Development of Newer Adsorbents: Activated Carbons Derived from

Article pubs.acs.org/IECR

Development of Newer Adsorbents: Activated Carbons Derived from Carbonized Cassia f istula Laxmi Gayatri Sorokhaibam,*,† Vinay M. Bhandari,* Monal S. Salvi, Saijal Jain, Snehal D. Hadawale, and Vivek V. Ranade Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pune 411008, India ABSTRACT: Development of newer adsorbent materials, especially from biomass, is most crucial to sustain growth of otherwise well established adsorption processes that already have hundreds of commercial adsorbents in practice. In the present study, newer adsorbents and their potential-carbonaceous adsorbents derived from the whole fruit of Cassia f istula (Golden shower) have been reported for applications in environmental pollution control using natural, renewable biomass as precursor. The potential of different forms of the derived adsorbents produced by thermal and chemical activation was examined for two representative cases, desulfurization of transportation fuels and wastewater treatment. The adsorbents were characterized by FTIR, XRD, XPS, and SEM techniques that indicate specific characteristics useful as an adsorbent. A successful application in the real industrial wastewater treatment and comparison with well recognized commercial adsorbents clearly highlights the utility of the developed newer adsorbents in separation science and technology.

1. INTRODUCTION The aim of the present study is to develop novel activated carbons from an inexpensive biomass based raw material, substantiate its good adsorptive performance, and investigate potential applications in environmental pollution control. Cassia f istula is a popular ornamental tree with attractive yellow flowers and widely found in India. The common chemical components in seeds of C. f istula have been reported by medicinal and analytical chemists as consisting of sugars,1 unsaturated fatty acids,2 cyclopropenoid fatty acids,3 and triacylglycerols.4 It can be visualized that high carbon content in the activated forms of Cassia f istula (CF) provides a good potential for applications in environmental pollution control, especially with reference to liquid phase adsorptive separation of nonpolar to slightly polar species. This is an interesting and important differentiating aspect, especially in view of the fact that there are many sources for preparation of carbon adsorbents such as coal and biomass precursors such as sugar cane bagasse,5 rice husk,6 coconut coir,7 etc. Further, activated carbon from Cassia f istula with well-developed pore structure and high surface area for multipurpose contaminant removal application has not been reported so far. Formulating activated carbon from this precursor is not known, and only the biomass treatment has been considered for adsorption in some cases.8,9 This precursor is anticipated to offer relatively high content of surface groups that can aid in developing specific surface properties for adsorption of various species after converting to activated carbons. The inherent properties can be further modified suitably through various known chemical/thermal treatments, with or without additional changes, for improving adsorption properties. Another objective of the study is that the developed adsorbents find useful applications in the real world. Therefore, the developed adsorbents were examined for specific applications in prominent research areas for removal of pollutants that encompass both organic and aqueous streams, © XXXX American Chemical Society

e.g., desulfurization of transportation fuels and industrial wastewater treatment. Deep desulfurization of transportation fuels has been an increasingly researched problem due to stricter governmental norms worldwide on the sulfur content in diesel, gasoline, and jet fuels. Sulfur content below 15 ppm (parts per million) in diesel and 30 ppm in gasoline has been mandated worldwide, and adsorptive processes are expected to play a crucial role in the development of technoeconomic process integration in this regard.10,11 The conventional hydrodesulfurization process involves catalytic reactors at high temperature and pressure and has been reported to be efficient in removing thiols, sulfides, and disulfides but less effective for thiophene and thiophene derivatives.12 Since most of the commercial adsorbents are not suitable for deep desulfurization, newer alternative adsorbents that can be made from easily available resources and are cost-effective with ease of preparation are highly sought after. In the present study, deep desulfurization using adsorbents developed from Cassia f istula has been investigated with model fuel (MF) comprising three different thiophenic compounds, namely, thiophene (T), benzothiophene (BT), and dibenzothiophene (DBT). The new materials so developed have neither been doped nor impregnated with other elements, except they have undergone heat and chemical treatment to alter the pore characteristics that may influence the nature of adsorption. Industrial wastewater treatment is a similarly challenging area where practically all important separation processes are required for the removal of various pollutants to meet stipulated stringent pollution control norms. Advancement in the physicochemical methods and biological methods can Received: August 11, 2015 Revised: November 2, 2015 Accepted: November 5, 2015

A

DOI: 10.1021/acs.iecr.5b02945 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Activated carbon from Cassia f istula (Golden shower): (a) picture of Golden shower plant bearing fruits; (b) pods of the plant being sun dried; (c) pods after heating in the oven to remove excess moisture; (d) minimal crushing of the pods; (e) pods after activation in the furnace; (f) powdered activated carbon obtained.

substantially improve overall technoeconomic feasibility of the effluent treatment plant.13,14 Adsorption, again, is an effective method here due to its selectivity to specific pollutants, low initial concentrations of the pollutants, and ability to bring down pollutant levels below prescribed norms. Activated carbons being hydrophobic in nature find wide applications for removal of organics that are nonpolar or only slightly polar. Surface modification is expected to alter the polarity/acidity of the surface, thereby effecting the selective removal of polar to nonpolar pollutants from wastewaters. This also provides enormous opportunity for specialty materials or material modifications to suit specific pollutant removal or the development of tailor-made adsorbent materials. In the present study, an important application of dye wastewater treatment has been presented for both synthetic dye removal of Congo red and for real industrial wastewaters. A comparison with commercial adsorbent for real industrial wastewater treatment has also been presented to highlight the effectiveness of the developed adsorbents.

CF-raw which are referred to as CF-450, CF-900, CFP-450, and CFP-900, where the numbers 450 and 900 refer to the temperature of activation and P stands for phosphoric acid treated forms after heating at the particular temperature. Phosphoric acid impregnation was carried out at 100 °C in 1:1 weight/volume ratio for 1 h and further at 500 °C for 2 h in the tube furnace. Prior to each adsorption experiment, the adsorbents were activated at 200 °C for a minimum of 2 h. The reproducibility of the carbons was checked and was found satisfactory. However, though it is possible that composition can change depending on the location/harvest of the plant, significant variation is not expected in the derived materials. 2.3. Characterization. Fourier transform infrared (FTIR) spectroscopy studies were carried out using FTIR-2000, PerkinElmer with the anhydrous KBr pellet method. XPS patterns of the adsorbent samples were evaluated with XPSVG-Microtech, Multi Lab instrument (model no. 800-003, serial no. 8546/1). XPS conditions involve sample chamber equipped with a TMP (turbomolecular pump), analyzer chamber with ion pump having vacuum of 12 μA and 3.9 × 10−9 Torr, X-ray source Al Kα (energy 1486.6 eV) with ray current 4.7 A, filament current 12 mA, and voltage of 12.5 kV along with a Channeltron detector. The surface morphology of the prepared samples was characterized by Quanta 200 3D dual beam environmental scanning electron microscope (ESEM) having a resolution of 3 nm at 30 kV with tungsten filament (W) as the electron source. ESEM was coupled with an energy dispersive X-ray spectrometer (EDS, FEI Quanta 200 3D) to carry out elemental analysis of the samples. Nitrogen adsorption−desorption isotherms were measured using an automatic adsorption unit, Autosorb-1 (Thermo Scientific), at −195.6 °C. Dubinin−Radushkevich method was applied for determination of micropore volume, while pore size distribution was determined by Barret−Joyner−Halenda (BJH) method. Powder XRD of the adsorbent samples was carried out using X’Pert Pro PANanalytical XRD with Cu Kα radiation (λ = 1.542 Å) in 2θ range of 10−80°. 2.4. Model Fuel (MF) and Analysis of S Content. Model fuel was prepared by adding known quantities of sulfur compounds into liquid n-octane. The model fuels so prepared are thiophene in octane (MF-T), benzothiophene in octane (MF-BT), dibenzothiophene in octane (MF-DBT), and ternary mixture consisting of equal concentrations of T, BT, and DBT in octane (MF-T/BT/DBT) where the total sulfur content was

2. MATERIALS AND METHODS 2.1. Chemicals and Raw Materials. The carbon precursors (whole fruit) of Golden shower tree (Cassia f istula) were collected from the local area of Pune, India. Chemicals noctane (Sigma-Aldrich, reagent grade, 98%), thiophene (≥99%, Loba Chemie), benzothiophene (Fluka Chemika, 95%), and dibenzothiophene (Sigma-Aldrich, 95%) and chemicals like phosphoric acid (Sigma-Aldrich, ACS reagent, 85% by weight in H2O), azo dye Congo red (Loba Chemie for microscopy) were used. Two commercial activated carbons, Norit (SigmaAldrich, USA) and SHIRASAGI TAC (Japan EnvironChem. Japan) were used. Industrial wastewater samples were procured from a major dye manufacturing unit in India. 2.2. Preparation of Activated Carbons. The carbon precursors (whole fruit) of Golden shower tree (Cassia f istula) were collected. The pods were sun-dried after thorough washing to remove any greasy or dust materials, crushed into fine powders, and kept in the oven at 80 °C for 72 h for complete removal of moisture. The powdered raw materials obtained from C. f istula fruit shell and seeds (referred as CFraw) were then carbonized in a temperature-programmed horizontally aligned electrical tube furnace (Nabartherm, Germany) at 450 and 900 °C for 4 h in the presence of air. Four different types of activated carbons were prepared from B


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Figure 2. Structural and material properties of activated carbon of Cassia f istula showing morphological changes with thermal and chemical treatment: (a, c, d, e, f) scanning electron microscope (SEM) images of CF-raw, CF-450, CFP-450, CF-900, and CFP-900, respectively; (b) energy dispersive X-ray spectroscopy (EDX) profile of CF-raw.

∼100 mg/L, 300 mg/L, and ∼500 mg/L. The study at ∼500 mg/L was investigated specifically to determine the effectiveness of the adsorbents at higher concentration. Batch adsorption studies were carried out using predetermined amounts of adsorbents and solution for a contact time of 16 h under ambient conditions. The sulfur content of thiophene, benzothiophene, and dibenzothiophene in the single and ternary model fuel was analyzed by gas chromatograph (Agilent GC 7890 A) equipped with a flame photometric detector (FPD) with CPSil 5CB for sulfur as column (30 m × 320 μm × 4.0 μm). Helium was used as a carrier gas with a flow rate of 2 mL/min and split ratio of 10:1 (20 mL/min flow rate). The injector temperature employed was 250 °C with an injection volume of 0.2 μL

and total analysis time of 25 min. The oven temperature was ramped at 20 °C/min from 40 to 100 °C and thereafter at 60 °C/min to 230 °C. Reproducibility check of the experimental results and cross-checking with total sulfur analyzer, TN-TS 3000 (Thermoelectron Co., Netherlands), were found satisfactory. 2.5. Adsorption in Aqueous System. Congo red was selected as a model dye pollutant for synthetic wastewater studies and two industrial wastewater samples from dye industry for real life case study. COD, ammoniacal nitrogen, and color reduction were measured using COD analyzer Spectroquant Pharo 100 spectrophotometer (Merck Limited); Spectroquant TR 320 was used as digester for digestion of samples for 2 h at 148 °C. Except for the pH study, all batch C


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As seen from Table 1, the material prepared at low temperature and without chemical treatment has a low proportion of oxygen content. Further, the chemically modified and high temperature activated forms, CFP-450 and CFP-900, have high carbon content. Usually the carbon content of the precursor material determines the carbon yield which in turn affects the properties of the activated carbon prepared thereof.15,16 The carbon content (weight) of the fresh CF-raw powder was found to be 18.5% which indicated that C. f istula can be a suitable precurssor for obtaining activated carbon. After activation by thermal and chemical treatment, there was an increase in carbon content to as high as 86.5% in CF-450 and 53% in CFP-450 which may be due to compacting as a result of loss of volatile matter.17 Characterization of the adsorbents indicated microporous to mesoporous structure. The channel sizes of activated carbons were in the range of 1−10 μm in CF-450 showing well-ordered arrangement of channels. Further, more well-structured pores are also observed in surface morphology of CF-900 carbonized at temperature of 900 °C for 4 h, as shown in Figure 2. Chemical activation by phosphoric acid at 450 and 900 °C produces structural changes, especially with the pore size (Table 2). However, it appears that with excessive heat treatment at 900 °C in CFP-900, there is a change in the total pore volume and reorganization of mesopore and micropore volume affecting the textural appearance. This can be attributed to the excessive burn-off of the precursor as well as partial collapse of certain open pores into closed ones.18 The average pore width decreased during thermal treatment alone. The carbonaceous adsorbents so prepared also have heteroatoms like P, K, Ca, and Fe which are considered to be beneficial in the adsorption process.19 EDX data show significant proportions of K, Ca, and Fe, while XPS shows presence of additional elements such as Si, Mg, Na in varying amounts as per the nature of treatment.These heteroatoms can influence the acidic and basic properties of the carbon surface and hence affect the adsorption behavior.20 For example, the presence of Ca may impart positive character21 and enhance the treatment efficiency for specific pollutants or increase the mesopore volume22 which is in agreement with Table 2 showing CF-450 with the least micropore volume (0.005 cm3/ g) and highest Ca content of 2.62%, while Fe is known for its ability to introduce acidic functional groups.23 The presence of heterogeneous elements like Mg (0.09−0.66 ppm) and Ca (0.51−1.62 ppm) and transition element like Fe (0.02−0.21 ppm) in the carbon matrix of the five adsorbents was also confirmed by ICP-OES analysis (Spectro-Arcos, ARCOS-FHS12). The maximum proportion of these elements was detected in CF-900. It is however pertinent to note that some of the alkali metals that are present in the raw precursor may get removed during the treatment; e.g., significant fraction of potassium can be lost by way of vaporization at temperatures

studies were performed at the natural pH. Shimadzu total organic carbon analyzer (TOC-L-CPH) with catalytically aided combustion oxidation and nondispersive infrared detector having detection limit of 4 μg/L was used for TOC analysis.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Adsorbent Materials. In Figure 1, steps a−f show the development of newer carbonaceous adsorbents from the dried pods of Cassia f istula. Table 1. Elemental Composition of Different Activated Carbons from EDX Data composition (wt %) element

CF-raw

CF-450

CFP-450

CF-900

CFP-900

C O P

18.5 64.5 0.51

86.5 9.8 0.11

53.0 22.5 19.4

45.7 22.8 2.7

54.1 30.2 13.5

The pores from scanning electron microscope (SEM) images indicate diameter in the micrometer (μm) range. These pores may be considered as channels to the microporous network. CF-raw depicted a rudimentary morphology with a highly amorphous structure (Figure 2). All the adsorbents have rough texture with heterogeneous surface and random pore size distribution. In Figure 2, the EDX inset of CF-raw shows the presence of elements like C, O, P, K, Ca, and Fe whose ratio varied in the different forms of the activated carbons developed (phosphoric acid treated carbons have proportionally higher levels of oxygen content). The results are given in Table 1. It is evident from the SEM images of Cassia f istula (Figure 2) that morphological changes take place on the surface under different conditions of activation mode. The SEM images display apparently cylindrical macropores in the activated carbon. With exception to CF-raw, all the four ACs showed well-developed porous surfaces at higher SEM magnification. The macropores have much larger pore sizes and play an important role of being the conduits through which access to the interior of the activated carbon and hence to the mesopores and micropores can be made. They are generally considered as being part of the external surface of the activated carbon. The macropores have size greater than 50 nm. Figure 2 depicts fizzy structure in CF-raw sample, which can be considered to be crucial to the production of activated carbon, since it enables the CF to absorb the chemical reagent which can activate pore formation inside. The SEM image of CF-450 (Figure 2) clearly shows that carbonization/charring the raw precursor at 450 °C for 4 h leads to the channeling of pores. The selection of the higher temperature of carbonization along with acid treatment in CFP-900 resulted in breaking down of the nearly homogeneous surface in CF-450, CF-900, and CFP-450 into smaller pieces as is evident from Figure 2f. Table 2. Structural Parameters

a

adsorbent

SBET a (m2/g)

VT b (cm3/g)

Vmic c (cm3/g)

average pore width (nm)

micropore surface area (m2/g)

adsorption energy (kJ/mol)

CF-raw CF-450 CF-900 CFP-450 CFP-900

69.34 214.1 594.6 1113.0 154.3

0.07 1.06 1.78 0.29

0.005 0.298 0.564 0.077

6.58 4.54 1.56 1.55 2.10

14.02 838.3 1584.0 218.62

3.94 5.73 16.65 16.69 12.33

SBET: BET surface area. bTotal pore volume (BJH cumulative adsorption pore volume). cMicropore volume (D−R method). D


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Figure 3. FTIR spectra of the adsorbents derived from Cassia f istula: (a) CF-raw, (b) CF-450, (c) CF-900, (d) CFP-450, and (e) CFP-900.

Figure 4. XRD pattern of adsorbents from C. f istula: CF-raw (black), CF-450 (red), CF-900 (blue), CFP-450 (green), CFP-900 (orange). Figure 5. Gas adsorption and desorption isotherms of activated carbons: CFP-450 (red), CF-900 (black), CFP-900 (blue), and CF450 (cyan).

higher than 350 °C. The exact mechanism of such loss is still not very clear and the loss could be due to various reasons that mainly include catalytic reactions and nature of medium such as inert gas environment or presence/absence of oxygen. The alkali metals in carbon themselves have catalytic effect in carbonization of biomass. Further, it is also possible that the loss occurs by other reactions with impurities that are present in precursor/biomass.24−26 The TG analysis has shown reduction/loss of weight at 230 and at 370 °C that can be mainly attributed to loss of volatiles and K respectively. The Kcontent of the raw precursor, Cassia f istula, as indicated by EDX analysis is significantly high (∼13%) and varies due to

thermal treatment in absolute amounts, and its proportion varies depending upon the relative amount of carbon in the modified materials. Thus, presence of metals in the carbon matrix can be varied depending on the temperature and nature of treatment that can affect the adsorption behavior. The FTIR transmission spectra of Cassia f istula derived adsorbents, CF-raw, CF-450, CF-900, CFP-450, and CFP-900, are presented in Figure 3. CF-raw shows a broad band at E


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Figure 6. Pore size distribution produced by different methods of activation of Cassia f istula. Data obtained by BJH method34 of analysis for CFP450 (red), CF-900 (black), and CFP-900 (blue). The insets in (a) and (b) are the respective differential pore size distribution curves of CF-900 and CFP-900 to indicate the difference of effect of chemical treatment at high temperature.

Figure 7. XPS patterns of adsorbents derived from Cassia f istula, CF-raw. The insets in (a), (b), and (c) are the XPS peaks for C 1s (285.3), O 1s (533.4), and Si 2p (102.2) found in CF-raw.

3368.2 cm−1, attributed to −OH stretching. A precise asymmetrical C−H stretching of alkane is located at 2926.2 cm−1, while the C−O stretching vibration of phenolic group is located at 1262 cm−1. In CF-raw, the broad band at 1622.7 cm−1 is due to carbonyl adsorption which is, however, shifted to lower wavenumber due to H bonding. The IR adsorption characteristic of C−O stretching of alcohol is located at 1048.94 cm−1. Finally, a well pronounced symmetric stretching due to monocarboxylates (−COO−) is located at 1417.16

cm−1. The −OH stretching band in CF-raw disappeared after heating the raw precursor at 450 °C and above in CF-450, CF900, and CFP-450. In CF-450, the peak at 1586 cm−1 is ascribable to the asymmetric stretching of (COO−) group bound to Mg2+ ion. The presence of Mg2+ ion has been confirmed by ESEM. Further, presence of aryl nitro group is confirmed by the peak at 1487 cm−1. The peak observed at 867.8 cm−1 with very weak intensity in the spectrum of CF-450 corresponds to (C−C) stretching vibration, fundamental of F


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Figure 8. Complete XPS patterns of the adsorbents indicating the presence of various heterogeneous elements: (a) CF-raw (black), (b) CF-450 (red), (c) CF-900 (olive green), (d) CFP-450 (blue), and (e) CFP-900 (orange).

Figure 10. Demonstration of percent sulfur removal capacity by CF450, CFP-450, and CFP-900 at high initial sulfur concentration (C0 ≈ 500 mg/L). Adsorbent loading of 75 g/L. Figure 9. Adsorption isotherm for CFP-450, showing initial total sulfur concentration in model fuel (MF) of 100 mg/L (single) and 300 mg/L (ternary).

stretching vibration mode of carbonate. This is again in agreement with EDAX data indicating the presence of Ca in the raw precursor. For CFP-900, a wide band centered at 3721 cm−1, associated with free −OH group, is observed as the precursor CF-raw. A peak at 3015.8 cm−1 corresponding to the C−H alkene stretching is also present. Additionally, the (COO−) peak is also detected in the spectrum of CFP-900 at 1532.3 cm−1. The presence of mineral elements like phosphorus as indicated by EDAX analysis is again substantiated by the bands at 1064 and 975 cm−1, assigned to P−O stretching modes, characteristic of PO43−. Figure 4 shows the XRD pattern of adsorbents developed from C. fistula. CF-raw shows a single distinctive peak at 2θ = 21.9° due to amorphous and crystalline mixtures of siliceous materials on the activated carbon surface.29 This is further verified by the presence of Si from XPS data and a broad one at 38.2°. X-ray diffraction profiles of CF raw showed broad peaks

acetaldehyde.27 CFP-450 shows almost similar spectrum to that of CF-450. However, some changes in the intensity and shifts in peak position are observed. In the spectrum of CFP-450, a new but a weak band at 1696.53 cm−1 is detected and is assigned to CO stretching vibration. A band observed at 1586 cm−1 due to contributions from (COO−) asymmetric stretching in CF-450 experiences a downshift in CFP-450 and is observed at 1564.75 cm−1, while the observed frequency at 1179.96 cm−1 can be identified due to C−C stretching vibration. Further, the weak intensity IR band at 987.04 cm−1 can be assigned to the rocking mode of NH2 group28 in CFP-450. A sharp IR peak at 1425.45 cm−1 is observed in the spectrum of CF-900, which can be identified due to the formation of calcite minerals in the sample as a result of activation at high temperature. This peak in conjunction with the frequency at 876.50 cm−1 can be attributed to CO G


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Figure 11. Adsorption isotherm of sulfur compounds in ternary model fuel using CF-450: sulfur as thiophene (black), benzothiophene (red), and dibenzothiophene (blue).

Figure 12. Adsorption isotherm in ternary model fuel mixture using CFP- 450: sulfur as thiophene (black), benzothiophene (red), and dibenzothiophene (blue). The composition of sulfur components in ternary model fuel was T/BT/DBT = 1:1:1.

and the absence of sharp peak indicating predominantly amorphous structure. After thermal/heat treatment at 450 °C, CF-450 shows a diffraction peak at 24.26° and new broad diffraction peaks are observed at higher angles of 29.42°, 44.7°, and 72.6°. CF-900 also exhibits the similar peaks at 24.33° but less intense and broader. Broad peaks observed at around 24° and 29° confirm nongraphitized form and the possibility of microporous structure, confirmed by the gas adsorption isotherms. The appearance of broad peaks around 24° in the XRD pattern of the powdered biomass activated as well as in the acid treated activated carbons indicates presence of carbon. However, the newly formed peaks at 29.53° and 44. 65° are sharper in CF-900. Another peak corresponding to the

presence of X-ray traceable compound, partially observed in CF-900 at 72°, becomes extremely prominent, intense and sharp in the acid activated forms, CFP-450 at 72.4° and CFP900 at 2θ = 72.37° (Figure 4) corresponding to (202) plane of phosphorus matched with orthorhombic end centered phosphorus. The XRD patterns of CFP-450 and CFP-900 indicate nearly the same peak position except for their difference in intensity. The peak at 24.45° in CFP-900 is the most intense of all and whose corresponding peaks are located at 24.45° for CFP-450. As indicated earlier, CFP-450 and CFP900 exhibit the same doublet peak around 41° and 44°. Figure 5 shows adsorption and desorption isotherms of the activated carbons prepared by thermal and chemical treatment. H


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Figure 13. Adsorption isotherms of ternary model fuel with CFP-900 as adsorbent.

It is clear that the steepness of the isotherm and the nature of hysteresis loop change with different treatment method. The carbon prepared by phosphoric acid treatment at 450 °C (CFP-450) exhibits the steepest rise in isotherm in initial pressure and a small rise after P/P0 value of 0.9. The amount of nitrogen adsorbed is considerably higher in CFP-450. Similarly, at relatively low pressure, P/P0 range of 0−0.02, adsorption rapidly rises, indicating the presence of micropores in CF-900. The shape reveals that CF-900 exhibits high microporosity with still large nitrogen uptake at relatively large pressures (above P/ P0 ≈ 0.9). CF-900 shows the presence of micropores, mesopores, and macropores. The overall shape of the isotherm remains the same in all the CF modifications and exhibits type I isotherm. However, the nature of the hysteresis loop differs slightly. CFP-450 and CFP-900 exhibit H4 type of hysteresis loop which indicates narrow slit pores,30,31 including pores in micropore region, and is observed for mesopores embedded in the matrix.32,33 Between CFP-900 and CFP-450, a wider hysteresis loop was associated with CFP-900 (not significant in Figure 5 due to scaling factor). CF-900 on the other hand displayed an almost overlapping adsorption and desorption isotherm. Figure 6 is a cumulative pore volume versus pore diameter plot for high temperature treated Cassia f istula and acid activated forms. The corresponding figures on the right are the differential curves, which indicate that the smaller pores have greater influence in the pore size distribution. The differential distribution curve of CFP-900 distinctly indicates asymmetrical distribution of the voids among the particles. The adsorbents in the present study showed the characteristics of both mesoporous and microporous nature which justifies the use of BJH method for study of pore size distribution. However, due to the limitations of BJH method in quantifying the distribution of microporous substances, HK (Horvath− Kowazoe) method was also studied in conjuction with BJH method to understand the microporous nature. The cumulative pore volume according to HK method for CF-900, CFP-450, and CFP-900 were respectively 0.26, 0.51, and 0.06 cc/g, which are much lower than those described by BJH method.

X-ray photoelectron spectroscopy (XPS) was used to analyze the surface elemental composition and the chemical environment/binding states of the elements present on the surface. XPS analysis indicated the presence of other heterogeneous elements like Si, Ca, Fe, Mg embedded in the carbon matrix, as shown in Figure 7, and the presence of carbon, oxygen, Si, and traces of elements like Ca, Fe, Mg, and Na. XPS spectra of CFraw reveal the C 1s Gaussian peak centered at 285.3 eV which is typical of sp3 hybridized carbon atoms. It also corresponds to C−N and CO interactions. From XPS results it is seen that Si peaks are more prominent and sharper in raw form. Actual aliphatic C 1s peak occurs at (284.6 eV),35 and the shifts are produced due to surrounding interaction with other elements. The binding energy responsible for the peak of C 1s in CF-450 (Figure 8) is located at 284.1 eV, signifying the presence of elemental carbon. It displays a Ca 2p peaks in doublet, confirming the presence of an oxide/hydroxide form of Ca.36 The O 1s peak of CF-450 at 531.2 eV can be attributed to CO bonds.37,38 XPS scan of CF-900 also indicated the presence of elements like Ca, Mg, Fe, Si, O, and C. Thus, with single raw material Cassia f istula, one can tailormake adsorbents in such a way that the developed materials have different porosities and surface areas. It is seen that CFP450 has the maximum surface area of 1113 m2/g, pore volume of 1.78 cm3/g, and micropore volume of 0.564 cm3/g and consequently the maximum micropore surface area. In general, the BET model has less applicability to micropores, especially for pores of less than 2 nm, and in the present study, materials such as CFP-450 and CF-900 have high micropore volume, indicating significant contribution of micropores in the adsorption. However, the high temperature treatment of chemically activated form, CFP-900, resulted in reduction of surface area and total pore volume/micropore volume. Further, CFP-900 indicated relative enlargement of the pore as compared to CF-900 and CFP-450. Simple charring of the oven-dried precursor at a lower temperature of 450 °C resulted in adsorbent with low surface area. Apart from that, CF-450 has the least adsorption energy. There is also a profound increase in the micropore surface area in CF-900 and CFP-450 which is reduced again in CFP-900, indicating that I


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Figure 14. Gas chromatography analysis of ternary model fuel having total sulfur content of 100 ppm. (A) GC trace of ternary fuel before adsorption. Three major peaks (retention time of 5.35, 12.27, and 21.96) corresponding to thiophene, benzothiophene, and dibenzothiophene are observed. (B) GC trace after adsorption with 0.4 g of CFP-450 shows similar peaks at retention times of 5.35, 12.27, and 21.96 but with lesser intensity. Peak at retention time of 21.96 min corresponding to DBT is almost negligible.

acid activation after heat treatment at 900 °C leads to closing of certain micropores. 3.2. Deep Desulfurization Using Newer Adsorbents. Adsorptive desulfurization using CF-450, CF-900, CFP-450, and CFP-900 was studied at ambient temperature of 30 °C by adding predetermined amount of adsorbents to 20 mL of model fuel with total equilibration time of 16 h using continuous stirring at 120 rpm. The sulfur removal capacity of CFP-450 was found to be highest among the different adsorbents. An adsorption capacity of ∼7.5 mg S/g adsorbent was obtained with CFP-450. This is quite comparable with commercially available sulfur specific adsorbents such as Shirasagi GH2x.11

Figure 9 shows individual adsorption isotherm of CFP-450. It is evident that the isotherm of BT and DBT in single solute model fuel using CFP-450 exhibits type I behavior of IUPAC classification which is characterized by an initial steep rise in adsorption, approaching a limiting value when the surface of adsorbent is saturated. However, the nature of adsorption isotherm for thiophene appears to be markedly different, though the initial region shows similar behavior as that of BT and DBT. The difference in isotherm characteristics of each adsorbate is crucial to selectivity. Figure 9 also depicts individual isotherm for T, BT, and DBT in ternary mixture, shown by hollow symbols, comprising T, BT, and DBT (initial concentration of 100 ppm each; total S concentration of 300 J


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no/negligible adsorption for DBT and the highest adsorption for thiophene. To elucidate the difference in the char form of carbon, CF450, the adsorption isotherm of the sulfur compounds thiophene (T), benzothiophene (BT), and dibenzothiophene (DBT) in the ternary MF using CF-450 char is shown in Figure 11. The nature of the curve depends on nature of adsorbent− adsorbate interaction. The capacities observed at lower S concentration with different components are rather low. Thus, the adsorption of the sulfur compounds, displaying a sigmoidal adsorption curve belonging to type V isotherm, indicates weak adsorbate−adsorbent interaction at low equilibrium concentration and increasing interaction/adsorption at higher equilibrium concentration which is due to capillary condensation. Adsorption of different sulfur compounds like thiophene, benzothiophene, and dibenzothiophene from model fuels in ternary system showed selectivity of the sulfur compound as DBT > T > BT. A selectivity reversal only with DBT is seen from the results of Figures 10 and 11 for CF-450, probably due to concentration effects. The equilibrium data for ternary model fuel with CFP-450 (Figure 12) indicate that individual loading of T and BT typically decreases in the presence of another component, possibly due to competitive adsorption. The trends observed for individual adsorption are similar to those observed in the individual/ternary isotherm data of Figure 9 and Figure 10. The results of adsorption in ternary system, however, indicate the possibility of using single component adsorption data in multicomponent adsorption. The results of ternary MF (1:1:1) for CFP-900 are shown in Figure 13. It can be observed that the trend in phosphoric acid modified carbons is similar, and the selectivity has order DBT > BT > T. Also, it can be seen that CFP-450 has the highest capacity for the removal of sulfur compounds as compared to CF-450 and CFP-900 that can be attributed to better pore characteristics of CFP-450 compared to others apart from phosphoric acid modification. The CFP-450 has the highest P content followed by CFP-900, which is confirmed by the FTIR and EDAX studies. DBT was not detected in the ternary mixture of the model fuel (100 ppm, total S) when the adsorbent loading of CFP-450 was increased to 20 g/L, indicating selective adsorption of DBT

Figure 15. Effect of adsorbent dose on TOC removal of dye wastewater: contact time, 6 h; Congo red concentration, 500 mg/L.

ppm). It is evident that the behavior in the mixture is similar to that in the single component systems. Since CFP-450 has high capacity for sulfur removal, which can be mainly attributed to acid modification by phosphoric acid, single component model fuel studies with this adsorbent were investigated in detail. However, for comparison, we have studied CF-450, CFP-450, and CFP-900 in ternary model fuel where the performance of the adsorbents can be compared with the single component adsorption. Among the five adsorbents investigated, CFP-450 has the maximum specific surface area of 1113.0 m2/g. We try to tailor the adsorbent surface and investigate the effect of calcinations temperatures at 450 °C and another at a slightly higher temperature of 900 °C in either the absence or the presence of H3PO4. Different modes of thermal activation were studied to find out whether improved volatilization or enhanced surface activation energy at higher temperatures could improve the adsorption. Desulfurization was good in CFP-450 and CFP900. The effectiveness of the adsorbents at a high initial sulfur concentration of C0 ≈ 500 mg/L (Figure 10) indicated that the phosphoric acid modified forms have higher sulfur adsorption for each of the components in the order of T < BT < DBT. The trend is however different for charred form, CF-450, showing

Figure 16. Effect of initial dye concentration on COD, color, and TOC removal, (a) using CFP-450 with adsorbent loading of 12.5 g/L and (b) using CFP-900 with adsorbent loading of 25 g/L and contact time of 6 h in both cases: COD (gray bars), color (pink bars), and TOC (yellow bars). K


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Figure 17. Effectiveness of various adsorbents in treating industrial effluents, (a) with industrial effluent stream 1 and (b) with industrial effluent stream 2. The adsorbent loading was maintained at 25 g/L for both cases.

of 50−500 mg/L. The results reveal that developed adsorbents can effectively remove dye and can be useful for dye− wastewater treatment. CFP-450 has a wide range of operating pH conditions, pH 2 to pH 9, with maximum COD reduction at pH 3. Interestingly, another maximum at pH 9 was also observed. Similar maxima at pH 2 and pH 9, corresponding to maximum COD/TOC (∼90%) and color removal (∼100%), were also observed for CFP-900. Figure 15 shows that decolorization with CFP-450 and CFP900 is independent of dye concentration up to 500 mg/L. Effect of dye concentration (50−500 mg·L−1) indicated the extent of removal of COD/TOC sharply rising with increasing initial concentration of Congo red (Figure 16). Effective decolorization of 90−99% was observed. The increased dye removal efficiency as represented by TOC/COD removal with increasing dye concentration for both adsorbents is useful from a commercial application point of view. 3.4. Application in Real Industrial Wastewater Treatment. Most studies in the literature report the application of adsorbents using synthetic wastewaters containing a single pollutant. However, it is important to evaluate the materials for the treatment of real industrial wastewaters.41 Considering the newer forms of adsorbent materials developed in this study, a case study using two different industrial wastewaters from a dye manufacturing unit in India was selected for removal of COD and ammoniacal nitrogen, and the results were compared with two commercially established activated carbon adsorbent materials: NORIT and TAC. Norit is a pellet form having surface area of 905 m2/g. TAC is a coconut shell derived granular activated carbon produced by steam activation and also has modification in the form of impregnation of specialty chemicals/metals to provide characteristic functions. It has surface area of the order of 1000 m2/g. The characterization of the industrial wastewater samples was carried out in detail, and the results are presented in Table 3. Adsorbent studies were carried out using the four different forms of developed adsorbent CF material and also using two commercial adsorbents, and the results of the removal of COD, ammoniacal nitrogen, and color are presented in Figure 17. It can be seen

Table 3. Characterization of Industrial Effluents Used in the Studya parameter

effluent 1

effluent 2

pH COD (mg/L) TDS (ng/L) salinity (ng/L) TOC (mg/L) color (Hazen units) NH4-N (mg/L)

6.2 48050 24.7 21.8 13285 3960, yellow color 9000

7.8 44650 11.3 9.9 11499 390000, dark brown 80

a

Statistical value of COD, color, and NH4-N exceed hugely maximum permissible limits described by environmental standards.

over BT and T. Further, the single solute model fuel of DBT with the same adsorbent loading showed the presence of DBT peak at 21.96 min with appreciable area corresponding to the residual concentration (Figure 14). Enhancing the weak physisorption between the adsorbent surfaces and weakly polarized organosulfur compounds has been a challenging issue. The activated carbon can have wide applicability, as they contain both hydrophobic graphene-like layers and hydrophilic functional groups.39,40 Figure 14 shows the GC-FPD chromatograms of the ternary model fuel, MF-T/BT/DBT analyzed before and after adsorption with 0.4 g of CFP-450. The results show total removal of DBT. The desulfurization capacity was the highest for DBT with each of the adsorbents. 3.3. Dye Wastewater Treatment. After preliminary screening and for the reasons mentioned earlier, adsorbents CFP-450 and CFP-900 were evaluated for dye removal by measuring the reduction in TOC, COD, and color (Figures 15, 16, and 17). A more detailed study was conducted using CFP450, as it has better dye removal characteristics even at lower adsorbent dose, although CFP-900 also showed high removal. Again, functionalization of the surface is found to play an important role as compared to the surface area or other physical characteristics. CFP-450 was found to demonstrate almost total removal of color (>99%) within a few minutes of contact with the dye in the selected domain of pH range and concentration L


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Industrial & Engineering Chemistry Research Author Contributions

that for effluent 1, CFP-450 and CFP-900 have shown a better ability for color and COD removal as compared to the two commercial adsorbents, whereas CF-900 has shown better ammoniacal nitrogen removal. However, for effluent 2, all the developed adsorbents were effective in removal of ammoniacal nitrogen though the ability for COD removal was limited. The results also confirm the previous inference that phosphoric acid modification plays a crucial role in the removal of pollutants. The difference in the removal of ammoniacal nitrogen for the two industrial streams can be attributed to the differences in the nature of nitrogen containing pollutants in the effluents. It is also evident that not all adsorbent materials are suitable for removal of either COD or ammoniacal nitrogen or both and that the selection of type of adsorbent is most critical to its application in industrial wastewater treatment, a factor crucial in the development of newer materials/adsorbents.

Laxmi Gayatri Sorokhaibam and Vinay M. Bhandari conceived and developed the project with Vivek V. Ranade coordinating the project. Monal S. Salvi, Saijal Jain, and Snehal D. Hadawale conducted part of the experimental work and characterization. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of Five Year Plan projects IndusMagic (CSC0123) and SETCA (CSC0113) of Council of Scientific and Industrial Research (CSIR), Government of India.

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4. CONCLUSIONS Development of newer adsorbent materials from a renewable biomass resource, Cassia f istula, has been successfully demonstrated. The results clearly highlight newer properties of the developed adsorbents and their utility in various applications from process separations to wastewater treatment. The important conclusions are the following. 1. A single raw material, Cassia f istula, can be used to tailormake adsorbents in such a way that the developed materials have different porosity and surface area. 2. CFP-450 has a maximum surface area of 1113 m2/g and excellent adsorption properties comparable to wellknown commercial adsorbents. 3. Phosphoric acid modified forms of CF have high sulfur adsorption capacity in the order of T < BT < DBT. 4. CFP-450 has excellent dye removal characteristics and has a wide range of operating pH condtions, from pH 2 to pH 9. Near total dye removal can be obtained using CFP-450 and CFP-900. 5. The developed adsorbents were demonstrated to have good potential in treating real industrial wastewaters from the dye industry. It was also shown that these compare well with or perform even better than some of the important and widely used commercial adsorbents. 6. The developed adsorbents can have wide applications for removal of COD and ammoniacal nitrogen from wastewaters of various industries. 7. The present study has clearly indicated newer forms of highly specific, tailored pore structures along with the hydrophobic nature of the adsorbents. With excellent capacity for removal of organics, it is believed that the material can be investigated further in various process separations and environmental pollution control applications.

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ABBREVIATIONS AC = activated carbon BT = benzothiophene CF = Cassia f istula CF-raw = dried powdered biomass of Cassia f istula CF-450 = dried powdered biomass of Cassia f istula heated at 450 °C CF-900 = dried powdered biomass of Cassia f istula charred at 900 °C CFP-450 = acid treated dried powdered biomass of Cassia f istula heated at 450 °C CFP-900 = acid treated dried powdered biomass of Cassia f istula charred at 900 °C COD = chemical oxygen demand DBT = dibenzothiophene MF = model fuel MF-T = thiophene in n-octane MF-BT = benzothiophene in n-octane MF-DBT = dibenzothiophene in n-octane MF-T/BT/DBT = ternary model fuel of thiophene, benzothiophene, and dibenzothiophene T = thiophene TOC = total organic carbon REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*L.G.S.: tel, +91 20 25902171; fax, +91 2025893041; e-mail, [email protected]. *V.M.B.: e-mail, [email protected]. Present Address †

L.G.S.: Applied Chemistry Department, Visvevaraya National Institute of Technology Nagpur, Maharastra 440010, India. M


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Development of Newer Adsorbents: Activated Carbons Derived from

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