Gasification of Granulated Scrap Tires for the ... - ACS Publications

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Gasification of Granulated Scrap Tires for the Production of Syngas and a Low-Cost Adsorbent for Cd(II) Removal from Wastewaters Antonio Molino,† Alessandro Erto,*,‡ Francesco Di Natale,‡ Antonio Donatelli,† Pierpaolo Iovane,† and Dino Musmarra§ †

ENEA, National Agency for New Technologies, Energy and Sustainable Economic Development, UTTRI S.S. 106 Ionica, km 419 + 500, 75026 Matera, Italy § Dipartimento di Ingegneria Civile Design Edilizia e Ambiente, Seconda Università di Napoli, Real Casa dell’Annunziata, Via Roma, 29, 81031 Aversa (CE), Italy ‡ Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università di Napoli Federico II, Piazzale Tecchio, 80, 80125 Napoli, Italy ABSTRACT: In this work, the steam gasification of scrap tires was investigated as a sustainable and cost-effective alternative to tire landfill disposal. Steam activation of the char derived from the tire residues of the gasification process was carried out at constant temperature and feeding ratio between gasifying agent and char, using different activation times (180 and 300 min). The complete characterization of all intermediate products of the processes, namely, raw material (tire), gasification products (char and syngas) and activation products (adsorbents, named Tirecarb), was performed. The adsorbents obtained from tire gasification were used to remove cadmium ions from aqueous solutions at 25 °C and pH 7. The experimental results showed that the higher the activation time, the higher the cadmium adsorption capacity. Experiments were carried out under the same conditions using a conventional activated carbon, and additional comparisons with other experimental results on Cd(II) adsorption on low-cost sorbents available in the relevant literature are also reported. In both cases, Tirecarb was found to show the highest cadmium adsorption capacity.

1. INTRODUCTION The disposal of end-of-life tires entails severe environmental and technical problems. It is estimated that more than 330 millions of waste tires, corresponding to more than 5 million tons, are generated each year in the world.1,2 Currently, most used tires are still discarded in landfills, even if this option has several drawbacks. In fact, because the mean decay time of tires is equal to hundreds of years, tires represent a continuous pollution source as a result of water leaching and a potential fire hazard. Furthermore, tire landfill disposal leads to high void fraction zones that can cause instability of the landfill, undesired floating of the wastes piled on top of the tires, and a general reduction of the available storage volume. Therefore, alternative solutions aimed at maximizing material and/or energy recovery such as retreading, recycling, incineration, or reuse in civil engineering applications (i.e., playground surfaces, rubber roofs, etc.) are highly desirable.2−4 The main technological obstacles to the management of tire residues are their complex mechanical structure and the same physicochemical properties of tires. In fact, tires are designed to be extremely resistant to physical, chemical, and biological degradation, so that any technique for their recycling and/or further processing is difficult to apply.2,3 The main component of tires is rubber, a mixture of styrene/ butadiene polymers in which the carbon content is as high as 75%. Some additives are also present, among which carbon black accounts for 20−30% of the total composition.2,3,5 This structure suggests that tire scraps can be valuable raw materials both for the synthesis of carbonaceous adsorbents through thermal treatments and for the production of a gas fuel © 2013 American Chemical Society

(syngas) whose calorific value depends on the process parameters employed.2,4,6 Currently, thermal treatments such as gasification and pyrolysis allow high efficiency in electricity production to be obtained, together with a low explosion risk.7−10 In the past few years, the production of activated carbon from waste materials has received increasing attention because this process could provide an alternative solution to reutilize such materials and to overcome the high cost of commercial activated carbons.11−14 Consequently, several investigators have evaluated the possibility of producing activated carbons from scrap tires through chemical or physical processes or a combination of the two, for liquid-phase removal of dyes,6,15 hydrocarbons,6,16 and heavy metals.3,17−20 The widespread contamination of water by heavy metals represents one of the major current environmental issues worldwide. In particular, cadmium is commonly considered to be one of the most dangerous heavy metals: it is extremely toxic and unanimously classified as carcinogenic. The high toxicity of cadmium is enhanced by its ability to bioaccumulate in the aquatic ecosystem and to reach humans through the food chain.21 Therefore, many efforts have been made to find suitable adsorbents for cadmium removal from water, such as agricultural wastes, 13,22 industrial byproducts, 13,23 fly ashes,14,24,25,26 and tires.20 Received: Revised: Accepted: Published: 12154

April 16, 2013 July 22, 2013 July 30, 2013 July 30, 2013 dx.doi.org/10.1021/ie4012084 | Ind. Eng. Chem. Res. 2013, 52, 12154−12160

Industrial & Engineering Chemistry Research

Article

5142), and higher heating value (IKA C5000 bomb calorimeter). The granular activated carbon (GAC) used as a comparison material to evaluate the performance of the adsorbents produced from scrap tires is commercially available under the name of Aquacarb 207EA, produced by Sutcliffe Carbon from a bituminous coal. The textural properties of the GAC were studied by means of N2 adsorption at −196 °C (Carlo Erba SORPTOMATIC 1900) and elemental analysis (EA 1110 CHNS-O, ThermoQuest). The BET surface area was found to be about 950 m2·g−1, and the average pore diameter was 24.5 Å. The GAC chemical composition revealed a high ash content (9.58%), and the value of the pH of the point of zero charge (pHpzc) of 8 showed that the activated carbon had a slightly basic nature. The main GAC properties are reported in Table 2, whereas a complete list of the chemical and physical characteristics was reported by Di Natale et al.30 The micropore and meso-/macropore surface areas were obtained from N2 adsorption data by the t-plot and Barrett−Joyner−Halenda (BJH) methods, respectively. 2.2. Gasification/Activation Pilot Plant. The gasification and activation of the scrap tires were carried out in a pilot plant having a total capacity of 10 kg·h−1. The plant was a rotating kiln reactor, heated electrically, with an internal diameter of 0.4 m, a heated-area length of 1 m, and a volume of 0.126 m3. The rotation speed was variable between 0.5 and 3 min−1, and the temperature ranged from 400 to 1000 °C.33,34 The residence time of the solid phase in the kiln was controlled by varying the slope and speed of rotation of the drum according to the formula35

Such low-cost sorbents might be particularly relevant for the remediation of wastewaters or groundwater in pump and treat techniques and permeable groundwater barriers,27 where large amounts of sorbents are needed to treat large amounts of water with Cd(II) concentrations that are relatively low, around 1−2 mg·L−1, but still well above the maximum levels allowable for a safe use of groundwater.26 This study considers an even more challenging objective: the simultaneous development of a new low-cost sorbent for cadmium removal at low concentrations, typical of groundwater remediation, and the production of syngas characterized by a good higher heating value (HHV), namely, in the range 8−10 MJ·Nm−3. This new adsorbent was obtained by the gasification of scrap tires carried out with high-temperature steam in a rotating electrically heated kiln and operated for different activation times. The complete characterization of all of the intermediates of the process, namely, raw material (tire), gasification products (char and syngas), and activation products (adsorbents named Tirecarb), is reported to provide a detailed description of the process and to highlight the energetic, environmental, and economic benefits of the proposed tire disposal solution. To test the performances of the synthesized adsorbents, cadmium adsorption tests were carried out at room temperature. In addition, a comparison with a commercial activated carbon, already tested for heavy-metal adsorption in previous works,28−31 is proposed for a meaningful evaluation of the experimental results.

2. EXPERIMENTAL SECTION 2.1. Materials. The material used in this work was a granulated scrap tire having a size ranging from 1.5 to 2 mm obtained by cold grinding after the removal of the fiber reinforcing steel. A preliminary characterization of the tire granules was carried out using an EA 1110 CHNS−O elemental analyzer (ThermoQuest), a 2950 thermobalance (TA Instruments), and an IKA C5000 bomb calorimeter. The main properties were typical of scrap tires32 and are listed in Table 1, in which the elemental analysis is reported on a dry ash-free (daf) basis, whereas the proximate analysis is reported on a wet basis (wb).

t = (1.77 θ BL)/(SND)

where θ is the angle response of the matrix, B is the coefficient that takes into account the possible presence of elements in the reactor, L is the length of the reactor (m), S is the slope of the reactor, N is the rotating speed of the reactor (min−1), and D is the diameter of the reactor (m). The feeding system consisted of a hopper with a capacity of up to 50 dm3 equipped with a system to prevent the formation of obstructions. A screw conveyor with a variable mechanical speed was located at the inlet of the reactor for constant feeding of the material directly to the reactor. The same apparatus was used for both gasification and subsequent activation of the produced char; both steps were conducted with saturated steam at 1 bar. The syngas produced was sent to the cleaning system, a sequence of a quiet room for the removal of heavier particles, a quencher for gas cooling and condensation of water vapor and tar, and an alkaline scrubber with a 4% NaOH solution to remove acidic components. After cleaning, the gas flowed through a hydraulic guard, which consisted of a 50-mm water column that provided a slight overpressure with respect to the ambient pressure of the upstream part of the plant to avoid unwanted introductions of harmful air from outside. Finally, the gas composition was analyzed using a gas chromatograph, the gas flow rate was quantified in a flow meter, and then the gas was sent to a safety burning torch. During the experiments, a known flow of nitrogen was sent to the system, to create an inert atmosphere during the heating of the reactor and to guarantee the uniformity of gas flow (carrier) in the reactor over time. In addition, a secondary current of nitrogen was sent to the seals of the reactor to prevent any air from entering the reactor itself.

Table 1. Main Properties of Scrap Tires elemental analysis (wt %, daf)

a

proximate analysis (wt %, wb)

C

85.2

moisture

1.2

H N S Oa

7.3 0.38 3.1 4.1

volatiles fixed carbon ashes HHV (MJ·kg−1)

61.3 33.5 4.4 37.1

(1)

By difference.

The total and fixed carbon percentages suggested that the samples could be suitably used as a precursor for the production of carbonaceous adsorbents, being consistent with previous experimental findings.2,3,5,15,17 The gasification (char) and final activation (adsorbent) products were characterized in terms of Brunauer−Emmett− Teller (BET) surface area (Carlo Erba SORPTOMATIC 1900), elemental analysis (EA 1110 CHNS-O, ThermoQuest), proximate analysis (CEN/TS 14774−14775 and ASTM D 12155

dx.doi.org/10.1021/ie4012084 | Ind. Eng. Chem. Res. 2013, 52, 12154−12160


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Table 2. Main Properties of the Commercial Activated Carbon Aquacarb 207EA and the Adsorbents Produced from Scrap Tires (Tirecarb_180 and Tirecarb_300) parameter activation method bulk density (kg·m−3) effective density (kg·m−3) average diameter (mm) porosity (% v/v) BET surface area (m2·g−1) meso-/macropore surface area (m2·g−1) micropore surface area (m2·g−1) moisture (% w/w) ultimate analysis (% w/w, dry basis) C H O N S ash

Aquacarb 207EA

Tirecarb_180

Tirecarb_300

steam activation 480−520 950 1.20 16.8 950 401 549 5 87.15 87.15 0.17 2.00 1.10 n.a. 9.58

steam activation 170 340 0.4 n.a. 775.1 675.6 99.5 2.2 92.1 92.1 0.19 4.5 0.19 n.a. 39.2

steam activation 163 310 0.4 n.a. 987.0 915.9 71.1 0.8 93.8 93.8 0.16 4.1 0.17 n.a. 65.4

Figure 1. Scheme of gasification/activation pilot plant.

steam was produced in a heat exchanger using the heat resulting from the combustion of a mixture of liquefied petroleum gas (LPG) and the syngas produced by the same gasification process. The water flow required for steam generation was provided through a valve, and a flow meter was placed directly on the water feed circuit to test the experimental power ratio. The process parameters employed were selected on the basis of previous experimental campaigns of tire gasification20,33 to achieve a good balance between the production of solid (char) and the energy content of gas (syngas). The char derived from gasification was stored in a tank located downstream of the rotating drum and discharged at the end of the process. At the end of each gasification run and after the reactor had been cooled, the nonfed granules of scrap tires and the water in the quencher tank were unloaded so as to accurately determine the total mass balance of the gasification process.

An emergency hydraulic guard was placed on an emergency line parallel to the process immediately prior to the quencher, which ensured the switching of the flow of gases to a flare in case of sudden pressure rise in the system. Figure 1 provides a complete schematic representation of the pilot plant. 2.2.1. Gasification Process. The setup of the system included the loading of the hopper with tire granules, the loading of the quencher tank with a known amount of water, and the preparation of the cleaning gas line (i.e., alkaline solution for the scrubber). The reactor was electrically heated from outside, and when the process temperature (850 °C) was reached, the granulated tire was sent to the reactor through the screw conveyor. The rotational speed of the feeding system was variable and was controlled by an inverter to obtain the desired mass flow. The gasification was conducted using steam as gasifying agent, with a constant feeding ratio of 0.67 kgH2O/kgtires. The 12156



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2.2.2. Char Activation. After the gasification process, the char produced was loaded into the feeder, the reactor was heated at the temperature of the activation process (920 °C), and then the char was introduced into the reactor through the screw conveyor. To guarantee the required residence time of the char into the reactor, necessary to reach the high surface area of the final product (i.e., the adsorbent), the activation step was performed in batch mode. The char was introduced into the reactor, and it was left under a current of water vapor for the entire duration of the activation process. The activation step was conducted with steam at a constant feeding ratio of 0.87 kgH2O/kgchar. As for the gasification process, the steam was produced in a heat exchanger using the heat produced by the combustion of a mixture of LPG and syngas. For all experimental tests, a nitrogen flow of 1 Nm3 h−1 was sent to the reactor. A variable activation time, namely, 180 and 300 min, was employed to test the influence of this parameter on the adsorbent yield and on its final physical and chemical properties. The obtained adsorbents are denoted as Tirecarb_180 and Tirecarb_300, respectively. After activation, the reactor vessel was cooled, and the solid residue was discharged into the reservoir and collected. The gas produced during the activation process was subjected to the same treatment as provided for the gasification step. Finally, the residual solid product (i.e., the adsorbent) was not further modified, and after being washed, it was used for cadmium adsorption tests. 2.3. Adsorption Tests. Experimental tests aimed at the determination of the cadmium adsorption capacities of both the synthesized adsorbents (Tirecarb_180 and Tirecarb_300) and the commercial GAC (Aquacarb 207EA) were carried out in a proportional−integral−derivative- (PID-) controlled thermostatic oven, using glass vessels (200 mL) as batch reactors. The cadmium solutions were obtained by dissolving a known amount of cadmium nitrate Cd(NO3)2 in distilled water. The initial cadmium concentration (10−50 mg L−1) and the carbon mass (0.−30.5 g) employed in each run were selected so that the equilibrium concentrations were in the typical range of contaminated water. Before each experimental run, the adsorbents were carefully rinsed with distilled water and oven-dried for 48 h at 120 °C. The adsorption tests were conducted at room temperature (T = 25 °C) and with a constant equilibrium pH of 7 ± 0.3. Preliminary tests indicated that, in the presence of both Tirecarb sorbents, the equilibrium cadmium solution pH equaled 10. Under these operating conditions, species such as Cd(OH)2 are likely to precipitate. Therefore, to obtain an equilibrium pH around 7, the initial cadmium solution pH was adjusted with 0.4 mL of HNO3 solution (0.01 M), and it was not further modified during the tesst. The pH value was measured with Orion ion analyzer EA920 electronic pH meter. After each test, the liquid solutions were filtered and analyzed by atomic absorption spectrophotometry (employing a Varian SpectrAA 220 apparatus), whereas the solid was leached with 1 M HCl solution to obtain the complete extraction of the adsorbed cadmium, thus providing for a direct measure of the cadmium uptake on the solid surface. The resulting solutions were analyzed by spectrophotometry, as described previously. The accuracy of the experimental runs was checked, and a maximum error of 5% in the cadmium mass balance was allowed.

Preliminary kinetic tests carried out under the same working conditions showed that a time of 48 h was sufficient to achieve equilibrium conditions for the Tirecarb sorbents. For the Aquacarb 207EA samples, the necessary time calculated under very similar experimental conditions (T = 20 °C and pH 7) was 60 h;25 hence, this value was also employed in the present study. Leaching tests were carried out to analyze the potential dissolution of any heavy metal derived from the addition of sorbent in fresh water, indicating that the Tirecarb sorbents did not lead to any undesired water pollution. To ensure the accuracy, reliability, and reproducibility of the collected data, all batch isotherm tests were performed in triplicate, and average values only are reported. In each test, the replicates showed a variance in cadmium concentration on the order of 4% and a variance in the adsorption capacity of around 6%. Blank tests without sorbent addition showed that the experimental procedure did not lead to any reduction of cadmium concentration and pH variation unrelated to sorbent effects.

3. RESULTS AND DISCUSSION The gasification of scrap tires was proposed here to provide an alternative solution to the disposal of tires in landfills, to identify an affordable path for a cost-effective and environmentally friendly valorization of this waste material, and to synthesize a low-cost adsorbent whose adsorption capacity is comparable to that of commercial GAC. To pursue all of these objectives, all of the steps of adsorbent production were followed, and a complete characterization of all intermediates of the process, namely, raw material (tire), gasification products (char and syngas), and activation products (Tirecarb sorbents), was performed. The gasification of granulated tires leads to the production of char and syngas; the employed experimental conditions provided gas and solid yields of 67.5% and 32.5%, respectively. After water condensation, the dry gas yield was reduced to 37%; considering that the density of the dry syngas obtained was 0.52 kg Nm−3, the volume of gas produced per ton of tires was equal to 712 N m3 ton−1. The syngas composition (on a dry basis) was characterized by a large fraction of combustible compounds (H2, 54.1; CO, 15.1; CH4, 23.4, expressed as vol %) and a small fraction of noncombustible compounds (CO2, 7.3 vol %).The HHV of the syngas, calculated as the weighted mean of the HHVs of its main components,36 was around 10.8 MJ·Nm−3. These properties, together with a very high gas yield, allowed the overall process to be sustained, thus making it costeffective. The char surface area was measured by BET analysis and was equal to 34.46 m2·g−1. Table 3 summarizes the main characteristics of the char produced, in which the elemental analysis is reported on a dry ash-free (daf) basis, whereas the Table 3. Main Chemical and Physical Properties of the Char elemental analysis (wt %, daf)

a

12157

proximate analysis (wt %, wb)

C

90.0

moisture

2.1

H N S Oa

0.25 0.24 2.3 7.2

volatiles fixed carbon ashes HHV (MJ·kg−1)

2.1 87.2 8.5 31.4

By difference. dx.doi.org/10.1021/ie4012084 | Ind. Eng. Chem. Res. 2013, 52, 12154−12160


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proximate analysis is reported on a wet basis (wb). The gasification process provided a considerable increase in the fixed carbon content with respect to that of the raw tires. To improve the adsorption properties of the synthesized solid, a steam activation step was performed, using a single feeding ratio between steam and char (0.87) and two different activation times (180 and 300 min), according to the pertinent literature.6,7,33 The burnoff value of the activation gives a direct indication of the consumption of the starting material and is defined as burnoff(t ) = [M 0char − Mads(t )]/M 0char

(2)

where M0char is the initial mass of char subjected to the activation process and Mads(t) is the mass of char remaining after t minutes of activation time (i.e., the mass of the resulting adsorbent). Experimental results showed that process burnoffs after 180 and 300 min led to the consumption of 78.3% and 87% of the char, respectively. The char activation involved the elimination of volatiles and the generation of high surface porosity as a consequence of steam gasification. The final properties of the adsorbents in terms of elemental analysis and textural properties are reported in Table 2. Both synthesized adsorbents showed a considerable improvement in superficial adsorption properties with respect to those of the char. As expected, the experimental results showed that an increase in the activation time led to an increase in the burnoff, due to the progressive occurrence of gasification reactions with steam. Simultaneously, the increase in the burnoff was associated with a significant increase in ash content and a further loss of the volatile fraction, as a non-negligible increase in the BET surface area with the activation time also occurred. However, it was found that the contribution of the micropores decreased, whereas the contribution of meso-/ macropores increased. This result is likely to be associated with a depletion of the pores that had previously formed while, at the same time, new macropores were produced within the carbonaceous surface. The upper limit of this process consisted of a carbon mainly formed by macropores with a very small number of micro- and mesopores, generally considered to be unsuitable for applications to aqueous-phase adsorption. For this reason, further experimental tests with higher activation times were not performed. The adsorbents obtained from tire gasification (i.e., Tirecarb_180 and Tirecarb_300) were tested for cadmium adsorption from aqueous solutions, to verify the possibility of a practical application. As a comparison, the cadmium adsorption isotherm on the commercial GAC Aquacarb 207EA was determined under the same experimental conditions. The Cd(II) adsorption isotherms of Tirecarb_180, Tirecarb_300, and GAC at 25 °C and pH 7 are shown in Figure 2. The experimental data showed that the tire adsorbents are very efficient for cadmium capture from aqueous solutions. In fact, for both samples, the cadmium adsorption capacity was about 3 times higher than the corresponding value for the commercial activated carbon (Aquacarb 207EA). This difference is likely related to the peculiar chemical properties of the solids, as inferred by the ultimate and proximate analysis data. In fact, both tire-derived adsorbents have higher oxygen contents than the commercial GAC. This means that higher numbers of functional groups potentially having high affinity toward cadmium are present on their surfaces.29,37 Moreover,

Figure 2. Cadmium adsorption isotherms at T = 25 °C and the equilibrium pH of 7 ± 0.3. Comparison between Langmuir model and experimental data.

these sorbents have ash contents significantly higher than that of the commercial activated carbon, reaching a maximum for Tirecarb_300 (cf. Table 2), which is expected to play a role in cadmium adsorption, even if its contribution cannot be isolated.14,26,29 The comparison between the solid properties and the adsorption data shows that cadmium adsorption was not proportional to the BET surface area. In fact, Aquacarb 207EA has a higher BET surface area but a lower cadmium adsorption capacity than Tirecarb_180. More specifically, a high micropore surface area seems to exert a detrimental effect on the cadmium adsorption capacity, as the solid with the highest micropore surface area was also characterized by the lowest adsorption capacity (i.e., Aquacarb 207EA). A high micropore surface area limits the accessibility of cadmium by a steric hindrance effect. As a confirmation of these correlations, it can be observed from the experimental data that the tire adsorbent synthesized with a higher activation time (i.e., Tirecarb_300) had the highest cadmium adsorption capacity. The two tire adsorbents had very similar compositions, particularly in terms of oxygen content, but Tirecarb_300 had a higher meso-/macropore area that is likely to be the part active in cadmium adsorption. In conclusion, even if a high BET surface area, and in particular, the actually accessible part, might provide a useful “support” to host a great number of adsorption sites, the superficial composition is likely to be the controlling parameter for cadmium adsorption.29,37 The experimental data were analyzed according to the Langmuir model and the corresponding parameters derived from a nonlinear regression analysis are reported in Table 4. Finally, it is worth performing a comparison of the adsorption performances of the tire adsorbents, and therefore, the survey was extended to other low-cost sorbents whose Table 4. Langmuir Model Parameters for Cadmium Adsorption on Tirecarb_180, Tirecarb_300, and Aquacarb 207EA

Tirecarb_180 Tirecarb_300 Aquacarb 207EA 12158

ωmax (mg·g−1)

K (L·mg−1)

R2

22.72 20.38 8.445

0.135 0.467 0.0371

0.983 0.993 0.992



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appear relatively long, but it can be significantly reduced by using different technologies with higher conversion efficiencies and higher heat- and mass-transfer rates, such as fluidized bed units.42−44 The adsorbents obtained from tire gasification were tested for cadmium adsorption from aqueous solutions and then compared with the performances of a commercial activated carbon (i.e., Aquacarb 207EA) under the same experimental conditions (T = 25 °C and pH 7) and with other low-cost adsorbents available in the relevant literature, confirming the high efficiency of the scrap-tire adsorbents synthesized in the present study. Of the two sorbents, the one synthesized at a longer activation time (i.e., Tirecarb_300) showed the highest cadmium adsorption capacity. This result is likely to be related to the peculiar chemical properties of the solids and to the micropore/mesopore surface area. The experimental data showed that the tire adsorbents were very efficient in cadmium capture: the maximum adsorption capacity was about 3 times higher than the corresponding value for the commercial activated carbon, but at lower concentration, around 1 mg L−1, which is typical of groundwater remediation systems, this difference becomes more than 5 times higher. The Tirecarb sorbents outperformed the other sorbents analyzed and reported in the relevant literature. Moreover, both Tirecarb sorbents considered in this study did not lead to any undesired emission of heavy metals in water, confirming their potential as sorbents for both wastewater and groundwater remediation.

cadmium adsorption data are available in the literature. In particular, this comparative analysis was extended to include solids of different natures and properties such as another commercial activated carbon (Calgon F400),38 a coal combustion ash (CCA),14 a fly ash derived from bagasse,39 a steam-gasified coal combustion ash (steam-gasified CCA),26 dried hazelnut shells, an agriculture waste,40 a biochar produced by fast pyrolysis of oak bark,38 and a pyrolyzed sewage sludge.41 The Cd(II) adsorption isotherms of these adsorbents, under the same experimental conditions as employed in this study (i.e., T = 25 °C and pH 7), are reported in Figure 3:

■

Figure 3. Cadmium adsorption isotherms at T = 25 °C for different materials: Tirecarb_180, Tirecarb_300, GAC Aquacarb 207EA (this study), GAC F400,38 oak bark char derived from pyrolysis,38 bagasse fly ash,39 dried hazelnut shell,40 pyrolized sewage sludge,41 steamgasified coal combustion ash (CCA),26 CCA.14

AUTHOR INFORMATION

Corresponding Author

*Tel.: +39 081 7682246. Fax +39 081 5936936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

As can be observed, the adsorbent samples produced in this study were highly efficient in the removal of Cd(II) ions from water, as their adsorption capacity was the highest of all those compared.

REFERENCES

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4. CONCLUSIONS To find a viable alternative to landfill disposal of scrap tires, a steam gasification process was proposed to achieve the simultaneous production of an adsorbent for liquid-phase applications (Tirecarb) and a syngas for energy savings. Scraptire gasification was conducted with a constant value of the feeding ratio between gasifying agent and tires so as to provide an optimal balance between solid adsorbent yield and energy content of the syngas. The syngas was composed mainly by H2 and CH4, with a low fraction of noncombustible compounds, with an HHV of around 10.8 MJ·Nm−3, allowing the overall process to be sustained. The char derived from gasification was then subjected to a subsequent steam activation step. A constant temperature (920 °C) and feeding ratio (0.87) were employed, whereas two different activation times (i.e., 180 and 300 min) were tested to investigate the influence on the physical and chemical properties of the adsorbents obtained. The experimental results showed that an increase in the activation time provided an increase in the burnoff and BET surface area, even though a reduction of the micropore surface area, likely associated with a depletion of pores that had previously formed, was observed. The activation time might 12159



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