Preparation and Application of Sewage Sludge Bio-char/zero Valent Iron (SSBC/ZVI) Composite for Improving the Biodegradability of a Real Chemical Synthesis-based Pharmaceutical Wastewater

A new kind of micro-electrolysis ller sewage sludge biochar/zero violent iron (SSBC/ZVI) composite was prepared for a real chemical synthesis based pharmaceutical wastewater (CSPW) pretreatment for improving the biodegradation index (BI). The optimal operation condition of micro-electrolysis system was obtained at HRT of 2 h, the initial pH of 3.0 and ller dosage of 100 g/L, with COD removal rate of 30.5%. Comparative analysis of raw and used SSBC/ZVI ller, GC-MS analysis of raw and treated pharmaceutical wastewater suggested that the pollutants removal was mainly attributed to the combination of reduction and oxidation, absorption of SSBC and occulation effect of iron sludge. In addition, SSBC/ZVI exhibited relative high stability and excellent reusability for COD removal and BI improvement of real pharmaceutical wastewater. The results of this study provide new ideas of sewage sludge utilization for real wastewater pretreatment.


Introduction
The pharmaceutical industry is one of the most polluting industries which generating huge amount of wastewater during the production process. The composition of wastewater produced from different pharmaceutical units differs obviously, and the organic content in chemical synthesis based pharmaceutical wastewater (CSPW) usually varies between 800 and 29,000 mg/L [1]. Moreover, pharmaceutical wastewater (PW) containing hazardous and recalcitrant compounds are often nonbiodegradable [2,3]. PW treatment is a very challenging task because of its varying composition, recalcitrant nature, high toxicity, and low biodegradability [4]. Various techniques have been developed for PW treatment including physicochemical methods such as adsorption, sedimentation, advanced oxidation processes (AOPs) and biochemical methods [5,6]. Although biochemical processes provide the cheapest option for wastewater treatment, it often exhibits limited pollutants removal e ciency for PW with high toxicity. AOPs provide an e cient option to treat e uents with low biodegradability. Therefore, varieties kinds of AOPs techniques were explored to remove the refractory pollutants and to improve the biodegradability index (BI, BOD5/COD) of PW [7].
Micro-electrolysis technique attracted much more attention recently in refractory wastewater pretreatment due to its low operation cost, simple process control and high pollutants removal e ciency [8]. Fe-C micro-electrolysis has been proven to be highly e cient and low-cost in the treatment of various wastewater, including pharmaceutical wastewater [9], petroleum wastewater [10], chemical wastewater [11], etc. In Fe-C micro-electrolysis system, Fe 0 and activated carbon (AC) act as the anode and cathode, respectively, and numerous microscopic galvanic cells will form spontaneously. In anode, Fe 0 loses electrons and dissolves into the solution in the form of Fe 2+ . For the cathode, AC accepts and transfers the electrons to the acceptors. Under anoxic conditions, H + will accept the electrons to form [H] with excellent activity for organic pollutants reduction. If oxygen exists, OH with strong oxidizing property will form and the organic pollutants will be oxidized. [12]. In addition, the pollutants removal by Fe-C microelectrolysis also includes the adsorption of AC and the occulation and precipitation effect by ferric and ferrous hydroxides. The pollutants removal mechanisms by micro-electrolysis pretreatment can be summarized as oxidation reduction, coagulation and occulation, physical adsorption [13]. Previous studies suggested that high COD removal e ciency can be achieved by micro-electrolysis pretreatment and the BI values of refractory wastewater with high organics concentration can be improved obviously [5].
Sewage sludge bio-char (SSBC) is one of the pyrolysis products of sewage sludge, which can be used as an adsorbent for wastewater treatment. In this study, a new kind of micro-electrolysis ller SSBC/ZVI composite was prepared using SSBC as the cathode for pretreating a real chemical synthesis-based pharmaceutical wastewater. Micro-electrolytic llers preparation using SSBC for PW pretreatment process is expected to realize the waste resources utilization and process pollution emission reduction. To the best of our knowledge, there was no report about Fe-C micro-electrolysis ller using SSBC for improving the biodegradability of a real PW. The SSBC/ZVI composites were characterized by scanning electron microscopy coupled to an energy dispersive X-ray spectrometry (SEM-EDX), fourier-transform infrared spectroscopy (FT-IR), x-ray diffraction (XRD) and speci c surface area analysis. The pretreatment effect of PW was evaluated by BI improvement.

Materials and reagents
Dewatered anaerobically digested SS with a moisture content of 82.1% (wt%) was obtained from Chongqing Sino French Water Company. The digested SS was oven dried at about 105°C for 12 h, and then milled and the particles with the size lower than 0.18 mm were collected. CSPW was provided by a chemical synthesis-based pharmacy company in Chongqing manufacturing cephalosporins and anticancer drugs. The wastewater used in this research was mainly generated from products manufacturing and equipment cleaning processes. Relevant characteristics of the CSPW were measured and the results were shown in Table 1 [14]. The obtained SSBC was milled and sieved again, and particles with the size below 0.15 mm were used for SSBC/ZVI composite preparation. The mass ratio of SSBC to ZVI was controlled at 2:1 due to the low carbon content of SSBC, and supplement level of ATP which acts as adhesive was according to the mechanical strength of the ller. In addition, certain amount of sodium bicarbonate was added to increase the porosity of the ller. For composite preparation, SSBC powder, ZVI powder, ATP and NaHCO 3 as main raw material in a mass percentage of 50, 25, 20 and 5 were granulated to particles with the diameter of 10 mm and then sintered at 800°C for 60 min under an oxygen-free circumstance. Before use, the ller was washed by deionized water with the solid liquid ratio of 1:10 for 12 h to remove the residues of poreforming agent.

Characterization of SSBC/ZVI composite
The surface area, porosity and pore volume of fresh and used SSBC/ZVI composite was determined by measuring the adsorption/desorption of N 2 at 77 K according to the Brunauer-Emmett-Teller (BET) principle using a Micromeritics ASAP 2460 (Micromeritics, America). FT-IR was performed to identify the functional groups of SSBC/ZVI composite. To obtain the FT-IR spectra, samples were milled and mixed with KBr and then pressed into pellets. FT-IR spectra were recorded in the range of 4000 − 400 cm − 1 with the resolution of 32 scans and 4 cm − 1 on a Shimadzu IR Prestige-21 Spectrometer. Scanning electron microscopy (SEM, Zeiss Sigma 300, Germany) equipped with an energy dispersive spectrometer (EDX, XFlash 6, Bruker, Germany) detector was used to characterize the microscopic morphology of SSBC/ZVI composite. The crystal structures of SSBC and SSBC/ZVI composite were determined by XRD (X'Pert Pro MPD, Cu Kα radiation). The data in the 2θ range from 5°to 90° were collected with a scanning velocity of 10°/min.

Batch experiment for CSPW treatment
Batch CSPW treatment was conducted in 250 mL of glass bottles placed in a water bath shaker at 25°C and 50 rpm. Each bottle contained 100 mL supernatant of CSPW after centrifugation at 2770 g RCF for 10 min. The initial pH was adjusted into desired values by sulfuric acid, and then the washed SSBC/ZVI composite was added into the reactor with different solid/liquid ratio. 1 mL of sample was taken at certain time intervals for analysis. After reaction, the SSBC/ZVI composite ller was taken out and dried at 105ºC for 120 minutes under vacuum condition for materials characterization.

Wastewater analysis
Chemical oxygen demand (COD) of CSPW was measured by COD analyzer (Lianhua Technology pH value was measured by a pH meter (Leichi Instrument Factory, Shanghai, China). Total organic carbon (TOC) and total nitrogen (TN) of CSPW was analyzed using a TOC analyzer (Shimadzu TOC-V CPH, Japan). Qualitative analysis of pollutants in CSPW was conducted on gas chromatography-mass spectrometry (Shimadzu GCMS-QP 2010 plus, Japan). Before analysis, the sample was centrifuged for 10 minutes at 8110 g RCF, and the supernatant was ltered through a 0.45 µm membrane lter for analysis. The injection volume of each analysis was controlled at 1 uL with split ratio of 10:1. The compounds were separated by Agilent DB-17 MS column (30 m×0.25 um×0.25 mm) with helium as the carrier gas. The injector was kept at 300ºC. The oven heating program was from 60ºC (holding for 2 min) to 300ºC (holding for 5 min) at a rate of 10ºC/min. MS analysis was performed in full scan acquisition mode, in the mass range 29-500 m/z at 1 scan/s. The main pollutants were identi ed by comparing the mass spectra with the NIST library and similarity index calculation results by the software. In addition, the qualitative analysis results are also con rmed by comparing with the pollutant types in the environmental assessment reports.

Characterization of SSBC/ZVI composite
In order to characterize the characteristics of SSBC/ZBI composite ller and identify the mechanism of pollutant removal, fresh and used SSBC/ZVI composite was characterized by SEM-EDX, FT-IR, XRD and speci c surface area analysis. The results and discussion about the characterization of materials were presented as follows.

SEM-EDX characterization
The microstructure of fresh SSBC/ZBI composite with 200 and 10000 times magni cations was presented in the SEM results in Fig. 1a. It can be found that the ller surface was heterogeneous due to the inconsistency of the particle size and density of the raw materials. The lamellar structure with relative smooth surface and short rod crystal pieces should be assigned to ferron particles and ATP, respectively.
Previous research suggested that the surface of Fe 0 in the fresh Fe-C micro-electrolytic ller is relatively smooth [15]. The ATP acts as the adhesive is short rod with 2 µm in length and about 100 nm in diameter [16]. The rest amorphous material of the ller which contains rough, irregular structures with very few pores is SSBC [17]. EDX element contents analysis (shown in Fig. 1b) made clear to us that the ller contains iron, carbon and oxygen derived from the organic matter of SS, aluminum and potassium derived from the inorganic components of SS, and magnesium, silicon, and oxygen in ATP.
Approximately, 32.15% iron and 13.56% carbon were measured in the SSBC/ZVI composites by EDX.  [19]. The weak peak at 770 cm − 1 was probably associated with C-H bending vibration, while the peak at 779 cm − 1 was correspond to the stretching vibration of aromatic C-H groups [20]. Diversity of functional groups explains that SSBC and SSBC/ZVI can be used as adsorbent for multiple pollutants removal.

XRD characterization
XRD characterization was conducted to investigate the crystal structures of Fe 0 and carbon on the surface of SSBC/ZVI composites. As the results shown in Fig. 1d, the SSBC/ZVI showed obvious peaks at 2θ of 44.9°, which corresponds to the re ection of Fe 0 [21]. The weak bands at 2θ of 25.9°indicated the carbonaceous structure presented in SSBC/ZVI [22]. The diffraction peaks of SiO 2 were often appeared in SSBC as previous reports [23]. XRD analysis showed that SiO 2 existed in both SSBC and SSBC/ZVI, and their content and morphology remained relatively stable.

Speci c surface area analysis
The evolution of the BET surface area of SSBC/ZVI ller can be observed in Table 2. As can be seen, the BET surface area of fresh SSBC/ZVI was 32.35 m 2 g − 1 , indicating the development of pores at the stage of carbonization and ller calcination. However, the BET surface area was smaller than commercially available Fe-C ller (165.638 m 2 g − 1 ) [24], which was due to the relative low speci c surface area of SSBC [25].

Evaluation of CSPW treatment effect under different condition
The pollutants removal e ciency of micro-electrolysis was closely related to operation conditions, such as reaction time, initial pH, ller dosage In this study, hydraulic retention time (HRT), initial pH, and ller dosage were chosen as operational factors to discuss their in uences on COD removal e ciency, and the results were illustrated in Fig. 2.

Pollutants removal e cacy with different HRT
HRT determines the reaction time of micro-electrolysis, and the in uence of HRT on COD removal e ciency of CSPW was depicted in Fig. 2a. The COD remove process could be divided into three stages.
In the rst stage (0-20 min), 15.2% of COD was removed within the 20 min and the COD removal rate was much higher than the residual two stages, which was due to the porous structure of SSBC/ZVI composite could adsorb the pollutants on its surface [26]. In the second stage (20-120 min), the growth of COD removal e ciency slowed down gradually. On the one hand, the adsorption capacity of SSBC/ZVI ller was limited, and the pollutants absorbed on the ller surface made a mass transfer resistance of electrons [5]. On the other hand, Fe 2+ released in the anode might gathered on the ller surface, which further increased the mass transfer resistance [12]. In the third stage (120-180 min), the COD removal e ciency kept at relative constant value. As the reaction time extended, the pH value of the reaction system will rise to neutral or even alkaline. The micro-electrolysis reaction will halt at the neutral or alkaline condition due to the precipitated iron in the form of hydroxides attached on the lling surface, and the accessibility of active [H] and OH will decrease. Base on the results above, a satis ed COD removal e ciency (30.5%) can be achieved at HRT of 2 h.

Pollutants removal e cacy under different initial pH condition
The pH value can affect the availability of active [H] and OH in the reaction system, which in turn affects the pollutants removal e ciency by iron-carbon micro-electrolysis. Five different pH levels (1, 3, 5, 7 and 9) with HRT of 2 h and ller dosage of 100 g/L were other condition, were operated to survey COD removal e ciency in batch experiments. The in uence of pH on the pollutants removal e ciency was illustrated in Fig. 2b. With the increase of the pH from 1 to 3, COD removal e ciency was marginally increased from 24.6-26.7%. When the pH increased from 3 to 9, the COD removal e ciency decreased gradually from 26.7-3.0%. The micro-electrolysis was acid promoted reaction and mainly occurred between anode and cathode materials with the electron transfer. During the micro-electrolysis, the Fe 0 was gradually corroded under the acid condition with electron lose. The electron transfer will be enhanced by increasing the H + concentration and the accessibility of active [H] and OH will be improved [27]. Large amount of Fe 2+ will be generated gradually from Fe0 corrosion, and then the precipitate Fe(OH)x will be created as the Fe 2+ oxidization and H + consumption. The iron hydroxides acts as the mass transfer resistance could be dissolved in the acidic condition, resulting in more reactive sites exposed. In neutral or alkaline condition, the micro electrolysis reaction will halt. It was generally accepted that the electron transfer rate would be inhibited remarkably in the alkaline condition, thus the COD removal e ciency continued to decline when the initial pH increased from 3.0 to 9.0. In order to achieve an effective COD removal e ciency, the initial pH of CPSW was adjusted to 3 before treatment.

Pollutants removal e cacy with different SSBC/ZVI ller dosage
The COD removal e ciency under different ller dosage was studied and the results were shown in Fig. 2c. As illustrated in Fig. 2c, COD removal e ciency increased gradually when the ller dosage increased from 50 to 150 g/L, and the maximum COD removal rate of 33.9% was achieved with 150 g/L ller dosage. This could be explained by the increased surface area, reactive sites and [H] formation when the ller dosage increased [27]. Improved pollutants removal rate will be achieved when the electrode materials dosage increased because the pollutants degradation generally occurs at the active sites of the anode surface [28]. However, the pollutant removal e ciency will decrease when the electrode material dosage is too high, as the contacting area between electrode materials particles and wastewater would decrease. Moreover, micro-electrolysis reactions would be weakened when excess iron reacted with H + [29]. The appropriate dosage of SSBC/ZVI composites was 100 g/L with corresponding COD removal rate of 26.7%.

Characterization of pollutants removal by GC-MS
To clarify the pollutants degradation pathways, GC-MS analysis was used to examine the pollutants and derived intermediates in CSPW. The pollutants species and derived intermediates in CSPW were listed in Table 3. Total of 17 main organic pollutant species were detected in raw CSPW, which were the major contributors to COD. For the untreated CSPW, 17 main organic pollutants could be divided into 7 types including alcohols (No. 1-5), ketones (No. 6), esters (No. 7-9), intermediate (No. 10), amines (No. 12-15), nitriles (N0. 17) and pyridines (No. 18,20). After micro-electrolysis treatment for 120 min, the number of main pollutant species increased to 23. Speci cally, there was no pollutant species were completely removed and 6 new pollutant species were generated after treatment. The above results suggested that the COD of CSPW could be partially removed owing to the abatement of organic pollutant species in SSBC/ZVI treatment. It was obvious that most pollutants presented in CSPW were degraded into smaller molecules after micro-electrolysis treatments, and the above discoveries supported the COD removal by SSBC/ZVI treatment.

Pollutants removal mechanisms analysis
In order to clarify the pollutants removal mechanism by the SSBC/ZVI micro-electrolysis system, the used SSBC/ZVI ller and iron sludge were characterized by EDX and FTIR rstly, and the results were shown in Fig. 3. The EDX analysis was carried out for used SSBC/ZVI composite to show the change of the ller after CSPW treatment (Fig. 3a). It is shown in EDX spectra that after SCPW treatment, the content of Fe in the composite decreased obviously while the content of carbon increased correspondingly, indicating that a great amount of Fe 0 was consumed after micro-electrolysis treatment.
The FTIR analysis of used SSBC/ZVI composite was also conducted to show the change of the ller after CSPW treatment (Fig. 3b). Compared with the FTIR spectrum of fresh SSBC/ZVI (Fig. 1c), several new absorption peaks were found in the FTIR spectrum of used SSBC/ZVI. The peak at 1420 cm − 1 was caused by C = O symmetric stretching vibration. The peak at 1320 cm − 1 corresponds to C-N stretching vibration of amines. In addition, the peak at 1063 cm − 1 covered by the two strong peaks at 1035 cm − 1 and 986 cm − 1 . The peak at 986 cm − 1 corresponds to C-O-C vibrations of proteins or polysaccharides.
The peak at 689 cm − 1 was ascribed to C-Cl stretching vibration. The peaks at 513 cm − 1 and 472 cm − 1 were associated with M-X (M-metal, X-halogen) stretching vibration. Diversity of functional groups in SSBC/ZVI explains SSBC/ZVI can be utilized for the adsorption removal of multiple pollutants in CSPW.
These new absorption peaks in FTIR spectra of SSBC/ZVI indicated that the pollutants in CSPW can be removed through adsorption. In addition, in order to investigate the pollutants removal by SSBC adsorption, separate adsorption experiment was also conducted. The results of adsorption experiments showed that 5.2% of COD could be removed within 2 h under neutral pH conditions and 100 g/L SSBC/ZVI dosage. SSBC/ZVI showed limited adsorption removal rate of pollutants in CSPW due to the low absorption capacity of SSBC [30].
Iron sludge containing the ferrous and ferric hydroxides was formed by oxidation and precipitation of Fe 2+ released from anode. Pollutants can be removed by iron sludge with the combinational effects of enmeshment, co-precipitation, adsorption and interparticle bridging in the process of coagulation [13]. FTIR characterization of iron sludge (Fig. 3c) showed that there were some new absorption peaks in addition to the infrared absorption peaks produced by the detrital materials exfoliated from SSBC/ZVI.
The new FTIR peak at 3055 cm − 1 caused by the C-H vibration of ethyl and methylene groups suggested that the pollutants with aliphatic chain could be removed by iron sludge occulation. The new peak at 1617 cm − 1 suggested that compounds containing benzene ring could be also removed by iron sludge occulation. However, some pollutants may volatilize during the drying process of sample preparation for FTIR characterization and cannot be detected. Figure 4 proposed the removal mechanisms of pollutants in CSPW treated by SSBC/ZVI microelectrolysis system. The mechanism of pollutants removal includes the adsorption of SSBC and the coagulation as discussed above, as well as micro-electrolysis [13,31]. In SSBC/ZVI micro-electrolysis system, SSBC acts as the cathode in which H + obtain electrons to generate [H] for pollutants reduction.
Fe O acts as the anode of micro-electrolysis system, and Fe O loses electrons to form Fe 2+ . OH radical formed in anode can oxide the pollutants to intermediates or CO 2 and H 2 O. In terms of newly generated organic species as shown in Table 3, the possible formation pathway can be illustrated as follows. Ethylamine (No. 16) was formed by de-isopropyl and de-ethyl of diisopropylethylamine (No. 13) and triethylamine (No. 14), respectively.

Reusability of SSBC/ZVI composite ller
Previous researches reported that the micro electrolysis llers can be gradually hardened after the repeated running process owing to the passivation, blockage, and mass transport limitation of the ller surface [5,32]. Long-term run of SSBC/ZVI system for CSPW treatment was conducted to verify whether the llers could become harden. The operation conditions for 6 runs within 720 min were set as follows with the initial pH of 3, HRT of 2 h, ller dosage of 100 g/L. The COD removal e ciency and BI values were presented in Fig. 5. The COD removal e ciency in SSBC/ZVI was stable and performed well within the rst 5 runs. While for the sixth run, COD removal e ciency decreased gradually from 29.1 to 23.9%, owing to the surface passivation, blockage, and mass transport limitation. The results suggested that SSBC/ZVI systems had relative high stability and excellent reusability for COD removal from CSPW. The COD of SSBC/ZVI treated CSPW was still high and the e uent cannot be discharged directly. We explore the feasibility using SSBC/ZVI as pretreatment technology before biochemical treatment, since microelectrolysis pretreatment could increase the BI value of wastewater, which is helpful for subsequent biological processes [33]. The BI value is normally used to express the biodegradability of wastewater. When BI value is higher than 0.3, the wastewater usually exhibits a better biodegradability [34]. The BI values after different operation runs were analyzed and the results were shown in Fig. 5. Raw CSPW cannot be biodegraded according to the low BI value of 0.17. Improvement in biodegradability was achieved with SSBC/ZVI micro electrolysis system in the rst 4 run operation, and BI value increased from initial 0.17 to 0.39, 0.36, 0.36 and 0.32, respectively. The results indicated the biodegradability of SSBC/ZVI system treated CSPW was enhanced. However, the BI value decreased to 0.29 and 0.27 after the 5th and 6th run operation. Therefore, after 4 run operation using SSBC/ZVI for CSPW pretreatment, necessary measures should be taken to ensure the stability of the system. Table 4 shows a comparison of different advanced oxidation processes (AOPs) as pretreatment methods of real chemical synthesis-based pharmaceutical wastewater, which mainly includes ozone oxidation, inner electrolysis, Fenton oxidation, catalytic oxidation and the combination technologies. The pretreatment effect of SSBC/ZVI system on real pharmaceutical wastewater is equivalent to the reported methods such as CoMnAl-CMO-Al 2 O 3 /H 2 O 2 oxides catalyst process [35], microwave assisted Fenton like process [36], but higher than the process using an Fe 2 O 3 / SBA-15 nano composite catalyst [37]. It should be emphasized that the raw material of SSBC/ZVI composites ller is waste sewage sludge, and the reaction process does not require the addition of chemical reagents such as H 2 O 2 . These results indicated that SSBC/ZVI micro electrolysis was suitable for CSPW pretreatment. 4 Conclusions

Comparison of different pretreatments of CSPW for biodegradability improvement
In this study, SSBC was used to establish micro-electrolysis systems for actual pharmaceutical wastewater pretreatment. The optimal parameters of SSBC/ZVI micro electrolysis system were HRT of 2 h, initial pH of 3 and the ller dosage of 100 g/L. The COD removal rate was 30.5%. According to the characterization results of SEM-EDS, FTIR, XRD analysis of raw and used SSBC/ZVI composites, and the GC-MS analysis of raw and treated CSPW, the removal of pollutants mainly includes adsorption, micro electrolysis and occulation. Besides, SSBC/ZVI possessed stability in repeated runs for BI improvement of CSPW. Therefore, SSBC/ZVI micro-electrolysis could be considered as a cost-effective, feasible, and promising method for real pharmaceutical wastewater pretreatment.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Author contribution
Gaoxiang Qi performed the preparation of materials, experiments design and manuscript preparation.

Ethics approval and consent to participate
No animals or human subjects were used in this research.

Consent for publication
Our manuscript does not contain any individual data in any form.   Proposed mechanism of CSPW treatment by SSBC/ZVI composite ller