1 Introduction

During crude oil extraction, a high amount of water is produced as a by-product that comes to the surface. This fluid contains high levels of inorganic and organic compounds, oils, metals and suspended and dissolved solids, which can cause negative impacts on nature and the environment (Bouafia et al., 2023). Currently, the production of water produced in the world has reached approximately 250 million barrels per year (Sher et al., 2023). Additionally, the RAP water–oil ratio (barrels of water generated/barrel of crude oil produced) worldwide is around 3 and 5. These values range between 10 and 14 for mature fields. In the case of Colombia, for each barrel of oil, an average of 12 to 13 barrels of water is produced (Almansa et al., 2018, Mesa et al., 2018). However, produced water must be treated for its final disposal (Carmona et al., 2017). Considering the volume of water associated with hydrocarbon production and its physicochemical properties, it is important to treat it so as not to affect the total water footprint. These conditions allow that, once treated, they can be reused in rural activities and projects influenced by the hydrocarbon sector. Likewise, the hydrocarbon sector is interested in reducing the contamination of surface discharges and their effect on drinking water sources (Mesa et al., 2018).

Wastewater from hydrocarbon extraction is used for different purposes: reinjection into the reservoir to maintain pressure, subterranean injection, discharge, agriculture and injection as final destination among others (Feodorov et al., 2016; Marathe et al., 2021; Jiang et al., 2022). Also, water quality depends on the region, geological formation and other fluids involved in the recovery process (Gómez et al., 2022, Belmont et al., 2017, Shahrashoub et al., 2021, Suresh et al., 2012). The different methods and subgroups applied for treatment of effluents in the oil industry are shown in Fig. 1 (Varjani et al., 2020).

Fig. 1
figure 1

Treatments in the process of wastewater from the petroleum industry

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Primary treatment allows the separation of the phases through physical principles, contributing to a water stream with minimal traces of crude oil and solids present. Secondary treatment continues with the cleaning of the effluent, allowing the separation of emulsions and traces of impurities present in the production water. This method includes gas flotation processes, using equipment that allows the entry of a specific gas into the treated fluid and whose bubbles adhere and float with the crude oil which needs to be removed. Finally, tertiary treatment is carried out through the use of adsorbent materials and chemical mechanisms in order to achieve the technical specifications required for the treated production water, depending on its final destination.

In addition, this method can use advanced oxidation and membrane technologies, but these types of procedures can have an excessive cost depending on the purity required. In the last stage, water treatment with adsorbent materials has attracted/awaken research interests, especially materials like clays, zeolites, activated carbons, exfoliated graphene, and graphene oxide (Kumari et al., 2020, Gul et al., 2021). Non-conventional adsorbent materials include natural products such as bentonite and some organo-clays, and residual vegetable biomass as waste from fruit growing and agriculture (Dawoud et al., 2021).

Regardless of the alternatives for reusing or discharging wastewater, it is important to treat it to comply with regulations for meeting significant concentrations of organic and inorganic pollutants. However, the treatment method depends on the physicochemical characteristics of the produced water. The environmental risks of water from the oil sector derive from its content of total solids, fats and oils, heavy metals, among others (Mesa et al., 2018, Naranjo 2023). Phenol is a representative compound of the organic substances present in oilfield production waters, and its presence is considered a serious environmental pollutant. To reduce this effect, studies related to the adsorption mechanism have been considered. Phenol removal with activated alumina and activated carbon from coconut coir and rice husk ash showed yields of 21.8%, 95.2%, and 94.23%, respectively (Mandal et al., 2019).

Therefore, to improve the purification stages, the use of adsorbent materials has been implemented, due to their high availability and economic viability in terms of cost/benefit ratio (Mariana et al., 2021). However, different mechanisms are required heavy metal and organic compound removal from effluents, which differ in specification and removal efficiency (Gul et al., 2021). For example, organic wastes such as rice husk and orange-peel powder were used as adsorbents for arsenic removal. The results show that arsenic removal was in the range of 82.2 to 96.38%, and the efficiency increases with the increasing particle size of the organic waste (Appiah et al., 2022).

Activated carbon is characterized as an adsorbent agent, capable of ensuring the removal of organic compounds, heavy metals and elimination of free chlorine in water treatment (Alfatah et al., 2021; Aniagor et al., 2021; Li et al., 2020). From the residual biomass and its nature, it is possible to obtain activated carbons by physical and/or chemical activation processes, and according to the process conditions, it is possible to obtain well-constituted pore structures, a large surface with organized macro-, meso- and microporous textural properties and active functional groups. León et al. 2022 developed the methodology for obtaining activated carbons with defined textural properties with specific surface area values in the range of 990 to 1217 m2 g−1 and mesoporous structure. The results in the research allowed removal efficiencies of methylene blue and ferric chloride in the range of 97.1 to 89.8 and 94.4 to 82.3%, respectively. Currently, this product is being widely studied, since the specific surface area properties obtained from this production chain’s waste exceed the results found by implementing commercial activated carbon (Dhahri et al., 2022; Tuomikoski et al., 2021).

On the other hand, for the improvement of water treatment, adsorbent materials with nanoparticles, which allow the elimination of materials with high selectivity, are of great interest (Orha et al., 2017; Thalgaspitiya et al., 2020). Matos et al. (2009) prepared activated carbons from Tabebuia Pentaphyla wood sawdust impregnated with TiO2 nanoparticles for the removal of 4-chlorophenol in polluted waters. The results of photoactivity with solar energy showed that the technology makes it possible to obtain clean water in short periods of time. In the same line of research, several authors have succeeded in synthesizing activated carbons from different vegetable biomass residues in the presence of nanoparticles. For example, the results reveal significant efficiencies in the removal of phosphorus contaminants with an improvement in the viscosity and calorific value of treated oil (Syahruddin et al., 2022).

Bayode et al. (2021) developed efficient photosensitive nanocomposites made of clay, Fe2O3 and graphene oxide (GO). These materials under visible light allowed the removal of steroidal estrogens within the range of 58–80%. Jain et al. (2018) synthesized activated carbons with iron oxide nanoparticles (Fe3O4/AC) through the coprecipitation method for the removal of Cr+4, Cu+2 and Cd+2 ions from polluted waters. The adsorption efficiency of the ions was in the range around 75 to 90%. In addition, desorption tests with 0.1 M HCl indicated that the adsorbent material (Fe3O4/AC) can be effectively regenerated. This process can be conducted in four adsorption–desorption cycles. In other studies, biochar from sweet lemon peel has been used for the removal of lead (Pb+2) in battery effluents. In this method, the removal efficiency was around 97.1% (Kumar et al., 2020).

Karnib et al. (2014) and Al-Zboon (2018) performed tests to remove lead, cadmium, nickel, chromium, zinc and phosphate from polluted water using activated carbon with SiO2 nanoparticles. The addition of nanoparticles significantly improved the removal efficiency of the activated carbon. The obtained correlation coefficient (R2) for different adsorbents suggested a good fit of the experimental data to the Freundlich adsorption isotherm. Yi et al. (2019) prepared hydrophobic silica aerogels, guided by the solgel exchange method, which were preheated at 500 °C. The water treatment and adsorption performance of the adsorbent material showed high efficiency in the removal of organic materials such as nitrobenzene, phenol and methylene blue. The adsorption capacity of methylene blue was higher, which could be associated with electrostatic interaction. Xing et al. (2020) prepared activated carbon membranes and SiO2 nanoparticles to decrease the environmental risk of atrazine in agricultural production water. The results showed that the porous membrane allows high selectivity and permeability of atrazine.

On the other hand, nanoparticles are implemented as an improved method in the water treatment process due to their larger surface area, smaller intra-article distance and a larger number of reaction sites differing from conventional methods, which are usually limited by the number of active sites or lack of selectivity to remove pollutants (Kotia et al., 2020; Qu et al., 2013). Nanoparticles obtained from metal oxides are capable of removing organic and inorganic compounds, considering their positive qualities such as high efficiency, high variety of magnetic properties and low cost (Xu et al., 2012). Despite presenting these potential characteristics for high adsorption efficiency, due to its high surface energy, instability of the material occurs and it is susceptible to agglomeration due to van der Waals force (Hua et al., 2012). For this reason, the structure of this nanomaterial tends to be modified with surfactants or be supported in a porous structure for its implementation.

Residual cocoa biomass is a source for the generation of value-added products in Colombia. In this sense, the department of Santander is the largest cocoa producer in Colombia (54,000 hectares planted). In addition, the yield is around 0.640 tons per hectare, generating significant amounts of residual biomass (Solarte et al., 2022; Rodriguez et al., 2023). Therefore, the efficient use of residual cocoa shell biomass is an opportunity to obtain activated carbon and its subsequent use in the treatment of wastewater from the oil industry. In order to reduce the negative impacts on the water resource and guarantee its sustainability over time, this research proposes the use of activated carbon from cocoa shells' residual biomass.

The research will support the flagship mission of the Sustainable Development Organization (SDO), focused on sustainable water use and minimization of pollution risks, and will also serve as a basis for studies on the implementation and utilization of residual biomass. In addition, the use of residual plant biomass can contribute to solving the challenges of the bioeconomy in the generation of value-added products. This research shows the adsorption behavior of activated carbon impregnated with silicon dioxide nanoparticles (SiO2) for the removal of pollutants detected in wastewater. The analysis was carried out through Fourier transform infrared spectroscopy (FTIR), specific surface area and UV–Vis spectroscopy, where the number of impregnated nanoparticles of two pollutants, phenol and 3,4-dimethoxybenzyl alcohol at different concentrations, was studied.

2 Experimental section

2.1 Materials and reagents

All reagents used were analytical grade reagents. The reactions were carried out in the presence of phosphoric acid (H3PO4, 95% purity), ethanol (C2H5OH, 96% purity), AEROSIL® 380 silicon dioxide nanoparticles (SiO2), phenol (C6H6O, 99.9% purity) and 3,4-dimethoxybenzyl alcohol (C9H12O3, 97.49% purity). All chemicals and reagents were purchased from Sigma-Aldrich©. To carry out the experiment, a shredder model TRF 300 (TRAPP©), a sieve model Gran Test (Pinzuar©), an oven model UN 110 (Carbolite Gero Ltd.©), a horizontal shaker model F340 (FALC Instruments S.R.©), a hot plate (RC Velp Scientifica©), a FTIR spectrophotometer model Nicolet IS50 (Thermo Fisher Scientific©), a centrifuge model 320R (Hettich GmbH & Co©), an ultrasonic cleaning unit model S300H (Elmasonic©), an UV–Vis spectrophotometer (Thermo Fisher Scientific©) and a sterilizer model SN30 (Memmert GmbH©) were used.

2.2 Experimental diagram

This research depicts the adsorption behavior of activated carbon impregnated with SiO2 nanoparticles for the removal of some pollutants detected in wastewater, which are phenol and 3,4-dimethoxybenzyl alcohol. This section describes the experimental phases developed for this research, as illustrated in Fig. 2.

Fig. 2
figure 2

Research workflow diagram

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2.3 Carbon activation

For the preparation of the activated carbons, the methodology developed by León et al. (2022) was used, where cocoa fruit shells from the Santander region were selected as precursor material. Initially, the cocoa shells were put through a grinding process where different particle diameters were obtained. The crushed material was dried in the sun for 72 h and then classified into the different particle diameters. The material went through a drying process in an oven at 105 °C for an 8-h period. This process was repeated until the weight of the sample was constant. Subsequently, the sample was subjected to two calcination processes, and the first was at 300 °C for a period of 1 h and the second at a temperature of 800 °C for 2 h of reaction. Finally, the sample was cooled in a desiccator and weighed so that it was ready for the activation process.

The biomass activation process was carried out using the chemical activation method. Biomass activation was carried out using H3PO4 at 95%, with a 1.5 impregnation ratio. During the activation stage, approximately 300 g of dry and shredded cocoa shells was weighed, and then, an amount of H3PO4 was added according to the impregnation ratio setting. During this process, the mixture was kept in constant agitation at 100 °C during 1 h of reaction. The preparation of activated carbon was carried out in an oven in a thermochemical process, in which 200 g of activated cocoa shells was selected. This thermochemical process was carried out in an inert atmosphere with nitrogen (N2) at a flow rate of 20 mL min−1. The temperature and reaction time were 480 °C and 30 min. Subsequently, the activated carbon samples were subjected to the washing process with water for a suitable period, where a pH in the interval of 6 to 7 was verified in the washing water. Finally, in the drying process, the samples were placed in an oven at a temperature between 100 and 105 °C for 24 h.

2.4 Preparation of activated carbon impregnated with SiO2 nanoparticles

Activated carbon/SiO2 was prepared using the coprecipitation method, in which suspensions in ethanol at concentrations of 1800, 3600 and 5400 mg L−1 SiO2 were put out using a sonicator to stabilize them. Subsequently, 4 g of carbon was added to the stabilized fluid and again subjected to the sonicator for one hour. The resulting impregnated carbon was refluxed for 3 h, washed and filtered. Finally, it was subjected to drying at 110 °C for 3 h.

2.5 Characterization of activated carbon impregnated with SiO2 nanoparticles

Infrared spectra were acquired on a Thermo Scientific© Nicolet IS50 FTIR in the mid-infrared (MIR) spectral range, which is coupled to an attenuated total reflectance (ATR) cell equipped with a single-pass diamond reflection crystal with a fixed 45° angle of incidence. The IR spectra of control activated carbon and impregnated carbons were obtained and normalized using OMNIC® software in txt format. For the determination of specific surface area and pore volume, the Brunauer, Emmett and Teller (BET) method were applied based on adsorption–desorption isotherms with nitrogen (N2) at 77 K. Activated carbon samples impregnated with different concentrations of nanoparticles were weighed in 9-mm-diameter cells made of borosilicate glass in the Vac Prep 061 MICROMERITICS® device.

Secondary electron (SE) and backscattered electron (BSED) imaging and energy-dispersive X-ray spectroscopy (EDS) analysis of activated carbon samples were carried out using a FEI QUANTA FEG 650 environmental scanning electron microscope (ESEM), under the following analytical conditions: magnification = 200X-3.000X, WD = 9.5 mm, HV = 15.00 kV, detector = BSED detector and Everhart–Thornley detector (ETD), signal = BSED in Z CONT mode and ETD in SE mode, and EDS detector EDAX APOLO X with resolution of 126.1 eV (in Mn Kα).

2.6 Procedure for the adsorption of persistent organic pollutants

The equilibrium adsorption isotherm was studied using the batch adsorption process. Equilibrium studies were carried out using the activated carbon/SiO2 (AC/SiO2) composite with two pollutants: phenol and 3,4-dimethoxybenzyl alcohol in a series of 15-mL centrifuge tubes. The effect of the concentration of nanoparticle impregnation on the carbon and the effect of the initial pollutant concentration (between 50 and 350 mg L−1) were studied, and contact time was kept constant. To each centrifuge tube, 5 mL of simulated wastewater solution was added at a constant temperature of 25 ± 1 °C. Subsequently, 0.1 g of AC/SiO2 composite was added to each tube and kept on a FALC F340 horizontal shaker at 150 rpm for 24 h (estimated time to reach equilibrium). Based on Lambert–Beer's law, a linear correlation was found between the absorbance and the concentration obtained from the calibration curves, obtaining the final concentration of each of the adsorption test samples.

Finally, the qe adsorption capacity of the activated carbons was determined from Eq. (1), where qe is the adsorption capacity (mg g−1), Co is the initial concentration of the solution (mg L−1), Ce is the final concentration of the solution (mg L−1), W is the mass of activated carbon (g) and V is the volume of solution (L).

$${q}_{e}= \frac{{C}_{o}-{C}_{e}}{W} \times V$$
(1)

When an adsorbent is in contact with a surrounding fluid containing species of a certain composition, the adsorption process occurs, which, after a certain contact time, reaches equilibrium between the adsorbent and the surrounding fluid. Adsorption isotherms represent the amount of material bonded to the surface (adsorbate) as a function of the properties of the adsorbent material at a given temperature (Puszkarewicz et al., 2020, Moradi et al., 2014). The results obtained were fitted with Langmuir, Freundlich and Temkin isotherm models. The linearized models are represented below:

  • Langmuir isotherm: It is represented by Eq. (2), where qmax and Ce represent the maximum adsorption capacity (mg g−1) of the adsorbent and the energy related to the heat of adsorption.

    $$\frac{{C}_{e}}{{q}_{e}}=\frac{{C}_{e}}{{q}_{\mathrm{max}}}+\frac{1}{{q}_{\mathrm{max}}.b}$$
    (2)

  • Freundlich Isotherm: It is represented by Eq. (3), where K is the equilibrium constant (mg g−1) and n is the affinity-related constant.

    $$\mathrm{ln} \left({q}_{e}\right) =\mathrm{ln} \left(K\right) +\frac{1}{n}.\mathrm{ln}({C}_{e})$$
    (3)

  • Temkin's isotherm: It is represented by Eq. (4), where K and nT are Temkin's equilibrium constant and the adsorption energy.

    $${q}_{e}={n}_{T}\mathrm{ln}\left(K\right) +{n}_{T}\mathrm{ln}\left({C}_{e}\right)$$
    (4)

3 Results and discussion

3.1 Characterization of activated carbon impregnated with SiO2 nanoparticles

3.1.1 Fourier transform infrared spectroscopy (FTIR)

Figure 3 shows the characterization of the four carbons synthesized in this research using FTIR spectroscopy, where the control AC refers to activated carbon without nanoparticles and C.A, C.B and C.C refer to activated carbon impregnated with SiO2 nanoparticles at concentrations of 1800, 3600 and 5400 mg L−1, respectively.

Fig. 3
figure 3

FTIR spectra of activated carbons impregnated with SiO2-NPs

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Table 1 shows the presence of functional groups associated with phenolic substances, carboxylic acids and aromatic rings in the results obtained (Nandiyanto et al., 2019). Also, a main absorption band can be evidenced that is increasing as the concentration of SiO2 nanoparticles increases in the range of 980 to 1200 cm−1, which are related to the bending of Si ↔ OH and stretching vibrations between Si ↔ O, reaching its highest intensity at 1098 cm−1, affirming the impregnation of SiO2 nanoparticles to the surface of the activated carbon (Mohammadi et al., 2019).

Table 1 Functional groups of activated carbons by FTIR spectroscopy
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3.1.2 Scanning electron microscopy (SEM)

The morphology of activated carbon without the presence of nanoparticles (control AC) and activated carbons impregnated by SiO2 nanoparticles at different concentrations: 1800 mg L−1 (C.A.), 3600 mg L−1 (C.B.) and 5400 mg L−1 (C.C.), were analyzed by scanning electron microscopy (SEM), and images are shown in Fig. 4. As shown in Fig. 4a, the activated carbon granules have structured surfaces, but with irregular sizes and disordered channels, leading to a possible decrease in the adsorption of organic compounds (Ma et al., 2022). However, this irregular structure facilitates the anchoring of the SiO2 nanoparticles on the surface of the activated carbon, as shown in Fig. 4b, c, d, where the presence of white clumps dispersed on the surface of the activated carbon is observed, indicating the adhesion of the nanoparticles, which in turn helps to increase the surface area of the activated carbon, resulting in a potential increase in the adsorption of organic compounds (Buchori et al., 2022; Yasin et al., 2021).

Fig. 4
figure 4

SEM images obtained from a control AC and impregnated carbons with b 1800 mg L-1, c 3600 mg L−1 and d 5400 mg L−1

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3.1.3 Energy-dispersive X-ray spectroscopy (EDS)

According to the information reported by EDS test and summarized in Fig. 5, the activated carbon without the presence of nanoparticles (Control AC) is essentially composed of carbon and oxygen, the mass fraction of element C is 80.89%, while that of O is 15.61%. The element phosphorus (P) was also detected on the surface of the activated carbon with a mass fraction of 3.50%, which appears due to the chemical method of carbon activation with H3PO4.

Fig. 5
figure 5

EDS analysis of activated carbons

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On the other hand, the activated carbons impregnated with nanoparticles (C.A, C.B and C.C), besides containing carbon, oxygen and phosphorus in their surface structure, the presence of silicon (Si) was detected, confirming the adhesion of the SiO2 nanoparticles onto the activated carbon surface. It is also shown that the mass fraction of Si and O increases proportionally as the concentration of SiO2 nanoparticles also increases. The EDS spectra of each of the samples are shown in Supplementary Information.

3.1.4 Determination of physical characteristics by specific surface area test

The specific surface areas of the samples were calculated by the Brunauer–Emmett–Teller (BET) method. Table 2 presents the magnitudes of specific surface area, pore volume and pore size obtained.

Table 2 BET characterization of activated carbons
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According to International Union of Pure and Applied Chemistry (IUPAC), carbons are classified according to pore size as macropores (> 50 nm diameter), mesopores (2–50 nm diameter) and micropores (< 2 nm diameter). Thus, the results show that the obtained carbons are classified as mesoporous, with specific surface area greater than 800 m2 g−1, where it was observed that the impregnation of nanoparticles in their structure increased the final specific surface area by up to 20%, which allows a greater interaction of the pollutants at the time of adsorption. From these characteristics shown, it can be indicated that the methodology used in the design of experiments allows obtaining activated carbons with outstanding specific surface areas. From the N2 adsorption–desorption isotherms shown in Fig. 6, it is determined that the obtained carbons show a type IV trend since the hysteresis effect can be evidenced in the behavior of the curve, which, in turn, evidences the presence of mesopores whose filling is produced by capillary condensation (Olivier, 2004).

Fig. 6
figure 6

Adsorption curves obtained from a control AC, b C.A., c C.B. and d C.C

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3.2 Phenol adsorption on activated carbon impregnated with SiO2 nanoparticles

Figure 7 shows the adsorption behavior of phenol on each of the carbons used for its removal from solution, which resembles a monolayer adsorption process, which was mathematically modeled by Freundlich adsorption isotherms, as in previous investigations (Abdelbassit et al., 2016; Abussaud et al., 2016; Asmaly et al., 2015; Orha et al., 2017). The adsorption isotherms of phenol represented with the Freundlich model according to the experimental data obtained in this research are shown in Supplementary Information.

Fig. 7
figure 7

Phenol adsorption isotherms using activated carbon impregnated with SiO2-NPs as adsorbent

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Table 3 shows that the experimental data can be better represented by the Freundlich model compared to the Langmuir and Temkin models, according to the R2 coefficient. Furthermore, Dobbs and Cohen (1980) suggested that the Freundlich model better represents the adsorption results when working with dilute solutions such as those used in this research.

Table 3 R2 correlation coefficients for phenol adsorption isotherms on activated carbons
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Terzyk et al. (2003) established that phenol adsorption with activated carbon occurred in three fundamental stages: (1) infinite dilution adsorption, which establishes that there is a linear correlation between the average adsorbability of phenol and the basic or acidic groups (–COOH) found on the carbon surface. At neutral pH, a positive surface charge is generated by the adsorption of protons from the strongest bases, increasing water adsorption and blocking the most active adsorbing parts of phenol. The presence of carboxylic groups at acidic pH, due to the presence of carboxylic groups that remove the π electrons present in the carbon matrix, generates a decrease in the interaction forces between the aromatic ring of the phenol molecule and the basal planes of the carbon, decreasing the adsorption of phenol (Salame et al., 2003). (2) The second stage consists of micropore filling which establishes a correlation between the phenol adsorption energy and surface groups and (3) adsorption in larger micropores in which the basic groups are active to adsorb phenol. On the other hand, the study by Feigenbaum et al. (1986) established that the bonding that occurs between phenolic substances and silica occurs due to hydrogen bonds and dipolar interactions that form between these two compounds.

3.3 Adsorption of 3,4-dimethoxybenzyl alcohol on activated carbon impregnated with SiO2 nanoparticles

Figure 8 represents the test behavior of carbon adsorption and 3,4-dimethoxybenzyl alcohol, which resembles a monolayer adsorption behavior and which, at the same time, is analyzed by linear mathematical models of adsorption for further understanding. To evaluate the linearity of the points, the coefficient of determination R2 was obtained for the adsorption of 3,4-dimethoxybenzyl alcohol in the presence of the four activated carbons with the Freundlich and Temkin isotherms. The R2 coefficients obtained with the Freundlich model were in the range of 0.979 to 0.992. Meanwhile, the experimental data adjusted with the Langmuir and Temkin model presented R2 values in the range of 0.934 to 0.981 and 0.964 to 0.979, respectively.

Fig. 8
figure 8

Adsorption isotherms of 3,4-dimethoxybenzyl alcohol using activated carbon impregnated with SiO2-NPs as adsorbent

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To simplify the analysis of the results, the Freundlich model has been selected to compare the adsorption parameters. The adsorption isotherms of 3,4-dimethoxybenzyl alcohol represented with the Freundlich model according to the experimental data obtained in this research are displayed in Supplementary Information. The presence of –OCH3 groups, strong activating agents bonded to the aromatic ring of the 3,4-dimethoxybenzyl alcohol and proximal to the OH group, causes a resonance effect in the molecule, which generates a higher charge in the electron cloud of the aromatic ring and attractive forces that favor adsorption. The oxygen atoms present in the carboxyl groups of the activated carbon interact with the aromatic ring of the adsorbate molecule by attaching it to the surface and forming π bonds. In addition, the SiO2 molecules interact with the associated groups present on the 3,4-dimethoxybenzyl alcohol through hydrogen bonds and dipole interactions (Feigenbaum et al., 1986).

Due to the existence of more interactions between adsorbent and adsorbate, the average removal percentage of the activated carbons was 97.9% and 98.5% for phenol and 3,4-dimethoxybenzyl alcohol, respectively, demonstrating that the implementation of this material is efficient for the adsorption of organic compounds with aromatic rings in their chemical structures. The percentage removal of phenol and 3,4-dimethoxybenzyl alcohol in solution was calculated using Eq. (5) as follows (Abdelbassit et al., 2016):

$$\% \mathrm{Removal}=\frac{\left({C}_{o}-{C}_{e}\right)\times 100}{{C}_{o}}$$
(5)

where Co (mg/l) and Ce (mg/l) are the initial and final concentrations at time t of the organic compounds in solution. The concentration of each component was determined using the UV–Vis spectroscopy technique. Also, Table 4 shows the parameters of the Freundlich isotherms adjusted for the adsorption of phenol and 3,4-dimethoxybenzyl alcohol on each of the activated carbons synthesized. The results of the equilibrium constant K (mg g−1) and the values of n suggest that there is a higher affinity and adsorption capacity of 3,4-dimethoxybenzyl alcohol compared to phenol in the presence of control activated carbon and impregnated carbons. However, for a concentration of 5400 mg L−1 of SiO2 there is a higher equilibrium constant, which indicates under these conditions there is a higher affinity for phenol compared to activated carbons with concentrations of 1800 and 3600 mg L−1.

Table 4 Parameters of Freundlich isotherms in the adsorption of phenol and 3,4-dimethoxybenzyl alcohol
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In general, considering the adsorption kinetics parameters obtained in the adsorption curves by the Freundlich method, it can be indicated that the abundance of OH, C=O and –CO functional groups and SiO2 content in the activated carbons influence the adsorption kinetics of the organic compounds studied. This could be due caused by the presence of enough vacant adsorption sites in the adsorbent material during the adsorption time, which is attributed to the increase in surface area and pore volume. Additionally, the adsorption results show that the removal efficiency of phenol and 3,4-dimethoxybenzyl alcohol depends on their chemical properties.

In general, results revealed that the preparation of activated carbons with the chemical method (H3PO4) and the benefit of the use of SiO2 nanoparticles improve the removal capacity of persistent organic compounds in wastewater treatment. Likewise, the effect of SiO2 nanoparticles found in this study is consistent with the results reported by several researchers (Al-Zboon et al., 2018; Saleh et al., 2019). The results indicated that cocoa shell‑based activated carbon—SiO2 nanoparticles are promising adsorbents for the removal of phenol and 3,4-dimethoxybenzyl alcohol in aqueous media.

4 Conclusions

The activated carbons synthesized with SiO2 nanoparticles from residual biomass of cocoa shells in the present investigation presented higher specific surface area values compared to the activated carbon synthesized without nanoparticles. On the other hand, it can be indicated that these materials present excellent textural properties and are classified as a mesoporous structure. These properties confer a high adsorption efficiency which allows the removal of a greater amount of phenol and 3,4-dimethoxybenzyl alcohol from the prepared solutions with removal percentages of 97.9% and 97.8%, respectively.

The addition of nanoparticles to the carbon surface generates an increase in its specific surface area, increasing the removal of pollutants. For this reason, activated carbons were evaluated at three concentrations of nanoparticles (1800, 3600 and 5400 mg L−1). It was found that phenol presents a higher adsorption capacity under a concentration of 5400 mg L−1 (qmax = 39.5 mg g−1), which provides an increase compared to the carbon without nanoparticles (qmax = 21.6 mg g−1) of 45.3%. In the case of 3,4-dimethoxybenzyl alcohol, at concentrations between 1800 mg L−1 and 3600 mg L−1 of nanoparticles, an increase in the maximum adsorption capacity (qmax = 66.9 mg g−1 and qmax = 59.8 mg g−1 respectively) is generated compared to carbon without nanoparticles (qmax = 56.6 mg g−1).

The adsorption of phenol and 3,4-dimethoxybenzyl alcohol on the four carbons was analyzed using the Langmuir, Freundlich and Temkin isotherms, finding that all three fit the adsorption data adequately; however, the Freundlich isotherm presented slightly higher values in terms of R2 correlation coefficients.

According to the results of the specific surface area test analysis, the obtained carbons had a multilayer behavior of type IV, which was identified due to the presence of hysteresis in the trend of the adsorption–desorption curves.

The maximum adsorption capacity of 3,4-dimethoxybenzyl alcohol was higher compared to that of phenol on activated carbons without nanoparticles. This is due to the presence of methoxy functional groups (-OCH3) bonded to the aromatic ring of 3,4-dimethoxybenzyl alcohol, which are strong aromatic ring activating agents responsible for a resonance effect on the molecule. Therefore, these properties generate a higher charge in the electron cloud of the ring and increase the attractive forces that favor adsorption and, consequently, the removal of the contaminant from the solutions.

The utilization of activated carbon with silicon oxide nanoparticles proved to be a significant and promising method for treating produced water in the petroleum industry. The impressive adsorption percentages achieved through this innovative approach demonstrate its potential as an efficient and effective means of removing organic compounds from oilfield effluent. By applying this advanced technique to treat wastewater, the petroleum industry can substantially reduce the negative environmental impacts associated with the discharge of organic pollutants. The removal of harmful contaminants from produced water not only ensures compliance with environmental regulations but also contributes to the conservation of ecosystems and the protection of water resources.

Finally, the environment-friendly nature of this method aligns well with the industry's growing commitment to sustainable practices. As the petroleum sector strives to minimize its water footprint, the application of activated carbon with silicon oxide nanoparticles offers a valuable solution for the responsible management of produced water. Moreover, this research provides a foundation for further exploration and optimization of this technology in the context of oilfield wastewater treatment. Researchers and industry stakeholders are encouraged to pursue further investigations to fully leverage the benefits of this innovative treatment method in petroleum-produced water management.