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The present research work is focussed on the treatment of leachate generated from crude oil-contaminated soil sites using wet air oxidation as an advanced oxidation process. The factors affecting the wet air oxidation (WAO) process, viz. temperature, pressure and time of treatment were optimized using central composite design and response surface methodology. The sig- nificant factors were optimized to maximize % COD removal from the leachate. The linear effects of pressure and temperature (p = 0.000); the square effects of pressure (p = 0.019) and time (p = 0.007) and the interaction effect of temperature–pressure (p = 0.002) were found to be significantly governing the % COD removal. The maximum COD removal of 76% was obtained at temperature = 244 °C, time = 30 min and pressure = 5 bar. Further, the biodegradability index (BOD /COD) increased from 0.14 ± 0.007 of the untreated leachate to 0.48 ± 0.02 of the wetox-treated leachate. Moreover, the degradation of recalcitrant hydrocarbons in initial leachate by WAO treatment was confirmed using GC–MS analysis. Keywords Crude oil-contaminated soil · Leachate · Recalcitrant hydrocarbons · Wet air oxidation · Response surface methodology · Biodegradability index Introduction increase in chances of crude oil spillage due to the trans- portation of oil, storage tank rupture, and pipeline leakage, Crude oil has been widely used for the production of differ - thereby causing soil and water pollution [5]. ent products like gasoline, diesel and heavy oil. Crude oil Land treatment unit (LTU) has been reported as a poten- is a complex mixture of aliphatic saturated hydrocarbons, tial bioremediation process for the treatment of crude oil cyclic saturated hydrocarbons, polycyclic aromatics hydro- contaminated soil [6]. In order to attain significant bioreme- carbons (PAH) and PAH containing nitrogen and sulphur diation using LTU the total petroleum hydrocarbon (TPH) compounds [1]. PAH are considered toxic, carcinogenic, and concentration of contaminated soil should be below 8%. In mutagenic [2]. Accumulation of long-chain saturated hydro- addition, for an effective on-site bioremediation, (1) con - carbons, viz. nonacosane ( C H ), hentriacontane (C H ), taminated soil should contain less than 5% TPH; (2) proper 29 60 31 64 and tritriacontane (C H ) are known to cause severe health irrigation system must be installed to maintain the soil mois- 33 68 problems in human [3] including skin irritation, eye irri- ture content for proper biodegradation and (3) the depth of tation, respiratory irritation, drowsiness or dizziness, and the soil should not be more than 18 in. [7]. During the infil- possibly cancer. Worldwide average oil demand for the year tration of water by irrigation and rainfall, water percolates 2016 was 96.1 mb/day, and it is estimated to reach up to through the contaminated soil and leaches out through LTU 97.6 mb/day and 99 mb/day for the year 2017 and 2018 [4]. [8]. These leachates contain emulsified and solubilized non- Increasing global demand for petroleum products leads to biodegradable and partially degraded eco-toxic hydrocar- bons which cause detrimental effects on the groundwater resources [9]. Scott and co-workers investigated the bio- * Rajesh Biniwale degradation of TPH in LTU leachate in California, USA rb_biniwale@neeri.res.in [9]. They also reported a very low biodegradation (12%) of CSIR-National Environmental Engineering Research TPH owing to their recalcitrant nature due to the presence of Institute (NEERI), Nehru Marg, Nagpur 440020, India long-chain hydrocarbons (≥ C20) in the leachates. The safe Rashtrasant Tukadoji Maharaj Nagpur University (RTMNU), Nagpur 440033, India Vol.:(0123456789) 1 3 238 International Journal of Industrial Chemistry (2019) 10:237–248 disposal of leachate generated from LTU is a key issue and soil. In this study, significant parameters, viz. temperature, certain pretreatment is necessary. pressure, and time of WAO treatment was optimized by In recent years, there has been an increasing interest in using response surface methodology (RSM) using central the use of advanced oxidation processes (AOPs) for the composite design (CCD). Further, the degradation of com- treatment of recalcitrant organic compounds in wastewater, plex hydrocarbons present in the leachate by WAO was con- and crude oil-contaminated soil. These AOP technologies firmed using GC–MS analysis. The specific objective of the include Fenton, Fenton-like process, ozonation, wet air study was to consider WAO as an advanced treatment pro- oxidation, photo-catalysis, etc. Wet air oxidation (WAO) cess for enhancing the biodegradability index of the leachate involves the aqueous phase oxidation of organic and some generated from crude oil-contaminated soil sites. oxidizable inorganic components at elevated temperature 150–325 °C and pressures 5–200 bar [10]. It involves the formation of highly reactive hydroxyl radical which has a Materials and methods high oxidation potential, i.e., E = 2.8 V [11]. WAO oxidizes recalcitrant organic component into biodegradable interme- Leachate collection diates or mineralizes to carbon dioxide and water [12]. WAO has been widely reported for the treatment of oil Crude oil-contaminated soil was collected from the crude sludge, dye degradation, municipal landfill leachate, and degradation of PAH in contaminated soil [13–16]. Jing oil refinery industry situated in North India. The soil sample was collected from near composite well site, where several et al. compared the wet air oxidation of oil sludge in pres- 2+ ence of catalyst (Ni ) and without catalyst by keeping tem- crude oil inlets merge and crude oil were processed for frac- tionation. Due to transportation and spillage of the crude perature, pressure and time constant. The oil sludge with −1 an initial COD of 20,000 mg l showed 99.7% and 88.7% oil, the soil was highly contaminated. The collected soil was analyzed for physico-chemical parameters, viz. parti- COD removal in presence of catalyst and without catalyst, respectively [13]. PAH degradation study was carried out cle size distribution, pH, electrical conductivity (EC), total petroleum hydrocarbons (TPH) concentration, total organic using WAO in presence and absence of free radical promoter by Rivas et al. [17]. Four PAH namely acenaphthene, phen- carbon (TOC), bulk density, porosity, maximum water hold- ing capacity (MWHC), cation exchange capacity (CEC), etc. anthrene, anthracene and fluoranthene underwent 80–100% conversion at a temperature of 190 °C, a pressure of 50 bar Leachate was generated from the contaminated soil using a trough-shaped unit (1 m × 1 m × 1 m). The unit was packed and reaction time of 80 min while the addition of hydroxyl radical promoter reduced the reaction time up to 60 min and to the depth of 0.5 m with the contaminated soil leaving the head space above for holding water. The unit was divided temperature up to 100–150 °C [17]. Some researchers have reported the applicability of WAO for the treatment of land- into three 15-cm-long segments starting from the base of the unit by inserting annular rings between them. The annular fill leachate. Rivas et al. investigated the differential impact of sulphate radical and hydroxyl radical promoted WAO of rings acted as baffles to reduce sidewall flow condition so −1 that water would pass evenly through the unit [19]. At the landfill leachate with an initial COD of 2700–7000 mg l . The temperature of 180–270 °C and pressure of 40–70 bar bottom of the unit, a layer of 10-cm gravels was placed fol- lowed by another layer of 5 cm sand for smooth percolation resulted in 20% COD conversion, while WAO using H O 2 2 provided 35% COD removal, and the addition of oxone of leachate. The schematic diagram of the trough unit used for the leachate generation is represented in Fig. 1. A con- resulted in 80% COD conversion [18]. In another study, factorial design methodology was used to optimize cata- tainer was kept at the bottom of the unit for leachate collec- tion. Leachate was collected in batches by passing a volume lytic wet air oxidation (CWAO) conditions for the effec- tive COD reduction of landfill leachate with an initial COD of water equal to the volume of soil in the unit. This process −1 was continued until the COD of leachate reached a constant of 4920 mg l and biodegradability index of 0.073. The temperature of 200 °C and time of 22 min had shown 78% low level. The collected leachate and contaminated soil were −1 2+ −1 stored in plastic barrels at 4 °C, to minimize any change COD reduction with 250 mg l Cu and 1500 mg l H O 2 2 loading at oxygen partial pressure of 25 bar [15]. However, in its physico-chemical and biological properties until the analysis and experiments were carried out. to the best of our knowledge, no such study has been con- ducted previously for the WAO treatment of leachate gener- The leachate was analyzed for different measurable parameters, viz. appearance, pH, chemical oxygen demand ated from crude oil contaminated soil sites which further establishes the novelty of the present study. (COD), biochemical oxygen demand (BOD), color, bio- degradability index (BI), total organic carbon (TOC). The present research work aims to develop a WAO-based process for physico-chemical treatment of recalcitrant com- Further, the leachate sample was analyzed for GC–MS characterization. pounds in leachate obtained from crude oil contaminated 1 3 International Journal of Industrial Chemistry (2019) 10:237–248 239 Fig. 1 Schematic representa- tion of the trough unit used for the collection of leachate from crude oil contaminated soil tube, a sampling port, a rupture disc as well as non-return Chemicals valve at the gas inlet. The reactor was also equipped with −1 a temperature controller unit and 2-kW electric heater was C –C hydrocarbon standard (500 µg ml ) and polycyclic 8 40 −1 used to heat the reaction mixture to the desired temperature. aromatic hydrocarbons (PAH) standard (1000 µg ml ), were The reactor was leakage proof and the valve of the reactor procured from Sigma Aldrich (USA) and was used for the was tightly sealed with an end cap. preparation of calibration curve in GC–MS analysis. Dichlo- The pH of the collected leachate was considered as initial romethane (DCM), hexane, and cyclohexane (HPLC grade) pH for the WAO pretreatment, therefore the pH of leachate were purchased from Finar (India) and used for the extrac- was not adjusted during the pretreatment. For each experi- tion of organic compounds from the contaminated soil and mental run, the reaction vessel was loaded with 500 ml of leachate. Reagents such as potassium dichromate (K Cr O ), 2 2 7 −1 leachate having initial COD of 2000 ± 10.64 mg l . All lines silver sulphate (Ag SO ), sulphuric acid (H SO ), ferroin 2 4 2 4 were properly closed, ensuring the absence of any leakage indicator, potassium dihydrogen phosphate (KH PO ), 2 4 from the reactor. Oxygen was supplied at the initial stage magnesium sulphate (MgSO ·7H O), dipotassium hydro- 4 2 of the reaction and the stirring speed was fixed at 400 RPM gen phosphate (K HPO ), disodium hydrogen phosphate 2 4 for all the reaction conditions. Preliminary experiments (Na HPO ·7H O), calcium chloride (CaCl ), ferric chloride 2 4 2 2 were performed to investigate the effect of pH and stirring (FeCl ·6H O), ammonium chloride (NH Cl), sodium thio- 3 2 4 speed on % COD removal. The results of the preliminary sulphate (Na S O ), sodium hydroxide (NaOH), etc., were of 2 2 3 experiments were described in detail under the “Result” and reagent grade and purchased from Fisher Scientific (India). “Discussion” sections. WAO parameters were optimized by varying temperature in the range of 150–220 °C, the oxygen Wet air oxidation setup pressure in the range of 10–25 bar and reaction time in the range of 10–25 min. The WAO pre-treated samples were The leachate was treated in a high pressure (HP) and high analyzed for pH, COD, BOD, BOD /COD ratio after each temperature (HT) wet air oxidation (WAO) reactor system experimental run. (Parr, USA) made of stainless steel having total capacity of 1.8 L. The reactor could operate up to a maximum tempera- Statistical design of experiments ture of 350 °C and the maximum pressure of 350 bars. The internal diameter of the reactor was 95 mm, and four-bladed Temperature, pressure, and time are the important param- turbine type impeller (I.D. 50 mm) was used for stirring. The eters influencing the WAO treatment. These factors were stirring speed was kept constant at 400 rotations per minute optimized using two-level three-factor central composite (RPM) throughout the experiment which ensured adequate design (CCD) [20–22]. Minitab software version 16.1 was mass transfer from gas to the liquid phase. The reactor was used for the design of experiment. A total of 20 experimental provided with a pressure indicator gauge, a gas sparging 1 3 240 International Journal of Industrial Chemistry (2019) 10:237–248 runs were carried out in order to maximize the response, multi-wavelength method as per APHA 2120 D method. The i.e., % COD removal from crude oil leachate. A model was sample analysis was performed at two pHs, one at origi- constructed based on the effect of independent variables on nal pH and other adjusted at pH 7 using sulphuric acid or % COD removal for which the quadratic equation is gener- sodium hydroxide. The sample was filtered using 0.22-µm ated as follows: filter in order to prevent the interference by particulate mat- ter during spectrophotometric method. The readings were 2 2 Y = + X + X + X + X + X taken for the 10 ordinates and calculations were performed 0 1 1 2 2 3 3 1,1 2,2 1 2 (1) as per APHA guideline. COD and BOD were determined + X + X X + X X + X X 3,3 1,2 1 2 1,3 1 3 2,3 2 3 according to APHA standard protocols. The % COD removal where Y (% COD removal) is the predicted response; X , of leachate from WAO treatment was calculated using the X , X are independent variables corresponding to tem- following formula: 2 3 2 2 2 perature, pressure and time, respectively; X , X , X are the 1 2 3 X − X i f squared effects of independent variables; X X , X X , X X %COD removal = × 100 (2) 1 2 1 3 2 3 are interaction effects; β is the constant term; β , β , β are 0 1 2 3 the linear regression coefficients for individual factors; β , 1,1 where X and X represent COD of initial and WAO-treated i f β , β are the coefficients for squared effect; β , β , β 2,2 3,3 1,2 1,3 2,3 leachate, respectively. are the regression coefficients corresponding to interaction For BOD analysis, the seed was isolated from crude oil effects. All the quadratic coefficients in the equation (Eq. 1) contaminated soil. In this process, contaminated soil suspen- were calculated by regression analysis of the experimental sion was incubated in Bushnell Haas medium containing responses. In the CCD RSM design, lower and higher values 0.5% crude oil as the sole carbon source and incubated at corresponding to each independent variable were fixed based ambient temperature for 1 week. The culture was enriched on the previously reported WAO studies (Table 1) [15]. The further by sub-culturing in medium containing a higher con- experiments were performed in the randomized order in centration of crude oil up to 1%. The final enriched culture duplicate. In order to minimize block effect on the predicted was used as the seed. response, all the experimental runs were performed within a Biodegradability index (BI) is the ratio of B OD : COD, week by the same individual [23]. The GC–MS analysis of which is also a measure of the extent to which leachate is WAO optimized leachate was carried out to determine the amenable to biodegradation. The COD and BOD of WAO degree of degradation of leachate components. treated leachate were analyzed as per the standard protocol provided by APHA, and compared with the initial leachate Analytical methods without WAO treatment. TPH was extracted from the contaminated soil by means The pH and electrical conductivity of all the samples were of soxhlet extraction (EPA method 3540 C) with dichlo- checked using a pH meter (Cyberscan Eutech 510, US) and romethane as extracting solvent. 10 g of soil was blended an EC meter (HI 8730, Hanna instrument, US). Particle with 10 g of anhydrous sodium sulphate and placed in an size distribution was analyzed by the Hydrometer method extraction thimble. After extraction, the collected extract (IS 2720 Part 4). Bulk density, porosity, maximum water was passed through anhydrous sodium sulphate column holding capacity (MWHC) were measured using K-R box and concentrated in a rotary evaporator at a temperature of method (USDA Gravimetric method). Cation exchange 35 °C. The final volume of the extract was around 2 ml. A capacity (CEC) was determined by the acetate extract tech- 1 µl of the sample was analyzed by GC MS. Organic com- nique (IS 2720 part 24). TPH in soil was gravimetrically pound from leachate were extracted by liquid–liquid extrac- analyzed by the EPA method 9071 B. Total organic carbon tion (EPA method 3510C). The aqueous leachate solution (TOC) was measured by means of Shimadzu TOC-L instru- was extracted 3 times with DCM. The organic fraction was ment equipped with an ASI-V autosampler. The color of passed through a packed sodium sulphate column to remove leachate sample was measured using the spectrophotometric moisture content and then concentrated up to 2 ml using a Table 1 Experimental factors Variables Factors Levels and levels for RSM design − α − 1 0 + 1 + α Temperature (°C) X 126 150 185 220 244 Pressure (bar) X 5 10 17.5 25 30 Time (min) X 5 10 17.5 25 30 1 3 International Journal of Industrial Chemistry (2019) 10:237–248 241 rotary evaporator. Finally, the sample was syringe filtered Table 2 Physico-chemical characterization of the crude oil-contami- nated soil through 0.22 µ filter and analyzed using GC–MS instrument. The GC–MS analysis was performed using Perkin Elmer Parameters Value Clarus 600 C Quadrupole gas chromatograph equipped with Particle size distribution mass spectrophotometer detector. A DB-5 MS capillary col- Sand % 98 umn (30 m × 0.25 mm ID × 0.25 µm) was used for the analy- Clay % 2 sis of TPH in extracted samples, while, DB 624 Ultra inert Texture class Sand (UI) capillary column (30 m × 0.32 mm ID × 1.80 µm) was pH 7.04 ± 0.15 used for the detection of acetic acid in the aqueous leachate −1 EC 206.5 ± 2.48 µs cm sample after WAO treatment. Helium was used as the carrier TPH 3.22 ± 0.18% −1 gas with a flow rate of 1 ml min . The GC injection tempera- TOC 6.64 ± 0.16% ture was set at 250 °C. For TPH analysis, the column tem- −3 Bulk density 1334 ± 14.64 kg m perature was fixed at 60 °C, for 1 min, followed by heating at Porosity 4.733 ± 0.12% −1 a rate of 6 °C min to 300 °C and hold for 20 min. For acetic MWHC 2.979 ± 0.24% acid analysis, the initial column temperature was maintained CEC 5.37 ± 0.26 meq/100 gm −1 at 110 °C and then it was increased at the rate of 8 °C min to 150 °C with 1 min hold time. Afterwards, the column was −1 heated at a rate of 8 °C min to 190 °C. The MS detector was operated in the EI mode (70 eV) and scanned from 40 to Table 3 Physico-chemical characterization of the leachate before and after wet air oxidation at optimized conditions (temperature = 244 °C, 500 amu for TPH and 40 to 200 amu for acid analysis. pressure = 5 bar, and time = 30 min) The calibration curves for C –C hydrocarbons and 8 40 Parameters Before WAO After optimized WAO PAHs were prepared using varying concentration ranges, −1 viz. 5, 10, 15, 20 and 25 ng µl and their respective peak pH 7.12 ± 0.08 6.98 ± 0.12 areas. All the standard solutions were prepared in DCM from −1 −1 COD 2000 ± 10.64 mg l 480 ± 9.63 mg l the stock solution. The concentration of compounds present −1 −1 BOD 280 ± 18.43 mg l 230 ± 11.68 mg l in untreated and WAO treated samples were determined on −1 −1 TOC 678 ± 15.83 mg l 250 ± 8.96 mg l the basis of respective calibration curves. Further, the % Color (at original pH) Greenish yellow Pale yellow degradation of compound was calculated using the formula: Color (at pH 7) Greenish yellow Pale yellow BI 0.14 ± 0.007 0.48 ± 0.02 Z − Z i f %degradation of compound = × 100, i BI, biodegradability index where Z and Z represent the initial and final concentration i f The pH of collected leachate was found to be of compounds before and after WAO, respectively. 7.12 ± 0.08. The recalcitrant nature of leachate is −1 e vident fr om high C OD = 2000 ± 10.64 mg l , −1 −1 BOD = 280 ± 18.43 mg l and TOC content ~ 678 mg l . Results and discussion The initial leachate has a biodegradability index (BI) of 0.14 which indicates its non-biodegradable nature. Characterization of crude oil‑contaminated soil On the other hand, it is clear from the characteriza- and leachate tion data (Table 3) that the COD drastically reduced to −1 480 ± 9.63 mg l after WAO treatment. The degradation The physico-chemical characteristics of crude oil-con- of organic matter present in the leachate was confirmed taminated soil and leachate are presented in Tables 2 and from the TOC data, showing an overall TOC reduction of 3, respectively. The particle size distribution showed that 63% after wet air oxidation (Table 3). Further, the BOD −1 percentage of sand was found to be around 98% in com- changes from 280 ± 18.43 to 230 ± 11.68 mg l after parison to clay (2%) on the soil texture triangle diagram. WAO and the biodegradability index (BI) was increased up The electrical conductivity of contaminated soil was found to 0.48 ± 0.02 after WAO corresponding to 71% increase −1 to be 206.5 µs cm because of the high TPH percentage. in the BI. The contaminated soil showed the pH of 7.04 ± 0.15 and The improvement in the biodegradability index (BI) after cation exchange capacity (CEC) of 5.37 ± 0.26 meq/100 g. WAO may be attributed by the degradation of recalcitrant The higher CEC indicates the presence of a large amount of compounds present in leachate and getting transformed into organic matter in soil. Further, the presence of long-chain the smaller organic compound like acetic acid, carbon diox- hydrocarbons was confirmed from the GC–MS analysis. ide, etc., making the leachate amenable to further biological 1 3 242 International Journal of Industrial Chemistry (2019) 10:237–248 treatment. The effect of WAO was also reflected in color of 70 the leachate which changed from greenish yellow to pale yellow (Table 3). Eec ff t of stirring speed Wet air oxidation involves two significant stages; first one is the mass transfer of oxygen from the gas phase to the liquid phase and second is a chemical reaction between the trans- ferred oxygen and organic compound present in the medium. Mass transfer of oxygen from gas to liquid phase plays a crucial role for the oxidation of long chain/complex organic 200 300 400 500 600 compounds into smaller compounds which further enhances Stirring Speed (RPM) the % COD removal [24]. However, for good mass transfer (a) Effect of stirring speed on % COD removal of oxygen from gas to liquid phase, a turbulence has been required in the aqueous phase which was generated using a COD before WAOCOD after WAO controlled agitation process. Further, in order to visualize the effects of agitation on % COD removal, the experiments were conducted at varying speed of agitation in the range of 200–600 rpm, and fixing the other variables as constant (temperature = 200 °C, pressure = 10 bar, time = 30 min) [24, 25]. The results depicts an increase in % COD removal with agitation speed with a maximum % COD removal of 57 ± 2% at 400 rpm as evident from Fig. 2a. Nevertheless, there is no significant effect in % COD removal by enhancing the agitation speed beyond 400 rpm (Fig. 2a). Therefore, all the optimization experiments were conducted at fixed agitation Acidic Alkaline Original speed of 400 rpm. pH (b) Effect of pH on COD of Initial and after WAO of leachate Eec ff t of pH Fig. 2 a Effect of stirring speed on % COD removal. b Effect of pH on COD of initial and after WAO of leachate The experiments were performed at three different pH, viz. acidic (3.5 ± 0.08), original (7.12 ± 0.08), and alka- line (9.5 ± 0.12) to determine the effect of pH on COD of Optimization and validation of optimized pre- and post-WAO-treated leachate. The experiments for conditions studying the effect of pH on COD were conducted at tem- perature = 200 °C, pressure = 10 bar and time = 30 min. Response surface methodology was used to evaluate the The initial COD of untreated leachate was found to be −1 −1 correlation between independent variables and their effect 2000 ± 10.64 mg l at original pH, 1940 ± 12.55 mg l at −1 on the dependent variable (% COD removal). The results acidic pH and 1960 ± 10.83 mg l at alkaline pH, respec- from individual runs were analyzed by regression in order to tively. From Fig. 2b, COD after WAO at acidic, origi- −1 optimize the significant factors influencing WAO conditions nal, and alkaline pH were found to be 535 ± 8.75 mg l , −1 −1 (Table 4). A quadratic polynomial equation was predicted 540 ± 7.68 mg l and 538 ± 9.24 mg l , respectively, which as follows: shows that no significant changes were observed in COD values. This may be due to the complex nature of leachate Y = 52.32 − 0.003X + 0.923X − 1.388X + 0.0004X 1 2 3 which was generated from crude oil contaminated soil. This 2 2 + 0.019X + 0.023X − 0.008X X + 0.003X X leachate contains solubilized and emulsified hydrocarbon 1 2 1 3 2 3 resistant to oxidation so that COD were not remarkably (3) + 0.008X X 2 3 reduced before WAO. After this experiment, the conclusion where X = temperature, X = pressure, X = time. was drawn that there is no need of pH adjustment of leachate 1 2 3 The experimental- and model-predicted values were before WAO. All the optimization experiments were per- found to be in close accordance with each other as evident formed at original pH without adjustment of pH. 1 3 COD (mg/L) COD removal (%) International Journal of Industrial Chemistry (2019) 10:237–248 243 Table 4 Central composite design (CCD) showing experimental run from Fig. 3. Table 5 represents the results of analysis of conditions and results variance (ANOVA). It was apparent from the ANOVA that the main effects of temperature and pressure ( p = 0.000); Std. order Independent vari- Experimental (% Predicted (% ables COD removal) Y COD removal) the square effects of pressure (p = 0.019), time (p = 0.007) and the interaction effects of temperature and pressure X X X 1 2 3 (p = 0.002) significantly affected the COD removal (p val- 1 150 10 10 56.30 55.47 ues < 0.05). Further, the coefficient of determination was 2 220 10 10 62.63 64.08 2 found to be R = 0.93 which reveals that 93% of the vari- 3 150 10 25 52.64 54.09 ance in COD removal has been predicted accurately by using 4 220 10 25 64.73 65.70 the independent variables. Figure 4a depicts the normal % 5 150 25 10 62.63 62.92 probability and Studentized residuals plot which indicates 6 220 25 10 63.64 63.46 satisfaction of the normality. The data point indicates the 7 150 25 25 63.48 63.31 linearity with observed and model predicted values. The Stu- 8 220 25 25 64.74 66.84 dentized residual and predicted % COD removal of leachate 9 126 17.5 17.5 55.45 55.62 plot is shown in Fig. 4b. The plot depicts the random scat- 10 244 17.5 17.5 67.80 65.83 tering of data points around the central line in the range of 11 185 17.5 5 61.66 61.84 ± 2 and resembles the data were accurate and trustworthy. 12 185 17.5 30 65.50 63.52 The plot showed no abnormality. 13 185 5 17.5 59.63 58.44 The optimized conditions from RSM were found to be 14 185 30 17.5 66.27 65.67 temperature = 244 °C, pressure = 5 bar, time = 30 min result- 15 185 17.5 17.5 59.12 59.09 ing in maximum predicted COD removal of 76% with the 16 185 17.5 17.5 59.36 59.09 desirability of 1. In order to further validate and re-check the 17 185 17.5 17.5 59.06 59.09 predicted optimum conditions and its effect on the response, 18 185 17.5 17.5 58.86 59.09 additional experimental trials were run only at the optimized 19 185 17.5 17.5 58.80 59.09 conditions and % COD removal was evaluated. The % COD 20 185 17.5 17.5 59.06 59.09 removal thus obtained after the final experimental run was found to be 75%, which is in close accordance with the X = temperature, X = pressure, X = time 1 2 3 model-predicted values under optimized conditions. Fig. 3 Experimental and predicted values for % COD removal 1 3 244 International Journal of Industrial Chemistry (2019) 10:237–248 Table 5 Analysis of variance (ANOVA) data for % COD removal Study of the interaction effects Source Degree of Adj MS F p The effect of interaction between different independent freedom variables and its impact on COD removal has been the key Regression 9 29.835 14.73 0.000 feature of the present study and was visualized by using Linear 3 64.136 31.67 0.000 contour plots. X 1 125.867 62.15 0.000 X 1 63.107 31.16 0.000 Eec ff t of temperature and pressure X 1 3.434 1.70 0.222 Square 3 12.484 6.16 0.012 From Fig. 5a, it is evident that both temperature and pressure X *X 1 4.793 2.37 0.155 1 1 significantly affect the response. % COD removal increases X *X 1 15.742 7.77 0.019 2 2 with increase in temperature and pressure. A high COD X *X 1 23.167 11.44 0.007 3 3 removal of 65–70% has been observed as the temperature Interaction 3 12.886 6.36 0.011 reaches above 220 °C and pressure increases to 27 bars with X *X 1 32.603 16.10 0.002 1 2 a hold time of 17.5 min. These experimental results were X *X 1 4.515 2.23 0.166 1 3 found to be in accordance with previous studies conducted X *X 1 1.540 0.76 0.404 2 3 by different researchers around the world, as reported by Residual error 10 2.025 – – Luck [26]. The possible explanation behind the enhanced Lack-of-fit 5 4.011 100.51 0.000 COD removal is as temperature and pressure increase inside Pure error 5 0.040 – – the reactor, hydroxyl radicals are generated in presence of Total 19 an O -rich environment, which in turn react with the hydro- carbon C–H bond and enhance the rate of decomposition R = 93%, X = temperature, X = pressure, X = time 1 2 3 [27]. Furthermore, the addition of molecular O initiates a chain reaction (initiation, propagation, and termination) as described below [27–29]: RH + O → R ⋅ +HO ⋅ (initiation) 2 2 R ⋅ +O → ROO ⋅ (propagation) ROO ⋅ +RH → ROOH + R ⋅ (propagation) ROOH → RO ⋅ +HO ⋅ (decomposition) RH + HO⋅ → R ⋅ +H O (propagation) -2 -1 21 0 2ROO⋅ → ROOR + O (termination) Studentized Residuals These reactions generate organic radicals, hydroxyl radi- (a) Studentized residuals and normal % probability plot cals and free radicals which may be the reason for higher % COD removal. Generally, the WAO reaction involves the breakdown of complex organic molecules into intermedi- ates with lower carbon atoms under high temperature and pressure. These molecular breakdown reactions leads to the formation of carboxylic acids, viz. acetic acid or formic acid which are eventually converted into CO and H O as the 2 2 final product [30]. -1 Eec ff t of time and pressure -2 54 57 60 63 66 Figure 5b presents the effects of pressure and time on % % COD Removal COD removal. From the figure, it is significant that % COD (b) % COD removaland Studentized residuals plot removal increased with increase in pressure and time which is also evident from the ANOVA results (Table 5) which Fig. 4 a Studentized residuals and normal % probability plot. b % reveals that both the individual effect (p = 0.000) and square COD removal and Studentized residuals plot 1 3 Normal % Probability Studentized Residual International Journal of Industrial Chemistry (2019) 10:237–248 245 Fig. 5 a Contour plot showing the effect of temperature and pressure perature (185 °C). c Contour plot showing the effect of temperature on % COD removal at a given time (17.5 min). b Contour plot show- and time on % COD removal at a given pressure (17.5 bar) ing the effect of time and pressure on % COD removal at a given tem- effects of pressure (p = 0.019) and the square effect of time Eec ff t of temperature and time (p = 0.007) are significant. Further, at lower pressure (5 bar, time = 30 min) % COD removal was found to be in the range Figure 5c represents the interaction effect of temperature of 60–62%, which is lower than the % COD removal at high and time at a given pressure. A COD removal of 72% pressure (30 bar, time = 30 min) which was 67–70%. The was obtained at a temperature of 240–245 °C and time plausible reason behind this may be explained on the basis of of 30 min with pressure hold value of 17.5 bars. It is evi- the fact that the organic compounds are degraded to recalci- dent from Fig. 5c that no significant effect on % COD trant organic compounds, i.e., low-molecular weight carbox- removal was observed if only reaction time is raised at any ylic acid with time which are resistant to further oxidation given temperature. However, the vice versa is not neces- [30]. sary the same. This is because when the temperature was 1 3 246 International Journal of Industrial Chemistry (2019) 10:237–248 Fig. 6 GC–MS chromatogram of a initial leachate, b WAO-treated leachate and c WAO-treated leachate showing peak of acetic acid 1 3 International Journal of Industrial Chemistry (2019) 10:237–248 247 Table 6 % degradation of long- Peak Compound Chemical formula Molecular weight % degradation chain hydrocarbon compounds −1 (g mol ) A Hexadecane C H 226 96 ± 1.24 16 34 B Heptadecane C H 238 93 ± 1.68 17 36 C Hentriacontane C H 436 68 ± 1.88 31 64 D Tritriacontane C H 464 66 ± 2.09 33 68 E Pentatriacontane C H 492 55 ± 1.98 35 72 increased and time was kept constant (for e.g., 10 min), from crude oil-contaminated soil for the first time. The opti- the maximum % COD removal was increased to 64–68% mization of WAO process was found to result in removal of (refer to Fig. 5c). This finding was further supported by 75% COD and 3.4-fold increase in biodegradability index the ANOVA results (Table 5), which also shows that the (BOD5/COD) of the crude oil leachate. The degradation of temperature has been a significant factor (p = 0.000). crude oil leachate components was corroborated by GC–MS However, most of the results of the present study could analysis. Thus, we propose WAO to be an effective pretreat- not be compared with the available literature, because there ment step for crude oil leachate, making it amenable to sub- are no data available for the treatment of leachate contain- sequent biological treatment. ing solubilized and emulsified hydrocarbons generated from Acknowledgements The authors gratefully acknowledge the financial crude oil-contaminated soil sites which further adds to the support from the Department of Biotechnology, Government of India novelty of this work. (No. BT/PR12687/BCE/08/1122/2015). The authors are thankful to the director, CSIR-NEERI, Nagpur, for his invaluable help in extend- GC–MS analysis ing all facilities at the institute. The authors would like to acknowl- edge the knowledge resource center (KRC), CSIR-NEERI, Nagpur, for plagiarism check for which the Accession Number is CSIR-NEERI/ The GC–MS chromatogram of initial and WAO-treated lea- KRC/2018/JAN/CTMD-EBGD-EISD-AID/1. chate is shown in Fig. 6a, b. The analysis of initial leachate showed the presence of long-chain aliphatic hydrocarbons, Compliance with ethical standards viz. hexadecane (C H ), heptadecane (C H ), hentriacon- 16 34 17 36 tane (C H ), tritriacontane (C H ), and pentatriacontane 31 64 33 68 Conflict of interest The authors declare that they have no conflict of (C H ). The leachate was treated by WAO under the oper- interest. 35 72 ating conditions optimized by RSM. The intensity of peaks Open Access This article is distributed under the terms of the Creative in WAO-treated leachate chromatogram was lower than the Commons Attribution 4.0 International License (http://creativecom- initial leachate indicating degradation of compounds. Few mons.org/licenses/by/4.0/), which permits unrestricted use, distribu- residual peaks in the treated leachate may correspond to rela- tion, and reproduction in any medium, provided you give appropriate tively recalcitrant compounds. The percentage degradation credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. of the above-mentioned compounds is shown in Table 6. The GC–MS analysis confirmed the presence of carbox- ylic acid like acetic acid in the WAO-treated leachate which is shown in Fig. 6c. It has been widely reported that, acetic References acid is recalcitrant to further oxidation [30] and contributes 1. 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International Journal of Industrial Chemistry – Springer Journals
Published: Jun 12, 2019
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