A large amount of wastewater is produced during oilfield exploitation. The effluents are mainly composed of phenols, alkanes, aromatic hydrocarbons, alcohols, aldehydes, ketones, esters, acids and nitrogen-containing compounds. Generally, phenols account for a large proportion of the total organic contaminants in the oilfield wastewater. The effective treatment of oilfield wastewater plays an important role in oilfield exploration and rational recovery of petroleum. Nowadays biochar-based adsorption technology attracts much attention in the field of wastewater treatment considering its relatively low-cost treatment and the recovery of VOCs in contrast to chemical and biological methods      . By adsorbing organic matter and large particles from wastewater, biochar-based adsorbents can achieve preferable removal of aqueous contaminants     .
Since synthesis of activated carbons is a costly process  , in recent years much attention      has centered on hydrochar which is produced by hydrothermal carbonization (HTC). HTC is a thermochemical conversion process in which biomass is converted to a high carbonaceous solid product after heating for 5 to 240 min in an aqueous closed system at a temperature of 180˚C - 250˚C and autogenous pressure of 2 - 6 MPa  -  . HTC is considered as a cost-effective approach to produce hydrochar due to its ability to transform wet biomass into hydrochar without predrying  as well as mild conditions   .
As a waste agricultural biomass, corncob is routinely used for the production of paper pulp, livestock feed and biochemicals. However, most of corncob is burned as agricultural fuel, causing tremendous waste of resources and serious environmental pollution  . Hydrochars derived from corncob are prepared through hydrothermal carbonization. Phenol, which is a dominant component of organic contaminants in oilfield wastewater, is selected as the representative pollutants to evaluate the adsorbents. Specific objectives of this work are to 1) determine the feasibility of the corncob hydrochar as a phenol adsorbent; and 2) elucidate the adsorption thermodynamics and kinetics of phenol onto the corncob hydrochar.
2. Materials and Methods
2.1. Instruments and Materials
Phenol, 4-aminoantipyrine, potassium ferricyanide, hydrochloric acid, and sodium hydroxide are analytical reagents and purchased from Aladdin Industrial Corporation (Shanghai, China).
UV-visible spectrophotometer (722N), electric blast drying oven (101-1EBS), electronic balance (TP-213), high-speed Chinese medicine pulverizer (HK-08B), acidity meter (PB-10) specific surface area and porosity Analyzer (Quantasorb SI).
2.2.1. Preparation of Hydrochar
The pristine corncob is washed with distilled water and then dried in an oven at 105˚C for 5 h. After the sample is slowly cooled to room temperature in air, it is crushed and sieved to 80 mesh. About 25 g of dried corncob granules are weighed and placed into a stainless-steel reaction vessel with a volume of 250 mL. The influence of various factors such as reaction temperature, residence time e and water amount on the adsorption capacity of the prepared hydrochar is investigated by orthogonal experiment. The optimum adsorption conditions are determined as follows: 100 ml of distilled water, 25 g of the raw corncob and reacting at 230˚C for 8 h. After the hydrothermal carbonization, the solid product is taken out, washed repeatedly with distilled water until the filtrate is nearly colorless. Afterwards, the resulting slurry is dried in an oven at the temperature of 105˚C for 5 h. The dried samples are ground and sieved to 80 mesh to obtain the corncob hydrochar granules.
2.2.2. Characterization of Hydrochar
The surface functional groups of the corncob hydrochar are identified by the Thermo Nicolet 6700 Fourier Transform Infrared Spectrometer (FTIR). The surface structures of the hydrochar samples are determined by a Gemini SEM 300 scanning electron microscope (SEM).
The element analyzer (Vario EL III, Elementar) is used to determine the contents of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) in samples. The internal crystal structure of the corncob hydrochar is analyzed by Analytical X-Ray Diffractometer (XRD).
2.2.3. Adsorption Experiment
0.05 g - 0.4 g of the corncob hydrochar granules are mixed with 50 mL of phenol solutions. The pH of the solution is adjusted to the desired value by adding concentrated HCl and/or NaOH. After stirring at set temperatures for 1 h, the mixture is filtered through a 0.45 μm filter, and then the concentration of phenol in the filtrate is measured by an ultraviolet-visible spectrophotometer (722 N). The maximum absorption wavelength of phenol is determined by UV-visible spectrophotometer (722N) as 520 nm by means of full wavelength scanning. A linear curve of standard phenol concentration vs. absorbance is constructed by employing a series of phenol solutions (0.25, 0.5, 1, 2, 4, 8 and 10 mg∙L−1). The standard curve is given by: y = 0.1022x − 0.0061, R2 = 0.9996, where y is the absorbance and x is the phenol concentration.
3. Results and Discussion
Elemental analysis of the pristine corncob and hydrochar is performed on a Vario EL III Elemental Analyzer, and the results are shown in Table 1. Compared to the raw corncob, the hydrogen, nitrogen and oxygen contents of the corncob hydrochar is reduced by 0.11%, 0.08% and 12.4%, respectively. This indicates the occurrence of decarboxylation, and dehydration during pyrolysis and hydrothermal carbonization of biomass  .
Table 1. The elemental analysis of samples.
Note: ad denotes air-dried basis, d means by difference.
The carbon contents of the corncob hydrochar are increased by 12.64% in comparison with those of the pristine corncob. Notably, the carbon contents of the corncob hydrochar (58.95%) are lower than those of some hydrochars from other raw materials such as wood and grass reported in literatures  , but higher than those of some hydrochars from rice straw  .
The corncob hydrochars have BET specific surface areas of 15.7858 m2·g−1 and total pore volumes of 0.0618 cm3·g−1. Especially, the BET surface areas of the corncob hydrochars are higher than those of some reported hydrochars produced from other raw materials such as prosopis africana shell and urban food waste   .
The FTIR spectra of the corncob hydrochar presented in Figure 1 are conducted to determine the functional groups on sample surface. The broad and strong band in the range of 3000 - 3600 cm−1 is assigned to the –OH stretching vibration in hydroxyl or carboxyl groups    . Additionally, the band at 1700 cm−1 in the FTIR spectra of the prepared hydrochars is denoted as the stretching vibration of –C = O in carboxyl, carbonyl, quinonyl or ester groups  . Its relative intensity in the hydrochars is more intense than the raw corncob, indicating the generation of new oxygen functional groups on the surface of the corncob hydrochar. The peaks at 1450, 1500 and 1600 cm−1 are assigned to the –C = C vibration in aromatic ring carbon  . Especially, the relative intensity of the peak at 1500 cm−1 is more intense in the hydrochars than the raw corncob, indicating the occurrence of aromatization process.
Figure 1. The FTIR spectra of the raw corncob and the representative hydrochar.
The peak at 1400 cm−1 is associated with the in-plane bending of carbonyl (–COH)  . The peak at 1062 cm−1 is related to the –C–O–R stretching in ester groups and –C–O stretching in hydroxyl groups  . The relative intensity of the above two peaks in the FTIR spectra of the prepared hydrochars is weaker than that of the raw corncob, implying the occurrence of dehydration during HTC  .
The SEM images in Figure 2 are used to assess the morphological structure of the raw corncob and the prepared hydrochars. It can be seen from Figure 2(a) that the raw corncob shows clustered aggregates and few pore structures on its surface, and its morphology is dramatically changed by HTC. The sponge-like structure and the enhanced porosity are clearly developed in the representative hydrochars (Figure 2(b)), in accordance with other studies   . The prepared hydrochars exhibits small fragments and pore structures with different diameters, possibly owing to the disintegration of the corncob matrix. The enhanced porosity of the corncob hydrochar is expected to effectively improve absorption of phenols when the hydrochar is applied for the treatment of oilfield wastewater.
Figure 2. The SEM images of samples. (A) Corncob biomass; (B) Corncob hydrochar.
3.2. Influence Factors of Adsorption of Phenol Bycorncob Hydrochar
3.2.1. Effect of Adsorbent Dosage
The effect of the prepared hydrochar dosage on the adsorption of phenols from aqueous solution is studied at 25˚C by using 0.05 - 0.4 g adsorbent dosage, 50 ml of phenol solution with a concentration of 50 mg/L for 60 min of agitation time at pH of 6. It is observed in Figure 3 that as the dose of the adsorbents increases from 0.05 to 0.15 g, the phenol removal rate increases from 50.29% to 77.73% before equilibrium is attained. The number of sites available for the adsorption of phenols is directly proportional to the amount of adsorbent used in the measurement. Accordingly, increasing the amount of adsorbent leads to an increase in the number of active sites and therefore to an approximately linear increase in the number of phenols adsorbed by the prepared hydrochar. However, no significant change is observed after a dosage of 0.15 g, suggesting that at this point phenols and adsorption sites have reached equilibrium. In other words, there is a threshold dosage, for which no significant increase will be observed in the uptake of phenols by the corncob hydrochar. As a result, 0.15 g of the prepared hydrochar is sufficient for the optimum removal of phenols.
Figure 3. The effect of adsorbent dosage on phenol removal rate.
3.2.2. Effect of Initial Phenol Concentration
The effect of initial phenol concentration is shown in Figure 4. The experiment is performed for 60 min at 25˚C by using 0.15 g adsorbent dosage, 50 ml of phenol solution with concentrations of 10 - 100 mg/L for 60 min of agitation time at pH of 6. There is a gradual increase in the adsorption capacity of the corncob hydrochar as the phenol concentration in the solution increases from 10 to 50 mg/L. At low phenol concentration (<50 mg/L), there is a high availability of adsorption sites, therefore almost all phenols bind to the prepared hydrochar. This stage is the rapid adsorption of phenols by the prepared hydrocarbon (Figure 4). However, as the phenol concentration exceeds 50 mg/L, the growth of the adsorption capacity for the corncob hydrochar slows down due to the saturation of the available adsorption sites. The interaction between phenol and the corncob hydrochar enters the slow absorption period (Figure 4).
Figure 4. The effect of initial concentration of solution on adsorption capacity.
3.2.3. Effect of Contact Time
The experiment is performed at 25˚C by using 0.15 g adsorbent dosage, 50 ml of phenol solution with a concentration of 50 mg/L for 60 min of agitation time at pH of 6, while the contact time is varied between 1 and 120 min in order to establish equilibrium. The effect of contact time on the adsorption of phenols from aqueous solution by the corncob hydrochar is shown in Figure 5. The adsorption of phenols increases rapidly within the first 30 min, followed by a gradual increase up to 60 min and remains almost constant thereafter for the prepared hydrochar. Most of the phenol removal occurrs within 30 min with an increase of phenol removal rate from 17.35% to 66.11% and an augmentation of the adsorption capacity of the corncob hydrochar from 2.89 mg∙g−1 to 11.02 mg∙g−1. The increase in adsorption at the initial stage is due to a large number of vacant adsorption sites, which becomes saturated with time and as a result, adsorption is less efficient after 30 min.
Figure 5. The effect of contact time on phenol removal rate and adsorption capacity.
3.2.4. Effect of pH
The solution pH is an important monitoring parameter governing an adsorption process. The effect of pH on the phenol removal rate and adsorption capacity is studied at 25˚C by using 0.15 g adsorbent dosage, 50 ml of phenol solution with a concentration of 50 mg/L for 60 min of agitation time at the pH range of 1 - 12. The results of pH study are demonstrated in Figure 6. Figure 6 shows that the adsorption of phenol increases with the increasing of pH. The maximum phenol removal rate (83.25%) and the highest adsorption capacity (13.87 mg/g) are at pH 6.0 for the prepared hydrochar. After that point the phenol removal rate as well as the adsorption capacity is found to decrease gradually. The equilibrium pH 6.0 is considered as optimum pH in case of the removal of phenols for further studies. At low pH, phenols are likely to be found as free molecules in equilibrium with protonated species in water  , while at a pH above 6.0 they tend to dissociate in water to give the phenolate anion and H+ due to the presence of increasing OH− groups. Two mechanisms can be used to describe adsorption onto the corncob hydrochar: the first one is coordination of the adsorbate with the functional groups on the surface of the prepared hydrochar which is affected by the pH value of the solution, while the second one is physical adsorption relating to the surface area and porosity of the hydrochar. The coordination route should play an important role in this study as the prepared hydrochar has small surface area with low porosity. In the low pH range (pH < 4), the decreasing pH dramatically reduces the adsorption capacity, due to the protonation of phenols in acidic medium  and the presence of excess H+ ions that compete with the cationic phenol molecules for adsorption sites. In the high pH range (pH > 8), an increasing competition exists between the phenolate anion derived from the dissociation of phenols and the OH− in solution with the increase of pH. Therefore, the experiments clearly show that the adsorption of phenols by the corncob hydrochar is strongly pH dependent, the prepared hydrochar exhibits a greater potential for the phenol adsorption at the pH 6.0 values tested.
Figure 6. The effect of pH on phenol removal rate and adsorption capacity.
3.3. Adsorption Kinetics
The adsorption kinetic characteristics are important for providing valuable insights into the adsorption mechanisms  . The experimental kinetic data of adsorption of phenols onto the corncob hydrochar are examined by using the pseudo-first-order and pseudo-second-order equations  .
The pseudo-first-order rate equation is expressed by
The pseudo-second-order rate equation is expressed by
where qe and qt are the amount of phenol adsorbed per unit mass of the adsorbent (mg/g) at equilibrium and time t respectively. k1 (min−1), and k2 (g∙mg−1∙min−1) are the pseudo-first-order, and pseudo-second-order rate constant respectively. The model constants are calculated by using origin 9.1 for pseudo-first-order and pseudo-second-order, respectively. The results of the adsorption kinetics are presented in Table 2 and Figure 7. The correlation coefficient (R2) values of the pseudo-second-order are higher than that of the pseudo-first-order kinetics model; it indicates that the experimental data fit well into the pseudo-second-order model, suggesting that it can be used for the entire adsorption process  . The kinetic process that is fitted by the pseudo second-order kinetic model has chemisorption as the rate-determining step and involves the chemical bonding between phenols and polar functional groups on the prepared hydrochar  .
Figure 7. The kinetics of phenol adsorption by hydrochar. (A) Pseudo-first-order kinetics fitting; (B) Pseudo-second-order kinetics fitting.
Table 2. The parameters of adsorption kinetics.
A detailed understanding of adsorption mechanisms facilitates a determination of the rate-limiting step. This information can then be used to optimize the design of adsorbents and adsorption conditions. The overall rate of adsorption can be described by the following three steps  : 1) film or surface diffusion where the sorbate is transported from the bulk solution to the external surface of sorbent, 2) intraparticle or pore diffusion, where sorbate molecules move into the interior of sorbent particles, and 3) adsorption on the interior sites of the sorbent. Since the adsorption step is very rapid, it is assumed that it does not influence the overall kinetics. The overall rate of adsorption process, therefore, will be controlled by either surface diffusion or intraparticle diffusion.
3.4. Adsorption Isotherms
The sorption data of phenols are correlated with Langmuir and Freundlich models (Equation (3) and Equation (4)).
where Ce is the equilibrium concentration of phenol in the water (mg/L), qe is the amount of phenol adsorbed per unit mass of the adsorbent at equilibrium state (mg/g), qmax is the maximum adsorption capacity (mg∙g−1). KL and KF are the coefficient of the adsorption rate (L·mg−1·min−1), n is the intensity of the adsorption constant. Figure 8 shows the fitting curves of the Langmuir and Freundlich isotherms. A summary of the theoretical parameters and the corresponding R2 values are shown in Table 3. The R2 values for the adsorption of phenol onto the prepared hydrochar are higher for the Freundlich model, implying that the adsorption data fit the Freundlich model better than the Langmuir model. Normally, in consideration of the heterogeneity of the hydrochars in physical-chemical properties (e.g., surface function groups and elemental compositions), it is reasonable to expect Freundlich model to give a better description for the isotherm curve. Constant n in Freundlich model gives a measure of favorability of adsorption: favorable (1 < n < 10), irreversible (n = 1), unfavorable (n < 1)  . In the current investigation, the values of n suggest that the phenol uptake by the prepared hydrochar can be considered to be a favorable adsorption process. Additionally, an increase in the value of n with the increasing of temperature reflects stronger interaction between phenol and the corncob hydrochar at higher temperatures. For the Freundlich constant, KF, which is related to the adsorption capacity, increases with temperature, indicating that the adsorption process is endothermic.
Table 3. The Langmuir and Freundlich isotherm parameters for the adsorption of phenol onto the corncob hydrochar.
Figure 8. The isothermal adsorption of phenol on the corncob hydrochar. (A) Langmuir model; (B) Freundlich model.
3.5. Thermodynamic Studies
When the thermodynamic parameters are properly assessed, they can provide in-depth information regarding the inherent energy and structural changes after adsorption. Thermodynamically, in an isolated system, energy cannot be gained or lost; the entropy change is the driving force. In the practice of environmental engineering, both energy and entropy factors should be considered for determining the processes that occur spontaneously. The thermodynamic parameters, the Gibbs free energy change of adsorption ΔG˚ (kJ∙mol−1), enthalpy (ΔH˚, kJ∙mol−1), and entropy (ΔS˚, J∙mol−1∙K−1) for the adsorption of phenol onto the prepared hydrochar are calculated by using the following equations:
where R is the universal gas constant (8.314 J∙mol−1∙K−1), T is the absolute temperature and KF is the Freundlich constant. The enthalpy (ΔH˚) and entropy (ΔS˚) values are estimated from the following equations:
The values of ΔG˚ are calculated from Equation (5). Reciprocal of temperature T is plotted against ΔG˚, which is found to be a straight line (Figure 9). The values of ΔH˚ and ΔS˚ are evaluated from the slope and intercept of the line in Figure 9. The Gibbs free energy (ΔG˚), enthalpy (ΔH˚) and entropy (ΔS˚) values for the adsorption of phenol onto the corncob hydrochar are presented in Table 4. The negative values of ΔG˚ denote that the adsorption of phenol onto the prepared hydrochar is spontaneous and thus thermodynamically favorable. The magnitude of ΔG˚ also increases with the increase of the temperature, which indicates that the adsorption is more favorable at higher temperatures. The value of ΔH˚ is positive, indicating the endothermic nature of the adsorption of phenol onto the corncob hydrochar at the temperature range of 298 - 318 K. One possible explanation of endothermicity of the enthalpy of adsorption is due to the reason that hydrogen bonds are formed between phenol and water molecules  . Phenols would not be adsorbed by hydrochars until the hydrogen bonds are destroyed. This rupture of hydrogen bonds between phenol and water molecules obviously requires energy. It is reasonably assumed that this energy of hydrogen-bond breaking exceeds the exothermicity of the phenol attaching to the surface of the corncob hydrochar. The positive values of ΔS˚ indicate the increase in randomness at the interface of the solid/liquid during the adsorption process, and shows the good affinity of the adsorbents for the adsorbed species. It should be noted that a chemisorbed layer could be followed by the physically adsorbed layer which may not be excluded in this study.
Figure 9. The plot of ΔGo versus T.
Table 4. The thermodynamic parameters for phenol adsorption onto the corncob hydrochar.
In this study, experiments are performed to evaluate the use of hydrochars prepared from the hydrothermal carbonization of corncob as an adsorbent for phenols in aqueous solution. It is found that the prepared hydrochars are rich in oxygen containing functional groups. The phenol removal rate and/or adsorption capacity are highly dependent on the pH of the aqueous solution, the amount of adsorbent used, initial phenol concentration as well as the contact time. Thermodynamic analysis shows that the adsorption process is endothermic and spontaneous in nature. The kinetics of the adsorption processes follow a pseudo-second-order rate law with the adsorption data being described by the Freundlich isotherm model. The results suggest that low cost corncob hydrochars obtained from an abundant natural resource may be used as effective adsorbents for the removal of phenols from oilfield wastewater.
We gratefully acknowledge financial support by the National Natural Science Foundation of China (No. 41472095), the Open Fund of Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. K2018-05) and the Foundation of Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology (Grant No. DMSM2018041).