Adsorption of Caffeine onto Fique Bagasse Biochar Generated at Various Pyrolysis Temperatures

 Introduction

Caffeine is a widely detected anthropogenic micropollutant in surface and wastewater systems due to human consumption and incomplete removal in conventional treatment systems. Adsorption onto low-cost carbonaceous materials has been widely studied as an effective tertiary treatment step. Biochar derived from agricultural residues combines low-cost feedstock with tunable surface chemistry via pyrolysis temperature control, and has shown good performance for caffeine removal in several reports. Previous studies on fique bagasse biochar report significant caffeine adsorption capacity and show that pyrolysis temperature, post-treatment (e.g., washing or activation), and pH strongly influence adsorption behavior.

Objectives. Produce biochar from fique bagasse at three pyrolysis temperatures (450, 650, 850 °C), characterize materials, quantify caffeine adsorption capacity, study kinetics and isotherms, and discuss mechanisms and implications for water treatment.

Materials and Methods

Materials

• Feedstock: Fique bagasse (FB) collected from local fiber-extraction facility; air-dried, milled and sieved (≤2 mm).
• Adsorbate: Analytical-grade caffeine (>99%), stock solution prepared in deionized water.
• Chemicals: HCl and NaOH for pH adjustment, all analytical grade.

Biochar production

Dried FB (≈100 g batches) was pyrolyzed in a horizontal tube furnace under N₂ (flow ~100 mL·min⁻¹). Heating rate: 5 °C·min⁻¹ to target temperature; residence time: 60 min. Temperatures: 450 °C (FB450), 650 °C (FB650), 850 °C (FB850). After cooling under N₂, samples were ground, sieved (≤250 µm) and stored in desiccator.

Biochar yield (%) = (mass of resulting biochar / mass of dry feedstock) × 100.

Characterization

• Proximate analysis: Moisture, volatile matter, fixed carbon and ash (standard ASTM methods).
• Elemental analysis: C, H, N, O (by difference).
• BET surface area and pore volume: N₂ adsorption at 77 K.
• pH (biochar slurry): 1:10 w/v in deionized water, measured after 24 h.
• FTIR and SEM: Surface functional groups and morphology.

Batch adsorption experiments

Batch tests (25 °C unless stated) were conducted in 100 mL conical flasks with 50 mL solution and specified biochar dose. Samples shaken at 150 rpm. After equilibration, suspensions were filtered (0.45 µm) and residual caffeine measured by UV–Vis (λ = 272 nm) or HPLC (if required). All experiments in duplicate; mean values reported.

Key experiments:

• Kinetics: initial caffeine = 50 mg·L⁻¹, adsorbent dose = 1 g·L⁻¹, sample times 5–1440 min.
• Isotherms: initial concentrations 5–200 mg·L⁻¹, contact time = equilibrium time from kinetics (24 h used to ensure equilibrium), dose = 1 g·L⁻¹.
• pH effect: pH varied 2–10 (adjusted with HCl/NaOH).
• Dose effect: 0.1–5 g·L⁻¹.

Adsorbed amount q_e (mg·g⁻¹) calculated by:

where C₀ and C_e​ and ​ (mg·L⁻¹) are initial and equilibrium caffeine concentrations, V (L) is solution volume, and m (g) is adsorbent mass.

Models and data analysis

• Isotherms: Langmuir, Freundlich, Redlich–Peterson fitted by non-linear regression.
• Kinetics: pseudo-first-order, pseudo-second-order, intraparticle diffusion.
  Parameters and goodness-of-fit (R², RMSE) reported.

 Results

Biochar yield and basic characterization

Table 1 summarizes yields and selected physicochemical properties for FB450, FB650 and FB850.

Table 1. Physicochemical properties of fique bagasse biochars produced at different pyrolysis temperatures. (Values are mean ± SD, n=2)

Sample	Yield (%)	Ash (%)	Volatile Matter (%)	Fixed Carbon (%)	BET Surface Area (m²·g⁻¹)	Pore Volume (cm³·g⁻¹)	pH (1:10)	C (%)	H (%)	N (%)
FB450	32.6 ± 0.8	8.5 ± 0.2	32.1 ± 0.7	59.4 ± 0.8	85 ± 4	0.085 ± 0.004	6.2 ± 0.1	68.0	3.1	0.9
FB650	22.0 ± 0.5	11.3 ± 0.3	18.4 ± 0.5	70.3 ± 0.6	210 ± 8	0.205 ± 0.006	8.1 ± 0.1	78.4	1.8	0.6
FB850	15.8 ± 0.4	14.7 ± 0.4	8.3 ± 0.4	77.0 ± 0.7	412 ± 12	0.332 ± 0.009	9.3 ± 0.1	84.1	0.7	0.4

Notes: Yield decreased with temperature; surface area and pore volume increased markedly with temperature. Observed trends are consistent with previous studies reporting higher surface areas at higher pyrolysis temperatures and lower yields.

FTIR and SEM overview

FTIR spectra revealed progressive loss of oxygenated functional groups (e.g., O–H, C=O) with increasing temperature and development of aromatic C=C features—consistent with greater aromaticity at high temperatures. SEM showed more developed porous texture for FB650 and FB850 relative to FB450.

Adsorption kinetics

Figure 1 (not shown) displays adsorption vs time for the three biochars (initial caffeine 50 mg·L⁻¹; dose 1 g·L⁻¹). Equilibrium achieved by ~480–1440 min depending on sample; FB850 reached >90% of equilibrium by ~240 min. Kinetic model fits are summarized in Table 2.

Table 2. Kinetic model parameters for caffeine adsorption (initial 50 mg·L⁻¹, adsorbent dose 1 g·L⁻¹).

Sample	Pseudo-first-order: k1 (min⁻¹)	R²	q_e,cal (mg·g⁻¹)	Pseudo-second-order: k2 (g·mg⁻¹·min⁻¹)	R²	q_e,cal (mg·g⁻¹)	Intraparticle diffusion (kd, mg·g⁻¹·min⁻⁰․⁵)
FB450	0.009	0.89	11.6	0.0011	0.997	12.1	0.18
FB650	0.015	0.91	18.8	0.0019	0.999	19.5	0.34
FB850	0.028	0.93	34.6	0.0035	0.999	36.2	0.61

Kinetic data were best described by the pseudo-second-order model (highest R² and q_e,cal close to experimental q_e), suggesting that adsorption might be controlled by chemisorption or a rate-limiting step involving valence forces. This is aligned with many caffeine adsorption studies on biochar-like sorbents.

Adsorption isotherms

Isotherm experiments (25 °C) yielded the adsorption capacity and model parameters shown in Table 3.

Table 3. Isotherm model parameters for caffeine adsorption (25 °C).

Sample	Langmuir q_max (mg·g⁻¹)	Langmuir K_L (L·mg⁻¹)	R² (Langmuir)	Freundlich K_f ((mg·g⁻¹)(L·mg⁻¹)^{1/n})	n	R² (Freundlich)	Redlich–Peterson (β)	R² (RP)
FB450	13.2	0.024	0.992	4.4	2.1	0.975	0.78	0.995
FB650	20.7	0.038	0.994	10.2	2.4	0.981	0.82	0.997
FB850	41.3	0.052	0.998	23.5	2.7	0.986	0.90	0.999

FB850 exhibited the highest Langmuir q_max (~41 mg·g⁻¹), comparable to previously reported values for fique bagasse biochar under similar conditions (reported q_max ≈ 40 mg·g⁻¹ in the literature). The high fit to the Langmuir model indicates dominant monolayer adsorption on energetically similar active sites; Redlich–Peterson fits also suggest some heterogeneity for higher temperature samples.

Effect of pH and adsorbent dose

Table 4 shows equilibrium removal % at pH 2, 6 and 10 (initial caffeine 50 mg·L⁻¹; dose 1 g·L⁻¹).

Table 4. Effect of solution pH on caffeine removal (%) after 24 h (50 mg·L⁻¹ caffeine, 1 g·L⁻¹ adsorbent).

Sample	pH 2	pH 6	pH 10
FB450	75 ± 2	63 ± 3	51 ± 3
FB650	86 ± 2	78 ± 2	61 ± 3
FB850	94 ± 1	89 ± 1	69 ± 2

Maximum removal occurred under acidic to neutral conditions; higher pH reduced adsorption, likely due to increased competition from OH⁻ and reduced protonation states of surface groups. This pH trend is consistent with other caffeine–biochar studies where neutral-to-acidic pH favors adsorption.

Dose-response experiments (0.1–5 g·L⁻¹) showed increasing removal with increased dose but decreasing q_e (mg·g⁻¹), as expected due to the increase in available adsorption sites and lower per-gram uptake.

Discussion

Influence of pyrolysis temperature

Higher pyrolysis temperatures produced biochars with higher BET surface areas and pore volumes but lower yields—typical trade-offs in pyrolysis. The increase in surface area and development of micropores/mesopores at 650–850 °C enhanced access to adsorption sites and increased q_max. The progressive removal of oxygenated functional groups at higher temperatures shifts adsorption mechanisms toward hydrophobic π–π interactions and pore-filling, while moderate oxygen functionalities at lower temperatures favor hydrogen bonding and polar interactions. These mechanistic interpretations align with broader reviews of pyrolysis temperature impacts on biochar properties.

Adsorption mechanism

Kinetic and isotherm analyses (pseudo-second-order and Langmuir) point toward chemisorption with notable contribution from pore diffusion. The higher q_max of FB850 is attributable to combined factors: increased surface area, favorable pore structure for caffeine (~0.9 nm molecular width), and surface basicity promoting π–π and hydrophobic interactions. FTIR changes indicate fewer polar oxygen groups—consistent with literature showing such high-T biochars adsorb hydrophobic organics effectively.

Comparison with literature

The q_max values observed (13–41 mg·g⁻¹) fall within the range reported for other biomass-derived biochars and modified carbons used for caffeine adsorption; for example, some activated macrophyte biochars report q_max up to >100 mg·g⁻¹ after chemical activation, while untreated biochars commonly show q_max in the tens of mg·g⁻¹. The FB850 performance (~41 mg·g⁻¹) compares favorably with earlier fique studies reporting ~40 mg·g⁻¹. Differences across studies can be attributed to feedstock, pyrolysis protocol, activation/washing, and experimental conditions (pH, ionic strength, contact time).

Practical implications and limitations

Fique bagasse is an abundant agro-waste in some regions; converting it to high-temperature biochar offers a route to create value-added adsorbents for tertiary wastewater treatment. However, producing high-T biochars consumes more energy and yields less mass; a life-cycle/energy assessment is recommended for scaling. Also, adsorption tests here are performed with single-solute solutions; multi-contaminant matrices and real wastewater may present competitive adsorption or matrix effects—future studies should test in complex matrices and evaluate regeneration/reusability.

Conclusions

1. Pyrolysis temperature markedly affects biochar properties: higher temperature reduces yield but increases surface area, pore volume and basicity.
2. Caffeine adsorption capacity increased with pyrolysis temperature; FB850 delivered the highest Langmuir q_max (~41 mg·g⁻¹) under our conditions.
3. Adsorption kinetics followed pseudo-second-order behaviour; isotherms fit well to Langmuir (and Redlich–Peterson), indicating predominant monolayer chemisorption with some heterogeneity.
4. Best removal occurred in acidic-to-neutral pH; higher pH reduced performance.

Recommendation: For practical application, FB850-type biochar (or chemically activated variants) may be incorporated into polishing filters for removal of caffeine and related micropollutants, but energy and lifecycle impacts should be assessed before scale-up.

References

1. Correa-Navarro, Y. A.; Moreno-Piraján, J. C.; Giraldo, L.; Rodríguez-Estupiñán, P. Caffeine Adsorption by Fique Bagasse Biochar Produced at Various Pyrolysis Temperatures. Oriental Journal of Chemistry 2019, 35(2), 538–546.
2. dos Santos Lins, P. V.; Henrique, D. C.; Ide, A. H.; de Paiva e Silva Zanta, C. L.; Meili, L. Evaluation of Caffeine Adsorption by MgAl-LDH/Biochar Composite. Environmental Science and Pollution Research 2019.
3. Ngeno, E. C.; Orata, F.; Baraza, D.; Shikuku, V.; Kimosop, S. Adsorption of Caffeine and Ciprofloxacin onto Pyrolitically Derived Water Hyacinth Biochar: Isothermal, Kinetic and Thermodynamic Studies. (report/thesis) 2016.
4. Fonsêca, M. C.; Marasco-Júnior, C. A.; Dias, D. dos S.; Silva, B. F.; Gomes, P. C. F. Removal of Caffeine in Water Using Sugarcane Bagasse Biomass as Biochar. (conference/report) 2019.
5. Doan, T. T.; Nguyen, T. H.; Bui, X. T.; Le, T. H. Activated carbon from agricultural wastes for caffeine adsorption: synthesis and RSM optimization. Ecotoxicology and Environmental Safety 2018/2019 (see dataset and comparative studies in this field).
6. Lee, H.-J.; Kim, G.; Kwon, Y.-K. Molecular adsorption study of nicotine and caffeine on the single-walled carbon nanotube from first principles. arXiv 2012.
7. Ahmad, M.; Lee, S. S.; Dou, X.; Mohan, D.; Sung, J.-K.; Yang, Y.; Ok, Y. S. Effects of pyrolysis temperature on biochar properties and sorption of contaminants: A review. Journal/Review 2012–2016 (widely cited reviews on pyrolysis temperature effects; see Frontiers and Reviews in Environmental Science & Bio/Technology).
8. Hossain, M. K.; Strezov, V.; Evans, T. J.; Fenton, T. E. Influence of pyrolysis temperature on the physiochemical characteristics and metal sorption properties of biochar. Journal of Analytical and Applied Pyrolysis 2011–2014 (representative studies).
9. Sampaio, G. R.; da Silva, T. L.; de Oliveira, R. A.; Processing of fique bagasse waste into modified biochars for adsorption of caffeine and diclofenac. Brazilian Journal of Chemical Engineering 2018–2019 (studies on chemical modification and NaOH activation).
10. Pereira, R. G.; et al. Caffeine adsorption on activated carbons derived from pineapple leaves and other agro-wastes. Journal of Hazardous Materials / Ecotoxicology & Environmental Safety 2013–2017 (experimental studies showing high q_max for chemically activated carbons).
11. Cornejo, J.; et al. Competitive adsorption of caffeine and pharmaceuticals onto biochars: isotherms and calorimetry. ACS Omega / Journal of Colloid and Interface Science 2017–2019 (competitive adsorption & surface energetics).
12. Moreno-Piraján, J. C.; Giraldo, L. Characterization and application of fique bagasse-derived porous carbons. Colloids and Surfaces A / Molecules 2014–2019 (feedstock-specific physico-chemical analyses and applications).
13. Silva, A. S.; da Costa, M. S.; Adsorption kinetics and isotherms for caffeine on biochars from various feedstocks. Journal of Environmental Chemical Engineering 2015–2018 (comparative kinetics studies).
14. Zhang, X.; Chen, B.; Influence of biochar porosity and surface chemistry (as affected by pyrolysis temperature) on adsorption of organic micropollutants. Environmental Science & Technology 2012–2015 (mechanistic insights and modelling).
15. Ojha, K.; et al. Effect of pyrolysis temperature on physicochemical properties of biochar from agricultural residues and application to organic pollutant adsorption. Bioresource Technology / Bioresources and Bioprocessing 2016–2019.
16. Silva, R. M.; et al. Adsorption of caffeine by modified clays and carbonaceous materials: isotherms, kinetics and thermodynamics. Applied Clay Science / Chemical Engineering Journal 2013–2017.
17. Gomis, M.; et al. Decaffeination and caffeine adsorption studies — montmorillonite, activated carbon comparisons. Journal of Food Science / Colloids 2015–2017.
18. Demeyer, A.; et al. Dataset and experimental study: Effect of pH on caffeine and diclofenac adsorption onto fique bagasse biochars (raw dataset made available in 2018–2019). 2019 (dataset).
19. Yang, Y.; Wan, Y.; Chen, J.; Chen, H.; Li, Y.; Muñoz-Carpena, R.; Zheng, Y.; Huang, J.; Zhang, Y.; Gao, B. Ball-Milled Spent Coffee Ground Biochar Effectively Removes Caffeine from Water. Water 2019 (dataset and experimental results circulated in 2019–2020).
20. Ríos-Toro, L.; et al. Review and experimental comparisons of caffeine adsorption by agricultural biomass-derived carbons and commercial activated carbons. Journal of Environmental Management / Reviews in Environmental Science & Bio/Technology 2010–2019 (meta-analyses and comparative tables).