1 Introduction

In recent decades, water contamination caused by dyes used in various industries such as textiles, pharmaceuticals, leather, cement, etc. is a principal concern because of their toxic effects on the environment and human health [1, 2]. The textile industry is the major source of waste, and it dumps around 92 million tons of waste, with dyes being the principal water pollutant [3]. Due to the complex chemical structure of dyes, it is difficult for them to be broken down in a natural environment. Hence, they accumulate and bring about detrimental impacts on the water environments, including an increase in the biological and chemical oxygen demand, pH, temperature, salinity, total dissolved solids (TDS), and other associated parameters [2, 4, 5]. It also affects the process of photosynthesis by reducing the amount of light that can penetrate water bodies, which has a negative impact on the aesthetic quality of the aquatic body [2, 4, 6]. Methylene blue (MB) is widely used in different industries [7]. In addition, MB is most readily released into water streams, as it does not fully adhere to the fibers [7]. The concentration of MB in the effluents of textile and printing industries may vary from 10 mg/L to 1000 mg/L. The permissible MB content, as the Environmental Protection Agency recommends, is 0.2 mg/L [7]. The MB dye has the potential to bring about a variety of adverse health effects in human beings, such as cyanosis, tissue necrosis, the formation of Heinz bodies, vomiting, jaundice, and a raised heart rate [8]. Thus, identifying feasible, simplified, more efficient, and cost-effective methods for tackling MB contamination in water sources is essential.

Currently the wastewater treatment from textiles are mainly based on hybrid methods of both physical and chemical treatments such as adsorption, coagulation, oxidation by both chemical and electrochemical methods, ozonation, ion exchange, membrane process, photocatalysis, sonication, and irradiation [9, 10]. Some of these methods such as coagulation, photocatalysis etc. are costly and may generate harmful byproducts [9]. But complete dye removal can be achieved with a number of advanced oxidation technologies such as ozonisation, Fenton reactions, electrochemical methods and photocatalysis. However, these processes are generally complicated, and the overall cost increases due to the need to remove the micropollutants [11]. Another alternative method for degrading dye is biodegradation, which leads to the mineralization of the dye pollutant. However, as methylene blue is a bio-resistant pollutant, traditional remediation methods such as ion exchange, membrane filtration, electrocoagulation, activated sludge method, biodegradation etc., are often not applicable [12]. On the other hand, adsorption technology is an effective, flexible and inexpensive method for the treatment of dye-contaminated wastewater [1, 13].

The different agricultural wastes can be converted into various carbonaceous materials, such as biochar, which is cost-effective, eco-friendly, and has a high surface area and porosity. [1, 14]. However, pyrolyzed biochar is often less efficient in removing dyes and other contaminants present in wastewater, limiting its potential uses [1]. So, to enhance the adsorption capacity of the biochar, various chemical and physical activation methods are being adopted, which involve the impregnation of biochar with known activating agents, such as KCNS, H2O2, KMnO4, K2CO3, ZnCl2, NaOH, KOH, H2SO4, and HNO3 [12]. Mahmoud et al. prepared kenaf fibre biochar with better pores by treating it with HCl after heating it at 1000 °C, which is used to remove methylene blue from wastewater [15]. Similarly, Xu et al., synthesized citric acid-modified water hyacinth biochar to maximize the adsorption capacity of MB to 395 mg/g [1]. Liu et al., prepared KOH-modified Walnut shell biochar to increase the adsorption capacity of the biochar to 701.3 mg/g of MB [16]. Mu et al., synthesized NaOH-modified mesoporous biochar from tea residue to improve MB adsorption capacity with maximum adsorption of 105.44 mg/g [17]. Manisha et al., prepared NaOH-activated Opuntia ficus-indica biochar to increase the oxygen-containing functional groups for the adsorption of different water-containing pollutants [18]. Puspa et al., activated rice husk biochar with NaOH to increase the adsorption capacity of the biochar for removing dye from water environments [19].

To achieve a twofold benefit, in this work, agricultural waste biomass rice straw, which causes severe air pollution due to stubble burning, has been used as a precursor to produce non-activated and sodium hydroxide base-activated biochar materials. Thereafter, the biochar adsorbent materials have been characterised by using XRD, FESEM, FTIR, Raman, XPS, CHNS, TGA, and nitrogen adsorption techniques. Batch adsorption experiment studies have been performed by varying adsorption parameters such as dose, pH, initial dye concentration, etc. Furthermore, the mechanism behind the adsorption has been studied with adsorption isotherms, kinetics, thermodynamics, and activation energy studies.

2 Materials and methods

2.1 Chemicals

The chemical reagent used in this study, sodium hydroxide (NaOH, LR grade), was obtained from Central Drug House (P) Ltd, and hydrochloric acid (HCl-37%, AR grade) and sodium chloride (NaCl, LR grade) were obtained from Merck. The dye Methylene blue trihydrate (C.I. 52,015) was purchased from Sisco Research Laboratories (SRL), and details are presented in Table S1. The entire experiment was carried out using pure (Type 2) water.

2.2 Preparation of base-modified biochar

The adsorbent was prepared using Dehradun rice straw as the initial raw material and collected from the NGO Live and Let Live in Jagatsinghpur, Odisha, India. The collected rice straw was washed extensively with purified water and dried in an oven at a temperature of 100 °C for 5 h. Then, the rice straw biomass (DR) was carefully cut, and 80 g of it was pyrolyzed in an anaerobic condition in a tubular furnace with a temperature of 600 °C for 1 h and a heating rate of 10 °C per minute. The obtained biochar was then passed through using a 38-micron sieve to ensure uniform non-activated biochar (DBCNa0). The yield of biochar was ~ 38%. The pyrolyzed non-activated biochar was activated using 5 M and 10 M NaOH with a ratio of 1:6 biochar to base volume (w/v). The mixture was then agitated in a magnetic stirrer with 500 RPM for a period of 18 h at room temperature. Thereafter, the base-treated biochar was subjected to sonication for a duration of 30 min. Subsequently, a membrane paper with a porosity of 0.45 μm pore size was filtered and washed with type 2 millipore water to acquire neutralized base-activated biochar. The sample was then dried in a hot air oven at a temperature of 120 °C for 5 h and stored in a desiccator for later use. The 5 M and the 10 M NaOH-activated biochar were denoted as DBCNa5 and DBCNa10, respectively. The process for synthesising the non-activated and base-activated biochar has been presented in Scheme 1.

Scheme 1
scheme 1

Schematic presentation of the synthesis of non-activated and base-activated biochar from the rice straw biomass

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3 Characterization/instrumentation

The field emission scanning electron microscope-FESEM (Zeiss Gemini SEM 450) with EDX detector, the X-ray diffraction spectrometer (Bruker D8 Advance), the Raman Spectrophotometer (RENISHAW), the FTIR (Thermo Scientific NICOLET iS5), the X-ray photoelectron spectroscopy (XPS-PHI VersaProbe III), the surface area analyzer (BELSORB Max II), thermogravimetric analyzer (HITACHI-TG/DTA-STA7200), CHNS analyzer (PerkinElmer 2400 Series II System) and UV–Visible spectrophotometer (Agilent Cary 3500) were used to analyze the base-activated and non-activated biochar adsorbent.

FESEM was used to determine the surface morphology of base-activated and non-activated rice straw biochar. EDAX was utilized to comprehend the elemental makeup of several types of rice straw. FTIR was employed to distinguish between the number of functional groups in the biochar. The sp2 (graphitic) and sp3 (amorphous) hybridization structures of the biochar were identified using Raman spectroscopy. The homogenous or heterogeneous surface of the biochar, as well as its crystalline structure, were determined by X-ray diffractometer (XRD). The surface containing the functional groups of the biochar materials was located using XPS. The surface area and pore structure analysis were measured using a surface area analyzer. The material's thermal stability was assessed using a thermogravimetric analyzer. Additionally, the adsorption capacity of biochar toward the cationic dye methylene blue was measured using the batch adsorption method. The concentration of MB was calculated using the UV–visible spectrophotometer both before and after adsorption.

4 Batch adsorption experiment

The adsorption experiment of MB was carried out using both base-activated and non-activated biochar as the adsorbent in a batch study. The experiments were conducted in a 100 ml conical flask using a shaker incubator at a constant temperature. After the equilibrium time was reached, the dye solution was taken out from the conical flask containing the reaction mixture and then centrifuged at a speed of 10,000 × g. The supernatant obtained after centrifugation was analyzed using a UV–visible spectrophotometer at a wavelength of (λmax) 664 nm. The following equation is used to determine the percentage of dye removal (R) and the adsorption capacity (qt) at different time points

$$\text{Percentage removal }(\text{\%}) (\text{R}) = \frac{{C}_{o}-{C}_{t}}{{C}_{o}}\times 100$$
(1)
$$\text{Adsorption capacity }(\text{qt}) =\frac{{(C}_{o}-{C}_{t})\times V}{M}$$
(2)

where Co = initial dye concentration in mg/L, Ct = dye concentration in different time intervals in mg/L. t = time in min, M = mass of adsorbent in g, V = volume of solution in liter (L), and qt = adsorption capacity in mg/g.

Also, control tests were conducted during the adsorption experiments, and the standard curve for varying pH levels of the dye solution was performed (Fig. S1).

4.1 Determination of pHPZC, effect of pH and MB dye concentration

The point zero charge (pHPZC) of the adsorbent was determined using pH drift method [20,21,22,23,24]. This method adjusted the pH value of 0.01 M NaCl solution within a pH range of 2–12 using 0.1 M NaOH and 0.1 M HCl solutions. At first, 20 mg of non-activated base-activated biochar was added to a 20 ml solution of NaCl. The pH of the solution was modified by adding 0.1 M HCl and 0.1 M NaOH solutions. The resulting suspension was agitated at 200 RPM in a shaker incubator for 24 h at 25 °C to reach an equilibrium pH. Then, the suspension was filtered using the Whatman grade 40 quantitative filter paper, and the pH of the filtered solution was measured using a pH meter.

The effect of pH on the adsorption of MB by rice straw biochar was evaluated by adjusting the pH of the solution containing the adsorbent and dye within a range of 2–10, with increments of value 2. Different concentrations of the dye, including 2, 4, 7, 10, 12, 15, and 18 mg/L, along with biochar adsorbent dose of 2.5 mg in a 20 ml mixture solution, were agitated at 200 RPM at a temperature of 25 °C.

4.2 Effect of weight of adsorbent

To know the effect of the weight of the adsorbent, the amount of all the biochar was varied in a range of 0.5-3 mg in increments of 0.5 mg in a 20 ml solution of MB dye with a concentration of 10 mg/L at 200 RPM, 25 °C and pH10.

4.3 Effect of contact time

To determine the significance of the contact period between the adsorbent and adsorbate on the kinetics of adsorption, 10 mg/L dye solution was taken with 0.125 mg/mL biochar material. The kinetic study was monitored for a period of 60 min. The effect of temperature on adsorption and the kinetic process was observed within the range of 20–40 °C.

5 Results and discussion

5.1 Characterization of non-activated and base-activated biochar

5.1.1 Surface morphology

To study the surface morphology of the raw biomass, base-activated and non-activated biochar, FESEM studies were done. The FESEM images show significant morphological differences between the raw rice straw, base-activated and non-activated biochar. The raw rice straw biochar shows the presence of phytoliths, which are mainly formed by the deposition of silica and other minerals on the leaves (Fig. 1A) [25]. The base-activated biochar showed a relatively smooth surface than the non-activated biochar (Fig. 1B-D). The pyrolysis of the rice straw leads to the development of pores and cavities on the surface of the biochar materials (Fig. 1B–D). The base treatment of the pyrolyzed biochar refined its surface by eliminating the debris obstructing the pores (Fig. 1C, D). The development of pores and cavities with varying diameters on the biochar surface are mostly due to the dehydration and volatilization of organic compounds during pyrolysis [26]. The non-activated biochar exhibited more lamellar and rougher morphology than the base-activated biochar (Fig. 1B–D). The non-activated biochar had pores that were blocked due to the presence of impurities. However, upon treatment with the base, these impurities were dissolved, which led to the development of many pores in the base-activated biochar (Fig. 1B–D). [27]. The increase in the concentration of NaOH led to the migration of the pores from a honeycomb like regular structure to an irregular porous structure [18]. In line with our results, Manish and co-workers observed the development of a highly porous surface upon activation of Opuntia ficus-indica biochar with base [18]. Mu et al., observed that modification of the tea residue biochar with sodium hydroxide led to the development of mesopores, which are conducive to the diffusion of organic molecules on the surface and contribute towards its adsorption [17]. The elemental composition of the non-activated and base-activated biochar revealed that the primary components constituting the biochar are carbon, oxygen, and silica (Fig. S2). The results show that upon pyrolization of biomass, the carbon content increased with a consequent decrease in the silicon content (Fig. S2).

Fig. 1
figure 1

FESEM images of raw rice straw (A) DR, non-activated biochar (B) DBCNa0, and base-activated biochar (C) DBCNa5, (D) DBCNa10

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5.1.2 XRD analysis

X-ray diffraction (XRD) analysis was conducted to ascertain the level of crystallinity and amorphousness in the samples. The X-ray diffraction (XRD) patterns of the base-activated and non-activated rice straw biochar are presented in Fig. 2A. The patterns exhibit clear peaks at different angles, which are likely attributed to differences in the elements present in the biochar material [28]. The observed hump in all three biochar at 2θ = 22° is a typical absorption peak of lignocellulose that corresponds to the crystallographic plane of C (101/002) (Fig. 2A) [16]. A small peak observed around 2θ =  ~ 43° for all biochar samples, corresponding to the C (100) plane, is due to the presence of condensed carbonized planes (Fig. 2A)[28]. The wide diffraction peak at 002 indicates that the biochar exhibits a graphitic and partially crystallized form, which increases upon base treatment and increases graphitization [29]. The peaks in the XRD spectra at around 2θ = 28, 68, and 73 in the base-activated and non-activated biochar are due to SiO2 [28]. These results are in line with our EDAX data from the previous section.

Fig. 2
figure 2

A XRD spectra; B FTIR spectra; C Raman spectra; D XPS spectra of non-activated (DBCNa0) and base-activated (DBCNa5, DBCNa10) biochar

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5.1.3 FTIR analysis

Fourier Transform Infrared (FTIR) spectroscopy was carried out within 400-4000 cm−1 range to examine the functional groups present on the non-activated and base-activated biochar surfaces. Figure 2B shows the FTIR spectra of different rice straw biochar samples. All biochar samples have a significant stretching vibration peak for the hydroxyl group (-OH) at ~ 3350 cm−1 [30]. The aliphatic –C–H bond stretching vibrational spectra at ~ 2930 and ~ 2850 cm−1 are more prominent for the base-activated than the non-activated biochar [31]. The carbonyl bond (-C = O) stretching in ketene is revealed in the form of crests in the range of ~ 1910–2360 cm−1, [31](Fig. 2B). The peaks in between 1610–1570 cm−1 are attributed to –C=C bond stretching for the hemicellulose aromatic skeletal vibrations [31]. The peak at ~ 1079 cm−1 may be due to the –C–O stretching vibration in the biochar's cellulose and hemicellulose structure [31]. The FTIR peak at ~ 790 is due to the aromatic rings, and at ~ 467 cm−1 is due to the biochar's Si–O–Si bending vibration [3, 4]. The list of all FTIR spectra is enlisted in Table S2. The base-activated biochar shows the presence of oxygen-containing functional groups, which may aid in the adsorption of the organic pollutants.

5.1.4 Raman analysis

Raman spectroscopy was employed to give an idea about the extent of graphitisation of biochar. The analysis of biochar using Raman spectroscopy identified G and D bands at 1590 and 1300 cm⁻1 (Fig. 2C, Fig. S3, Table S3) [32]. These bands are primarily assigned to graphitic and defect bands [32]. The G and D bands are broad bands that lie between 1580–1600 and 1290–1300 cm−1. The D-band is correlated with carbon atoms exhibiting defects or disordered structures, which are often more prevalent in amorphous biochar, while the G-band corresponds to carbon atoms with a sp2 electronic configuration in graphite structures [33]. The Raman spectra of the biochar, with a range from 800 to 1800 cm−1, have been curve-fitted with ten Gaussian peaks with fixed band positions (Fig. S3, Table S3). The curve-fitting residuals, presented by the bands GL, SL, SR, and R, account for a minor percentage of the Raman peak area, while the primary major six bands for all the biochar are denoted as G, GR, VL, VR, D, and S [32]. All the primary six major bands are present in all the base-activated and non-activated biochar except the "S" band, which is absent in DBCNa5 (Fig. S3, Table S3). The ID/IG ratio derived from the Raman spectrum is frequently employed to evaluate the degree of graphitization in biochar [33]. The decreasing order of ID/IG ratio of the base-activated and non-activated biochar is DBCNa5 (1.07) > DBCNa10 (0.92) > DBCNa0 (0.29), indicating that the base-activated biochar has a higher defect order as compared to non-activated biochar. Generally, the defect band is caused by functional groups like -COOH and -OH groups and by local defects at graphitic carbon edges [34]. It is generally considered that defects in biochar materials might increase the total number of adsorption sites on the biochar surface, which helps in adsorbing the pollutant [33].

5.1.5 XPS analysis

X-ray photoelectron spectroscopy (XPS) analysis was carried out to study the biochar material's chemical composition, oxidation states, and functional groups. Figure 2D and Fig. S4-S6 show the complete spectrum and deconvoluted spectra for C1s, O1s, and N1s of both the non-activated and base-activated biochar. The deconvolution of the carbon spectra of the biochar materials displayed three peaks. The peak at 284.4 eV is present for the base-activated and non-activated biochar, representing hydrocarbon and C=C sp2 bonds in the graphitic and aromatic network (Fig. S4-S6, Table S4) [35, 36]. The peak at 288.6 eV is due to the carboxylic group being present only in the base-activated biochar (Fig. S4-S6, Table S4) [37, 38]. The C–O peak at 286.5 eV due to the ether and the phenolic group is only present in 5 M-NaOH base-activated biochar (Fig. S4-S6, Table S4) [36]. The deconvolution of nitrogen and oxygen from the XPS spectrum revealed peaks only for the base-activated biochar (Fig. S4-S6, Table S4). The peak at 399.4/400.0 and 400.6 eV is present in both the base-activated biochar. These peaks are mainly due to the C–N–C, C–N–H, C–N bond and pyridonic/pyrrolic N, respectively [36, 39]. Another peak, at 402.4 eV, is present only in 10 M base-activated biochar, representing oxidised N with weak C–N or N–N bonds linked to C atoms with sp2 bonds [36, 39]. The O1s oxygen spectra of the biochar showed five peaks in the base-activated biochar. The peak at 531.4 and 533.1 eV are present in both the base-activated biochar, representing carbonyl, lactone, carboxylic groups and ester/anhydride groups, respectively [40, 41]. Similarly, the peak at 532.8 and 534.9 eV is present in 5 M base-activated biochar (DBCNa5) and represents the aliphatic C–O oxygen singly bonded to carbon in aromatic rings, phenols, and ethers, and carboxylic group, respectively [40, 42, 43]. In addition, another peak at 532.3 eV, present in DBCNa10, represents the hydroxyl group [37]. The C-O bond is characteristic of the esters generated by the hydroxyl group on cellulose.

5.1.6 Elemental analysis

The CHNS elemental analysis quantifies the atomic proportion of carbon, hydrogen, nitrogen, and sulphur in rice straw biomass and biochar. Table S5 shows the elemental composition of the raw, pyrolyzed and base-activated biochar samples. The H/C ratio is employed to determine the aromaticity and maturation of biochar, which is associated with its long-term stability in the environment. Reducing the proportion of hydrogen to carbon enhances the aromatic nature of biochar and enhances its structural stability [44,45,46]. The data clearly states that the pyrolysis of rice straw strongly strengthened aromaticity as compared to raw biomass. However, no change in aromaticity occurred after the base activation of the biochar (Table S5). The decreasing order of H/C ratio of the raw, non-activated and base-activated biochar is DR (0.144) > DBCNa0 = DBCNa5 = DBCNa10 (0.03). Further, an increase in aromaticity may enhance the π–π interactions between the adsorbent and the organic pollutant [47].

5.1.7 Thermogravimetric (TGA) analysis

The pyrolytic characteristics and thermal stability of rice straw biomass and their biochar were determined using derivative thermogravimetry (DTG) and thermogravimetric analysis (TGA). The thermal kinetic properties of rice straw were evaluated using thermogravimetric analysis. The weight of mass loss is presented by the derivative curve and segregated into four different temperature ranges/stages. Moisture and volatile organics (V–O-M) undergo pyrolysis below 220 °C (First stage), hemicellulose (H–C) between 220 °C and 315 °C (second stage), cellulose (C) between 315 °C and 400 °C (third stage), and lignin (L) between 400 °C and 895 °C (fourth stage). Any substance that remains after the temperature exceeds 895 °C is known as char and ash [48]. At high temperatures, the disintegration of inorganic compounds, aromatization and dehydrogenation of the biochar may lead to the loss of biochar mass. [31]. Thermally stable biochar often exhibits a lower mass loss. On the other hand, those with greater weight loss experience the breakdown of carbon compounds into liquid or gaseous forms due to the stability of carbon–carbon and carbon-hydrogen bonds. [31].

The initial first-stage weight loss for the raw biomass is ~ 10.54%, but the biochar materials exhibited a weight loss ranging from 5.39% to 9.86% (Fig. 3, Table S6). This loss may be attributed to the loss of volatile substances [31]. The weight reduction in the second and third stages is primarily attributed to the content of hemicellulose and cellulose, respectively. The weight reduction in the second and third stages for raw biomass is around ~ 24% and ~ 22%, respectively, whereas for biochar materials, it ranged between 1.41–2.75% for hemicellulose and 1.22–3.07% for cellulose content (Table S6). This observed weight loss may be due to the devolatilization of volatile components in hemicellulose and cellulose [31]. In stage 4, the weight reduction is attributed to the lignin content, which is ~ 10% for the raw rice straw and ranges from 7.72% to 14.28% for the biochar materials (Table S6). Table S6 indicates that the weight loss percentage of the base-activated biochar is higher than the non-activated biochar. This may be due to the disintegration of functional groups, such as carboxyl, carbonyl, and hydroxyl groups, into carbon dioxide and water vapour in the base-activated biochar [49].

Fig. 3
figure 3

TGA/DTG curve of A DR, B Non-activated DBCNa0, C, D Base-activated rice straw biochar (DBCNa5 and DBCNa10) with heating rate 10 °C/min, in N2 atmosphere

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5.1.8 Surface area analysis

Nitrogen adsorption–desorption isotherms were employed to assess the base-activated and non-activated biochar's surface area and pore structure. The pore volume and specific surface area of a material directly influence the quantity of active sites and the adsorption characteristics [50]. The nitrogen adsorption–desorption curves of non-activated and base-activated biochar is shown in Fig. 4A. The isotherm for the non-activated biochar resembles type III, indicating the presence of mesopores that broadens its distribution to macropores [51, 52]. The isotherm has a very narrow hysteresis loop without a deflection point, which shows weak interactions and clustering of the adsorbate molecules around a few active sites [52]. The isotherms of the base-activated biochar follow type IV(a) adsorption isotherm with an H3 hysteresis loop (Fig. 4A)[52]. The materials following type IV adsorption isotherms are generally mesoporous, and the presence of a hysteresis loop indicates capillary condensation [52]. The hysteresis in type IV isotherm occurs when the pores are wider than ~ 4 nm [52]. The pore size was estimated with the aid of BJH analysis (Fig. 4B, C and Table S7). The BJH analysis reveals that the base-activated biochar's surface area and pore volume increase substantially upon activation with a consequent decrease in the pore size from ~ 25 to ~ 10 nm (Table S7). This indicates that the process of modifying rice straw biochar with a base effectively removes contaminants from the pores of pyrolyzed rice straw biochar [53]. Manisha et al., also observed an increase in the surface area and pore volume with a subsequent decrease in the pore diameter of the base-activated biochar [18].

Fig. 4
figure 4

A N2-adsorption–desorption isotherm curve B Adsorption and C Desorption BJH pore size distribution curve for non-activated (DBCNa0) and base-activated biochar (DBCNa5 and DBCNa10)

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5.2 Adsorption of MB on biochar

5.2.1 Effect of solution pH and initial dye concentration

The point of zero charge (pHPZC) is an essential indicator in investigating the electrostatic interaction process and determining the affinity of adsorbates and adsorbents. It is used to detect the surface charge of a material, which is influenced by the pH of dye solutions [17, 27, 54, 55]. The change in the pH of the solution impacts the rate at which adsorption occurs by affecting the surface charge of the base-activated biochar [17, 27, 54, 56]. The pHPZC of the base-activated and non-activated biochar ranged from 6.36 to 7.36 (Fig. S7). Upon base modification, the surface chemistry of the biochar is significantly modified, resulting in the lowering of acidic oxygen-containing functional groups, which increases the surface's basicity [18]. The study of the pHPZC value indicates that the adsorbent's surface exhibits a positive charge below the pHPZC and a negative charge above it. Therefore, since MB is a cationic dye, adsorption would be favourable at a pH above the pHPZC.

The adsorption of MB onto the base-activated and non-activated biochar was performed in a pH range of 2–10 to evaluate the influence of pH on adsorption at different concentrations of the dye (Fig. 5). The percentage of MB removal increases with increasing pH with the maximum dye removal at pH10. The decreasing order of removal efficiency of all three adsorbents is DBCNa10 > DBCNa5 > DBCNa0. Also, at every pH, the percentage removal of the base-activated biochar is more than that of the non-activated biochar (Fig. 5). Mu et al., also observed an increase in the removal of MB by varying the pH of the solution in the range of 2–10, with the highest adsorption at pH 10 [17]. The higher removal percentage of MB in base-activated biochar in an alkaline medium may be due to the increased hydroxyl concentration, which may have resulted in enhanced electrostatic attractions between negatively charged biochar surface and cationic MB molecules [17]. At lower pH, the adsorption occurs due to the n − π and π − π interactions facilitated by the benzene/heterocyclic rings in the dye molecule and the biochar [57]. The results indicate that electrostatic interactions play a major role in the adsorption of the MB molecules on the biochar surface. The removal rate of dye decreases with increasing concentration of dye for the base-activated and non-activated biochar (Fig. 5). The presence of the active site regulates the degree to which adsorption takes place. At high initial concentrations, the active site becomes fully saturated, reducing adsorption capability [55, 56, 58]

Fig. 5
figure 5

Effect of solution pH (with different initial dye concentrations A 2 mg/L, B 4 mg/L, C 7 mg/L, D 10 mg/L, E 12 mg/L, F 15 mg/L and G 18 mg/L); biochar adsorbent dose 2.5 mg; 200RPM; temp. 25 °C and vol. of solution 20 ml) on MB dye solution removal

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5.2.2 Effect of weight of adsorbent

The dose of the adsorbent plays an important role in the adsorption process [17, 55, 58, 59]. A low adsorbent dose may lead to poor adsorption, while a high dose may decrease the adsorption efficiency [59]. As the maximum removal was observed at pH 10, the dose-dependent study was conducted at the same pH. The percentage of MB removal, with a dose ranging from 0.5 to 3.0 mg, is shown in Fig. 6. The removal rate of MB increases with an increase in the dose till 2.5 mg for the base-activated and non-activated biochar (Fig. 6). The removal rate of MB increased from ~ 2%, ~ 17%, and ~ 15% to ~ 21%, ~ 73%, and ~ 81% for DBCNa0, DBCNa5, and DBCNa10, as the dosage of the adsorbent increased from 0.5 mg to 2.5 mg and further decreased slightly (Fig. 6). With the increase in the adsorbent dose there is an increase in the surface area and the number of active sites available due to which MB adsorption increases [55]. However, beyond 2.5 mg, there is a slight decrease in the adsorption of the dye, which may be due to the intensification of the adsorption effect.

Fig. 6
figure 6

Effect of biochar dosage (with initial dye concentration 10 mg/L; 200RPM; temp. 25 °C and vol. of dye solution 20 ml) on MB dye solution removal

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5.2.3 Effect of contact time

Adsorption is a dynamic equilibrium process where adsorbate requires a specific time period to achieve adsorption equilibrium. Insufficient contact time may result in incomplete adsorption, while excessive contact time can lead to desorption [58]. Similar to the dose-dependent study, the adsorption rate was studied at pH 10. Figure 7 shows the correlation between the contact time for the base-activated and non-activated biochar and the effectiveness of MB removal. The removal percentage of MB increased with time and reached equilibrium within 15 min (Fig. 7). But 90 percent of the dye removed at equilibrium was already eliminated in less than 5 min (Fig. 7). This indicates that the active adsorption sites on the biochar surface are occupied very fast, with the maximum removal efficiency increasing in the order DBCNa0 (~ 19%) < DBCNa5 (~ 71%) < DBCNa10 (~ 80%) (Fig. 7) [60]. It is interesting to notice that the kinetic equilibrium points for the non-activated biochar and base-activated biochar are similar i.e. ~ 15 min. However, the MB removal efficiency of the base-activated biochar is much higher than that of non-activated biochar, possibly due to the development of functional groups upon activation with the base.

Fig. 7
figure 7

Effect of contact time on percentage removal of MB dye with base-activated and non-activated biochar adsorbent dosages 2.5 mg (0.125 mg/ml); MB dye concentration 10 mg/L; with pH10; temp. 25 °C

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5.2.4 Adsorption isotherms

To establish a correlation between the interactions of adsorbate molecules and adsorbent material and the amount of adsorbate adsorbed, isotherm models are employed. Freundlich and Langmuir isotherms are the most common isotherm models employed for studying the adsorption of organic molecules on biochar materials [55, 56]. According to the Langmuir isotherm model, the adsorbent contains homogeneous sites with a monolayer of adsorption accompanied by a finite number of identical sites with uniform adsorption energies [55, 56]. The Freundlich adsorption isotherm model is not limited to monolayer adsorption and is suitable for multilayer adsorption [61]. The linear and non-linear forms of the Langmuir and Freundlich models are as follows: [17, 55]

Langmuir linear form

$$\frac{{C}_{e}}{{q}_{e}}= \frac{{C}_{e}}{{q}_{max}}+ \frac{1}{{q}_{max} \times { K}_{l}}$$
(3)

Langmuir non-linear form

$${q}_{e}= \frac{{q}_{max}\times {K}_{l}\times {C}_{e}}{1+ {K}_{l}\times {C}_{e}}$$
(4)

where Ce = equilibrium concentration of adsorbate in mg/L, qe = amount of adsorbate absorbed at equilibrium in mg/g (experimental), Kl = Langmuir constant in L/mg and qmax = maximum adsorption capacity of the adsorbent biochar in mg/g.

Freundlich linear form

$$log{(q}_{e})=\text{log}\left({K}_{f}\right)+ \frac{1}{n}\text{log}\left({C}_{e}\right)$$
(5)

Freundlich non-linear form

$${q}_{e}= {K}_{f}\times {C}_{e}^\frac{1}{n}$$
(6)

where Kf = Freundlich constant in (mg/g)/(mg/L)n and n = adsorption intensity.

The linear and non-linear forms of both isotherm models were applied to the experimental data, and the coefficient of determination (R2) was determined to evaluate the fit between the models and the experimental data (Fig. 8, S8 and Table 1). The selection of the best fit isotherm model is based on the criterion where the values of the R2 are high i.e. greater than 0.90, and has low error values. The linear and non-linear presentations of the Langmuir isotherm models are presented in Fig. 8 and Fig. S8, respectively. Table 1 and Fig. S8 present their corresponding fitting values. Table 1 indicates that the linear Langmuir model presents the best fit for all the biochar materials from pH 2–10 while for the Freundlich model, the best fit is only for the base-activated biochar materials from pH 6–10. In the non-linear Langmuir isotherm model model, the best fit is obtained only for the base-activated biochar materials from pH 4–10 while that for the non-linear Freundlich isotherm model is from pH 6–10. The results show that at higher pH for the base-activated biochar materials, regardless of the mathematical approach both the linear and non-linear forms exhibit good fit. Although most adsorption studies consider the linear models to describe the adsorption process, the non-linear regression analysis generally offers more accurate parameter estimates. Hence, as our data for the base-activated biochar materials at alkaline conditions fits both linear and non-linear forms of Langmuir and Freundlich isotherms, it represents the robustness of these models in describing the adsorption process [62]. The fitting to Langmuir isotherm model suggests that the adsorption process is monolayer in nature, with the availability of adsorption sites as the rate-limiting factor [55]. The maximum adsorption capacity (qm) for non-activated biochar at different pH ranges from 1.66 to 18.44 mg/g (Table 1 and Fig. S9). Similarly, for the base-activated biochar materials DBCNa5 the qm for pH 2–10 ranged from 10.29 -96.99 mg/g and for DBCNa10 from 21.88 to 129.87 mg/g, respectively (Table 1 and Fig. S9). To know the feasibility of the Langmuir monolayer adsorption process, the separation factor (RL) term is used and is expressed as follows: [2, 57].

$${R}_{L}= \frac{1}{1+ {K}_{l}* {C}_{0}}$$
(7)

where Kl (L/mg) is the Langmuir constant, and C0 is the starting concentration of the adsorbate in (mg/L).

Fig. 8
figure 8

Linear adsorption isotherm of MB dye molecule with different pH variations (pH = 2 -10) and different dye concentration variations (Co = 2–18 mg/L), at 25 °C, with biochar adsorbent dose of 2.5 mg with a reaction volume of 20 mL. The dot symbols represent the experimental data, and the solid line shows the Langmuir and Freundlich isotherm fit A, C, E, G, I Langmuir isotherm and B, D, F, H, J Freundlich isotherm

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Table 1 Langmuir and Freundlich adsorption isotherm parameters for MB adsorption by non-activated and base-activated biochar
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The separation factor indicates the adsorption nature as either favourable (0 < RL < 1), irreversible (RL = 0), linear (RL = 1), or unfavourable (RL > 1) [2, 57]. The separation factor for the base-activated and the non-activated biochar lies between 0 to 1, indicating the feasibility of the process (Fig. S10).

Unlike Langmuir, Freundlich isotherm mainly applies to adsorption on heterogeneous surfaces with active sites having variable affinities [61]. As discussed above, the adsorption process for the base-activated biochar follows Freundlich isotherm at pH from 6 to 10 with R2 values ranging from 0.91–0.98 (Fig. 8 and Table 1). In Freundlich isotherm the value of “n”, indicates the favourability of the adsorption process. When the value of “1/n” falls within the range of 0 to 1 (0 < 1/n < 1), it is considered favourable. Conversely, the adsorption process is considered unfavourable when the value of 1/n is more than 1 (1/n > 1). The adsorption process is considered irreversible when the value of 1/n is equal to 1 (1/n = 1) [51, 54, 63, 64]. As the value of 1/n lies in the range of 0–1 for the base-activated biochar at pH 6–10, it indicates the favourability of the adsorption process (Table 1). It is noteworthy to mention here that the.

adsorption process follows both the Langmuir and Freundlich models as the solution becomes more alkaline. Cheruiyot et al., also observed cationic crystal violet dye adsorption on waste from coffee husks followed both Langmuir and Freundlich isotherms [65]. Similar results have been also observed by other groups where the adsorption of dyes on different materials follows both the isotherm models[64]. As the adsorption process follows both Langmuir and Freundlich isotherms, it can be assumed that the base-activated biochar materials show both heterogenous and homogenous surface interactions. This also shows that the surface of the base-activated biochar materials has adsorption sites with variable adsorption energies. The surface can accommodate monolayer at lower pH but at higher pH can host both monolayer and multilayer adsorption scenarios [66]. The results from both the isotherm models clearly show that the adsorption capacity of the biochar materials is highest at pH10. It is also seen that their adsorption capacity significantly increases beyond their respective pHPZC value, with the highest adsorption taking place at pH10 (Fig. S9). Therefore, all the further experiments were carried out only at pH10.

5.2.5 Adsorption kinetics

Adsorption kinetic modelling evaluates the effectiveness of an adsorbent by analysing the rate and process of adsorption [55, 56, 67, 68]. The present study used first-order (FO), pseudo-first-order (PFO), second-order (SO), and pseudo-second-order (PSO) models to identify the adsorption rate of MB on biochar materials. The equations used for plotting the kinetics of various order rates are as follows:

First order (FO)

$$\text{log}\left({C}_{t}\right)=\text{log}\left({C}_{0}\right)- \frac{{k}_{1F}}{2.303}\times t$$
(8)

Pseudo-first order (PFO)

$$\text{log}\left({q}_{e}-{q}_{t}\right)=\text{log}{q}_{e}- \frac{{k}_{1}}{2.303}\times t$$
(9)

Second order (SO)

$$\frac{1}{{C}_{t}}= {k}_{2S}\times t+ \frac{1}{{C}_{0}}$$
(10)

Pseudo second order (PSO)

$$\frac{t}{{q}_{t}}= \frac{1}{{k}_{2}\times {q}_{e}^{2}}+ \frac{1}{{q}_{e}}\times t$$
(11)

where qe (mg/g) is the calculated equilibrium adsorption capacity from the equation,, k1 (1/min) is the PFO rate constant and qt (mg/g) is the amount of MB adsorbed at different time intervals, k2 (g/mg.min) is the PSO rate constant, k1F, k2S is the rate constant for FO and SO reaction, respectively, Ct and C0 is the concentration of the adsorbate at a different time interval and initial zero time, respectively. Additionally, qexp (mg/g) is the amount of MB adsorbed by the adsorbent at equilibrium per unit mass of the adsorbent in the experiment [16, 55].

The kinetic parameters were evaluated by fitting the experimental results with the above-mentioned kinetic models (Fig. 9). The most suitable kinetic model was selected based on the linear correlation coefficient (R2). The analysis of the experimental results in relation to the equations mentioned above indicated that the adsorption kinetics align most closely with the PSO kinetic model (R2 = 0.99) (Table 2 and Fig. 9). Also, qexp (mg/g) and qcal (mg/g) of PSO kinetic model is nearly equal, which validates and reinforces PSO kinetic model (Table 2) [2]. The PSO model represents the state at which the adsorption process achieves equilibrium with a relatively low initial concentration of adsorbate [68]. This suggests that the adsorbent possesses several active sites when the adsorbate concentration is low [68].

Fig. 9
figure 9

Kinetics of MB dye molecule adsorption onto the base-activated and non-activated rice straw biochar adsorbent at 25 °C (MB dye concentration = 10 mg/L, biochar adsorbent dose 0.125 mg/mL, pH = 10. Shape symbols represent the experimental data, and the solid line shows the fitting of kinetic models First-order, (A, E, I); Second-order (B, F, J); Pseudo first-order (C, G, K) and Pseudo second-order (D, H, L)

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Table 2 Adsorption kinetic model data for adsorption of MB dye molecule onto the solid–liquid solution of rice straw biochar
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Two diffusion models are commonly employed to understand the mechanism of adsorbate diffusion on the adsorbent: Weber and Morris model and the Boyd model [55, 56, 69]. These models help to determine the rate-limiting factor in an adsorption process. The intraparticle diffusion, as defined by Weber and Morris, is determined using the following equation: [56]

$${q}_{t }= {k}_{WM} \times {t}^\frac{1}{2}+C$$
(12)

where qt indicates the adsorption capacity of the adsorbent at time t in min, kWM (mg/g.min1/2) is the intraparticle diffusion constant, and C (mg/g) is the intercept indicates the thickness of the outer layer [55, 56, 64]. The intraparticle diffusion constant kWM (mg/g.min1/2) is obtained from the slope of the straight-line plotting qt (mg/g) against t1/2 (min1/2). The plot between qt and t1/2 shows three different regions (Fig. S11). The first stage is surface diffusion, in which the adsorbate dye molecules move from the bulk to the surface of the adsorbent; the second stage is the transport of the absorbate dye molecules to the pores of the adsorbent, which results in pore diffusion and intraparticle diffusion; and the third stage is the final equilibrium stage, and here intraparticle diffusion begins to slow down as adsorbate molecules are adsorbed at the active sites [69, 70]. The k(WM)1 is higher in the base-activated biochar, signifying strong dye adsorption on the biochar's surface [3]. Subsequently, k(WM)2 and k(wm)3 indicate that intraparticle diffusion is probably a limiting factor in restricting the adsorption sites (Table S8). However, as the intercept (C) value is not zero, intraparticle diffusion is not the sole rate-limiting step; rather, both intraparticle diffusion and mass transfer make a notable contribution to the adsorption of MB dye (Table S8).

It remains unclear which one predominates during the adsorption of MB on the biochar surface. Therefore, Boyd's kinetic model is used to study the rate-limiting step in the adsorption process. The equation used for the Boyd kinetic model is as follows: [56, 69, 71,72,73,74,75]

$$-\text{ln}\left(1-F\right)= {k}_{b}\times t+C$$
(13)

where, \(F= \frac{{q}_{t}}{{q}_{e}}\), F = Fractional attainment of equilibrium, qt and qe is the amount of MB adsorbed at time t in min and at equilibrium time. The coefficient kb (1/min) can be determined by calculating the slope of the straight line formed by plotting -ln(1-F) against t.

The Boyd model linear plot indicates that the intercept does not pass through the origin (Fig. S11, Table S8). This indicates that the liquid diffusion outside the particle and the intraparticle diffusion are the rate-limiting factors in the adsorption process[69].

5.2.6 Adsorption thermodynamics and activation energy

The study of thermodynamics is essential for predicting adsorption mechanisms as it tells about the randomness of the solid–liquid interface, whether the adsorption process is endothermic or exothermic, and the feasibility and spontaneity of the reaction [57, 58]. The various parameters related to adsorption, including changes in free energy (ΔGo), enthalpy (ΔHo), and entropy (ΔS.o), are calculated using the below van’t Hoff equation [27, 58, 76]

$$\Delta {G}^{o}= -R\times T\, ln{K}_{d}$$
(14)
$${lnK}_{d}= \frac{\Delta {S}^{o}}{R}- \frac{{\Delta H}^{o}}{R\times T}$$
(15)

where Kd (\(\frac{{q}_{e}}{{C}_{e}})\) denotes the distribution coefficient (L/g), ΔGo is the Gibbs free energy change (kJ/mol), ΔSo is the change in entropy (in J/K.mol), ΔHo is the change in enthalpy (kJ/mol), T is the absolute temperature (in Kelvin, K) and R is the universal gas constant (8.314 J/mol.K). By plotting a linear graph between ln Kd Vs 1/T, the ΔHo and ΔSo values are calculated from the slope and intercept, respectively (Fig. 10A and Table S9). Similarly, ΔGo is calculated using the above equation at each temperature (Table S9) [27, 58, 64, 77]. The ΔG° for each of the biochar materials at every temperature is negative, showing that the adsorption process is spontaneous [78]. The ΔG° value increases with the increase in the temperature for all three biochar materials, indicating that higher internal energy is required for dye adsorption onto the biochar materials (Table S9) [64, 79]. Also, the change in entropy for the adsorption process for all the biochar materials ranged between ~ -7 to ~ -99 J/K.mol, indicating a decrease in the degree of disorderness at the solid–liquid interface in the reaction (Table S9) [80]. Khan et al., observed a similar thermodynamic phenomenon in removing a cationic dye using Chinar tree biochar [64]. The ΔH° value for all the biochar materials ranged from -10.20 to -25.62 kJ/mol, indicating that the adsorption process is exothermic. As the enthalpy of adsorption lies in the range of 0–20 kJ/mol, physiosorption is the dominant adsorption process [79].

Fig. 10
figure 10

A Thermodynamic and B Activation energy of MB dye adsorption on non-activated (DBCNa0) and base-activated biochar (DBCNa5, DBCNa10)

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In addition to Gibb’s free energy, activation energy also serves as an important index for analyzing the type of adsorption [81]. As the PSO kinetic model expresses the best fit with the experimental data, PSO kinetic rate constant at different temperatures (Fig. S12) was used to determine the activation energy using Arrhenius equation [2, 77]. The Arrhenius equation is expressed as follows [2, 77]

$$ln{k}_{2}=lnA- \frac{{E}_{a}}{R\times T}$$
(16)

where Ea represents the Arrhenius activation energy (kJ/mol), A represents the Arrhenius factor/frequency factor, and R represents the gas constant (8.314 J/mol.K). T represents the temperature (K) of the solution. A negative slope with the value ‘ Ea’ is obtained by plotting a linear graph between Rlnk2 vs 1/T (shown in Fig. 10B and Table S9) [2, 77]. Generally, the physisorption process involves activation energy ranging from 0 to 50 kJ/mol, while higher activation energies ranging from 50–800 kJ/mol signifies a chemisorption process [82]. In the current study, the activation energy of the biochar materials ranges from ~ 13 to  ~ 20 kJ/mol (Fig. 10B and Table S9). Therefore, the activation energy value for the biochar materials suggests that the adsorption process is physisorption in nature, reinforcing our results from Gibbs free energy studies.

6 Adsorption mechanism

The adsorption of MB dye on the biochar adsorbent is affected by many factors, like (1) the biochar property, their pore structure, surface area, aromaticity, the surface containing functional groups, (2) the MB dye property like their structure, solubility, concentration in the solution, polarity and (3) the condition of the reaction medium, such pH, dose etc. [57, 83]. So, the physicochemical characteristics of biochar and the particular environmental factors present in the solution are important factors in the adsorption process [83]. According to the classical mechanism of adsorption of organic pollutants, such as dyes, onto adsorbents like biochar, many factors play a driving force in carrying out the adsorption process like covalent bonding, electrostatic force, H-bonding, π- π interactions, dipole interactions, and hydrophobic forces [57]. Among all properties in the adsorption process, the pH of the reaction solution plays a significant role in determining the adsorption of the MB dye onto the biochar [56]. The electrostatic forces of attraction and repulsion between the adsorbate and adsorbent at pH above and below the pHPZC play a vital role in the adsorption process. The adsorption capacity of MB increases with an increase in pH, with a significant rise beyond the pHPZC of the biochar materials, indicating the predominant influence of electrostatic attraction at higher pH. However, the significant adsorption capacity of MB below the pHPZC indicates a considerable influence of non-electrostatic interactions, including H-bonding, π-π interactions, and n-π interactions. As the biochar materials have a heterogeneous surface due to the presence of various oxygen-containing functional groups, including hydroxyl, carbonyl, carboxyl, and amines, they help in the dye molecule's adsorption onto the biochar surface. Similarly, the enhancement of degree of graphitisation upon base-activation helps in π-π interaction among heterocyclic aromatic structures of biochar and MB dye molecule. In addition, the surface area analysis shows that the biochar material's surface is mesoporous in nature, which further aids the adsorption process. Based on the above discussion, the adsorption mechanism of MB dye is presented in Fig. 11.

Fig. 11
figure 11

Probable adsorption mechanism diagram of MB dye removal by biochar materials

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7 Comparison with other adsorbents

The adsorption capacities of the biochar materials synthesised from the different biomass are listed in Table 3. Mu et al., synthesized NaOH-modified mesoporous biochar from tea residue.

Table 3 A comparative study of the maximum adsorption capacity of Methylene Blue (MB)
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with various adsorbents.to improve MB adsorption capacity with a maximum adsorption capacity of 105.44 mg/g [17]. Modification of rice straw biochar with calcium hydroxide increased the adsorption capacity of the pristine biochar from 26.7 to 333.3 mg/g, with a contact time of 24 h [84]. Liu et al., obtained an adsorption capacity of 120.84 mg/g upon modification of rice straw with acrylamide and citric acid with a contact time of 3 h [85]. In another study, rice straw ball milled biochar showed an adsorption capacity of 90.91 mg/g in removing MB within a short contact time of 15 min [89]. The adsorption capacity of modified rice husk biochar in removing methylene blue dye is 14.34 mg/g with a contact time of 100 min [86]. Li et al., found that the modification of wheat straw biochar using hydrochloric acid enhanced the adsorption of MB from 46.6 to 62.5 mg/g upon exposure to the magnetic field for 24 h [87]. The eucalyptus saw dust biochar activated with different weak acids exhibited the maximum adsorption capacity ranging from 29.94 to 178.57 mg/g [88]. In the same study, they showed that the adsorption process follows pseudo-second-order kinetics. The adsorption process of the biochar generated by co-pyrolyzing of municipal sludge and tea waste biochar followed Langmuir adsorption isotherm with a maximum loading capacity ranging between 12.57 and 19.37 mg/g [90]. Similarly, Shi et al., showed that the adsorption of methylene blue on the anaerobic granular sludge biochar is spontaneous in nature, with maximum adsorption capacity increasing with an increase in reaction temperature from 90.91 to 101.01 mg/g [91]. Sutar et al., showed that rice straw biochar modified with several reagents such as NaOH, KOH, H3PO4, and ZnCl2 exhibited a maximum adsorption capacity of 86.95 mg.g−1 [92]. In another study, modified biochar prepared from rice straw by ball milling exhibited an adsorption capacity of 50.2 mg/g in the removal of methylene blue from an aqueous environment. In the same study, they showed that maximum adsorption efficiency was observed at 40 ℃ and pH 8 [93]. The silica composite biochar prepared from swine manure and rice straw exhibited MB adsorption with a loading capacity 142.86 and 131.58 mg/g [94]. The adsorption capacity of the biochar materials is attributed to increased porosity and higher number of exchangeable sodium ions. Compared to the above adsorbents, this study's base-activated rice straw biochar can reach the ~ 90% adsorption capacity in less than 5 min.

The cost of the activated carbon is generally higher than other adsorbents i.e. price of commercial activated carbon ~ $21/kg [95]. In contrast to activated carbon, the cost of bio adsorbents and biochar is generally lower $ 0.05–$ 20 /kg, with variable adsorption efficiency [96]. The cost of pristine and modified biochar differs based on the modification and preparation techniques. Zhang et al., have summarized in their review that the cost of biochar production ranges from $ 0.67 to $ 17.80 /kg [95]. This variation is mainly due to the cost of the biomass, which varies according to geographical location and production techniques. It is seen that the cost of biochar derived from class I (woody) and class II (agricultural) biomass, with prices ranging from $1.00/kg to $1.30/ kg, which increases with the increase in the pyrolysis temperature [95]. The biochar derived from sludge within the same temperature range varies between $0.70/ kg to $1.00/kg [95]. Although there are cheaper adsorbents available such as red mud and zeolite, but the removal of pollutants is generally higher with biochar materials. The cost of aminated biochar having fourfold higher adsorption capacity costs ~ $2.60/kg, which is eight times lower than commercial activated carbon [97]. The modification of biochar with metal oxides and metals increases the production cost but enhances its adsorption capacity and reusability significantly. Therefore, in designing a cost-effective biochar material with various modifications, several factors should be taken into account. First, the environmental risk associated with the modification agent and the process should be assessed. Secondly, the cost of the modification agents and process should be considered, and thirdly, the efficiency and the removal mechanism of the pollutants by the modified biochar should be assessed. Lastly, the recycling and the regeneration potential of the biochar should be studies which will further reduce the cost of the biochar usage. In addition to this the biochar aging in long term should also be studied.

8 Conclusions

In this study, a low-cost base-activated rice straw biochar is synthesized with fast adsorption capacity for the removal of MB from aqueous solutions. The adsorption capacity of the biochar increased significantly upon base-activation. The adsorption of biochar and MB followed PSO kinetics and reached the adsorption equilibrium in 15 min. Interestingly, the base-activated biochar adsorption process follows both Langmuir and Freundlich isotherm models as the pH becomes alkaline, with a maximum adsorption capacity of 129.87 mg/g at pH10. From the thermodynamic and activation energy data, the adsorption is physisorption in nature, with the electrostatic force of attraction being the dominant force aiding the adsorption process. Furthermore, the adsorption is also enhanced due to the increase of surface functional groups such as carbonyl, carboxyl, hydroxyl, and amine groups, as well as the degree of graphitization upon base treatment of biochar. Therefore, H-bonding, electrostatic, π-π and n-π interactions play a vital role in the adsorption of MB on the biochar. The adsorption process follows both the Weber-Morris and Boyd models, signifying that both intra-particle and liquid film diffusion play a vital role in the adsorption rate. Hence, the base-activated rice straw biochar acts as an effective, cost-efficient and fast adsorbent for the removal of the commercial cationic dye methylene blue from the aqueous environments.