Adsorption of Organic Pollutants From Wastewater Using Biochar: A Mechanistic Study on Competitive Adsorption Behavior

ABSTRACT

This study investigates the adsorption of methyl orange (MO), methylene blue (MB), and acetaminophen (ACT) using biochars produced from Douglas fir and Miscanthus at different temperatures and with different post‐pyrolysis treatments that added surface functional groups. Compounds were adsorbed separately and in mixtures to examine the competitive nature of the adsorption processes. MO is known to interact with MB and ACT, whereas MB and ACT are not likely to interact due to both having electron‐donating groups. When comparing the biochar adsorption capacities for these compounds when alone and mixed, biochars with both hydroxyl and carbonyl surface functional groups had higher adsorption capacities for the tested compounds when they were in mixed solutions. Biochars with only hydroxyl groups exhibited competing adsorption mechanisms and poorer adsorption capacities of aromatic compounds in complex solutions. This provides an understanding of how competing adsorption mechanisms of aromatic compounds by biochars vary depending on the dominant adsorption mechanisms of the biochar, which will allow for more effective real‐world applications for water purification in the future.

Summary

• Adsorption capacities for organic compounds may increase/decrease when adsorbed together.

• Oxygen functional groups provide adsorption sites for organic compounds.

• Competition for adsorption sites on biochar results in lower adsorption.

• Strong intermolecular interactions between adsorbates may result in higher adsorption capacities.

Methyl orange (MO), methylene blue (MB), and acetaminophen (ACT) were adsorbed separately and in mixtures by biochars with varying surface properties to examine the competitive nature of the adsorption processes. Biochars with both hydroxyl and carbonyl surface functional groups had higher adsorption capacities when in mixed solutions, while hydroxyl groups alone exhibited competing adsorption mechanisms and poorer adsorption. This provides an understanding of real‐world competing adsorption mechanisms that vary depending on the dominant adsorption mechanism.

1. Introduction

Rising environmental and health concerns due to water contamination from organic pollutants due to industrial and agricultural wastes are driving research into sustainable methods of water treatment (Luo et al. 2022; Qu et al. 2023). Organic dyes, antibiotics, petroleum hydrocarbons, and persistent organic matter, including various pharmaceuticals, are all examples of common contaminants found in wastewater for which adsorption has proven to be the most effective and sustainable method for water treatment (Qu, Meng, et al. 2023; Tang et al. 2022; Zhou et al. 2019; Qu, Liu, et al. 2023). Different adsorbents have been investigated in the past, including activated carbon, zeolite, silica gel, modified chitosan, and carbon; however, biochar has the advantage of both economic feasibility and sustainability (Li et al. 2019; Sinaei Nobandegani et al. 2022; Benvenuti et al. 2019; Z. Liu et al. 2010; Ai et al. 2019; Zhang et al. 2025; B. Liu et al. 2024). Biochar is a low‐cost carbonaceous by‐product of the pyrolysis of waste biomass, which has been utilized in a wide variety of applications including soil amendment, carbon sequestration, energy storage/conversion, advanced materials, and water filtration (Luo et al. 2022; Ai et al. 2019; Purakayastha et al. 2019; He et al. 2021; Gujre et al. 2021). Biochar is increasingly being considered for use in adsorption applications due to its unique physicochemical properties including high microporous volumes, high aromaticity, abundant surface oxygen‐containing functional groups, huge specific surface area (SSA), and low cost (Luo et al. 2022; G. Liu et al. 2019; Zhang et al. 2017; Ahmed and Hameed 2018; Varma 2019; Moreira et al. 2017). In particular, there has been a large amount of recent research into the adsorption of organic compounds by biochar (Qiu et al. 2022; Stylianou et al. 2021; Luo et al. 2022). However, many of the existing studies have only looked at the adsorption of individual organic species, whereas pollutants in the environment exist in the presence of many other chemical species (Peng et al. 2014; Jin et al. 2023). The adsorption mechanisms depend on the physical characteristics of the biochar, which are impacted by the biomass properties and pyrolysis conditions (e.g., temperature, heat‐up rate, gaseous environment, and residence time) (Qiu et al. 2022; L. Liu et al. 2020).

To investigate the effect that different adsorption mechanisms may have on competitive adsorption, five biochars were investigated in this work with varying properties, which result in different adsorption mechanisms including $\pi$–$\pi$ electron donor‐acceptor (EDA) interactions and electrostatic interactions. These interactions and mechanisms are influenced by the physical characteristics of the biochar including aromaticity and oxygen‐containing functional groups. Methyl orange (MO), methylene blue (MB), and acetaminophen (ACT) were used as test organic compounds, where three competitive adsorption tests of each possible pairing were done for each of the five biochars. These compounds were chosen because they are common pollutants found in water sources, aromatic, and active within the UV–visible spectrum (Gupta et al. 2021; Santoso et al. 2020; Waghchaure et al. 2022). Although MB and ACT have electron‐donating groups, MO has electron‐withdrawing groups and thus can form charge transfer interactions with one another. MB and MO, on the other hand, cannot undergo these interactions and thus do not interact. The biochars chosen for investigation were specifically selected such that they each would display different adsorption mechanisms and physical characteristics. Pyrolysis temperatures of 500°C and 800°C were chosen, as these temperatures were previously determined to have the most differing physical characteristics including surface area and functionality (Loebsack et al. 2025). Miscanthus‐derived biochar at lower pyrolysis temperature was chosen because of its lower aromaticity and high hydroxyl groups, giving the biochar strong electrostatic interactions. Douglas Fir biochar pyrolyzed at higher temperatures was also investigated, as well as the biochar after oxidative post‐treatment with H₂O₂ and KOH to investigate the effect of the increased oxygen content resulting from these treatment methods (Tan et al. 2019; W. Chen et al. 2020).

Limiting adsorption studies only to individual compounds provides an incomplete understanding of the biochar's potential for the removal of organic compounds in a multispecies/multicomponent system. The competitive adsorption under multispecies systems has received very little attention. Some studies have been done on the competitive adsorption of metals; however, the competitive adsorption of complex mixtures of organic compounds remains sparse (Lee and Shin 2021; Meng et al. 2022; Deng et al. 2020). Of the few studies, there were mainly antagonistic effects observed whereby compounds compete for adsorption sites. However, there is little investigation into the effects of intermolecular interactions between compounds and how these are affected by the adsorption mechanisms of the biochar (Martínez‐Costa et al. 2018; Reguyal and Sarmah 2018). The aim of this study is to investigate the sorption of three organic compounds in multicomponent solutions in order to observe under which conditions adsorption increases or decreases relative to when adsorbed individually (Tan et al. 2019; W. Chen et al. 2020).

2. Materials and Methods

2.1. Materials

Douglas fir (DF) biomass was supplied by a local furniture manufacturer in London, Ontario, Canada, and miscanthus (MC) biomass was collected in fields near London, Ontario, Canada. Before the pyrolysis, the biomass was pulverized using a blade grinder mill and sieved to the particle size range of 0.3–0.6 mm using standard test sieves (W.S. Tyler, United States). Compressed CO₂ gas used for pyrolysis was purchased from Praxair Canada Inc. Products 85% methylene blue (MB) (Product #7220793), 85% methyl orange (MO) (Product #1002790460), > 98% ibuprofen (IBU) (Product #102051532), and > 98% acetaminophen (ACT) (Product #1002893831) used as adsorbates were purchased from Sigma Aldrich, Canada.

2.2. Preparation of Biochars

DF and MC biomasses were pyrolyzed in a tube furnace by heating at 10°C/min until 500°C or 800°C and held for 3 h under a carbon dioxide atmosphere with a gas flow rate of 2 NL/min. Four DF biochars were produced, one at 500°C, which was then treated with KOH, and three at 800°C, one of which was treated with KOH, one with H₂O₂, and the other was not modified further. These post‐treatments were chosen based on common chemical oxidative post‐treatments reported in similar studies on biochar functionalities (Loebsack et al. 2025). Each adsorbent is referred to with the abbreviation representing the feedstock biomass followed by the pyrolysis temperature, the pyrolysis gas, and the post‐treatment, if applicable (i.e., DF800‐CO₂‐H₂O₂). MC was only pyrolyzed at 500°C to produce a biochar referred to as MC500‐CO₂.

2.3. Biochar Post‐Treatments

2.3.1. H₂O₂ Post‐Treatment

H₂O₂ post‐treatment was based on the method proposed by Huff and Lee (2016), where treatment of DF800‐CO₂ biochar was done by mixing 1 g of biochar with 50 mL of 9.8‐M H₂O₂ at room temperature for 1 h before filtering it through Whatman Filter Paper No. 1 and washing it with DI water. The oxidized biochars were then dried at 105°C overnight and referred to as DF800‐CO₂‐H₂O₂.

2.3.2. KOH Post‐Treatment

KOH post‐treatment was based on the procedure proposed by Lü et al. (2022), where 5 g of DF500‐CO₂ and DF800‐CO₂ biochars was each separately placed in 100 mL of 5‐M KOH and left to mix for 1 h at room temperature before being filtered through Whatman No. 1 filter paper, washed with DI water, and dried overnight at 105°C. The biochars were then activated in a tube furnace at 700°C with a ramp rate of 10°C/min for 1 h under 2 NL/min flow of N₂. After activation, the biochars were then washed again with DI water and dried overnight at 105°C. The post‐treated biochars were then referred to as DF500‐CO₂‐KOH and DF800‐CO₂‐KOH.

2.4. Biochar Characterization Methods

2.4.1. CHNS‐O Elemental Analysis

Proximate analysis was done for all the biomass samples and was performed following the American Society for Testing and Materials (ASTM) standard (ASTM D1762‐84) procedure for proximate analysis using a Netzsch Vulcan D‐550 muffle oven (Klasson 2017). The moisture content was determined by the mass difference after heating 1 g of biochar in a ceramic crucible to 105°C for 2 h, and the volatile matter was determined by the mass difference of heating the crucible with a lid to 950°C for 7 min. Finally, the ash content was found after heating the crucible without a lid at 750°C for 6 h, and the fixed carbon was found using the following equation: % fixed carbon = 100 − [% moisture + % volatile matter + % ash content] (Klasson 2017). This method was developed to provide information on the moisture content, volatile matter, fixed carbon, and the ash content of woody biomass by measuring the mass difference at increasing temperatures (Klasson 2017). Elemental analysis of carbon, hydrogen, nitrogen, and sulfur was carried out using the model Thermo Flash EA 1112 series CHNS analyzer (Thermo Fisher Scientific, United States). Samples were combusted at 900°C under helium with a known amount of oxygen. The oxygen content in the sample was calculated by mass difference.

2.4.2. FTIR Spectroscopy

The surface organic functional groups present in the samples were detected using Fourier Transform InfraRed (FT‐IR) spectroscopy using a platinum attenuated total reflectance (Pt‐ATR) attachment equipped with a diamond crystal in the main box of a Bruker Tensor II spectrometer. During the analysis, the sample in powder form was scanned within the range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹ under transmittance mode and 32 accumulations for each spectrum.

2.4.3. BET Surface Analysis

Samples were tested for Brunauer–Emmett–Teller (BET) surface area with a Nova 2000e Surface Area and Pore Size Analyzer (Quantachrome Instrument, Florida, United States). The physisorption measurements were performed via N₂ adsorption–desorption experiments at 77.35 K. Samples were degassed at 105°C for 1 h to remove moisture; then, the temperature was increased to 300°C and maintained for at least 3 h before each analysis.

2.5. Adsorption Tests

2.5.1. Adsorption Mechanisms Test

The adsorption mechanisms were investigated by placing 25 mg of each adsorbent biochar sample in a 20‐mL test tube containing 15 mL of a 3 mM H₃PO₄ buffer solution and 100 ppm of the test compound at the desired pH. Adsorption experiments were carried out at relative pH values of 2, 6, 8, and 11. The pH was controlled with a phosphate buffer, prepared with 3‐mM H₃PO₄, and the pH was adjusted by adding NaOH. Samples were agitated for 1 h at 25°C and filtered through Whatman No. 1 filter paper. The pH of the filtrate was determined after filtering using a pH meter (Thermo Scientific, Orion Star). The concentration of adsorbate remaining in the filtrate was determined at 220, 243, 464, and 668 nm for IBU, ACT, MO, and MB, respectively, using a UV–visible spectrophotometer (Thermo Scientific, Evolution 220). Samples were analyzed in triplicate, and their average absorbances were recorded. The adsorption capacity was defined as the amount of chemical adsorbed per unit mass of the tested adsorbent (mg/g).

2.5.2. Competitive Adsorption Test

The competitive adsorption mechanisms were investigated by placing 25 mg of adsorbent (biochar) in 20‐mL test tubes containing 15 mL of a 3‐mM H₃PO₄ solution containing 100 ppm of each of the two compounds without adjusting for pH so that the solution remains acidic as MO only has EWGs under these conditions. The performance under these conditions was compared to the adsorption of each compound individually at acidic pH. Adsorption tests were performed with the following mixtures for each biochar: MO and MB, MO and ACT, and ACT and MB. Samples were agitated for 1 h at 25°C and filtered through Whatman No. 1 filter paper. The pH of the filtrate was determined using a pH meter (Thermo Scientific, Orion Star). The concentration of adsorbate remaining in the filtrate was determined at 243, 464, and 668 nm for ACT, MO, and MB, respectively, using a UV‐visible spectrophotometer (Thermo Scientific, Evolution 220). Samples were analyzed in triplicates, and their average absorbances were recorded. The adsorption capacity was defined as the amount of chemical adsorbed per unit mass of the tested adsorbent (mg/g).

3. Results and Discussion

3.1. Adsorption Mechanisms of Biochars

Previous studies have found that the characteristics that have the biggest impact on the adsorption mechanisms of aromatic contaminants are the aromaticity and the oxygen functional groups (B. Chen et al. 2008; Xiao et al. 2016; Tan et al. 2021). The aromaticity is often characterized by the H/C ratio, calculated by the elemental analysis. The oxygen functional groups, which can be characterized by FTIR spectroscopy, commonly consist of carbonyl groups that have electron‐withdrawing effects or hydroxyl groups that have electron‐donating effects. Hydroxyl groups are also the main moieties that give biochar its highly negative surface charge, acting as negatively charged sites upon deprotonation, whereas carbonyl groups are not able to significantly contribute to electrostatic interactions (Zhang, Xiong, et al. 2024). This means that a greater number of hydroxyl groups will result in both greater electrostatic interactions and $\mathit{\pi }$–$\mathit{\pi }$ EDA interactions with compounds containing EWGs. Conversely, a greater number of carbonyl groups relative to hydroxyl groups could mean that an increase in $\mathit{\pi }$ – $\mathit{\pi }$ EDA interactions with compounds with EWGs will also result in a decrease in electrostatic interactions. Table 1 shows the surface areas of the biochars produced. Although surface area impacts the overall adsorption capacity, the adsorption mechanisms are more likely to be influenced by surface functionalities. Given these trends, five biochars were chosen to represent the various types of physical characteristics influential towards adsorption. MC500‐CO₂ was chosen because it is a biochar with low aromaticity and a relatively higher amount of oxygen groups giving it very strong electrostatic interactions. DF800‐CO₂ was chosen due to its high aromaticity but relatively lower oxygen content, whereas DF800‐CO₂‐KOH and DF500‐CO₂‐KOH were selected as they had similarly high aromaticities to DF800‐CO₂, with mainly hydroxyl and carbonyl groups, respectively. DF800‐CO₂‐H₂O₂ was also investigated due to its high aromaticity and strong electrostatic interactions based on the observed strong repulsion to MB with increasing pH. All these biochars, along with their adsorption mechanisms, are characterized in Table 1.

TABLE 1.

Biochars investigated, adsorption mechanisms, and physical characteristics.

Biochar	Adsorption mechanisms		Elemental analysis (%mass)						BET SSA (m2/g)	Average pore volume (cm3/g)	Average pore diameter (nm)
	Strong	Weak	C	H	N	O a	Ash	H/C
DF500‐CO2‐KOH	Electrostatic EDA with EDGs	$\pi$–$\pi$ stacking	80.3	1.23	0.01	15.6	2.89	1.5 × 10−2	395	0.22	2.18
DF800‐CO2	$\pi$–$\pi$ stacking	Electrostatic EDA with EWGs	57.2	1.00	0.69	7.07	34.0	1.7 × 10−2	461	0.29	0.248
DF800‐CO2‐H2O2	$\pi$‐$\pi$ stacking Electrostatic	EDA with EDGs	64.7	1.85	1.28	12.1	20.1	2.9 × 10−2	676	0.40	2.37
DF800‐CO2‐KOH	Electrostatic EDA with EWGs	$\pi$–$\pi$ stacking EDA with EDGs	65.0	0.01	0.05	21.5	13.4	1.5 × 10−4	606	0.36	2.35
MC500‐CO2	Electrostatic	$\pi$–$\pi$ stacking EDA with	63.1	1.12	0.64	29.4	8.84	2.4 × 10−2	64	0.055	3.42

^a Measured by difference (total mass‐C‐H‐N‐Ash=O).

DF800‐CO₂ had very high aromaticity, evidenced by the low H/C ratio (Table 1), which resulted in strong $\pi$–$\pi$ interactions with the adsorbents. The carbonyl groups that were present on the surface (Figure 1) resulted in strong adsorption of compounds with EDGs demonstrated by the higher adsorption of MO under basic conditions compared to acidic conditions, as shown in Figure 2a. This is due to the carbonyl groups acting as EWGs, which attract MO due to $\pi$–$\pi$ EDA interactions and the fact that there are few hydroxyl groups to act as negatively charged sites to repel it. These findings explain the low electrostatic interactions; in the case of strong electrostatic interactions, MO would have had lower adsorption when anionic in basic pH.

FIGURE 1.

FTIR spectra of biochars.

FIGURE 2.

Adsorption capacities of ibuprofen, acetaminophen, methyl orange, and methylene blue with Increasing pH for (a) DF500‐CO₂‐KOH, (b) DF800‐CO₂, (c) DF800‐CO₂‐H₂O₂, (d) DF800‐CO₂‐KOH, and (e) MC500‐CO₂.

Biochar produced at 500°C with KOH post‐treatment (DF500‐CO₂‐KOH) also has a similarily high aromaticity and low H/C ratio and therefore $\pi$–$\pi$ interactions. However, it was found to have a greater peak for O‐H stretches while also having a distinct peak for a carbonyl C=O stretch in the FTIR spectrum presented in Figure 1. This suggests a larger amount of hydroxyl groups on the surface along with the carbonyl groups, giving the biochar slightly stronger electrostatic interactions while maintaining dominantly strong $\pi$–$\pi$ EDA interactions with compounds with EDGs such as ACT. As shown in Figure 2b, ACT had the highest adsorption of all the compounds tested with DF500‐CO₂‐KOH. Unlike DF800‐CO₂, the presence of hydroxyl groups also increased electrostatic interactions, indicated by the decrease of IBU and increase of MB adsorption with increasing pH.

Biochar that was produced at 800°C and treated with KOH (DF800‐CO₂‐KOH) had high aromaticity and contained primarily hydroxyl groups with no carbonyl groups observed in the FTIR spectrum (Figure 1). This biochar showed the highest adsorption capacities for MO and MB at low pH compared to IBU and ACT in Figure 2c. This is due to electrostatic interactions along with $\pi$–$\pi$ EDA interactions with EWGs present in MO and MB.

After H₂O₂ treatment, the DF800‐CO₂ biochar became more aromatic with slightly more oxygen groups, however fewer oxygen groups than KOH treatment or MC biochar. This is evidenced by the fact that the relatively high H/C ratio in Table 1 for the DF800‐CO₂‐H₂O₂ and the second‐lowest oxygen content at 12.1%. According to the results on the functionality of the biochar in the FTIR spectrum in Figure 1, there is a strong peak correlating to the C‐O‐C stretch and another for the O‐H stretch indicating that the oxygen content that is present in the biochar is made of hydroxyl and ether groups, both of which are EDGs. Looking at the adsorption results in Figure 2c, this biochar had high adsorption of MO at low pH, which decreased significantly at higher pH and high adsorption of MB. This means there are strong $\pi$–$\pi$ EDA interactions with compounds with EWGs, such as MO, but, more importantly, there are stronger electrostatic interactions, which are responsible for the high adsorption of MB and the decreasing adsorption of MO.

Lastly, MC500‐CO₂ had one of the lowest aromaticity of the biochars, and the greatest oxygen content, comprised mainly of ether and hydroxyl groups given the C‐O‐C and O‐H stretches around 1000 and 3000 cm⁻¹ in the FTIR spectra in Figure 1. Thus, MC500‐CO₂ showed significantly greater electrostatic interactions than $\pi$–$\pi$ interactions, given that MB was the only compound that was significantly adsorbed, due to the electrostatic interaction between the negatively charged sites from the deprotonated hydroxyl groups on the biochar and the cationic moieties on the MB.

3.2. Competitive Adsorption of Methyl Orange and Methylene Blue

The different biochar samples were used to adsorb MO and MB individually and in a mixture. This was done to attempt to emulate a more real type of scenario where practical application of biochar for adsorption of pharmaceuticals will occur where there are mixtures of organic compounds interacting with one another as well as the adsorbents. The structures of MO and MB, schematically shown in Figure 3a, can help explain a certain kind of the behavior observed in these practical scenarios. MB can undergo a charge transfer interaction with MO given the electron‐donating amine group bonded to the aromatic ring of the MB and the electron‐withdrawing sulfate group on the MO (Waghchaure et al. 2022; Santoso et al. 2020; Cheng et al. 2022; M. Chen et al. 2023). MO is also able to electrostatically interact with MB through their ionic charges (Pan et al. 2021). The adsorption of MO and MB individually and in a mixture is shown in Figure 4 for all the biochars investigated. DF500‐CO2‐KOH and MC500‐CO2 biochars both showed an increase in the adsorption of MO and MB when in a mixture, indicating that the combination of these compounds results in a synergetic increase in adsorption. This could be due to a complex that is formed by the intermolecular interactions of MO and MB, which has a stronger affinity towards the biochar than the compounds do individually. Another explanation may be that the mechanisms through which MO and MB are adsorbed are different, so they do not compete for adsorption sites. Third, another possibility is that when MO is adsorbed via $\pi$–$\pi$ EDA interactions, MB will then be attracted to the MO on the biochar surface. In the case of MC500‐CO₂, MB would be adsorbed onto the surface first via electrostatic interactions and MO would then be attracted to the adsorbed MB because this biochar has shown stronger electrostatic interactions as previously stated. DF500‐CO2‐KOH has strong electrostatic interactions and strong EDA interactions with compounds with EDGs and EWGs, whereas MC500‐CO2 has very strong electrostatic interactions and weak $\pi$–$\pi$ interactions. MO was not adsorbed at all by MC500‐CO₂; therefore, the increase in adsorption of MB by adding MO is most likely due to the MB adsorbing first onto the surface of the biochar, thus providing sites for adsorption of MO. Once MO adsorbs to the surface, MB will be adsorbed further allowing for increased adsorption of both compounds. This would be less likely to happen when either compound is adsorbed alone, as MB is more likely to interact with MO due to its withdrawing groups than itself, given that aromatic compounds are less likely to stack if they both have donating or both withdrawing groups.

FIGURE 3.

Schematic representation of possible intermolecular Interactions between (a) MO and MB, (b) MO and ACT, and (c) ACT and MB.

FIGURE 4.

Adsorption capacities of biochar in a solution containing MO, MB, or mixtures of these compounds for biochars (a) DF500‐CO₂‐KOH, (b) MC500‐CO₂, (c) DF800‐CO₂, (d) DF800‐CO₂‐H₂O₂, and (e) DF800‐CO₂‐KOH.

For biochar produced at 800°C with no post‐treatment (DF800‐CO₂), the adsorption of MO increased, and the adsorption of MB remained the same when they were combined as can be seen in Figure 4c. Given the strong $\pi$–$\pi$ EDA interactions, especially with compounds with EDGs, and weak electrostatic interactions, this biochar has strong adsorption of MB through $\pi$–$\pi$ EDA interactions. A proposed explanation for the increase in MO when combined with MB is that the MO is likely to interact with MB, which is well absorbed by the biochar. Because MB did not increase when adsorbed with MO, like what was observed for DF500‐CO₂‐KOH and MC500‐CO₂, DF800‐CO₂ may adsorb the compounds via similar adsorption sites. If there is competition for adsorption sites, then MB will be adsorbed at a higher capacity, because DF800‐CO₂ has a higher affinity for this compound due to electrostatic interactions. Therefore, the adsorption capacity of MB would not change; however, the adsorption of MO will be increased by the presence of MB.

When biochar was treated with H₂O₂ (e.g., DF800‐CO₂‐H₂O₂), the resulting biochar had high aromaticity and contained hydroxyl functional groups only. The adsorption of MB in an MB/MO mixture was lower compared to MB only, whereas the adsorption of MO increased slightly, as shown in Figure 4d. Due to hydroxyl surface groups, this biochar has strong electrostatic interactions and EDA interactions with compounds with EWGs. Therefore, there may be competition between adsorption sites, where MO is preferentially adsorbed as the $\pi$–$\pi$ EDA interactions are stronger than electrostatic interactions. Because MB decreases in adsorption, however, this means that there is more competition for adsorption sites and fewer intermolecular interactions between compounds. The $\pi$–$\pi$ EDA interactions observed for DF800‐CO₂‐H₂O₂ can then be said to be stronger than those of DF800‐CO₂, as they are great enough that the compounds more favorably interact with the biochar than each other.

Biochar treated with KOH was used to adsorb MO and MB, and the results are shown in Figure 4e. The adsorption of MO and MB alone when combined followed different trends compared to those of DF500‐CO2‐KOH and MC500‐CO2. With KOH‐treated biochar, the adsorption of MO and MB decreased when adsorbed together. This suggests that the MO and MB are adsorbed by the same sites, thus competing for adsorption. MO and MB are both able to interact with aromatic groups that have hydroxyl substituents: MO via $\pi$–$\pi$ EDA interactions and MB through electrostatic interactions. This would lead to competitive adsorption, explaining the decrease in adsorption that was observed. The FTIR spectrum of DF800‐CO₂‐KOH in Figure 1 shows stretches that are characteristic of hydroxyl groups, further suggesting that $\pi$–$\pi$ EDA and electrostatic interactions both compete for adsorption at adsorption sites where these functional groups are present.

3.3. Competitive Adsorption of Methyl Orange and Acetaminophen

ACT has both an amide and a hydroxyl group, both of which are electron‐donating groups, making this compound likely to have stronger charge transfer interactions with MO, as seen in Figure 3b (Gupta et al. 2021). ACT is also neutral at low pH, meaning that the complex formed by MO and ACT would have a zero net charge, decreasing potential electrostatic interactions (Waghchaure et al. 2022; M. Chen et al. 2023; Gupta et al. 2021). The adsorption of MO and ACT individually and in binary mixtures was tested for the different biochars and presented in Figure 5. Looking at the adsorption capacity of MO by DF500‐CO2‐KOH, shown in Figure 5a, when in a solution of equal parts ACT and MO, there was a significant decrease in adsorption of both MO and ACT. This is the opposite result that was seen when MO was adsorbed with MB by this biochar, as shown in the previous section. This would imply that ACT and MO interact more with one another than the biochar, which would result in both being poorly adsorbed by the biochar. Given that the most significant difference between MB and ACT is the positive charge on MB, the complex formed from MO and MB is likely adsorbed by electrostatic interactions, which are not able to adsorb the MO and ACT complex as it has a net neutral charge. The decrease in adsorption also indicates that there may be competition for adsorption sites. Because DF500‐CO₂‐KOH was shown to have strong $\pi$–$\pi$ EDA interactions, which would adsorb MO and ACT via different sites, a possible explanation would be that MO and ACT are more likely to undergo intermolecular charge transfer interactions between themselves than the $\pi$–$\pi$ EDA interactions with the biochar. This interaction would be equally as likely for MO and MB; however, because MB can be adsorbed by electrostatic interactions unlike ACT, there was the opportunity for MO and MB to interact with both each other and the biochar thus increasing both their adsorption capacities.

FIGURE 5.

Adsorption capacities of biochar in a solution containing MO, ACT, or mixtures of these compounds for biochars (a) DF500‐CO₂‐KOH, (b) MC500‐CO₂, (c) DF800‐CO₂, (d) DF800‐CO₂‐H₂O₂, and (e) DF800‐CO₂‐KOH.

The adsorption capacity of MO increased relative to when adsorbed alone when combined with ACT in the case of DF800‐CO₂, whereas the adsorption capacity of ACT remained the same, as shown in Figure 5c. This is similar to the results for MO combined with MB for this biochar, except MO increases more when combined with ACT as opposed to with MB. This indicates that the MO adsorption capacity is higher when adsorbed with ACT, as it can interact with ACT, which has a stronger affinity towards absorption than the MO. Also, because this is the same trend as was seen for MB, this implies that the adsorption mechanisms through which this occurs are not the electrostatic interactions but $\pi$–$\pi$ EDA interactions, as this is the only interaction through which both ACT and MB can be absorbed. This is consistent as this was determined to be the strongest adsorption mechanism for this biochar.

For the DF800‐CO₂‐H₂O₂, when adsorbing both MO and ACT together, it resulted in relatively higher adsorption of MO, whereas the adsorption of ACT remained the same as when adsorbed alone. Figure 5d shows that the adsorption capacity of MO increased more when combined with ACT than MB, and ACT was not seen to decrease in adsorption when combined with MO as MB did. Because ACT does not have a positive charge, it instead has stronger electron‐donating groups, and therefore, the greater increase in adsorption of MO with ACT may be attributed to stronger $\pi$–$\pi$ EDA interactions towards ACT compared to MB. ACT and MO can also be said to adsorb via different adsorption sites, whereas MB adsorbs at the same sites as MO and therefore competes for adsorption. This is because ACT will be adsorbed at aromatic rings with electron‐withdrawing carbonyl groups, whereas MO and MB will be adsorbed at aromatic groups with hydroxyl groups, due to both their negative charge and electron‐donating character. This would result in competition between MO and MB but not MO and ACT.

The adsorption capacities for DF800‐CO₂‐KOH interestingly displayed the opposite trends with respect to those observed for DF800‐CO₂‐H₂O₂ and DF800‐CO₂ when adsorbing MO and ACT together. Looking at Figure 5e, the adsorption of MO decreased, whereas the adsorption of ACT remained the same as when adsorbed separately. When MO was adsorbed with MB, the adsorption capacity of both compounds decreased relative to when they were adsorbed separately, possibly due to competition for adsorption sites. When MO was adsorbed with ACT, on the other hand, only the adsorption of MO decreased, whereas ACT had no change in adsorption, suggesting that the ACT adsorption mechanisms were stronger than that of MO. There was less evidence for competition for adsorption sites than was seen for the DF800‐CO₂‐H₂O₂ and DF800‐CO₂ biochars, where there was an increase in the adsorption of MO. DF800‐CO₂‐H₂O₂ and DF800‐CO₂ have more carbonyl groups, according to FTIR spectra in Figure 1, which only contribute to $\pi$–$\pi$ EDA interactions with compounds with EDGs, whereas DF800‐CO₂‐H₂O₂ was found to predominantly have hydroxyl groups that contribute to both electrostatic and $\pi$–$\pi$ EDA interactions with compounds with EWGs. This would mean that the presence of carbonyl groups and hydroxyl groups allows for multiple adsorption mechanisms without competition, therefore increasing the adsorption capacities of the compounds because they can form intermolecular interactions with one another. However, the greater the number of hydroxyl groups relative to carbonyl groups, the more electrostatic and $\pi$–$\pi$ EDA interactions with EWGs will compete. This could result in the decreasing adsorption of one or both compounds, depending on which adsorption mechanism is stronger.

Lastly, MC500‐CO2 showed no change in the adsorption of MO or ACT when combined, as seen in Figure 5e. This is most likely due to the finding that this biochar did not show strong adsorption of either when adsorbed alone; consequently, the combination of the two did not affect adsorption.

3.4. Competitive Adsorption of Acetaminophen and Methylene Blue

The compounds MB and ACT both have electron‐donating groups, as seen in Figure 3c, and are less able to interact via a charge transfer interaction (Santoso et al. 2020; Gupta et al. 2021). There is however the possibility of a weaker cation–$\pi$ intermolecular interaction that can happen between ACT and the positively charged MB (Peiris et al. 2022). The adsorption of ACT and MB was investigated both separately and in binary mixtures for each of the five biochars tested, as shown in Figure 6. When looking at the change in adsorption capacities of ACT and MB when adsorbed together, DF500‐CO₂‐KOH, MC500‐CO₂, DF800‐CO₂, and DF800‐CO2‐KOH all showed no significant change in the adsorption of MB or ACT, except for a small decrease in adsorption for both compounds. The compounds may compete for adsorption sites on the biochar, making their adsorption capacities decrease slightly if not remaining the same.

FIGURE 6.

Adsorption capacities of biochar in a solution containing MB, ACT, or mixtures of these compounds for biochars (a) DF500‐CO₂‐KOH, (b) MC500‐CO₂, (c) DF800‐CO₂, (d) DF800‐CO₂‐H₂O₂, and (e) DF800‐CO₂‐KOH.

Figure 5d shows that the adsorption capacities of ACT by DF800‐CO₂‐H₂O₂ slightly increased when adsorbed with MB, which is well adsorbed given strong electrostatic interactions. This may be due to ACT and MB forming weak cation–$\pi$ interactions, which would cause ACT to be adsorbed at a higher capacity with MB (Peiris et al. 2022). This interaction is notably weaker because MB and ACT do not interact as strongly and was only found for DF800‐CO₂‐H₂O₂, which is the biochar with the greatest number of hydroxyl groups, reported in Figure 1, and strongest electrostatic interactions.

4. Conclusions

The adsorption of three aromatic compounds, MO, MB, and ACT, individually and in mixtures was investigated using specific biochars with different properties. Although this combination of compounds is not representative of all possible interactions that will occur in realistic applications, it provides a valuable insight into how some interactions have not yet been heavily researched. In general, it was found that the biochars that contained different types of oxygen functional groups on the surface, that is, both hydroxyl and carbonyl groups, had greater synergetic adsorption when adsorbing compounds in a mixture as they showed less competition for adsorption sites. Biochars with only hydroxyl groups exhibited competing adsorption mechanisms and poorer adsorption capacities of aromatic compounds in complex solutions.

MO is known to be able to interact with MB both through a $\pi$–$\pi$ charge transfer and ionic charge interactions, and they are the most likely pairing to increase adsorption when adsorbed together. Because of this, it is more likely that MO and MB can interact both with each other and the biochar. So long as the compounds do not compete for adsorption sites, the adsorption of both compounds should be greater when adsorbed together, as was seen for DF500‐CO₂‐KOH. In the case where there is competition for adsorption sites, there is lower adsorption of the compound with weaker attraction to the biochar, as was seen for DF800‐CO₂ and DF800‐CO₂‐H₂O₂. However, if only one of the compounds is strongly adsorbed by the biochar and the other is not, it was found that the adsorption of the other compound increased relative to when adsorbed alone, as shown by the increased adsorption of MO for DF800‐CO₂ and MC500‐CO₂.

Both MO and ACT are also more likely to be adsorbed by $\pi$–$\pi$ EDA interactions and not likely to compete for adsorption sites but are likely to interact with one another. DF800‐CO₂ and DF800‐CO₂‐H₂O₂ displayed increased adsorption of these compounds when adsorbed together, suggesting that the $\pi$–$\pi$ EDA interactions between the compounds and the biochar must be stronger than the intermolecular interactions between MO and ACT.

Lastly, ACT and MB both have EDGs and are not likely to interact with one another as well as being likely to compete for adsorption sites. MB is also able to be adsorbed by electrostatic interactions; however, this does not increase ACT when adsorbed together because there is not a strong enough electrostatic interaction for most biochars. DF800‐CO₂‐H₂O₂ and MC500‐CO₂ were seen to be exceptions to this, where ACT increased when adsorbed with MB. This may be because this biochar allowed for weak cation–$\pi$ intermolecular interactions between the compounds. The findings that the adsorption mechanisms of a biochar are dependent on both the physical properties of the biochar and the physical properties of the compounds can be used to devise methods for selective adsorption of compounds from mixtures. This is a suggested avenue for future studies.

Author Contributions

Griffin Loebsack: conceptualization, investigation, methodology, validation, writing – original draft. Ken K.‐C. Yeung: conceptualization, supervision, funding acquisition, writing – review and editing. Franco Berruti: conceptualization, supervision, funding acquisition, writing – review and editing. Naomi B. Klinghoffer: conceptualization, supervision, funding acquisition, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Loebsack, G. , Yeung K., Berruti F., and Klinghoffer N.. 2025. “Adsorption of Organic Pollutants From Wastewater Using Biochar: A Mechanistic Study on Competitive Adsorption Behavior.” Water Environment Research 97, no. 8: e70164. 10.1002/wer.70164.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.