High purity and analytical grade chemicals and reagents (K₂Cr₂O₇, NaOH, HCl, and ethanol), purchased from Shalimar Scientific Store, Pakistan, were used in this study. All glassware like conical flasks were washed with distilled water before use, and an adsorption experiment was performed using 500-ml Erlenmeyer conical flasks on the shaker (Orbi-Shaker™) filled with 250 ml of solution; the pH of the working solutions was adjusted using 0.1 M HCl/NaOH and measured by using a pH multimeter (pHTestr^®# Z-527831). The concentrations of methyl orange and chromium were calculated from the calibration curve using absorbance data by UV-visible spectroscopy (Genesys 20 spectrophotometer) at 540 and 460 nm wavelengths using the 1.5-diphenylcarbazide method.

Semi-dried poultry manure was collected from a poultry farm in Rawalpindi, Pakistan. Poultry manure samples were semi-decomposed in the botanical garden, University of Gujrat, Gujrat, Pakistan. Afterward, the oven-dried samples were passed through a sieve to produce a required particle size. The crushed material was taken into alumina crucible, covered with an aluminum sheet, then placed in a muffle furnace (Thermo Scientific™ FB1310M), was programed at 550°C for 3 h at a ramping rate of 10°C/min, and then, the crucible was left to cool. The final material was washed using ethanol and DI water, dried at 105°C overnight. The obtained yield of biochar was packed in an airtight plastic zip-lock bag and was later used in adsorption (Figure 1). The prepared carbonaceous materials (poultry manure biochar) were characterized by various advanced techniques. The structural properties were analyzed by using a scanning electron microscope (SEM; Hitachi S-3000N, Hitachi Scientific Instruments, Tokyo, Japan, and surface functional groups) and identified by Fourier transform infrared spectroscopy (Model: FTIR Spectrum 100 PerkinElmer) at the spectral range of 4,000–400 cm⁻¹. The Brunauer−Emmett−Teller (BET) surface area and pore size of the prepared biochar were analyzed by N₂ adsorption−desorption isotherms at 77 K using a Micromeritics (ASAP 2020; Norcross, GA, United States) analyzer.

To study the adsorption efficiency, a series of batch experiments were conducted using 250 ml of working solution in 500-ml volume conical flasks at neutral pH except for analyzing the effect of pH. The working solution was prepared by dissolving methyl orange powder and K₂Cr₂O₇ in deionized water. To evaluate the effect of adsorbent dosage, (1–3 g) sorbent with 100 mg/L pollutant concentration working solution was shaken in an orbital shaker at 125 rotations per minute agitation speed for 120 min at room temperature. Following the same way, a series of experiments were conducted using working solution with pH of 4, 5, 5.5, 6, 6.5, 7, 7.5, 8, and 9 adjusted with 0.1 M HCl/NaOH reagent to analyze its effect. For the time effect, the mixture was withdrawn at the desired interval (0–180 min) and afterward placed in centrifugation tubes at 1,000 rpm, and later, the supernatant was filtered using a 0.45-μm PVDF filter and analyzed by UV-Vis spectroscopy for adsorption efficiency. To investigate the effect of temperature, different experiments were conducted at 298, 303, 313, and 323 K temperature and analyzed for removal efficiency using 150 mg/L pollutant concentration at neutral pH and 2 g adsorbent dose for 120 min. In a similar mode, experiments were conducted with a different initial concentration of pollutants from 10 to 300 mg/L. For the reusability test experiment, 1 g of BC with pollutant loadings was mixed with 30 ml of the 0.1 M HCl/NaOH reagent; after centrifugation, the solution was filtered, and the concentration was analyzed in solution after desorption.

The maximum adsorption capacity and efficiency used as indicators for overall adsorption performance and removal efficiency were calculated using Eq. 1 and the maximum adsorption capacity qe (mg/g) from Eq. 2, where Ci and Ce correspond to initial and final concentrations of pollutants (mg/L), respectively. V indicates the working solution volume (L), and m is the sorbent quantity. % R =     ( C i − C e ) C i   × 100 , (1) q e = ( C i − C e ) × V   m . (2)

The surface structural properties of biochar have been investigated by scanning electron microscopy, and it has a rough surface with a scar-like heterogeneous structure (Figures 2A–D). This scar-like surface of the poultry manure biochar had a high porosity structure (Figure 2A), but the surface of BC was uneven and char-deposited (Figures 2C,D). Moreover, there are many large stomata-like pores and attachment sites on the surface of BC (Figure 2D), which corresponds to high surface area due to volatilization of organic matter in biomass. With the increasing surface area, the adsorption capacity also increases (Jin et al., 2018; Coleman et al., 2019).

The structure of the prepared biochar was analyzed by N₂ adsorption isotherms. S _BET was calculated from N₂ adsorption at 77 K using the Brunauer−Emmett−Teller (BET) method. However, the other physical properties such as pore volume and pore size are listed in Table 1, indicating that poultry manure-derived biochar has S _BET 16.0921 m²/g and pore size 1.84 nm from the BET method. The plot of the adsorption isotherm, pore volume distribution, and BET surface area are given in Figure 3. The experimental results indicate that poultry manure-derived biochar comprises micropores completely, which means it has a microporous structure. The volatilization of organic matter caused micropores in biochar. Generally, particles with size <2 nm diameter are microporous, 2–50 nm range mesoporous, and >50 nm are known as macroporous structures.

The surface structural properties and functional groups were identified by Fourier transform infrared (FTIR) spectra. The biochar is used as an adsorbent material for treating wastewater as a high surface area and hole-like structure can perform an essential role in surface adhesion of methyl orange and chromium (Nguyen and Oh, 2019). The IR spectra of the poultry manure-derived biochar are shown in Figure 4. The IR spectra of PMB represent that it has diverse functional groups across the different ranges of absorption peaks. The strong peak between 3,500 and 3,300 cm⁻¹ appeared after adsorption was assigned as the −OH group compared to before adsorption. The C=C double bond referred to the absorption peak at 1,630 cm⁻¹ after adsorption and the peak strength reduced at pyrolysis temperature of 550°C compared to the previous study of Zolfi Bhovariani et al. (2019) at a lower temperature biochar. The weak peak was found at 1,406 cm⁻¹, which was changed to 1,412 cm⁻¹ after adsorption assigned to −CH₂ based on the aldehyde group. The strength of the peak was enhanced due to higher sorption of pollutants. Moreover, the peak between 1,000 and 1,100 cm⁻¹ was known as the deformed C–O bond. The double conjugated bond C=O appeared at 1,650 and 1,500 cm⁻¹ (Mahamad et al., 2015). The band at 870 cm⁻¹ is C–H bond vibration in aromatic compounds; this peak vanished after adsorption, and vibration between 700 and 800 cm⁻¹ has been assigned as the C–H bond in hetero-aromatic compounds (Lustosa Filho et al., 2017; Daramy et al., 2020). A weak absorption peak between 450 and 700 cm⁻¹ corresponds to Si–O–Si.

The contact time between the sorbent and adsorbate is a critical parameter in the adsorption process to evaluate the capacity of the prepared sorbent; it gives information about how much time is needed for the maximum adsorption of the pollutant which can help in developing wastewater treatment. To evaluate the time effect, a series of experiments were conducted under different times of contact (from 0–180 min), at same conditions of 2 g/L adsorbent dose, neutral pH, and at room temperature in an orbital shaker under 125 rpm of agitation speed. The results of the contact time effect are shown in Figure 5D. The calculated values of removal efficiency for MO and Cr were 85.48 % and 88.5%, respectively, and the adsorption capacities qe were 10.68 and 11.06 mg g⁻¹ for MO and Cr, respectively. As the time increased from 40 to 120 min, the removal rate increased rapidly to maximum 85.48 % and 88.5% from 47.9 % and 48.35% for MO and Cr, respectively, and further contact did not catalyze the efficiency. It is conferred that the removal efficiency of methyl orange and chromium increased as the time of contact increased and then became stable reaching at 120 min to validate the results in the literature. Liu et al., (2020) confirmed that the adsorption capacity of polyethyleneimine-modified corncob biochar increased for removing MO and Cr synergistically as the contact time increased. The adsorption reached to an equilibrium stage when 120 min of contact time passed, and the values of removal efficiencies were described earlier. Upon elucidating the results, it was seen that maximum sorption occurred rapidly in the first 120 min, and these data were used for kinetic study, as discussed in the adsorption kinetics section. Generally, it is known that increasing contact time have an impact on getting high adsorption rate but up to a limit because active sites on the surface of biochar become saturated, and no further adhesive behavior was showed by the sorbent (Coleman et al., 2019; Jang and Kan, 2019). Thus, based on the experimental results, the optimum contact time for the conducted study was 80 min for the maximum adsorption efficiency.

The quantity of the added dose of the sorbent has significant effect on the adsorption capacity investigated and is shown in Figure 5A. The removal of MO and Cr was seen to increase as the sorbent dose increased. In the experimental work, adsorbent dosages of 1, 1.5, 2, 2.5, and 3 g were used. The maximum removal reached to 79.17 % and 74.54% for MO and Cr at 2.5 g. Increasing the dose further does not improve the removal, while according to the results of Ghorbani-Khosrowshahi and Behnajady (2016), by increasing the adsorbent dose to 0.4 g from 0.2 g, the adsorption rate also increased. The improvement in removal referred to high attachment sites on the BC surface, and driving force initially caused diffusion of pollutants to the biochar surface (Pan et al., 2016; Bardestani et al., 2019; Sun et al., 2019). Adding more adsorbent did not improve the efficiency of MO and Cr removal due to surface saturation between the adsorbed ions and solution. On the basis of the obtained results, the optimized dose of the sorbent was considered to be 2.5 g. Other studies conducted by Cheng et al., (2021) confirmed that the efficiency improved from 36.4 to 93.3% as the dose increased toward 4 g of the sorbent.

In Figure 5B, the pH effect on the efficiency of MO and Cr removal is shown. A series of experiments conducted across the range of 4.0–9.0 to delineate the pH dependence of the adsorption process and other factors remain fixed. The results revealed that the PMB adsorption efficiency was maximum at 88.80 % and 90.23% for MO and Cr, respectively, at the acidic pH. As it becomes alkaline, the efficiency decreases to 39.88 % and 38.19% at 9.0 pH, respectively, and the results by Dong et al., (2021) showed that the capacity of adsorbent material reached the maximum in acidic pH. This could be due to surface species on the adsorbent surface. Therefore, the optimal pH for higher results is found to be 4.0. The pH nature has a significant effect on the functional groups of sorbent materials. The high efficiency at the acidic pH may be described by the presence of H⁺ ions and charge of the sorbent material, which have significant impact on the electrostatic interaction of the sorbent material with the pollutant molecule (Alshameri et al., 2014). The literature reported that acidic pH between 2 and 6 Cr exists in the form of HCrO₄ ⁻ and Cr₂O₇ ²⁻, which can be rapidly adsorbed onto the BC surface by an electrostatic interaction because the solution has H⁺ ions; so it increases the attraction between the sorbent and adsorbate material. Acidic pH causes protonation of the sorbent surface functional groups which resulted in high electrostatic interaction. At the alkaline pH conditions, deprotonation of the surface functional groups occurs, which means that there is a higher density of OH⁻ ions, thereby indicating that higher negative charges on the surface of the sorbent material might affect the pollutant molecules to attach and induce competition of adsorption for CrO₄ ²⁻. Moreover, it produces electrostatic repulsion among similarly charged ions which resulted in lower adsorption. The removal rate of Cr (VI) decreases as the pH of a working solution becomes alkaline pH, as reported by Dong et al., (2017). The main redox reactions in the adsorption mechanism are as follows: Cr 2 O 7 2 - + H 2 O → 2 HCr O 4 - , (3) HCr O 4 - → Cr O 4 2 - +  H + , (4) 2 Cr O 4 2 - + 2 H + →  Cr 2 O 7 2 - +  H 2 O , (5) Cr 2 O 7 2 - +   14 H + + 6 e - → 2 Cr 3 + + 2 H 2 O, (6) Cr O 4 2 - + 4 H 2 O + 3 e -   → Cr ( OH ) 3 + 5 OH - . (7)

The initial concentration of the working solution has significant effect on the removal of MO and Cr, so a series of experiments were conducted using 10–300 mg/L concentrations (Figure 5C). According to the results, highest efficiency reached to 87.94% and 89.87% for MO and Cr, respectively, at 10 mg/L, and when the concentration of 300 mg/L was used, the efficiency become low reaching 50.034% and 50.45% for MO and Cr, respectively. These results supported by the study of Ghorbani-Khosrowshahi and Behnajady (2016) indicated that an increase in the initial concentration impacted the adsorption efficiency to decrease. These results could be described as at a low pollutant concentration, there is a high surface area, and attachment sites are present to attach the large number of molecules, which resulted in high efficiency, while the efficiency decrease as the pollutant concentration become high. This occur because of the competition among molecules to attach on the restricted surface of the sorbent material (Oh and Seo, 2016). Many previous studies reported that the saturated pollutant concentration caused reduction in the overall efficiency due to the attachment competition on the sorbent material (Kang et al., 2019).

Figure 5E showed the effect of temperature on the adsorption of MO and Cr. In this work, a series of experiments were conducted at 298, 303, 313, and 323 K temperature using the 150 mg/L initial concentration and 2 g adsorbent dose at neutral pH for 120 min. From the experimental results, increase in temperature improved the removal efficiency. When the temperature increased from 298 to 323 K, the removal efficiency reached to 76.02% and 77.57% from 56.47% and 56.05% for MO and Cr, respectively. These results indicated that an increase in temperature of the adsorption reaction may increase the kinetic energy of Cr₂O₇ ²⁻ and SO₃ ⁻ ions to the biochar’s affinity level, which leads to active attachment of both the pollutants on the BC surface, and the endothermic adsorption process occurred, and this description can be supported by (Cheng et al., 2021).

Langmuir, Freundlich, and Tempkin models were plotted for isotherm studies by Origin Pro 8.1 (Origin Lab, United States); nonlinear (Eqs 8, 11, 13) and linear (Eqs 9, 12, 13) forms of models are given in the following equations.

The Langmuir equation is as follows: q e = q m K L C e 1 + K L C e , (8) 1 qe =   1 K L qmax . 1 Ce + 1 qmax , (9)   R L =   1 1 + K L . C e . (10)

The Freundlich equation is as follows: q e = K f C e 1 / n , (11) Logqe = Log Kf +   1 n LogCe . (12)

The Tempkin equation is as follows: q e = R T b T l n a T C e , (13) qe = RT bT lnAT + ( RT bT ) lnCe . (14)

In the aforementioned equations, q _max indicates the monolayer sorption capacity (mg/g), Langmuir constant K_L (L/mg) describes the affinity of the adsorbent toward the adsorbate, and R_L is the separation factor, while in the Freundlich linear equation, Kf corresponds to the Freundlich constant (mg/g or L/mg) which quantifies the adsorption strength, n is the adsorption intensity parameter, the Tempkin model has the AT binding constant (L/g), bT indicates the Tempkin constant, and R indicates the universal gas constant (8.314 J/mol/K) at T temperature of 298 K.

All the values of the parameters of linear and nonlinear fitting of isotherm models are shown in Table 2. The Freundlich plot logCe vs. logqe, Langmuir plot 1/Ce vs. 1/qe, and a plot of lnCe against qe for the Tempkin isotherm model and nonlinear fitting are given in Figures 6A–D, respectively. The values of maximum adsorption capacity (q _max) R_L and R ² , calculated by the Langmuir model, are presented in Table 2. The correlation coefficient (R ² ) of MO and Cr from the linear Langmuir model was considerably higher (R ² ≥ 0.99; ≥0.99) than that of both the Freundlich (R ² ≥ 0.932; ≥0.935) and Tempkin models (R ² ≥ 0.97; ≥0.977). The coefficient value for the Freundlich isotherm was less than the Tempkin model. According to both the linear and nonlinear fitting, the best fit of the Langmuir model indicated that monolayer physical sorption occurred, and it better described the MO and Cr adsorption on PMB (Figure 6B). When seen, the heterogeneity constant (1/n) values from the Freundlich model were 0.564 and 0.536 for MO and Cr, respectively (if 1/n = 1 means linear adsorption, 1/n > 1 chemical sorption, and 1/n < 1 physical favorable adsorption take place) (Zeng et al., 2019). Another important parameter R_L was calculated to evaluate using Eq. 10; the adsorption efficiency and the value of separation factor R_L were 0.177 and 0.139 for MO and Cr, respectively, which meant that sorption is favorable (when R_L is 0, it indicates irreversible, and when it is great than 0 and less than 1, it means sorption is favorable, and when it is equal to unity, it is the linear form of sorption). The pollutants being adsorbed produces the monolayer on the surface of the sorbent material, and it becomes limited. When all the attachment sites are filled, no more adsorption occurred. The Langmuir model fit best with the sorbent which has limited surface attachment sites (Huang et al., 2016; Coleman et al., 2019). The maximum adsorption capacities (q _max) derived from the linear fit Langmuir model for MO and Cr were 20.8 and 19.09 mg g⁻¹, respectively, and the nonlinear fitting derived values are 20.8 and 20.33 mg g⁻¹, respectively.

The adsorption kinetics of methyl orange (MO) and chromium (VI) were analyzed by fitting the kinetic models and calculating the correlation coefficient R ² values to check the higher fitting of the model.

The linear and non-linear pseudo–first-order is as follows: ln ( q e − q t ) = ln ( K 1 q e ) − K 1 t , (15) qt = qe ( 1 -  exp - KLt ) . (16)

The linear and nonlinear pseudo–second-order is as follows: t q e =   1 K 2 q e 2 +   t q e , (17) q t = K 2 . q e 2 . t 1 + K 2 . q e . t . (18)

Generally, the pseudo–first-order (PFO) state, the initial stage of the adsorption process, and the pseudo second-order model (PSO) are applied for the understanding of the whole adsorption process based on the adsorption capacity. The linear and nonlinear forms of both the models are presented in Eqs. 15–18. The plot fit of the nonlinear and linear pseudo–first-order and pseudo–second-order models are given in Figures 7A–D. From the experimental data, the value of K₁ was acquired from the slope method of the linear plot of ln (qe-qt) vs. t, and the values of K₂ and qe were determined by the slope of the linear plot of t/qt vs. t. (Figures 7C,D). The nonlinear pseudo second-order model is highly fitted with the experimental data with a high correlation coefficient R ² value of 0.97 and 0.974 for MO and Cr, respectively, than PFO as compared to nonlinear fitting (Figure 7D). The values of K₂ are 71.37 and 57.49 for MO and Cr, respectively (Dhiman and Kondal, 2021). These fitting results suggested that closed chemisorption occurs, which involve electron sharing between the pollutant and biochar. All the detailed values of other important parameters of the kinetic models are presented in Table 3.

The reaction temperature has significant effect on the adsorption efficiency. In this work, the adsorption experiment was conducted in temperature from 298–323 K and analyzed its effects on the adsorption process by calculating the thermodynamic parameters like change in Gibbs free energy change ΔG°, heat of enthalpy ΔH°, and reaction entropy ΔS° change. The values of all the thermodynamic parameters are presented in Table 4, and the Van’t Hoff plot is given in Figure 9. The Gibbs free energy change was calculated using the Van’t Hoff Eq. 19 Δ G ° = − R T l n K c , (19) K C =   C i − C e C e . (20)

Here, R represents the universal gas constant, T is the temperature, and Kc is the thermodynamic equilibrium constant (calculated using Eq. 20). Moreover, ΔH° and ΔS° are calculated from the slope method using Eq. 21. ln ⁡ K c Δ S ° R − Δ H ° R T . (21)

All the values of ΔG° of both the pollutants are negative, which indicate that the adsorption process was spontaneous and favorable at higher temperature. The value of change in heat of enthalpy of the adsorption reaction greater than 0 means the adsorption is endothermic, and irreversible reaction occurred. These values exceeding the range of physical adsorption heat values (8.37–62.8 KJ/mol) correspond that chemical sorption occurs as the hexavalent chromium reduced to trivalent chromium (Yu et al., 2018). The values of ΔS° are also greater than 0, which indicate that the arbitrary movement of ions occurred and the freedom of degree of pollutants increased with temperature (Guo et al., 2020). These results confirmed that the whole adsorption process was endothermic and favorable at high temperature.

In the literature, adsorption is a widely used method to treat dye and heavy metals from the aqueous solution, isotherm, and kinetic modeling being used primarily to investigate the mechanism of adsorption. To delineate the adsorption mechanism of MO and Cr onto the biochar isotherm, the kinetic models were applied, and the important parameters were calculated. Tables 2, 3 showed the values of all important parameters. According to the experimental fitted data, the mechanism followed the pseudo–second-order kinetic model which corresponds to the adsorption of MO and Cr via the chemisorption method, involving the covalent interactions between the pollutant and surface functional groups (−OH, −COOH, and C=O) because electrons were shared to Cr (VI) for the reduction to Cr (III) form, whereas the Langmuir model highly conform the data which indicate the physical sorption of MO and Cr. From the results, optimum conditions for high adsorption of both the pollutants was found to be acidic pH of 4.0 because of higher electrostatic attraction between the pollutant and biochar surface. At an acidic condition, the surface charge and protonation of the surface functional groups enhance the adsorption capacity. The oxygen-containing functional groups such as −OH and Si−O−Si provide electrons to hexavalent chromium and reduce it to trivalent chromium ions, which may be released into the working solution.

From the FTIR results, it can be seen that the presence of oxygen-containing functional groups attracts more HCrO₄ ⁻ and Cr₂O₇ ²⁻ compared to SO₃ ⁻ of MO and electrostatic affinity of HCrO₄ ⁻ and Cr₂O₇ ²⁻ toward functional groups, which is higher (Mahamad et al., 2015). Guo et al., (2020) reported that the reduction of hexavalent chromium to trivalent chromium is an electron sharing process under acidic conditions due to presence of hydroxyl and aromatic functional groups. Thus, following this mechanism, a following reduction path Eqs 22, 23 may occur. R - O H 2 + + HCr O 4 - →  R - OH 2 Cr O 4 - +  H + , (22) H C r O 4 − + 3 e − + 7 H + → C r 3 + + 4 H 2 O. (23)

After the adsorption process, a peak appeared at 3,310 and 870 cm⁻¹, while the peak 2,104 cm⁻¹ slightly shifted to 2,108 and 1,406 cm⁻¹ to 1,412, 1,018 to 1,016 cm⁻¹, and 706 cm⁻¹ to 650 cm⁻¹, respectively.

Reusability of an adsorbent is an important property as the prepared sorbent proved high adsorption efficiency for MO and Cr simultaneously. The desorption efficiency of the two reagents is shown in Figure 8A, in which 0.1 M NaOH had a higher efficiency than 0.1 M HCl, which reached up to 63.095 % and 71.037% for MO and Cr, respectively. Hence, 0.1 M NaOH was used as the desorbing agent, and the BC was reused for adsorption again. Figure 8B showed the number of reusability cycles of the poultry manure biochar after being desorbed using 0.1 M HCl, for the removal of MO and Cr using 100 ml of solution. The biochar was reused for three cycles after desorption. The efficiency of the sorbent decreased by 6.5%, 7.59%, and 23.3% for MO and 5.9%, 16%, and 22.75%, for Cr, respectively after the 1st, 2^nd, and 3rd cycle compared to the adsorption efficiency before the desorption experiment, while in the study of Liu et al., (2020), the efficiency of the adsorbent decreased up to 3% after reuse. These results proved the good reusability of the prepared poultry manure-derived sorbent. The desorption efficiency was calculated according to Eq. 23. % Desorption = ( V d × C d ) ( C i − C e ) × 100.

In the aforementioned equation, Vd indicates the desorbing agent volume (ml), Cd is the concentration of the pollutant after desorption (mg/L), and Ci and Ce are the initial and final concentrations of the pollutant after adsorption (mg/L), respectively.