Method for manufacturing an electrochemical sensor to detect heavy metals from plastic waste

Abstract

The present invention relates to a method for manufacturing an electrochemical sensor based on activated carbon obtained from polyethylene terephthalate (PET) waste, which is prepared as a carbon paste for use in detecting and quantifying heavy metals in aqueous solutions using the square wave voltammetry method. The method comprises using different concentrations of activating agents and activation temperatures, with double heat treatment in oxygen prior to carbonisation.

Description

[0001] METHOD FOR DEVELOPING AN ELECTROCHEMICAL SENSOR FOR DETECTING HEAVY METALS FROM PLASTIC WASTE

[0002] FIELD OF INVENTION

[0003]

[0001] The present invention falls within the field of carbonaceous materials for electrochemical sensors and discloses a method for preparing an electrode as an electrochemical sensor for determining the concentration of heavy metal ions in aqueous solution. Specifically, it discloses a method for preparing an electrode material based on activated carbon obtained from single-use plastic waste called polyethylene terephthalate (PET).

[0004] STATE OF THE ART

[0005]

[0002] Heavy metal pollution is a global problem due to its high toxicity and slow degradation, affecting natural resources such as water and crops, as well as human health. The need to monitor the presence of these metals has driven the development of electrochemical sensors, which are notable for their low cost, high reproducibility, linearity, sensitivity, selectivity, stability, and ease of reuse, compared to more expensive and complex techniques such as gas chromatography, fluorimetry, and other spectroscopies.

[0006]

[0003] Electrochemical sensors often employ carbon materials, such as activated carbon (AC), with petroleum- and coal-derived precursors being common and environmentally unfriendly. In contrast, carbon materials obtained from agricultural waste, agricultural sludge, rubber byproducts, and plastics offer an economical and ecological alternative. Plastic waste, in particular, is an abundant but underutilized source of carbon, with types such as PVC, PS, PET, PE, PFR, LDPE, HDPE, PAN, and PP. These can be transformed into carbonaceous materials through various methods, including inert atmosphere carbonization, high-pressure carbonization, hydrothermal carbonization with H2O2, Joule flash heating, carbothermic shock, and microwave conversion.

[0007]

[0004] In the prior art, there is the article “Promising post-consumer PET-derived activated carbon electrode material for non-enzymatic electrochemical determination of carbofuran hydrolysate,” which describes the use of activated carbon (AC) materials prepared from waste PET bottles as an indirect carbofuran detection platform. The electrochemical activity of the glassy carbon electrode was modified with the obtained PET-AC (PET-AC / GCE), where the carbon paste is formed by incorporating a mineral oil (mineral oil, Nujol, or paraffin) and a conductive solution (KCl) at 0.1 mol-L. 1 They used a conventional three-electrode setup using modified and bare GCE as the working electrode, Ag / AgCl (in saturated KCl) as the reference electrode, and a large platinum (Pt) sheet as the counter electrode. All electrochemical experiments were carried out in an inert atmosphere at room temperature.

[0008]

[0005] We also have patent document CN103121678A, published on May 29, 2013, for “Method for preparing nitrogen-doped activated carbon using PET waste material”; the invention describes a method for obtaining activated carbon from PET waste that was pre-oxidized in an air (oxygen) atmosphere at 300 °C for 18 h, and mixed in a mass ratio of 1:1. The resulting mixture was carbonized at 12 °C min' 1 in a nitrogen atmosphere at room temperature at a heating rate of 8°C min' 1 up to 500 °C for 3 h. After mixing the cooled carbonized product with the activator K2CO3 in a mass ratio of 1:2, the temperature is raised to an activation temperature of 900 °C and a constant temperature of 1 h under a nitrogen flow rate of 12 L h' 1 and a heating rate of 8 °C min' 1The product is then washed with an excess of hydrochloric acid. It also indicates an application in a working sensor with a polytetrafluoroethylene binder and conductive carbon black, with a small amount of absolute ethanol in a mass ratio of 85:5:10, where the Pt electrode is the counter electrode and the mercury / mercury sulfate electrode is the reference electrode. The electrolyte is a 1 mol L⁻¹ H₂SO₄ solution. 1 The volt-ampere cyclic curve is tested in the voltage range of -0.4 to 0.5 V, and the Land system test voltage range is -0.4 to 0.5 V.

[0009]

[0006] Another article considered as a precedent is “Electrochemical study of the removal of Cd (II) ions in aqueous solutions using activated carbon obtained from orange peel”, published on October 30, 2022; the invention describes a process for the removal of Cd 2+Electrochemical measurements were performed using square-wave voltammetry (VOC) with a three-electrode glass electrochemical cell. A graphite rod was used as the counter electrode, and an Ag / AgCl electrode (3 mol L⁻¹) was used as the electrode. 1 KCl) as a reference electrode and a carbon paste electrode (orange peel carbon) as a working electrode. Electrochemical evaluation was performed in different support electrolytes such as HCl, H2SO4, acetate buffer, Britton Robinson buffer, phosphate buffer, acetic acid, KCl, KOH, and NaCl, and in the presence of Cd 2+ 0.01 mol L' 1 for obtaining the optimal electrolyte and then the optimal time for Cd removal was obtained 2+

[0010]

[0007] On the other hand, the article “Removal of heavy metal ions using a functionalized single-walled carbon nanotube: a molecular dynamics study” highlights a characteristic of the significant modification and / or functionalization of carbonaceous materials with functional groups: carboxyl, hydroxyl, and amide. The evaluation of the materials reports an adsorption capacity of between 150% and 230% of metal ions (Pb 2+ , Cu 2+ ' CD 2+ and Hg 2+ ) with the carboxyl groups.

[0011]

[0008] Regarding activated carbon materials from PET, we found patent document CN101979316A, which mentions washing the residual PET with deionized water, placing it in an oven, and baking it at 40 °C for 24 h. After drying, the residual PET is pulverized to obtain an 80-mesh PET powder. Weigh 4 g of K2CO3 activator into 20 mL of deionized water. After dissolving, add 1 g of PET powder and mix thoroughly. After mixing, place in an oven and bake at 90 °C for 24 h to obtain a solid mixture. Then, place the mixed raw materials in a tubular heating oven, under nitrogen protection with a flow rate of 100 mL / min. 1 It is heated to a carbonization temperature of 500 °C at a heating rate of 10 °C min -1 and is maintained for 120 min. Finally, the charred and activated samples are washed with a 0.5 mol L HCl solution 1for 5 times and then washed with deionized water for 5 times. Likewise, patent CN 101708843A describes an alternative use of plastic waste to obtain a high specific surface area, through alkaline hydrolysis and activation in an inert atmosphere.

[0012]

[0009] Among the sensors and methods for obtaining AC from PET reported are the use of materials such as: modified glassy carbon, noble metals deposited on carbonaceous materials and / or organic modifications; reaching values ​​of: 15.4 x 10' 9 mol L' 1 of Pb 2+ , 0.5 x 10' 6 g L' 1 of Pb 2+ and 0.8 x 10' 6 g L' 1 from Cd 2+However, no studies have been found indicating a melting stage in an air or oxygen atmosphere prior to the chemical activation process. Additionally, no research has been reported on the application of activated carbon obtained from recycled PET as a working electrode in electrochemical sensors, nor on the use of a three-electrode system for the detection of heavy metals such as Cd. 2+ and Pb 2+ , with a detection limit of up to 9 x 10' 12 mol L' 1 unlike other electrochemical sensors reported in the state of the art.

[0013] DESCRIPTION OF THE INVENTION

[0014]

[0010] Based on the aforementioned technical problem, the present invention was developed as a solution, which proposes an electrochemical sensor for the detection of heavy metals from plastic waste comprising the following steps: a. Washing and cutting polyethylene terephthalate (PET) waste into pieces; b. Performing a first heat treatment in the presence of oxygen on the cut PET pieces at a temperature between 200 and 400 °C, until liquid PET is obtained; c. Mixing the liquid PET with an activating agent, where the ratio of PET to activating agent is: 1:1, 1:2, 1:3 or 1:4, at a temperature between 80 and 100 °C; obtaining an activated solid phase; d. Adding to the activated solid phase a volume of ethyl alcohol at a concentration of 50-90% v / v (made up with water), under stirring, and then drying at a temperature between 50-60 °C; obtaining a dry solid phase; e.Perform a second heat treatment, in the presence of oxygen, with the dry solid phase at a temperature between 300-400 °C, then cool and pulverize it; obtaining a solid mixture; f. Carbonize the solid mixture at a temperature between 700-900 °C in an inert atmosphere, obtaining activated carbon; the activated carbon is washed with an acidic or neutral solution and dried; g. Form the carbon paste, incorporating mineral oil, Nujol or paraffin, along with a conductive solution; and, h. Form the electrochemical sensor, where the electrochemical sensor includes three electrodes: an auxiliary electrode, a reference electrode, a working electrode that includes the carbon paste from the previous step, and an electrolyte.

[0015]

[0011] Electrochemical detection of Cd metal ions 2+ and Pb 2+The experiment is conducted by assembling a 5-cell electrochemical cell, placing the reference electrode, the counter electrode, and the working electrode. Each of these electrodes is connected to the potentiostat / galvanostat wires. The electrochemical results are obtained through the current / potential response. The electrochemical technique used is square wave voltammetry, employing an amplitude of 75 mV and a frequency of 10 Hz.

[0016]

[0012] The present invention provides for the preparation of an electrochemical sensor composed of activated carbon from PET, which achieves a detection limit of up to 9 x 10' 12 mol L -1 ; using the developed electrochemical sensor and employing the carbon paste electrode (CPE) containing carbon obtained at 900 °C (CA-900), and a supporting electrolyte of 0.1 mol L- HCl 1obtaining the lead signal at a potential of -0.6 V. These nanomolar-scale traces are due to the high sensitivity of the carbonaceous material used. The activated carbon of the present invention has a high variety of mesopores and electrical conductivity, represented by a high conductivity obtained by electrochemical impedance spectroscopy measurements by plotting the data obtained on the Nyquist diagram, whose results show a greater electron transfer compared to other developed materials.

[0017]

[0013] Regarding the technical contribution, it is important to highlight that, in the proposed method, the mixture of a basic medium, such as NaOH, and ethanol produces a synergistic effect due to transesterification reactions that cleave the long polymer chains and anchor the ethanol to the PET surface. However, some oligomers were suspended on the PET surface when the ethanol substituted the ester groups of the long polymer chains, allowing for hydrophilicity. This contributes positively and significantly to the final properties of the electrochemical sensor.

[0018]

[0014] Furthermore, one of the advantages of the first heat treatment is that it allows deeper penetration of the activating agent into the surface of the activated solid phase (carbon) through its primary pores. Additionally, the double heat treatment increases surface roughness and the number of surface oxygen functional groups (carbonyl or quinone). Likewise, the first heat treatment contributes to the formation of carbonaceous material, while the second treatment helps to remove residual compounds.

[0019]

[0015] It should be noted that the present invention considers the appropriate amount of heat to avoid the major formation of CO and CO2, due to the decarbonylation of carbonite groups (radical reaction) and carboxyl groups (concerted reaction), respectively.

[0020]

[0016] It is also shown that PET can be used as a raw material for the synthesis of activated carbon.

[0021]

[0017] The use of an electrochemical sensor employing PET detects Pb metal ions 2+ and CD 2+ with high sensitivity and fast response.

[0022]

[0018] The active material (AC) synthesis method is low cost, easy to develop and contributes to the circular economy because it includes the use of plastic waste, which generally ends up damaging various natural ecosystems.

[0023] BRIEF DESCRIPTION OF THE FIGURES

[0024]

[0019] The accompanying drawings illustrate only exemplary embodiments of the invention and should not be considered as limiting its scope; the invention may admit other equally effective embodiments.

[0025] FIGURE 1. Flow diagram of the synthesis of activated carbon obtained from PET plastic waste

[0026] FIGURE 2. SEM Morphography of sample CA-700, activated with NaOH at 700 °C. Roughness and porosity are observed on a micrometer scale, which can be related to the activation of the material.

[0027] FIGURE No. 3. SEM Morphography of the CA-800 sample, activated with NaOH at 800°C, showing the formation of pores on the surface of the material on a micrometer scale as a result of the activation process.

[0028] FIGURE 4. SEM micrograph of sample CA-900, activated at 900 °C using NaOH as the activating agent. The formation of porosity on the surface of the material as a consequence of the activation process is evident.

[0029] FIGURE 5. Raman spectra (a), FT-IR infrared spectra (b), X-ray diffraction patterns. FIGURE 6. a) N2 adsorption / desorption isotherm at 77K of samples CA-700, CA-800 and CA-900 and b) pore size distribution vs. pore volume.

[0030] FIGURE 7. Comparison of carbon paste electrodes (CPE) formed from CA-700, CA-800 and CA-900 using different electrolytes in the presence of Cd 2+ To determine the electrolyte with the best electrochemical response, as evidenced by the current height. The dotted line indicates that HCl is the optimal electrolyte.

[0031] FIGURE 8. Comparison of carbon paste electrodes (CPE) formed from CA-700, CA-800 and CA-900 using different electrolytes in the presence of Pb 2+ To determine the electrolyte with the best electrochemical response, as evidenced by the current height. The dotted line indicates that HCl is the optimal electrolyte.

[0032] FIGURE 9. Cyclic voltammograms of the carbon paste electrodes (CPE) formed from samples CA-700, CA-800, and CA-900. Using a speed of 25 mV s -1 and as a supporting electrolyte 5 mmol L -1 of Faith 2 7Fe3+ in 0.1 mol L- 1 from KCI.

[0033] FIGURE 10. Electrochemical impedance spectra of carbon paste electrodes (CPE) formed from samples CA-700, CA-800 and CA-900. Using 5 mmol L as the supporting electrolyte -1 of Faith 2 7Fe 3+ in 0.1 mol L -1 from KCI.

[0034] FIGURE No. 11. Cyclic voltammetry at 25 mV s _1 of the CA-700 / EPC, CA-800 / EPC and CA-900 / EPC sensors in the presence of Cd 2+ and Pb 2+ using 0.1 mol L as the supporting electrolyte 1 of HCI

[0035] FIGURE No. 12. Square wave voltammetry at 75 mV and 10 Hz of the CA-700 / EPC, CA-800 / EPC and CA-900 / EPC sensors in the presence of Cd 2+ and Pb 2+ using 0.1 mol L as the supporting electrolyte -1 of HCI

[0036] FIGURE 13. Stability of the CA-700 / EPC, CA-800 / EPC and CA-900 / EPC sensors in the presence of Cd 2+ and Pb2+ using 0.1 mol L as the supporting electrolyte 1 of HCI

[0037] FIGURE 14: Optimal parameters of the CA-700 / EPC, CA-800 / EPC and CA-900 / EPC sensors, in the presence of Cd 2+ and Pb 2+ using 0.1 mol L- as the supporting electrolyte 1 Figure 15. Calibration curves of the CA-700 / EPC sensor, in the presence of Cd 2+ and Pb 2+ using 0.1 mol L as the supporting electrolyte 1 of HCI

[0038] FIGURE No. 16. Calibration curves of the CA-800 / EPC sensor, in the presence of Cd 2+ and Pb 2+ using 0.1 mol L as the supporting electrolyte 1 of HCI

[0039] FIGURE No. 17. Calibration curves of the CA-900 / EPC sensor, in the presence of Cd 2+ and Pb 2+ using 0.1 mol L as the supporting electrolyte -1 of HCI

[0040] FIGURE 18. Comparison of carbon paste electrodes formed from different grades of ethyl alcohol (10 o , 50° and 100°) using Pb electrolyte 2+ to determine the electrolyte with the best electrochemical response evidenced by the current height.

[0041] DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

[0042] Preferred method for obtaining activated carbon from PET

[0043]

[0020] The present invention describes a method for the production of activated carbons from PET plastic waste and their use as an electrode material for an electrochemical sensor for the detection of heavy metals.

[0044]

[0021] The preparation of active materials as a component of electrodes for sensors is carried out using the following method developed by the inventors, which comprises the following steps: a. Washing and cutting into pieces some polyethylene terephthalate (PET) waste; b. Performing a first heat treatment in the presence of oxygen on the cut PET pieces at a temperature between 200 and 400 °C, until liquid PET is obtained; c. Mixing the liquid PET with an activating agent, where the ratio of PET to activating agent is: 1:1, 1:2, 1:3 or 1:4, at a temperature between 80 and 100 °C; obtaining an activated solid phase; d. Adding to the activated solid phase a volume of ethyl alcohol at a concentration of 50-90% v / v (made up with water), under stirring, and then drying at a temperature between 50-60 °C; obtaining a dry solid phase; e.Perform a second heat treatment, in the presence of oxygen, with the dry solid phase at a temperature between 300-400 °C, then cool and pulverize it; obtaining a solid mixture; f. Carbonize the solid mixture at a temperature between 700-900 °C in an inert atmosphere, obtaining activated carbon; the activated carbon is washed with an acidic or neutral solution and dried; g. Form the carbon paste, incorporating mineral oil, Nujol or paraffin, along with a conductive solution; and, h. Form the electrochemical sensor, where the electrochemical sensor includes three electrodes: an auxiliary electrode, a reference electrode, a working electrode that includes the carbon paste from the previous step, and an electrolyte.

[0045]

[0022] This process can be divided into 3 main stages:

[0046]

[0023] First stage: Conditioning of the precursor. It begins with the collection of PET, washing with distilled water and disinfection preferably with 1% NaClO, then the PET plastic is cut into pieces and dried at a temperature between 40 - 60 °C for 12 - 24 h.

[0047]

[0024] Second stage: first pretreatment where the first smelting is carried out and the activation that is carried out in a second smelting, which includes the following steps:

[0048] - In step b) the temperature of the first heat treatment is between 200 - 400 °C for 20 to 30 min,

[0049] - In step c) the activating agent can be: KOH, CO3 2 HCOs 1- or NaOH,

[0050] - In step c) the ratio of PET to the activating agent is: 1:1, 1:2, 1:3 and 1:4, with the temperature between 80-100 °C for 8-12 h,

[0051] - In step d) the concentration of the ethyl alcohol solution is preferably 70%, - In step d) the volume of ethyl alcohol is between 10 - 20 mL and the volume of water is 5 - 10 mL, for a PET mass of 5 g.

[0052] - In step e) the second heat treatment is carried out at a temperature between 300 - 400 °C for 2 - 3 h.

[0053]

[0025] Third stage: Pyrolysis of the activated product. This is carried out at a high temperature in an inert atmosphere, obtaining activated carbon as the product.

[0054] - In step f) the carbonization temperature is between 700 - 900 °C applied preferably for 1 h and the inert atmosphere is a nitrogen or argon gas.

[0055] - In step f) the reagents used for washing the activated carbon are: ultrapure water and 0.1 - 6 mol L HCl -1 , and the drying temperature is 40 - 60 °C for 12 to 24 h.

[0056] Method for preparing an electrode for an electrochemical sensor from activated carbon obtained from PET plastic waste

[0057]

[0026] The preparation of the electrochemical sensor is carried out following these steps:

[0058] - In step g) the additive can be mineral oil, nujol or paraffin and the volume is 50 to 70 pL, preferably 60 pL, for a mass of 2 mg of activated carbon from PET.

[0059] - In step g) the preferred conductive solution is KCl 0.05 mol L -1 with a volume of 0.20 to 0.30 mL, preferably 0.25 mL, for a mass of 2 mg of activated carbon from PET.

[0060] - In step h) the mixture is deposited on the carbon paste electrode.

[0061] - In step h) the reference electrode can be Ag / AgCl, Hg / Hg2Cl2 or Pt / H2 / H + .

[0062] - In step h), the counter electrode can be a graphite rod, steel, or platinum wire. - In step h), the electrolyte can be any of the following solutions: HCl, H2SO4, CH3COOH, phosphate buffer, acetate buffer, Britton Robinson buffer, KCl, NaCl, KNO3, KOH, or NaOH.

[0063]

[0027] This sensor simultaneously detects Cd ions 2+ and Pb 2+ using the electrochemical technique of square wave voltammetry employing an amplitude of 75 mV and a frequency of 10 Hz.

[0064] Example of another preferred way of carrying out the invention

[0065]

[0028] For the synthesis of activated carbon from single-use plastic waste (PET), the following procedure is used: plastic bottles are cut into approximately 1 cm x 1 cm pieces, washed with ultrapure water, and dried in an oven at 60 °C for 24 hours. Then, 5 g of the plastic material is weighed and heated in a muffle furnace at 300 °C for 30 minutes. The resulting sample is activated with a solution of 5 g of NaOH in 20 mL of ultrapure water, with a 1:1 ratio of activating agent to PET. The mixture is heated to 100 °C for 10 hours, and ultrapure water is added to maintain a constant volume of 100 mL. A 2:1 ratio of ethanol to water is then added, the mixture is stirred for 30 minutes, and finally dried in an oven at 60 °C. Immediately afterwards, the solid sample is taken back to the muffle furnace for 3 hours at a temperature of 400 °C, for melting.The sample is then carbonized in an N2 atmosphere system at a rate of 5 °C min. -1 The sample, designated CA-800, is heated to a maximum temperature of 800 °C for 1 hour and then cooled to room temperature. Finally, CA-800 is washed to a neutral pH (6-7) and dried at 80 °C for 24 hours. Activated carbons treated at different carbonization temperatures of 700, 800, and 900 °C are obtained, designated CA-700, CA-800, and CA-900, respectively.

[0066] Physicochemical characterization of activated carbons

[0067]

[0029] The physicochemical characterization of the CA-PET was performed using scanning electron microscopy (SEM) (Figures 2-4), which revealed its morphology. The functional groups were determined using Fourier transform infrared spectroscopy (FTIR) (Figure 5). Raman spectroscopy (Figure 5) was used to determine the characteristic vibrational modes in its graphitic lattice, specifically the G and D bands. Finally, X-ray diffraction (XRD) (Figure 6) was used to determine the crystallinity, grain size, and Bravais lattices; these results are shown in Figure 4. The material's porosity and surface area were determined by N2 adsorption / desorption analysis at 77 K (Figure 6).

[0068] Electrochemical characterization of activated carbon

[0069]

[0030] 50 mg of synthesized activated carbon (CA-700, CA-800 or CA-900), previously ground manually to a fine powder, is weighed and mixed with 250 pL of 0.05 mol L' KCl 1 It is then dried in an oven at 60 °C and 60 pL of Nujol is added, thus obtaining the activated carbon paste.

[0070]

[0031] The paste is then deposited onto a carbon paste electrode (an electrode with a 3 mm diameter and 2 mm deep hole and a Teflon-coated steel contact). The working electrodes obtained are: CA-700 / EPC, CA-800 / EPC, and CA-900 / EPC. Electrochemical characterization is performed using a 0.1 mol L⁻¹ HCl electrolyte. 1 , a graphite counter electrode and an Ag / AgCl reference electrode (KCl 3 mol L- 1The electrochemical behavior of the working electrodes is evaluated using electrochemical techniques: cyclic voltammetry (VC), square wave voltammetry (VOC), and electrochemical impedance spectroscopy (EIE).

[0071] Simultaneous detection of Cadmium (Cd 2+ ) and Lead (Pb 2+ ) using the CA-700 / EPC, CA-800 / EPC and CA-900 / EPC sensors.

[0072]

[0032] For electrochemical detection, the choice of electrolyte is important; therefore, the following electrolytes were evaluated: HCl, H2SO4, CH3COOH, phosphate buffer, acetate buffer, Britton Robinson buffer, KCl, NaCl, KNO3, KOH, and NaOH, using the electrochemical VOC technique (Figures 5-6). The measurement parameters were: 75 mV amplitude and 10 Hz frequency. To evaluate the conductivity of the sensors, the EIE and VC techniques were used in an Fe solution. 2+ / Faith 3+ 0.5 mol L' 1 in KCI 0.1 mol L' 1in a frequency range of 0.1 Hz - 100 KHz and 10 mV amplitude and the open circuit potential (OCP) (Figure 7 - 8).

[0073]

[0033] To evaluate the sensitivity of the sensors (CA-700 / EPC, CA-800 / EPC, CA-900 / EPC) the VOC technique is used employing the electrolyte HCl 0.1 mol L' 1 and adding volumes of Pb ions 2+ and CD 2+ in a concentration range of 10' 10 up to 10' 2 mol L' 1 (Figure 9-10). For the analysis, a 10 mL electrolyte volume was used, with an amplitude of 75 mV, a frequency of 10 Hz, a potential step of 5 mV, and an equilibrium time of 10 s. These parameters were repeated for each electrolyte. The optimal electrolyte is 0.1 mol L⁻¹ HCl. 1 with which the stability of the CA-700 / EPC, CA-800 / EPC, CA-900 / EPC sensors is evaluated, using the VOC technique in the presence of Pb ions 2+ and CD 2+ (Figure 11).

[0074] Additional examples of implementation with variation of ethanol concentration:

[0075] Example 1. Use of 10% ethanol in the synthesis of PET- 900°C activated carbon

[0076] Activated carbon synthesis

[0077]

[0033] Disinfected and cut PET (12 g) was heated between 350-380 °C, then activated with 1 mol L' NaOH 1 (activating agent). The activating agent:water wt / wt ratio was 100 / 5 and the mixture was kept in contact for 10 h at 100 °C. 20 mL of 10% ethanol was added and stirred for 30 min.

[0078] It was left in the oven at 60 °C for drying and then placed in the muffle furnace at 400 °C for 3 h. Activation was carried out under a flow of N2 at 5 °C min' 1 by 1 ha 900 °C. The sample obtained, called activated carbon, was allowed to cool to room temperature and treated with HCl (0.1 mol L' 1) until reaching a pH of 7. Subsequently, the activated carbon was left in an oven at 60 °C for 24 h for drying.

[0079] Electrochemical characterization of activated carbon

[0080]

[0034] A mixture of 22.5 mg of activated carbon, previously ground manually to a fine powder, was made with 2.5 g of graphite, 80 pL of 0.5 mol L' HCl 1 The mixture was dried in an oven at 60 °C for 30 min, and then 60 pL of Nujol was added to obtain a mixture called activated carbon paste. The activated carbon paste was then placed on a carbon paste electrode. Electrochemical characterization was performed using a 0.5 mol L⁻¹ HCl electrolyte. 1 , a graphite counter electrode and an Ag / AgCl reference electrode (KCl 4 mol L' 1 ); in a potential window between -0.7 to -0.4 V. Figure 18 shows the voltammogram obtained, where the detection of Pb is shown. 2+10' 4 mol L' 1 with additions of 100 to 400 pL.

[0081] Example 2

[0082] Use of 50% ethanol in the synthesis of PET-900°C activated carbon. Activated carbon synthesis

[0083]

[0035] Disinfected and cut PET (12 g) was heated between 350-380 °C, then activated with 1 mol L' NaOH 1 (activating agent). The activating agent:water wt / wt ratio was 100 / 5 and the mixture was kept in contact for 10 h at 100 °C. 20 mL of 50% ethanol was added and stirred for 30 min.

[0084] It was left in the oven at 60 °C for drying and then placed in the muffle furnace at 400 °C for 3 h. Activation was carried out under a flow of N2 at 5 °C min' 1 by 1 ha 900 °C. The sample obtained, called activated carbon, was allowed to cool to room temperature and treated with HCl (0.1 mol L' 1until a pH of 7 was reached. Subsequently, the activated carbon was left in an oven at 60 °C for 24 h for drying. Electrochemical characterization of activated carbon

[0085]

[0036] A mixture of 22.5 mg of activated carbon, previously ground manually to a fine powder, was made with 2.5 g of graphite, 80 pL of 0.5 mol L' HCl 1 The mixture was dried in an oven at 60 °C for 30 min, and then 60 pL of Nujol was added to obtain a mixture called activated carbon paste. The activated carbon paste was then placed on a carbon paste electrode. Electrochemical characterization was performed using a 0.5 mol L⁻¹ HCl electrolyte. 1 , a graphite counter electrode and an Ag / AgCl reference electrode (KCl 4 mol L' 1 ); in a potential window between -0.7 to -0.4 V. Figure 18 shows the voltammogram obtained, where the detection of Pb is shown. 2+ 10' 4 mol L-1 with additions of 100 to 400 pL.

[0086] Example 3: Use of 100% ethanol in the synthesis of PET- 900°C activated carbon

[0087] Activated carbon synthesis

[0088]

[0037] Disinfected and cut PET (12 g) was heated between 350-380 °C, then activated with 1 mol L' NaOH 1 (activating agent). The activating agent:water wt / wt ratio was 100 / 5 and the mixture was kept in contact for 10 h at 100 °C. 20 mL of 100% ethanol was added and stirred for 30 min.

[0089] It was left in the oven at 60 °C for drying and then placed in the muffle furnace at 400 °C for 3 h. Activation was carried out under a flow of N2 at 5 °C min -1 by 1 ha 900 °C. The sample obtained, called activated carbon, was allowed to cool to room temperature and treated with HCl (0.1 mol L' 1) until reaching a pH of 7. Subsequently, the activated carbon was left in an oven at 60 °C for 24 h for drying.

[0090] Electrochemical characterization of activated carbon

[0038] A mixture of 22.5 mg of activated carbon, previously ground manually to obtain a fine powder, was made with 2.5 g of graphite, 80 pL of 0.5 mol L' HCl 1 The mixture was dried in an oven at 60 °C for 30 min, and then 60 pL of Nujol was added to obtain a mixture called activated carbon paste. The activated carbon paste was then placed on a carbon paste electrode. Electrochemical characterization was performed using a 0.5 mol L HCl electrolyte. -1 , a graphite counter electrode and an Ag / AgCl reference electrode (KCl 4 mol L -1 ); in a potential window between -0.7 to -0.4 V. Figure 18 shows the voltammogram obtained, where the detection of Pb is shown. 2+ 10' 4 mol L'1 with additions of 100 to 400 pL.

Claims

CLAIMS 1. A method for preparing an electrochemical sensor for detecting heavy metals from plastic waste, characterized by comprising the following steps: a. Washing and cutting polyethylene terephthalate (PET) waste into pieces; b. Performing a first heat treatment in the presence of oxygen on the cut PET pieces at a temperature between 200 and 400 °C, until liquid PET is obtained; c. Mixing the liquid PET with an activating agent, wherein the ratio of PET to activating agent is 1:1, 1:2, 1:3, or 1:4, at a temperature between 80 and 100 °C; obtaining an activated solid phase; d. Adding a volume of ethyl alcohol at a concentration of 50-90% v / v to the activated solid phase, under stirring, and then drying at a temperature between 50-60 °C; obtaining a dry solid phase; e.Perform a second heat treatment, in the presence of oxygen, with the dry solid phase at a temperature between 300-400 °C, then cool and pulverize it; obtaining a solid mixture; f. Carbonize the solid mixture at a temperature between 700-900 °C in an inert atmosphere, obtaining activated carbon; the activated carbon is washed with an acidic or neutral solution and dried; g. Form the carbon paste, incorporating mineral oil, Nujol or paraffin, along with a conductive solution; and, h. Form the electrochemical sensor, where the electrochemical sensor includes three electrodes: an auxiliary electrode, a reference electrode, a working electrode that includes the carbon paste from the previous step, and an electrolyte.

2. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the activating agent is KOH, COs 2 ' , HCO3 1 Or NaOH.

3. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the conductive solution is KCl 0.05 mol L' 1 .

4. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the inert atmosphere is nitrogen or argon.

5. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the reference electrode is Ag / AgCl, Hg / Hg2Cl2 or Pt / H2 / H + 6. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the activated carbon is washed with ultrapure water and 0.1 - 6 mol L' HCl 1 , and the drying temperature in stage f) is 40 - 60 °C for 12 to 24 h.

7. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the counter electrode is a graphite, steel or platinum wire rod.

8. The method of preparing the electrochemical sensor, according to claim 1, characterized in that the electrolyte is any of the following solutions: HCl, H2SO4, CH3COOH, Phosphate buffer, Acetate buffer, Britton Robinson buffer, KCl, NaCl, KNO3, KOH or NaOH.

Images