Investigation of the Adsorption Capacity of H₃PO₄-Activated Biochar from Eucalyptus Harvest Waste for the Efficient Removal of Paracetamol in Water

Abstract

The present study showed that it is possible add value to eucalyptus harvest waste, obtained in large quantities, from the cellulose industries, without known economic use, for the production of an activated biochar. The biochar, produced from the impregnation of eucalyptus harvest waste with H₃PO₄, and subsequently pyrolyzed at 600 °C for 1 h, was successfully used as a bioadsorbent in the removal of paracetamol, an emerging pollutant present in wastewater. The biochar showed a high specific surface area with micro- and mesopores and functionalized surface. The optimal conditions for the removal of paracetamol achieve an efficiency around 88–93%. The Langmuir and the pseudo-first-order models best fit the experimental data, with a maximum adsorption capacity of approximately 27.8 mg g⁻¹, at 25 °C. The thermodynamic showed that adsorption occurred spontaneously, endothermally and randomly at the solid–liquid interface. In addition, the bioadsorbent showed excellent reusability and no significant difference in adsorption capacity was observed in more complex aqueous matrices. Thus, the activated biochar produced in this study proved to be an efficient, low-cost and environmentally friendly bioadsorbent, capable of removing paracetamol from contaminated water, with great potential for use in water treatment plants, on a large scale and economically, contributing to the improvement of water quality and minimizing residual biomass in the environment.

1. Introduction

Water contamination by emerging pollutants has become a global ecological problem in recent decades. Among these pollutants, pharmaceutical products have gained special attention as they are widely consumed by the world population [1,2]. Studies show that these compounds are normally found in trace concentrations (ng L⁻¹-µg L⁻¹) in different aquatic ecosystems [1,3,4]. Prolonged exposure to these drugs can have various negative effects on the environment and public health, including environmental changes, toxicity at different levels of the trophic chain, changes in the reproductive and behavioral system, mutations, cancer and even deaths [2,5].

Paracetamol, also known as acetaminophen, is widely used due to its analgesic, anti-inflammatory and antipyretic effects [6,7]. It is also easy to obtain without the need for a doctor’s prescription [8]. Similarly to other drugs, it is not completely metabolized by the body but excreted into the environment in its biologically active form through urine and feces. About 58–68% of the drug is excreted from the human body during medical treatments and enters the available water bodies [9,10]. In addition, the improper discharge of wastewater from industry and hospitals also contributes to the pollution of ecosystems [11].

Despite the low concentrations at which it is normally found in various aquatic resources (in the order of ng·L⁻¹ to µg·L⁻¹), it can cause aquatic toxicity, endocrine disruption, genotoxicity and imbalance at various levels of the trophic chain, jeopardizing human and animal health [8,11,12]. According to some studies, the paracetamol molecule can be converted into some metabolites such as paminophenol, 1,4-benzoquinone and N-acetyl-p-benzoquinone-imine, at trace concentrations higher than those of paracetamol itself, which are described in the literature as highly toxic and carcinogenic compounds [13,14,15].

Paracetamol is found in virtually all water sources: wastewater treatment plants, underground and surface waters and even drinking water [3,8,13]. Due to its high consumption, high persistence in various aquatic sources and harmful effects on the environment and human health, the elimination of the paracetamol compound in the ecosystem is therefore crucial. However, conventional methods of waste and water treatment are not effective in removing this and other emerging pollutants [6,16].

There are several technologies for removing pollutants from contaminated water, ranging from physical and chemical methods to biological processes. The main technologies include adsorption, membrane filtration, advanced oxidative processes, electrochemical treatment and bioremediation. All have advantages and disadvantages, and the choice of the ideal technology depends on the type and concentration of pollutants, as well as the characteristics of the water to be treated [8,17]. Membrane separation is one of the technologies that has gained prominence in recent years in wastewater treatment, and the construction and optimization of MXene-MOF membranes present characteristics that make them an ideal material for applications in oil–water separation and dye removal in saline environments [18,19].

On the other hand, adsorption seems to overcome most of the disadvantages presented by other technologies currently under study. Among the advantages presented are low operating costs, no production of toxic byproducts, high efficiency in removing emerging pollutants, great versatility, simplicity of design, low energy consumption, low waste production and the possibility of adsorbent regeneration [8,20,21].

The most commonly adsorbent used is commercial activated carbon, which is derived from petroleum waste, wood, lignite and peat from finite sources and has a high cost [4,14,22]. In this sense, the development and improvement of activated carbon from comparatively cheaper, highly efficient and more available sources has increased in the scientific community [22]. Agro-industrial waste, which tends to be a source of pollution, has been explored in the scientific community as a cheap and highly available source for the production of activated carbon [6,14,22].

In general, the production of activated carbon from local waste sources is considered an excellent option. However, since the final properties of the produced adsorbent depend on the specific properties of the precursor material and the parameters used in the thermochemical process, studies that incorporate the properties of the produced adsorbent and its performance in the adsorption process with respect to a specific pollutant are always necessary for scientific progress [20]. As an example, we can cite the residues of eucalyptus harvests, which is widely cultivated in Brazil and around the world for the production of cellulose and paper, as well as for the wood industry. These residues are deposited and accumulated in the soil itself and have no industrial use. The discarded waste, which represents about 23% of the harvested material, consists of leaves (5%), branches (6%) and bark (12%), is an inexpensive biomass. The use of these residues is not yet well documented in the scientific community [13]. To our knowledge, there is no study addressing the use of biochar from eucalyptus waste for the removal of paracetamol. In the present study, eucalyptus waste was used to produce a bioadsorbent with high porosity and surface area. Phosphoric acid (H₃PO₄) was used as an activating agent before the pyrolysis. Chemical activation has three advantages over physical activation: it allows a lower temperature range (400–600 °C) and significantly shorter times for pyrolysis as well as a higher yield of the solid phase [4,13,23]. H₃PO₄ was chosen as the impregnating agent because, according to the literature, it is environmental friendly, less expensive and safer [5,24,25].

This study has two objectives: firstly, to show that solid waste can produce a bioadsorbent material with high added value and satisfactory capacity for removing paracetamol from contaminated water; and secondly, to evaluate the capacity for removing paracetamol using the activated biochar produced from eucalyptus harvest waste, activated with H₃PO₄ and pyrolyzed at 600 °C for 1 h. In this evaluation, the equilibrium isotherms, kinetic behavior and thermodynamic properties will be assessed, as well as its reusability and adsorption capacity when more complex aqueous matrices are used.

2. Material and Methods

2.1. Chemicals and Reagents

Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH, 97%), phosphoric acid (H₃PO₄, 85%) and methanol (CH₃OH, 99.9%) of analytical grade are from VETEC (Duque de Caxias, Brazil). The paracetamol with purity 99% was purchased from Sigma-Aldrich (Barueri, Brazil), and its molecular structure and main characteristics are shown in Figure 1. To improve the solubility of paracetamol in distilled water, stock solution was prepared with 1% (w/w) methanol. This is the standard procedure employed in several studies found in the literature [10,12,14]. A paracetamol stock solution with a concentration of 25 g L⁻¹ was prepared, and its natural pH remained close to neutrality (7.6). The solution was used without pH adjustment.

2.2. Obtaining the Biomass

The biomass used in this study comes from the pruning of eucalyptus cultivated in forests destined for the timber industry. It is the forest residue of Eucalyptus saligna, grown in the city of Viamão–State of Rio Grande do Sul, Brazil. This residue consists of bark, branches and leaves that are deposited on the ground after harvesting and was taken directly from the soil. After collection, the raw sample was dried at room temperature and then crushed in a knife mill and passed through a 200 mm sieve.

2.3. Production and Characterization of the Activated Biochar

To produce the activated biochar, the biomass was first impregnated with H₃PO₄ 85% in a ratio of 5:1 (w/w). The mixture of phosphoric acid and biomass was left for 24 h at room temperature. The material was then dried in an oven at 105 °C for 24 h and then pyrolyzed in a fixed bed laboratory reactor at 600 °C for 1 h, employing a heating rate of 20 °C min⁻¹ and N₂ flow rate of 1 L min⁻¹. The pyrolyzed sample was cooled to room temperature under N₂ flow and washed thoroughly with distilled water until the wash effluent was nearly neutral. The sample was then dried in an oven at 105 °C for 24 h and stored in a desiccator. The percentage yield of activated biochar was calculated by dividing the mass of biochar obtained by the original mass of the precursor material.

The activated biochar, produced in this study, was characterized by Fourier transform infrared spectroscopy–FTIR (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) to identify the functional groups. The sample was analyzed in the infrared range of 400–4000 cm⁻¹ using the potassium bromide (KBr) disk method. The specific surface area and pore size distribution of the adsorbent were determined in a Micromeritics Tristar Kr 3020 instrument (Norcross, GA, USA) using the Brunauer–Emmett–Teller (BET) method. The surface morphology of the prepared bioadsorbent was observed using scanning electron microscopy JEOL JSM-6060 (Tokyo, Japan). The zero charge potential (pHpzc) was determined using the method known as the “11-point experiment” [26]. The pHpzc allows evaluating the ionization of the functional groups on the surface of the adsorbent so that the distribution of electrical charges on the surface of the adsorbent can be predicted. Its value depends on the amount and type of functional groups, i.e., the formation of negative species on the surface of the adsorbent decreases the pHpzc value, while the formation of cationic species increases the pHpzc. Thus, if the adsorbent is negatively charged, the adsorbent surface may interact with positive species present in the solution when the pH is higher than pHpzc. And if the pH is lower than pHpzc, the surface will be positively charged, allowing interaction with negative species in the solution [27].

2.4. Preliminary Adsorption Tests

The batch adsorption experiments were carried out on a rotary shaker (Marconi MA-420, São Paulo, Brazil) with a stirring speed of 180 rpm. First, the influence of the interaction time between adsorbate and adsorbent (15 to 240 min) and paracetamol concentration of 10 to 50 mg L⁻¹ were investigated. These experiments were performed in a suitable flask containing 45 mL of solution, with the following conditions: 25 °C, natural pH of the solution (7.6) and 1.75 g L⁻¹ adsorbent (value based on preliminary studies). Subsequently, the effect of adsorbent dosage was evaluated by adding different dosages of the activated biochar from 0.6 to 2.5 g L⁻¹ into a solution of 25 mg L⁻¹ (a value obtained from the previous test), at a natural pH (7.6), 25 °C and interaction time of 60 min (a value also obtained from the previous test). Finally, the effect of pH in the range of 2.0 to 12 (adjusted with NaOH or HCl solutions 0.1 mol L⁻¹) was studied under previously selected conditions: interaction time of 60 min, initial paracetamol concentration of 25 mg L⁻¹, adsorbent dosage of 1.75 g L⁻¹, at 25 °C. To determine the concentration of paracetamol remaining in the solutions after each test, aliquots were taken and filtered with a nylon syringe filter (0.22 µm). Analytical quantification measurements were carried out with a UV–Vis spectrophotometer (Lambda 265, PerkinElmer, São Paulo, Brazil), at a maximum wavelength of 257 nm using a previously established calibration curve for the drug (R² = 0.9996). All the analyses were performed in two series: the first in triplicate and the second in duplicate, thus performing tests in quintuplicate. The removal efficiency (E%) and adsorption capacity at equilibrium (mg g⁻¹) were determined using Equations (1)–(3), respectively.

$$E\left(\%\right)={\displaystyle \frac{(C_0-C_e)}{C_0}}100$$

(1)

$$q_e={\displaystyle \frac{(C_{0 }-C_{e })}{w}} V$$

(2)

$$q_t={\displaystyle \frac{(C_{0 }-C_{t })}{w}} V$$

(3)

where C₀ and C_e are the adsorbate concentrations at the initial and equilibrium times, respectively, expressed in mg L⁻¹, q_e and q_t are the adsorption capacities at equilibrium at each measurement time “t”, respectively, expressed in mg g⁻¹, V is the volume of the solution in liters and w is the mass of the adsorbent in grams.

2.5. Effect of Temperature and Thermodynamic Study

The previous tests were carried out at a temperature of 25 °C. However, studying adsorption at different temperatures is crucial for understanding the kinetics and equilibrium of the process Temperature variation allows us to determine the ideal conditions for optimizing the adsorption of a given adsorbent. Thereby the effect of temperature on the adsorption of paracetamol by the activated biochar produced was studied at four temperatures (25, 35, 45 and 55 °C), maintaining the best conditions previously determined by preliminary adsorption tests. With the data obtained, the thermodynamic parameters (standard change in Gibbs free energy (ΔG°), standard change in enthalpy (ΔH°) and standard change in entropy (ΔS°) could be calculated. The standard Gibbs free energy for a system in chemical equilibrium can be calculated using Equation (4).

$$∆G^{°}=-RT\mathit{ln}{K}_{eq}$$

(4)

where K_eq is the thermodynamic equilibrium constant (dimensionless), T is the absolute temperature (in K) and R is the universal gas constant (8.314 J mol⁻¹ K⁻¹).

Similarly, the standard Gibbs free energy change can be calculated the Equation (5):

$$\Delta G^{°}=\Delta H°-T\Delta S°$$

(5)

Substituting Equation (4) into Equation (5), it is obtained Equation (6), i.e., the Van’t Hoff equation.

$$\mathit{ln}{K}_{eq}={\displaystyle \frac{-\Delta H°}{RT}}+{\displaystyle \frac{\Delta S°}{R}}$$

(6)

Plotting ln (K_eq) against 1/T should give a straight line where the slope gives the standard enthalpy change and the intercept gives the standard entropy change. In this work, the value of K_eq is determined using Equation (7).

$$K_{eq}={\displaystyle \frac{1000 M_i{C}_{io}{K}_L}{{\gamma }_i}}$$

(7)

where M_i is the molar mass of the adsorbate, C_io is the standard molar concentration of the adsorbate (1 mol L⁻¹), γ_i is the activity coefficient (dimensionless) assumed to be 1 for dilute solutions and K_L is the Langmuir constant taken from the Langmuir isotherms obtained from data collected at different temperatures (in our case at 25, 35, 45 and 55 °C). In the literature, other authors have also used the same equation to determine the value of K_eq [28,29,30,31,32].

2.6. Adsorption Isotherms

The study of adsorption isotherms was carried out using the best previously determined conditions. The adsorption performance of activated biochar, at different initial paracetamol concentrations (10–50 mg L⁻¹), was investigated at a pH of 7.6 (natural pH of the paracetamol solution) and an adsorbent dosage of 1.75 g L⁻¹ (value from the adsorbent dosage evaluation). Four temperatures were used for the study (25, 35, 45 and 55 °C). The samples were shaken at 180 rpm and the aliquots were taken after 60 min of interaction time. At the appropriate time, the aliquots were withdrawn and filtered to subsequently quantify the remaining paracetamol in the solutions using UV-Vis spectrophotometry at 257 nm.

The Langmuir model is based on the assumption that adsorption occurs on a homogeneous surface of the adsorbent, with the formation of a monolayer on the surface of the adsorbent, in which all sites are energetically equivalent, without interaction between the adsorbate molecules or ions. Equation (8) describes the expression proposed by Langmuir in its linear form.

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{K_L q_{max}}}+{\displaystyle \frac{C_e}{q_{max}}}$$

(8)

where C_e is the residual concentration of the solute at equilibrium (mg L⁻¹), q_e is the adsorption capacity at equilibrium (mg g⁻¹), q_max is the theoretical maximum adsorption capacity related to a complete monolayer (mg g⁻¹) and K_L is the Langmuir adsorption equilibrium constant (L mg⁻¹), related to the interaction strength between the adsorbate and the adsorbent surface, that is, the higher the K_L value, the greater the interaction between adsorbate and adsorbent. Plotting C_e/q_e against C_e results in a straight line whose intercepts gives the value of q_max and slope gives the value of K_L.

In order to check whether the adsorption process is favorable or not, the dimensionless constant R_L, known as the separation factor, can be estimated using Equation (9).

$$R_L={\displaystyle \frac{1}{(1+K_L{C}_0)}}$$

(9)

where K_L is the Langmuir constant (L mg⁻¹) and C₀ is the initial concentration of the adsorbate in the solution (mg L⁻¹).

Table 1. Separation factor and type of isotherm.

Separation Factor RL	Isotherm
RL > 1	unfavorable
RL = 1	linear
0 < RL < 1	favorable
RL = 0	irreversible

shows the type of isotherm that can be determined based on the R_L value obtained.

Another widely used model is that of Freundlich, which assumes that: (i) the surface of the adsorbent is heterogeneous both in terms shape and energy; (ii) there is unlimited interaction between the adsorbed molecules, so that multilayers are formed and no saturation of the adsorbent surface occurs. The Freundlich isotherm in its linearized form (Equation (10)) makes it possible to determine the value of the Freundlich constant (K_F), expressed in [(mg g⁻¹) (L mg⁻¹)^1/n], by its slope, which gives an indication of the adsorption capacity. The other parameter, defined by n (dimensionless), results from the intersection of the linear plot and provides information about the favorability of the interaction between adsorbent and adsorbate. Values of n greater than unity mean that the adsorption process is favorable.

$$\mathit{ln}{q}_e={\displaystyle \frac{1}{n}}\mathit{ln}{C}_e+\mathit{ln}{K}_F$$

(10)

And the third most commonly used isotherm model is Temkin which is described by Equation (11), in its linear form. The value of two constants can be read from the plot of q_e versus lnC_e: K_T, the Temkin constant (L mol⁻¹), which corresponds to the maximum binding energy, and b, which refers to the heat of adsorption (J·mol⁻¹).

$$q_e={\displaystyle \frac{RT}{b}}\mathit{ln}{K}_T+{\displaystyle \frac{RT}{b}}\mathit{ln}{C}_e$$

(11)

2.7. Kinetic Study

For the kinetic study, a series of experiments were performed under similar conditions to the isothermal study, except that the initial paracetamol concentration was kept constant at 25 mg L⁻¹ (a value chosen to proceed with the adsorption experiments) and all experiments were performed at 25 °C. The mixtures were shaken for different time intervals covering a range from 15 to 240 min to investigate the influence of reaction time.

Three different kinetic models were used to describe the experimental data. The first was the pseudo-first-order (PFO) kinetic model proposed by Lagergren, which is mathematically represented in its linear form by Equation (12).

$$ln \left(q_e-q_t\right)=\mathit{ln}{q}_e-k_{1}t$$

(12)

where q_t and q_e are the adsorbed quantities (mg g⁻¹) at time t and at equilibrium, respectively, and k₁ (min⁻¹) is the pseudo-first-order kinetic rate constant.

The second kinetic model applied was the pseudo-second-order (PSO) model proposed by Ho and McKay [33], which is mathematically represented in its linear form by Equation (13).

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(13)

where k₂ (g mg⁻¹ min⁻¹) is the pseudo rate constant of the second order.

The third widely used kinetic model is the one described by the Elovich equation, which was originally presented in 1939 [34,35]. Its mathematical representation in linearized form is shown in Equation (14).

$$q_t={\displaystyle \frac{1}{\beta }}\mathit{ln}\left(\alpha \beta \right)+{\displaystyle \frac{1}{\beta }}\mathit{ln}t$$

(14)

where α represents the initial rate of the adsorption process (mg g⁻¹ min⁻¹) and β corresponds to the desorption constant (g mg⁻¹).

2.8. Study on Desorption and Reusability

The quality of an adsorbent in an adsorption process depends on two factors: its adsorption capacity and how well it can be regenerated and reused. With this in mind, desorption cycle experiments were conducted to evaluate the regeneration capacity of the activated biochar as well as the subsequent stability and cost benefit. For these tests, the best previously determined conditions were used (25 mg L⁻¹ of paracetamol, 1.75 g L⁻¹ of adsorbent, 60 min interaction time, natural pH of the solution and 25 °C of temperature). After each adsorption experiment, the paracetamol-loaded adsorbent was heat-treated at 400 °C for 30 min in a muffle furnace, in the presence of atmospheric air to thermally degrade the paracetamol molecule. The calcination temperature was determined based on a study on the thermal decomposition of several drugs, which showed that paracetamol starts to decompose at 326 °C [36]. The heating time was determined experimentally, choosing a value that would not affect the structure of the activated biochar. After each regeneration process, a new adsorption test was performed under the same conditions as the initial treatment. FTIR was used with the aim of qualitatively comparing the spectra of the paracetamol molecule, the activated biochar after the paracetamol adsorption test and activated biochar after the heat treatment in order to observe possible degradation of the drug after calcination.

2.9. Application in Real Water Samples

To investigate the application of the produced activated biochar in real and more complex water environments, its performance was evaluated using tap and stream water, in line with other studies [5]. The tap and stream water solutions were prepared with a concentration of 25 mg L⁻¹ paracetamol and 1.75 g L⁻¹ biochar. The system was shaken sequentially at 180 rpm and 25 °C, with aliquots of the solutions being withdraw at regular intervals over a total period of 180 min. The solution was then filtered with syringe filters (0.22 µm) and analyzed using a UV-Vis spectrometer to determine the residual concentration of paracetamol. The pH values of the samples were not adjusted, so that the original pH value of the respective system was maintained, i.e., 6.88 for tap water and 7.80 for stream water.

3. Results and Discussion

3.1. Characterization of the Activated Biochar

The yield of biochar activated with H₃PO₄ in this work was about 38%, a value considered high compared to other activating agents. Several drugs have also been removed by using biochar activated with H₃PO₄ for various residual biomasses [5,27]. Studies in the literature indicate that phosphoric acid is the most effective activating agent to hydrolyze, dehydrate and attack the precursor biomass and increase the yield of the solid phase, as it reduces the losses due to volatilization during heat treatment and produces an activated biochar with important functional groups for the adsorption process, a large specific area and a more heterogeneous surface [5,25].

To characterize the biochar activated with H₃PO₄, a texture analysis was performed using N₂ adsorption–desorption isotherms at the nitrogen boiling point. The results of BET surface area and pore volume are shown in

Table 2. Textural parameters obtained for different adsorbents.

Adsorbent	SBET (m2 g−1) a	Vp (cm3 g−1) b
Activated commercial carbon	826 ± 15	0.32 ± 0.01
Biochar without activation	17 ± 2	not detected
H3PO4 activated biochar	1187 ± 20	0.89 ± 0.01

Note(s): ^a S_BET = BET surface area; ^b V_p = total pore volume.

. From the isotherms shown in Figure 2, the pore volume increases significantly at low pressure, indicating the presence of a larger amount of available micropores. On the other hand, the isotherms also show hysteresis, which only occurs in the presence of mesopores. Considering the presence of both micropores and mesopores, BJH and DFT analyzes were performed (Inset Figure 2). The BJH curve indicates the presence of small mesopores with a diameter of less than 4 nm, which was confirmed by the DFT analysis, showing an area with small mesopores and also a proportion of small micropores with a diameter of less than 2 nm. Surface area, volume and distribution of pores is essential to determine the adsorbent capacity to remove molecules of different sizes when studying the effect of different concentrations of H₃PO₄, on the removal of paracetamol and tetracycline [37]. The paracetamol molecule has a diameter in the order of a few angstroms, i.e., length, width and thickness are about 14.61, 9.93 and 6.11 Å, smaller than the average pore size of the biochar produced [12]. The micro and mesoporous properties make it a potential adsorbent for the removal of molecules such as paracetamol and favor the transfer of molecules into the adsorbent.

For comparison, a biochar without activation with H₃PO₄ was produced by subjecting the same residual biomass only to the pyrolysis process at 600 °C for 1 h. The biochar without activation and the commercially available activated carbon were also analyzed texturally and the values are listed in Table 2.

The activated biochar was subjected to SEM analysis. A typical image can be seen in Figure 3 and shows that the material has a surface roughness with apparent porosity at micrometric and sub-micrometric levels, which is due to the pretreatment with H₃PO₄ before the pyrolysis process. This roughness is responsible for a high specific surface area available for the adsorption process, favoring a possible interaction between pollutant molecules and the surface of the pore-filling type bioadsorbent. These properties make the activated biochar, produced in this study, a promising material that can be used as a cost-effective adsorbent for the removal of paracetamol molecules in aqueous media.

Qualitative identification of the chemical functional groups on the surface of the activated biochar was carried out using FTIR analysis, and the most important peaks are shown in the spectra in Figure 4.

Some functional groups have been identified: 3377 cm⁻¹ corresponding to a broad peak characteristic of stretching of the functional group -OH, which occurs in carboxylic acids and phenols and is also due to the water adsorbed on the surface [4,7,25]; 1600 cm⁻¹ can be assigned to the stretching of the C=C double bond in aromatic rings; 1159 cm⁻¹ due to the stretching of the C-O or C-O-C bonds in carboxylic acids, phenols, esters and ethers [4,31,38]; 1070 cm⁻¹ can be attributed to the stretching of the P-O-P bond of phospho-carbonaceous and polyphosphate species formed by the reaction of H₃PO₄ with biomass [5,23,25,31,39]; and 620 cm⁻¹ due to the C-H bond in the aromatic ring [4,31].

A comparison of the spectrum obtained with those of other studies reported in the literature concludes that the activated biochar produced has a graphitized structure capable of interacting with paracetamol molecules, which also have an aromatic structure, through π-π type interactions [5,24]. In addition, oxygen- and phosphorus-containing functional groups have also been identified on the surface of the bioadsorbent, which may act as active sites for the n-π interactions. Finally, hydrogen bonding may occur due to the interaction between the H of the functional groups (-O-H and –COO-H) and the N atom in the paracetamol molecules [32].

The H₃PO₄ acts as an acidic catalyst that promotes the cleavage of bonds and the formation of crosslinks through dehydration processes such as cyclization and condensation [40]. The activating agent that remains in the biomass during heat treatment can serve as a template for the formation of micro- and mesopores, as shown in the texture analysis (Figure 2). In addition, H₃PO₄ can combine with organic species in the biomass to form carbonaceous phosphorus and polyphosphate compounds that crosslink the biopolymer fragments of the lignocellulosic biomass enhancing the interactions with the paracetamol molecules. The phosphate groups inserted in the matrix cause expansion, creating a porous structure, rich in functional groups and accessible for the adsorption of organic pollutants [37]. Thus, pretreatment with H₃PO₄ leads to significant changes in surface area, morphology and chemical composition that favor interaction with the functional groups of organic pollutants [5,21].

3.2. Results of Preliminary Adsorption Tests

3.2.1. Effect of Contact Time

The contact time between adsorbate molecules and the surface of an adsorbent is one of the most important parameters in water treatment systems from an economic point of view. To evaluate the effect of contact time, the following conditions were used: Initial paracetamol concentrations of 10, 20, 25, 30, 40 and 50 mg L⁻¹, natural pH of the paracetamol solution (7.6), 25 °C and an adsorbent dosage of 1.75 g L⁻¹. As can be seen in Figure 5, increasing the contact time after 30 min does not lead to a significant increase in the adsorption capacity of the drug. Although the plateau is reached after about 30 min, the remaining experiments were performed at 60 min to ensure that the adsorption process had truly reached equilibrium. Similar results were found for the removal of paracetamol with activated carbon from tea waste [22]. Removal of paracetamol with activated charcoal extracted from spent tea leaves achieved similar results to this study, with a saturation time of about 60 min [41]. An equilibrium time of 120 min for the adsorption of paracetamol with activated charcoal as adsorbent, achieving a removal of about 93% and an adsorption capacity of 3.6 mg g⁻¹, was found elsewhere [42].

3.2.2. Effect of Initial Paracetamol Concentration

The influence of the initial concentration of paracetamol on the adsorption process was carried out with a contact time of 60 min, 1.75 g L⁻¹ adsorbent, a natural pH of the solution (7.6), a temperature of 25 °C and an initial concentration of paracetamol between 10 and 50 mg L⁻¹. As can be seen in Figure 6, the adsorption capacity increases with increasing paracetamol concentration, while the percentage of removal decreases. This shows that the removal of the drug depends on its initial concentration in the solution [29,43,44]. An initial concentration of 25 mg L⁻¹ paracetamol was chosen for the following studies, with which an adsorption capacity around 12.8 mg g⁻¹ and an efficiency of 88.1% was achieved, representing the best compromise between adsorption capacity and percentage removal of the adsorbate.

3.2.3. Effect of Adsorbent Dosage

The adsorbent dosage is an important parameter as it has a direct influence on the adsorption capacity and the process efficiency with regard to the cost–benefit ratio. Figure 7 illustrates the effect of biochar dosage on the removal of paracetamol. The effect was studied by varying the amount of biochar from 0.6 to 2.5 g L⁻¹ while keeping all other parameters constant (initial concentration of 25 mg L⁻¹ paracetamol, natural pH of the solution, contact time of 60 min and temperature of 25 °C). An increase in removal efficiency from 54% to 94% is observed when the dosage of adsorbent is increased from 0.6 g L⁻¹ to 2.5 g L⁻¹. This trend can be explained by the increase in the number of active sites in the adsorbent. Conversely, the adsorption capacity decreased from about 22.0 mg g⁻¹ to 9.4 mg g⁻¹, as found in similar studies [41,42,43,44]. To continue the study, a biochar dosage of 1.75 g L⁻¹ was chosen, which resulted in an approximate removal of 86% of the drug and an adsorption capacity of around 12.28 mg g⁻¹.

Experimental conditions: interaction time of 60 min, initial paracetamol concentration of 25 mg L⁻¹; natural pH of solution (7.6) and 25 °C.

3.2.4. Effect of pH

In general, the pH of the solution is a crucial parameter that significantly influences adsorption efficiency [41]. This is due to both its influence on the charge distribution on the surface of the adsorbent and the determination of the degree of ionization of the chemical species present in the solution that need to be removed. In this way, the effect of pH was studied in the range from 2 to 12, keeping all other factors constant (contact time of 60 min, initial paracetamol concentration of 25 mg L⁻¹ adsorbent dosage of 1.75 g L⁻¹ and temperature of 25 °C). Figure 8 shows a gradual increase in the percentage of removal as the pH increases from 2.0 to 10, reaching an efficiency of about 89%. From this value, a sharp drop in the removal percentage is observed, with the efficiency falling to around 30%. A similar trend has been described by other authors who have studied the removal of paracetamol with other types of adsorbents [4,14,41,45]. Given this behavior, it was decided to continue the study with the paracetamol solution at its natural pH (7.6), making the process more economical and environmentally friendly. At this pH, the removal efficiency reaches about 88% and the adsorption capacity about 12.4 mg g⁻¹.

An important factor in studying the effect of pH on an adsorption process is the point of zero charge potential (pHpzc), which indicates the ionization of the surface functional groups and their electrical interaction with the adsorbate species in solution. If the pH of the solution is higher than the pHpzc, the surface of the adsorbent becomes negatively charged, which favors the adsorption of positively charged species. If the pH of the solution is lower than the pHpzc, the surface of the adsorbent becomes positively charged, which favors the adsorption of negatively charged species in the solution. Figure 9 shows the determination of pHpzc, which is illustrated by the plateau of the curve of the final pH value compared to the initial pH value of the solution. It is determined by the arithmetic mean of the values obtained for the final pH values of the adsorbent and its measurement provides the pH value at which the surface charge of the adsorbent is neutral.

Adsorption efficiency and adsorption capacity, as a function of pH, are related to both, the point of potential zero charge (pHpzc) of the adsorbent and the acid dissociation constant (pKa) of the paracetamol molecule. It is known that the pKa of paracetamol molecules is 9.38. Up to a pH value of 9.38, the paracetamol molecule is in the neutral, non-dissociated form, while at pH values above 9.38, its deprotonated (anionic) form predominates [4,5,7,14,41,43].

Figure 9 shows that at pH values above 10, an abrupt decrease in adsorption efficiency and capacity is observed, as the negatively charged surface of the adsorbent repels the anionic species of paracetamol present in the solution, preventing its adsorption. In addition, at alkaline pH values, the -OH groups and the paracetamol anions compete for the active sites of the adsorbent, which also contributes to the reduction in the percentage removal [4,41].

3.3. Effect of Temperature and Evaluation of Adsorption Thermodynamics

Temperature can increase or decrease the adsorption capacity of an adsorbate on a given adsorbent. In this study, the influence of temperature on the adsorption process of paracetamol on the surface of the activated biochar produced was studied at four temperatures: 25, 35, 45 and 55 °C, at a contact time of 60 min, an initial concentration of 25 mg L⁻¹, adsorbent dose of 1.75 mg L⁻¹ and a solution pH of 7.6 (conditions determined in the preliminary adsorption experiments). The results obtained are shown in Figure 10, from which, a slight increase in adsorption capacity from ~12 mg g⁻¹ to ~14 mg g⁻¹, with increasing temperature from 25 to 55 °C, can be seen. In previous studies on the adsorption of paracetamol in different adsorbents, it was suggested that the pores of the adsorbent enlarge when the temperature of the solution is increased [21,42]. Such an increase in size can facilitate the diffusion of molecules through the pore network on the surface of the adsorbent. In addition, as the temperature increases reduces the viscosity of the solution and increases the kinetic energy and diffusion rate of paracetamol molecules through the pores of the bioadsorbent [38,44].

Regarding thermodynamic parameters, shown in

Table 3. Thermodynamic parameters for adsorption of paracetamol using the activated biochar produced.

T (K)	Keq × 104	ΔG° (kJ mol−1)	ΔH° (kJ mol−1)	ΔS° (kJ mol−1 K−1)
298	8.255 ± 0.720	−27.49 ± 3.19	−3.09 ± 0.50	0.094 ± 0.011
308	7.762 ± 0.635	−28.59 ± 3.43
318	7.225 ± 0.963	−30.09 ± 3.61
328	7.147 ± 0.786	−30.48 ± 3.66

, it was found that the ΔG° values were negative, indicating favorable and spontaneous adsorption of paracetamol on the surface of the adsorbent. Furthermore, Figure 11 shows that ΔG° becomes more negative with increasing temperature, indicating that the process is favored at higher temperatures, as previously shown in Figure 10. The ΔG° value varied between, approximately, −27.49 and −30.09 kJ mol⁻¹. From Figure 12 and Equation (6), enthalpy has a positive value (about 3.90 kJ mol⁻¹), demonstrating the endothermic character of the adsorption process studied. Additionally, a positive value for entropy (0.094 kJ mol⁻¹ K⁻¹) means that the randomness at the solid/solution interface increases, being responsible for the driving force of the adsorption process studied. Similar results were presented in other studies for the adsorption of paracetamol on adsorbents of different nature [5,10,42].

3.4. Adsorption Isotherms Results

The study of adsorption isotherms aims to explain the nature of the interaction between the adsorbent and the adsorbate (monolayer or multilayer), as well as the nature of the adsorbent surface, and with this knowledge, to be able to optimize the process [46]. The adsorption isotherm resulting from the relationship between the adsorption capacity (q_e) and the concentration of paracetamol (C_e), under equilibrium conditions, at different temperature values (25, 35, 45 and 55 °C), is shown in Figure 13. In the low paracetamol concentration range, a rapid increase in adsorption capacity is observed, followed by a lower slope region until finally a plateau is reached in the curve, which can be related to the saturation of the active sites. In this way, the maximum experimental adsorption capacity can be observed at the plateau [47]. The value obtained for the system studied at 25 °C was 20.9 mg g⁻¹. For all four temperature values, the same trend was observed. Furthermore, it can be noted that as the temperature increased, the adsorption capacity increased slightly, too. This behavior is consistent with that exhibited in Figure 11, which shows that the variation in standard Gibbs free energy decreases as the temperature increases, highlighting the spontaneity of the process with increasing the temperature.

In order to obtain a better understanding of the nature of the interactions between the paracetamol and the surface of the adsorbent produced, three of the best-known isothermal models (Langmuir, Freundlich and Temkin) were used to analyze the experimental data obtained. The most important parameters resulting from the isotherms at 25, 35, 45 and 55 °C, are listed in

Table 4. Langmuir, Freundlich and Temkin isotherm parameters for the adsorption of paracetamol on activated biochar produced in this study.

Isotherm	Parameters	25 °C	35 °C	45 °C	55 °C
Langmuir	KL (L mg−1)	0.546 ± 0.031	0.513 ± 0.033	0.478 ± 0.041	0.473 ± 0.045
	Q[max]calc (mg g−1)	27.8 ± 3.6	30.5 ± 4.3	33.8 ± 3.2	37.2 ± 4.1
	R2	0.9815	0.9885	0.990	0.9896
	RL	0.07	0.07	0.08	0.08
Freundlich	KF (mg g−1)·(L mg−1)1/n	10.79 ± 0.76	11.27 ± 0.89	11.80 ± 0.79	12.64 ± 0.82
	n (dimensionless)	2.67	2.49	2.32	2.19
	R2	0.9256	0.9413	0.9490	0.9480
Temkin	KT (L mol−1)	4.75 ± 0.27	4.27 ± 0.28	3.84 ± 0.34	3.69 ± 0.34
	b (J mol−1)	6.36 ± 0.60	7.14 ± 0.70	8.07 ± 0.81	9.03 ± 0.89
	R2	0.9664	0.9790	0.9850	0.9881

.

Table 4 shows that the Langmuir isotherm, for both four temperatures, is the one that best fits the experimental data, with the highest values of R². Moreover, the values found for R_L are between zero and one, indicating that the adsorption process of paracetamol on the surface of the activated biochar, produced in this study, is favorable, as found in other studies [17,41]. This model assumes the formation of a monolayer of paracetamol molecules in energetically equivalent active sites of the activated biochar produced, that is, each active site adsorbs a single molecule and the surface of the adsorbent is uniform, with a dynamic equilibrium between the adsorption and desorption processes. The maximum adsorption capacity of the monolayer, obtained at 25 °C, was 27.8 mg g⁻¹. It can be observed that the adsorption capacity increases from 27.8 to 37.2 mg g⁻¹, as the temperature increases from 25 to 55 °C. However, the increase is not significant enough to economically justify the use of a temperature higher than 25 °C.

Table 4 shows, also, that the coefficients of determination (R²) of the Langmuir and Temkin models present the highest values and are very close to each other. When this occurs, it can be assumed that the interactions between the adsorbate molecules and the adsorbent surface are weak and that surface heterogeneity is limited. And, under such conditions, the two isotherms can yield close R² values, since the assumptions of both converge.

3.5. Adsorption Kinetics

Figure 14 shows the kinetic curve of paracetamol adsorption by the activated biochar produced. The curve obtained indicates a rapid adsorption process at the beginning, followed by a decrease in the adsorption rate as the adsorbent surface becomes saturated. The kinetic curve shows that the equilibrium time was reached in about 30 min.

The kinetic parameters and coefficient of determination (R²) calculated with the pseudo-first-order model, the pseudo-second-order model, and the Elovich model are listed in

Table 5. Kinetic parameters of the pseudo-first-order, pseudo-second-order and Elovich models for the adsorption of paracetamol on activated biochar produced.

Model	Parameters	Values
Pseudo-first-order model	K1 (min−1)	0.059 ± 0.007
	qe (mg g−1)	12.84 ± 0.30
	R2	0.9598
Pseudo-second-order model	K2 (g mg−1 min−1)	0.0071 ± 0.010
	qe (mg g−1)	13.71 ± 0.00
	R2	0.9241
Elovich kinetic model	α (mg g−1 min−1)	3.12 × 102
	β (g mg−1)	0.638 ± 1.18
	R2	0.4878

. The correlation coefficients (R²) show that the pseudo-first-order model is the best kinetic model to fit the experimental data obtained for paracetamol adsorption. This behavior indicates that the rate-limiting step of adsorption is the diffusion of adsorbate molecules to the active sites on the adsorbent surface, suggesting a physisorption process, with mainly van der Waals interactions, although paracetamol is smaller than the micropore diameter. The dimensions of the paracetamol molecule (length; width; thickness (14.61; 9.93; 6.11 Å) [12], are smaller than the average pore size of activated biochar, prepared in this study (from 20 to 40 Å, as shown in Figure 2. This result is consistent with other studies investigating a low-cost adsorbent from orange peel residue [4], KOH-super activated carbon waste [14], activated carbon from Cannabis sativum [43] and ginkgo leaf biochar activated with H₃PO₄ [48].

However, looking more closely at Table 5, it is possible see that the coefficients of determination (R²) for both the pseudo-first-order and pseudo-second-order kinetic models are similar. When this occurs, the interaction between the adsorbate molecules and the adsorbent surface involves a combination of steps, such as adsorbate diffusion to the adsorbent surface (a simpler step, based on physisorption), followed by a chemical binding (a slower process, involving complex formation or other chemical reactions on the adsorbent surface). In this case, both steps are important and influence the adsorption rate, and both models can provide good fits to the experimental data.

3.6. Possible Adsorption Mechanisms

In short, the adsorption mechanisms of aromatic pollutants in porous carbonaceous materials generally involve interactions of different nature that can occur simultaneously. The nature of the interactions depends on the conditions of the adsorption process (interaction time, dosage of the adsorbent, initial concentration of the pollutant, pH and temperature of the solution). In addition, the properties of the adsorbate (solubility, molecular size, pKa, etc.) are also of great importance, as are the characteristics of the adsorbent itself (surface area, pore area and volume, as well as the amount and nature of surface functional groups).

In this study, it can be firstly hypothesized that the large surface area and well-developed pore structure of biochar (confirmed by the N₂ adsorption/desorption isotherms) provide ample active sites for the adsorption of paracetamol molecules, whose size is smaller than that of the pores on the surface of the adsorbent. This condition suggests that paracetamol molecules can easily diffuse through the internal pore network via the pore-filling mechanism, a type of physical adsorption based on weak attractive forces, such as van der Waals forces, between the adsorbate molecules and the adsorbent surface, observed mainly in mesoporous and microporous adsorbents. Another type of interaction that can also be considered is that the paracetamol molecule has an aromatic ring in its structure, and according to the FTIR analysis, the activated biochar, produced in this study, has a certain degree of graphitized structure, so that the π-electrons (donor) of the activated biochar prepared and the π-electrons (acceptor) of the paracetamol molecules can establish an π-π type interaction. In addition, FTIR analysis showed that the structure of the biochar produced has functional groups containing oxygen and phosphorus, the latter due to the H₃PO₄ treatment performed. Thus, these groups or atoms (containing non-bonding electrons) can interact with the aromatic structure of the paracetamol molecule via n-π interactions. Hydrogen bonding is another type of interaction that can occur, due to the interactions between the hydrogen atom present in polar functional groups, of the type -O-H and -COO-H) of the functional groups existing on the surface of the adsorbent and the N atoms in the paracetamol molecules. Finally, the electrostatic interaction between the adsorbent surface and the adsorbate molecules can also be considered a possible mechanism. At pH values above pHpzc, the surface of an adsorbent is negatively charged, and below this value, it is positively charged. It is known that the pKa of the paracetamol molecule is 9.38, and thus in solutions with a pH above this value, the molecule is ionized, existing in the anionic form. Therefore, as seen in Figure 8, above this value, electrostatic repulsion occurs between the anionic species of paracetamol in solution and the negatively charged surface of the adsorbent, leading to a rapid drop in adsorption capacity and removal efficiency.

3.7. Desorption and Reusability Study

The reuse of the adsorbent was investigated to obtain information on its stability and cost–benefit ratio. To fulfill this requirement, in this study, we opted for thermal treatment of the bioadsorbent loaded with pollutant molecules in the presence of atmospheric air to thermally degrade the paracetamol molecules and subsequently reuse the bioadsorbent in a new adsorption test. The degradation temperature of paracetamol is about 326 °C [36]. Therefore, a temperature of 400 °C was chosen for a period of 30 min. Preliminary tests have shown that longer times are not effective.

Figure 15 shows that the removal efficiency of paracetamol remained practically constant at about 93% during the first four cycles and then dropped to about 79%. In this way, it can be demonstrated that the activated biochar, produced in this study, has an excellent capacity for thermal regeneration, which allows its reuse.

A explanation for the reduction in the paracetamol removal efficiency as a function of the number of use cycles is that the heat treatment can alter the chemical structure of the activated biochar produced, generating a possibly more aliphatic structure, and thus reducing its affinity for aromatic organic pollutants [5]. Another explanation could be the lower desorption capacity of paracetamol molecules, as the number of adsorption–desorption cycles increases and thus the number of active sites decreases [10].

To verify the regeneration of biochar after each adsorption–desorption cycle, FTIR spectroscopy was performed. Figure 16 shows three spectra: the first being from pure paracetamol, the second from biochar after an adsorption experiment and the third from biochar after heat treatment. A simple qualitative comparison of the peaks in each spectrum shows the adsorption of paracetamol onto the structure of the activated biochar produced and its removal after heat treatment, regenerating the biochar surface for a new adsorption process. Although the bioadsorbent comes from available, renewable and sustainable solid waste, its reusability is always a factor with a major economic impact. As we have found, the activated biochar produced has excellent regenerative ability, making it a promising material with potential for commercialization in water treatments, as also found in another study using a bioadsorbent produced from activated ginkgo leaf biochar [31].

3.8. Application in Real Water Environments

With tap water, it was found that the removal of paracetamol over a period of 180 min was about 95%. This value is higher than the value found for the synthetic solution prepared with distilled water, which was around 88%. A practically constant and high value of approx. 99% was found for the stream water. The activated biochar produced, thus, showed excellent performance in the removal of paracetamol when used in real matrices. It can be concluded that biochar has considerable tolerance to samples containing contaminants of a more complex nature and has great potential for use in wastewater treatment systems. The presence of other compounds and ions in a complex matrix can reduce the adsorption of the organic molecule (paracetamol, in this case), competing with it for adsorption at the same site on the adsorbent. On the other hand, they can facilitate the adsorption of the organic molecule, as observed in this study. This behavior could be explained by considering the effect of complexation, that is, some solutes present in contaminated water can form complexes with the organic molecule, altering its physicochemical properties and facilitating its adsorption. The formation of complexes can increase the hydrophilicity of the organic molecule, for example, making it more prone to adsorption on a hydrophilic surface, formed by the presence of functional groups on the surface of the activated biochar produced. A similar result on the reuse of spherical biochar prepared from pure glucose in different aqueous environments was found elsewhere [7].

4. Conclusions

In this study, an activated biochar was produced by impregnating eucalyptus harvest residues with H₃PO_4, and subsequent pyrolyzed at 600 °C for 1 h, and then used as a bioadsorbent for the removal of paracetamol from contaminated water.

The characterizations carried out with the bioadsorbent (FTIR, BET, SEM) revealed surface functional groups and a large specific surface area composed of micro- and mesopores, important characteristics for the adsorption process.

The optimal experimental conditions for the removal of paracetamol were: a contact time of 60 min, an initial paracetamol concentration of 25 mg L⁻¹, a dosage of 1.75 mg g⁻¹ for the adsorbent, the natural pH of the paracetamol solution (7.6) and 25 °C of temperature. In this way, a removal efficiency of around 88–93% was achieved.

The adsorption process was found to be spontaneous (∆G°), endothermic (∆H°) and with a greater randomness of the paracetamol molecules at the solid/liquid interface (∆S°). Moreover, it was observed that the Langmuir and pseudo-first-order models, satisfactorily, describe the equilibrium isotherm and kinetics, respectively, with a maximum adsorption capacity of approximately 27.8 mg g⁻¹, at 25 °C.

Furthermore, the study of the regeneration of the biochar after each adsorption–desorption cycle showed excellent reusability of the bioadsorbent produced, with around 93% of paracetamol being removed until the fourth cycle, showing potential for commercialization.

Finally, the ability to remove paracetamol in aqueous matrices of more complex nature showed no significant difference, proving that the activated biochar, produced in this study, has a high environmental compatibility.

In conclusion, this study has shown that it is possible to add value to eucalyptus harvest residue, an available solid waste, with no known economic use, and convert it into an efficient, low-cost and eco-friendly bioadsorbent, making it a promising material for the removal of paracetamol, an emergent pollutant widely found in contaminated water, contributing to the development of another new and efficient bioadsorbent of great importance to the environmental area.