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The effects of adsorbing organic pollutants from super heavy oil wastewater by lignite activated coke
Journal of Hazardous Materials 308 (2016) 113–119
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
The effects of adsorbing organic pollutants from super heavy oil wastewater by lignite activated coke Kun Tong a,b,d,∗ , Aiguo Lin a,b,∗ , Guodong Ji c,∗∗ , Dong Wang d , Xinghui Wang d a
College of Chemical Engineering, China University of Petroleum, Qingdao 266555, China National University Science Park, China University of Petroleum, Dongying 207062, China Department of Environmental Engineering, Peking University, Beijing 100871,China d Liaohe Petroleum Exploration Bureau, China National Petroleum Corporation, Beijing 102206,China b c
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• LAC adsorption can increase pH and B/C of SHOW that benefit for biodegradation. • FTIR, Boehm titrations, GC–MS, and LC–OCD were employed for mechanisms analysis. • The organic pollutions that removed are HOCs, complicated structures or toxic. • The mechanisms of adsorption are due to hydrogen bonding and the functional groups.
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 28 December 2015 Accepted 8 January 2016 Available online 12 January 2016 Keywords: Adsorption Lignite activated coke Super heavy oil wastewater Biodegradability
a b s t r a c t The adsorption of organic pollutants from super heavy oil wastewater (SHOW) by lignite activated coke (LAC) was investigated. Specifically, the effects of LAC adsorption on pH, BOD5 /CODCr (B/C), and the main pollutants before and after adsorption were examined. The removed organic pollutants were characterized by Fourier transform infrared spectroscopy (FTIR), Boehm titrations, gas chromatography–mass spectrometry (GC–MS), and liquid chromatography with organic carbon detection (LC–OCD). FTIR spectra indicated that organic pollutants containing COOH and NH2 functional groups were adsorbed from the SHOW. Boehm titrations further demonstrated that carboxyl, phenolic hydroxyl, and lactonic groups on the surface of the LAC increased. GC–MS showed that the removed main organic compounds are difficult to be degraded or extremely toxics to aquatic organisms. According to the results of LC–OCD, 30.37 mg/L of dissolved organic carbons were removed by LAC adsorption. Among these, hydrophobic organic contaminants accounted for 25.03 mg/L. Furthermore, LAC adsorption was found to increase pH and B/C ratio of the SHOW. The mechanisms of adsorption were found to involve between the hydrogen bonding and the functional groups of carboxylic, phenolic, and lactonic on the LAC surface. In summary, all these results demonstrated that LAC adsorption can remove bio-refractory DOCs, which is beneficial for biodegradation. © 2016 Published by Elsevier B.V.
1. Introduction ∗ Corresponding author at: College of Chemical Engineering, China University of Petroleum, Qingdao 266555, China. Fax: +86 546 8391778. ∗∗ Corresponding author. Fax: +86 10 62755914 87. E-mail addresses: [email protected], [email protected] (A. Lin), [email protected] (G. Ji). http://dx.doi.org/10.1016/j.jhazmat.2016.01.014 0304-3894/© 2016 Published by Elsevier B.V.
Activated coke (AC) can be produced from various natural carbonaceous materials, such as lignite [1] [termed lignite activated coke (LAC)], petroleum coke [2], wood [3], and other biomasses [4].
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Table 1 Characteristics of SHOW. Parameter
Concentration (mg/L)
Parameter
Concentration (mg/L)
CODCr BOD5 Oil NH3 -N TN TP SS
467.8 79.53 24.19 10.53 14.31 0.206 50.77
Cl− SiO2 Ca2+ Mg2+ Total salt pH TOC
460.1 140.3 33.21 14.62 3060 7.098 152.3
diameter was 2.61 nm, as measured by N2 adsorption isotherms using an ASAP 2010 micromeritics instrument (Table 2). Prior to the adsorption experiments, the LAC was heated in an electric furnace. Then it was washed for several times with distilled water until visibly clear water was obtained in order to remove the organic gases [9]. The LAC was then dried in an oven at 378 K for 72 h and then cooled to the room temperature in an air-tight glass bottle. The LAC remained was stored in an air-tight glass bottle until needed. 2.3. Adsorption experiments
In particular, LAC represents a potential alternative material for activated carbon and is a less expensive adsorbent. Previously, LAC has been extensively applied to treat the toxic gas-phase materials such as H2 S [3], SOx , NOx [4,5], and Hg◦ [6], and the bio-refractory wastewaters contaminated with trinitrotoluene [7], coke [8], and heavy oil [9]. Though LAC is generally utilized as an advanced treatment after biodegradation processes, the capacity for LAC to improve the biodegradability of SHOW, and especially its effect on the properties of wastewater, have not been extensively reported. After most mineral oils and surfactants have been demulsified and flocculated in SHOW, chemical oxygen demand (COD) is generally lowered to 500 mg/L. However, large amounts of heavy oils and surfactants still exist, so a further treatment of SHOW is needed. Correspondingly, biological methods have become increasingly popular in the treatment of oilfield wastewater due to their high efficiency, cost effectiveness, and environmental friendliness [10–13]. However, pretreatment of SHOW is needed to improve its biodegradability [14,15] and high toxicity [16]. LAC has a lower iodine sorption value (300–500 mg/g) compared with many other active carbons. It is also characterized by mesopores with a diameter of 403 m and a pore volume of 0.271 cm3 /g. Thus, these pores can selectively absorb large molecules and longchain organic compounds. The adsorption capacity of LAC for COD is approximately 100 mg/g [9]. It can remove organic compounds in effluents and alleviate biodegradation load. The objective of this study was to investigate the capacity of the LAC removing organic pollutants in SHOW. The factors evaluated included pH and B/C. A systematic characterization of the effects of LAC on the main pollutants was also conducted. Regarding the latter, GC–MS and LC–OCD were used to characterize the organic compounds removed. In addition, the mechanisms that improved the biodegradability of LAC to treat SHOW were analyzed. These results provided the basis for the full-scale usage of LAC in treatments of SHOW. 2. Materials and methods 2.1. SHOW The SHOW used in the experiments was obtained from a heavy oil wastewater treatment plant, the Liaohe oilfield (Liaoning province, China), which had already been treated by demulsification and flocculation. The SHOW received was a colorless and transparent liquid with no peculiar smell and it had a COD value of less than 500 mg/L. Table 1 summarizes the important chemical and physical parameters of the SHOW used. 2.2. LAC LAC prepared by lignite was obtained from the Beijing Guodianfutong Technology Development Co., Ltd. The particle size and surface area for the LAC ranged from 0.074 mm to 0.83 mm and from 500 m2 /g to 600 m2 /g, respectively. In addition, the total pore volume ranged from 0.48 cm3 /g to 0.52 cm3 /g and the average pore
LAC powder (0.5 g) was added into each sealed 1000 mL glass bottle, which contained 500 mL of heavy oil wastewater. Then the bottles were placed in an air-bath shaker at 150 rpm and 303 ± 0.2 K for 360 min. The initial pH was unchanged. The samples were subsequently filtered and the resulting filtrate was analyzed. All of the experiments were performed in duplicated. 2.3.1. Effect of LAC dosage on pH Various doses of LAC powder (1.0, 2.0, 3.0, 3.5, and 4.0 g/L) were added to each 1000 mL sealed glass bottle, which contained 500 mL of heavy oil wastewater. Experiments were performed in an airbath shaker at 150 rpm and 303 ± 0.2 K for 360 min, and the initial pH was unchanged. 2.3.2. Effect of LAC dosage on B/C Various doses of LAC powder (1.0, 2.0, 3.0, 4.0, and 5.0 g/L) were added to each 1000 mL sealed glass bottle, which contained 500 mL of heavy oil wastewater. These experiments were also performed in an air-bath shaker at 150 rpm and 303 ± 0.2 K for 360 min, and the initial pH remained unchanged. 2.4. Analytical methods Biochemical oxygen demand after 5 days (BOD5 ) was determined according to Chinese standards (GB 7488-87). COD was used to evaluate SHOW treatment efficiency and pH was determined by the dichromate method (Water Quality-Determination of the Chemical Oxygen Demand-Dichromate method, GB11914-89) with a pH meter (pH-3D, Leici Corporation, China). Mineral oil concentration was measured by infrared spectroscopy (F2000, Jilin China). Oil components in the wastewater were determined by a national standard method (GB/T16488-1996). Ions were detected by an ion chromatograph (DionexIonpac, USA). Concentrations of Cl− , Mg2+ , and Ca2+ , as well as other pollutants, were determined by a national standard method [17]. The removal rates and adsorption of COD at equilibrium were calculated according to the following formulas: = qe =
(C0 − Ce ) × 100% C0 (C0 − Ce )V W
(1) (2)
where (%) is the removal rate, C0 (mg/L) is the COD concentration in the raw SHOW, Ce (mg/L) is the COD concentration after adsorption, qe (mg/g) is the amount of maximum COD adsorbed per unit mass by the LAC, V (L) is the volume of the SHOW, and W (g) is the LAC weight. 2.5. Characterizations A HITACHI S-450 scanning electron microscope (Hitachi, Japan) operated at 10 kV was employed to observe the surface morphology of the LAC before adsorption [18].
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Table 2 Activated coke technical index. Parameter
Concentration (mg/L)
Parameter
Concentration (mg/L)
Particle diameter (mm) Loading density (g/L) Pore volume (mL/g) Surface area (m2 /g) Iodine value (mg/g)
Powder (<0.3 mm) 500–600 0.48–0.52 500–600 400–500
Methylene blue value (mg/g) Water content (%) Fixed carbon (%) Volatile matter (%) Ash content (%)
95–103 <4 >67 <4 <20
FTIR spectra of LAC were obtained with a Perkin Elmer spectrum 100. In brief, the powder samples were mixed with spectroscopic grade KBr and pressed into disks with a diameter of 10 mm and a thickness of 1 mm. Scanning was performed from 4000 cm−1 to 450 cm−1 . The surface functional groups of the LAC before and after adsorption were examined with Boehm titrations [19,20]. In brief, LAC powder (3.00 g) was put in a 2500 mL flask and 2000 mL of raw SHOW was added. The experiments were performed at 303 ± 0.2 K in an air-bath shaker (150 rpm) for 6 h, with the initial pH unchanged. The samples were filtered and the filtration residues were analyzed by Boehm titrations. All of the experiments were performed in duplicated. GC–MS analysis of the organic components in the influents and effluents of the wastewater were performed using an Agilent 7890A/5975C GC–MS system (Agilent Technologies Co., Ltd., USA). Wastewater samples (500 mL each) were extracted with 60 mL of dichloromethane for three times at pH 2, 7, and 12 in sequence. These organic phases were merged and dehydrated with an anhydrous sodium sulfate capillary column, and then were concentrated to 1 mL by purging with nitrogen. Pretreated samples (1 L) were analyzed by GC–MS with pure helium (99.999%) used as the carrier gas at a constant flow rate of 1.1 mL/min. An ADB-5MS capillary column (30 m × 0.25 mm × 0.25 m, J&W Co., Ltd., USA) was used in the separation system. The oven temperature was programmed to increase from 50 ◦ C (5 min) to 290 ◦ C (10 min) at a rate of 10 ◦ C/min, and the electron energy and electron double voltage were set to 70 eV and 1365 V, respectively. Size exclusion chromatography was used to analyze wastewater samples, which were collected before and after adsorption with LAC using a LC–OCD, Model 8 (DOC-Labor, Germany) with organic carbon, nitrogen, and UV254 detectors. A volume of 1000 mL was applied to a Toyopearl TSKHW-50S column (particle size, 30 mm; length, 250 mm; internal diameter, 20 mm) with a phosphate buffer mobile phase (2.5 g/L KH2 PO4 and 1.5 g/L Na2 (HPO4 )2 ·H2 O) at pH 6.8. The analysis provided a quantification of the organic matter fractions, including biopolymers (BP), humics (HS), building blocks (BB), low molecular weight (LMW) organic acids (OA), and LMW Neutral (NEU), as well as the total DOC concentration [21].
3.2. Effect of LAC dosage on the properties of the treated SHOW 3.2.1. Effect of the LAC dosage on pH The experiments performed were conducted at 303 ± 0.2 K with different doses of LAC at the original pH after agitating for 6 h. Fig. 1 presents the changes in pH that were detected for the different LAC doses. When the LAC dosage increased from 0 g/L to 2.0 g/L, a rapid increase in pH from 7.098 to 7.833 was observed. When the LAC dose increased from 2.0 g/L to 4.0 g/L, a slow increase in pH from 7.833 to 7.981 was observed. Therefore, LAC adsorption can increase pH of the SHOW. The factors influencing the rates of microbial degradation for hydrocarbons include temperature, pH, salinity, oxygen, nutrients, and the physical and chemical composition of petroleum [22]. Furthermore, microorganisms are able to grow in neutral or near alkaline environments, and the optimal range pH for the biodegradation of petroleum hydrocarbons is from 7.5 to 7.8 [23]. Thus, the addition of 1–2 g/L LAC can potentially increase the pH of SHOW, which is beneficial for the process of petroleum hydrocarbon biodegradation. 3.2.2. Effect of LAC dosage on B/C Generally, the biodegradability of wastewater is assessed by a B/C ratio. When the B/C ratio is ≤0.1, the wastewater is extremely difficult to degrade. When the B/C ratio is ≥0.1 and ≤0.3, then the wastewater is biodegradable. Finally, if the B/C ratio is ≥0.3, the wastewater is easily to be degraded [24]. In the present study, the SHOW is difficult to degrade because the abundance of large and non-biodegradable organic molecules is present [25]. Therefore, the treatment to improve biodegradability of SHOW is needed [26]. After the treatment of the SHOW with increased dose of LAC, a change in the B/C ratio from 0.171 to 0.275 was observed (Fig. 2), and a reduction in COD concentrations and BOD5 from 467.8 mg/L to 236.4 mg/L and 80 mg/L to 65 mg/L, respectively was also observed. Thus, the biodegradability of the SHOW substantially increased following the treatment with LAC.
3. Results and discussion 3.1. Wastewater quality analysis To evaluate the initial quality of the untreated SHOW, the COD, BOD5 , total N (TN), total P (TP), salinity, ion concentrations, and other physical and chemical characteristics were examined. The results are listed in Table 1. Measurements of COD and BOD5 were 467.8 mg/L and 79.53 mg/L, respectively. The B/C ratio was 0.170, which indicated that the organic substrates in the wastewater were bio-refractory and would be difficult to treat using conventional biological processes.
Fig. 1. Effects of the LAC dosage on pH.
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Fig. 2. Effects of the LAC dosage on B/C. Fig. 4. FTIR characterizations of LAC before and after adsorption.
Table 3 Pollutants concentrations changed before and after adsorption. Parameter
Oil CODCr SS pH NH3 -N TN TP Sulfide
Concentration (mg/L) Before
After
24.18 467.8 50.77 7.098 10.53 14.54 0.206 0.048
15.23 394.8 34.21 7.697 8.321 11.28 0.158 0.028
Parameter
Volatile Phenol Mg2+ Ca2+ Cl− Total Hardness Total salt Cr Anionic Surfactants
Concentration (mg/L) Before
After
0.003 14.62 33.21 460.1 387.4 3060 0.064 0.160
0.002 11.23 23.31 339.6 232.2 2190 0.009 0.041
Fig. 3. Environmental scanning electron micrographs of LAC before adsorption.
3.2.3. Changes in pollutant concentrations The data in Table 3 indicate that LAC not only removed organic pollutants such as COD, NH3 -N, and oils, but also removed inorganic pollutants such as Mg2+ , Ca2+ , and other salts. The pH increase was observed, which indicated LAC adsorption can remove some acidic substances in the SHOW. 3.2.4. SEM The surface morphology and characteristics of LAC before adsorption were characterized by SEM. SEM images showed that the LAC surface is porous with apertures greater than 10 m and nearly 50 m (Fig. 3). LAC is macroporous and mesoporous pos-
sessing a small micropore volume which benefits adsorption of contaminants from the liquid phase [1]. The images also show a great deal of irregular structure similar to the fish-scale structure on the surface of LAC. This structure has stronger adsorption activity [27]. 3.2.5. FTIR characterization of LAC before and after adsorption Fig. 4 shows the FTIR spectra of LAC before and after adsorption. The peaks at 796 cm−1 and 874 cm−1 prior to adsorption represent out of plane bending vibrations of the C H bonds, while 1077 cm−1 may represent a stretching vibration of the C O or primary alcohols [28]. The peaks at 1386 cm−1 and 1460 cm−1 correspond to CH3 , the antisymmetry stretching vibration of C H, or the bending vibration band of CH2 [29]. The peak at 1632 cm−1 corresponds with the double bonds of the C C stretching vibration area [30], whereas the peak at 2346 cm−1 corresponds to the C H triple bond or the cumulated double bonds stretching vibration area [18]. The peak at 3418 cm−1 represents an H O stretching vibration band or a displaced amino group. Some adsorption peaks drifted, appeared, or disappeared after adsorption. The adsorption peak at 800 cm−1 is due to out of plane bending vibrations of the C H bond. The 1074 cm−1 peak may represent the stretching vibration of C O bonds or the primary alcohols, and the 1386 cm−1 peak can be assigned to CH3 . The peak at 1596 cm−1 is for the C C bond stretching vibration area [31], and the peaks at 2916 cm−1 and 3422 cm−1 represent COOH and the H O stretching vibration band or displaced amino group, respectively [32–34]. Fig. 4 indicates that the surface functional groups in LAC are C O, primary alcohols, CH3 , C C, H O, or a displaced amino group prior to adsorption. After adsorption, COOH and NH2 groups appeared and were detected at the LAC surface, which indicated some of the organic compounds were adsorbed from the SHOW contained these functional groups. 3.2.6. Boehm titrations Boehm titrations were employed for analysis of the changes in surface functional groups of the LAC used before and after adsorption. The results are listed in Table 4. Prior to adsorption, an abundance of carboxyl, phenolic hydroxyl, and lactonic groups were detected. After adsorption, the amounts of these surface functional groups all increased. Especially, the amount of phenolic hydroxyl groups was increased by 91.87%. The concentration of total acidic functional groups increased from 0.3892 mmol/g to 0.4966 mmol/g, and the concentration of total
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Table 4 Surface functional groups in LAC before and after adsorption. Surface functional groups
Before (mmol/g)
After (mmol/g)
Change rate (%)
Carboxyl Phenolic hydroxyl group Lactonic group Total acidic functional groups Total alkaline groups
0.1998 0.0332 0.1562 0.3892 1.207
0.2531 0.0637 0.1798 0.4966 1.102
26.68 91.87 15.11 27.60 −8.70
alkaline groups decreased from 1.207 mmol/g to 1.102 mmol/g. These results indicate that the LAC adsorbed many organic compounds contain carboxyl and phenolic hydroxyl groups. The above results are consistent with the FTIR and GC–MS data. Moreover, surface functional groups, such as carboxyls, have been shown to play a vital role in the environmental applications of surfaces [36]. 3.2.7. GC–MS analysis Fig. 5 shows the results obtained from a GC–MS analysis of the organic species, which were extracted from the SHOW before and after LAC adsorption. Forty types of organic compounds were detected before adsorption, and dibutyl phthalate had the highest concentration (Table 5). After adsorption, only fifteen types of organic compounds were detected. Dibutyl phthalate remained the most abundant compound, although its abundance decreased from 380,000 to 272,000 after adsorption. Many organic compounds were removed by LAC adsorption, including: phenol, aniline, heptanoic acid, o-toluidine, 3-methyl-benzenamine, cyclopentaneacetic acid, 2-ethyl-phenol, 1,2-benzenediol, 2-(1-methylethyl)phenol,2-ethyl-4-methyl-phenol, 3-methyl-benzoic acid, quinoline,2,3-dihydro-1H-inden-5-ol,3,5-dimethyl-benzoicacid, 1-ethyl-3-(1-methyleth)-benzene,3-phenyl-furan,4methyl-2-(3-methyl-but-2-enyl, 1,2,3,5,6,7,8,8aoctahydro-naphthalene,6-(3-isopropenylcycloprop-1-eny, 3-t-butyl-oct-3-ene-1,5-diyne,4-methyl-2(1H)-quinolinone, 3,4-dihydro-2-naphthalenamine, 1,2,3,6-tetrahydro-1pyridine,7H-pyrazolo[4,3-d]pyrimidin-7-o, tetradecanamide, and 5,8,11,14-eicosatetraynoic acid. Petroleum hydrocarbons have been divided into four classes: saturates, aromatics, asphaltenes (e.g., phenols, fatty acids, ketones, esters, and porphyrins), and resins (e.g., pyridines, quinolines, carbazoles, sulfoxides, and amides) [37]. Saturates have exhibited the most rapid biodegradation rates, followed by the light aromatics. In contrast, high molecular weight aromatics and polar compounds exhibit extremely low rates of degradation, with resins and
Fig. 5. GC–MS results before and after adsorption (a) before (b) after.
Fig. 6. Analysis chromatogram by LC–OCD of SHOW.
asphaltenes being extremely resistant to degradation by microorganisms [38]. Table 5 shows that after LAC adsorption, most of the resins and asphaltenes, such as the amines, pyridines, polycyclic aromatic hydrocarbons, and quinolines were completely removed. Previously, phenolics, organic acids, and other organic pollutants have exhibited lower removal rates, yet have undergone biodegradation by an activated sludge process [39]. In addition, alkanes and esters that remain in SHOW can be degraded by biological processes [40]. However, aniline, o-toluidine, and 3-methyl-benzenamine are extremely toxic to aquatic organisms and exert an additional oxygen demand due to nitrification reactions occur during their biodegradation [41]. Based on the present results, it can be concluded that LAC can remove organic compounds with large molecular weights and with complicated structures, including resins, asphaltenes, and toxic compounds. Moreover, similar results have been observed in other studies [9,14]. Thus, pretreatment with LAC adsorption can improve the biodegradability of SHOW.
3.2.8. LC–OCD LC–OCD with ultra violet (UV) and online organic carbon (OC) detection (UVD and OCD, respectively) was used for DOC characterization [21]. LC–OCD separates chromatographable organic carbon (CDOC) into fractions of different molecular weights [43]. Water samples were analyzed after filtration through 0.45 m filters, and the LC–OCD results obtained indicate that dissolved organic carbon (DOC) in the SHOW consisted of a large number of HOCs and NEU. A small amount of HS and BB were also refractory organic matters that were present, and trace amounts of BP and OA represented biogenic organic pollutants that were present (Fig. 6, Table 6). DOC (30.37 mg/L), which accounted for 24.26% of the total amounts was removed by LAC adsorption, including 25.03 mg/L HOCs (19.97%). The remaining compounds were NEU, BB, and OA, and the amounts removed were 2.57 mg/L, 1.188 mg/L, and 1.154 mg/L, respectively. HOCs were the most abundant DOCs that were removed, and they accounted for 82.42% of the total amount of DOCs presented. HOCs can be natural hydrocarbons, sparingly soluble “humins” of the humic substances family, or other complex organic compounds that are inert and difficult to degrade [38,44]. Hence, the removal of HOCs, NEU, BB, and bio-refractory DOCs by LAC adsorption demonstrates the ability of LAC to improve SHOW biodegradability, thereby enhancing the biodegradation process.
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Table 5 Main organic compounds analysis before and after adsorption. Components
Species
Quantity
Removal rates (%)
Before adsorption
After adsorption
Saturates Aromatics
Alkanes Aromatics Amines Pyridines
1 1 5 1
1 – – –
0 100 100 100
Resins
Polycyclic aromatic hydrocarbons Quinolines Phenolics
1 2 12
– – 8
100 100 33.33
Asphaltenes
Organic acids Esters
8 1
3 1
62.50 0
Others Total
/ /
8 40
2 15
75.0 62.50
References
[36,42]
Note: “–” means no measured. Table 6 Main organic compounds analysis before and after adsorption.
Before (mg/L) After (mg/L) Removal amount (mg/L) Removal rate (%)
DOC
HOC
CDOC
BP MW >20000
HS MW ∼1000
BB MW 300–450
NEU MW <350
OA MW <350
125.2 94.83 30.37 24.26
55.06 30.03 25.03 45.46
70.17 64.80 5.37 7.653
1.078 0.832 0.246 22.82
8.829 8.619 0.21 2.378
7.423 6.235 1.188 16.00
50.69 48.12 2.57 5.070
2.149 0.995 1.154 53.70
Fig. 7. The models of hydrogen bonding between LAC and organic pollutants of SHOW.
3.3. Mechanisms of adsorption The mechanisms associated with the adsorption process are based on electrostatic interactions, hydrogen bond forces, hydrophilie bonds, electron donor–acceptor interactions, and – dispersion interactions that occur [45]. In our previous study, an equilibrium isotherm analysis indicated that LAC treatment of SHOW involved chemical adsorption [9]. In the present study, Boehm titrations (Table 6), GC–MS, FTIR, and LC–OCD analyses demonstrated that the main adsorption groups of LAC include carboxylic, phenolic hydroxyl, and lactonic groups. Furthermore, the hydrogen atoms of these groups can bond with the nitrogen and oxygen atoms of the organic pollutants of SHOW. Meanwhile, the oxygen atoms of the groups in LAC can act by hydrogen bonding with hydrogen atoms of the COOH, OH and NH2 groups in the organic pollutants of SHOW (Fig. 7). With larger specific surface area and more abundant pore structure, the LAC has stronger ability of absorption [27]. Based on our previous analysis and the results of the Boehm titrations analysis performed in the present study, as well as FTIR, GC–MS, and LC–OCD analyses conducted before and after adsorption, the mechanisms of adsorption by LAC were found
to be related to the functional carboxylic, phenolic, and lactonic groups on the surface of the LAC [35].
4. Conclusion In this study, it was demonstrated that LAC is a porous material suitable for the adsorption of organic compounds from SHOW. LAC adsorption can increase pH and B/C of SHOW. FTIR spectra indicated that a subset of organic compounds containing functional groups of COOH and NH2 was adsorbed from the SHOW. Boehm titrations analysis showed that the LAC adsorbed many organic compounds containing carboxyl and phenolic hydroxyl groups. GC–MS and LC–OCD analysis demonstrated that LAC can remove organic compounds with high molecular weights and complicated structures or extremely toxic to aquatic organisms. The mechanisms of adsorption were found to involve hydrogen bonding and the functional carboxylic, phenolic, and lactonic functional groups on the surface of LAC. LAC adsorption is beneficial for biodegradation of SHOW.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
The effects of adsorbing organic pollutants from super heavy oil wastewater by lignite activated coke Kun Tong a,b,d,∗ , Aiguo Lin a,b,∗ , Guodong Ji c,∗∗ , Dong Wang d , Xinghui Wang d a
College of Chemical Engineering, China University of Petroleum, Qingdao 266555, China National University Science Park, China University of Petroleum, Dongying 207062, China Department of Environmental Engineering, Peking University, Beijing 100871,China d Liaohe Petroleum Exploration Bureau, China National Petroleum Corporation, Beijing 102206,China b c
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• LAC adsorption can increase pH and B/C of SHOW that benefit for biodegradation. • FTIR, Boehm titrations, GC–MS, and LC–OCD were employed for mechanisms analysis. • The organic pollutions that removed are HOCs, complicated structures or toxic. • The mechanisms of adsorption are due to hydrogen bonding and the functional groups.
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 28 December 2015 Accepted 8 January 2016 Available online 12 January 2016 Keywords: Adsorption Lignite activated coke Super heavy oil wastewater Biodegradability
a b s t r a c t The adsorption of organic pollutants from super heavy oil wastewater (SHOW) by lignite activated coke (LAC) was investigated. Specifically, the effects of LAC adsorption on pH, BOD5 /CODCr (B/C), and the main pollutants before and after adsorption were examined. The removed organic pollutants were characterized by Fourier transform infrared spectroscopy (FTIR), Boehm titrations, gas chromatography–mass spectrometry (GC–MS), and liquid chromatography with organic carbon detection (LC–OCD). FTIR spectra indicated that organic pollutants containing COOH and NH2 functional groups were adsorbed from the SHOW. Boehm titrations further demonstrated that carboxyl, phenolic hydroxyl, and lactonic groups on the surface of the LAC increased. GC–MS showed that the removed main organic compounds are difficult to be degraded or extremely toxics to aquatic organisms. According to the results of LC–OCD, 30.37 mg/L of dissolved organic carbons were removed by LAC adsorption. Among these, hydrophobic organic contaminants accounted for 25.03 mg/L. Furthermore, LAC adsorption was found to increase pH and B/C ratio of the SHOW. The mechanisms of adsorption were found to involve between the hydrogen bonding and the functional groups of carboxylic, phenolic, and lactonic on the LAC surface. In summary, all these results demonstrated that LAC adsorption can remove bio-refractory DOCs, which is beneficial for biodegradation. © 2016 Published by Elsevier B.V.
1. Introduction ∗ Corresponding author at: College of Chemical Engineering, China University of Petroleum, Qingdao 266555, China. Fax: +86 546 8391778. ∗∗ Corresponding author. Fax: +86 10 62755914 87. E-mail addresses: [email protected], [email protected] (A. Lin), [email protected] (G. Ji). http://dx.doi.org/10.1016/j.jhazmat.2016.01.014 0304-3894/© 2016 Published by Elsevier B.V.
Activated coke (AC) can be produced from various natural carbonaceous materials, such as lignite [1] [termed lignite activated coke (LAC)], petroleum coke [2], wood [3], and other biomasses [4].
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Table 1 Characteristics of SHOW. Parameter
Concentration (mg/L)
Parameter
Concentration (mg/L)
CODCr BOD5 Oil NH3 -N TN TP SS
467.8 79.53 24.19 10.53 14.31 0.206 50.77
Cl− SiO2 Ca2+ Mg2+ Total salt pH TOC
460.1 140.3 33.21 14.62 3060 7.098 152.3
diameter was 2.61 nm, as measured by N2 adsorption isotherms using an ASAP 2010 micromeritics instrument (Table 2). Prior to the adsorption experiments, the LAC was heated in an electric furnace. Then it was washed for several times with distilled water until visibly clear water was obtained in order to remove the organic gases [9]. The LAC was then dried in an oven at 378 K for 72 h and then cooled to the room temperature in an air-tight glass bottle. The LAC remained was stored in an air-tight glass bottle until needed. 2.3. Adsorption experiments
In particular, LAC represents a potential alternative material for activated carbon and is a less expensive adsorbent. Previously, LAC has been extensively applied to treat the toxic gas-phase materials such as H2 S [3], SOx , NOx [4,5], and Hg◦ [6], and the bio-refractory wastewaters contaminated with trinitrotoluene [7], coke [8], and heavy oil [9]. Though LAC is generally utilized as an advanced treatment after biodegradation processes, the capacity for LAC to improve the biodegradability of SHOW, and especially its effect on the properties of wastewater, have not been extensively reported. After most mineral oils and surfactants have been demulsified and flocculated in SHOW, chemical oxygen demand (COD) is generally lowered to 500 mg/L. However, large amounts of heavy oils and surfactants still exist, so a further treatment of SHOW is needed. Correspondingly, biological methods have become increasingly popular in the treatment of oilfield wastewater due to their high efficiency, cost effectiveness, and environmental friendliness [10–13]. However, pretreatment of SHOW is needed to improve its biodegradability [14,15] and high toxicity [16]. LAC has a lower iodine sorption value (300–500 mg/g) compared with many other active carbons. It is also characterized by mesopores with a diameter of 403 m and a pore volume of 0.271 cm3 /g. Thus, these pores can selectively absorb large molecules and longchain organic compounds. The adsorption capacity of LAC for COD is approximately 100 mg/g [9]. It can remove organic compounds in effluents and alleviate biodegradation load. The objective of this study was to investigate the capacity of the LAC removing organic pollutants in SHOW. The factors evaluated included pH and B/C. A systematic characterization of the effects of LAC on the main pollutants was also conducted. Regarding the latter, GC–MS and LC–OCD were used to characterize the organic compounds removed. In addition, the mechanisms that improved the biodegradability of LAC to treat SHOW were analyzed. These results provided the basis for the full-scale usage of LAC in treatments of SHOW. 2. Materials and methods 2.1. SHOW The SHOW used in the experiments was obtained from a heavy oil wastewater treatment plant, the Liaohe oilfield (Liaoning province, China), which had already been treated by demulsification and flocculation. The SHOW received was a colorless and transparent liquid with no peculiar smell and it had a COD value of less than 500 mg/L. Table 1 summarizes the important chemical and physical parameters of the SHOW used. 2.2. LAC LAC prepared by lignite was obtained from the Beijing Guodianfutong Technology Development Co., Ltd. The particle size and surface area for the LAC ranged from 0.074 mm to 0.83 mm and from 500 m2 /g to 600 m2 /g, respectively. In addition, the total pore volume ranged from 0.48 cm3 /g to 0.52 cm3 /g and the average pore
LAC powder (0.5 g) was added into each sealed 1000 mL glass bottle, which contained 500 mL of heavy oil wastewater. Then the bottles were placed in an air-bath shaker at 150 rpm and 303 ± 0.2 K for 360 min. The initial pH was unchanged. The samples were subsequently filtered and the resulting filtrate was analyzed. All of the experiments were performed in duplicated. 2.3.1. Effect of LAC dosage on pH Various doses of LAC powder (1.0, 2.0, 3.0, 3.5, and 4.0 g/L) were added to each 1000 mL sealed glass bottle, which contained 500 mL of heavy oil wastewater. Experiments were performed in an airbath shaker at 150 rpm and 303 ± 0.2 K for 360 min, and the initial pH was unchanged. 2.3.2. Effect of LAC dosage on B/C Various doses of LAC powder (1.0, 2.0, 3.0, 4.0, and 5.0 g/L) were added to each 1000 mL sealed glass bottle, which contained 500 mL of heavy oil wastewater. These experiments were also performed in an air-bath shaker at 150 rpm and 303 ± 0.2 K for 360 min, and the initial pH remained unchanged. 2.4. Analytical methods Biochemical oxygen demand after 5 days (BOD5 ) was determined according to Chinese standards (GB 7488-87). COD was used to evaluate SHOW treatment efficiency and pH was determined by the dichromate method (Water Quality-Determination of the Chemical Oxygen Demand-Dichromate method, GB11914-89) with a pH meter (pH-3D, Leici Corporation, China). Mineral oil concentration was measured by infrared spectroscopy (F2000, Jilin China). Oil components in the wastewater were determined by a national standard method (GB/T16488-1996). Ions were detected by an ion chromatograph (DionexIonpac, USA). Concentrations of Cl− , Mg2+ , and Ca2+ , as well as other pollutants, were determined by a national standard method [17]. The removal rates and adsorption of COD at equilibrium were calculated according to the following formulas: = qe =
(C0 − Ce ) × 100% C0 (C0 − Ce )V W
(1) (2)
where (%) is the removal rate, C0 (mg/L) is the COD concentration in the raw SHOW, Ce (mg/L) is the COD concentration after adsorption, qe (mg/g) is the amount of maximum COD adsorbed per unit mass by the LAC, V (L) is the volume of the SHOW, and W (g) is the LAC weight. 2.5. Characterizations A HITACHI S-450 scanning electron microscope (Hitachi, Japan) operated at 10 kV was employed to observe the surface morphology of the LAC before adsorption [18].
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Table 2 Activated coke technical index. Parameter
Concentration (mg/L)
Parameter
Concentration (mg/L)
Particle diameter (mm) Loading density (g/L) Pore volume (mL/g) Surface area (m2 /g) Iodine value (mg/g)
Powder (<0.3 mm) 500–600 0.48–0.52 500–600 400–500
Methylene blue value (mg/g) Water content (%) Fixed carbon (%) Volatile matter (%) Ash content (%)
95–103 <4 >67 <4 <20
FTIR spectra of LAC were obtained with a Perkin Elmer spectrum 100. In brief, the powder samples were mixed with spectroscopic grade KBr and pressed into disks with a diameter of 10 mm and a thickness of 1 mm. Scanning was performed from 4000 cm−1 to 450 cm−1 . The surface functional groups of the LAC before and after adsorption were examined with Boehm titrations [19,20]. In brief, LAC powder (3.00 g) was put in a 2500 mL flask and 2000 mL of raw SHOW was added. The experiments were performed at 303 ± 0.2 K in an air-bath shaker (150 rpm) for 6 h, with the initial pH unchanged. The samples were filtered and the filtration residues were analyzed by Boehm titrations. All of the experiments were performed in duplicated. GC–MS analysis of the organic components in the influents and effluents of the wastewater were performed using an Agilent 7890A/5975C GC–MS system (Agilent Technologies Co., Ltd., USA). Wastewater samples (500 mL each) were extracted with 60 mL of dichloromethane for three times at pH 2, 7, and 12 in sequence. These organic phases were merged and dehydrated with an anhydrous sodium sulfate capillary column, and then were concentrated to 1 mL by purging with nitrogen. Pretreated samples (1 L) were analyzed by GC–MS with pure helium (99.999%) used as the carrier gas at a constant flow rate of 1.1 mL/min. An ADB-5MS capillary column (30 m × 0.25 mm × 0.25 m, J&W Co., Ltd., USA) was used in the separation system. The oven temperature was programmed to increase from 50 ◦ C (5 min) to 290 ◦ C (10 min) at a rate of 10 ◦ C/min, and the electron energy and electron double voltage were set to 70 eV and 1365 V, respectively. Size exclusion chromatography was used to analyze wastewater samples, which were collected before and after adsorption with LAC using a LC–OCD, Model 8 (DOC-Labor, Germany) with organic carbon, nitrogen, and UV254 detectors. A volume of 1000 mL was applied to a Toyopearl TSKHW-50S column (particle size, 30 mm; length, 250 mm; internal diameter, 20 mm) with a phosphate buffer mobile phase (2.5 g/L KH2 PO4 and 1.5 g/L Na2 (HPO4 )2 ·H2 O) at pH 6.8. The analysis provided a quantification of the organic matter fractions, including biopolymers (BP), humics (HS), building blocks (BB), low molecular weight (LMW) organic acids (OA), and LMW Neutral (NEU), as well as the total DOC concentration [21].
3.2. Effect of LAC dosage on the properties of the treated SHOW 3.2.1. Effect of the LAC dosage on pH The experiments performed were conducted at 303 ± 0.2 K with different doses of LAC at the original pH after agitating for 6 h. Fig. 1 presents the changes in pH that were detected for the different LAC doses. When the LAC dosage increased from 0 g/L to 2.0 g/L, a rapid increase in pH from 7.098 to 7.833 was observed. When the LAC dose increased from 2.0 g/L to 4.0 g/L, a slow increase in pH from 7.833 to 7.981 was observed. Therefore, LAC adsorption can increase pH of the SHOW. The factors influencing the rates of microbial degradation for hydrocarbons include temperature, pH, salinity, oxygen, nutrients, and the physical and chemical composition of petroleum [22]. Furthermore, microorganisms are able to grow in neutral or near alkaline environments, and the optimal range pH for the biodegradation of petroleum hydrocarbons is from 7.5 to 7.8 [23]. Thus, the addition of 1–2 g/L LAC can potentially increase the pH of SHOW, which is beneficial for the process of petroleum hydrocarbon biodegradation. 3.2.2. Effect of LAC dosage on B/C Generally, the biodegradability of wastewater is assessed by a B/C ratio. When the B/C ratio is ≤0.1, the wastewater is extremely difficult to degrade. When the B/C ratio is ≥0.1 and ≤0.3, then the wastewater is biodegradable. Finally, if the B/C ratio is ≥0.3, the wastewater is easily to be degraded [24]. In the present study, the SHOW is difficult to degrade because the abundance of large and non-biodegradable organic molecules is present [25]. Therefore, the treatment to improve biodegradability of SHOW is needed [26]. After the treatment of the SHOW with increased dose of LAC, a change in the B/C ratio from 0.171 to 0.275 was observed (Fig. 2), and a reduction in COD concentrations and BOD5 from 467.8 mg/L to 236.4 mg/L and 80 mg/L to 65 mg/L, respectively was also observed. Thus, the biodegradability of the SHOW substantially increased following the treatment with LAC.
3. Results and discussion 3.1. Wastewater quality analysis To evaluate the initial quality of the untreated SHOW, the COD, BOD5 , total N (TN), total P (TP), salinity, ion concentrations, and other physical and chemical characteristics were examined. The results are listed in Table 1. Measurements of COD and BOD5 were 467.8 mg/L and 79.53 mg/L, respectively. The B/C ratio was 0.170, which indicated that the organic substrates in the wastewater were bio-refractory and would be difficult to treat using conventional biological processes.
Fig. 1. Effects of the LAC dosage on pH.
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Fig. 2. Effects of the LAC dosage on B/C. Fig. 4. FTIR characterizations of LAC before and after adsorption.
Table 3 Pollutants concentrations changed before and after adsorption. Parameter
Oil CODCr SS pH NH3 -N TN TP Sulfide
Concentration (mg/L) Before
After
24.18 467.8 50.77 7.098 10.53 14.54 0.206 0.048
15.23 394.8 34.21 7.697 8.321 11.28 0.158 0.028
Parameter
Volatile Phenol Mg2+ Ca2+ Cl− Total Hardness Total salt Cr Anionic Surfactants
Concentration (mg/L) Before
After
0.003 14.62 33.21 460.1 387.4 3060 0.064 0.160
0.002 11.23 23.31 339.6 232.2 2190 0.009 0.041
Fig. 3. Environmental scanning electron micrographs of LAC before adsorption.
3.2.3. Changes in pollutant concentrations The data in Table 3 indicate that LAC not only removed organic pollutants such as COD, NH3 -N, and oils, but also removed inorganic pollutants such as Mg2+ , Ca2+ , and other salts. The pH increase was observed, which indicated LAC adsorption can remove some acidic substances in the SHOW. 3.2.4. SEM The surface morphology and characteristics of LAC before adsorption were characterized by SEM. SEM images showed that the LAC surface is porous with apertures greater than 10 m and nearly 50 m (Fig. 3). LAC is macroporous and mesoporous pos-
sessing a small micropore volume which benefits adsorption of contaminants from the liquid phase [1]. The images also show a great deal of irregular structure similar to the fish-scale structure on the surface of LAC. This structure has stronger adsorption activity [27]. 3.2.5. FTIR characterization of LAC before and after adsorption Fig. 4 shows the FTIR spectra of LAC before and after adsorption. The peaks at 796 cm−1 and 874 cm−1 prior to adsorption represent out of plane bending vibrations of the C H bonds, while 1077 cm−1 may represent a stretching vibration of the C O or primary alcohols [28]. The peaks at 1386 cm−1 and 1460 cm−1 correspond to CH3 , the antisymmetry stretching vibration of C H, or the bending vibration band of CH2 [29]. The peak at 1632 cm−1 corresponds with the double bonds of the C C stretching vibration area [30], whereas the peak at 2346 cm−1 corresponds to the C H triple bond or the cumulated double bonds stretching vibration area [18]. The peak at 3418 cm−1 represents an H O stretching vibration band or a displaced amino group. Some adsorption peaks drifted, appeared, or disappeared after adsorption. The adsorption peak at 800 cm−1 is due to out of plane bending vibrations of the C H bond. The 1074 cm−1 peak may represent the stretching vibration of C O bonds or the primary alcohols, and the 1386 cm−1 peak can be assigned to CH3 . The peak at 1596 cm−1 is for the C C bond stretching vibration area [31], and the peaks at 2916 cm−1 and 3422 cm−1 represent COOH and the H O stretching vibration band or displaced amino group, respectively [32–34]. Fig. 4 indicates that the surface functional groups in LAC are C O, primary alcohols, CH3 , C C, H O, or a displaced amino group prior to adsorption. After adsorption, COOH and NH2 groups appeared and were detected at the LAC surface, which indicated some of the organic compounds were adsorbed from the SHOW contained these functional groups. 3.2.6. Boehm titrations Boehm titrations were employed for analysis of the changes in surface functional groups of the LAC used before and after adsorption. The results are listed in Table 4. Prior to adsorption, an abundance of carboxyl, phenolic hydroxyl, and lactonic groups were detected. After adsorption, the amounts of these surface functional groups all increased. Especially, the amount of phenolic hydroxyl groups was increased by 91.87%. The concentration of total acidic functional groups increased from 0.3892 mmol/g to 0.4966 mmol/g, and the concentration of total
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Table 4 Surface functional groups in LAC before and after adsorption. Surface functional groups
Before (mmol/g)
After (mmol/g)
Change rate (%)
Carboxyl Phenolic hydroxyl group Lactonic group Total acidic functional groups Total alkaline groups
0.1998 0.0332 0.1562 0.3892 1.207
0.2531 0.0637 0.1798 0.4966 1.102
26.68 91.87 15.11 27.60 −8.70
alkaline groups decreased from 1.207 mmol/g to 1.102 mmol/g. These results indicate that the LAC adsorbed many organic compounds contain carboxyl and phenolic hydroxyl groups. The above results are consistent with the FTIR and GC–MS data. Moreover, surface functional groups, such as carboxyls, have been shown to play a vital role in the environmental applications of surfaces [36]. 3.2.7. GC–MS analysis Fig. 5 shows the results obtained from a GC–MS analysis of the organic species, which were extracted from the SHOW before and after LAC adsorption. Forty types of organic compounds were detected before adsorption, and dibutyl phthalate had the highest concentration (Table 5). After adsorption, only fifteen types of organic compounds were detected. Dibutyl phthalate remained the most abundant compound, although its abundance decreased from 380,000 to 272,000 after adsorption. Many organic compounds were removed by LAC adsorption, including: phenol, aniline, heptanoic acid, o-toluidine, 3-methyl-benzenamine, cyclopentaneacetic acid, 2-ethyl-phenol, 1,2-benzenediol, 2-(1-methylethyl)phenol,2-ethyl-4-methyl-phenol, 3-methyl-benzoic acid, quinoline,2,3-dihydro-1H-inden-5-ol,3,5-dimethyl-benzoicacid, 1-ethyl-3-(1-methyleth)-benzene,3-phenyl-furan,4methyl-2-(3-methyl-but-2-enyl, 1,2,3,5,6,7,8,8aoctahydro-naphthalene,6-(3-isopropenylcycloprop-1-eny, 3-t-butyl-oct-3-ene-1,5-diyne,4-methyl-2(1H)-quinolinone, 3,4-dihydro-2-naphthalenamine, 1,2,3,6-tetrahydro-1pyridine,7H-pyrazolo[4,3-d]pyrimidin-7-o, tetradecanamide, and 5,8,11,14-eicosatetraynoic acid. Petroleum hydrocarbons have been divided into four classes: saturates, aromatics, asphaltenes (e.g., phenols, fatty acids, ketones, esters, and porphyrins), and resins (e.g., pyridines, quinolines, carbazoles, sulfoxides, and amides) [37]. Saturates have exhibited the most rapid biodegradation rates, followed by the light aromatics. In contrast, high molecular weight aromatics and polar compounds exhibit extremely low rates of degradation, with resins and
Fig. 5. GC–MS results before and after adsorption (a) before (b) after.
Fig. 6. Analysis chromatogram by LC–OCD of SHOW.
asphaltenes being extremely resistant to degradation by microorganisms [38]. Table 5 shows that after LAC adsorption, most of the resins and asphaltenes, such as the amines, pyridines, polycyclic aromatic hydrocarbons, and quinolines were completely removed. Previously, phenolics, organic acids, and other organic pollutants have exhibited lower removal rates, yet have undergone biodegradation by an activated sludge process [39]. In addition, alkanes and esters that remain in SHOW can be degraded by biological processes [40]. However, aniline, o-toluidine, and 3-methyl-benzenamine are extremely toxic to aquatic organisms and exert an additional oxygen demand due to nitrification reactions occur during their biodegradation [41]. Based on the present results, it can be concluded that LAC can remove organic compounds with large molecular weights and with complicated structures, including resins, asphaltenes, and toxic compounds. Moreover, similar results have been observed in other studies [9,14]. Thus, pretreatment with LAC adsorption can improve the biodegradability of SHOW.
3.2.8. LC–OCD LC–OCD with ultra violet (UV) and online organic carbon (OC) detection (UVD and OCD, respectively) was used for DOC characterization [21]. LC–OCD separates chromatographable organic carbon (CDOC) into fractions of different molecular weights [43]. Water samples were analyzed after filtration through 0.45 m filters, and the LC–OCD results obtained indicate that dissolved organic carbon (DOC) in the SHOW consisted of a large number of HOCs and NEU. A small amount of HS and BB were also refractory organic matters that were present, and trace amounts of BP and OA represented biogenic organic pollutants that were present (Fig. 6, Table 6). DOC (30.37 mg/L), which accounted for 24.26% of the total amounts was removed by LAC adsorption, including 25.03 mg/L HOCs (19.97%). The remaining compounds were NEU, BB, and OA, and the amounts removed were 2.57 mg/L, 1.188 mg/L, and 1.154 mg/L, respectively. HOCs were the most abundant DOCs that were removed, and they accounted for 82.42% of the total amount of DOCs presented. HOCs can be natural hydrocarbons, sparingly soluble “humins” of the humic substances family, or other complex organic compounds that are inert and difficult to degrade [38,44]. Hence, the removal of HOCs, NEU, BB, and bio-refractory DOCs by LAC adsorption demonstrates the ability of LAC to improve SHOW biodegradability, thereby enhancing the biodegradation process.
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Table 5 Main organic compounds analysis before and after adsorption. Components
Species
Quantity
Removal rates (%)
Before adsorption
After adsorption
Saturates Aromatics
Alkanes Aromatics Amines Pyridines
1 1 5 1
1 – – –
0 100 100 100
Resins
Polycyclic aromatic hydrocarbons Quinolines Phenolics
1 2 12
– – 8
100 100 33.33
Asphaltenes
Organic acids Esters
8 1
3 1
62.50 0
Others Total
/ /
8 40
2 15
75.0 62.50
References
[36,42]
Note: “–” means no measured. Table 6 Main organic compounds analysis before and after adsorption.
Before (mg/L) After (mg/L) Removal amount (mg/L) Removal rate (%)
DOC
HOC
CDOC
BP MW >20000
HS MW ∼1000
BB MW 300–450
NEU MW <350
OA MW <350
125.2 94.83 30.37 24.26
55.06 30.03 25.03 45.46
70.17 64.80 5.37 7.653
1.078 0.832 0.246 22.82
8.829 8.619 0.21 2.378
7.423 6.235 1.188 16.00
50.69 48.12 2.57 5.070
2.149 0.995 1.154 53.70
Fig. 7. The models of hydrogen bonding between LAC and organic pollutants of SHOW.
3.3. Mechanisms of adsorption The mechanisms associated with the adsorption process are based on electrostatic interactions, hydrogen bond forces, hydrophilie bonds, electron donor–acceptor interactions, and – dispersion interactions that occur [45]. In our previous study, an equilibrium isotherm analysis indicated that LAC treatment of SHOW involved chemical adsorption [9]. In the present study, Boehm titrations (Table 6), GC–MS, FTIR, and LC–OCD analyses demonstrated that the main adsorption groups of LAC include carboxylic, phenolic hydroxyl, and lactonic groups. Furthermore, the hydrogen atoms of these groups can bond with the nitrogen and oxygen atoms of the organic pollutants of SHOW. Meanwhile, the oxygen atoms of the groups in LAC can act by hydrogen bonding with hydrogen atoms of the COOH, OH and NH2 groups in the organic pollutants of SHOW (Fig. 7). With larger specific surface area and more abundant pore structure, the LAC has stronger ability of absorption [27]. Based on our previous analysis and the results of the Boehm titrations analysis performed in the present study, as well as FTIR, GC–MS, and LC–OCD analyses conducted before and after adsorption, the mechanisms of adsorption by LAC were found
to be related to the functional carboxylic, phenolic, and lactonic groups on the surface of the LAC [35].
4. Conclusion In this study, it was demonstrated that LAC is a porous material suitable for the adsorption of organic compounds from SHOW. LAC adsorption can increase pH and B/C of SHOW. FTIR spectra indicated that a subset of organic compounds containing functional groups of COOH and NH2 was adsorbed from the SHOW. Boehm titrations analysis showed that the LAC adsorbed many organic compounds containing carboxyl and phenolic hydroxyl groups. GC–MS and LC–OCD analysis demonstrated that LAC can remove organic compounds with high molecular weights and complicated structures or extremely toxic to aquatic organisms. The mechanisms of adsorption were found to involve hydrogen bonding and the functional carboxylic, phenolic, and lactonic functional groups on the surface of LAC. LAC adsorption is beneficial for biodegradation of SHOW.
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