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Biomass-derived activated carbon for removal of 222Rn from air
Sustainable Chemistry and Pharmacy 14 (2019) 100193
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Biomass-derived activated carbon for removal of
222
Rn from air
Parimal Chandra Bhomick a, Supongtoshi Jamir a, Upasana Bora Sinha a, B.K. Sahoo b, Dipak Sinha a, * a b
Department of Chemistry, Nagaland University, Lumami, 798627, India Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
A R T I C L E I N F O
A B S T R A C T
Keywords: 222 Rn Activated carbon Breakthrough time Adsorption coefficient Regeneration
Radon (222Rn) and its decay products are considered harmful to humans because of their toxicity and is regard as a prior cause of lung cancer in non-smokers. This research thus focuses on the application of activated carbon synthesised from pinecone for mitigating the risk of radon in indoor environments. Characterization of the prepared carbon was evaluated using FESEM, TGA, and BET surface area and total pore volume analyzer. BET surface area of the prepared carbon was found to be 839 m2 g-1 and a total pore volume of 0.476 cm3 g-1. Fixedbed adsorption method was used to estimate the adsorbent efficiency by varying process parameters such as bed length, bed diameter and flow rate. On the basis of these parameters, breakthrough curves were generated to calculate the breakthrough time for obtaining the adsorption coefficient(K) of the prepared carbon which was found to be in the range of 3.05–4.90 m3 kg-1. Regeneration studies were done at different temperatures for 50 min which showed that heating at 70–90 � C resulted in complete degassing of the adsorbed 222Rn, with K equivalent to that of the pristine carbon.
1. Introduction Radon (222Rn), a radioactive gas having a half-life of 3.8235 days is the densest naturally occurring member of the noble gas family that continuously emanates from uranium-bearing minerals in rocks and soils (Fry and Thoennessen, 2013). 222Rn enters the indoor environment chiefly through exhalation from the soil, rocks, and building materials, and this infiltration is considered as the most significant cause of resi dential radon (Sahoo and Sapra, 2015). It is reported that indoor radon tends to saturate in enclosed spaces such as underground mines or houses and contributes about ~ 52% of the ionising radiation dosage received by individuals in general (Sahoo and Sapra, 2015). Epidemio logical studies have provided substantial evidence of lung cancer due to indoor radon exposure even at relatively low radon levels (Darby et al., 2005; Krewski et al., 2006; Obenchain et al., 2019; Sahoo et al., 2011, 2010; WHO, 2009). Though most of the radon gas inhaled is exhaled, the radon progeny decaying from radon adheres to the respiratory tract leading to tracheobronchial deposition of radon progeny in the human body causing lung cancers (Yu et al., 2000). Radon (222Rn) and its decay products are considered harmful to humans because of their toxicity, usually in the occupational sectors of thorium processing facilities and uranium mills that usually contains radioactivity levels above the
permissible level (Karunakara et al., 2015). Presently, radon is acknowledged to be the number one cause of lung cancer in non-smokers and second in smokers (Frutos et al., 2019; Obenchain et al., 2019). In this regard, the WHO has suggested to cut down the reference level of indoor 222Rn from the existing 200 Bqm-3 to 100 Bqm-3 (WHO, 2009). In recent years, therefore, the adverse effects caused by exposures to radon (222Rn) and its decay products on human as well as non-human biota have been receiving considerable attention in the world community, thereby creating considerable attention on the sub ject of 222Rn monitoring around the globe (Chen and Ford, 2017; Cinelli et al., 2015; Mick et al., 2016; Sahoo and Sapra, 2015; Singh et al., 2016; Vives et al., 2017). In this regard, the removal of radon from the air becomes quintes sential. Activated carbon was first used at the beginning of the 20th century by Rutherford in one of his experiments using a tube filled with cocoa nut charcoal for adsorption of 222Rn (Rutherford, 1906). This experiment has been used even recently not only for 222Rn removal systems but also for atmospheric 222Rn sampling and measurement of indoor and outdoor 222Rn concentration in the air (Boyle, 1907; Cosma et al., 2006, 1999; Gaul and Underhill, 2005; Hammon et al., 1975; Nakayama et al., 1994; Scarpitta, 1995; Zikovsky, 2001, 1998). How ever, though activated carbon is found to be a suitable adsorbent and
* Corresponding author. E-mail address: [email protected] (D. Sinha). https://doi.org/10.1016/j.scp.2019.100193 Received 13 July 2019; Received in revised form 7 November 2019; Accepted 12 November 2019 Available online 19 November 2019 2352-5541/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Experimental set up for radon adsorption.
material of interest for 222Rn adsorption, till date only a few experiments for radon removal have been investigated using commercially available carbon (Cosma et al., 2006; Scarpitta, 1995; Zikovsky, 2001, 1998). For instance, Karunakara et al. (2015) have used commercially available coconut based activated charcoal for radon removal from radiation hazard areas and reported an adsorption coefficient value in the range of 2.3–4.12 m3 kg-1 (Karunakara et al., 2015). Nagarajan et al. (1990) have also developed a model for 222Rn sampling using commercially available coarse activated carbon powder for monitoring of Rn in the air (Nagarajan et al., 1990). Hence it can be seen that only a few available works on adsorption of radon from the air have been reported, and all of the works have used commercially available carbon. This limits the effectiveness of the protocol because, consideration of the cost of the adsorbent is significant for the economic feasibility of the process. Considering the risk of indoor radon and also the importance of low cost activated carbon, the present work emphasises on the preparation of activated carbon from an easily available biomass. The activated carbon was synthesised from cones of Pinus kesiya which was thereafter used for remediation of radon from the air. Pinus cone was selected as a precursor of interest for preparing activated carbon for this piece of work as it is a readily available biomass, and considerable quantities of pinecones are formed annually as agricultural by-product around the globe which are usually treated as a waste and discarded without proper utilisation(Aksakal and Ucun, 2010; Sen et al., 2011). Further, since remediation of gases requires microporous carbon, KOH activation was
selected for this work as KOH is one of the widely used activating agents that produces microporous activated carbon. The prepared activated carbon was used for remediation of 222Rn which was drawn continually from the soil. Soil gas was used in this study as a model for a real-world case. Thus, this study explores the utilisation of biomass material, Pinus kesiya cones for the preparation of granular activated carbon for effec tive use in remediation of radon from the air. Granular activated carbon was used in this study as it reduces the effect of pressure drop during gas adsorption. The current work also highlights the characterisation results of the prepared granular activated carbon. It also explains the efficiency of activated carbon prepared from pine cones biomass for 222Rn reme diation by tuning the process parameters like adsorber bed length, diameter, and flow rate. The study also puts light on the understanding of breakthrough time, the adsorption coefficient brought about by variation of these process parameters. A mathematical model was used to calculate the adsorption coefficient and correlation between the mass of activated carbon and breakthrough time was also evaluated. Besides, desorption studies were also pursued on the spent adsorbent for un derstanding its regeneration capability.
Fig. 2. Diagrammatic representation of the experimental step up for radon adsorption. 2
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2. Materials and methods
developed by Bhabha Atomic Research Centre, Mumbai, India, using scintillation technology (Gaware et al., 2013, 2011b; 2011a). The mechanism for detection of radon inside the radon monitor is explained elsewhere (Gaware et al., 2013; Karunakara et al., 2015).
2.1. Materials Pinus kesiya cones used for the synthesis of granular activated carbon were collected locally (Nagaland, India). Potassium hydroxide was purchased from Fisher Scientific India, Hydrochloric acid (38%) was purchased from HiMedia Laboratories Pvt. Ltd, India.
2.4. Evaluation of
Pinecones collected were crushed into smaller pieces and repeatedly washed with double distilled water to remove any impurities adhered to it. It was then allowed to dry in an oven at 110 � C until all the moisture evaporates. For the preparation of activated carbon, first, the sample was carbonized in an electric furnace from room temperature to 500 � C and kept at this temperature for 2 h at a heating rate of 10 � C min-1 under continuous nitrogen flow. The carbonized sample was then mixed with KOH of 1:2 (w/w) ratio. The mixture was then stirred in a magnetic stirrer for 3 h and kept in an oven until all moisture evaporates and was then carbonized at 700 � C under a continuous nitrogen atmosphere for 2 h. The carbonized activated black sample was then soaked in 0.5 M HCl solution to completely remove any by-products and washed multi ple times with hot water and finally with cold distilled water until the wash water final pH becomes neutral. Finally, the granular pine cone activated carbon (PCAC) was dried in a hot air oven for 24 h at 110 � C and stored in an airtight container. The prepared carbon was charac terised using various method and techniques (details about the charac terization is presented in the supplementary information).
220
Rn concentration
2.5. Mathematical modeling For any fixed bed studies, evaluation of breakthrough time by investigation of the breakthrough curve is of utmost importance as it underpins the operation and performance of the bed adsorption process. For this work, we have considered the adsorber bed length as “L ” and volume of the bed as “ᴠ.” In general, the breakthrough curve at the exit is presumed to be a distinct step function outcome for a steady administration at the entry point provided that it has an infinitesimal small gas dispersion and fast adsorption through the carbon particles (Karunakara et al., 2015). In this condition, it is known that the con centration front distributes through the cylindrical bed as a sharp line with retarded velocity (v). However, owing to the effect of molecular, scattering, and tempestuous diffusion along with the saturation of charcoal bed, infinite gas dispersion will be noticed. This will result in a stretched S-shaped line on a graph instead of sharp step function curve (Karunakara et al., 2015). In this regard, the breakthrough time (τ50 %) may be taken as the time taken for obtaining half of the saturation value (Karunakara et al., 2015). Since velocity is the distance travelled with respect to time taken, thus, taking into consideration that the length ‘L ’ of the PCAC bed and, the effective transit time ‘τ’, the time taken for travelling along the bed by 222Rn to reach the outlet can be expressed as
Rn adsorption
The experimental setup for the current work is shown in Fig. 1 and its diagrammatic representation is illustrated in Fig. 2. The pinecone acti vated carbon was tightly packed in polyethylene columns by wobbling thoroughly for ensuring attainment of homogeneous packing inside the column used as an adsorber bed for the experiments. For the present study, radon gas was pumped from the soil which comes along with carrier gas, i.e., air in this case. Most of the work re ported earlier have used nitrogen gas or dry air as a medium for carrying 222 Rn and 220Rn (Cosma et al., 2006, 1999; Fusamura et al., 1964; Gaul and Underhill, 2005; Giibeli and Stori, 1955; Przytycka, 1964; Scarpitta, 1995; Strong and Levins, 1979; Zikovsky, 2001). However, in this study, air containing 222Rn and 220Rn in high concentrations from the soil was pass through the adsorber bed with the aid of a dry air pump. Radon from soil was taken into consideration for this work in order to inves tigate the efficiency of the adsorber bed in extreme conditions (high humid value of ~90%). This condition was chosen as it has the advan tage in real workplace applications where the average humidity is ex pected to be comparatively less than the soil gas values. Also, the adsorption efficiency would be more when compared to the calculated adsorption efficiency. In the present study, though 220Rn was present with 222Rn, however, only removal of222Rn was chosen as the subject of interest because of its half-life period by injecting air containing 222Rn and 220Rn, into the PCAC bed. A soil gas probe was inserted into the soil to a certain depth (~1.0 m), to which a pump was connected to an adjustable flow meter to collect the soil gas. Before passing through the pump, it was passed through a drier and a progeny filter made of glass fibre to adsorb water droplets and moisture. Soil source was chosen for this study, as it pro vides a constant, high and static concentration of 222Rn (~100 kBqm-3) gas flow, which is otherwise difficult to obtain from a commercially available radon source. Measurements for concentrations of 222Rn and 220 Rn were done continuously at the inlet and outlet of the granular carbon column using the Smart Rn Duo-portable Radon Monitor
τ¼L
=
222
Rn and
A steady concentration of 222Rn and 220Rn is important for a smooth experiment and for obtaining a good result. Therefore, to check the steadiness of 222Rn and 220Rn concentration, soil gas was pumped continuously at 5 LPM flow rate for ~10 h 222Rn and 220Rn concentra tions from soil gas concerning time are illustrated in Fig. 4. It can be observed that both 222Rn and 220Rn concentrations remained static having an average of~142 kBq m-3 for 222Rn, and ~120 kBq m-3 for 220 Rn. Once the steady supply of 222Rn and 220Rn gases were established before each experiment, their adsorption characteristics in the PCAC columns were investigated.
2.2. Preparation of granular activated carbon
2.3. Experimental setup for
222
v
1
Also, an expression relating to breakthrough time, the mass of the adsorbent and flow rate and adsorption coefficient can be deduced taking into account the bulk density, adsorption coefficient, and su perficial velocity, based on this, the (retarded) effective velocity of 222Rn inside the fixed carbon column is specified by the relation (Sahoo et al., 2017; Sudeep Kumara et al., 2017), v¼
U K ρb
2
where, Kρb is the retardation factor, U (m s-1) is the superficial velocity, K (m3 kg-1) is the adsorption coefficient and ρb (kg m-3) the bulk density respectively. Equation (2). ignores the effects of diffusion and is built on the assumption that transport of radon gas occurs by convection as well as adsorption, corresponding travelling time to breakthrough time, which in the present case is τ50 .Substituting equation (1) in equation (2), τ may be associated to the adsorption coefficient as
τ¼
L K ρb U Considering velocity as flow rate equation (3) can be rewritten as;
3
3
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Sustainable Chemistry and Pharmacy 14 (2019) 100193
Table 1 Physico-chemical properties of the pinecone activated carbon. Ash content %
Moisture Content %
Particle Size (mm)
Bulk Density (kg m-3)
pH
6.59� 0:03
10.12�0:40
�1 mm
487�10
7.34�0:02
a
BET Surface area (m2 g-1)
Total Pore Volume (cm3 g-1)
Elemental composition % C
H
N
Oa
839�04
0.476�0:16
88.61
1.66
0.71
2.43
By difference.
K¼
qτ m
6
3. Results and discussion 3.1. Characterization of prepared activated carbon The physicochemical properties of the carbon produced from pine cone biomass are given in Table 1. It is observed that the prepared activated carbon has low ash content, low moisture content, and neutral pH, making it a suitable adsorbent for gas adsorption applications. Moreover, the synthesised carbon has a significant BET surface area with high pore volume, which accounts for the porosity in the adsorbent primarily responsible for the adsorption of gases (Ello et al., 2013; Jeon et al., 2016; Sethia and Sayari, 2016). The results of elemental analysis of granular activated carbon from Pinus biomass show higher carbon content revealing the fact that the activated carbon can be used for removal studies. The prepared activated carbon was subjected for FESEM analysis to understand its surface morphology and its micrograph is shown in Fig. 3. The prepared carbon is filled with pores of all size and shapes mostly micropores which is reported to be suitable for application in gas adsorption. Microporous carbon provides high adsorptive capacity and selectivity thus, it is preferred for gas adsorption (Baklanova et al., 2003; Burchell, 1999; Ello et al., 2013; Rodríguez-Reinoso, 2001; Sethia and Sayari, 2015). For application of the adsorbent in an environment including indoors and high-temperature mines, it is essential to understand its thermal stability. To understand the thermal stability and the decomposition pattern of the prepared activated carbon thermogravimetric analysis was employed. The thermal stability of the prepared activated carbon was found to be up to 700 � C. Details regarding TGA studies of the prepared carbon is supplied in the supplementary information section 1.2.
Fig. 3. FESEM micrograph of PCAC.
τ¼
π r 2 L K ρb q
4
Since the mass of carbon ‘m’ is πr2 L ρb then equation (4) becomes,
τ¼
Km q
5
This equation (5) provides the central equation for calculating the adsorption coefficient K of pinecone activated carbon. Since τ is dependent on flow rate, the diameter of the bed and length, therefore, in this present work, effect on these process parameters were evaluated and the adsorption coefficient was calculated using the rearranged form of equation (5) as:
Fig. 4. (a) Effect of bed length (Bed diameter ¼ 4.5 cm and flow rate ¼ 5LPM) (b) effect of bed diameter (Bed length ¼ 21 cm and flow rate ¼ 5LPM) on adsorption of 222 Rn from air. 4
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Table 2 Adsorption coefficient corresponding to the breakthrough time at flow rate of 5LPM at different bed diameter and length. Diameter of PCAC bed (m)
Length of PCAC bed (m)
τ50 (min)
K (m3 kg-1)
0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 Average � Std Dev Geometric Mean
0.1 0.1 0.1 0.1 0.15 0.15 0.15 0.15 0.21 0.21 0.21 0.21 0.3 0.3 0.3 0.3
48.37 71.68 89.6 101.97 71.47 103.62 134.47 156.74 72.48 89.96 211.62 214.14 102.94 130.41 303.98 354.76
4.59 4.90 4.56 4.57 4.40 4.28 3.92 3.62 3.45 3.05 4.53 3.98 3.81 3.63 3.92 3.59 4.05 � 0.51 4.02
rate constant), it was observed that with an increase in the bed diameter, a shifting of breakthrough time takes place to the greater side (Fig. 4(b)). These occurrences could be explained based on the understanding that as the bed length and diameter increase, the total mass of the carbon also increases, which in turn increases the availability of more pores. With the availability of more pores, more quantity of the radon gas can be adsorbed, thus it takes longer time to reach the breakthrough time. Moreover, adsorption of radon is a dynamic process, as adsorption of other gases present in the carrier gas (air) also takes place. Further, since radon is in a mixture of other gases present in the air and its half-life is only 3.8 days, it is a combination of both adsorption and radioactivity decay which brings about the steady-state outlet concentration as ob tained in Fig. 4. Further, in order to understand the adsorption of radon onto the activated carbon, determination of BET surface area, moisture content, ash content, and total pore volume have been evaluated after radon adsorption (details about the analysis is provided in the supple mentary information). Flow rate is known to play an essential role in achieving break through times, and it is observed that an increase of inlet gas flow rate causes the breakthrough time to occur faster. Three flow rates (5, 10, and 15 LPM) were studied in order to un derstand its effect on the adsorption of radon by PCAC bed by keeping adsorbent column length (30 cm) and diameter (4.5 cm) constant. It is interesting to note that breakthrough time occurs faster for higher flow rate, as the transmigration of 222Rn through the carbon column becomes faster at higher flow rate. It was observed that breakthrough time was increased expressively with a decrease in the flow rate. This phenome non is because, at a low flow rate, 222Rn had more time to migrate through the pores of the adsorbent, resulting in a more prolonged period of stay of 222Rn in the carbon column. From Fig. 5, it is evident that a linear correlation exists with the proper fit of R2 ¼ 0.99 suggesting that the variation of τ50 with the dimension of the carbon bed follows the theoretical assumptions.
Fig. 5. Log-log plot of illustrating the variation of τ50 with (a) length of carbon column at a constant diameter of 4.5 cm and (b) diameter of carbon column at a constant length of 0.21 m at different flowrates.
3.2. Effect of process parameters on222Rn adsorption In order to understand the adsorption coefficient of the prepared carbon, effect of process parameters such as length of PCAC bed (10 cm, 15 cm, 21 cm, and 25 cm), bed diameter (2.8 cm, 3.6 cm, 4.5 cm, and 5.9 cm), and the flow rate (5, 10, and 15 LPM) were evaluated and breakthrough curve was obtained after running the experiments. Length of the bed is important as it gives an idea about setting up a bed for real-time application in the adsorption of radon from air below its permissible limit (100 Bqm-3). In this work, four different dimensions (10 cm, 15 cm, 21 cm, and 30 cm) had been used, and during this study, both diameters of the carbon bed and flow rate were kept constant at 4.5 cm and 5 LPM respectively. As expected, with an increase in the bed length, radon breakthrough time was extended to a higher side as shown in Fig. 4(a). This is due to presence of the higher amount of carbon in the longer carbon bed as compared to shorter bed lengths. Also, when the study was done at different bed diameter (keeping bed length and flow
3.3. Evaluation of adsorption coefficient Adsorption coefficient (K) indicates the efficiency of an adsorbent in terms of its performance in fixed-bed studies. Adsorption coefficient (K) or Adsorption capacity of radon in adsorbent is defined as the ratio of radon adsorbed activity per unit mass of activated carbon (Bq kg-1) to the radon activity concentration in the airspace of the adsorbent (Bqm3) and thus “K” is express as m3kg-1. The reason for using m3kg-1 is due to unmeasurable low atomic concentration of radon in air (~ 10-6 ppb) through chemical/physical technique but measurable radioactivity 5
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Fig. 6. Linear regression to the plot of breakthrough time again mass of PCAC (Flow rate ¼ 5 LPM) showing a good linear correlation between two parameters.
concentration (~100 Bq m-3) through radiation measurement tech nique. Herein, the equation (eq. (5)) for adsorption coefficient was first reported by Pocar in his work, which gave an expression for the deter mination of adsorption coefficient using breakthrough time as τ50 and not the exhaustion time (Pocar, 2003). Table 2 highlights the adsorption coefficient values along with the breakthrough time of 222Rn adsorption experiment with varying column length and diameter. All experiments were performed by drawing 222Rn gas directly from the soil as the presence of water vapour and mixture of other gases present in the air affects the adsorption process significantly, and re duces the site of adsorption on the adsorbent. Thus, experiments were carried out to evaluate the efficiency of prepared activated carbon with varying bed dimensions keeping the bulk density of the carbon bed constant. Adsorption coefficient for the present study was found in be tween 03.05 and04.90 m3 kg-1 having arithmetic average of 4.05 and a standard deviation of 0.51 at a temperature of 26�3 � C. A good correlation between the mass of activated carbon and breakthrough time was found to exist with an R2 value of 0.97. Fig. 6 shows the breakthrough time with respect to the mass of carbon in the adsorber column, and it can be seen that with an increase in the mass of carbon, breakthrough time shifts to a higher range. This could be explained based on the fact that with an increase in the mass of carbon, there is an increase in the number of carbon grains having more pores available for adsorption of 222Rn. Study on the variation of adsorption coefficient with flow rate is very vital for any real time mitigation of Radon in an occupational facility. Thus, experiments were carried out to evaluate the efficiency of pre pared activated carbon with varying bed dimensions and keeping the bulk density constant at a different flow rate as shown in Fig. 7. By keeping the bulk density constant, adsorption coefficient K was found to rise with an increase in the rate of air flow with maximum adsorption coefficient of ~5.6 m3 kg-1at flow rate of 15 LPM which could be probably due to the homogeneously mixing of 222Rn with the activated carbon granules in the column leading to use of the mass of carbon efficiently. Though this phenomenon has still not been confirm thor oughly, this information would be important in fixed-bed design con siderations. Similar findings on the effect of flow rate on length and diameter of the bed had been reported by Karunakara et al. (2015), using coconut shell activated carbon (Karunakara et al., 2015). On the other hand, when the effect of a single flow rate was considered, there was no significant effect on the adsorption coefficient (Karunakara et al.,
Fig. 7. Effect of flow rate on adsorption coefficient K on (a) length of bed (b) bed diameter.
2015; Strong and Levins, 1979). 4. Regeneration studies For any adsorption studies, once the columns breakthrough is exhausted, it either needs to be changed or regenerated for continuing its performance. Reuse rather than replace is the essential requirement for cost-effectiveness in industrial applications. Thus, regeneration of exhausted activated carbon was investigated for its reusability. Regen eration studies were done at different temperatures for 50 min. In this experiment, the spent carbon was immediately removed from the col umn, put in a utility tray and then heated at different temperatures (30 � C, 50 � C, 70 � C, 90 � C, and 110 � C). The heated carbon was cooled and refilled in the same column, and the adsorption study was repeated keeping the same experimental arrangement (Flow Rate ¼ 5 LPM, bed diameter ¼ 4.5 cm, and bed length ¼ 20 cm). The result of the 6
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regeneration experiment is shown in Fig. 8. It was observed that regeneration of the exhausted activated carbon shows higher break through time for those carbons which were heated at temperature above 70 � C. The breakthrough time was almost same with the earlier break through time of ~200 min under the same experimental conditions. Thus, it can be stated that 70–90 � C can be a suitable temperature range for complete regeneration of the exhausted carbon. 5. Comparative study with other carbon adsorbent To understand the effectiveness of the prepared adsorbent, an array of comparison is done with various charcoal adsorbent available in the literature which were used for adsorbent of radon at a temperature range of 20–35 � C and is illustrated in Table 3. It can be observed that the prepared granular activated carbon has a good adsorption coeffi cient K of 3.05–4.90 m3 kg-1 which is at par with other adsorbents re ported earlier. 6. Conclusions From all the discussions, it can be thus concluded that synthesis of granular activated carbon from pine cone can be utilized as an efficient adsorbent for mitigation of radon in workplaces and indoor environ ments. FESEM micrographs show well-developed pores. Adsorption characteristics were effectively established using breakthrough time in an adsorber column system using 222Rn from soil gas. Effect of bed dimension, flow rate along with mass of adsorber column on the breakthrough times were found to commensurate with each other. Adsorption coefficient was found to be in the range of 3.05–4.90 m3 kg-1 with a mean of 4.05 � 0.51. Increase in the K value was found with an increase in flow rate with increase up to mean of 5.66 at a flow rate of 15 LPM. Regeneration of the exhausted carbon in the range 70–90 � C shows an excellent performance on the reusability of the adsorbent. All the parameters examined in this work thus summarize that pine cone acti vated carbon can be efficiently utilized for 222Rn mitigation applications. In future, work on the effect of particle size on the adsorption coef ficient can be evaluated for 222Rn mitigation applications which will help in reducing the cost of the bed by increasing the adsorption capacity.
Fig. 8. Graph showing breakthrough time for regenerated activated carbon at different temperatures. Table 3 Comparison of pine cone activated carbon with other carbon adsorbent used for radon adsorption. Adsorbent
Mode of transport Gas
Adsorption Coefficient (m3kg-1)
References
Pine Cone Activated Carbon
Air
Present Study
Coconut activated charcoal Commercial activated carbon Active Carbon
Air
3.05–4.90(at 5 LPM) 5.66(at 15 LPM) 2.3–4.12
Ambient Air
0.94–4.26
Dry Air
3.563
Coconut shell
Dry Air
3.53
Polish Charcoal Elm-wood charcoal
Dry Air Nitrogen
4.6–5.6 7.084
Elm-wood charcoal
CO2
1.282
Elm-wood charcoal
H2
10.976
Various commercial activated carbon ART2515 Dry air
0.98
Ro-2F
Dry air
1.02
Merck
Dry air
1.05
PZ
Dry air
1.52
George
Dry air
2.75
Calgon BPL
Dry air
2.75
Norit-R1
Dry air
2.9
Calgon PCB
Dry air
3.61
Ro-1G
Dry air
4.02
Carboxen (2 mm)
Dry air
6.92
Carboxen (3 mm)
Dry air
7.1
Karunakara et al. (2015) Zikovsky (1998) Fusamura et al. (1964) Strong and Levins (1979) Przytycka (1964) Giibeli and Stori (1955) Giibeli and Stori (1955) Giibeli and Stori (1955)
Declaration of competing interest The authors declare no conflict of interest. Acknowledgments
Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006)
PC Bhomick is thankful for DST-INSPIRE Fellowship and S Jamir to BRNS-DAE Project Fellowship. Financial assistance from BRNS-DAE is duly acknowledged (Grant No: 36(4)/47/2015-BRNS). Support from DST-FIST is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scp.2019.100193. References Aksakal, O., Ucun, H., 2010. Equilibrium, kinetic and thermodynamic studies of the biosorption of textile dye (Reactive Red 195) onto Pinus sylvestris L. J. Hazard Mater. 181, 666–672. https://doi.org/10.1016/j.jhazmat.2010.05.064. Baklanova, O.N., Plaksin, G.V., Drozdov, V.A., Duplyakin, V.K., Chesnokov, N.V., Kuznetsov, B.N., 2003. Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin. Carbon N. Y. 41, 1793–1800. https://doi.org/10.1016/S00086223(03)00149-0. Boyle, R.W., 1907. The absorption of the radioactive emanations by charcoal. J. Phys. Chem. 12, 484–506. https://doi.org/10.1021/j150097a002.
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Nakayama, Y., Nagao, H., Mochida, I., Kawabuchi, Y., 1994. Adsorption of radon on activated carbon. Carbon N. Y. 32, 1544–1547. Obenchain, R., Young, S.S., Krstic, G., 2019. Low-level radon exposure and lung cancer mortality. Regul. Toxicol. Pharmacol. 107, 104418. https://doi.org/10.1016/j. yrtph.2019.104418. Pocar, A., 2003. Low Background Techniques and Experimental Challenges for Borexino and its Nylon Vessels. Ph.D Thesis. Princet. Univ. Przytycka, R., 1964. Sorption of radon on Polish charcoals. Nukl. 6(1), 23. Chem. Abstr. 60, 2354. Rodríguez-Reinoso, F., 2001. Activated carbon and adsorption. Encycl. Mater. Sci. Technol 22–34. https://doi.org/10.1016/B0-08-043152-6/00005-X. Rutherford, E., 1906. Adsorption of the radio-active emanations by charcoal. Nature. Sahoo, B.K., Mayya, Y.S., Sapra, B.K., Gaware, J.J., Banerjee, K.S., Kushwaha, H.S., 2010. Radon exhalation studies in an Indian uranium tailings pile. Radiat. Meas. 45, 237–241. https://doi.org/10.1016/j.radmeas.2010.01.008. Sahoo, B.K., Sapra, B.K., 2015. Advances in measurement of indoor 222Rn and 220Rn gas concentrations using solid state nuclear track detectors. Solid State Phenom. 238, 116–126. https://doi.org/10.4028/www.scientific.net/SSP.238.116. Sahoo, B.K., Sapra, B.K., Gaware, J.J., Kanse, S.D., Mayya, Y.S., 2011. A model to predict radon exhalation from walls to indoor air based on the exhalation from building material samples. Sci. Total Environ. 409, 2635–2641. https://doi.org/10.1016/j. scitotenv.2011.03.031. Sahoo, B.K., Sudeep Kumara, K., Karunakara, N., Gaware, J.J., Sapra, B.K., Mayya, Y.S., 2017. Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 1): development of theoretical models for design considerations. J. Environ. Radioact. 172, 237–248. https://doi.org/10.1016/j. jenvrad.2017.03.015. Scarpitta, S.C., 1995. A theoretical model for 222Rn adsorption on activated charcoal canisters in humid air based on Polanyi’s potential theory. Health Phys. 68, 332–339. Sen, T.K., Afroze, S., Ang, H.M., 2011. Equilibrium, kinetics and mechanism of removal of methylene blue from aqueous solution by adsorption onto pine cone biomass of Pinus radiata. Water Air Soil Pollut. 218, 499–515. https://doi.org/10.1007/s11270010-0663-y. Sethia, G., Sayari, A., 2016. Activated carbon with optimum pore size distribution for hydrogen storage. Carbon N. Y 99, 289–294. https://doi.org/10.1016/j. carbon.2015.12.032. Sethia, G., Sayari, A., 2015. Comprehensive study of ultra-microporous nitrogen-doped activated carbon for CO2 capture. Carbon N. Y 93, 68–80. https://doi.org/10.1016/j. carbon.2015.05.017. Singh, P., Sahoo, B.K., Bajwa, B.S., 2016. Theoretical modeling of indoor radon concentration and its validation through measurements in South-East Haryana, India. J. Environ. Manag. 171, 35–41. https://doi.org/10.1016/j. jenvman.2016.02.003. Strong, K.P., Levins, D.M., 1979. Dynamic adsorption of radon on activated carbon. In: Proceedings of the Fifteenth DOE Nuclear Air Cleaning Conference. Sudeep Kumara, K., Sahoo, B.K., Karunakara, N., Gaware, J.J., Sapra, B.K., Mayya, Y.S., 2017. Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 1): development of theoretical models for design considerations. J. Environ. Radioact. 172, 237–248. https://doi.org/10.1016/j. jenvrad.2017.03.015. Vives, J., Ulanovsky, A., Copplestone, D., 2017. Science of the Total Environment A method for assessing exposure of terrestrial wildlife to environmental radon (222 Rn) and thoron (220 Rn). Sci. Total Environ. 605–606, 569–577. https://doi.org/ 10.1016/j.scitotenv.2017.06.154. WHO, 2009. Who handbook on indoor radon - a public health perspective. World Heal. Organ. 110 https://doi.org/10.1080/00207230903556771. Yu, K.N., Balendran, R.V., Koo, S.Y., Cheung, T., 2000. Silica fume as a radon retardant from concrete. Environ. Sci. Technol. 34, 2284–2287. https://doi.org/10.1021/ es991134j. Zikovsky, L., 2001. Temperature dependence of adsorption coefficients of 222Rn on activated charcoal determined by adsorption-desorption method. Health Phys. 80, 175–176. https://doi.org/10.1097/00004032-200102000-00011. Zikovsky, L., 1998. Determination of adsorption coefficients of 222Rn on activated charcoal by adsorption-desorption in a simple laboratory apparatus. Health Phys. 313–314.
8
Contents lists available at ScienceDirect
Sustainable Chemistry and Pharmacy journal homepage: http://www.elsevier.com/locate/scp
Biomass-derived activated carbon for removal of
222
Rn from air
Parimal Chandra Bhomick a, Supongtoshi Jamir a, Upasana Bora Sinha a, B.K. Sahoo b, Dipak Sinha a, * a b
Department of Chemistry, Nagaland University, Lumami, 798627, India Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
A R T I C L E I N F O
A B S T R A C T
Keywords: 222 Rn Activated carbon Breakthrough time Adsorption coefficient Regeneration
Radon (222Rn) and its decay products are considered harmful to humans because of their toxicity and is regard as a prior cause of lung cancer in non-smokers. This research thus focuses on the application of activated carbon synthesised from pinecone for mitigating the risk of radon in indoor environments. Characterization of the prepared carbon was evaluated using FESEM, TGA, and BET surface area and total pore volume analyzer. BET surface area of the prepared carbon was found to be 839 m2 g-1 and a total pore volume of 0.476 cm3 g-1. Fixedbed adsorption method was used to estimate the adsorbent efficiency by varying process parameters such as bed length, bed diameter and flow rate. On the basis of these parameters, breakthrough curves were generated to calculate the breakthrough time for obtaining the adsorption coefficient(K) of the prepared carbon which was found to be in the range of 3.05–4.90 m3 kg-1. Regeneration studies were done at different temperatures for 50 min which showed that heating at 70–90 � C resulted in complete degassing of the adsorbed 222Rn, with K equivalent to that of the pristine carbon.
1. Introduction Radon (222Rn), a radioactive gas having a half-life of 3.8235 days is the densest naturally occurring member of the noble gas family that continuously emanates from uranium-bearing minerals in rocks and soils (Fry and Thoennessen, 2013). 222Rn enters the indoor environment chiefly through exhalation from the soil, rocks, and building materials, and this infiltration is considered as the most significant cause of resi dential radon (Sahoo and Sapra, 2015). It is reported that indoor radon tends to saturate in enclosed spaces such as underground mines or houses and contributes about ~ 52% of the ionising radiation dosage received by individuals in general (Sahoo and Sapra, 2015). Epidemio logical studies have provided substantial evidence of lung cancer due to indoor radon exposure even at relatively low radon levels (Darby et al., 2005; Krewski et al., 2006; Obenchain et al., 2019; Sahoo et al., 2011, 2010; WHO, 2009). Though most of the radon gas inhaled is exhaled, the radon progeny decaying from radon adheres to the respiratory tract leading to tracheobronchial deposition of radon progeny in the human body causing lung cancers (Yu et al., 2000). Radon (222Rn) and its decay products are considered harmful to humans because of their toxicity, usually in the occupational sectors of thorium processing facilities and uranium mills that usually contains radioactivity levels above the
permissible level (Karunakara et al., 2015). Presently, radon is acknowledged to be the number one cause of lung cancer in non-smokers and second in smokers (Frutos et al., 2019; Obenchain et al., 2019). In this regard, the WHO has suggested to cut down the reference level of indoor 222Rn from the existing 200 Bqm-3 to 100 Bqm-3 (WHO, 2009). In recent years, therefore, the adverse effects caused by exposures to radon (222Rn) and its decay products on human as well as non-human biota have been receiving considerable attention in the world community, thereby creating considerable attention on the sub ject of 222Rn monitoring around the globe (Chen and Ford, 2017; Cinelli et al., 2015; Mick et al., 2016; Sahoo and Sapra, 2015; Singh et al., 2016; Vives et al., 2017). In this regard, the removal of radon from the air becomes quintes sential. Activated carbon was first used at the beginning of the 20th century by Rutherford in one of his experiments using a tube filled with cocoa nut charcoal for adsorption of 222Rn (Rutherford, 1906). This experiment has been used even recently not only for 222Rn removal systems but also for atmospheric 222Rn sampling and measurement of indoor and outdoor 222Rn concentration in the air (Boyle, 1907; Cosma et al., 2006, 1999; Gaul and Underhill, 2005; Hammon et al., 1975; Nakayama et al., 1994; Scarpitta, 1995; Zikovsky, 2001, 1998). How ever, though activated carbon is found to be a suitable adsorbent and
* Corresponding author. E-mail address: [email protected] (D. Sinha). https://doi.org/10.1016/j.scp.2019.100193 Received 13 July 2019; Received in revised form 7 November 2019; Accepted 12 November 2019 Available online 19 November 2019 2352-5541/© 2019 Elsevier B.V. All rights reserved.
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Sustainable Chemistry and Pharmacy 14 (2019) 100193
Fig. 1. Experimental set up for radon adsorption.
material of interest for 222Rn adsorption, till date only a few experiments for radon removal have been investigated using commercially available carbon (Cosma et al., 2006; Scarpitta, 1995; Zikovsky, 2001, 1998). For instance, Karunakara et al. (2015) have used commercially available coconut based activated charcoal for radon removal from radiation hazard areas and reported an adsorption coefficient value in the range of 2.3–4.12 m3 kg-1 (Karunakara et al., 2015). Nagarajan et al. (1990) have also developed a model for 222Rn sampling using commercially available coarse activated carbon powder for monitoring of Rn in the air (Nagarajan et al., 1990). Hence it can be seen that only a few available works on adsorption of radon from the air have been reported, and all of the works have used commercially available carbon. This limits the effectiveness of the protocol because, consideration of the cost of the adsorbent is significant for the economic feasibility of the process. Considering the risk of indoor radon and also the importance of low cost activated carbon, the present work emphasises on the preparation of activated carbon from an easily available biomass. The activated carbon was synthesised from cones of Pinus kesiya which was thereafter used for remediation of radon from the air. Pinus cone was selected as a precursor of interest for preparing activated carbon for this piece of work as it is a readily available biomass, and considerable quantities of pinecones are formed annually as agricultural by-product around the globe which are usually treated as a waste and discarded without proper utilisation(Aksakal and Ucun, 2010; Sen et al., 2011). Further, since remediation of gases requires microporous carbon, KOH activation was
selected for this work as KOH is one of the widely used activating agents that produces microporous activated carbon. The prepared activated carbon was used for remediation of 222Rn which was drawn continually from the soil. Soil gas was used in this study as a model for a real-world case. Thus, this study explores the utilisation of biomass material, Pinus kesiya cones for the preparation of granular activated carbon for effec tive use in remediation of radon from the air. Granular activated carbon was used in this study as it reduces the effect of pressure drop during gas adsorption. The current work also highlights the characterisation results of the prepared granular activated carbon. It also explains the efficiency of activated carbon prepared from pine cones biomass for 222Rn reme diation by tuning the process parameters like adsorber bed length, diameter, and flow rate. The study also puts light on the understanding of breakthrough time, the adsorption coefficient brought about by variation of these process parameters. A mathematical model was used to calculate the adsorption coefficient and correlation between the mass of activated carbon and breakthrough time was also evaluated. Besides, desorption studies were also pursued on the spent adsorbent for un derstanding its regeneration capability.
Fig. 2. Diagrammatic representation of the experimental step up for radon adsorption. 2
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Sustainable Chemistry and Pharmacy 14 (2019) 100193
2. Materials and methods
developed by Bhabha Atomic Research Centre, Mumbai, India, using scintillation technology (Gaware et al., 2013, 2011b; 2011a). The mechanism for detection of radon inside the radon monitor is explained elsewhere (Gaware et al., 2013; Karunakara et al., 2015).
2.1. Materials Pinus kesiya cones used for the synthesis of granular activated carbon were collected locally (Nagaland, India). Potassium hydroxide was purchased from Fisher Scientific India, Hydrochloric acid (38%) was purchased from HiMedia Laboratories Pvt. Ltd, India.
2.4. Evaluation of
Pinecones collected were crushed into smaller pieces and repeatedly washed with double distilled water to remove any impurities adhered to it. It was then allowed to dry in an oven at 110 � C until all the moisture evaporates. For the preparation of activated carbon, first, the sample was carbonized in an electric furnace from room temperature to 500 � C and kept at this temperature for 2 h at a heating rate of 10 � C min-1 under continuous nitrogen flow. The carbonized sample was then mixed with KOH of 1:2 (w/w) ratio. The mixture was then stirred in a magnetic stirrer for 3 h and kept in an oven until all moisture evaporates and was then carbonized at 700 � C under a continuous nitrogen atmosphere for 2 h. The carbonized activated black sample was then soaked in 0.5 M HCl solution to completely remove any by-products and washed multi ple times with hot water and finally with cold distilled water until the wash water final pH becomes neutral. Finally, the granular pine cone activated carbon (PCAC) was dried in a hot air oven for 24 h at 110 � C and stored in an airtight container. The prepared carbon was charac terised using various method and techniques (details about the charac terization is presented in the supplementary information).
220
Rn concentration
2.5. Mathematical modeling For any fixed bed studies, evaluation of breakthrough time by investigation of the breakthrough curve is of utmost importance as it underpins the operation and performance of the bed adsorption process. For this work, we have considered the adsorber bed length as “L ” and volume of the bed as “ᴠ.” In general, the breakthrough curve at the exit is presumed to be a distinct step function outcome for a steady administration at the entry point provided that it has an infinitesimal small gas dispersion and fast adsorption through the carbon particles (Karunakara et al., 2015). In this condition, it is known that the con centration front distributes through the cylindrical bed as a sharp line with retarded velocity (v). However, owing to the effect of molecular, scattering, and tempestuous diffusion along with the saturation of charcoal bed, infinite gas dispersion will be noticed. This will result in a stretched S-shaped line on a graph instead of sharp step function curve (Karunakara et al., 2015). In this regard, the breakthrough time (τ50 %) may be taken as the time taken for obtaining half of the saturation value (Karunakara et al., 2015). Since velocity is the distance travelled with respect to time taken, thus, taking into consideration that the length ‘L ’ of the PCAC bed and, the effective transit time ‘τ’, the time taken for travelling along the bed by 222Rn to reach the outlet can be expressed as
Rn adsorption
The experimental setup for the current work is shown in Fig. 1 and its diagrammatic representation is illustrated in Fig. 2. The pinecone acti vated carbon was tightly packed in polyethylene columns by wobbling thoroughly for ensuring attainment of homogeneous packing inside the column used as an adsorber bed for the experiments. For the present study, radon gas was pumped from the soil which comes along with carrier gas, i.e., air in this case. Most of the work re ported earlier have used nitrogen gas or dry air as a medium for carrying 222 Rn and 220Rn (Cosma et al., 2006, 1999; Fusamura et al., 1964; Gaul and Underhill, 2005; Giibeli and Stori, 1955; Przytycka, 1964; Scarpitta, 1995; Strong and Levins, 1979; Zikovsky, 2001). However, in this study, air containing 222Rn and 220Rn in high concentrations from the soil was pass through the adsorber bed with the aid of a dry air pump. Radon from soil was taken into consideration for this work in order to inves tigate the efficiency of the adsorber bed in extreme conditions (high humid value of ~90%). This condition was chosen as it has the advan tage in real workplace applications where the average humidity is ex pected to be comparatively less than the soil gas values. Also, the adsorption efficiency would be more when compared to the calculated adsorption efficiency. In the present study, though 220Rn was present with 222Rn, however, only removal of222Rn was chosen as the subject of interest because of its half-life period by injecting air containing 222Rn and 220Rn, into the PCAC bed. A soil gas probe was inserted into the soil to a certain depth (~1.0 m), to which a pump was connected to an adjustable flow meter to collect the soil gas. Before passing through the pump, it was passed through a drier and a progeny filter made of glass fibre to adsorb water droplets and moisture. Soil source was chosen for this study, as it pro vides a constant, high and static concentration of 222Rn (~100 kBqm-3) gas flow, which is otherwise difficult to obtain from a commercially available radon source. Measurements for concentrations of 222Rn and 220 Rn were done continuously at the inlet and outlet of the granular carbon column using the Smart Rn Duo-portable Radon Monitor
τ¼L
=
222
Rn and
A steady concentration of 222Rn and 220Rn is important for a smooth experiment and for obtaining a good result. Therefore, to check the steadiness of 222Rn and 220Rn concentration, soil gas was pumped continuously at 5 LPM flow rate for ~10 h 222Rn and 220Rn concentra tions from soil gas concerning time are illustrated in Fig. 4. It can be observed that both 222Rn and 220Rn concentrations remained static having an average of~142 kBq m-3 for 222Rn, and ~120 kBq m-3 for 220 Rn. Once the steady supply of 222Rn and 220Rn gases were established before each experiment, their adsorption characteristics in the PCAC columns were investigated.
2.2. Preparation of granular activated carbon
2.3. Experimental setup for
222
v
1
Also, an expression relating to breakthrough time, the mass of the adsorbent and flow rate and adsorption coefficient can be deduced taking into account the bulk density, adsorption coefficient, and su perficial velocity, based on this, the (retarded) effective velocity of 222Rn inside the fixed carbon column is specified by the relation (Sahoo et al., 2017; Sudeep Kumara et al., 2017), v¼
U K ρb
2
where, Kρb is the retardation factor, U (m s-1) is the superficial velocity, K (m3 kg-1) is the adsorption coefficient and ρb (kg m-3) the bulk density respectively. Equation (2). ignores the effects of diffusion and is built on the assumption that transport of radon gas occurs by convection as well as adsorption, corresponding travelling time to breakthrough time, which in the present case is τ50 .Substituting equation (1) in equation (2), τ may be associated to the adsorption coefficient as
τ¼
L K ρb U Considering velocity as flow rate equation (3) can be rewritten as;
3
3
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Sustainable Chemistry and Pharmacy 14 (2019) 100193
Table 1 Physico-chemical properties of the pinecone activated carbon. Ash content %
Moisture Content %
Particle Size (mm)
Bulk Density (kg m-3)
pH
6.59� 0:03
10.12�0:40
�1 mm
487�10
7.34�0:02
a
BET Surface area (m2 g-1)
Total Pore Volume (cm3 g-1)
Elemental composition % C
H
N
Oa
839�04
0.476�0:16
88.61
1.66
0.71
2.43
By difference.
K¼
qτ m
6
3. Results and discussion 3.1. Characterization of prepared activated carbon The physicochemical properties of the carbon produced from pine cone biomass are given in Table 1. It is observed that the prepared activated carbon has low ash content, low moisture content, and neutral pH, making it a suitable adsorbent for gas adsorption applications. Moreover, the synthesised carbon has a significant BET surface area with high pore volume, which accounts for the porosity in the adsorbent primarily responsible for the adsorption of gases (Ello et al., 2013; Jeon et al., 2016; Sethia and Sayari, 2016). The results of elemental analysis of granular activated carbon from Pinus biomass show higher carbon content revealing the fact that the activated carbon can be used for removal studies. The prepared activated carbon was subjected for FESEM analysis to understand its surface morphology and its micrograph is shown in Fig. 3. The prepared carbon is filled with pores of all size and shapes mostly micropores which is reported to be suitable for application in gas adsorption. Microporous carbon provides high adsorptive capacity and selectivity thus, it is preferred for gas adsorption (Baklanova et al., 2003; Burchell, 1999; Ello et al., 2013; Rodríguez-Reinoso, 2001; Sethia and Sayari, 2015). For application of the adsorbent in an environment including indoors and high-temperature mines, it is essential to understand its thermal stability. To understand the thermal stability and the decomposition pattern of the prepared activated carbon thermogravimetric analysis was employed. The thermal stability of the prepared activated carbon was found to be up to 700 � C. Details regarding TGA studies of the prepared carbon is supplied in the supplementary information section 1.2.
Fig. 3. FESEM micrograph of PCAC.
τ¼
π r 2 L K ρb q
4
Since the mass of carbon ‘m’ is πr2 L ρb then equation (4) becomes,
τ¼
Km q
5
This equation (5) provides the central equation for calculating the adsorption coefficient K of pinecone activated carbon. Since τ is dependent on flow rate, the diameter of the bed and length, therefore, in this present work, effect on these process parameters were evaluated and the adsorption coefficient was calculated using the rearranged form of equation (5) as:
Fig. 4. (a) Effect of bed length (Bed diameter ¼ 4.5 cm and flow rate ¼ 5LPM) (b) effect of bed diameter (Bed length ¼ 21 cm and flow rate ¼ 5LPM) on adsorption of 222 Rn from air. 4
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Table 2 Adsorption coefficient corresponding to the breakthrough time at flow rate of 5LPM at different bed diameter and length. Diameter of PCAC bed (m)
Length of PCAC bed (m)
τ50 (min)
K (m3 kg-1)
0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 0.028 0.036 0.045 0.059 Average � Std Dev Geometric Mean
0.1 0.1 0.1 0.1 0.15 0.15 0.15 0.15 0.21 0.21 0.21 0.21 0.3 0.3 0.3 0.3
48.37 71.68 89.6 101.97 71.47 103.62 134.47 156.74 72.48 89.96 211.62 214.14 102.94 130.41 303.98 354.76
4.59 4.90 4.56 4.57 4.40 4.28 3.92 3.62 3.45 3.05 4.53 3.98 3.81 3.63 3.92 3.59 4.05 � 0.51 4.02
rate constant), it was observed that with an increase in the bed diameter, a shifting of breakthrough time takes place to the greater side (Fig. 4(b)). These occurrences could be explained based on the understanding that as the bed length and diameter increase, the total mass of the carbon also increases, which in turn increases the availability of more pores. With the availability of more pores, more quantity of the radon gas can be adsorbed, thus it takes longer time to reach the breakthrough time. Moreover, adsorption of radon is a dynamic process, as adsorption of other gases present in the carrier gas (air) also takes place. Further, since radon is in a mixture of other gases present in the air and its half-life is only 3.8 days, it is a combination of both adsorption and radioactivity decay which brings about the steady-state outlet concentration as ob tained in Fig. 4. Further, in order to understand the adsorption of radon onto the activated carbon, determination of BET surface area, moisture content, ash content, and total pore volume have been evaluated after radon adsorption (details about the analysis is provided in the supple mentary information). Flow rate is known to play an essential role in achieving break through times, and it is observed that an increase of inlet gas flow rate causes the breakthrough time to occur faster. Three flow rates (5, 10, and 15 LPM) were studied in order to un derstand its effect on the adsorption of radon by PCAC bed by keeping adsorbent column length (30 cm) and diameter (4.5 cm) constant. It is interesting to note that breakthrough time occurs faster for higher flow rate, as the transmigration of 222Rn through the carbon column becomes faster at higher flow rate. It was observed that breakthrough time was increased expressively with a decrease in the flow rate. This phenome non is because, at a low flow rate, 222Rn had more time to migrate through the pores of the adsorbent, resulting in a more prolonged period of stay of 222Rn in the carbon column. From Fig. 5, it is evident that a linear correlation exists with the proper fit of R2 ¼ 0.99 suggesting that the variation of τ50 with the dimension of the carbon bed follows the theoretical assumptions.
Fig. 5. Log-log plot of illustrating the variation of τ50 with (a) length of carbon column at a constant diameter of 4.5 cm and (b) diameter of carbon column at a constant length of 0.21 m at different flowrates.
3.2. Effect of process parameters on222Rn adsorption In order to understand the adsorption coefficient of the prepared carbon, effect of process parameters such as length of PCAC bed (10 cm, 15 cm, 21 cm, and 25 cm), bed diameter (2.8 cm, 3.6 cm, 4.5 cm, and 5.9 cm), and the flow rate (5, 10, and 15 LPM) were evaluated and breakthrough curve was obtained after running the experiments. Length of the bed is important as it gives an idea about setting up a bed for real-time application in the adsorption of radon from air below its permissible limit (100 Bqm-3). In this work, four different dimensions (10 cm, 15 cm, 21 cm, and 30 cm) had been used, and during this study, both diameters of the carbon bed and flow rate were kept constant at 4.5 cm and 5 LPM respectively. As expected, with an increase in the bed length, radon breakthrough time was extended to a higher side as shown in Fig. 4(a). This is due to presence of the higher amount of carbon in the longer carbon bed as compared to shorter bed lengths. Also, when the study was done at different bed diameter (keeping bed length and flow
3.3. Evaluation of adsorption coefficient Adsorption coefficient (K) indicates the efficiency of an adsorbent in terms of its performance in fixed-bed studies. Adsorption coefficient (K) or Adsorption capacity of radon in adsorbent is defined as the ratio of radon adsorbed activity per unit mass of activated carbon (Bq kg-1) to the radon activity concentration in the airspace of the adsorbent (Bqm3) and thus “K” is express as m3kg-1. The reason for using m3kg-1 is due to unmeasurable low atomic concentration of radon in air (~ 10-6 ppb) through chemical/physical technique but measurable radioactivity 5
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Fig. 6. Linear regression to the plot of breakthrough time again mass of PCAC (Flow rate ¼ 5 LPM) showing a good linear correlation between two parameters.
concentration (~100 Bq m-3) through radiation measurement tech nique. Herein, the equation (eq. (5)) for adsorption coefficient was first reported by Pocar in his work, which gave an expression for the deter mination of adsorption coefficient using breakthrough time as τ50 and not the exhaustion time (Pocar, 2003). Table 2 highlights the adsorption coefficient values along with the breakthrough time of 222Rn adsorption experiment with varying column length and diameter. All experiments were performed by drawing 222Rn gas directly from the soil as the presence of water vapour and mixture of other gases present in the air affects the adsorption process significantly, and re duces the site of adsorption on the adsorbent. Thus, experiments were carried out to evaluate the efficiency of prepared activated carbon with varying bed dimensions keeping the bulk density of the carbon bed constant. Adsorption coefficient for the present study was found in be tween 03.05 and04.90 m3 kg-1 having arithmetic average of 4.05 and a standard deviation of 0.51 at a temperature of 26�3 � C. A good correlation between the mass of activated carbon and breakthrough time was found to exist with an R2 value of 0.97. Fig. 6 shows the breakthrough time with respect to the mass of carbon in the adsorber column, and it can be seen that with an increase in the mass of carbon, breakthrough time shifts to a higher range. This could be explained based on the fact that with an increase in the mass of carbon, there is an increase in the number of carbon grains having more pores available for adsorption of 222Rn. Study on the variation of adsorption coefficient with flow rate is very vital for any real time mitigation of Radon in an occupational facility. Thus, experiments were carried out to evaluate the efficiency of pre pared activated carbon with varying bed dimensions and keeping the bulk density constant at a different flow rate as shown in Fig. 7. By keeping the bulk density constant, adsorption coefficient K was found to rise with an increase in the rate of air flow with maximum adsorption coefficient of ~5.6 m3 kg-1at flow rate of 15 LPM which could be probably due to the homogeneously mixing of 222Rn with the activated carbon granules in the column leading to use of the mass of carbon efficiently. Though this phenomenon has still not been confirm thor oughly, this information would be important in fixed-bed design con siderations. Similar findings on the effect of flow rate on length and diameter of the bed had been reported by Karunakara et al. (2015), using coconut shell activated carbon (Karunakara et al., 2015). On the other hand, when the effect of a single flow rate was considered, there was no significant effect on the adsorption coefficient (Karunakara et al.,
Fig. 7. Effect of flow rate on adsorption coefficient K on (a) length of bed (b) bed diameter.
2015; Strong and Levins, 1979). 4. Regeneration studies For any adsorption studies, once the columns breakthrough is exhausted, it either needs to be changed or regenerated for continuing its performance. Reuse rather than replace is the essential requirement for cost-effectiveness in industrial applications. Thus, regeneration of exhausted activated carbon was investigated for its reusability. Regen eration studies were done at different temperatures for 50 min. In this experiment, the spent carbon was immediately removed from the col umn, put in a utility tray and then heated at different temperatures (30 � C, 50 � C, 70 � C, 90 � C, and 110 � C). The heated carbon was cooled and refilled in the same column, and the adsorption study was repeated keeping the same experimental arrangement (Flow Rate ¼ 5 LPM, bed diameter ¼ 4.5 cm, and bed length ¼ 20 cm). The result of the 6
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regeneration experiment is shown in Fig. 8. It was observed that regeneration of the exhausted activated carbon shows higher break through time for those carbons which were heated at temperature above 70 � C. The breakthrough time was almost same with the earlier break through time of ~200 min under the same experimental conditions. Thus, it can be stated that 70–90 � C can be a suitable temperature range for complete regeneration of the exhausted carbon. 5. Comparative study with other carbon adsorbent To understand the effectiveness of the prepared adsorbent, an array of comparison is done with various charcoal adsorbent available in the literature which were used for adsorbent of radon at a temperature range of 20–35 � C and is illustrated in Table 3. It can be observed that the prepared granular activated carbon has a good adsorption coeffi cient K of 3.05–4.90 m3 kg-1 which is at par with other adsorbents re ported earlier. 6. Conclusions From all the discussions, it can be thus concluded that synthesis of granular activated carbon from pine cone can be utilized as an efficient adsorbent for mitigation of radon in workplaces and indoor environ ments. FESEM micrographs show well-developed pores. Adsorption characteristics were effectively established using breakthrough time in an adsorber column system using 222Rn from soil gas. Effect of bed dimension, flow rate along with mass of adsorber column on the breakthrough times were found to commensurate with each other. Adsorption coefficient was found to be in the range of 3.05–4.90 m3 kg-1 with a mean of 4.05 � 0.51. Increase in the K value was found with an increase in flow rate with increase up to mean of 5.66 at a flow rate of 15 LPM. Regeneration of the exhausted carbon in the range 70–90 � C shows an excellent performance on the reusability of the adsorbent. All the parameters examined in this work thus summarize that pine cone acti vated carbon can be efficiently utilized for 222Rn mitigation applications. In future, work on the effect of particle size on the adsorption coef ficient can be evaluated for 222Rn mitigation applications which will help in reducing the cost of the bed by increasing the adsorption capacity.
Fig. 8. Graph showing breakthrough time for regenerated activated carbon at different temperatures. Table 3 Comparison of pine cone activated carbon with other carbon adsorbent used for radon adsorption. Adsorbent
Mode of transport Gas
Adsorption Coefficient (m3kg-1)
References
Pine Cone Activated Carbon
Air
Present Study
Coconut activated charcoal Commercial activated carbon Active Carbon
Air
3.05–4.90(at 5 LPM) 5.66(at 15 LPM) 2.3–4.12
Ambient Air
0.94–4.26
Dry Air
3.563
Coconut shell
Dry Air
3.53
Polish Charcoal Elm-wood charcoal
Dry Air Nitrogen
4.6–5.6 7.084
Elm-wood charcoal
CO2
1.282
Elm-wood charcoal
H2
10.976
Various commercial activated carbon ART2515 Dry air
0.98
Ro-2F
Dry air
1.02
Merck
Dry air
1.05
PZ
Dry air
1.52
George
Dry air
2.75
Calgon BPL
Dry air
2.75
Norit-R1
Dry air
2.9
Calgon PCB
Dry air
3.61
Ro-1G
Dry air
4.02
Carboxen (2 mm)
Dry air
6.92
Carboxen (3 mm)
Dry air
7.1
Karunakara et al. (2015) Zikovsky (1998) Fusamura et al. (1964) Strong and Levins (1979) Przytycka (1964) Giibeli and Stori (1955) Giibeli and Stori (1955) Giibeli and Stori (1955)
Declaration of competing interest The authors declare no conflict of interest. Acknowledgments
Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006) Cosma et al. (2006)
PC Bhomick is thankful for DST-INSPIRE Fellowship and S Jamir to BRNS-DAE Project Fellowship. Financial assistance from BRNS-DAE is duly acknowledged (Grant No: 36(4)/47/2015-BRNS). Support from DST-FIST is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scp.2019.100193. References Aksakal, O., Ucun, H., 2010. Equilibrium, kinetic and thermodynamic studies of the biosorption of textile dye (Reactive Red 195) onto Pinus sylvestris L. J. Hazard Mater. 181, 666–672. https://doi.org/10.1016/j.jhazmat.2010.05.064. Baklanova, O.N., Plaksin, G.V., Drozdov, V.A., Duplyakin, V.K., Chesnokov, N.V., Kuznetsov, B.N., 2003. Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin. Carbon N. Y. 41, 1793–1800. https://doi.org/10.1016/S00086223(03)00149-0. Boyle, R.W., 1907. The absorption of the radioactive emanations by charcoal. J. Phys. Chem. 12, 484–506. https://doi.org/10.1021/j150097a002.
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