Comparison of the Surface Features of the Three Chemically Modified Silk Cotton Hull Activated Carbons

Introduction

Various methods are used to manufacture activated carbons.  The most common precursors are coal, wood, agricultural waste by-product, peat, and polymers ^[1].  When the so-called physical activation is applied, organic material is first carbonized and then activated at high temperature with steam or CO₂ to introduce porosity ^[2,3].  Another method is based on chemical transformations during carbonization ^[4].  In this method, compounds such as sulphuric acid, hydrogen peroxide, and ammonium persulphate are used as activating agents at relatively low temperature.  This method leads to a very unique sorbent with maximum surface area (around 100 m²/g) and a very high volume of pores, all smaller than 30 A°.

The surface features of carbons such as porosity, surface chemistry, density, and mechanical stability govern the application of activated carbons as adsorbents ^[5].  The objective of this paper is to compare the surface features of three SCHACs.  The difference in the method of activation results in dramatically different structural and chemical properties of sorbents.  One carbon is obtained using the traditional treatment of SCH with sulphuric acid, the second one with the mixture of sulphuric acid and hydrogen peroxide (1:0.4) and the third one with the mixture of sulphuric acid and ammonium persulphate (1:0.1).  From the characterization studies it is demonstrated that SCHACs can be applied when sorption or separation of heavy metal ions is taken into consideration.

2. Experimental Section

2.1. Materials

The silk cotton hull was collected from the campus, Anna University, Tamil Nadu (India).   It was washed thoroughly with double distilled water to remove dust and soluble impurities.  The samples were cut into small pieces, dried in sunlight until the hulls became crisp.  Then the material was powdered by using a jaw crusher and kept it in a hot air oven at 60°C for 24 hours.  The completely dried material was then used for the preparation of carbon.  According to the literature study ^[6] the SCH were soaked in the mixture of a concentrated sulphuric acid, hydrogen peroxide and ammonium persulphate in proper ratios for AC1 & AC2.  The material is divided into three equal parts: the first part was soaked in concentrated H₂SO₄ (W/V) at room temperature (30±2°C) over night.  The carbonized material was then washed with distilled water several times to remove the free acid.  It was finally dried in a hot air oven at 120°C for 6 hours (AC).  The second part of the material was soaked in the mixture of concentrated sulphuric acid and hydrogen peroxide  in the ratio 1: 0.4 and kept in a hot air oven at 80°C ± 5 for 12 hours, and the same procedure as mentioned above (AC1) was followed.  The third part was soaked in the mixture of concentrated sulphuric acid and ammonium persulphate in the ratio 1: 0.1 and kept in a hot air oven at 80°C ± 5 for 12 hours, and the same procedure as mentioned above (AC2) was followed.  Finally, the dried activated carbons were sieved to get AC, AC1 and AC2, having an average particle size of 120µm and stored in an air tight container for subsequent experiments.

Methods

Adsorbent characterization was performed by means of spectroscopic and quantitative analyses.    The textural characteristics of the adsorbents were determined by the Brunauer Emmette Teller (BET) method, using the Quantachrome version 5.02 instruments.

Surface acidic functional groups and basic sites of the adsorbents determined by the acid-base titration method proposed by Boehm ^[7,8].  A half-gram of carbon and 15mL of NaHCO₃ (0.1M), Na₂CO₃ (0.05M) or NaOH (0.1 M) solution was mixed in conical flask, and agitated at 100 rpm for more than two days in room temperature.  An aliquot of the solution for each sample was back titrated with HCl (0.1M).  The NaHCO₃ neutralizes only carboxylic groups on the carbon surface, Na₂CO₃ does carboxylic and lactonic, and NaOH reacts with carboxylic, lactonic and hydroxyl groups.  Accordingly, the difference between the groups neutralized by NaHCO₃ and Na₂CO₃ becomes lactones, the difference between those neutralized by Na₂CO₃ and NaOH is hydroxyls.  The same procedure was carried out for the mixture of 0.5g of the carbons and 15mL of HCl (0.1M) solution to determine the basic sites of the carbon surface.  The remaining HCl solution was titrated with NaOH (0.1M) after neutralization.  Neutralization points were fixed using methyl orange solution indicator for weak base titrated with strong acid, and phenolphthalein solution for strong acid and strong base combination.

Thermal analysis was done using a TA instruments thermal analyzer SDT Q600.   An identification of the functional groups was performed, using a Fourier Transform Infrared (FTIR) spectrometer (Shimadzu FTIR-8400) in the range of 400 – 4000 cm^-1.  A scanning electron microscope (SEM) (Joel JSM-6360) coupled with energy dispersive X-ray (EDX) spectroscopy was used to obtain the surface images and the elemental composition in the adsorbent.

The pH of zero-point charge (pH_zpc) of the sample was carried out as follows: 50ml of a 0.1M NaCl was placed in a closed Erlenmeyer flask.  The effect of pH on metal ions biosorption can be explained in terms of pH_zpc determined by the method of Leyva-Ramos et al. proposed ^[9].  The pH was adjusted to a value between 2 and 12 by adding 0.1M HCl or NaOH.  Then, 0.02 g of each sample was added, and the final pH was measured after 24 h under agitation at room temperature. The pH_zpc is the point where the curve pH_final versus pH_initial crosses the line pH_initial = pH_final.

The bulk densities of all materials were determined following the procedure proposed by No et al., ^[10].  One gram of each material was placed in a 15ml tapered graduated centrifuge tube and packed gently by tapping the tube 10 times on the bench top.  The volume was recorded and the bulk density expressed in grams per milliliter (g/ml).  This procedure was carried out in triplicate and the means reported.

The chemical oxygen demand (COD) in water was determined by means of the dichromate method.

Table 1. Physical characteristics of SCHACs

Characteristics	AC	AC1	AC2
pHzpc	5.1	5.0	4.7
Particle size (µm)	< 120	< 120	< 120
Bulk Density (g/ml)	0.71	0.45	0.42
Surface area (BET) (m2/g)	102.78	107.23	57.68
Total pore volume (cc/g)	7.621	9.968	6.987
Average pore radius (A°)	5.2491	3.1579	7.9385
Total surface acidity (mequiv/g)	0.1298	0.1444	0.1609
Carboxyl (mequiv/g)	0.0571	0.0420	0.0605
Lactone (mequiv/g)	0.0679	0.1000	0.0900
Phenolic (mequiv/g)	0.0048	0.0024	0.0104
Total Surface basicity (mequiv/g)	0.1531	0.1356	0.1269

Results and Discussion

The physico-chemical characteristics of the adsorbent SCHACs are given in Table 1.  The adsorption process is affected by surface properties such as surface area and polarity.  The BET results revealed that the SCHACs are a microporous material, with a significant specific surface area.  Although the three carbons AC, AC1 and AC2 were obtained from the same wood precursors, different methods of activation resulted in completely different pore structures.  The surface area and micropore volumes of AC1 are higher than AC and AC2.  For AC2, the surface area and pore volume decreased with significant changes in the range of larger pores.  It is noteworthy that the average pore radius is higher than AC and AC1.  The observed changes in the pore sizes are the effects of the destruction of some small pore walls and creation of functional groups on the carbon surface.  The surface chemistry of carbons studied was evaluated using the “wet” methods providing information about acidic centers (Bronsted type) used here are Boehm.  The results of the Boehm titration in terms of the numbers of carboxylic acids, phenols, lactones and bases are collected in Table 1.  The results clearly show all the three initial carbons have significant number of surface groups; the overall lower acidity of the AC compared to AC1 and AC2 is demonstrated by its smaller number of strong acids and larger number of base.  The oxidation resulted in significant changes in surface chemistry toward an increase in acidity.  In the case of AC1 and AC2 the number of acidic groups increased significantly and its total value is found to be greater than total surface basicity.  The pH_zpc of AC1 & AC2 decreased markedly due to an increase in the amount of the surface acidic oxygen-containing groups ^[11] and this was caused by the increase in carboxylic acid groups and protonation of functional groups as indicated by the following equations.

-RCOOR¹  +  H₂0       H⁺           RCOOH + R-OH

-ROH   +   H₃O⁺                           -ROH₂⁺    +   H₂O

Activated carbons with acidic character and lower pH_zpc value, can release protons to form net negative surface charges, which is beneficial for the aqueous metal cation adsorption.  Bulk Density may not directly relate to the carbon’s effectiveness for removing heavy metals but they are important to their commercial utilization.

Table 2. Weight loss at various temperatures (%) using TG method.

Temperature	AC	AC1	AC2	Proposed group
41 – 95°C	14.32	19.45	24.53	Chemisorbed water
250 – 487°C	34.25	41.44	34.76	Carboxylic acid
479 – 627°C	8.65	18.55	12.77	Lactone
685 – 760°C	–	6.88	16.20	Carboxylic anhydride
773 – 864°C	8.17	4.35	10.85	Phenolic hydroxyl
765°C	–	–	0.33	Residue
> 984°C	26.17	8.09	–	Residue may be quinone

Table 3 FTIR peak assignments of functional groups on carbon surfaces

Group or Functionality	Wave number assignments (cm-1)
Carboxylic acids
C=O (Stretching)	1600 – 1800
O-H (Stretching)	3420
Quinones	1616
C-H (Stretching)	2913
Lactones
C=O (Stretching)	1709
Phenolic groups
C-OH (stretching)	1207
O-H (Stretching)	2500 – 3620
Acidic Hydroxyl group	3851
Hydrogen bond with water molecules	2421
Sulphate group	1115/610
Organic Sulphates	1378
Isocyanate	2279

Thermal Analysis

The effect of oxidation on surface chemistry is also demonstrated from the analysis of DTA curves obtained from the measurement in nitrogen.  The TGA for AC, AC1 and AC2 reveals abrupt changes located around 70, 360, 588, 708 & 827ºC as shown in figure 1.  This behavior demonstrates the successive decomposition of different species (e.g., H₂O, CO₂, CO).  Using the data obtained from, ^[12] calculations were made to show the presence of chemisorbed water, carboxylic, lactone, phenolic, carbonyl and quinone group (Table 2).  On the TGA curve for AC, AC1 and AC2 indicates a loss of chemisorbed water 14.32, 19.45 and 24.53% between 41 – 95ºC.  Here, the water is adsorbed by the surface functional groups on the carbon by hydrogen bonding.  Once the water molecule is adsorbed on the primary centre, it becomes a secondary adsorption site and the clusters are formed thus enhancing the heavy metal adsorption.  Also, we noted that maximum weight losses (34 – 42%) at 360ºC which is due to the decomposition of carboxylic group with the evolution of CO₂.  The maximum weight loss percentage is found in AC1 (41.44%).   The other three significant weight losses, the first (500 – 600ºC), the second (600 – 750 ºC) and the third (800ºC) corresponds to the presence of lactone, carboxylic anhydride (another form of carboxylic acid), and phenolic group resulting in the emission of CO₂ and CO.  Here, the maximum weight losses occur in AC1, 18.55% (509ºC) for lactone and for AC2, 16.20% (730 ºC) for carboxylic anhydride and 10.85% (810ºC) for phenolic group.  At the end of the analysis, 26.17 & 8.09% for AC & AC1 (984ºC) and 0.33% for AC2 (765ºC) of the starting sample was left (Table 3).  Here, the residue weight percentage decreased from AC < AC1 < AC2 at 984ºC.  This shows that the formation of more acidic oxygenated groups in AC1 and AC2 occurred during modification.  Although the total weight losses for these carbons are similar, the differences in surface chemistry are pronounced in the larger weight loss at lower temperature where, strong acids are supposed to decompose and the smaller at higher temperature for AC2 and AC1 compared to AC.  As expected based on the titration results, the AC2 and AC1 is more acidic than AC.

Fourier Transform Infrared Spectrum

Fig.2 presents the fundamental peaks of the adsorbent.  The broad band at 3420 – 3450 cm^-1 is attributed to the stretching absorption of the O-H due to inter and intramolecular hydrogen bonding of the polymeric compounds, such as alcohols, phenols and carboxylic acids.  The sharp peak at 3851cm^-1 is due to the presence of strongly acidic hydroxyl groups.  The band at 1600 – 1800cm^-1 suggests the presence of the stretching C=O vibrations arising from groups such as carboxylic acids, lactones and quinones.  The small peak observed at 2913cm^-1 denotes the presence of the stretching C–H vibration in the quinone group.  The bands at 1709cm^-1 show the presence of the stretching C=O vibration from the lactone group.  The sharp bands at 1616 cm^-1 are due to the asymmetric stretching of carboxylate anion vibrations ^[13].  The vibration at 1207cm^-1 can be attributed to the deformation of C=O, and the stretching formation of the –OH of carboxylic acids and phenols.  The strong C–O band at 1037 cm^-1 due to    –OCH₃ group, also confirms the presence of the lignin structure ^[14] which was found in AC1 and AC2.  The additional peak at 810 cm^-1 can be assigned to the bending modes of aromatic compounds.

Also, the peaks at 1378cm^-1 and 1115cm^-1 indicate the presence of sulphur-oxy compounds which possess a positive charge on the sulphur and a negative charge resonating on the oxygen atoms, making them excellent for adsorption where ionic species are involved ^[15].  A peak at 2279 cm^-1 is found in AC2 which shows the formation of isocyanate group during the modification process.  Thus FTIR analysis revealed that –OH, C-H bending, and -C=O, COO-, O-H, S-O, -N=C=O stretching were mainly responsible for the adsorption by the SCHAC.

SEM images and EDX spectra

The surface morphology and porous structure of the different carbons were studied by SEM.  It is clear from Fig.3 that SCHACs has the microporous structure.  From EDX analysis, the carbon content is found to be decreasing from AC < AC1 < AC2, suggesting that during the modification process, caused the release of organic compounds such as hemicellulose and lignin.  The results observed in this study are in complete agreement with the findings of Chen and Wu ^[16].  Thus from the EDX spectra there is reduction in carbon but an increase in oxygen content.

3.9. The effect of the modification process

To evaluate the toxicity of the adsorbed solutions, their initial and final COD values were determined.  It was found that for the metal ion solution adsorbed by AC, AC1 and AC2, the COD decreases sharply from 282 mg O₂ L^-1 for AC to 60 mg O₂ L^-1 for AC1 and 50 mg O₂ L^-1 for AC2 respectively, indicating the lower toxicity of the solutions after adsorption.

4. Conclusion

In this study, the low values of temperature (80°C) used during the heat-treatment process make this material potentially very useful for applications to remove contaminants from water in a cheapway.  From our data, it can be concluded that the adsorbents treated with various oxidising agents enable us to modify the chemico-physical properties of the materials, and it greatly enhances the adsorption capacity.  The FTIR, TGA analyses and Boehm titration results confirmed the modification process.  Also, it decreased the released COD of the SCH to an accepable level for wastewater.  The BET surface area increased for AC1, whereas it decreased for AC2 after oxidation; however, the total adsorption capacity increased by the total surface acidity facilitating the retention of heavy metal ion that form a complex with the surface groups ^[17]. Also, these render the SCHACs having more acidic sites to release protons to exchange with heavy metal ions in the adsorption process. 

Nomenclature

SCH                Silk Cotton Hulls

SCHACs         Silk Cotton Hull Activated Carbons

SEM                Scanning Electron Microscope

EDX                Energy Dispersive X-ray Spectroscopy

FTIR               Fourier Transform Infrared Spectroscopy

BET                 Brunauer Emmette Teller

COD               Chemical Oxygen Demand

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