Activated Carbons from Waste Tyre Pyrolysis: Application

Abstract

The development of better and efficient methods of consuming less and/or wasting little resource materials is becoming more important. In this study, pyrolytic waste tyre carbon black residue and commercial grade activated carbon were characterized and evaluated against adsorption of mercury vapor. The performance of the raw carbon black residue and the activated carbon against mercury vapor generated in the laboratory was determined using a designed reactor system. The adsorption of Hg+ was investigated at temperatures ranging from 200 to 280°C for 6 hours. Batch experiments were conducted for the different carbon residue samples and characterization analysis were done before and after adsorption using the spectroscopic, microscopic, and structural techniques to elucidate the structural arrangements and properties of the carbonaceous materials. Spectroscopic analysis of these carbonaceous residues showed a C=C stretching vibration attributed to the lignocellulose aromatic ring at 1657–2000 cm−1. Comparatively, it was also observed that the Infrared spectrum of raw carbon black exhibits less functional groups as compared to the H2SO4-AC and H2O2-AC carbonaceous residues prepared.

1. Introduction

The continually growing global economies inevitably results in a steady increase of the automobile industry, which unavoidably produces huge amounts of end-of-life waste tyres. It is estimated that about 1.5 billion waste tyres are produced annually worldwide [1]. In South Africa, the majority of these waste tyres are landfilled and do not readily decompose, contributing to both land and air pollution [2]. The carbon black currently produced via pyrolysis has not been put to any significant use as to justify its economic/environmental production from the plant operations. On the hand, mercury (Hg) is a known neurotoxin and can be dangerous once it is released into the environment [3]. It is a toxic heavy metal that bio accumulates in organisms and causes brain and liver damage if digested or inhaled by human beings. The efficiency and sustainability of control and management of mercury vapor release from compact flourescent lamp waste in South Africa, however is not adequately documented and therefore specific studies aimed at developing cheaper and locally available technology to control this are necessary.

The phenomenon of mercury, (Hg vapor) Hg-vapor sorption is well recognized and accepted. However, vapor sorption of many vapors is a well-recognized physical principle. Gaseous materials such as Hg vapor diffuse into a solid host material because of differences in concentration. This driving force for adsorption continues until equilibrium conditions are established between the vapor phase and the solid phase. The solubility of one material in another is generally governed by the rule “like dissolves like.” Mercury vapor exists as a nonpolar molecule that would likely be most soluble in nonpolar substances such as other metals and nonpolar hydrocarbons [4]. At first glance the new energy efficient lighting technology appears to be the better way to reduce energy consumption and protecting the environment. However, the serious mercury management concerns associated with use of CFL lamps needs to be considered and addressed. Depending on the type of lamp, a longitudinal compact fluorescent lamp may contain mercury from greater than 0.1 up to 100 milligrams (mg). Although cleaning and recovery methods have been developed and are implementation stages, efficient levels of the containment of the released mercury vapor during the recycling processes are not clearly known. Therefore there is a need for the development of a cheaper and locally available mercury recovery adsorbent material from waste tyres pyrolytic carbonaceous residues for use at recycling facilities in South Africa. This study therefore seeks to investigate the use of carbon black residue from waste tyre pyrolysis as an adsorption agent for the removal of mercury vapor from fluorescent lamp waste recycling, yet another waste hurdle, the country is grappling on.

2. Materials and methods

2.1 Preparation of waste tyre carbon materials

Two forms of carbon residues were considered during this research study on the characterization and development of a sustainable adsorption material for mercury vapor recovery. One form of the carbon residue was sourced from waste tyre pyrolysis by-product, the carbon black residue by RECO, a waste to energy Solution Company based in Pretoria, Gauteng province. The other carbon residue sample was a commercially available activated carbon collected from RECLITE, a fluorescent lamp waste recycling company based in Germiston, Johannesburg, Gauteng Province in South Africa. The company, RECLITE, has imported the fluorescent lamp recycling plant, including the activated carbon filter cartridges from Sweden. The commercial grade Activated Carbon was produced and supplied by MRT Systems, Swedish based company specializing in technology and equipment for the safe disposal/recycling of mercury containing waste products. All chemicals and reagents were obtained from Sigma-Aldrich Merck and used as received unless stated otherwise. Sulfuric acid (95–99% H₂SO₄), hydrogen hydroxide (H₂O₂), and 32% hydrochloric acid (HCl). The experimental materials include the following: Electric Oven (0–300°C), 100 mL volumetric flasks, de-ionized water, desiccator, vacuum filter, drying oven (Temperature 90–120°C for drying Carbon black C\char), and the Carbon black (CB) was collected RECOR. The carbon black was taken into the oven (Scientific Oven Series 2000 Labotec SA) at 120°C to dry. After drying, it was crushed and sieved into smaller pieces of about 0.85–0.1 mm internal diameter (Figure 1A and B). The as-prepared sieved carbon black material was stored in an Oven at 100°C prior to activation.

Figure 1.

(A) Grinding of commercial grade AC; (B) prepared raw and commercial AC samples ready for characterization.

2.2 Activation of carbon black

Activated carbon (AC) was prepared from waste tyre pyrolytic carbon black using one-step chemical activation method [5]. The two different chemical activating agents, Sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) were used in this study, in a 1:4 ratio (carbon black: agent).

2.3 Activation of carbon black (CB) using sulfuric acid and hydrogen peroxide

In a typical experiment, approximately 15 g of dried-sieved carbon black was impregnated with 100 mL of H₂SO₄ and the impregnation mixture was kept for 12 hours in an oven at 100°C as shown in Figure 2. Therefore, the mixture was further heated in an oven for 6 hours at temperatures ranging from 200 to 280°C. Consequently, a black bubbles-like structure of activated carbon was produced and was allowed to cool. Upon cooling, the activated carbon was rinsed with hot distilled water to remove acid from the activated carbon until the pH of the resulting solution was neutral, that is, pH = 7. The resultant activated carbon was further dried in an oven at 120°C overnight. The resultant product was thus named: Sulfuric acid-based carbon black-activated carbon (CB-AC 1). On the other hand, the hydrogen peroxide-based activated carbon was rinsed with cold distilled water to remove hydrogen peroxide residue from the activated carbon. The resultant activated carbon was further dried in an oven at 120°C overnight. The resultant product was thus named: Hydrogen peroxide-based carbon black-activated carbon (CB-AC 2).

Figure 2.

Samples of raw carbon black in an electric oven for chemical activation.

2.4 Characterization of carbon residues

Spectroscopic, microscopic and structural characterization methods were applied to determine the qualitative and structural properties of the carbonaceous materials in this study. The analytical methods employed have been chosen as the most suitable ones considering the instrument, material availability, and cost. The selected methods and techniques will then be adapted to enhance the accuracy and precision of analysis of the material as far as possible. The as-prepared activated carbons were characterized using Fourier transform infrared spectroscopy (Perkin Elmer, System 2000 FT-IR, USA), scanning electron microscopy/energy dispersive X-ray spectroscopy (Jeol, JSM-6390 LV SEM with Noran system Six software, USA), powder X-ray diffraction (Brker X8 Proteum pXRD), and Brunnauer Emmett–Teller methods.

2.5 Adsorption of Hg using carbon black residues

A sample of liquid mercury (4.89 g) in a 250 mL flask was placed in a water bath with controlled water temperature of 50–100°C at atmospheric conditions. The determination of the mercury vapor adsorption of the carbon residues was done through an experimental set up where the mercury vapor was generated through placing liquid mercury contained in a flask on temperature-controlled water bath. The flask was connected to a reactor glass column filter packed with the carbon residue samples and connected to an active peristaltic air vacuum pump. The experiment was run for 6 hours, Figure 3. This was repeated for all the 4 samples of the carbon residues (raw CB; saturated commercial AC; sulfuric acid activated CB; hydrogen peroxide activated CB). The same experiment set up was repeated using an electric hotplate as a heat source (0–300°C) replacing the water bath. This experiment set up was placed and run under an active laboratory fume hood cupboard to ensure safe extraction and emission of all potential fugitive mercury vapor.

Figure 3.

Experimental set up for the extraction generation and adsorption of mercury vapor on the developed AC from waste Tyre pyrolysis. (a peristaltic vacuum pump connected to the filter unit to extract the mercury vapor under the fume wood cupboard).

3. Results and discussion

3.1 Characterization of carbon black materials

The results of FT-IR analysis (FT-IR spectra) of the carbon black and its activated carbons are shown in Figure 4 (A-B carbon black-based activated carbons), (C Raw carbon black or tyre waste), and (D Saturated commercial carbon black). In Figure 4A, absorption bands around 1100 cm⁻¹ with an overtone at 1430 cm⁻¹ corresponding to Si-O-Si asymmetric stretch [6]. The C=C stretching vibration attributed to the lignocellulose aromatic structure appeared at 1657–2000 cm⁻¹ in carbon black and in activated carbons [6, 7]. Another vibration bands at 2345 cm⁻¹ appeared in both carbon black and activated carbons can be assigned to the alkyne groups [8]. However, it was also observed that the IR spectrum of raw carbon black showed less functional groups as compared to the sulfuric acid and hydrogen peroxide-based activated carbons [9]. The latter is due to the fact that, during/after activation, various functional groups are formed. The C-O-C stretching vibrations in ether, phenol and esters groups appeared around 1041–1248 cm⁻¹. In their study, [7] also reported acyclic C-O-C groups around 1300–1000 cm⁻¹ coupled with aromatic structures (C=C). The stretching vibration band at 1000 cm⁻¹ attributed to C-O group is distinct in sulfuric acid-based activated carbon. It has been reported that chemical activation with hydrogen peroxide oxidant normally yield carbonaceous structures with oxygen-functionalities on the surface [7]. In this study, oxygen-functionalities were observed in the surface of the activated carbon and are believed to have participated in the activated carbon-Hg interaction, and subsequently influenced the Hg uptake and adsorption rate of Hg by activated carbons.

Figure 4.

The FT-IR spectra of carbonaceous materials. A, B: Sulfuric acid and hydrogen peroxide-based activated carbons, respectively. C-raw carbon black, and D-saturated commercial carbon black.

BET analysis results revealed that activated carbon prepared via impregnation with hydrogen peroxide exhibits high surface area, high micropore volume, and average pore volume diameter. However, both activated carbons (CB-AC 1 and CB-AC 2) prepared via chemical activation in this study showed improved surface area and micropore volume. It is believed that the activating agents were good enough to percolates the interior of the waste tyre carbon black, breaks the lateral bonds in the cellulose molecules (increases Inter and Intra miscelle voids) and able to influence the surface properties [9]. The BET results obtained confirmed the successfulness of the chemical activation method and the influence of the chemical activating agents on the surface of carbon black. Based on the surface area and micropore volume results obtained, raw carbon black and the two as-prepared activated carbons were further used for the adsorption of Hg vapor. It has been reported that the uptake of an gaseous or liquid materials by an adsorbent normally rely on the BET surface area and pore volume, which all indicate the degree of microporosity in an carbonaceous material [7, 9].

Pyrolytic carbon black and as-prepared activated carbons were also studied under pXRD to check the crystalline arrangement of the carbonaceous materials. In the untreated carbon black (Figure 5), peaks at 2θ = 27 and 58° corresponds to the reflections from (111) and (222) planes for cubic ZnS. Additionally, peaks at 2θ = 51 and 64° corresponding to the reflections (102) and (110) planes, respectively, are of the wurtzite phase of ZnO (F.A. [10]). It has been reported that Zinc Oxides are normally added as vulcanization agents/catalysts during tire manufacturing and they decompose to different forms of ZnS during pyrolysis [6]. However, as shown in Figure 5, the diffraction patterns depict broad bands centered around 2θ = 24, 25 and 45°, associated or attributed to (002) and (100 and/or 101) planes, respectively [11]. These denote the stacking height Lc and the lateral size of the crystallites La in the carbonaceous material [12]. The presence of a predominantly porous structure, especially in H₂O₂-AC, is confirmed by the sharp peak around 2θ = 25° [12]. Naturally, carbon black is a lignocellulosic material (containing interlinked cellulose, hemicellulose, and lignin structures). However, the introduction/use of an activant (i.e. H₂O₂) normally brings the breakdown and alteration of the 3D linkages between the chemical components of carbon black (cellulose, hemicellulose, lignin), thereby forming skeletons of fully disorganized carbonaceous materials (activated carbon). Furthermore, the X-ray diffraction patterns of saturated carbonaceous material (SATU-CB, Figure 5C) shows relatively broad, weak and low intensity peaks as compared to raw carbon black or H₂O₂-AC, confirming the interaction of carbonaceous material with adsorbate. In comparison, [6] also studied the crystallographic structure of activated carbon prepared via KOH activation. A broad hump peak was observed around 2θ = 22°, which was attributed to the amorphous nature of the carbon material before activation. After KOH treatment, sharp peaks around 2θ = 23° were observed and assigned to the turbostratic structure and corresponds to the (002) reflection.

Figure 5.

Powder XRD patterns of pyrolytic carbon black and activated carbons.

The surface morphology and elemental content of the carbon black and as-prepared activated carbons were determined using field emission scanning electron microscopy-coupled with energy-dispersive X-ray spectroscopy (FESEM/EDX). Figure 6A and B shows the SEM image and the EDX of carbon black used in this study, respectively. According to Figure 6A, carbon black exhibits a dense and rough porous structure with very limited pores. EDX confirms a highly carbon-dominated material (carbonaceous), having high carbon content and minimal presence of other elements such as sodium (Na), silicon (Si), and Sulfur (S). Upon activation with hydrogen peroxide and sulfuric acid, the as-prepared activated carbons were also analyzed with FESEM/EDX. Figure 6A and A1 shows the FESEM/EDX results of the activated carbon prepared by hydrogen peroxide activation before adsorption of Hg. As shown in the image (Figure 6A), the surface morphology of the resultant activated carbon is dominated by increased and improved irregular cracks and pores with many shapes and sizes [9]. This type of porous morphology confirms that activation with hydrogen peroxide was successful and has produced carbonaceous material with improved porous structure (as shown in the BET analysis). The advantage of field-emission scanning electron microscope (FESEM) is that, it studies the structures of carbonaceous materials to microporous level. After adsorption it was expected that some Hg will be spotted and noticed on the surface of the activated carbon. As shown in Figure 7B and B1, the surface of the activated carbon is covered by dense and light materials of Hg. To that effect, the light spots of Hg infused in the pores of the activated carbon were also deeply checked and focused with high resolution of the microscopy and EDX (Figure 7). Figure 8 illustrates the round-ball like structure of Hg found inside the pores of the activated carbons. Furthermore, the backscatter electron technique was applied to further analyze the surface of the materials against Hg. Light and heavy Hg was seen and confirmed by EDX analysis (Figure 7C and D, respectively). Mercury is known to show dense and lighter spots when screened by the backscatter electrons (Figure 9). In the study by [13], it was revealed through the SEM/EDX analysis that high elementary mercury vapor concentration was found in synchronicity with high sulfur content in a mercury-saturated carbon material as very white spots in the SEM images [13]. In this study, it can been concluded that the interactions between Hg and the activated carbons were influenced and facilitated by heterogeneous surface (oxygen-functionalities) of the adsorbent and its narrow micropores (Figure 9). Due to the heterogeneous surface nature of the activated carbons and variety of oxygen-functionalities, the adsorption of Hg is believed to have involved both the physisorption and chemisorption valence forces in both multilayer and monolayer fashion. Recently, reported the preparation and subsequent chemical activation of carbon black via pyrolysis of waste scrap tyres. In order to achieve complete activated material, they employed pyrolysis temperature of 600°C, and further KOH chemical activation at 800°C. It was observed that KOH solution chemical activated predominantly affected the porous structure of the resultant activated material.

Figure 6.

FESEM/EDX of pyrolytic carbon black.

Figure 7.

FESEM/EDX of CB-AC 2 before and after adsorption of Hg.

Figure 8.

FESEM/EDX of Hg in the activated carbons prepared.

Figure 9.

FESEM/EDX of CB-AC 1 before and after adsorption of Hg.

4. Conclusion

The current study presents chemical preparation, characterization and performance evaluation of pyrolytic carbon black and its as-prepared activated carbons as potential adsorbents for mercury (Hg) vapor. FTIR, BET, SEM/EDX and pXRD were employed to elucidate the structural arrangements and properties of the carbonaceous materials. FTIR analysis of these carbonaceous materials showed a C=C stretching vibration attributed to the lignocellulose aromatic ring at 1657–2000 cm⁻¹. Comparatively, it was also observed that the IR spectrum of raw carbon black exhibits less functional groups as compared to the H₂SO₄-AC and H₂O₂-AC carbonaceous materials prepared. BET analysis confirmed the effectiveness of the chemical activation method and the influence of the chemical activants on the surface of carbon black.

It was observed that the porous morphology of activated carbons prepared via hydrogen peroxide activation showed carbonaceous material with improved porous structure and was complimented by BET analysis results obtained. However, after adsorption it was expected that some Hg will be spotted and noticed on the surface of the activated carbon. However, backscatter electron analysis was employed and round-ball like structures of Hg were found inside the pores of the activated carbons. Additionally, light and heavy Hg was seen and confirmed by EDX analysis. Furthermore, the pXRD patterns confirmed that the use of an activant such as H₂O₂ normally alter the 3D linkages between the chemical components of carbon black (cellulose, hemicellulose, lignin), thereby forming skeletons of fully disorganized carbonaceous materials, thus resulting in increased porosity of the carbon residue.

Acknowledgments

The authors would like to acknowledge RECOR and RECLITE companies for providing the pyrolytic Waste Tyre and a commercial grade activated carbon, respectively.