Adsorptive removal of dimethyl phthalate using peanut shell-derived biochar from aqueous solutions: equilibrium, kinetics, and mechanistic studies

Rise in polymer industry and extensive use of their products leads to leaching of phthalate esters and distributed into the different matrices of the environment. This chemical group has the potential to hamper the life of living organisms and ecosystem. Thus, it is essential to develop cost-effective adsorbents capable of removing these harmful compounds from the environment. In this work, peanut hull-derived biochar was taken as the adsorbent, and DMP was selected as the model pollutant or adsorbates. The biochars of different properties were produced at three pyrolysis temperatures (i.e., 450, 550, and 650 °C) to check how temperature affected the adsorbent properties and adsorption performance. Consequently, the performance of biochars for DMP adsorption was thoroughly studied by the combination of experiments and compared with commercial activated carbon (CAC). All the adsorbents are meticulously characterized using various analytical techniques and used for adsorption DMP from aqueous solutions. The results suggested that adsorption was favoring chemisorption with multi-layered adsorption as adsorption kinetics and isotherm are in good alignment with pseudo-second-order kinetics and Freundlich isotherm, respectively. Further, thermodynamic study revealed DMP adsorption on adsorbent is physically spontaneous and endothermic. The removal efficiency order of four adsorbent was as follows: BC650 > CAC > BC550 > BC450 with maximum efficiency of 98.8% for BC650 followed by 98.6% for CAC at optimum conditions. And as it is a short carbon chain PAE, dominant mechanisms of adsorption for DMP onto porous biochar were H-bonding, π-π EDA interactions, and diffusion within the pore spaces. Therefore, this study can provide strategies for the synthesis of biochar for effectively removing DMP from aqueous solution.


Introduction
Phthalate esters (PAEs) have received significant attentions for their wide applications as plasticizer to improve flexibility and workability of polymers (Jing et al. 2011;Li et al. 2020a, b;Staples et al. 1997). Worldwide, annual production of PAEs is about 6-8 million tons because it is mostly used in manufacturing of plastic containers, cosmetics, adhesives, oils, and grease (Net et al. 2015;Pang et al. 2021;Schettler et al. 2006). Hence, because of their wide applications, PAEs became a ubiquitous environmental pollutant. PAEs are endocrine disrupting compounds because they can mimic and disrupt the functions of hormones (Li et al. 2016). It was also reported that phthalates have the potential to disturb molecular signaling and genetic expressions (Benjamin et al. 2017). There are 23 types of PAEs that have been recognized as the hazardous substance by the WHO (Gao and Wen 2016;He et al. 2013). And surprisingly, in a study, it was found that phthalate metabolite concentrations of 389 ng ml −1 in urine samples and 506 µg g −1 in creatinine sample were detected in the samples collected from India, indicating widespread human exposure to phthalates 1 3 (Guo et al. 2011a, b). Therefore, it is urgent to bring some robust processes that can remove these pollutants from the environment. Among the series of well-developed separation processes for the PAEs, adsorption was intensively studied because it is energy efficient and easy to operate and has high removal efficiency (Wei et al. 2016). Several researchers have worked with various conventional adsorbents for the removal of organic contaminants. Some scholars have focused on the application of carbonaceous materials such as activated carbon, carbon nanotubes, and graphene for removal of phthalate esters (Abdul et al. 2017;Gao et al. 2013;Lu et al. 2018).
Recently, biochar has evolved as a potential alternative over the conventional adsorbents for removal of variety of pollutants such as PPCPs, Dyes, EDCs, and heavy metals (Dai et al. 2019;Hoslett et al. 2020;Zazycki et al. 2018). The specific advantages of biochar are as follows: low cost, large surface area, presence of multiple functional groups, and eco-friendly, and also provide a pathway for waste to resource conversion (Inyang et al. 2016;Lee et al. 2013;Ulrich et al. 2017). Now, depending on the physicochemical characters and polarity, it was found that biochar could adsorb PAEs through variety of interactions such as hydrogen bonding, pore diffusion and pore filling, π-π EDA interaction, and hydrophobic interaction (Julinová and Slavík, 2012;Ngo et al. 2015;Ye et al. 2021). However, physicochemical characteristics of the biochar solely depend on the feedstock types and pyrolysis temperature, as it regulates the physical properties (i.e., surface area and porosity) as well as the availability of functional groups on the surface (Li et al. 2020a, b;Ruan et al. 2019;Tripathi et al. 2016). Therefore, suitable feedstock type and pyrolysis temperature are required for the synthesis of efficient biochars which not only has large surface area and pore volume but should also hold multiple functional groups onto the surface for bringing the interactions between the adsorbents surface and the PAEs.
However, information on removal of PAEs by synthesizing low-cost carbonaceous materials, i.e., biochars using suitable feedstock type and pyrolysis temperature, is limited in the literature. It was reported that peanut shells have the potential to serve as an energy source (Tomul et al. 2020). The heating value of peanut shells (18.54 MJ kg −1 ) is greater than other residuals like, olive stones (17.88 MJ kg −1 ) and almond shells (18.20 MJ kg −1 ) (Perea-Moreno et al. 2018). Now, the conversion of energy from biomass was recognized as a feasible route for minimizing the emission of greenhouse gases. Perea-Moreno et al. (2018) projected, using peanut shells as the energy source might reduce CO 2 emissions by up to 18.22 k tonnes in China and 4.08 k tonnes in India. Various processes such as thermal gasification, torrefaction, and pyrolysis were established for converting biomass to energy. Among these processes, pyrolysis has the potential to produce important product, i.e., charcoal or biochar (Azargohar et al. 2019;Sewu et al. 2020). Here, peanut shells were used as raw material to synthesize biochar for remediation of PAEs from the aqueous systems. Dimethyl phthalate (DMP) was selected as the model pollutant because it is frequently identified in diverse environmental samples like, fresh water, and sediments, and high concentration of about 300 mg L −1 was reported in landfill leachate (Mersiowsky 2002). And USEPA has listed it as a priority pollutant (USEPA 1992) because it has a strong endocrine disrupting potential. Therefore, the specific objectives of our study are as follows: first, systematically examine the impact of pyrolysis temperature on the synthesis of biochars so as to interactions with the DMP; second, study the adsorption kinetics behavior of DMP with the synthesized biochars and CAC; third, analyze the adsorption mechanisms of DMP through adsorption isotherms study; and fourth, determine the thermodynamic behavior during interactions of DMP with the adsorbents. The findings from our study will provide useful information for the synthesis of highly effective biochar adsorbent and will also provide an economical pathway for the removal of DMP.

Materials
Peanut hulls were collected from hostel mess of Indian Institute of Technology, Bombay (IITB). All reagents used for experiment were of analytical grade. DMP (C 10 H 10 O 4 ; log K ow = 1.56; molar weight = 194.18 gmol −1 ; purity ≥ 99%), commercial activated carbon (CAC), and other chemicals, such as HCl and NaOH, were procured from Sigma-Aldrich, India. Deionized water was used for the preparation of all the reagents.

Synthesis of biochar
Biochars were produced from peanut hulls using pyrolysis at different temperatures. The development process of peanut shell derived biochar is illustrated in Fig. 1. A muffle furnace was used to maintain oxygen free environment. The name of the biochars was given as per their synthesis temperature. BC450 was pyrolyzed at 450℃, BC550 at 550℃, and BC650 at 650℃ with a heating rate of 5 °C min −1 for all pyrolysis processes to maintain slow pyrolysis condition. After synthesis, all biochars were washed with 1 M HCl and deionized water to remove impurities present in the biochar, then dried in hot air oven and stored in containers at 25℃ for future use.

Characterization
FTIR analysis was achieved to identify presence of functional groups on synthesized biochar using a HYPER-ION-3000 Spectrometer (Bruker, Germany). The FTIR spectra of all three types of biochar (i.e., BC450, BC550, and BC650) were plotted under wavelength of 400-4000 cm −1 . The X-ray diffractogram study of synthesized biochars was carried out by PANalytical (X'Pert PRO) X-ray powder diffraction system using the Cu-X-ray tube of wavelength 1.54 Å. Diffractogram of biochars was attained in the 2θ range of 5-90°. Surface morphology of all the three types of biochar was analyzed with the help of a Zeiss EVO 40 scanning electron microscope at an operating voltage of 20,000 kV mA −1 . A BET analyzer (Micrometric ASAP-2020 machine USA) was used to evaluate the surface properties of synthesized biochars (i.e., BET surface area, pore volume, and pore diameter). A CHN analyzer (LECO CHNS-932, Michigan, USA) was used to calculate the elemental composition such as, carbon, hydrogen, and nitrogen contents in the adsorbents. Combustion tube and reduction tube temperatures for elemental analysis were 1150 °C and 850 °C, respectively, while combustion tube alone was employed for O mode analysis. Finally, the pHzpc of the biochars was calculated using pH drift method (Debnath and Mondal 2020;Paunovic et al. 2019) as follows: a 50 mL solution of 0.1 M KNO 3 was added to several beakers, and initial pH values was maintained between 1 to 10 by adding 0.05 N HNO 3 and 0.1 N KOH. Certain amount of biochars was added into the flasks and kept for continuous shaking at a particular temperature. Afterwards, final pH values of the supernatant were used to calculate zero-point charge of adsorbents.

Batch adsorption studies
Adsorptions were performed in a batch reactor, a stock solution of 1000 mg L −1 of dimethyl phthalate (DMP) was prepared in deionized water. All the tests were completed in a reactor having volume of 1 L by adding 10 mg L −1 of biochar in 5 mgl −1 of DMP solution. The experiments were performed for 0.5-8 h using a mechanical shaker to promote homogenous interaction of DMP with biochar and then centrifuged at 10,000 rpm for 10 min. Now, final concentration of DMP in the solution was measured using the collected supernatant in an UV-VIS Spectrophotometer (Merck Spectroquant Prove 300) at 230 nm. Now, adsorption kinetics were checked by changing the contact time from 0.5 to 8 h. Other parameters such as concentration, dosage, and temperature were kept constant at 5 mg L −1 , 10 mg L −1 , and 25 °C. For the isotherm analysis, the concentration of DMP was varied from 5 mg L −1 to 25 mg L −1 and the other factors were kept constant. Similarly, pH was kept constant in all the experimental runs (7 ± 0.5) but it was varied only for determining the effect of pH on the adsorption performance. Further, the following equations were used to compute the quantity of DMP adsorbed per gram (g) of adsorbent at equilibrium (q e ) and at a specific time (q t ): where the initial and final concentrations of DMP are C 0 and C e , respectively (mg L −1 ). V is the solution's volume in liters, and m is the amount of adsorbent used (g). The removal rate of DMP was determined using the formula: Now, the experimental results were interpreted using different kinetic models such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion. Similarly, an isotherm analysis for adsorption was carried out using Langmuir and Freundlich models. Now, to examine the type of adsorption, thermodynamic study was also performed by varying the temperature of the solutions to 30℃, 40℃, 50℃, and 60℃. In addition, the possible adsorption mechanisms of DMP into the synthesized biochars were established.

Characterization of adsorbents
The surface area is one of the critical parameters studied for the characterization of adsorbents. Additionally, surface morphology, existence of functional groups, crystallinity of materials, their elemental composition, and pH ZPC may affect overall performance of the materials. Therefore, various analytical tools were used to characterize the materials and characterization results were discussed. Fourier  (Abdul et al. 2017), and a peak at 870 cm −1 could be due to out-ofplane C-H deformation of aromatic ring (Kim et al. 2013). It was also found that intensity of peaks in synthesized biochars has decreased with decrease in pyrolysis temperature; i.e., peaks in BC650 are more pronounced whereas peaks in BC450 are least pronounced. Thus, it can be concluded that higher pyrolysis temperature attributes to complete decomposition of organic matter into micro molecules and monomers. That is why, more sharp peaks were detected in BC650 as it was pyrolyzed at 650 °C unlike BC450 pyrolyzed at 450 °C. X-ray powder diffraction (pXRD) analysis helps in identifying crystallinity of materials as well as chemical composition and physical properties. The diffractogram of the biochars and CAC is represented in Fig. 3. A broad peak of diffuse bands could be seen at 2θ of around 26.56° due to reflection from the (002) (micro) graphitic surfaces Malhotra et al. 2018). This signifies development of improper stacking of turbostratic graphite materials in the pyrolyzed sample (Muniandy et al. 2014;Tomul et al. 2020). Interestingly, it was found that the peak intensity was higher for the adsorbents synthesized at higher pyrolysis temperature compared to the adsorbents synthesized at lower pyrolysis temperature. This high and low peak intensity represents stack of parallel aromatic layers and systematic alignment of individual layered structure, respectively (Hadoun et al. 2013). Therefore, it can be considered that with rise in pyrolysis temperature the layers between the stacked graphite layers are broken into individual sheets, resulting into the formation of graphitic-like carbon materials. Whereas, in CAC a broad diffraction peak at around 2θ = 24° and 42° was observed due to (002) and (100) planes, respectively (Song et al. 2013). Therefore, XRD pattern revels the amorphous nature of CAC and poorly graphitized.
Scanning electron microscopy (SEM) helps to examine surface morphology of materials. SEM images of biochars are represented in Fig. 4. This micrograph shows an irregular amorphous surface with porous network-like structure of pyrolyzed lignocellulose materials (Tomul et al. 2020). Therefore, the synthesized biochars contains pores and grooves, formed by the volatilization of volatile organic compounds during pyrolysis process resulting into improved surface area and surface porosity for the efficient adsorption of pollutants (Salman and Hameed 2010a, b, c).
Pyrolysis temperature plays a prominent effect on the physical as well as chemical properties of biochars. In this study, it was found that biochars synthesized at higher temperature having higher porosity and leads to higher surface area of the biochars. Decomposition of cellulose, hemicellulose, lignin, and pectins during high pyrolysis temperature helps to generate such porous structures. From the BET-N 2 sorption isotherms, it was found that surface area of BC450, BC550, and BC650 was 8.1 m 2 g −1 , 237.43 m 2 g −1 , and 365.38 m 2 g −1 , respectively. Similarly,

Fig. 3 XRD diffractogram of synthesized biochars and CAC
the pore volume of synthesized biochars was as follows: BC650 > BC550 > BC450. Therefore, rise in pyrolysis temperature might help to achieve higher surface area and pore volume in biochars (Elnour et al. 2019). In summary, large surface area and pore volume can encourage physical adsorption of DMP molecules resulting into trapping of DMP molecules into biochar surfaces. The results of BET surface area analysis are tabulated in Table 1.
In addition, biochar yield was calculated as a percentage of biochar output with respect to feedstock input (yield of biochar/mass of feedstock × 100%). It was found that with rise in pyrolysis temperature, ash content of the biochars increases as decomposition of organic matter is higher. These findings tell us that increase in pyrolysis temperature favors the formation of inorganic matters and graphitic carbon (Keiluweit et al. 2010). In this study, the ash content of peanut hull biochars was 2.1-3.77%. The CHNO (carbon, hydrogen, nitrogen, and oxygen) analysis revealed rise in carbon and nitrogen concentration and a fall in hydrogen and oxygen concentration from BC450 to BC650. This could be due to complete carbonization of feedstock materials leading to enhancement in surface area and pore spaces. The elemental composition and ash content of all three types of biochars are tabulated in Table 2.

Adsorption kinetic studies
Adsorption kinetic studies were performed by varying the contact time of the DMP to check the nature of adsorption. In this study, three established kinetic models were used, viz., pseudo-first-order, pseudo-second-order, and intraparticle diffusion to predict kinetics of the interaction happening in solid-liquid interface. The non-linear kinetic model equations were used to fit kinetic data and the equations are as follows: where k 1 (min −1 ), k 2 (g mg −1 min −1 ), and K id (mg g −1 min 0.5 ) are rate constants of kinetic models and q e (mgg −1 ) and q t represent the amount of DMP adsorbed on biochars at equilibrium and at time t, respectively. Table 3 displays kinetic fitting parameters calculated from the models, and the fitting curves of the models are demonstrated in Figs. 5, 6, and 7.
Intraparticle dif fusion ∶ q t = K id t 0.5 Therefore, from the non-linear fitting curves, it can be concluded that DMP adsorption data was fitting well with all the three models. However, on the basis of coefficient of  determination (R 2 ) and error functions analysis, the pseudosecond-order kinetic model was slightly superior than intraparticle diffusion and pseudo-first-order model and the similar trend was reported by Chen et al. 2019a, b, Malhotra et al. 2018and Salman et al. 2011a Hence, the rate limiting step of the process was governed by pseudo-second order kinetics, signifying that adsorption mechanism was chemisorption and is regulated by functional groups present on surface of biochars (Salman and Hameed 2010a, b, c). In addition, pore diffusion also plays a prominent role in the adsorption of DMP on porous biochar surfaces. The errors of kinetic models are showcased in Table 4.

Effect of contact time on DMP removal
Peanut shell-derived biochar was synthesized as an adsorbent at different pyrolysis temperature, and the effect of time was examined to find out equilibrium time and the removal of DMP using synthesized biochars and CAC. The experiments were performed for 0.1-8 h. It was found that with the increase in contact time, the removal rate of DMP for the all four adsorbents showed the same trend, which firstly increased in the initial 60 min and then elevated slowly till reaching equilibrium at 360 min. The rate of adsorption in the initial phase was high because of high concentration gradient and availability of empty sites in the adsorbent. Among the four adsorbents, the removal rate of CAC was higher followed by BC650, BC550, and BC450. This trend was reported because CAC and BC650 have the maximum surface area and pore spaces compared to other synthesized biochars. When the contact time was 6 h, it was found that the removal rate of CAC and BC650 was the almost same. The removal rate of DMP by four adsorbents was in the following order: CAC (79.6%) > BC650 (77.4%) > BC550 (70.4%) > BC450 (66.2%), and the corresponding order of adsorption capacities (q t ) was CAC (3.98) > BC650 (3.87) > BC550 (3.52) > BC450 (3.31). The effect of contact time for the removal of DMP is shown in Fig. 8.

Adsorption isotherm studies
An adsorption isotherm helps to know about solute-surface interaction and the amount of pollutant accumulates on the surface of adsorbent at given temperature (Kalam et al. 2021). In the present study, two standard isotherms, viz., Langmuir and Freundlich, were used to check experimental data. Langmuir isotherm helps to calculate maximum adsorption capacity (q max ) assuming monolayer adsorption of DMP on adsorbent surfaces (Langmuir 1918). The equation for Langmuir adsorption isotherm is represented as follows: where K L (Lmg −1 ) is Langmuir adsorption constant, q max (mgg −1 ) is Langmuir equilibrium capacity, and R 2 is model correlation coefficient. Similarly, Freundlich isotherm assumes heterogeneous surface having exponential distribution of active sites with multi-layer adsorption and non-uniform heat distribution over adsorbent surfaces (Freundlich 1907). The equation for Freundlich adsorption isotherm is represented as follows: where K f (Lmg −1 ) is the Freundlich adsorption capacity, 1∕n is the heterogeneity factor (i.e., intensity of adsorption), and R 2 is the model correlation coefficient. The fitting curves are shown in the supplementary information provided (Fig. S3-S6), and the associated parameters are showcased in Table 5. The coefficients of determination (R 2 ) for Freundlich isotherm were higher than those of the Langmuir model, signifying that DMP adsorption by both biochars and CAC was multilayered in nature. Now, this interaction was largely attributed by the π-π stacking effects of short carbon chain PAEs (i.e., DMP) and the biochar surfaces (Chen et al. 2019a, b;Doufene et al. 2019;Ye et al. 2021). In addition, values of Freundlich model constant (K f ) followed the order of CAC > BC650 > BC550 > BC450. Thus, the larger K f value indicates the higher q e and promotes spontaneous adsorption of DMPs (Ahmadi et al. 2016;Chen et al. 2019a, b;Doufene et al. 2019;Salman et al. 2011a, b).

Adsorption thermodynamics
A thermodynamic study was performed to calculate change in enthalpy, entropy, and Gibbs energy for DMP adsorption on the four adsorbents (i.e., BC450, BC550, BC650, and CAC). These parameters were calculated using the following equations: Negative Gibb's free energy ( ΔG • ) value represents DMP adsorption feasibility and spontaneous nature. Thus, with increase in solution temperature, the ΔG • was decreasing for all the adsorbents, suggesting the spontaneity in adsorption process, and similar results were reported by Malhotra et al. (2018) and Salman and Hameed (2010a, b, c). Further, change in enthalpy (ΔH) and entropy (ΔS) were calculated from the slope and intercept of the van't Hoff plot represented in Fig. 9.
The positive change in enthalpy signifies that DMP accumulation on adsorbent is endothermic in nature. That means with rise in temperature in the adsorption system the adsorption efficiency of DMP increases as formerly reported by Jing et al. (2018) and Salman and Al-Saad (2012). However, Abdul et al. (2015) reported that adsorption mechanism using carbon nanotubes is spontaneous but exothermic. The increase in adsorption capacity with increase temperatures might result from increase in kinetic energy of the molecules such that it will effectively diffuse within the pore spaces of adsorbents and their collision will favor various linkages such as H-bonding and π-π electron donor-acceptor (EDA) interactions between aromatic rings of DMP and biochars. In addition, the positive change in entropy also indicates DMP affinity for the adsorbent and increase in stochasticity at adsorption interface during DMP uptake; a similar trend was reported by Chen et al. (2019a, b) and Ganie et al. (2021). The outcomes of thermodynamic study are tabulated in Table 6.

Effect of solution temperature on DMP removal
As the thermodynamic study disclosed endothermic adsorption process, adsorptive removal percentage will increase with increase in solution temperature. It was found that at 303 K the removal rate of CAC and BC650 was higher compared to that of BC450 and BC550 because of having more pore spaces as BC650 was synthesized at higher pyrolysis temperature and the removal was around 80% as represented

Fig. 9
Van't Hoff plot to determine thermodynamic parameters of adsorption systems in Fig. 10. Interestingly, it was found that with increasing the temperature to 323 K, the removal percentage was sharply increased to 96.8% and 97.6% for BC650 and CAC, respectively. This is because as the temperature of the system increases, the movement of DMP will also increase, which lead to increase the frequency of their mutual collisions, and result in improved adsorption capacity and adsorption efficiency. And at 333 K, the removal was marginally increased to 97.2% and 97.8% for BC650 and CAC, respectively. Therefore, the adsorption system was reaching equilibrium at 323 K, and temperature plays a critical role to drive the adsorption of the DMP and regulating the process.

Effect of pH on DMP removal
Effect of pH for adsorption of DMP by the synthesized biochars and CAC was evaluated, and the results are plotted in Fig. 11. It was found that when pH increases from 3 to 11, the removal rate decreases for all the adsorbents. The order of maximum adsorption recorded for each adsorbent is as follows: BC650 (98.8%) > CAC (98.6%) > BC550 (94.6%) > BC450 (93.8%). Therefore, the maximum removal was shown by BC650 at an acidic medium, i.e., at pH 3. A similar result was reported by the earlier studies conducted for the adsorption of DMP using various adsorbents ). Now, reduction in adsorption behavior with increase in pH was might be due to the following reasons: at alkaline conditions, the medium has more OH − ions and it will turn the surface charge of adsorbent into negative. Now, the negative charge of phenolic group of the DMP and the adsorbent will create electrostatic repulsion and subsequently the rate of adsorption will get reduce. Similarly, at acidic conditions, the medium has more H + ions and the surface of the adsorbent will get protonated. And it will facilitate adsorption between positively charged adsorbent surface and DMP molecules having negatively charged phenolic groups on the surface (Doufene et al. 2019;Julinová and Slavík, 2012). From the pH ZPC analysis, it was found that for the synthesized biochars the pH ZPC was in the range of 3.9-4.2 and for CAC it was 6.4 (Fig. 12). The pH at which adsorbent's net surface charge is zero is designated as pH ZPC . The adsorbent's surface charge is positive at pH levels below pH ZPC , while at pH more than pH ZPC , surface will be negatively  charged, and also, maximum DMP removal was detected at pH 3. Thus, it can be concluded that surface of biochars is positively charged at pH lower than pH ZPC and favors adsorption of DMP. This confirms the above-mentioned findings of the study and also have good agreement with the earlier studies (Ahmadi et al. 2016;Doufene et al. 2019).

Effect of co-existing ions
In a real-world scenario, wastewater contains a lot of additional co-existing ions along with the model pollutant (i.e., DMP). Generally, co-existing ions interfere with the adsorbents as well as with the pollutants. It was reported that co-existing ion blocks the active sites of the adsorbents and changes the chemistry of the whole system (Pang et al. 2021). Therefore, it is important to study the effect of coions (cations: Ca 2+ , Mg 2+ , and Na + ; anions: Cl − , HCO 3 2− , SO 4 2− , NO 3 − , and PO 4 3− ) for the adsorption of DMP. The study was performed using different reagents such as NaNO 3 , Mg (NO 3 ) 2 ·6H 2 O, NaCl, Al (NO 3 ) 3 ·9H 2 O, Na 2 SO 4 , and Na 2 HPO 4 using BC650 as a potential adsorbent, and the results are shown in Figs. 13 and 14. The results revealed that around 81 to 91% removal of DMP is achieved in the presence of co-existing anions (initial concentration of 50 mg L −1 ), and the removal keeps on reducing with increase in the concentration. Thus, it implies the presence of competitive influence in the system between the co-existing anions and DMP. In addition, removal efficiency of DMP is less in systems having higher concentration of cations than anions. It was found that around 73%, 82%, and 77% removal happened for Ca 2+ , Na + , and Mg 2+ , respectively. The reason might be the repulsion between positively charged co-ions and protonated DMP molecules in acidic pH. Our results also found that among various anions, HCO 3 − showed the lowest removal efficiency due to its amphoteric nature leading to shifting the solution pH towards alkalinity (Ganie et al. 2021). In summary, greater the concentration of coexisting ions, more will be the competition for adsorbing into adsorption sites of the adsorbent.

Regeneration study
Desorption is an important step in the regeneration of spent adsorbents for further applications. In this context, regeneration efficiency of the adsorbent was quantified using the following equation: where q e, desorption and q e, adsorption (mgg −1 ) are the capacity of desorption and adsorption, respectively (Sen et al. 2021).
The study was conducted on BC650 as its showing maximum removal efficiency using 0.1-1 M NaOH and 0.1-1 M NaCl as the eluting agent. It was found that around 76.1 to 87.6% of regeneration achieved using 0.1, 0.2, 0.5, and 1 M NaOH concentration; similarly, 61.7 to 74.18% regeneration achieved under varied NaCl concentration, as represented in Fig. 15 Fig. 16. The decrease in removal efficiency is because of blocking of active sites by the adsorption of DMP in the initial cycles and also by the reduction in spent adsorbent properties during regeneration. Therefore, this study revealed the successful regeneration and reuse of BC650 up to two cycles and fruitful application of peanut shell derived biochar for the removal of DMP from aqueous solutions in a sustainable manner.

Possible mechanisms of DMP adsorption
From the kinetics and isotherm study, it was found that both physisorption and chemisorption processes are happening between DMP and biochars. In addition, from the IR spectra, it was found that biochar holds numerous oxygen containing Similarly, in acidic conditions, DMP molecules gets protonated and interact with the electron dense regions of stacked graphitic carbon and graphene layers (i.e., π-electron clouds) present on the surfaces of biochars. This generates π-π electron donor-acceptor (EDA) interactions, as illustrated in Fig. 19. Now, from the intraparticle diffusion model, it was clear that pore diffusion plays a significant role in adsorption of DMP into biochar pore spaces. High BET surface area and pore volume of biochars can greatly promote the DMP adsorption. DMP easily diffuses into the biochar pores as shown in Fig. 20. Therefore, both physical and chemical interactions are possible for DMP sorption (Hameed et al. 2009).

Conclusions
The efficacy of biochars synthesized from peanut hulls was investigated by checking the adsorption capacity and mechanisms for DMP. It was concluded that pyrolysis temperature plays a tremendous role in developing physicochemical characteristics of biochar as well as in adsorption performance. With the rise in pyrolysis temperature, the surface area, pore volume, and the aromatic carbon content of the biochars increase. The adsorption performance of DMP on the adsorbents was as follows: BC650 > CAC > BC550 > BC450 with an efficiency of 98.8% for BC650 followed by 98.6% for CAC at 333 K, pH 3 and after 6 h of contact time. From the adsorption isotherm analysis, it was found that Freundlich model (R 2 > 0.99) fits well compared to Langmuir, suggesting that adsorption was multilayered in nature. Similarly, based on coefficient of determination and error function analysis, it was found that among kinetic model's pseudo-second-order kinetics (R 2 > 0.998) fits well compared to pseudo-first-order, signifying chemisorption in the biochar surfaces. Also, the intraparticle diffusion model disclosed good fit with the adsorption results, exhibiting diffusion within pore spaces of biochars. Thermodynamic analysis revealed that adsorption of DMP on biochar surfaces was endothermic and spontaneous. In addition, from the adsorption mechanism, it could be concluded that the dominant mechanisms for the adsorption was the interaction that taking place between the DMP and the adsorbent such as H-bonding, π-π electron donor-acceptor (EDA) interactions, and pore diffusion. Overall, the outcome of the study might show a possible direction for the synthesis of low-cost porous biochar materials capable of adsorbing DMP.