Synthesis of hierarchical MgO based on a cotton template and its adsorption properties for efficient treatment of oilfield wastewater

A biomorphic MgO nanomaterial was fabricated via a facile and low-cost immersion method using cotton as the template. The obtained materials were characterized via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and N2 adsorption–desorption analysis. The as-prepared MgO retained the structure of cotton, with a porous hierarchical structure and a high specific surface area, which endowed it with great potential due to its excellent adsorption properties for the adsorption of additives in oil field wastewater. It also exhibited the maximum adsorption capacity of 391.36 mg g−1 for sulfonated lignite. The adsorption process of sulfonated lignite on biomorphic MgO was systematically investigated and was found to obey the pseudo-second-order rate equation and the Langmuir adsorption model. The negative values of Gibbs free energy change (ΔG) showed that the adsorption process was feasible and spontaneous. The endothermic process was depicted with a positive value for ΔH.


Introduction
With the rapid development of China's industry, there has been a rapid increase in exploration and exploitation drilling for oil and gas. The amount of drilling wastewater discharge is also growing, resulting in serious environmental problems. Drilling wastewater is industrial wastewater resulting from the process of petroleum drilling, which has a complex composition. It generally contains toxic and harmful substances such as dissolved and dispersed oil sediment, organic and inorganic chemical treatment agents, heavy metal salts, radioactive substances and bacteria, which accumulate in the effluent and in the environment. Furthermore, the impact on ora and fauna is serious due to the direct or chronic toxicity of these compounds and/or their degradation products. Therefore, it is very important to develop technologies to prevent further drilling uid contamination.
Sulfonated humic acid is a derivative of humic acid and it is also known as sulfonated lignite (SHA), which is formed by the agent (sulfate, sulte or bisulte), acid, alkali, formaldehyde, or other inorganic salts under appropriate temperature conditions. Sulfonated lignite is widely used in the drilling mud, which signicantly affects the biochemical oxygen demand (BOD), chemical oxygen demand (COD) 1 and sulfur content of drilling wastewater. Therefore, the study of economical methods for the treatment of sulfonated humic acid in oil eld wastewater is of great signicance for the protection of water resources and environmental health. To remove these pollutants from drilling wastewater, several conventional methods such as chemical precipitation, oxidation-reduction, electrochemical deposition, ltering, ion exchange, and adsorption have been applied. 2,3 Among these methods, adsorption, as a more competitive treatment process on an industrial scale, has become an attractive option for industrial wastewater treatment due to its exibility, simplicity of design and ease of operation. 4 Numerous advanced porous structure materials have been employed as efficient adsorbents, such as activated carbon. 5,6 However, its slow sorption kinetics, low regeneration rate and a limited temperature range for optimal operation limit its application in a wide range. Activated carbon is easily affected by the environment during adsorption and leads to adsorbed substances being easily desorbed and released, which again pollute the environment. Chitosan can chemically or physically entrap various metal ions due to the presence of amine and hydroxyl groups that can serve as the chelating and reaction sites. 7 However, it is difficult to directly apply raw chitosan in the removal of metal ions in wastewater treatment because of its disadvantages such as swelling, solubility in acidic conditions, and unsatisfactory mechanical properties. As a non-toxic, economical, and environmentally friendly material, MgO has already been widely used to treat wastewater 8 (e.g., for the removal of organic dyes, 9 toxic uorides, 10 heavy metal ions 11,12 and arsenic). 13 Despite these successes, the adsorption capacities of the reported MgO materials were not signicantly higher than those reported on other adsorbents. Commercial MgO with a small specic surface area and grain size is usually produced by the thermal decomposition of magnesium hydroxide or carbonate, which is disadvantageous for application in heterogeneous systems. 14 Recent research has shown that hierarchical metal-oxide nanostructured materials as adsorbents can afford remarkable advantages in environmental applications for water purication owing to their large surface areas, high available surface adsorption site density, special functionality, and well-dened morphology. The bio-diversity of natural materials endows us with a wide range of choices for templates with different microstructures. As such, biological templates have attracted signicant attention for the fabrication of porous structures. Compared with inorganic templates, biological templates can provide a well-dened morphology in the preparation of microand nano-materials. Moreover, the adsorbent synthesized by biological templates can possess a hierarchically porous structure with higher surface area and offers more active sites during the adsorbent process. For example, on employing sorghum straw as a biotemplate, biomorphic porous CaTiO 3 was prepared by the sol-gel method for adsorbing heavy metal ions from wastewater. 15 Zhang's group presented the controllable biomimetic fabrication of MgO complex nanostructures with the outstanding ability to adsorb dyes. 16 In this paper, we developed an effective strategy to synthesize porous hierarchical MgO using cotton as a bio-template. The method involves the impregnation of a matrix with an aqueous precursor solution and the adsorption performance was evaluated by the removal of sulfonated lignite in oileld wastewater. 17 Since the effectiveness of adsorption relies on the operational conditions, parameters that may affect the adsorption, including pH and adsorption temperature, were evaluated. Both kinetic and equilibrium isotherm models were applied to establish the rate of adsorption and the adsorption capacity. 18 The mechanism of scale-up can also serve as the base-line for adsorbent treatment of sulfonated lignite effluents.

Synthesis of the material
Magnesium acetate tetrahydrate (Mg(Ac) 2 $4H 2 O) was purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. Cotton was obtained from Yanggu Jingyangguang Sanitary Material Co. Ltd. Sulfonated lignite was purchased from Tarim, China. All the reagents were of analytical grade and were used as received without further purication. Distilled water was used for all syntheses and treatment processes.

Synthesis of biomorphic MgO
Typically, different dosages of Mg(Ac) 2 $4H 2 O were dissolved in 300 mL distilled water with vigorous stirring to form a homogeneous solution and then 6.7 g cotton was added to the solution. The mixed solution was aged for 24 h at room temperature and dried in a blast drying oven at 70 C for 12 h. Finally, the sample was calcined at 450, 500 C and 550 C for 3 h in an air atmosphere, respectively (with ramping rate 10/ min). The template was removed aer calcination and biomorphic MgO was thus obtained.

Characterization of materials
The morphology of the prepared material was elucidated using a scanning electron microscope (SEM, JSM-6390A, JEOL, Japan) with an accelerating voltage of 20.0 kV. The phase structures of the calcinated material were analyzed using an X-ray diffraction device (JDX-3530, JEOL, Japan) with an X-ray tube having copper (Cu) as a target and releasing K a radiation when accelerated at 40 mA and 40 kV, at 10-90 with a scanning speed of 2 min À1 . Surface area and pore property measurements were performed by N 2 physisorption using a Micromeritics ASAP 2010 instrument at 77 K. The surface areas were calculated using the BET equation in the pressure of range P/P 0 ¼ 0.02-0.2, and the pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method. The infrared spectra of the samples were measured in the form of KBr (ratio 1 : 100) powder pellets on a Nicolet 5700 Fourier transform infrared (FT-IR) spectrophotometer (Thermo Electron Co., USA) under ambient conditions.

Adsorption experiments
In a typical adsorption experiment procedure, a certain amount of adsorbent was added to 50 mL sulfonated lignite solutions with the initial concentration of 100 mg L À1 . The mixture was stirred at room temperature (303 K) for 2 h to ensure that adsorption equilibrium was obtained. Aer that, the sample solutions were ltered, the residual concentrations of sulfonated lignite were measured by a UV-vis spectrophotometer at a maximum adsorption wavelength of 300 nm. The effects of various parameters, such as the solution pH and the dosage on the adsorption capacity in the solution, were studied. The pH of the solution was adjusted with 0.1 mol L À1 HCl or 0.1 mol L À1 NaOH. For accurate adsorption results, experiments were repeated three times so the average values were represented. The amount of sulfonated lignite adsorbed was calculated based on the formula as given below: where C 0 and C e (mg L À1 ) are the initial and equilibrium sulfonated lignite concentrations respectively; V is the volume of solution (mL), and M is the mass of adsorbent (g). , indicating that pure MgO can be obtained via the thermal decomposition of the Mg(CH 3 COOH) 2 $4H 2 O precursor at 500 C. Moreover, the relatively broad diffraction peaks revealed that the MgO particles were small crystallites and the grain size was determined to be 11.7 nm by the Scherrer equation (D ¼ Kl/ B cos q). The intensity of the peaks revealed that both samples were crystalline, while the peaks of the biotemplated MgO showed some broadness as compared to commercial MgO, signifying a smaller crystalline size and good dispersion.

Results and discussion
3.1.2 Specic surface area and porosity analysis. To analyze the porous structure of the as-obtained MgO, N 2 adsorption-desorption measurements were carried out on the sample derived from the cotton template impregnated in 0.01 mol L À1 Mg(Ac) 2 $4H 2 O and calcined at 500 C. The N 2 adsorption-desorption isotherms and the corresponding BJH pore size distribution curves of the samples are illustrated in Fig. 2. The N 2 adsorption/desorption isotherm of the biomorphic MgO presented the typical type IV isotherm with a H3-type hysteresis loop, which indicated the presence of mesopores and the adsorption capacity increased rapidly at higher relative pressure due to the existence of macropores. Combined with the pore size distribution curve of biomorphic MgO, it was found that the pore structure distribution of MgO (Fig. 2) exhibited a large number of mesopores and some macropores, conrming the hierarchical meso-macroporous structure of the obtained biomorphic MgO. However, for commercial MgO a small hysteresis loop and narrow pore size distribution indicated its poor pore properties. The specic surface areas, average pore size and total pore volumes of the samples are listed in Table 1. The BET surface area increased from 12.51 cm 2 g À1 to 90.73 cm 2 g À1 and a signicant increase from 0.049 to 0.290 cm 3 g À1 in the mesopore volume was observed when we compared commercial MgO with biomorphic MgO. There was a signicant overall increase in the surface and porosity of the biomorphic MgO composite, indicating that the utilization of the bio-template technique resulted in the large surface areas of the biomorphic metal oxides.
3.1.3 SEM analysis. The morphologies of cotton ber, commercial MgO and biomorphic MgO prepared aer calcination at 500 C were investigated by SEM. The cotton had a long tubular microstructure as shown in Fig. 3a with an average diameter of about 10 mm. Fig. 3b illustrates the morphology of the biomorphic MgO. Compared with the diameter of the raw    cotton, an apparent volume contraction and collapse were observed, caused by the removal of carbon and volatile organic matter in the calcination process. The biomorphic MgO retained the tubular morphology of cotton and a diameter of about 5 mm by high-temperature calcination. Fig. 3c presents the image of commercial MgO oxides without the template, which revealed an irregular sheet-like structure that was stacked together in a disorderly manner.

Adsorption performance
3.2.1 The effects of different adsorbents. ZSM-5 and 3A molecular sieves, commonly used as adsorbents, were used to compare the adsorption performance with biomorphic MgO in sulfonated lignite solution. As can be seen from Fig. 4, the performances of the different adsorbents on the adsorption capacity of sulfonated lignite were evaluated at pH 6 at 25 C, and the initial concentration of sulfonated lignite was 100 mg L À1 . The maximum adsorption capacity was 86.58 and 84.94 mg g À1 with the maximum removal rate of 43.29% and 42.27% for ZSM-5 and 3A molecular sieves, respectively. The biomorphic MgO showed better adsorption activity and a 94.33% removal efficiency of sulfonated lignite was obtained, which indicated its good adsorption performance.

3.2.2
The effect of the calcination temperature. A signicant variable affecting the removal efficiency of adsorption material is the calcination temperature. The adsorption capacities of biomorphic MgO oxides prepared at the calcination temperatures of 450 C, 500 C, and 550 C were compared, using the following reaction conditions: pH ¼ 6, adsorbent dosage of 0.025 g, and initial concentration of 500 mg L À1 . As shown in Fig. 5, the percentage removal of sulfonated lignite increased from 87.04 to 93.88% when the calcination temperature increased from 450 to 500 C by the complete conversion of the precursor to MgO(O), while the percentage removal gradually decreased to 44.5% on further increasing the calcination temperature to 550 C. Compared with the pore properties as shown in Table 1, the biomorphic MgO with large pore size and average volume exhibited the best adsorption efficiency, which further conrmed the strong relationship between the pore size and adsorption property of the adsorbent.
3.2.3 The effect of the precursor concentration. The concentration of the metal salt in the impregnating solution is a vital factor for ascertaining the adsorption rate. The effects of the precursor concentration on the adsorption capacity of sulfonated lignite were investigated at the adsorbent dosage of 0.025 g, 25 C and initial concentrations of 100, 200, 400 mg L À1 . The results are presented in Fig. 6. The    experimental results indicate that the biomorphic MgO prepared with 0.2 mol L À1 magnesium acetate exhibited the highest adsorption capacity with the percentage removal of sulfonated lignite reaching 97.33%. Further increasing the concentration of the precursor resulted in the decrease in the removal rate, due to the increased concentration of metal salt ions causing the enhancement of solution viscosity by incomplete mixing with the template to block the pore structure of the cotton template.
3.2.4 The effect of adsorbent dosage. The biomorphic MgO concentration is a vital factor for ascertaining the adsorption rate. Fig. 7 shows the adsorption performance of biomorphic MgO investigated by altering the concentration range from 50 mg L À1 to 2 g L À1 at pH ¼ 6, the temperature of 25 C, and the initial concentration of sulfonated lignite was 100 mg L À1 . As seen from the results, the percent adsorption of sulfonated lignite with MgO(O) followed an increasing trend from 37.74 to 94.17%, attributable to the active and available adsorption sites on the porous materials. The adsorption efficiency, as well as the rate of adsorption, was signicantly improved when the adsorbent dose reached 250 mg L À1 at the sulfonated lignite concentration of 100 mg L À1 . This can be explained by the positively charged surface of the obtained MgO(O), which facilitates the accessibility of the sulfonated lignite to its large number of active sites and makes favorable conditions for sulfonated lignite adsorption even in the presence of a low adsorbent dosage. In this case, the adsorption on the surface is quickly saturated, showing a high uptake capacity of synthesized MgO(O). At higher particle concentrations, the absorption percent slightly decreased to 91.93%, which is due to the active sites exceeding the demand of the saturated adsorption and resulting in the reduction of the adsorption capacity. 19 It was concluded that a further increase in the adsorbent dose did not cause any signicant increase in the % removal of sulfonated lignite.
3.2.5 The effect of pH. The surface charge of the adsorbent is affected by the pH values of the sulfonated lignite solution, so it is very important to investigate the inuence of the hydroxyl/ hydronium ion concentration on the adsorption process. 20 The effect of the solution pH on the adsorption capacity of sulfonated lignite over the biomorphic MgO adsorbent was evaluated with the initial pH ranging from 3.0-9.0, the adsorbent dosage of 0.025 g, set temperature of 25 C and an initial concentration of 100 mg L À1 . The results are displayed in Fig. 8. The pH of the solution affected the adsorption performance of this adsorbent. The adsorption uptake of biomorphic MgO for sulfonated lignite decreased with the increase in the pH value to the maximum at pH ¼ 3, then it decreased from 194.3 mg g À1 to 77.3 mg g À1 with an increase in the pH. The isoelectric point (IEP) for magnesium oxide was about 10.5, 21 which could be because at low pH, the metal oxide surface may attain a positive charge due to the formation ions in a strongly acidic solution. Therefore the electrostatic attraction of the positively-charged metal oxide surfaces and negatively charged adsorbent molecule may lead to the increased uptake efficiency of the sulfonated lignite on the adsorbent. 22 However, as the pH increases, humic acid may be partially ionized in the solution, resulting in a series of anions, which are difficult to adsorb on the solid surface, thus resulting in the retardation of diffusion and adsorption. 23 3.2.6 Kinetics of adsorption. To analyze the adsorption rate of sulfonated lignite adsorption onto Mg(O), the kinetic data were modeled using the pseudo-rst-order, 24,25 pseudo-secondorder 26,27 and intra-particle diffusion models. 28 The linear equations of these three kinetic models are listed below: logðq e À q t Þ ¼ log q e À k 1 t 2:303 (3) Fig. 8 The effect of pH on the adsorption of sulfonated lignite over biomorphic MgO. Fig. 9 The effect of adsorption time on the adsorption efficiency. Fig. 10 The effect of adsorption time on removal efficiency.
This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 28695-28704 | 28699 Paper RSC Advances where q t is the amount of adsorption at time t; k 1 (min À1 ) and k 2 (g g À1 min) are the rate constants of the pseudo-rst-order and pseudo-second-order models, respectively; k di and c i are the intra-particle diffusion model rate parameter and the intercept of stage i, respectively. Sulfonated lignite adsorptions on biomorphic MgO at various contact times in solution with initial sulfonated lignite concentrations of 100, 150 and 200 mg L À1 are shown in Fig. 9. It can be clearly observed that the adsorption capacity was rapidly enhanced with the contact time increasing from 0 to 16 min. Aer 40 min of adsorption, the sulfonated lignite adsorption capacity on biomorphic MgO reached 189.63 mg g À1 , 279.38 mg g À1 and 386.34 mg g À1 , respectively. Further extending the contact time did not signicantly improve the sulfonated lignite adsorption capacity, indicating that the hierarchical porous structure can provide more available adsorption sites during the adsorption process (Fig. 10).
To further understand the adsorption process, the adsorption kinetics data were tted to the three models. The results of contact time for the sorption of sulfonated lignite onto the MgO(O) were tted with the pseudo-rst and pseudo-secondorder kinetic models and are shown in Fig. 11 and 12. From the slope and intercept of the straight line obtained, the kinetic parameters for the removal of sulfonated lignite were determined, as compiled in Tables 2 and 3. It is apparent that when the pseudo-rst-order kinetic model was used, the experimental data (q e ) were much higher than the calculated values, which could be attributed to the low correlation coefficients R 2 in the range 0.8856-0.9605. On the contrary, the pseudo-second-order model showed close agreement between the experimental and calculated adsorption capacity values and gave high R 2 values (R 2 > 0.99), indicating that this kinetic model is suitable for describing the adsorption behavior of sulfonated lignite onto the adsorbents in this study, which showed a monolayer adsorption system. To ensure the adsorption mechanism, the intraparticle diffusion model was investigated based on the resulting sorption data at varying contact times. The intraparticle model kinetics is shown in Fig. 13. Taken as a whole, the linear relations of q t versus t 1/2 are not good. The fact that none of the lines pass through the origin reveals that the intra-    Table 3 Adsorption kinetic parameters of the intra-particle diffusion model for the adsorption of sulfonated lignite onto biomorphic MgO Intra-particle diffusion model particle diffusion is not the rate-controlling step for the whole adsorption process. 3.2.7 Adsorption isotherms. The analysis of equilibrium data to construct adsorption isotherms is usually important for the design of adsorption systems. Adsorption isotherms express the mathematical relationship between the quantity of adsorbate and the equilibrium concentration of adsorbate remaining in the solution at a constant temperature. Several types of isotherms have been developed for the determination of the equilibrium adsorption capacity of the adsorbent. In this study, the Langmuir (eqn (6)), Freundlich (eqn (7)) and Dubunin-Radushkevich (eqn (8)) models were used to analyze equilibrium adsorption data. The linear forms of these models are expressed as follows: 29,30 D-R:ln q e ¼ ln q max À b3 2 where q max (mg g À1 ) is the theoretical maximum adsorption capacity, K L is the Langmuir constant related to the energy of adsorption, K F is the Freundlich constant related to adsorption capacity, 1/n is the empirical parameter related to adsorption intensity. b is a constant associated with the adsorption energy, and 3 is the Polanyi potential, which is equal to RT ln(1 + 1/C e ).
To evaluate the adsorption capacities of sulfonated lignite onto biomorphic MgO, we described the adsorbent-adsorbate interactions and adsorption mechanism. The adsorption isotherms at different temperatures while varying the initial concentrations from 200-500 mg L À1 are depicted in Fig. 14. It can be seen that increasing the experimental temperature led to an increase in the adsorption capacity. The maximum adsorption capacity increased from 912.10 to 935.98 mg g À1 with the temperature increase from 25 C to 35 C at the initial concentration of 500 mg g À1 . The results indicate that the adsorption reaction of sulfonated lignite adsorbed by biomorphic MgO is an endothermic process. This indicated that high temperature favored the adsorption of sulfonated lignite onto the active sites of Mg(O).
The isotherm constants of the Langmuir isotherm, Freundlich isotherm and D-R adsorption isotherm presented in Table  4 were calculated from the slope and intercept of linear plots obtained as shown in Fig. 15-17, respectively. It was concluded   31 The parameter R L indicates the nature of the shape of the isotherm: R L > 1 unfavorable adsorption, 0 < R L < 1 favorable adsorption, R L ¼ 0 irreversible adsorption, R L ¼ 1 linear adsorption. The value of R L in the present investigation was 0 < R L < 1; therefore, the adsorption process was very favorable and the adsorbent employed exhibited good potential for the removal of sulfonated lignite from aqueous solution. 32 Thus the Langmuir model is the most applicable for describing sulfonated lignite adsorption onto the biomorphic MgO adsorbent. 3.2.8 Adsorption thermodynamics. During any adsorption process, the changes in energy and entropy should be considered to determine what process will take place spontaneously and whether the process is spontaneous or not. The values of thermodynamic parameters have great meaning for practical application for adsorption processes. 33 Experiments were carried out using 500 mg g À1 of sulfonated lignite solution and the adsorbent dosage of 0.025 g at temperatures of 20, 30 and 40 C, respectively. The thermodynamic parameters Gibb's free energy (DG , kJ mol À1 ), enthalpy (DH , kJ mol À1 ) and entropy (DS , kJ mol À1 K À1 ) were calculated using the following equations: 34 where m is the adsorbent dose (g L À1 ), C e is the concentration of metal ions in solution at equilibrium (mg L À1 ), q e is the amount of metal ions at equilibrium in unit mass of adsorbent (mg g À1 ), q e /C e is called the adsorption affinity. R is the universal gas constant (8.314 J mol À1 K À1 ) and T is the temperature (K). The values of DG and DH can be calculated from the slope and intercept of the van't Hoff plot (ln q e m/C e vs. 1/T) shown in Fig. 18. The results of DG , DS and DH are shown in Table 5.
The results showed that theDG values are negative and their absolute values increased with temperature. This suggests that a high temperature is favoured for the adsorption of sulfonated lignite onto biomorphic MgO, which indicates a spontaneous adsorption process. Moreover, the decrease in DG as the temperature rises indicates that the adsorption was more favorable at higher temperatures, which is consistent with the results of the more efficient absorption capacity at higher temperatures as shown in Fig. 18. It is well known from the thermodynamics study that if DG values fall in the range of À20-0 kJ mol À1 , then it is the physisorption process, whereas the DG values found in this range support physisorption in Mg(O). This observation was further validated by the isotherm studies as described before. The positive values of DH suggest the endothermic nature of the adsorption of sulfonated lignite onto biomorphic MgO, indicating the endothermic nature of the adsorption process. The positive values of entropy DS show the increasing randomness at the solid/solution interface during the adsorption process. Also, the positive entropy of adsorption reects the affinity of the adsorbent for the sulfonated lignite. 35

Adsorption mechanism
To better understand the adsorption process, we studied its adsorption mechanism, the FTIR spectra of biomorphic MgO   before and aer adsorption are shown in Fig. 19. The peak near 1638 cm À1 could be attributed to the bending vibration of interlayer water molecules and the peak at 1434 cm À1 to the bending vibration of Mg(OH) 2 . 37 The FTIR spectra of biomorphic MgO aer adsorption showed new peaks at 1353 and 1385 cm À1 , ascribed to the S-O stretching of the sulfonate group in sulfonated lignite and the characteristic symmetrical bending vibration of the carboxylate COO-, suggesting the adsorption of sulfonated lignite onto biomorphic MgO due to the defects of magnesium (V, V À , V 2À ) and oxygen holes (F, F + , F 2+ ) on the surface of nano-magnesium oxide. 36 In conclusion, the adsorption of sulfonated lignite onto biomorphic MgO could be speculated to occur at the surface of the adsorbent through electrostatic attraction to sulfonated lignite as suggested in Fig. 20.

Conclusions
In summary, the biomorphic hierarchical MgO was synthesized through an immersion method using cotton as a bio-template. Cotton plays a crucial role in the formation of the biomorphic MgO hierarchical microstructures. The adsorption of sulfonated lignite onto biomorphic MgO depends on the time of contact, the solution pH, and the initial concentrations of sulfonated lignite solution, which followed the pseudo-secondorder kinetic model. In addition, under the conditions of 100 mg L À1 and 50 mL sulfonated lignite solution with 250 mg adsorption dose, the calcined sample showed better adsorption activity and 94.17% removal efficiency of sulfonated lignite could be obtained. The adsorption of sulfonated lignite onto biomorphic MgO was successfully t to Langmuir isotherm models over all the concentration ranges studied. The negative values for DG suggested that the adsorption was spontaneous in nature. The positive values for DH and DS indicated an endothermic adsorption process and increased randomness at the surface-solution interface, respectively. This facile preparation method and the improved properties derived from the cotton template are of great signicance in the synthesis and application of nanomaterials.

Conflicts of interest
There are no conicts to declare.  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 28695-28704 | 28703