Use of Industrial Coal Waste Materials as Adsorbents for Textile Effluent Remediation

This paper presents experimental study on six carbonaceous industrial waste samples that were obtained from a local industry in Saskatchewan, Canada. Hereafter, the samples are coded as ES1, ES2, ES3, PU, RPS and SS1 and were characterized using IR and C solid state NMR spectroscopy, nitrogen porosimetry, TGA, metal leaching analysis using ICP and point-of-zero-charge. Adsorption studies were conducted using two types of adsorptive dye probes (p-nitrophenol, PNP; and methylene blue; MB) at pH 4.60 and pH 7.00.


Introduction
Uncontrolled release of effluent from the textile industry is one of the major point sources of water pollution through discharge into aquatic environments [1].Dyes are the main chemical constituents employed in this industry and decolourization [2] of the effluent or removal of dye species is required [1].New policies are anticipated that will be enforced to address wastewater treatment on textile industries prior to discharge into aquatic environments [3].
Various techniques have been employed (cf.Table 1 in Ref [4]) for the removal of dyes from textile wastewater effluent.A recent review proposed a combination of adsorption and ozonation techniques as an effective approach [5].
Activated carbon (AC) was shown to be among the most efficient adsorbent materials for the removal of dyes via adsorption-based methods.Properties of AC include its high surface area, enhancement of electrochemical dye oxidation, coagulation, and reductive catalysis of dyes [6] [7].However, AC is relatively costly in contrast to industrial carbonaceous waste materials, as evidenced by the Journal of Materials Science and Chemical Engineering

Characterization
Thermogravimetric analysis (TGA): Thermal weight loss profiles of the samples were analyzed using a TA Instruments Q50 TGA system at a heating rate of 5˚C min −1 to a maximum temperature of 900˚C using nitrogen as the carrier gas.
The thermal stability of the respective components of the materials are reported as first derivative plots of weight/temperature (%/˚C) against temperature (˚C).
Powdered samples were mixed with pure spectroscopic grade KBr in a 1:100 wt.% ratio followed by grinding in a small mortar and pestle.Multiple scans were recorded and corrected relative to a background of pure KBr. 13  Porosimetry: Nitrogen adsorption results were obtained using a Micromeritics ASAP 2020 (Norcross, GA) to evaluate the surface area and pore structure properties with an estimated accuracy of ±5%.Approximately, 1 g of the sample was degassed at an evacuation rate of 5 mm Hg/s in the sample chamber until the outgas rate became stabilized (<10 mmHg/min).The degas temperature for the samples was maintained ~90˚C until the degas rate was below 10 μmHg/min.
Alumina, and silica-alumina standards (Micromeretics) were used to check the calibration of the instrumental parameters for low and high surface area materials, respectively.The BET surface area was calculated from the adsorption isotherm where 0.162 nm 2 was used for the surface area for gaseous molecular nitrogen [13] [14].The micropore surface area was obtained using a t-plot (de Boer method) [15].The Barrett-Joyner-Halenda (BJH) method was used to estimate the pore volume and pore diameter from the adsorption isotherm [16].
The BJH method uses the Kelvin equation and the assumption of slit-shaped pores [13] [14].Journal of Materials Science and Chemical Engineering Inductively coupled plasma-optical emission spectrometry (ICP-OES): The samples were analyzed using an Agilent 7900 ICP-MS (SOP number: Chm-

522) which followed a Standard Method for the Examination of Water and
Wastewater, Part 3125, APHA-AWWA-WEF; without modification.
Point-of-Zero-Charge (PZC): The PZC for ES1 was determined according to a method described in a previous report [17].A stock solution of NaCl (0.01 M) was prepared and 25 mL portions were transferred into 125 mL Erlenmeyer flasks.The pH of the solutions was adjusted between 2 and 10 using NaOH/HCl such that each flask had a different pH value.Approximately 100 mg of ES1 was added to each solution and was equilibrated for 48 h before the final pH was measured.A graph of final pH vs initial pH was plotted and the intersection point was recorded as the pH for point of zero charge (pH zpc ).

Sorption
To determine the adsorption capacity of the carbonaceous materials with two q m (mmol•g −1 ) is the monolayer adsorption capacity at equilibrium, K s (L•mmol −1 ) is Sips isotherm constant related to energy of adsorption and n s is the surface heterogeneity parameter.

Results and Discussion
Several factors related to the physicochemical properties of an adsorbent contribute to its effectiveness as an adsorbent which relate to textural properties and surface chemistry [18] [19].Therefore, several complementary methods were employed to test the carbonaceous industrial materials as potential adsorbents with model organic dyes (MB and PNP).

Characterization
TGA is a suitable method for materials characterization since well resolved thermal events can provide insight on the composition of components in composite materials such as supported materials [20].Mohamed et al. illustrated the Journal of Materials Science and Chemical Engineering utility of TGA for estimating the composition of cross-linker and polysaccharide in cross-linked polymer materials [21].Figure 1 illustrates the TGA results for the various carbonaceous materials where three thermal events with weight loss profiles are observed across the temperature range.The first event below 100˚C relates to desorption of water and/or vapours since such types of coal materials are known to be hygroscopic [22].The second event between 200˚C -600˚C is due to the release of volatile matter such as light hydrocarbons and/or aliphatic components, while the third event above 600˚C is attributed to decomposition of graphitic components and/or heavier hydrocarbons and non-condensable gases [23].Each of the six materials display unique thermal profiles that indicate variable composition and thermal stability of the components.The presence of heteroatoms and trace metals are anticipated to contribute to variable thermal stability of the carbonaceous framework, as indicated in studies of polyaniline and iron oxide supported activated carbon [20] [24].
Figure 2 shows the normalized IR spectra that reveals the relative differences in functional groups between the materials with variable composition.All materials exhibit -OH (3800 -3000 cm −1 ), aliphatic hydrocarbons; -CH 3 and CH 2 (3000 -2800 cm −1 , 1390 cm −1 ), carbonyl-bearing group (~1700 cm −1 ), aromatics, analyzed using this method of acid digestion in water (cf.Table 1), where the ICP results reveal that the material contains various mineral phases.The main contributions relate to aluminum, barium, boron, iron, strontium, titanium and zinc.Based on the leaching test, aluminum (1.2 ppm), boron (4 ppm) and zinc (0.35 ppm) appear to be the main mineral species leached into water by acid digestion.In all cases, there appears to be greater leaching from the carbonaceous solid using acid digestion over water and this may relate to the present of amorphous domains of the carbonaceous phase that undergoes greater dissolution over water, as reported for activated carbon materials [19].pore volume while the isotherm shape indicates a type IV isotherm, in agreement with the nature of mesoporous adsorbents according to IUPAC [31] [32].
The variable P/P˚ values (0.42 -0.75) for the hysteresis loops infer that there are differences in evaporation versus condensation within the pores [33].The capillary condensation occurs within the mesopore domains.The surface area (SA) of the carbonaceous materials is generally low and ranges between 1.67 -4.27 m 2 /g (cf.Table 3) while the average pore width ranges between 97.2 -116 Å and confirms that the materials are mesoporous with low pore volume.Tabulated values from the BET analysis are given in Table 3 for the various carbonaceous materials.
Determination of the point where the net surface charge of a material is zero is important for an understanding of the electrostatic interactions at material surfaces, especially for charged species.At pH > pH zpc , there is adsorption of positively charged ions such as H + ions or other cations due to ionization effects at the material surface due to deprotonation.The opposite is true when the pH < pH zpc , where the adsorption of OH − ions and/or other anion species occurs due to the build-up of positive charge.The results obtained in Figure 5 reveal that the pH zpc of ES1 is ca.6.40.This implies that coal materials may be more suitable for the uptake of cation species such as MB, when pH > pH pzc .This occurs near ambient pH conditions (pH ~7).Thus, various model dyes were examined to probe the adsorption affinity of neutral and cationic dyes with the various carbonaceous materials to evaluate their efficacy as potential adsorbents.The following dyes, PNP (pH 4.60) and MB (pH 7.00), were studied at variable pH conditions to understand the role of surface charge effects.In the case of MB adsorption with the carbonaceous materials (cf.[34] are of comparable magnitude (≈10 2 mg/g).The greater overall uptake of MB over PNP is indicative of the presence of Lewis base sites on the surface of the carbon framework, in agreement with the IR and NMR spectral results above.In contrast to commercial activated carbon, the carbonaceous industrial wastes reported herein show promise as adsorbents for neutral dyes and cationic species.The uptake of PNP in its ionized state was comparatively low for pH conditions above the pK a value for PNP (results not shown) which provides further evidence that Lewis base sites are present on the sorbent surface.The industrial carbonaceous waste materials reported herein are markedly less expensive relative to commercial activated carbon.The limited need of further activation or modification of such adsorbents for removal of aromatic dyes from wastewater effluent illustrates their potential utility and valorization as alternative sorbent materials.

Conclusion
Several types of carbonaceous waste materials from SaskPower were structurally characterized and their adsorption properties with PNP and MB was determined.Variable uptake and binding affinity of a neutral phenolic dye (PNP) and a cationic dye (MB) were observed at equilibrium conditions.The difference in adsorption capacity was related mainly to differences in the surface chemistry of the materials due to the presence of heteroatoms or mineral phases on the carbon framework surface sites.This work demonstrates the utility of such industrial carbonaceous waste as an alternative low cost adsorbent material for the controlled remediation of wastewater effluents containing dye-based contaminants.
types of dyes at equilibrium conditions, various initial concentrations (C 0 ) of PNP (pH 4.60) and MB (pH 7.0) were prepared in the range 0.5 -30 mM and 0.05 -3.0 mM, respectively.Approximately, 10 mg of adsorbent was mixed with 7 mL of MB dye solution at variable concentration and the mixtures were equilibrated on a horizontal shaker (SCILOGEX SK-O330-Pro) in batch mode for 24 h.The supernatant solutions were analyzed by measuring UV-Vis absorbance (Varian Cary 100) at 317 nm (PNP) and 664 nm (MB) to determine the dye concentration after adsorption (C e ).Adsorption isotherms were generated using Equation (1) and evaluated by the Sips isotherm model with Equation(2)

Figure 1 .Figure 2 .
Figure 1.Differential thermal analysis (DTA) plots (weight loss/˚C vs temperature) of TGA data for the industrial carbonaceous materials.

Figure 3 Figure 3 .
Figure 3 reveals the 13 C NMR spectral results for solids obtained under crosspolarization (CP) and magic angle spinning (MAS) conditions for the carbonaceous materials.The 13 C signatures of the industrial solids show evidence of aliphatic (0 -65 ppm) and aromatic (95 -165 ppm) carbon atoms [29] [30], in agreement with the above IR results.Peak area analyses for the two types of hydrocarbons (cf.Table 2) reveal that 13 C aromatic content exceeds the aliphatic contributions for each of the industrial materials.The carbonaceous solids reveal the presence of carbon attached to heteroatoms as evidenced by a carbonyl signature (C−O−R; ~178 ppm), along with the 13 C signature ca.60 -90 ppm, in support of the presence of C−O groups.The unique structure of the carbonaceous materials is also supported by the 13 C signatures of the framework according to variable intensity and chemical shifts in Figure 3.The 13 C NMR results are further supported by the TGA and DRIFTS results above.The nitrogen adsorption-desorption isotherms for the carbonaceous materials are illustrated in Figure4.Each of the various materials reveal hysteresis loops that close near a relative pressure (p/p˚ ≈ 0.4) that is indicative of mesoporous character.The magnitude of nitrogen uptake is relatively low which suggests low

ES1
Journal of Materials Science and Chemical Engineering

Figure 5 .
Figure 5. Point of zero charge for ES1.

Figure 6 and
Figure 6 and Figure 7 illustrate adsorption isotherms of PNP at pH 4.60 and MB at pH 7.00 with the carbonaceous materials.It should be noted that PNP

Figure 6 .
Figure 6.Sorption of PNP at pH 4.60 and 295 K using the coal materials.

Figure 7 .
Figure 7. Sorption of MB at pH 7 and 295 K using carbonaceous materials

7
(a), Figure7(b)), the isotherms were shown to reach saturation ca.0.10-0.20 mM with a sharp rise in the uptake.This trend indicates a high adsorption affinity of the carbonaceous materials for this cation dye species.Figure8shows decolourization of MB with ES2 from the isotherm in Figure7(a).The Q m values (cf.

Table 1 .
Metal analysis using ICP-MS for ES1 material using acid digestion and conventional leaching in water.
2.1.MaterialsSix coal waste samples labelled as ES1, ES2, ES3, PU, RPS and SS1, were obtained from a local industry (SaskPower) in Saskatchewan, Canada.The samples were used without any modification/purification. Methylene blue (MB), p-nitrophenol (PNP) and potassium bromide were obtained from Sigma-Aldrich Canada Ltd. Nitric acid and hydrochloric acid were purchased from EMD USA.All chemicals were used as received without further purification unless stated otherwise.

Table 3 .
BET parameters obtained from adsorption of nitrogen for carbonaceous materials at 77 K.

Table 4 .
Q m and K s values obtained for PNP (pH 4.60) and MB (pH 7.00) using Sips isotherm adsorption model at 295 K.