nueva página del texto (beta)
Inglés (pdf)
Artículo en XML
Referencias del artículo
Traducción automática
Enviar artículo por email
Citado por SciELO 
Similares en
SciELO 
Permalink
INTRODUCTION
Waste from electrical and electronic equipment (WEEE) grew significantly over the last 20 years (Tan et al. 2017). The plastic fraction of WEEE (WEEEP) represents about 20 % of the total mass (Wäger et al. 2012). A real sample of WEEEP is a complex mixture of different types of polymers; yet, three types are predominant, acrylonitrile butadiene styrene (ABS, 30-35 %), high impact polystyrene (HIPS, 20-25 %) and polypropylene (20-30 %). Also, polycarbonate (PC) is present (Schlummer et al. 2007, Dimitrakakis et al. 2009a, Wäger et al. 2012, Peeters et al. 2014, Maris et al. 2015). It is widely know that this plastic contains heavy metals (Tamaddon and Hogland 1993, Dimitrakakis et al. 2009b, Stenvall et al. 2013), brominated flame retardants (BFRs) (Schlummer et al. 2005), polymer decomposition substances, and other hazardous compounds.
BFRs are persistent organic pollutants that can be accumulated and detected in humans and the environment (Meironyte et al. 1999, Covaci et al. 2006, Law et al. 2006). Over the last decades, concerns have been raised over the toxicity of some BFR compounds and their risk to human health (Darnerud 2003, Dunnick et al. 2017). Currently, it is difficult to recycle WEEEP and it is treated by incineration or landfilling (Ilankoon et al. 2018).
However, WEEEP could be used to develop novel recycled materials (Schlummer et al. 2006, Buekens and Yang 2014, Luhar and Luhar 2019), in particular novel building materials (Siddique et al. 2008, Saikia and de Brito 2012, Gu and Ozbakkaloglu 2016, Senthil-Kumar and Baskar 2018). An example of these are the recycled plastic aggregates (RPA) (Hannawi et al. 2010, Lakshmi and Nagan 2010, 2011, Wang and Meyer 2012, Alqahtani et al. 2014, 2017a, b, Nowek 2016), and the use of WEEEP as coarse aggregate for concrete (Senthil-Kumar and Baskar 2015a, b, c) and fiber for concrete mixture (Gu and Ozbakkaloglu 2016).
It is particularly important to analyze the impact of both contaminants (heavy metals and polybrominated organic compounds) in the final recycled material. In this way, Senthil-Kumar et al. (2016) and our research group considered the presence of metals in the leaching of Portland cement base materials made with WEEEP (Peisino et al. 2019). Also, we recently puy a study about the behavior of the BFRs present in WEEEP (specifically tetrabomobisphenol-A [TBBPA]), in a Portland cement system (Gómez et al. 2020). We have determined that the hazardous organic contaminant (TBBPA) leached in a higher proportion when WEEEP was in contact with the basic medium of the Portland cement. However, this effect was solved with the use of activated charcoal (AC) as a stabilizing additive.
AC are composed of carbonaceous matter with a porous structure. They are generally manufactured in two stages: firstly by carbonization (pyrolysis) of the organic matter (essentially wood of different types), and then by an activation process (Yang 2003). The pyrolysis stage removes elements other than carbon, producing a porous material. The activation stage is necessary in order to increase the adsorption capacity of the material. There are many kinds of activation procedures that can be performed: physical activation (in presence of water vapor and air under pressure) or chemical activation (usually by treatment with phosphoric acid) (Yin et al. 2007). AC are particularly attractive for the capture of inorganic and organic contaminants due to their porosity (pore size ranging from 4.5 to 60 Å), good adsorption capabilities, relatively low production cost, and large specific surface area (300-4000 m2/g) (Yang 2003). In addition, it is possible to modify the adsorption characteristics of activated carbons changing the raw material to be carbonized and by optimization of the carbonization process (Laine and Yunes 1992) and activation steps (Yin et al. 2007).
In this work we present the results on the deepening of the analysis of activated charcoal types and dosage. A core-shell approach was applied, where the particles of plastic wastes were covered with a mixture of masonry cement and activated charcoal. We evaluated the leached of TBBPA and bisphenol-A (BPA) in the curing water of the different formulations of RPAs. To achieve this, the extracts were analyzed by gas chromatography (GC) to determine the concentration of organic compounds leached and so evaluate the capability of the different activated charcoals to stabilize pollutants.
MATERIALS AND METHODS
Reagents and materials
WEEEP (WP) was supplied in crushed form by a local recycling company which collects and processes all types of waste of electric and electronic equipment. The apparent specific gravity of the used WEEEP was 510 kg kg/m3. Masonry cement (MC) from Holcim was employed. Powder activate charcoals were obtained from different suppliers and were used as received. CAE 061, MMF, CAE Ultra and Clarisorb B were obtained from Clarimex; Merck was obtained from Merck. Powder vegetal charcoal (“Vegetal”) was obtained from a local supplier. In order to obtain a homogeneous sample, the carbon was screened with a 75-µm sieve (200 mesh).
Milli-Q water was employed for the preparation of RPAs and to carry out the curing of these materials. Analytical grade n-hexane, ethyl acetate, and hydrochloric acid (HCl, 37 %) were used as they were received from the supplier. For iodine number determination the following reactants were used: potassium iodate (KIO3, GR for analysis), potassium iodide (KI, GR for analysis), iodine (I2, GR for analysis) sodium thiosulphate (Na2S2O3, GR for analysis), and starch.
TBBPA was obtained by column chromatography isolation in our laboratory with 99 % purity determined by GC-MS. For quantification, two standards were employed: docosane ≥ 99 % (C22H24), and 9-bromoanthracene ≥ 99 % (C10H15Br), both from Fluka.
RPA manufacture
The following procedure was used to manufacture the core-shell RPA, as described in a previous work (Gómez et al. 2020):
First, the WP (core) was placed in the mixer and water was added to wet the plastic particles.
Next, the cementitious mixture for the shell was added slowly and mixed by hand with enveloping movements.
The shell was built in three layers, with a waiting time of 24 h between layers. In addition, particles were screened with a 4.8 mm sieve (4 mesh) between each layer.
Finally, RPAs were cured by water immersion for 14 days.
Twenty-seven different RPAs were prepared. All samples were made using 250 g of WP covered with a mixture prepared with 300 g of MC, different amounts and types of AC and 100 mL of water (Table I).
TABLE I RECYCLED PL ASTIC AGGREGATES (RPAs) FORMULATIONS AND COMPOSITIONS1
| Entry | Sample | Carbontype | Amount of carbon2(g, %) |
| 1 | WP@MC | - | - |
| 2 | WP@MC:CAE-1 | CAE 061 | (6.25, 1.1) |
| 3 | WP@MC:CAE-2 | CAE 061 | (12.50, 2.2) |
| 4 | WP@MC:CAE-3 | CAE 061 | (18.75, 3.3) |
| 5 | WP@MC:CAE-4 | CAE 061 | (25.00, 4.3) |
| 6 | WP@MC:MMF-1 | MMF | (6.25, 1.1) |
| 7 | WP@MC:MMF-2 | MMF | (12.50, 2.2) |
| 8 | WP@MC:MMF-3 | MMF | (18.75, 3.3) |
| 9 | WP@MC:MMF-4 | MMF | (25.00, 4.3) |
| 10 | WP@MC:CAEU-1 | CAE Ultra | (6.25, 1.1) |
| 11 | WP@MC:CAEU-2 | CAE Ultra | (12.50, 2.2) |
| 12 | WP@MC:CAEU-3 | CAE Ultra | (18.75, 3.3) |
| 13 | WP@MC:CAEU-4 | CAE Ultra | (25.00, 4.3) |
| 14 | WP@MC:CLARI-1 | Clarisorb B | (6.25, 1.1) |
| 15 | WP@MC:CLARI-2 | Clarisorb B | (12.50, 2.2) |
| 16 | WP@MC:CLARI-3 | Clarisorb B | (18.75, 3.3) |
| 17 | WP@MC:CLARI-4 | Clarisorb B | (25.00, 4.3) |
| 18 | WP@MC:MERCK-1 | Merck | (6.25, 1.1) |
| 19 | WP@MC:MERCK-2 | Merck | (12.50, 2.2) |
| 20 | WP@MC:MERCK-3 | Merck | (18.75, 3.3) |
| 21 | WP@MC:MERCK-4 | Merck | (25.00, 4.3) |
| 22 | WP@MC:VEG-1 | Vegetal | (6.25, 1.1) |
| 23 | WP@MC:VEG-2 | Vegetal | (12.50, 2.2) |
| 24 | WP@MC:VEG-3 | Vegetal | (18.75, 3.3) |
| 25 | WP@MC:VEG-4 | Vegetal | (25.00, 4.3) |
| 26 | WP@MC:VEG-19 | Vegetal | (125.00, 18.5) |
| 27 | WP@MC:VEG-31 | Vegetal | (250.00, 31.3) |
1All the RPAs were prepared with 250 g of WEEEP and 300 g of masonry cement. The shell was prepared in three layers. Between layers the waiting time for cement setting was of 24 h. In addition, between each layer, particles were screened with a 4.8 mm sieve (4 mesh).
2The percentage is relative to the amount of WEEEP.
Chemical analyses
Quantitative analyses of organic compounds were performed by gas chromatography using a Shimadzu GC-2010 gas chromatograph with flame ionization detector, equipped with a VF-5ms column (30 m × 0.25 mm × 0.25 µm) using N2 as gas carrier. Quantification of TBBPA was made using the internal standard calibration method (r2 > 0.99) with authentic samples. Limit of detection (LOD = 84 mg/L) was calculated using the standard deviation of the residuals from the calibration curve (three times this standard deviation). Two internal standards were used, thus the LOD informed was a mean of these two calibrations. Limit of quantification (LOQ = 279 mg/L) was calculated using the standard deviation of the residuals from the calibration curve (10 times this standard deviation).
Pore water from the cured process of the RPAs was obtained with the following procedure: immersion of RPA containing 250 g of WP in 500 mL of milli-Q water for 14 days at room temperature (25 ºC) without stirring. Then the RPA (solid) was filtered and the solution was acidified with concentrate hydrochloric acid at pH = 2. The aqueous phase was extracted with n-hexane (4 × 125 mL), the organic phase was dried with sodium sulphate, and the solvent was evaporated in vacuum. Standards were added to the organic phase and it was dissolved in ethyl acetate (10 mL); the organic compounds (TBBPA and BPA) were quantified by GC-FID using an internal standard method.
Iodine number determination was carried out for all carbon samples by a titration method described in the ASTM D 4607 standard (ASTM 2006). The values are shown in table II.
TABLE II IODINE NUMBER AND SPECIFIC SURFACE AREA OF THE ACTIVATED CARBON SAMPLES.
| Entry | Sample | Manufacturer | Iodine number1 | SBET 2 (m2/g) |
| 1 | CAE 061 | Clarimex | 552 | 544 |
| 2 | MMF | Clarimex | 764 | 755 |
| 3 | CAE Ultra | Clarimex | 792 | 783 |
| 4 | CLARISORB B | Clarimex | 794 | 785 |
| 5 | Merck | Merck | 777 | 768 |
| 6 | Vegetal | Local supplier | 124 | 112 |
1Determined according to the ASTM D 4607 standard (ASTM 2006).
2Specific surface area (SBET) estimated from the iodine number (IN): SBET = (0.9946 × IN) - 4.91 (Mianowski et al. 2007).
RESULTS AND DISCUSSION
RPAs manufacturing
The main polymers in WEEEP samples were HIPS (34.5 %), ABS (62.0 %) and ABS/PC (2.2 %), which were determined by FT-IR in a previous report (Peisino et al. 2019). Better results in term of shapes and size homogeneity were achieved when three layers of cementitious covering were applied (Gómez et al. 2020). Then, RPAs were prepared according to Gómez et al. (2020). The identity of the activated charcoal used, and the dosage were modified to obtain samples that allowed establishing the most effective stabilization dosage for each activated carbon assayed.
Iodine number
The iodine number (IN) is the mass of iodine (in milligrams) consumed by 1 g of a chemical substance, commonly used to estimate the insaturation grade of oils and fatty acids, but it is also related to the porous characteristics of sorbents materials such as activated carbon, and fly ash, among others (Mianowski et al. 2007). Although the sorbent capability of a substance depends on variables such as porosity, pH, zeta potential, and temperature, for the proposed use conditions of the activated carbon, it is more related to porosity, since pH would be determined by the cement matrix and temperature is difficult to control and monitor in the production of construction materials (Mianowski et al. 2007).
In addition, a specific surface area can be estimated from the IN values, according to Mianowski et al. (2007).
As can be observed on table II, it is possible to divide the activated carbons employed in three categories: low IN (Vegetal), medium IN (CAE 061) and high IN (the rest of the carbons tested). This means that Vegetal charcoal is the less porous, while Merck, MMF, CAE Ultra and Clarisorb B had the highest porous character.
Chemical analysis
TBBPA has two ionizable aromatic hydroxyl groups (pKa1 = 7.5 and pKa2 = 8.5) (Bergman et al. 2012). When TBBPA is in contact with cement shell, which is a well-known strong base (Birchall et al. 1978), it reacts immediately to form the anionic species of TBBPA (Fig. 1), thus increasing leaching to the aqueous phase. The same behavior applies to BPA, which is also a phenol.
Table III shows the concentrations of organic compounds for each leachate measured by CG-FID analysis. Figure 2 shows data from table III represented in a graphic bar. From the analysis of table III and figure 2 it appears that all activated carbons tested, in the proposed dosages (1.1 to 4.3 %), produce leachates below the LOD for TBBPA. This is consistent with the high value and specific surface area estimated by IN for these carbons. Activated carbon with a specific surface area of 896 m2/g determined by BET was one of the most effective sorbents used for the removal of BPA from aqueous solutions in a study carried out by Zhou et al. (2012). This is in a good agreement with the efficiency that we obtained for activated carbons with specific surface area values between 500 to 800 m2/g for the leaching of TBBPA and BPA in pore waters. Taking into account these results, we propose that a dosage of 1.1 % for all the activated carbons, which has an IN greater than 500, is effective for the stabilization of TBBPA and to prevent its leaching to cure waters, and eventually to the environment.
TABLE III ORGANIC COMPOUND EXTRACTION OF DIFFERENT RECYCLED PLASTIC AGGREGATES (RPAs)1.
| Entry | Sample | [TBBPA]2 | TBBPA3 | BPA4 |
| 1 | WP@MC | (105±2) | (41.8±0.7) | 1.00 |
| 2 | WP@MC:CAE-1 | < LOD5 | < LOD | 0.48 |
| 3 | WP@MC:CAE-2 | < LOD | < LOD | 0.63 |
| 4 | WP@MC:CAE-3 | < LOD | < LOD | 0.90 |
| 5 | WP@MC:CAE-4 | < LOD | < LOD | 0.48 |
| 6 | WP@MC:MMF-1 | < LOD | < LOD | 0.60 |
| 7 | WP@MC:MMF-2 | < LOD | < LOD | 0.45 |
| 8 | WP@MC:MMF-3 | < LOD | < LOD | 0.33 |
| 9 | WP@MC:MMF-4 | < LOD | < LOD | 0.33 |
| 10 | WP@MC:CAEU-1 | < LOD | < LOD | 0.48 |
| 11 | WP@MC:CAEU-2 | < LOD | < LOD | 0.29 |
| 12 | WP@MC:CAEU-3 | < LOD | < LOD | 0.27 |
| 13 | WP@MC:CAEU-4 | < LOD | < LOD | nd6 |
| 14 | WP@MC:CLARI-1 | < LOD | < LOD | 0.46 |
| 15 | WP@MC:CLARI-2 | < LOD | < LOD | 0.37 |
| 16 | WP@MC:CLARI-3 | < LOD | < LOD | 0.22 |
| 17 | WP@MC:CLARI-4 | < LOD | < LOD | 0.37 |
| 18 | WP@MC:MERCK-1 | < LOD | < LOD | 0.41 |
| 19 | WP@MC:MERCK-2 | < LOD | < LOD | 0.32 |
| 20 | WP@MC:MERCK-3 | < LOD | < LOD | 0.04 |
| 21 | WP@MC:MERCK-4 | < LOD | < LOD | 0.03 |
| 22 | WP@MC:VEG-1 | (100.0±0.3) | (40.0±0.1) | 1.06 |
| 23 | WP@MC:VEG-2 | (85.0±0.5) | (34.0±0.2) | 0.88 |
| 24 | WP@MC:VEG-3 | (63.5±0.5) | (25.4±0.2) | 0.85 |
| 25 | WP@MC:VEG-4 | (58.3±0.5) | (23.3±0.2) | 0.86 |
| 26 | WP@MC:VEG-19 | < LOQe | < LOQ | 0.70 |
| 27 | WP@MC:VEG-31 | < LOD | < LOD | 0.15 |
1Water for curing the RPA. Immersion of RPA containing 250 g of WEEEP in 500 mL of milli-Q water for 14 days at 25 ºC without stirring was performed.
2Concentration of TBBPA (x 101 mg/L) in the concentrated solution (total volume of 10 mL).
3TBBPA leachate from samples in mgTBBPA/kgWEEEP (mg/kg).
4BPA leachate from samples relative to WP@MC.
5LOD = 84 mg/L, LOQ = 279 mg/L.
6Not determined.
On the other hand, it can be observed that the non-activated vegetal charcoal does not perform as well as other carbons, which is totally in agreement with the low IN and specific surface area shown in table II, and with the fact that this charcoal lacks an activation process.
Considering these findings, it can be said that for charcoals with low IN and specific surface area (IN lower than 150), an effective dosage in the production of RPAs must be of at least 31.3 %, which can be seen in more detail in figure 3.
Fig. 3 Tetrabomobisphenol-A (TBBPA) leached in curing water of the different RPA covered with increasing percentages of “Vegetal” charcoal.
Figure 4 shows a relative quantification of BPA leached from the curing waters. The general tendency for the stabilization of BPA, the amount of activated carbon used in the production of RPAs is similar to that observed in TBBPA. The greater the dosage, the lower the BPA leached. However, it is important to notice that while all activated carbons show similar behaviors, the Merck carbon proved to be more effective for the stabilization of BPA, though its IN or specific surface area is not the largest of all tested carbons. This could be evidence that, to some degree, other parameters such as the zeta potential or temperature can affect the efficiency of coal as a stabilizing agent. Still, it has been proven that IN is a robust parameter to anticipate the performance of a given carbon for stabilizing BFRs in the production of RPAs.
For the use of non-activated vegetal charcoal, the analysis of BPA leaching in pore waters was similar to the one observed for TBBPA. The amount of BPA leached in dosages of 1.1 to 18.5 % is considerable, but it can be said that in the dosage of 31.3 % stabilization of BPA is efficient.
CONCLUSIONS
In this work we used several activated carbons to produce RPAs with the plastic fraction of WEEE. Through chemical analysis and GC-FID we were able to measure the amount of TBBPA and BPA leached.
The IN was determined for each type of activated carbon used. Based on our findings, we can state that all carbons assayed with an IN higher than 500 can be used in a proportion of 1.1 % in the dosage for the production of RPAs with an effective stabilization of the organic contaminant present in the polymer matrix (BFR). Even though the IN is not related directly to the sorbent capability of the substance, but rather to its porosity, this index can be useful to predict the dosage needed for using a carbon not previously assayed, only by its simple and rapid determination.
Although vegetal non-activated charcoal needs to be added in a higher percentage (31.3 %) due to its low porosity (determined by the IN), this material can be easily obtained from a traditional charcoal factory. It is important to notice that charcoal was not subjected to an activation process, so it was unnecessary to use a power demanding processes like vapor treatments or acids. For this reason, the use of charcoal represents a more environmentally friendly, robust, and less expensive option for local ventures or cooperatives that want to implement the new proposed technology .
In further experiments we will study the effect of CA in the setting cement reaction. This will be crucial to determine the feasibility of using vegetable carbon in such high dosages.
