Abstract

Preparation and characterization of activated carbons (ACs) from oily sludge by physical and chemical activation using steam, ZnCl2 and FeCl3 were investigated. The characteristics of produced adsorbents were determined by iodine number, Brunauer–Emmett–Teller (BET) equation, Fourier transform infrared spectrometry and scanning electron microscopy analyses. Batch adsorption experiments for phenol and phosphate were performed to evaluate the efficiency of adsorbents. The optimum porous structure of adsorbents with a BET surface area of 1,259 m2 g−1, total pore volume of 1.22 cm3 g−1 and iodine number of 994 mg g−1 was achieved by ZnCl2 activation at 500 °C and impregnation ratio of 1:1. The adsorption data were well fitted to the pseudo-second-order kinetic model (R2>0.99) and Freundlich isotherm (R2>0.99). The maximum adsorption capacity of phenol (238 mg g−1) and phosphate (102 mg g−1) based on the Langmuir model was achieved at pH of 6.0 and adsorbent dose of 1 g L−1. Thermodynamic parameters were negative and showed that adsorption of phenol and phosphate onto the AC was feasible, spontaneous and exothermic. The results suggested that prepared AC was an effective adsorbent for removal of phenol and phosphate ions from the polluted water.

INTRODUCTION

Among all pollutants that are likely to be present in the industrial wastewater, phenol is considered as one of the most hazardous compounds for the environment due to high toxicity and low biodegradability (Gundogdu et al. 2012). Also, the increase of phosphate in the aquatic ecosystems leads to eutrophication in water bodies (Yang et al. 2018). Therefore it is necessary to explore an efficient technology to eliminate phenol and phosphate from water. There are many advanced technologies (e.g. electrochemical oxidation, photo-oxidation, ozonation and membrane processes) and conventional methods (e.g. liquid–liquid extraction, biodegradation, adsorption, solid-phase extraction) for phenolic wastewater treatment (Villegas et al. 2016). Also, there are several methods for phosphate removal from wastewater including physico-chemical processes (e.g. precipitation, sorption, ion exchange mechanisms) and biological treatments (Rybicki 1998). However, several studies pointed out that adsorption is a more economical and attractive method for phenol and phosphate removal from aqueous solutions (Gundogdu et al. 2012; Yang et al. 2018).

Activated carbon (AC) could be used as a sorbent of pollutants such as phosphate due to its high porosity, large specific surface area and presence of tunable surface functional groups (Wang et al. 2012). The use of suitable wastes as raw materials can reduce the AC production costs. Oil sludge, as an industrial waste with a high carbon content, has the potential to become porous carbon through different synthesis approaches (Mohammadi & Mirghaffari 2015). These wastes are landfilled or incinerated, which can cause various environmental problems (Hu et al. 2013). Hence, conversion of these materials into porous carbons could be an environmentally and economically promising approach (Mohammadi & Mirghaffari 2015). ACs are made by carbonization of various materials in an inert atmosphere and either chemical, physical (thermal) or physicochemical activation methods (Patil & Kulkarni 2012). The carbonization step enriches carbon content and creates initial porosity, while the activation process develops the pore structure and increases the size of the pores (Ma et al. 2017).

Considering the environmental problems of oily sludges and the importance of their management, particularly reuse of these materials, this study was carried out to evaluate the possibility of preparation of high surface area ACs from oil wastes using different activation methods. The second objective was to use the produced materials for treatment of phenol and phosphate polluted effluents through the adsorption process. Although the production of AC from oil sludge was previously reported by Mohammadi & Mirghaffari (2015), the surface area was not as high as the commercial sorbent. Therefore, this study focused on improving the characteristics of the produced ACs through chemical activation by zinc chloride (ZnCl2) and ferric chloride (FeCl3) and physical activation by steam. During the activation process, ZnCl2 and FeCl3 promote the decomposition of carbonaceous material due to their acid characteristics. Also, they restrict the formation of tar and prevent the clogging of pores and consequently increase the specific surface area (Özhan et al. 2014). ZnCl2 is one of the most widely used activating agents in chemical activation processes (Altintig & Kirkil 2016; Bouguettoucha et al. 2016), whereas the literature contains only a few studies about the synthesis of AC with FeCl3 (Oliveira et al. 2009; Theydan & Ahmed 2012; Mohedano et al. 2014; Bedia et al. 2017). In this regard, ZnCl2 and FeCl3 were chosen to analyze their effect on the structure and textural features of obtained adsorbents under different synthesis conditions including activation temperature and impregnation ratio. Furthermore, equilibrium, kinetic and thermodynamic data of the phenol and phosphate adsorption on the synthesized ACs were investigated.

EXPERIMENTAL SECTION

Materials

All the chemicals used were of analytical grade from Merck Company (Germany). The oil refinery sludge (ORS) was collected from Isfahan Oil Refinery (Iran).

Carbonization of raw material

The ORS was pyrolized under inert atmosphere (N2, 150 cm3 STP·min−1) at a heating rate of 10 °C min−1 up to 500 °C in a tubular furnace for 1 h, yielding ORS char.

Synthesis of ACs by physical (steam) activation

In the physical activation steps, a certain weight of prepared ORS char was placed in a vertical tube furnace under an inert atmosphere (N2 gas) and heated up to the selected activation temperature (700–900 °C) at heating rate of 10 °C min−1. After that the steam was passed through for 1 h. At the end of the activation period, the sample was cooled under nitrogen atmosphere.

Synthesis of ACs by chemical activation with ZnCl2 and FeCl3

In the case of chemical activation, two series of ACs were produced with ZnCl2 and FeCl3 in a horizontal tubular furnace. A specified weight of ORS char was physically mixed with corresponding weight of ZnCl2 and FeCl3 at different impregnation ratios of 1:1, 2:1 and 3:1 (mass ratio of activating agent to carbonized sample) in a ceramic crucible. The mixtures were heated up to different activation temperatures (500, 600 and 700 °C) at a heating rate of 10 °C min−1 and maintained at this temperature for 1 h under nitrogen flow (150 cm3 STP·min−1). After that, they were cooled down under the same nitrogen flow. In order to remove impurities and the remains of activating agents, the samples were washed with hot deionized water (80 °C) for 1 h in a magnetic stirrer. Then the samples were washed and filtered several times with cold double-distilled water until the pH of solution reached around 7. Finally, the samples were dried in an oven for 24 h at 60 °C.

Characterization of the ACs

Adsorption characteristics of all samples were determined by N2 adsorption–desorption at −196 °C (Micromeritics TriStar II 3020). The specific surface area was calculated using the Brunauer, Emmett and Teller (BET) equation based upon the N2 adsorption isotherm. The micropore volume (Vmic) and the micropore surface area (Smic) were measured by t-method. The total pore volume (Vt) was estimated from the amount of nitrogen adsorbed at a relative pressure of 0.99. Surface physical morphology of selected ACs was obtained by scanning electron microscopy (SEM, Tescan, Vega III). Fourier transform infrared (FTIR) spectroscopy (Tensor 27, Brucker) was done to identify the surface functional groups of the ACs in the wave number range of 4,000 to 400 cm−1.

Phenol and phosphate adsorption

Adsorption experiments

Initial adsorption experiments were performed using 1.0 g per litre of each prepared AC added to a set of flasks containing 50 mL of phenol solution (100 mg L−1) at pH = 6–7. The same procedure was done for phosphate adsorption experiments in 50 mg L−1 phosphate solution. The suspension was stirred at 25 °C in a shaking water bath with constant speed of 200 revolutions per minute (rpm). At the end of the reaction time (4 h), the content of each flask was filtered through 0.45 µm syringe filters and the concentrations of sorbate in solutions were quantified using an ultraviolet–visible spectrophotometer (Varian, Model Cary 1E). The λmax was 270 nm and 840 nm for phenol and phosphate, respectively. The amount of phosphate was determined using molybdenum blue phosphorus method. After these initial tests, the ACs of each series with the highest phenol and phosphate adsorption capacities were selected for further batch adsorption experiments including the adsorption isotherms, kinetic and thermodynamic studies.

The adsorption capacity (qe) was calculated using the following formula: 
formula
(1)
where qe is the adsorption capacity of phenol or phosphate (mg of phenol or phosphate per g of AC), C0 and Ce are the initial and final phenol or phosphate concentrations in the solutions (mg L−1), respectively, m is the mass of AC (g) and V is the volume of phenol or phosphate solution (L).
Pseudo first order (Equation (2)) and pseudo second order models (Equation (3)) were used to investigate adsorption kinetics. 
formula
(2)
 
formula
(3)
where and (mg g−1) are the amount of phenol and phosphate adsorbed at time t and at equilibrium, respectively, (min−1) is the rate constant for pseudo first order adsorption and (g·mg−1·min−1) is the rate constant for the pseudo second order adsorption.
Freundlich (Equation (4)) and Langmuir (Equation (5)) models were used to describe the adsorption equilibrium: 
formula
(4)
 
formula
(5)
where KF is the Freundlich constant associated with the adsorption capacity of the activated carbon, Ce is the equilibrium concentration of solute in bulk solution (mg L−1) and n is the constant related to the sorption intensity, indicative of the effect of the adsorbate concentration. Values of n higher than 1 indicate favorable adsorption conditions. KL is the equilibrium constant (L mg−1), related to the enthalpy of adsorption, and qm is the equilibrium concentration of the adsorbate (mg g−1) on the solid phase corresponding to a complete coverage of the adsorbent surface (adsorption capacity of the monolayer).
Thermodynamic parameters such as free energy change (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were estimated according to the following equations (Wang et al. 2012): 
formula
(6)
 
formula
(7)
 
formula
(8)
where R is the gas constant (8.314 J mol−1 K−1), T is the temperature (K), Kd is the distribution coefficient, qe and Ce are the adsorption capacity (mg g−1) and equilibrium concentration of phenol or phosphate in the solution (mg L−1), respectively.

RESULTS AND DISCUSSION

Characterization of adsorbents

Table 1 summarizes the percentage yields, iodine number and the characteristic parameters of the porous texture of ACs synthesized with steam (CPh), ZnCl2 and FeCl3 activation (CZ and CF respectively).

Table 1

Porous texture parameters, iodine number and percentage yield of ACs obtained by physical and chemical activation

Sample SBET (m2 g−1Smic (m2 g−1Sext (m2 g−1Vt (cm3 g−1Vmic (cm3 g−1Iodine number (mg g−1Yield (wt%) 
ORS char 14 – 14 0.04 – 8.4 13.1 
CPhT700 54 12 42 0.07 0.003 50.11 11.56 
CPhT800 150 117 32 0.123 0.05 127 9.17 
CPhT900 422 234 188 0.5 0.1 321.19 8.24 
CZ1T500 1,259 773 486 1.22 0.365 994.3 36.9 
CZ1T600 876 394 482 1.080 0.178 728.5 36.6 
CZ1T700 550 327 222 0.564 0.153 498.5 33.1 
CZ2T500 1,015 511 504 1.078 0.234 820.8 36.4 
CZ2T600 757 276 481 1.004 0.120 592.6 34.7 
CZ2T700 494 214 280 0.631 0.096 421.6 32.1 
CZ3T500 925 584 341 0.891 0.275 761.1 35.4 
CZ3T600 714 434 280 0.754 0.203 628.2 31.5 
CZ3T700 478 299 179 0.433 0.143 404.7 28.2 
CF1T500 161 112 49 0.159 0.052 197.6 43.7 
CF1T600 215 129 85 0.232 0.060 301.7 40.6 
CF1T700 428 249 178 0.456 0.121 443.5 35.2 
CF2T500 344 267 76 0.269 0.126 391.0 40.4 
CF2T600 477 374 103 0.371 0.176 428.6 38.7 
CF2T700 683 428 254 0.684 0.201 608.8 34.0 
CF3T500 183 129 53 0.252 0.060 259.9 38.7 
CF3T600 422 240 118 0.420 0.112 395.5 35.3 
CF3T700 584 326 258 0.657 0.151 512.8 32.8 
Sample SBET (m2 g−1Smic (m2 g−1Sext (m2 g−1Vt (cm3 g−1Vmic (cm3 g−1Iodine number (mg g−1Yield (wt%) 
ORS char 14 – 14 0.04 – 8.4 13.1 
CPhT700 54 12 42 0.07 0.003 50.11 11.56 
CPhT800 150 117 32 0.123 0.05 127 9.17 
CPhT900 422 234 188 0.5 0.1 321.19 8.24 
CZ1T500 1,259 773 486 1.22 0.365 994.3 36.9 
CZ1T600 876 394 482 1.080 0.178 728.5 36.6 
CZ1T700 550 327 222 0.564 0.153 498.5 33.1 
CZ2T500 1,015 511 504 1.078 0.234 820.8 36.4 
CZ2T600 757 276 481 1.004 0.120 592.6 34.7 
CZ2T700 494 214 280 0.631 0.096 421.6 32.1 
CZ3T500 925 584 341 0.891 0.275 761.1 35.4 
CZ3T600 714 434 280 0.754 0.203 628.2 31.5 
CZ3T700 478 299 179 0.433 0.143 404.7 28.2 
CF1T500 161 112 49 0.159 0.052 197.6 43.7 
CF1T600 215 129 85 0.232 0.060 301.7 40.6 
CF1T700 428 249 178 0.456 0.121 443.5 35.2 
CF2T500 344 267 76 0.269 0.126 391.0 40.4 
CF2T600 477 374 103 0.371 0.176 428.6 38.7 
CF2T700 683 428 254 0.684 0.201 608.8 34.0 
CF3T500 183 129 53 0.252 0.060 259.9 38.7 
CF3T600 422 240 118 0.420 0.112 395.5 35.3 
CF3T700 584 326 258 0.657 0.151 512.8 32.8 

CPh: activated carbon by steam, CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature in degrees Celsius, Sext: external surface area.

Production yield

According to the results of carbonization yield (Table 1), weight loss of ACs prepared by chemical activation are much lower than that of the ACs produced by physical activation due to release of more volatile matters in a continual carbonization process. The obtained yields were in the range of 43.7–28.2% and 11.56–8.24% for chemical and physical activation, respectively. As expected the higher temperature in the physical activation caused the lower yields. The maximum yields of CZ (36.9%) and CF (43.7%) were obtained at the lowest activation temperatures (i.e. 500 °C) and impregnation ratios of 1:1.The yield decreases with increasing activation temperatures as a consequence of a deeper devolatilization of the carbon.

As the impregnation ratio increases, a greater amount of activating agent is used. Since the activating agents act as dehydrating agents that affect pyrolytic decomposition, it seems that the higher amounts of such chemical agents with dehydrogenation properties inhibit the formation of tar and reduce the carbon yield (Ahmed & Theydan 2012).

N2 adsorption–desorption isotherms

To obtain a more accurate knowledge about the porous structure of the synthesized ACs, N2 adsorption–desorption isotherms at −196 °C were measured. According to the IUPAC classification, the N2 adsorption–desorption isotherm of CPhs prepared at 900 °C (Figure 1(a)) was isotherm type ІV showing mesoporous structure. The experimental data proved that physical properties of ACs, including BET surface area and micro-pore and total pore volume, depended on the final temperature of the physical activation process.

Figure 1

N2 adsorption–desorption isotherms for (a) steam activation derived ACs at different temperatures, ZnCl2 and FeCl3 derived ACs prepared at (b) different activation temperatures and an impregnation ratio of 1:1 and (c) different impregnation ratios and activation temperatures of 500 and 700 °C, respectively (c). CPh: activated carbon by steam, CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature in degrees Celsius.

Figure 1

N2 adsorption–desorption isotherms for (a) steam activation derived ACs at different temperatures, ZnCl2 and FeCl3 derived ACs prepared at (b) different activation temperatures and an impregnation ratio of 1:1 and (c) different impregnation ratios and activation temperatures of 500 and 700 °C, respectively (c). CPh: activated carbon by steam, CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature in degrees Celsius.

According to IUPAC classification, N2 adsorption–desorption isotherms of CZs and CFs belonged to the type І and type ІV, respectively (Figure 1(b) and 1(c)). These isotherms demonstrated the co-existence of both micro- and mesopores in the structure of the sorbents. The hysteresis loops in all isotherms, resulting from capillary condensation, could be an indicator of the existence of mesopores (Muniandy et al. 2014). Also progressive increase in the isotherms' slopes at relative pressures higher than 0.4 supports the existence of mesopores (Gao et al. 2013).

The activation temperature shows different trends when using ZnCl2 and FeCl3 activation agents (Figure 1(b)). For the former, a decrease in the amount of N2 adsorbed with increasing activation temperature is observed. This can be explained by the contraction of the ACs structure during thermal treatment (Allwar et al. 2008). In contrast, in the case of activation with FeCl3 the amount of N2 adsorbed increases with the activation temperature. It seems that FeCl3 needs higher temperatures to produce its activating effect (Bedia et al. 2018).

The effect of the impregnation ratio at activation temperatures of 500 °C for ZnCl2 and 700 °C for FeCl3 (the optimum activation temperatures for each activating agent) can be observed in Figure 1(c). In the case of ZnCl2 activation, lower impregnation ratios result in a higher porous development. However, in the case of activation with FeCl3, the highest N2 volumes adsorbed at impregnation ratio equal to 2. This result is in agreement with the optimal impregnation ratio founded by Bedia et al. (2018) for the FeCl3 activation of Tara gum.

As shown in Table 1, the surface area of carbon without any modification was very low (14 m2 g−1). The maximum surface area of 1,259 m2 g−1 was achieved by chemical activation with ZnCl2 at 500 °C and an impregnation ratio of 1:1, which may be attributed to the strong dehydrator function of ZnCl2.

Increasing temperature above 500 °C or impregnation ratio above 1:1 causes a considerable reduction in the surface area due to fast progress in reorganization of carbon structure (Oishi et al. 2012). Impregnation ratio of 1:1 was also reported as the optimum impregnation ratio in synthesis of AC by ZnCl2 activation from periwinkle shell (Owabor & Iyaomolere 2013). In the case of the activation temperature with ZnCl2, other studies (Ahmadpour & Do 1997; Ahmed & Theydan 2012) also recommended temperature of 500 °C for the maximum porous development. Above 500 °C, pore widening causes collapse of pore walls; hence the surface area and pore volumes begin to decrease. This indicates that pore formation is affected by both the reaction between incorporated ZnCl2 and the precursor and thermal pyrolysis of the precursor (Hsu & Teng 2000; Demiral et al. 2016). Chemical activation with ZnCl2 usually results in mesoporous ACs due to enlargement of micropores by fusion and vaporization of activating agent. Furthermore, higher activation temperatures can produce contraction of pore structure, resulting in considerable decrease in the porosity of obtained products, as previously observed for the activation of bamboo chips (Owabor & Iyaomolere 2013) with this activating agent. The increase of the impregnation ratio yielded a wider porosity because of deformation of the AC skeleton and destruction of the pore structure, which cause decrease in the ratio of the micropore volume to the total pore volume (Kamandari et al. 2015).

SEM observation

Figure 2 shows the SEM micrograph of the obtained ACs with the highest surface areas (ORS char, CPh900, CF2T700 and CZ1T500 ACs). The SEM images show quite different surface morphology and structure for the char (Figure 2(a)) and ACs (Figure 2(b)–2(d)). The char presented a smooth surface characteristic of non-porous materials. In contrast, the SEM images of the ACs show more heterogeneous surfaces with the presence of holes and thin walls randomly generated and distributed, caused by the loss of organic volatile matter as usually observed for porous carbon materials.

Figure 2

SEM of the (a) ORS char, (b) CPhT900, (c) CF2T700 and (d) CZ1T500 generated from oil sludge. CPh: activated carbon by steam CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature.

Figure 2

SEM of the (a) ORS char, (b) CPhT900, (c) CF2T700 and (d) CZ1T500 generated from oil sludge. CPh: activated carbon by steam CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature.

FTIR analysis

In FTIR spectra of both obtained ACs (Figure 3), broadened peaks at around 3,400–3,500 cm−1 are associated with the bands of the bonded –OH groups as a result of the vibration of water molecules (Tinti et al. 2015). The weak peak around 2,350 cm−1 is because of OH stretching vibration of the carboxylic acid groups (Krumins et al. 2012). The bands at 1,620 and 1,640 cm−1 can be attributed to ionized COO carbonyl groups and/or C═C bonds of sp2 carbon for CF and CZ, respectively. Another peak at 1,400 cm−1 is assigned to symmetric stretching of COO carbonyl (Baikousi et al. 2012). The peak which is presented at 1,115 cm−1 can also correspond to NH2 stretching. The band found at around 600 cm−1 arises due to the C-S vibration group (De Salvi et al. 2015). Phenol and its derivatives may be adsorbed on AC via a ‘donor–acceptor complex’ mechanism that involves carbonyl surface-oxygen groups acting as electron donor, and the aromatic ring of the solute acting as acceptor. Phosphate could be also removed by the ACs through interacting with the functional groups such as –OH and NH2 groups, the peak of the latter appearing at 1,115 cm−1.

Figure 3

FTIR spectra of ACs prepared with FeCl3 at temperature of 700 °C and impregnation ration of 2:1 (CF) and ZnCl2 at temperature of 500 °C and impregnation ratio of 1:1 (CZ).

Figure 3

FTIR spectra of ACs prepared with FeCl3 at temperature of 700 °C and impregnation ration of 2:1 (CF) and ZnCl2 at temperature of 500 °C and impregnation ratio of 1:1 (CZ).

Adsorption of phenol and phosphate

Initial adsorption tests were performed in order to select the ACs with the most suitable properties for phenol and phosphate removal in aqueous phase. Figure 4 presents the equilibrium phenol and phosphate adsorption capacities versus SBET of different ACs obtained by physical and chemical activation. The adsorption capacity of phenol and phosphate on ORS char was 14.28 and 2.1 mg g−1, respectively. The maximum adsorption capacities of ACs prepared by chemical activation process were 79 mg g−1 and 40 mg g−1 for phenol and phosphate, respectively. Hence, the adsorption capacities of ACs were considerably enhanced by the chemical activation through increasing the surface area, porosity structure and surface chemical groups. The maximum adsorption capacity of phenol and phosphate by AC prepared using physical activation was 31 and 8 mg g−1, respectively, which was significantly lower than that of chemical activation.

Figure 4

Adsorption capacities of (a) phenol and (b) phosphate versus SBET of different ACs obtained from oil sludge using physical activation and by ZnCl2 and FeCl3 chemical activation ([phenol]o = 100 mg L−1; [phosphate]o = 50 mg L−1; contact time = 4 h; [AC] = 1 g·L−1; pH was not adjusted).

Figure 4

Adsorption capacities of (a) phenol and (b) phosphate versus SBET of different ACs obtained from oil sludge using physical activation and by ZnCl2 and FeCl3 chemical activation ([phenol]o = 100 mg L−1; [phosphate]o = 50 mg L−1; contact time = 4 h; [AC] = 1 g·L−1; pH was not adjusted).

Figure 4(b) confirms the clear relationship between adsorption capacities and surface area although it should be mentioned that the FeCl3 and ZnCl2 derived carbons do not follow the same trend. It seems that the ACs obtained by FeCl3 activation have higher adsorption capacities than those obtained by ZnCl2 activation, with similar surface area and therefore similar porous development. Iron modification may cause different valence states of iron in carbon, so that the FeCl3 activation creates in carbon a more favorable surface chemistry for the adsorption of phosphate. Studies indicated that iron(III) has a high affinity toward phosphorus species and is very selective (Shi et al. 2011). The characterization results and adsorption capacities reveal that chemical activation is preferred over physical activation. So the experiments continued with ACs prepared by chemical activation.

We analyzed kinetic, isotherm and thermodynamics of phenol and phosphate adsorption on CZ1T500 and CF2T700, which displayed the highest adsorption capacities and porous development among their corresponding series. The adsorption capacity of ACs depends on various factors such as porous texture and surface functional groups of adsorbent; the polarity, solubility and molecular size of adsorbate; pH of solution or the presence of other ions in the solution. It has been reported that the surface area (SBET) is not the only factor which affects adsorption capacity. Other factors such as the volume of ACs pores and their size (e.g. with width smaller than 1.4 nm) could affect the phenol adsorption capacity (qm) (Lorenc-Grabowska 2016). In our study, microporosity was low, so the adsorption of phenol did not correlate with the BET surface area of CZ and CF.

Adsorption kinetics

Figure 5 shows the kinetic curves of phenol adsorption on CZ1T500 and CF2T700 and phosphate adsorption on CZ1T500 and CF2T700. The parameters obtained by fitting the experimental data with both kinetics models are summarized in Table 2. The results showed that the correlation coefficients for the pseudo second order kinetic model were higher than for the pseudo first order kinetic model in all the cases. Therefore, phenol and phosphate adsorption could be described by the pseudo second order kinetic model. These results are in agreement with previous studies about the adsorption of phenol and phosphate on different ACs (Shi et al. 2011; Ren et al. 2015; Li et al. 2017; Tao et al. 2017). Furthermore, the values of the equilibrium adsorption capacities of phenol and phosphate calculated by the model, qe CALC, are similar to those obtained experimentally, qe EXP, supporting the suitability of the second order model to describe the experimental data.

Table 2

Kinetic parameters for the removal of phenol and phosphate by CZ1T500 and CF2T700

   First order kinetic model
 
Second order kinetic model
 
AC, Sorbate C0 (mg L−1qe EXP (mg g−1K1 qe CALC (mg g−1R2 K2 qeCALC (mg g−1R2 
CZ1T500, phenol 50 45.63 0.131 40.0 0.986 0.0031 54.3 0.997 
100 79.68 0.144 75.4 0.993 0.0016 98.0 0.997 
200 149.85 0.158 138.1 0.988 0.0011 169.5 0.999 
CF2T700, phenol 50 39.43 0.119 30.2 0.983 0.0043 42.0 0.995 
100 71.91 0.122 54.1 0.980 0.0025 75.8 0.996 
200 137.86 0.147 120.7 0.978 0.0011 161.3 0.993 
CZ1T500, phosphate 10 7.87 0.016 5.40 0.880 0.036 8.41 0.991 
20 15.73 0.022 12.56 0.973 0.0046 18.69 0.990 
30 21.65 0.014 16.62 0.928 0.0026 24.44 0.990 
CF2T700, phosphate 10 9.77 0.017 5.09 0.789 0.012 9.92 0.994 
20 18.83 0.017 12.16 0.896 0.002 19.88 0.993 
30 27.75 0.016 20.22 0.954 0.0009 29.23 0.997 
   First order kinetic model
 
Second order kinetic model
 
AC, Sorbate C0 (mg L−1qe EXP (mg g−1K1 qe CALC (mg g−1R2 K2 qeCALC (mg g−1R2 
CZ1T500, phenol 50 45.63 0.131 40.0 0.986 0.0031 54.3 0.997 
100 79.68 0.144 75.4 0.993 0.0016 98.0 0.997 
200 149.85 0.158 138.1 0.988 0.0011 169.5 0.999 
CF2T700, phenol 50 39.43 0.119 30.2 0.983 0.0043 42.0 0.995 
100 71.91 0.122 54.1 0.980 0.0025 75.8 0.996 
200 137.86 0.147 120.7 0.978 0.0011 161.3 0.993 
CZ1T500, phosphate 10 7.87 0.016 5.40 0.880 0.036 8.41 0.991 
20 15.73 0.022 12.56 0.973 0.0046 18.69 0.990 
30 21.65 0.014 16.62 0.928 0.0026 24.44 0.990 
CF2T700, phosphate 10 9.77 0.017 5.09 0.789 0.012 9.92 0.994 
20 18.83 0.017 12.16 0.896 0.002 19.88 0.993 
30 27.75 0.016 20.22 0.954 0.0009 29.23 0.997 

CZ: activated carbon by ZnCl2, CF: activated carbon by FeCl3, T: the activation temperature in degrees Celsius.

Figure 5

Kinetic curves of phenol adsorption on CZ1T500 (a) and CF2T700 (b), and phosphate adsorption on CZ1T500 (c) and CF2T700 (d) (adsorbent dose = 1 g·L−1, pH was not adjusted).

Figure 5

Kinetic curves of phenol adsorption on CZ1T500 (a) and CF2T700 (b), and phosphate adsorption on CZ1T500 (c) and CF2T700 (d) (adsorbent dose = 1 g·L−1, pH was not adjusted).

Adsorption isotherms

Table 3 presents the phenol and phosphate adsorption isotherm data for CZ1T500 and CF2T700 ACs. The experimental data were fitted to the well-known Freundlich and Langmuir models (Equations (4) and (5), respectively). In the Langmuir model, adsorption sites are assumed to be energetically homogeneous, and a monolayer surface is formed without any interaction between the adsorbed molecules. In contrast, the Freundlich model is used for heterogeneous surfaces, and it takes place with a non-uniform distribution of heat of adsorption. Therefore, in the Freundlich model the adsorption is heterogeneous and multilayer.

Table 3

The Langmuir and Freundlich model constants and correlation coefficients for adsorption of phenol and phosphate on CZ1T500 and CF2T700

 Langmuir isotherm
 
Freundlich isotherm
 
Sorbate, AC qm (mg g1KL (L mg1R2 KF (mg g1) (L mg1)1/n n R2 
Phenol, CZ1T500 238.09 0.025 0.9768 91.6 1.2 0.9953 
Phenol, CF2T700 212.76 0.017 0.9688 5.4 1.2 0.9997 
Phosphate, CZ1T500 95.23 0.097 0.938 13.9 2.07 0.993 
Phosphate, CF2T700 102.04 0.073 0.945 9.15 1.55 0.995 
 Langmuir isotherm
 
Freundlich isotherm
 
Sorbate, AC qm (mg g1KL (L mg1R2 KF (mg g1) (L mg1)1/n n R2 
Phenol, CZ1T500 238.09 0.025 0.9768 91.6 1.2 0.9953 
Phenol, CF2T700 212.76 0.017 0.9688 5.4 1.2 0.9997 
Phosphate, CZ1T500 95.23 0.097 0.938 13.9 2.07 0.993 
Phosphate, CF2T700 102.04 0.073 0.945 9.15 1.55 0.995 

As can be seen, the Freundlich model showed a better fit to the adsorption data than the Langmuir model. The Langmuir model provides the values of qm, which could be considered as the saturation or maximum adsorption capacity of adsorbate onto the adsorbents at equilibrium.

Table 4 summarizes phenol and phosphate maximum adsorption capacities, BET surface areas and adsorption temperatures for different ACs reported in the literature. According to the results, the phenol and phosphate adsorption capacities were quite high, which confirmed the sorption efficiency of ACs prepared from oil sludge by chemical activation. It has been reported that the adsorption capacity of small molecules such as phenol onto the inner surface of carbon correlates with the content of micropores and BET surface area, while for mesoporous ACs the substituent group in the phenol and nature of the carbon controlled the phenol adsorption as well (Caturla et al. 1988).

Table 4

Phenol and phosphate maximum adsorption capacities obtained from Langmuir model, BET surface areas and adsorption temperatures for different activated carbons reported in the literature

Adsorbent  SBET (m2 g−1Adsorption temperature (°C) qm (mg g−1Reference 
Tea waste AC by chemical activation with ZnCl2 phenol 1,066 20 142.9 Gundogdu et al. (2012)  
Anthracene oil pitch derived AC phenol 1,117 24 303.0 Lorenc-Grabowska (2016)  
AC from roots by steam physical activation phenol 1,185 25 145.0 Altenor et al. (2009)  
AC from oil sludge by ZnCl2 chemical activation phenol 1,259 25 238 This work 
AC loaded with Fe(III) oxide phosphate – 25 98.39 Shi et al. (2011)  
Iron-impregnated biochar derived from sludge phosphate – 25 111 Yang et al. (2018)  
AC from oil sludge by FeCl3 chemical activation phosphate 683 25 102 This work 
Adsorbent  SBET (m2 g−1Adsorption temperature (°C) qm (mg g−1Reference 
Tea waste AC by chemical activation with ZnCl2 phenol 1,066 20 142.9 Gundogdu et al. (2012)  
Anthracene oil pitch derived AC phenol 1,117 24 303.0 Lorenc-Grabowska (2016)  
AC from roots by steam physical activation phenol 1,185 25 145.0 Altenor et al. (2009)  
AC from oil sludge by ZnCl2 chemical activation phenol 1,259 25 238 This work 
AC loaded with Fe(III) oxide phosphate – 25 98.39 Shi et al. (2011)  
Iron-impregnated biochar derived from sludge phosphate – 25 111 Yang et al. (2018)  
AC from oil sludge by FeCl3 chemical activation phosphate 683 25 102 This work 

Additionally the adsorptions of phenol as a weak acid can be carried out on ACs with basic and neutral surface characteristics. At pH values less than the pHpzc, water molecules donate more H+ than OH groups, so the adsorbent surface has a net positive charge and attracts anions such as phenol. Phenol adsorption driving forces on AC are due to ππ dispersion interactions between the aromatic ring of phenol and the basal plane of carbon (Dabrowski et al. 2005).

Effect of temperature on thermodynamic parameters of adsorption

In order to investigate the influence of temperature, phenol and phosphate adsorption was studied at 25–40 °C. It could be observed from Table 5 that the adsorption of phenol and phosphate decreased with increasing the temperature. This probably indicates that, at higher temperature, the chemical interactions between absorbate and surface functionalities of AC are weak.

Table 5

Thermodynamic parameters for the adsorption of phenol and phosphate onto the produced ACs at pH 6

Sorbate, AC Temperature (K) C0 (mg L−1qe (mg L−1ln Kd ΔG° (kJ mol−1ΔH° (kJ mol−1ΔS° (J kmol−1
 298 50 44.06 −5.12 −68.57 −212.9 
Phenol 303 50 41.84 1.63 −4.06   
CZ1T500 308 50 37.73 1.12 −2.99   
 313 50 33.26 0.686 −1.93   
 298 50 34.23 0.77 −2.158 −45.07 −144.65 
Phenol 303 50 31.09 0.49 −1.438   
CF2T700 308 50 26.83 0.115 −0.718   
 313 50 23.88 −0.09 −0.002   
 298 30 21.65 0.95 −2.59 −51.76 −165.54 
Phosphate 303 30 19.81 0.66 −1.76   
CZ1T500 308 30 16.43 0.19 −0.94   
 313 30 14.91 −0.01 −0.11   
 298 30 27.76 2.51 −9.43 −98.83 −300.65 
Phosphate 303 30 25.82 1.81 −7.93   
CF2T700 308 30 22.88 1.16 −6.43   
 313 30 19.79 0.65 −4.93   
Sorbate, AC Temperature (K) C0 (mg L−1qe (mg L−1ln Kd ΔG° (kJ mol−1ΔH° (kJ mol−1ΔS° (J kmol−1
 298 50 44.06 −5.12 −68.57 −212.9 
Phenol 303 50 41.84 1.63 −4.06   
CZ1T500 308 50 37.73 1.12 −2.99   
 313 50 33.26 0.686 −1.93   
 298 50 34.23 0.77 −2.158 −45.07 −144.65 
Phenol 303 50 31.09 0.49 −1.438   
CF2T700 308 50 26.83 0.115 −0.718   
 313 50 23.88 −0.09 −0.002   
 298 30 21.65 0.95 −2.59 −51.76 −165.54 
Phosphate 303 30 19.81 0.66 −1.76   
CZ1T500 308 30 16.43 0.19 −0.94   
 313 30 14.91 −0.01 −0.11   
 298 30 27.76 2.51 −9.43 −98.83 −300.65 
Phosphate 303 30 25.82 1.81 −7.93   
CF2T700 308 30 22.88 1.16 −6.43   
 313 30 19.79 0.65 −4.93   

The standard Gibbs free energies (ΔG°) of adsorption were negative in all cases, indicating the spontaneous and exothermic nature of phenol and phosphate adsorption onto ACs (Fu et al. 2009). The values of ΔG° for physisorption and chemisorption are between −20 and 0 kJ mol−1 and −80 and −400 kJ mol−1, respectively (Fu et al. 2009). As can be seen in Table 5, the free energy values were in the range between −0.002 and −9.43 kJ mol−1 indicating that phenol and phosphate adsorptions occur by physisorption. In addition, the decrease in the magnitude of ΔG° at the higher temperatures showed the diminishing of the spontaneous nature of the process, so the adsorption was not favorable at higher temperatures. Moreover, the values of ΔH° and ΔS° were obtained from the slope and the intercept of plot between lnK0 versus 1/T for initial phenol and phosphate concentration of 50 and 30 mg L−1, respectively. The range of enthalpy change (ΔH°) was from −68.57 to −45.07 kJ mol−1 for phenol adsorption and from −51.76 to −98.83 kJ mol−1 for phosphate adsorption. The negative values of ΔH° represented the exothermic nature of the adsorption process. Furthermore, the negative values of ΔS° showed the decreased randomness at the solid–solution interface in the adsorption system.

CONCLUSION

This study reported the preparation of different ACs from oil sludge using various physicochemical activation methods. Comparative tests demonstrated that preparation conditions strongly affected the AC surface area and its adsorption capacity. It was found that chemical activation of oil sludge using ZnCl2 and FeCl3 was more effective than the physical activation to synthesize highly porous ACs. The largest surface area (1,259 m2 g−1) and pore volume of activated carbon prepared from ORS using ZnCl2 activation processes occurred at 500 °C and impregnation ratio of 1:1. Adsorption kinetics followed a pseudo second order kinetic model while equilibrium adsorption followed the Freundlich model for both phenol and phosphate. The maximum phenol and phosphate adsorption capacities were 238 and 102 mg g−1, respectively. The negative ΔG° values were observed in all cases, indicating the spontaneous and exothermic nature of phenol and phosphate adsorption onto AC in a chemically controlled process. The results of this work present a viewpoint of oily sludge potential as a low cost precursor for production of high surface area ACs with well-developed porosity and considerable adsorption properties. However, the environmental effects during production of ACs from these hazardous wastes and heavy metal leaching of the sorbent as well as regeneration and disposal of the sorbents should be investigated in future studies.

REFERENCES

REFERENCES
Allwar
A. B. M. N.
Noor
M. A. B. M.
Nawi
M. A. M.
2008
Textural characteristics of activated carbons prepared from oil palm shells activated with ZnCl2 and pyrolysis under nitrogen and carbon dioxide
.
Journal of Physical Science
19
(
2
),
93
104
.
Altenor
S.
Carene
B.
Emmanuel
E.
Lambert
J.
Ehrhardt
J. J.
Gaspard
S.
2009
Adsorption studies of methylene blue and phenol onto vetiver roots activated carbon prepared by chemical activation
.
Journal of Hazardous Materials
165
(
1–3
),
1029
1039
.
Baikousi
M.
Dimos
K.
Bourlinos
A.
Zbořil
R.
Papadas
I.
Deligiannakis
Y.
Karakassides
M.
2012
Surface decoration of carbon nanosheets with amino-functionalized organosilica nanoparticles
.
Applied Surface Science
258
(
8
),
3703
3709
.
Bedia
J.
Monsalvo
V.
Rodriguez
J.
Mohedano
A.
2017
Iron catalysts by chemical activation of sewage sludge with FeCl3 for CWPO
.
Chemical Engineering Journal
318
,
224
230
.
Bedia
J.
Belver
C.
Ponce
S.
Rodriguez
J.
Rodriguez
J.
2018
Adsorption of antipyrine by activated carbons from FeCl3-activation of Tara gum
.
Chemical Engineering Journal
333
(
1
),
58
65
.
Caturla
F.
Martin-Martinez
J. M.
Molina-Sabio
M.
Rodriguez-Reinoso
F.
Torregrosa
R.
1988
Adsorption of substituted phenols on activated carbon
.
Journal of Colloid and Interface Science
24
,
234
528
.
Dabrowski
A.
Podkoscielny
P.
Hubicki
Z.
Barczak
M.
2005
Adsorption of phenolic compounds by activated carbon – a critical review
.
Chemosphere
58
,
1049
1070
.
Demiral
I.
Aydın Şamdan
C.
Demiral
H.
2016
Production and characterization of activated carbons from pumpkin seed shell by chemical activation with ZnCl2
.
Desalination and Water Treatment
57
(
6
),
2446
2454
.
De Salvi
D. T.
Job
A. E.
Ribeiro
S. J.
2015
New flexible and transparent solution-based germanium-sulfide polymeric materials
.
Journal of Brazilian Chemical Society
26
(
5
),
992
1003
.
Gundogdu
A.
Duran
C.
Basiri Senturk
H.
Solylak
M.
Ozdes
D.
Serencam
H.
Imamoglu
M.
2012
Adsorption of phenol from aqueous solution on a low-cost activated carbon produced from tea industry waste: equilibrium, kinetic, and thermodynamic study
.
Journal of Chemical and Engineering Data
57
(
10
),
2733
2743
.
Kamandari
H.
Rafsanjani
H. H.
Najjarzadeh
H.
Eksiri
Z.
2015
Influence of process variables on chemically activated carbon from pistachio shell with ZnCl2 and KOH
.
Research on Chemical Intermediates
41
(
1
),
71
81
.
Krumins
J.
Klavins
M.
Seglins
V.
Kaup
E.
2012
Comparative study of peat composition by using FT-IR spectroscopy
.
Materials Sciences and Applied Chemistry
26
,
106
113
.
Ma
H. T.
Ho
V. T. T.
Pham
N. B.
Naguyen
D. C. H.
Vo
K. T. D.
Lay
C. D.
Phan
T. D.
2017
Effect of the carbonization and activation process on the adsorption capacity of rice husk activation carbon
.
Vietnam Journal of Science and Technology
55
(
4
),
485
493
.
Mohedano
A.
Monsalvo
V.
Bedia
J.
Lopez
J.
Rodriguez
J.
2014
Highly stable iron catalysts from sewage sludge for CWPO
.
Journal of Environmental Chemical Engineering
2
(
4
),
2359
2364
.
Oliveira
L. C. A.
Pereira
E.
Guimaraes
L. R.
Vallone
A.
Pereira
M.
Mesquita
J. P.
Sapag
K.
2009
Preparation of activated carbons from coffee husks utilizing FeCl3 and ZnCl2 as activating agents
.
Journal of Hazardous Materials
165
(
1–3
),
87
94
.
Owabor
C.
Iyaomolere
A.
2013
Evaluation of the influence of salt treatment on the structure of pyrolyzed periwinkle shell
.
Journal of Applied Sciences and Environmental Management
17
(
2
),
321
327
.
Patil
B.
Kulkarni
K.
2012
Development of high surface area activated carbon from waste material
.
International Journal of Advanced Engineering Research and Studies
1
,
109
113
.
Rybicki
S. M.
1998
New technologies of phosphorus removal from wastewater. In: Proceedings of a Polish-Swedish Seminar, Joint Polish Swedish Reports, Report (No. 3)
.
Tinti
A.
Tugnoli
V.
Bonora
S.
Francioso
O.
2015
Recent applications of vibrational mid-infrared (IR) spectroscopy for studying soil components: a review
.
Journal of Central European Agriculture
16
(
1
),
1
22
.
Villegas
L. G. C.
Mashhadi
N.
Chen
M.
Mukherjee
D.
Taylor
K. E.
Biswas
N.
2016
A short review of techniques for phenol removal from wastewater
.
Current Pollution Reports
2
(
3
),
157
167
.
Wang
Z.
Nie
E.
Li
J.
Yang
M.
Zhao
Y.
Luo
X.
Zheng
Z.
2012
Equilibrium and kinetics of adsorption of phosphate onto iron-doped activated carbon
.
Environmental Science and Pollution Research
19
(
7
),
2908
2917
.
Yang
Q.
Wang
X.
Luo
W.
Sun
J.
Xu
Q.
Chen
F.
Zhao
J.
Wang
S.
Yao
F.
Wang
D.
Li
X.
Zeng
G.
2018
Effectiveness and mechanisms of phosphate adsorption on iron-modified biochars derived from waste activated sludge
.
Bioresource Technology
247
,
537
544
.