Activated carbon (AC) can efficiently remove organic pollutants such as taste and odor compounds, algal toxins, synthetic organics, endocrine disruptors, synthetic drug precursors and disinfection by-products. The application of biological activated carbon (BAC) in environment field can trace back to early 1970s when Weber et al.  treated the wastewater with microbial degradation method. BAC can remove organic compounds by adsorbing biodegradable organic matter, during which the microbial metabolism is capable of extending the life of AC significantly. In general, regeneration of AC is essential to extend the life of BAC and reduce the cost of treatment. Currently, the most popular method for regeneration is thermal regeneration, but its large carbon loss and heavy energy consumption restrict the regeneration efficiency . Meanwhile, pore structure rupture can lead to the contamination of surface due to the decomposition of substrate. As a potential alterative, microwave regeneration attracts much attention because of its uniform instant heating and controllable temperature. The large temperature gradient inside AC can benefit the regeneration with less energy consumption . Compared with traditional thermal regeneration, the consuming time of microwave regeneration method is short. Many researchers have focused on microwave regeneration of AC. Ondon  reported that the regeneration efficiency of AC could reach 98 % even after 10 regeneration cycles, with higher regeneration efficiency and better temperature control. Jie  found that both iodine value and methylene index recovered to 90 % of those of UAC after microwave regeneration, and that the CODMn removal was very close to that of new carbon.
This study focused on microwave regeneration of SAC from the BAC pools of a water treatment plant. Orthogonal experiment L9(3 4) (four factors and three levels) was designed to determine the optimum conditions for microwave regeneration. Performance comparison between RAC and UAC was conducted with surface characterization and methylene blue adsorption. Additionally, the removal effects of pollutants in raw water from the plant were investigated through the evaluation indexes including CODMn, UV254 and DOC.
2 Materials and methods
2.1 Materials and instruments
Reagents: Methylene blue (AR, Qingdao Ren Xing Experimental Technology Co., Ltd.), KI (AR, test four Hervey, Shanghai Chemical Co., Ltd.), I2 (AR, Tianjin Bodie Chemical Co., Ltd.), NaOH (AR, Shanghai Chemical Reagent Co., Ltd. joint trial), Na2CO3 (AR, Tianjin Bodie Chemical Co., Ltd.), HCl (AR, Laiyang Economic Development Zone Fine Chemical Plant), H2SO4 (AR, Laiyang City Kant Chemical Co., Ltd.). The SAC was gathered from the BAC pools after running for 10 months.
Experiment instruments: UV-visible spectrophotometer (TU-1800 Beijing Purkinje General Instrument Co., Ltd.), SHZ-82 bath oscillator (HY-8 Experimental Analysis Instrument Factory Changzhou Bo Yuan), Electric oven thermostat, coagulation Experiment mixer (Shenzhen Hydra Water Industry and Technology Development Co., Ltd.), thermal Field fever scanning electron microscope (SUPRA55 German Zeiss), the specific surface area and pore size analyzer (V-sorb 2800P Gold Egypt Spectrum Technology Co., Ltd.), hand-made microwave regeneration apparatus (shown in Figure 1).
In the microwave regeneration apparatus, a household microwave oven of 700 W is equipped with a quartz reactor mounted inside; gas flow is controlled by a rotameter and the exhaust can be cleaned with solution; N2 gas creates an inert environment for the regeneration of AC by anaerobic reactions.
2.2 Preparation methods
2.2.1 Pretreatment of the microwave-regenerated activated carbon (PRAC)
SAC sample was rinsed with 5 % HCl for 120 min and then washed to be neutral with distilled water to remove extra surface functional groups . Afterwards, the sample was dried for two hours before microwave regeneration.
2.2.2 Microwave regeneration of activated carbon (RAC)
To determine the optimal regeneration conditions, the impacts of regeneration time, microwave power, gas flow rate and carbon content on the regeneration effect were investigated, and iodine value was chosen as the indicator to evaluate the regeneration effect. Furthermore, methylene blue was selected as the adsorbate to compare the performance of RAC and UAC.
2.3 Characterization methods
The determination of iodine values was based on the “Standard Test Method for Determination of Iodine Number of Activated Carbon” ASTM D4607-1994 (2006). The surface topography and the pore structure of AC were observed by thermal field emission scanning electron microscope (SUPRA55, Carl Zeiss AG, Germany) after vacuum coating. The specific surface area, pore volume, average pore size and pore size distribution were detected by the N2 adsorption method with BJH, DFT and DH analysis .
2.4 Adsorption test
2.4.1 Methylene blue adsorption test
Methylene blue solution (1.5 g/L) was adsorbed for 180 min by 0.05 g, 0.1 g, 0.15 g, 0.2 g, 0.3 g, 0.6 g and 1 g 100-mesh AC respectively. Specially, 0.3 g AC was used to adsorb methylene blue solution (1.5 g/L) for 5 min, 10 min, 30 min, 1 h, 2 h, 3 h, 4 h, 6 h and 9 h respectively. The residual concentration was also determined by spectrophotometry at 665 nm.
2.4.2 Raw water adsorption test
0.05 g, 0.1 g, 0.15 g, 0.2 g, 0.3 g, 0.6 g and 1 g 100-mesh RAC and UAC were used to adsorb 0.45 micron membrane-filtrated raw water for 180 min, respectively. Specially, 0.3 g AC was used to adsorb the membrane-filtrated raw water for 5 min, 10 min, 30 min, 1 h, 2 h, 3 h, 4 h, 6 h and 9 h, respectively. The residual concentration was determined by spectrophotometry, and DOC was measured by the TOC analyzer.
3. Results and discussion
3..1 Orthogonal design of microwave regeneration
The microwave regeneration of SAC may be influenced by regeneration time, microwave power, gas flow rate and the mass of AC. These uncorrelated factors were chosen as orthogonal parameters for experiment design, as shown in Table 1 .
Table 2 presents the results of orthogonal test. From the value of R, it can be seen that the affecting factors significantly follow the descendent order as: regeneration time, microwave power, gas flow rate and AC mass. According to the average value of factor levels k1, k2 and k3, the preferred conditions are regeneration time of 30 min, microwave power of 500 W, gas flow rate of 0.25 m3/h and AC mass of 10 g.
3..2 Iodine values
According to the adsorption isotherms (Figure 2), the iodine adsorption values of UAC, SAC and RAC are 869 mg/g, 507 mg/g and 853 mg/g respectively when the remaining iodine is 0.02 mol/L. After microwave regeneration, the iodine value recovers to 98.1 % of that of UAC, which indicates that the monolayer adsorption capacity is almost recovered completely.
3..3 Surface topography
Figure 3 shows the SEM images of UAC, SAC and RAC. As can be seen, the pore of RAC is more regular and uniform than those of UAC and SAC. On the surface of RAC, the pores are surrounded with obvious burning marks and more micropores can be observed due to the high temperature of microwave heating. Moreover, the pores of RAC are cleaner than those of SAC, and many microbes can be found in them (Table 3).
3..4 Surface characterization
The specific surface area measurement results are shown in Table 4. Compared with UAC, the BET specific surface area of RAC decreases by 11.69 % after microwave regeneration, and the microporous surface area from t-plot calculation (pore diameter less than 2 nm) decreases by 44.49 %. Accordingly, the microporous volume drops from 0.163 to 0.103. That is to say, some micropores transform into mesopores or macropores after regeneration, which results in the decrease of specific surface area .
3..5 Boehm titration
Boehm titration is a widely used method to quantify functional groups on the surface of ACs . Oxygen-containing functional groups are the major active adsorption sites on ACs, and they are usually formed during incomplete carbonization or activator bonding on the surface of ACs. The functional groups are mainly comprised of carboxyl, phenolic hydroxyl, carbonyl, lactone, quinone, ether, etc. , and their chemical structure is shown in Figure 4.
As Figs. (figure 4c) and (d) show, the surface of AC can be activated after 5 % HCl pickling for two hours, and carboxyl and phenolic hydroxyl groups present a significant increase. During microwave regeneration, the carboxyl groups decompose into CO2 due to the high temperature. At the same time, the nitrogen environment of microwave regeneration can produce basic functional groups containing nitrogen. After microwave regeneration, acidic groups, phenol hydroxyl groups and carboxyl groups are significantly decreased, whereas basic groups are increased. The decrease of acidic groups and the increase of basic groups can weaken surface hydrophilicity and enhance surface hydrophobicity, which facilitates the adsorption of organic matter on ACs.
3..6 Methylene blue adsorption
Table 5 gives the isothermal fitting results of AC adsorption. As can be seen, the adsorption isotherm of RAC has a higher correlation coefficient than UAC for either Langmuir equation fitting or Freundich equation fitting . Meanwhile, both UAC and RAC prefer Freundich equation fitting rather than Langmuir equation fitting, which means that the adsorption mechanism of ACs in this study is multiplayer adsorption. In the Freundich equation, smaller 1 /n value usually means higher adsorption capacity . Since the 1 /n values of UAC and RAC both range between 0.1 ~ 0.5, both of them can be acceptable as good adsorbents. Besides, the 1 /n value of RAC is smaller than that of UAC, which means the better adsorption capacity of RAC. As shown in Figure 5 and Figure 6, both RAC and UAC can reach the adsorption equilibrium of methylene blue within 3 h, and the removal efficiency of RAC and UAC is 90.7 % and 99.7 %, respectively. These results mean that most adsorption capacity of AC for small organic molecules can be restored by microwave regeneration.
3..7 Raw water adsorption
The raw water was taken from the same water treatment plant and the water quality is shown in Table 6 and Table 7. It can be seen that organic matter with molecular weight less than 0.5 kD accounts for 44.26 % of the total organic matter in the water samples. That is to say, small organic compounds are dominated in the raw water.
Figure 7 shows the adsorption isotherms of CODMn, DOC and UV254 for UAC and RAC, and Table 8 gives the fitting constants of Freundich equation. Based on the 1/n value, the adsorption capacity of RAC for UV254 is the most significant and the removal efficiency is the highest. In fact, UV254 is a substitution parameter of total organic carbon (TOC), dissolved organic carbon (DOC), trihalomethanes formation potential (THMFP) and other indicators . Hence, the low 1 /n value indicates the good adsorption effect of RAC on such water quality. As Figure 8 shows, the removal rate of organic compounds by both UAC and RAC is fast and the equilibrium state can be achieved within two hours. Notably, the removal effects of RAC on CODMn, UV254 and DOC are more significant than those of UAC. These results agree well with the above analysis of isotherm fitting.
(1) Orthogonal experiments suggested the optimum microwave regeneration conditions were regeneration time of 30 min, microwave power of 500 W, air flow rate of 0.25 m3/h, and AC mass of 10 g. The significant order of these factors was regeneration time > microwave power > gas flow rate > AC mass. The iodine value of RAC was 853 mg/g.
(2) After regeneration, the number of acidic functional groups in the chemical adsorption was decreased while that of basic functional groups was increased, which made RAC more hydrophilic and more favorable for the adsorption of organic matter. Meanwhile, the increasing pore number and pore size benefited the adsorption capacity of organic matter.
(3) According to the Freundich isotherm, RAC had better removal efficiency than UAC, so that it was qualified for the drinking water treatment.
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About the article
Published Online: 2017-02-10
Published in Print: 2017-01-01