Biochar was produced by pyrolysing palm tree bark biomass at 500 °C for the removal of rhodamine B (RhB) and metronidazole (MET). Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller, X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray analyses were used to characterize the biochar. The biochar obtained was crystalline, mesoporous (SBET: 189.157 m2 g−1; pore diameter: 2.207 nm), clustered with prominent O–H and C = O functional groups. The pHpzc of the biochar was 7.98, and it adsorbed RhB and MET maximally at pH 3.4 and 7.2, respectively. The Langmuir and Freundlich isotherms described RhB and MET adsorption, respectively, with maximum adsorption capacities (qmax) of 31.81–224.30 mg/g for RhB and 95.44–26.76 mg/g for MET from 303 to 313 K. Both adsorbates exhibit favourable physisorption processes with pseudo-second-order kinetics, as the most appropriate. The thermodynamic parameter (−ΔG°) demonstrates spontaneous adsorption processes for RhB and MET, with spontaneity increasing with temperature for RhB and decreasing with increasing temperature for MET. The adsorption process was endothermic (+ΔH°) for RhB and exothermic (−ΔH°) for MET. Given its reusability of 96 and 95% for RhB and MET, respectively, mesoporous biochar derived from palm trees is a more promising adsorbent.

  • Biochar was prepared from palm tree biomass and characterized to be mesoporous and crystalline.

  • Adsorption of rhodamine B and metronidazole followed Langmuir and Freundlich isotherms, respectively.

  • Adsorption capacities increased from 31.81 to 224.30 mg/g for RhB and decreased from 95.44 to 26.76 mg/g for MET with temperature.

  • The kinetics of both adsorption processes followed pseudo-second order.

  • Adsorption was spontaneous.

Untreated toxic effluents are being released into the environment, especially into water sources, at an increasing rate due to urbanization and industrialization (Sarkhosh et al. 2019; Asgari et al. 2020; Akbari & Adibzadeh 2022). In view of the fact that industrial wastewater contains high levels of toxic, carcinogenic, and mutagenic pollutants, it is crucial that these pollutants be removed before being released into the environment (Duan et al. 2020; Azeez et al. 2022, 2023). Rhodamine B and metronidazole are pollutants found in surface and groundwater. They originate majorly from industries, such as paper and pulp, textiles, leather, and pharmaceuticals. Rhodamine B is a persistent dye while metronidazole is an emerging contaminant (Azeez et al. 2020; Ahmadfazelia et al. 2021; Bingül 2022).

Rhodamine B (RhB) is a light-resistant, heat-stable synthetic cationic dye used in textiles and dietary products (Azeez et al. 2018; Li et al. 2018). RhB has been linked to skin irritation, pulmonary inflammation, haemolysis, liver and kidney degeneration, and similar disorders once it exceeds the maximum acceptable concentration (tolerance limit) of 140 μgL−1 (Albanio et al. 2021; da Silva et al. 2022; Li et al. 2022; Sharma et al. 2022; Zhou et al. 2022). Due to the aromatic rings in the RhB structure, it is difficult to degrade. Its pollution of water and accumulation in aquatic organisms have increased as a result (Sridhar et al. 2022; Usman & Khan 2022; Li et al. 2023).

Metronidazole (MET) is an imidazole antibiotic that treats bacterial and protozoal infections in humans and animals. Due to its wide use and high solubility in water, it accumulates in aquatic environments (Nasseh et al. 2019; Asgari et al. 2020; Ahmadfazelia et al. 2021). It has been reported to cause cancer and mutations in humans as well as damage to the brain, lymph, and blood, especially when the tolerance limit of 171.54 ng mL−1 is exceeded (Hanna et al. 2018; Li et al. 2019; Bonyadi et al. 2021; Esmaili et al. 2023).

In developing countries such as Nigeria, there is no legislation regulating the concentration of RhB and MET in wastewater at present. Nonetheless, RhB and MET levels as low as 140 μg L−1 and 10 ng/L make aquatic environments unsuitable for animals, while lethal levels between 14 and 24 mg/L RhB and between 11 and 13 ng/mL MET could cause 50% toxicity to animals as previously reported (Nasseh et al. 2019; Ahmadfazelia et al. 2021; Sridhar et al. 2022; Usman & Khan 2022; Li et al. 2023).

The continuous release of pollutants into water bodies has made clean water increasingly difficult to obtain. As well, a lack of proper regulation on effluent disposal and improper treatment may adversely affect Sustainable Development Goals (SDG 6) (clean water and sanitation) (Mukherjee et al. 2023a, 2023b).

The most effective, efficient, and safest method of dealing with emerging pollutants is adsorption due to its superiority to other treatment methods including its efficiency, convenience, simplicity, and cost-effectiveness (Albanio et al. 2021; Le et al. 2021; da Silva et al. 2022; Li et al. 2022, 2023; Azeez et al. 2023). However, commercially available adsorbents are usually expensive to purchase while agriculture continually generates large amounts of biomass that can cause environmental and ecological challenges if not properly disposed of (Adeniyi et al. 2019; Vigneshwaran et al. 2021; Zhang et al. 2021; Li et al. 2022; Zhou et al. 2022; Burezq & Davidson 2023). It is, therefore, necessary to recycle agricultural biomass and rid the environment of waste. Biochar can be made from biomass due to its high lignin, hemicellulose, and cellulose contents (Burezq & Davidson 2023). The use of agricultural biomass for biochar production can be beneficial for waste management, climate change mitigation, and soil amendment (Vigneshwaran et al. 2021). A variety of materials have been used to produce biochar including rice husk, Chinese medicine herb residues, cement waste, coconut shell, rice bran, sugarcane bagasse, olive biomass waste, and date palm residue (Adeniyi et al. 2019; Albanio et al. 2021; Le et al. 2021; Vigneshwaran et al. 2021; Zhang et al. 2021; da Silva et al. 2022; Li et al. 2022; Burezq & Davidson 2023). Biochar is produced by pyrolysis of biomass under oxygen-limited conditions and has many benefits, including renewability, biodegradability, regeneration, and high porosity with a large surface area. Biochar's cost-effectiveness, availability, and ease of preparation make it an appealing adsorbent for wastewater treatment (Adeniyi et al. 2019; Olorunfemi et al. 2019; Yaashikaaa et al. 2019; Vigneshwaran et al. 2021). The threat of these pollutants to aquatic environments and human health has been demonstrated both individually and jointly, this study investigated the adsorption of RhB and MET on biochar made from palm tree bark to achieve environmental sustainability by converting waste biomass into useful biochar with minimal combustion and ash content and maximum carbon content.

Biochar production and characterization

Palm tree bark was collected from a farm in Ilesa, Osun State, Nigeria. The bark was sun-dried for 24 h and then ground using a Eurosonic blender (Nigeria). The pulverized palm tree bark was pyrolysed in the muffle furnace at 500 °C peak for 5 h and cooled for 2 h following the method of Salem et al. (2021). Biochar functional groups were determined using a Fourier-transform infrared (FTIR) spectrometer (Shimadzu FTIR 8400S, Japan) by scanning from 400 to 4,000 cm−1. In addition, scanning electron microscopy (SEM Phenom PRO X, Phenom world BV, Netherlands) was used to analyse morphological parameters, and energy-dispersive X-ray fluorescence was used to determine the surface elemental composition of biochar (EDXRF ARL XTRA Thermoscientific, Switzerland). In addition, an X-ray diffractometer (XRD Thermofisher, Switzerland) was used to ascertain the crystallinity and particle size was calculated using Debye–Scherrer formula (Equation (1)). The surface area, porosity, and shape of the isotherm were determined using Brunauer–Emmett–Teller (BET, Quantachrome Autosorb 1 series, USA).
formula
(1)
where D is the particle size, is the wavelength of X-ray, is the full-width half maximum, and is the diffraction angle.

Reagents and adsorbate characteristics

Rhodamine B (98%), metronidazole (99.5%), NaOH (99.5%), HCl (99%), and CH3CH2OH (99.5%) are analar grade reagents purchased from Sigma-Aldrich, Germany. A UV–Visible spectrometer was used to confirm the purity of RhB and MET purchased. Their maximum absorption was found to be at 555 and 320 nm, respectively, as previously reported (Ahmadfazelia et al. 2021; Azeez et al. 2022).

pH point of zero charge (pHpzc)

To measure the pH point of zero charge (pHpzc), the pH of 0.1 g of mesoporous biochar in 0.1 M NaCl solution was adjusted from 1 to 12 using 0.1 M HCl/NaOH at room temperature. The pHpzc was calculated by measuring the final pH after 24 h and extrapolating from the point of intersection in the plot of the final pH against the initial pH. A Jenway 3,505 pH meter was used to measure pH.

Adsorption studies of rhodamine B and metronidazole

The influence of the initial pH of RhB/MET solution (2–11), contact time (10–200 min), biochar dosage (0.1–0.5 g), and initial RhB/MET concentration (10–50 mg/L) on the adsorption of RhB/MET onto biochar was investigated. The working solutions of RhB and MET were adjusted to the desired pH values with 0.1 M HCl/NaOH. In a 100 mL conical flask, 0.5 g biochar was added to 50 mL of 50 mg/L RhB solution. A thermostatically controlled water bath shaker (Uniscope water bath shaker) was used to agitate the flask at 1,500 rpm for 100 min at 30 °C. The same procedure was repeated for MET. A Jenway 6405 UV–Visible spectrophotometer (Buch Scientific Inc., USA) was used to measure the residual RhB and MET concentrations at 555 and 320 nm, respectively. Equations (2) and (3) were used to calculate the percentage removal and quantity adsorbed per gram of biochar.
formula
(2)
formula
(3)
where is the amount of RhB/MET adsorbed per biochar gram (mg/g); Ct, Ci, and Cf are residual, initial, and final RhB/MET concentrations (mg/L); V is the volume of RhB/MET solution (L); M is the biochar mass (g).

Adsorption isotherms, kinetics, and thermodynamics of RhB and MET

Adsorption data were described using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models (Table 2) while adsorption kinetics were described with pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intra-particle diffusion model equations (Table 3). These models are required to explain adsorption isotherms and kinetics of adsorbates for preference on homogenous or heterogeneous surfaces of adsorbents, and the distribution of energy levels relative to their removal kinetics. Thermodynamic parameters obtained from Van't Hoff's equations (Table 5) were used to explain the energetics and spontaneity of the adsorption process for the removal of RhB/MET from mesoporous biochar produced from palm tree biomass. The best model was determined by root mean square error (RMSE, Equation (4)), Chi-square (χ2, Equation (5)), and coefficient of determination (R2, Equation (6)).
formula
(4)
formula
(5)
formula
(6)
where x denotes the of each model and y denotes the experimental .

Reusability and desorption study

This experiment examined biochar regeneration/reusability following its initial use for RhB/MET removal in ethanol and water. Briefly, 0.1 g biochar was added to 25 mL of 50 mg L−1 RhB and MET, with their pH adjusted to 3.4 and 7.2 (the maximum adsorption pH) and stirred at 303 K for 1 h before centrifuging at 300 rpm. After centrifugation, desorption was carried out with deionized-distilled water and ethanol, and the residual concentration of RhB/MET was determined. Three additional cycles were done. The reusability/desorption percentage was calculated using the following equation.
formula
(7)

Characterization of biochar produced from palm trees

FTIR spectrum of biochar showed distinct peaks at 3,441 cm−1 attributable to the stretching O–H possibly from carboxyl, phenol, and alcohol functional groups. The peaks at 1,684, 1,559, and 1,653 cm−1 correspond to the C = O of conjugated ketone, carboxylic acid and C = C groups, respectively (Figure 1). The peak at 1,111 cm−1 is assigned to the C–O of alcohol. This result is consistent with previously reported FTIR spectra of biochar produced from rice rusk, Chinese herb medicine residue, and sugarcane bagasse (Li et al. 2019, 2022, 2023; Albanio et al. 2021; Le et al. 2021; Zhang et al. 2021).
Figure 1

FTIR spectrum of biochar produced from palm tree bark.

Figure 1

FTIR spectrum of biochar produced from palm tree bark.

Close modal

According to the SEM micrograph (Figure S1a; supplementary document), the biochar is primarily oval-shaped with some cylindrical areas and amorphous with rough edges containing C, K, Cl, Ca, Fe, Ni, Cu, and Zn as revealed by the EDXRF analysis (Figure S1b; supplementary document). The SEM micrograph showed cavities that would favour the adsorption of pollutants. Usually, the elemental composition of biochar depends on its source (da Silva et al. 2022; Li et al. 2022).

X-ray diffraction (XRD) analysis of biochar revealed monoclinic-shaped particles with an average particle size of 34.21 nm and an average interplanar spacing of 2.172, matching Joint Committee on Powder Diffraction Standard (JCPDS) card number 00-712-6280 (Figure 2). The lattice parameters are a = 31.0593 Å, c = 21.8198 Å with six notable diffraction peak angles at 2θ = 28.41°, 40.57°, 50.24°, 58.71°, 66.45°, and 73.77°. These peaks correspond to hkl crystal planes of (111), (102), (110), (200), (112), and (202). This aligns with the report of Albanio et al. (2021).
Figure 2

XRD of biochar produced from palm tree bark.

Figure 2

XRD of biochar produced from palm tree bark.

Close modal

A type II isotherm (Figure S2a; supplementary document), N2 adsorption/desorption cycle with H4 hysteresis (Figure S2b; supplementary document), and mesoporous pore diameter distribution (Figure S2c; supplementary document) were determined with BET at 77 K. These results suggest a mesoporous material with macroporous slits that could be useful for unrestricted monolayer adsorption or multilayer adsorption. The SBET, total pore volume, average pore diameter, and micropore surface area (Table 1) indicate a mesoporous biochar, with an average pore diameter of 2.207 nm appearing within a range of 2–50 nm (Li et al. 2022, 2023; Azeez et al. 2023).

Table 1

Textural properties of biochar produced from palm tree

BET surface area (m2 g−1189.157 
Micropore surface area (m2 g−1189.318 
Barret-Joyner-Halenda (BJH) total pore volume (cc g−10.097 
BJH average pore diameter (nm) 2.207 
BET surface area (m2 g−1189.157 
Micropore surface area (m2 g−1189.318 
Barret-Joyner-Halenda (BJH) total pore volume (cc g−10.097 
BJH average pore diameter (nm) 2.207 

pH point of zero charge (pHpzc)

The mesoporous biochar had a pHpzc of 7.98 (Figure 3), which implies that its surface was positively charged and attracting anions below pHpzc, and negatively charged and attracting cations above pHpzc (Azeez et al. 2020, 2022; Ren et al. 2020; Bonyadi et al. 2021). The surface functionality of the biochar, as represented by pHpzc, determines the strength of attraction between it and RhB/MET, which depends on functional groups, cavities, and electrostatic interaction between the adsorbent and adsorbate (Sridhar et al. 2022; Zhou et al. 2022; Nayak et al. 2023).
Figure 3

pH point of zero charge (pHpzc) of biochar produced from palm tree bark.

Figure 3

pH point of zero charge (pHpzc) of biochar produced from palm tree bark.

Close modal

Influence of pH on RhB and MET adsorption

pH changes may either decrease or increase the adsorption percentage, depending on the surface charge of an adsorbent and mobility of the adsorbate (Elsayed et al. 2022; Liu et al. 2022). RhB removal increased from 90 to 93% with pH from 2 to 3.4 (Figure S3; supplementary document) and decreased to the minimum (45%) at pH 9. RhB exists in three forms; two as cationic RhB at pH < 1 as , and between pH 1 and 3 as RhBH+. The zwitterionic form (RhB±) exists at pH > 4 where the positive and negative charges are located in = N+ and COO. The electrostatic interaction between biochar and RhB at pH 3.4, where maximum adsorption occurred, is thus greatly impacted by electron transfer from COO of RhB to mesoporous biochar surface due to its cationic surface charge at this pH (Azeez et al. 2018, 2020, 2023; da Silva et al. 2022; Liu et al. 2022; Zhou et al. 2022). As the pH increased, biochar surface became less cationic as it approached its pHpz, and then anionic above it leading to repulsion between RhB and biochar, thus low percentage adsorption. The percentage removal of MET increased from 62 to 96% when the pH was increased from 5 to 7.2 with sinusoidal adsorption percentages. This is consistent with results of MET adsorption on biomass-derived porous aminated graphitic nanosheets with maximum adsorption at pH 8 (Bonyadi et al. 2021). Metronidazole (MET) has two pKa values (pKa1 = 2.38 and pKa2 = 14.48). It exists as a protonated species (MET-H+) at pH < 4, owing to the positively charged imidazoline nitrogen. Between pH 4 and 12, MET exists as a neutral species due to deprotonated imidazoline nitrogen while at pH > 12 the OH in MET is ionized, resulting in a negatively charged MET species (Ahmadfazelia et al. 2021; Bonyadi et al. 2021; Esmaili et al. 2023; Nayak et al. 2023). Thus, MET adsorption at this pH involved ππ interaction and electrostatic attraction between MET OH and biochar as well as hydrogen bonding interactions between the MET ring and cationic surfaces of the adsorbent (Vasseghian et al. 2022; Ghosh et al. 2023; Haghighat et al. 2023). RhB and MET adsorption per unit gram of biochar (, Figure S3) follow a similar pattern as percentage adsorption.

Influence of adsorbent dosage

An assessment of the influence of mesoporous biochar dosage on RhB and MET removal (Figure S4; supplementary document) shows a dosage-dependent increase for RhB removal while a sinusoidal increase was obtained for MET. As the biochar dosage increased from 0.1 to 0.5 g, RhB adsorption increased from 49 to 74% while MET adsorption rose from 8 to 89%. Moreover, the quantity of RhB/MET adsorbed per gram of biochar (, Figure S4) follows a similar trend, with RhB adsorption increasing from 0.022 to 0.248 mg/g and MET adsorption increasing from 0.132 to 0.199 mg/g. This increase could be attributed to an availability of more binding sites for the adsorbates. Similar results have been reported by Ahmadfazelia et al. (2021) and Esmaili et al. (2023).

Influence of initial RhB/MET concentrations and adsorption isotherms

The results of the influence of initial RhB/MET concentrations at different temperatures on the percentage uptake of RhB and MET on mesoporous biochar (Figure 4) showed that the percentage removal of RhB and MET decreased with increasing concentration of the adsorbates whereas the amount adsorbed (qe) increased with increasing solute concentrations, as also previously reported by Azeez et al. (2018, 2023). Also, temperature variations significantly affected their removal correlating with higher adsorption for RhB and decreasing removal for MET as the temperature rose from 303 to 313 K. Possibly, the significant removal of RhB/MET at the beginning can be explained by the interaction between both adsorbates and the surface of mesoporous biochar at low initial concentrations (Vigneshwaran et al. 2021). As RhB/MET concentrations increased, fewer adsorption sites became available, resulting in RhB/MET molecules competing for adsorption sites, thus, the reduction (Ren et al. 2020; Vigneshwaran et al. 2021; Li et al. 2022, 2023). Azeez et al. (2018, 2023) reported previously that RhB adsorption increased with increasing concentrations due to higher concentrations. The combined effects of concentration and temperature on the adsorption of RhB/MET can be explained as an effect of the energy of interaction between the adsorbates and the adsorbent as well as the kinetic energy of the molecules (Bello et al. 2019). At higher temperatures, the kinetic energy of the molecules increases, and this promotes desorption, leading to a lower adsorption capacity (Wu et al. 2020). RhB/MET and biochar interacted more strongly at 308 K than at 303 K, resulting in greater adsorption and higher percentage adsorption. Higher adsorption occurred at 308 K due to the adsorbate molecules possessing sufficient thermal energy to overcome the activation barrier and bind effectively to the adsorbent surface. The adsorption process at 330 K was further hindered by a reduced kinetic energy at lower temperatures. As the temperature increased from 308 to 313 K, the energy on interaction increased with an increase in kinetic energy, which encourages desorption and reduces adsorption (Sun et al. 2018).
Figure 4

Percentage adsorption of RhB and metronidazole (MET).

Figure 4

Percentage adsorption of RhB and metronidazole (MET).

Close modal

The adsorption process was described using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models (Figures S5a–h; supplementary document). The isotherm models specify the adsorbent affinity for RhB and MET, as well as the adsorption energetics and surface reactivity. A model's fitness is measured by its coefficient of determination (R2) and error function (RMSE). For RhB at 303 K, Langmuir (R2 = 0.990 and RMSE = 0.013) > Freundlich (R2 = 0.726 and RMSE = 0.247) > Temkin (R2 = 0.636 and RMSE = 0.491) > Dubinin–Radushkevich (R2 = 0.881 and RMSE = 0.558) (Table 2). At 308 K, the order is Langmuir (R2 = 0.988 and RMSE = 0.002) > Freundlich (R2 = 0.814 and RMSE = 0.164) > Temkin (R2 = 0.555 and RMSE = 0.393) > Dubinin–Radushkevich (R2 = 0.728 and RMSE = 0.240) while at 313 K, the trend follows Langmuir (R2 = 0.991 and RMSE = 0.001) > Freundlich (R2 = 0.812 and RMSE = 0.163) > Temkin (R2 = 0.551 and RMSE = 0.388) > Dubinin–Radushkevich (R2 = 0.722 and RMSE = 0.241). Based on these results, the Langmuir isotherm described the adsorption of RhB on a homogenous monolayer surface of mesoporous biochar with the highest R2 and the lowest RMSE at all temperatures and is the most suitable (Li et al. 2018, 2022, 2023; Azeez et al. 2020; Ren et al. 2020). For MET at 303 K, the most appropriate model follows Freundlich (R2 = 0.895 and 0.394) > Langmuir (R2 = 0.691 and RMSE = 4.476) > Temkin (R2 = 0.689 and RMSE = 1.167) > Dubinin–Radushkevich (R2 = 0.610 and RMSE = 1.461) while at 308 K, the model is ranked as Freundlich (R2 = 0.709 and RMSE = 0.914) > Langmuir (R2 = 0.687 and RMSE = 3.923) > Dubinin–Radushkevich (R2 = 0.450 and RMSE = 1.724) > Temkin (R2 = 0.439 and RMSE = 1.761) and at 313 K, Freundlich (R2 = 0.977 and RMSE = 0.108) > Langmuir (R2 = 0.931 and RMSE = 0.325) > Temkin (R2 = 0.897 and RMSE = 0.349) > Dubinin–Radushkevich (R2 = 0.652 and RMSE = 1.173) (Table 2). According to these results, the Freundlich isotherm was the most appropriate model for the adsorption of MET on heterogeneous mesoporous biochar. This was also the finding of Asgharzadeh et al. (2019), Li et al. (2022), and Esmaili et al. (2023).

Table 2

Isotherm models' parameters for the removal of RhB and MET at different temperatures on mesoporous biochar

RhB
MET
IsothermsIsothermal equationsParameters303 K308 K313 K303 K308 K313 K
Langmuir   (mg g−131.81 173.12 224.30 95.44 46.65 26.76 
  KL (L mg−1127.24 217.22 118.45 0.03 0.28 0.10 
  RL 0.07 0.20 0.09 0.06 0.21 0.05 
  R2 0.990 0.988 0.991 0.691 0.687 0.931 
  RMSE 0.013 0.002 0.001 4.476 3.923 0.325 
Freundlich   2.56 1.35 1.30 1.35 1.69 2.05 
  Kf 2.30 2.30 4.34 4.17 3.89 3.54 
  R2 0.712 0.814 0.812 0.895 0.709 0.977 
  RMSE 0.247 0.164 0.163 0.394 0.914 0.108 
Temkin  B 2.265 7.642 7.064 8.667 7.192 5.105 
  A (L g−11.849 1.503 0.677 1.289 1.039 1.429 
  b (J mol−11,112.12 329.63 356.58 290.63 350.26 493.42 
  R2 0.654 0.555 0.551 0.689 0.439 0.897 
  RMSE 0.491 0.393 0.388 1.167 1.761 0.349 
Dubinin–Radushkevich   (mg g−14.409 21.356 17.740 19.73 13.98 13.88 
  β ×10−7 (mol2 kJ−20.262 0.406 0.588 4.444 4.07 4.33 
  E (kJ mol−11.437 1.351 1.292 1.061 1.107 1.074 
  R2 0.350 0.728 0.722 0.610 0.450 0.652 
  RMSE 0.558 0.240 0.241 1.461 1.724 1.173 
RhB
MET
IsothermsIsothermal equationsParameters303 K308 K313 K303 K308 K313 K
Langmuir   (mg g−131.81 173.12 224.30 95.44 46.65 26.76 
  KL (L mg−1127.24 217.22 118.45 0.03 0.28 0.10 
  RL 0.07 0.20 0.09 0.06 0.21 0.05 
  R2 0.990 0.988 0.991 0.691 0.687 0.931 
  RMSE 0.013 0.002 0.001 4.476 3.923 0.325 
Freundlich   2.56 1.35 1.30 1.35 1.69 2.05 
  Kf 2.30 2.30 4.34 4.17 3.89 3.54 
  R2 0.712 0.814 0.812 0.895 0.709 0.977 
  RMSE 0.247 0.164 0.163 0.394 0.914 0.108 
Temkin  B 2.265 7.642 7.064 8.667 7.192 5.105 
  A (L g−11.849 1.503 0.677 1.289 1.039 1.429 
  b (J mol−11,112.12 329.63 356.58 290.63 350.26 493.42 
  R2 0.654 0.555 0.551 0.689 0.439 0.897 
  RMSE 0.491 0.393 0.388 1.167 1.761 0.349 
Dubinin–Radushkevich   (mg g−14.409 21.356 17.740 19.73 13.98 13.88 
  β ×10−7 (mol2 kJ−20.262 0.406 0.588 4.444 4.07 4.33 
  E (kJ mol−11.437 1.351 1.292 1.061 1.107 1.074 
  R2 0.350 0.728 0.722 0.610 0.450 0.652 
  RMSE 0.558 0.240 0.241 1.461 1.724 1.173 

RhB, rhodamine B; MET, metronidazole; , dye equilibrium concentration (mg L−1); , the quantity of RhB/MET adsorbed (mg g−1) per unit mass; , maximum monolayer adsorption capacity (mg g−1); , Langmuir adsorption constant (L mg−1); , unitless constant for favourability; , Freundlich constant; n, heterogenicity factor; B and E are the heat and energy of adsorption, respectively. R2 and RMSE are the correlation coefficient and root square mean, respectively.

The maximum monolayer adsorption capacities () for RhB were 31.181, 173.12, and 224.30 mgg−1 at 303, 308, and 313 K, increasing with temperature. For MET, the maximum monolayer adsorption capacities () were 95.44, 46.65, and 26.76 mgg−1 at 303, 308, and 313 K, respectively (Table 2). Adsorption capabilities for both adsorbates were affected by temperature. The highest temperature favoured RhB removal and the lowest temperature was suitable for MET. As previously reported, temperature has a significant influence on the adsorption of pollutants, so an increase would either increase or decrease their removal (Bello et al. 2019; Albanio et al. 2021). Based on these findings, it appears that temperature plays a significant role in increasing dye mobility on the mesoporous biochar surface through increased diffusion and intramolecular interactions. Biochar produced from palm trees for the removal of RhB and MET (Table 3) had significantly higher adsorption capacities than previously used biochar and some adsorbents for the removal of RhB and MET. This demonstrates that this adsorbent is a more promising option for dyes and pharmaceuticals removal given its adsorption capability, environmental friendliness, and ease of production. Both adsorbates have values in the range of 0 < < 1, indicating a favourable adsorption process (Azeez et al. 2022). The Freundlich plot showed that the adsorption strength () was > 1 for both adsorbates at all temperatures, lending further support to the favourability of the adsorption process and physiosorption adsorption process (Liu et al. 2022). The represents varied affinities of the adsorbent for RhB and MET at all temperatures and is proportional to the adsorption capacity (Bello et al. 2019; Vasseghian et al. 2022; Ghosh et al. 2023). In the Dubinin–Radushkevich plot, the adsorption energy (E) values ranged from 1.221 to 1.437 kJmol−1 for RhB and 1.061 to 1.107 kJmol−1 for MET, implying both adsorption processes were physisorption. Adsorption that occurs when E > 8 kJmol−1 is known as chemiosorption whereas when E < 8 kJmol−1, it is physiosorption (Bonyadi et al. 2021; Azeez et al. 2023; Haghighat et al. 2023).

Influence of contact time on RhB and MET removal and adsorption kinetics

RhB and MET adsorption onto biochar was influenced by contact time, one of the most important variables in batch adsorption (Figure 5). The plot demonstrates that the adsorption capacity () rose with time reaching equilibrium for MET at 100 min and RhB at 90 min. The increase in qt of RhB rose from 0.34 mg/g at 10 min to 1.68 mg/g at 90 min at 303 K. As for MET, the value increased from 1.10 mg/g at 10 min to 2.39 mg/g at 100 min at 303 K. RhB/MET removal was initially rapid due to the availability of adsorptive sites on mesoporous biochar prior to equilibrium being reached when most of the sites were occupied (Bello et al. 2019; Bakry et al. 2022; Zhou et al. 2022).
Figure 5

Effect of contact time on the adsorption of RhB and MET on biochar.

Figure 5

Effect of contact time on the adsorption of RhB and MET on biochar.

Close modal

RhB and MET removal kinetics and mechanisms were described using PFO, PSO, Elovich, and intra-particle diffusion equations. A model fitness estimate was determined by contrasting experimental and calculated for PFO, PSO, and Elovich kinetics. Furthermore, the preferred model for describing adsorption kinetics is supported by a relatively lower χ2 and higher coefficient of determination (R2). The adsorption data for RhB and MET are best fitted to PSO kinetics with the highest R2, lowest χ2, and closest to (Table 4 and Figure S6 (supplementary document). In the studies carried out by Bonyadi et al. (2021), Elsayed et al. (2022), Li et al. (2022), Azeez et al. (2023) and Li et al. (2023), PSO was found to be the most effective kinetic method for the removal of RhB and MET. The mechanism governing the rate-determining step is a multilinear profile plots with three layers that did not pass through the origin in the intra-particle diffusion plot (Figure S7; supplementary document), suggesting that adsorption and intramolecular diffusion processes were also involved in the removal of RhB and MET (Li et al. 2022). RhB/MET diffusion from solution to biochar's surface was attributed to the significant increase in the first step, while intra-particle diffusion of RhB/MET was attributed to the considerable rise in the second step (Figure S7). A decrease in active sites on biochar may have resulted in a lower adsorption rate () in the second step compared to the first. In the final stage, RhB/MET adsorbed onto the internal surface of the biochar, which is referred to as the equilibrium stage (Ren et al. 2020; Ghibate et al. 2021). Moreso, surface adsorption through the boundary layer (C) played a prominent role as well. The larger boundary layer could be the reason RhB adsorption was higher than MET (Ghibate et al. 2021).

Table 3

Comparison of adsorption capacity of RhB and MET on mesoporous biochar with previously published results

RhB
MET
Adsorbent (mgg−1)ReferenceAdsorbent (mgg−1)Reference
Hierarchically porous tobacco midrib-based biochar 658.2 Zhang et al. (2022)  AMGG (amine-modified green-graphene) 416.7 Bonyadi et al. (2021)  
 Activated cement waste biochar 531.84 da Silva et al. (2022)  Fe3O4-chitosan nano adsorbent 97.06 Asgari et al. (2020)  
 Olive biomass waste biochar 263.71 Albanio et al. (2021)  Mesoporous palm tree biochar 95.44 This study 
 Mesoporous palm tree biochar 224.30 This study Ammonia-modified activated carbon 66.22 Ahmadfazelia et al. (2021)  
 ABL@ZnCl2 190.63 Li, P et al. (2023)  ZIF-8 30 Haghighat et al. (2023)  
 ABL@H3PO4 184.70 Li, P et al. (2023)  Sugarcane bagasse biochar 23.61 Sun et al. (2018)  
 Cassava slag biochar 105.3 Wu et al. (2020)  Rice bran biochar 21.33 Asgharzadeh et al. (2019)  
 NaOH-treated rice husk 83.00 Khan & Shanableh (2022)  Spirulina platensis microalgae 20 Esmaili et al. (2023)  
 Sulfur-doped biochar 33.10 Vigneshwaran et al. (2021)  Cu-ZIF-8 16 Haghighat et al. (2023)  
 Fe–N biochar 12.14 Li, X et al. (2022)     
RhB
MET
Adsorbent (mgg−1)ReferenceAdsorbent (mgg−1)Reference
Hierarchically porous tobacco midrib-based biochar 658.2 Zhang et al. (2022)  AMGG (amine-modified green-graphene) 416.7 Bonyadi et al. (2021)  
 Activated cement waste biochar 531.84 da Silva et al. (2022)  Fe3O4-chitosan nano adsorbent 97.06 Asgari et al. (2020)  
 Olive biomass waste biochar 263.71 Albanio et al. (2021)  Mesoporous palm tree biochar 95.44 This study 
 Mesoporous palm tree biochar 224.30 This study Ammonia-modified activated carbon 66.22 Ahmadfazelia et al. (2021)  
 ABL@ZnCl2 190.63 Li, P et al. (2023)  ZIF-8 30 Haghighat et al. (2023)  
 ABL@H3PO4 184.70 Li, P et al. (2023)  Sugarcane bagasse biochar 23.61 Sun et al. (2018)  
 Cassava slag biochar 105.3 Wu et al. (2020)  Rice bran biochar 21.33 Asgharzadeh et al. (2019)  
 NaOH-treated rice husk 83.00 Khan & Shanableh (2022)  Spirulina platensis microalgae 20 Esmaili et al. (2023)  
 Sulfur-doped biochar 33.10 Vigneshwaran et al. (2021)  Cu-ZIF-8 16 Haghighat et al. (2023)  
 Fe–N biochar 12.14 Li, X et al. (2022)     

RhB, rhodamine B; MET, metronidazole; Fe–N biochar, iron–nitrogen biochar; ABL@ZnCl2, zinc chloride functionalized Atropa belladonna L. biochar and ABL@H3PO4–H3PO4 functionalized Atropa belladonna L. biochar. (ZIF-8) – C8H10N4 Zn metal–organic frameworks, (Cu-ZIF-8) – C8H10N4ZnCu metal–organic frameworks. Bold values represent results of this study.

Table 4

Adsorption kinetics of RhB and MET on mesoporous biochar

(mg g−1)Kinetic equationsParametersRhB1.693MET2.347
Pseudo-first-order (PFO)   (mg g−14.416 3.004 
  K1 (min−10.063 0.032 
  R2 0.753 0.680 
  χ2 1.679 0.143 
Pseudo-second-order (PSO)   (mg g−12.078 2.801 
  K2 (g mg−1 min−10.406 0.014 
  R2 0.919 0.967 
  χ2 0.071 0.073 
Elovich  β (mg g−1 min−12.156 1.954 
  α (g mg−10.156 0.383 
  R2 0.917 0.901 
Intra-particle diffusion   0.376 0.230 
 0.182 0.199 
 0.054 0.046 
  C1 (mg g−11.105 0.256 
  C2 (mg g−10.824 0.781 
  C3 (mg g−11.927 1.512 
   0.964 0.956 
   0.948 0.963 
   0.226 0.715 
(mg g−1)Kinetic equationsParametersRhB1.693MET2.347
Pseudo-first-order (PFO)   (mg g−14.416 3.004 
  K1 (min−10.063 0.032 
  R2 0.753 0.680 
  χ2 1.679 0.143 
Pseudo-second-order (PSO)   (mg g−12.078 2.801 
  K2 (g mg−1 min−10.406 0.014 
  R2 0.919 0.967 
  χ2 0.071 0.073 
Elovich  β (mg g−1 min−12.156 1.954 
  α (g mg−10.156 0.383 
  R2 0.917 0.901 
Intra-particle diffusion   0.376 0.230 
 0.182 0.199 
 0.054 0.046 
  C1 (mg g−11.105 0.256 
  C2 (mg g−10.824 0.781 
  C3 (mg g−11.927 1.512 
   0.964 0.956 
   0.948 0.963 
   0.226 0.715 

RhB, rhodamine B; MET, metronidazole. is the quantity of adsorbate adsorbed per unit time (mg g−1), , and are rate constants for the pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models, respectively. Elovich constant α represents the initial desorption rate while β is a desorption constant. R2 and χ2 are correlation coefficient and Chi-square, respectively.

Influence of temperature and adsorption thermodynamics

There was a temperature-dependent increase in RhB adsorption and a decrease in MET adsorption between 303 and 343 K (Figure 6). With an increase in temperature from 303 to 343 K, the RhB adsorption percentage increased from 47 to 87%, while the MET adsorption percentage decreased from 67 to 32%. For RhB, the adsorption trend with temperature indicates an endothermic adsorption process, whereas for MET, it suggests an exothermic process (Sun et al. 2018; Bello et al. 2019; Ren et al. 2020; Wu et al. 2020; Bonyadi et al. 2021; Ghibate et al. 2021; Bakry et al. 2022; Khan & Shanableh 2022; Li et al. 2022, 2023; Zhang et al. 2022; Azeez et al. 2023).
Figure 6

Effect of temperature on the adsorption of RhB and MET on mesoporous biochar.

Figure 6

Effect of temperature on the adsorption of RhB and MET on mesoporous biochar.

Close modal

A description of the energetics and spontaneity involved in adsorption as determined by thermodynamic parameters in the Van't Hoff's plot (Figure S8; supplementary document) presents the values obtained for ΔG°, ΔH°, and ΔS° (Table 5). Positive values of ΔH° and ΔS° for RhB removal imply an endothermic process with a greater degree of randomness whereas for MET adsorption, both ΔH° and ΔS° were negative indicating an exothermic adsorption process with a decreased degree of randomness (Khan & Shanableh 2022; Zhang et al. 2022). The values of ΔG° ranged from −1.553 to −7.726 kJ mol−1 for RhB and from −19.385 to −17.938 kJ mol−1 for MET, indicating both adsorption processes were spontaneous at all temperatures with adsorption of RhB becoming more feasible with an increase in temperature while it became less feasible for MET as temperature increased (Li et al. 2023). The values of ΔH° obtained are +45.206 and −30.343 kJmol−1 for RhB and MET, respectively, confirming the physical adsorption processes for the removal of MET (ΔH° < 40 kJmol−1) and chemisorption for RhB (ΔH° > 40 kJmol−1) (Sun et al. 2018; Wu et al. 2020).

Table 5

Thermodynamic parameters of RhB and MET on mesoporous biochar

RhB
MET
Temperature (K)Thermodynamic equationsΔG° (kJ mol−1)ΔH° (kJ mol−1)ΔS° (J K−1 mol−1)ΔG° (kJ mol−1)ΔH° (kJ mol−1)ΔS° (J K−1 mol−1)
303  −1.553   −19.385   
308  −2.325 45.206 154.32 −19.204 − 30.343 − 36.17 
313  −3.097 −19.023 
318  −3.868 −18.843 
323  −4.640 −18.662 
328  −5.411 −18.481 
333  −6.183   −18.300   
338  −6.955   −18.119   
343  −7.726   −17.938   
        
        
RhB
MET
Temperature (K)Thermodynamic equationsΔG° (kJ mol−1)ΔH° (kJ mol−1)ΔS° (J K−1 mol−1)ΔG° (kJ mol−1)ΔH° (kJ mol−1)ΔS° (J K−1 mol−1)
303  −1.553   −19.385   
308  −2.325 45.206 154.32 −19.204 − 30.343 − 36.17 
313  −3.097 −19.023 
318  −3.868 −18.843 
323  −4.640 −18.662 
328  −5.411 −18.481 
333  −6.183   −18.300   
338  −6.955   −18.119   
343  −7.726   −17.938   
        
        

RhB, rhodamine B; MET, metronidazole; R, gas constant (8.314 J mol−1K−1); T, temperature (K); , entropy change; , enthalpy change; = and , change in free energy.

Reusability/regeneration

After four desorption studies, the percentage desorption bars of RhB-loaded biochar in water and ethanol (Figure S9a; supplementary document) demonstrate regeneration/desorption rates as high as 96 and 80% in water and ethanol, respectively. This suggests that water is a more effective solvent to desorb RhB. Better regeneration and higher desorption obtained for water could be due to its universal nature. Additionally, over 96% of RhB was removed after the fourth cycle, indicating better results than many previously reported adsorbents. Likewise, four desorption studies revealed that biochar loaded with MET can be regenerated/desorbed to as high as 95 and 69% in water and ethanol, respectively (Figure S9b; supplementary document). As the number of cycles increased, the reusability decreased for MET, but it was better than some previously reported adsorbents.

Proposed mechanism of RhB and MET adsorption on mesoporous biochar

Mesoporous biochar's cationic surface was electrostatically attracted to COO of RhB during RhB adsorption, followed by hydrogen bonding between OH and CO of the biochar and COOH and N+ of RhB (Figure S10; supplementary document) (Haghighat et al. 2023). A similar mechanism has been proposed by Oladoye et al. (2022) for dye adsorption. Moreover, intramolecular and intra-particle diffusion played a role. The mechanism of MET adsorption was more prominent with electrostatic attraction and hydrogen bonding interaction between deprotonated MET and biochar (Figure S10) (Ghosh et al. 2023).

Palm tree biomass was successfully pyrolysed to produce biochar. The biochar produced contained functional groups O–H and C = O as revealed in the FTIR spectrum. The SEM micrograph also shows clustered images with EDXRF results showing the presence of carbon and other elements. Brunauer-Emmett-Teller suggests a type II isotherm with SBET of 189.157 m2 g−1 and pore diameter of 2.207 nm implying mesoporous nature. XRD spectrum shows six distinct peaks indicating high crystallinity that match JCPDS card number 00-712-6280. pH results show the mesoporous biochar has a cationic surface that interacted with COO of RhB and ππ ring of MET ring. Adsorption processes increased with an increase in biochar dosage and decreased with an increase in RhB/MET concentrations. Moreover, a decreased adsorption percentage was obtained for MET while an increase was obtained for RhB with an increase in temperature signifying exothermic and endothermic adsorption processes, respectively. Based on coefficients of determination and root mean square error, Langmuir and Freundlich isotherms accurately described RhB and MET. RhB had its maximum adsorption capacity (qmax) ranged from 31.81 to 224.30 mg/g increasing with temperature and 95.44 to 26.76 mg/g for MET decreasing with temperature from 303 to 313 K. Other adsorption isotherm parameters indicate a favourable physiosorption process with pseudo-second-order kinetics as the most appropriate for both adsorbates. The adsorption process was spontaneous for both adsorbates as shown by −ΔG° following the endothermic process (+ΔH°) for RhB removal and exothermic (−ΔH°) for MET.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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