Mesoporous structure regulation of activated carbons and effects on the synergistic efficacy of synchronous adsorption and bio-degradation in biological activated carbon process

Mesoporous activated carbon MCGL-4 was tailored for simultaneous enhancement of adsorption and bio-degradation by multistage depth-activation (MDA). Synergistic efficacy of synchronous adsorption and bio-degradation was evaluated in pilot-scale bio-enhanced activated carbon (BEAC) system. Results identified that MCGL-4 obtains synchronously well-developed meso(0.7605 cm/g), micro(0.2655 cm/g) and macro-porous (0.143 cm/g) structures. Higher volume during 20.4–208.2 Å (0.6848 cm/g) ensured higher adsorption capacities for natural organic matters (NOM). The initial immobilized biomass and stabilities on MCGL-4 were also significantly promoted. Rapid small-scale column tests system (RSSCTs) tests showed that adsorption capacities for humiclike organics were 67,725.32 mg·DOC/(kg·carbon) at 39.50 m·H2O/(kg·carbon). In BEAC system, MCGL-4 achieved higher removal efficiency for fulvic acid, humic acid and aromatic organic matters than commercial carbons. At 39.50 m·H2O/(kg·carbon), cumulative uptake of organic-pollutants achieved by MCGL-4 was 94,850.51 mg·DOC/(kg·carbon). The proportion occupied by biodegradation were 31,674.70 mg·DOC/(kg·carbon). It also confirmed that bio-degradation ability was much higher than commercial carbons after mesoporous structures regulation by MDA process.

High adsorption capacities for natural organic matter (NOM) and trace organic-pollutants (TrOPs) require synchronously well-developed micro-and meso-porosity distribution of carbons (Gong et al. ). Studies suggested that meso-pores (2-50 nm) and secondary micro-pores (1-2 nm) principally controlled adsorption of NOM from water (Velten et al. ). NOM suffered severely competitive adsorption with trace organic-pollutants (TrOPs) on carbon surfaces with primarily small micro-pores (<1.0 nm) (Quinlivan et al. ). On the other hand, synchronous adsorption and bio-degradation is the core mechanism of organic-pollutants removal in BEAC process (Velten et al. ; Gibert et al. ). And it can be significantly affected by pore structure distribution of meso-and macros-pores (Aktaş& Çeçen ).
Therefore, tailoring an efficient carbon with a perfect matching between adsorption and bio-degradation (both biomass and bio-activity levels on carbon surfaces) was a significantly important, and pore structure regulation is the promising way to improve adsorption and bio-degradation efficiency simultaneously (Sulaymon et al. ). Studies suggested that carbons with simultaneously well-developed meso-and micro-porous structures may achieve better matching between adsorption and bio-degradation (Singh et al.

Activated carbon preparation
Three new carbons were prepared by multi-step procedure in Figure 1, including precursor-blending, re-agglomeration, potassium hydroxide (KOH) impregnation, carbonization and multistage depth-activation (MDA) (Li et al. ; Gong et al. a, b). Take carbon MCGL-4 as an example, the detailed preparation parameters were described as the following: coalblending (35% Taixi coal þ30% Shen Fu coal) and coconut shell (30%) were first mixed and crushed to 100-200 mesh.
The mixture was agglomerated using refined tar (5%) as agglomerate. The agglomerated material was crushed and screened again to 2-10 mm. Impregnation was conducted before carbonization using 10% KOH (100 rpm, 60 min). Carbonization was consequently conducted at 600 C in N 2 protection environment for 30 min. MDA was conducted in activation apparatus, and MDA can be divided into three stages: CO 2 activation (650 C/30 min), steam activation (900 C/120 min) and depth-steam activation (950 C/20 min).

Activated carbon characterization
Nitrogen isotherms were measured by ASAP2020 Sorptometer at 77 K and the specific surface area (S BET ) was calculated by  T 7702.7-2008, 7702.6-2008 and 7701.2-2008).

Commercial carbons
Three kinds of commercial carbons in Table 1 were also employed as control group, which were commonly used in a water purification plant in China.

Experimental apparatus
Testing apparatus of functional bacteria immobilization As shown in Figure 2(   Utilization rates of dissolved oxygen (DO) during biodegradation by different carbons were determined using apparatus in Figure 2(b) in both adsorption system (column A) and bio-enhancement system (column B with functional bacteria immobilization). The influent raw water was sterilized by ultraviolet and fully aerated. Dissolved organic carbon (DOC) and NH 4 þ -N were adjusted to 4.0 ± 0.25 mg/L and 0.50 ± 0.16 mg/L, respectively, using sodium acetate and ammonium chloride. After stable operation for a week, changes of DOC, DO and NH 4 þ -N were determined for 5 days, using HQ30D portable DO analyzer and TOC analyzer (TOC-VCPH).
Herein, bio-degradation can be ignored in column A and the DOC and NH 4 þ -N removal can be attributed to   conducted according to the methods described by Zhang (Zhang et al. ). Ultraviolet sterilization (100 W·m À2 ) was employed in raw water tank to minimize the bacteria interference. Difference between BAC and BEAC system was the formation of bio-degradation. Bio-degradation formed naturally during operation. Therefore, there was no ultraviolet sterilization in the raw tank. Operation parameters and influent water qualities of BEAC and BAC system are summarized in Table 2.
Carbon samples were pre-treated as follows: (1) carbons were thoroughly washed with deionized water until all dust was removed and the pH of the wash water was steady; (2) well-washed carbons were dried at 150 C for 4 h; (3) carbon water slurry was heated to boiling for 30 min and loaded into the column after cooling to room temperature.
The feed water qualities were listed in Table 2.

Sampling and analysis
Removal efficiencies of DOC and UV 254 were determined in pilot-scale and RSSCT system. Three-dimensional fluorescence spectra (3D-EEM) of water samples were scanned with 5 nm increments by a fluorescence spectrophotometer (FP-6500) (Gong et al. ). Gas chromatography-mass spectrometry analysis (GC-MS) was performed using chromatography-mass spectrometer (6890GC-5973/5975MSD).

Cumulative uptake of micro-pollutants
The concentration levels of micro-pollutants (DOC) were reported as functions of the operational time and water volume fed to carbon columns divided by mass of GAC (KBV). The KBV can be described by the following equation: The KBV of pilot-scale system reached up to 39.50 m 3 ·H 2 O/(kg·carbon) and it was equivalent to 246 days of operation. Performance of micro-pollutants removal from water by carbons in BEAC/BAC and RSSCTs system was also represented by the cumulative uptake DOC (QC) as a function of KBV.

Adsorption capacities of NOM
As shown in Figure S1(a)-(c), pore volumes during 52.5 to 408 Å was well correlated with adsorption capacity of humic acid (R 2 > 0.96), when average molecular weight were below 1,600 Da. As mentioned above, pore volumes during 52.5∼208.2 Å were much more higher than 208.2∼408 Å.
Therefore, mesoporous volume during 52.5∼208.2 Å may mainly provide surface adsorption site. When average molecular weight increased to 2,700∼5,100 Da ( Figure S1(d)-(f), adsorption capacities also increased with the increase of mesoporous volume during 52.5∼208.2 Å. However, the linear correlation was not significant. Well linear correlation (R 2 > 0.96) was observed during 408∼903 Å, it suggested macroporous structure may also play an important role.

Immobilization properties for functional bacteria
Another goal of present work is to improve the properties of bio-enhancement on carbon surface. Studies suggested that initially immobilized biomass of functional bacteria in BEAC process can significantly shorten the biofilm formation, and consequently improve bio-degradation of organic-pollutants. As shown in Figure S2(a), biomass (phospholipid content) on carbons increased over time of cyclic loading.
After 24 hours, MCGL-4 obtained 9.13 mmol-P/g of phospholipid content, higher than other 6 carbons. After surface backwashing (5 minutes), phospholipid content on MCGL-4 decreased to 8.06 mmol-P/g. However, it was still higher than other carbons. It indicated that immobilized biomass and stabilities on MCGL-4 were significantly promoted. As shown in Figure S2(b), after 240 hours of continuous culture, MCGL-4 achieved higher phospholipid content (30.36 mmol-P/g), with average growth rate of 2.213 mmol-P/g/d.
Water purification performance of MCGL-4 As mentioned above, it suggested that new carbon MCGL-4 has advantages in NOM adsorption, immobilization capability and biodegradability in BEAC process. Therefore,  Figure 4(a).
In pilot-scale BAC system (Figure 4  The total intensity of influent water of pilot-scale system was 1.27 × 10 5 , with higher proportion in Region III (fulvic acid-like compounds, 3.5 × 10 4 , 27.56%) and V (humic-like organic compounds, 4.2 × 10 4 , 33.08%). Therefore, removal of fulvic acid-and humic acid-like organics were the core objectives. The total fluorescence intensities (removal     Figure S4 suggested that adsorption of humic acids by MCGL-4/3 followed pseudo-second-order kinetics (R 2 > 0.998) when average molecular weight ranged from 2,700 to 5,100 Da. It indicated that a key step of velocity control was the adsorption reaction stage, rather than pore diffusion stage.
However, adsorption process achieved by MCGL-2/1 followed Weber-Morris kinetic model (R 2 > 0.998), and it indicated that adsorption process was mainly influenced by the diffusion of adsorbents in the pores of the adsorbents.
The adsorption kinetics results confirmed that mesoporous regulation during 20.4∼408 Å promoted adsorption capacities and kinetics.

Comprehensive quantitative indicators of carbons
Selecting or tailoring an excellent carbon for bio-enhancement is a complex process, especially when the pilot-scale or production experimental data were scarce. Lots of factors can influence the carbon selection such as raw water quality, biomass, bio-activities and parameters of filtration, etc.
Therefore, methods which can be used to prejudge  of IMC BEAC were assigned as follows: S BET ! 1,400 m 2 /g, V total ! 1.90 cm 3 /g, V micro ! 0.60 cm 3 /g, V mes ! 1.00 cm 3 /g, V mar ! 0.25 cm 3 /g, D ! 40 Å, V IN ! 1,400 mg/g, V MB ! 350 mg/g, V HA ! 2.0 mg/g, strength ! 90% and density ! 500 g/L. Based on the IMC BEAC , Comprehensive Quantitative Indicators (CQI) was proposed consequently to evaluate the initial immobilization capacity of biomass (BI) by different carbons. It was identified using Equation (11).
Here, V i was parameters of carbons used in present work (Tables 1 and 3); W i was the linear correlation coefficients (R 2 ) in Figure 5(a), V i-IMC was parameters of IMC BEAC as mentioned above.

ZJ15(2.51). Linear correlation analysis between CQI
and BI was conducted in Figure 5 utilization efficiency during bio-degradation were determined in Figure 5 and Table 5 according to methods described earlier.
As shown in Table 5, considerable differences were observed in surface affinity for DO during adsorption process (ΔDO CS ) in column-A. And it indicated that ΔDO CS were negatively correlated with thermal ablation rate of carbons.
Fitting results also showed that linear correlation coefficient between ΔDO CS and surface oxygen content was 0.9725.     Cumulative uptake of DOC (QC) and RSSCT tests were introduced in present work to quantify synergistic efficacy between adsorption and bio-degradation.

Cumulative uptake of DOC (QC)
The changing curves of QC calculated from data in Although it is difficult to directly quantify the relationship between adsorption and bio-degradation, it is clear that effects of bio-degradation caused by immobilization of functional bacteria in BEAC system can be first evaluated based on data from BAC system. Variable α and β were consequently employed as a function of KBV. Variable α is described by the following Equation (12): Here, parameter i represents carbon MCGL-4, SX-10 or ZJ15. Results suggests α C-MCGL-4 was always higher than 1.0 even at the initial stage of operation; however, it required 3.78 m 3 ·H 2 O/(kg·carbon) for α C-SX-10 and 3.09 m 3 ·H 2 O/(kg·carbon) for α C-ZJ15, respectively. It indicated that simultaneous bio-degradation and adsorption occurred in B-MCGL-4 after bacteria immobilization, while B-SX-10 and B-ZJ15 still required a short operational time to achieve effective bio-degradation.