g. However, the COD removal rate decreased with the dosage continuous increased from 0.03 g to 0.05 g. This may be ascribed that the ash and unreacted lauric acid on biochar microspheres dissolved into solution, resulting in the increase of COD concentrations.

Besides, the effects of adsorption time and initial pH on the adsorption abilities were also explored in Figure 5 (c-d). In Figure 5 c, the COD removal rate increased with the increase of adsorption time, and reached the adsorption equilibrium (84.6%) gradually at 30 min with the adsorption quantity of 889 mg/g. The results indicated that 30 min is the suitable adsorption time, and the Fe₃O₄@L-ABM₅₀₀ had great adsorption efficiency for oily wastewater. In Figure 5d, it can be seen that the initial pH can affect the adsorption ability significantly. When the initial pH was 5.8, the Fe₃O₄@L-ABM₅₀₀ showed the highest adsorption capacity for oily wastewater, in which the COD removal rate was 84.5% (887 mg/g). However, the COD removal rate decreased significantly with the increase of pH value. When the pH was 9.5, the COD removal rate decreased to 73%. Because the carboxyl groups (-COOH) were converted into -COO^- under alkaline conditions, which can promote its water solubility. Hence, the affinity of Fe₃O₄@L-ABM₅₀₀ for oil decreased.

3.3. Adsorption Kinetics

The adsorption kinetic behavious of Fe₃O₄@L-ABM₅₀₀ for oily wastewater were explored as Figure 6, and the relative parameters were displayed in Table 1. It can be seen that the linear correlation coefficients (R²) of Pseudo-first order equation and Pseudo-second order equation were 0.79 and 0.99, respectively. The great linear relationship between t/Q_t value and adsorption time (t) indicate that the adsorption process was more suitable for the Pseudo-second order equation.

Moreover, the equilibrium absorption quantity (Q_e) of Pseudo-second order equation in Table 1 was also close to the experimental value. Because the Pseudo-second order equation is based on the assumption that the adsorption rate is linearly related to the concentration of the two reactants, reflecting the chemisorption process [23,24]. Hence, the chemisorption was the main adsorption manner of Fe₃O₄@L-ABM₅₀₀ for oily wastewater.

Based on the above discussion and analysis, the possible adsorption mechanism of oil agent onto biochar microsphere was shown in Figure 7. The adsorption process is divided into three steps, including surface adsorption, inner diffusion and adsorption binding onto the biochar. Firstly, the oil agent can be transferred easily from the solution to outer surface of biochar due to the alkyl chain on the surface of biochar (surface adsorption). Then, the oil agent can furtherly diffuse into the micropore of biochar microsphere from the outer surface to the inner surface (inner diffusion). Thus, the oil agent was adsorbed into biochar microspheres efficiently (adsorption binding onto biochar). In this process, the introduced alkyl chains from lauric acid, pore structures and high degree of graphitization can improve its adsorption efficiency [25].

4. Conclusion

In this study, the biochar microspheres were prepared from waste bamboo by hydrothermal reaction, KOH activation and lauric acid grafting modification. The prepared biochar microsphere (Fe₃O₄@L-ABM₅₀₀) had high adsorption efficiency for oily wastewater, and the maximum COD removal rate reaches 86.06%. Moreover, the biochar microspheres (Fe₃O₄@L-ABM₅₀₀) can be quickly separated from solution under the magnetic field. The structural etching, graphitization degree and lauric acid grafting played an important role in the adsorption process. Based on the adsorption kinetics analysis, chemisorption was the main adsorption manner. The high removal efficiency and low preparing cost of biochar microsphere (Fe₃O₄@L-ABM₅₀₀) may promote the large-scale utilization of waste biomass, and decrease the treatment cost of oily wastewater.