Study of adsorption of Congo red on ferric oxide@straw magnetic composites

In this paper, citric acid-modified corn stalk and Fe3O4 were used to make magnetic straw adsorbent to realize rapid separation of adsorbent and wastewater. The influences of pH, dosage, reaction time, and initial stress on the Congo red adsorption rate were investigated. Then, we used adsorption kinetics and isothermal linear adsorption adjustment to explore the adsorption mechanism. Studies have shown the increasing doses of magnetic adsorbents on straw. When the dose was 8 g/L, the commercial adsorption rate was 95%. The adsorption process of magnetic straw adsorbent for Congo red was exothermic. Adsorption was mainly chemisorption, and the adsorption process was relatively fast, reaching adsorption equilibrium after about 90 min.


Introduction
In recent years, China's protection of the ecological environment has been gradually strengthened, and the Emission Standards for Water Pollutants in the Textile Dyeing and Finishing Industry have been revised, with more stringent requirements for the discharge of dye wastewater [1].Improper treatment of dye wastewater not only pollutes the ecological environment but also jeopardizes human health [2].In recent years, the main methods for treating dye wastewater include adsorption, chemical methods, biological methods, and membrane separation methods [3].Compared with other techniques, the adsorption method is an efficient and low-cost technology, which is widely used for the purification of dye wastewater.
China is a large agricultural production country, and behind the huge food production, there are mountains of straw [4].A large amount of straw is not effectively and reasonably utilized and is casually discarded and burned, resulting in a serious waste of resources, destruction of soil structure, atmospheric environmental pollution, and other problems [5].Corn stover itself has a large number of pore structures as well as groups, which are good materials as adsorbents.When the adsorption method is utilized to treat dye wastewater, to increase the adsorption sites of adsorbent, adsorbents with smaller particles are generally prepared to increase their surface area, which leads to difficulties in solid-liquid separation.As a new type of adsorbent material, Fe 3 O 4 nanomaterials can be easily separated from wastewater and desorbed adsorbent under the action of the external magnetic field, and the adsorption efficiency will not be significantly reduced.In addition, magnetic materials have the advantages of small particle size, good magnetism, and stable performance, which have attracted extensive attention from researchers.Sun found that the maximum adsorption of magnetic bentonite can reach 27.48 mg/g, which can be recovered or removed magnetically.Wang et al. used the co-precipitation method to prepare Fe 3 O 4 @SBc composites, which are superparamagnetic and can be used to achieve rapid solid-liquid separation using an applied magnetic field.Wei et al. utilized nano zero-valent ferromagnetic chitosan material made from chitosan, which increased the dispersion of the adsorbent and had a better removal effect of Cu(II) in solution.
The paper took corn stover as the research object, used sodium hydroxide and citric acid to modify corn stover, synthesized Fe 3 O 4 @straw magnetic adsorbent by co-precipitation method, and investigated the adsorption effect of Fe 3 O 4 @straw adsorbent on Congo red in wastewater.The results of the research provide reference experience for the efficient use of straw, the preparation of biomass adsorbent, and the rapid separation of adsorbent from wastewater.

Preparation of Fe 3 O 4 @straw magnetic composites
2.3 g of modified corn stover core was weighed and dissolved in distilled water, and ultrasonicated and shaken with an ultrasonic cleaner for 2 h to make it well dispersed.Subsequently, 4.34 g of FeSO4 and 8.42 g of FeCl3-6H2O solution were slowly added to the MCS solution and sonicated for 2 h.The temperature was raised to 60°C, ammonia was added dropwise until the pH of the solution was 9, and the solution was aged for 30 min and then sonicated for 1 h.The magnetic stover adsorbent was cooled down to room temperature, washed to neutrality by using distilled water and ethanol under the effect of an applied magnetic field, and then dried in an oven at 55°C.The magnetic modified corn stover core was milled to obtain the magnetic modified corn stover core.It was dried in an oven at 55°C and ground to obtain magnetically modified straw sorbent (MMCS).Figure 1 shows the IR spectra of CS, MCS, and MMCS, respectively.From the figure, it can be seen that the peaks of CS at 3354 cm -1 and 2916 cm -1 are the N-H and O-H telescopic vibration peaks on the carboxyl group, respectively; the absorption peak at 1601 cm -1 is the C=C telescopic vibration peak in the benzene ring structure of maize stover [6].The absorption peaks of MCS at 1640 cm -1 and MMCS at 1644 cm -1 are the N-H telescopic vibration peaks on the carboxyl group of stover.The absorption peaks of MCS at 1057 cm -1 and MMCS at 1053 cm -1 are the N-H telescopic vibration peaks on the carboxyl group of stover.The absorption peaks of MCS at 1057 cm -1 and MMCS at 1053 cm -1 are the stretching vibration peaks of the carboxyl group on straw [7]; the characteristic peak at wavelength 592 cm -1 corresponds to the stretching vibration peaks of the Fe -O group, which may be the successful magnetization of straw.

3.1.FTIR characterization analysis of CS, MCS, and MMCS
MCS and MMCS Compared with CS, MCS has obvious absorption peaks at 1640 cm -1 and 1057 cm -1 , which may be caused by ester bonding, and the ester bonding is the esterification reaction between citric acid and hydroxyl group on the corn stover cores, which can provide carboxylic functional groups for the corn stover cores' chemical structure and provide favorable conditions for the adsorption reaction.The MMCS peaks at 1644 cm -1 and 1053 cm -1 are carboxylic groups, which indicates that a large number of carboxylic groups are attached to the surface of Fe 3 O 4 corn stover adsorbent.1644 cm -1 and 1053 cm -1 peaks are carboxyl groups, which indicates that a large number of carboxylic acid groups are attached to the surface of Fe 3 O 4 corn stover adsorbent.Therefore, MCS and MMCS relied on the large number of pore structures of the corn stover cores themselves during the adsorption of the dyes, and on the other hand, utilized the carboxylic groups and other groups that were added after being modified to favor adsorption to improve the adsorption rate of Congo red.

Analysis of the effect of different influencing factors on the adsorption effect 3.2.1. Analysis of the effect of pH on the adsorption properties of MCS and MMCS.
As shown in Figure 2, both MCS and MMCS showed the best adsorption of Congo red at pH 2-4.A reason is because of the role of pH on the morphology of adsorbates present in Congo red solution, when acidity is strong, it is easier to get these ions decomposed into a free state, so it is easier to adsorb.Secondly, it is because at low pH conditions, it is easier to adsorb anionic dyes such as Congo red because of more hydrogen ions (H + ) in the simulated dye solution and therefore the surface potential of the adsorbent is positively charged [8].

Effect of dosage on the adsorption performance of MCS and MMCS.
As shown in Figure 3, the removal of Congo red by MCS and MMCS was similar.With the increase in the dosage of MCS and MMCS, the adsorption rate of Congo red by MCS and MMCS showed an overall increasing trend.When the dosage of MMCS and MCS was 8 g/L, the highest adsorption rates were 95% and 91%.However, when the dosage was greater than 8 g/L, the adsorption rates of MMCS and MCS on Congo red tended to stabilize.This is because the adsorption sites increased with the increase of the dosage of adsorbent, and the increase of adsorption sites is favorable for the surface adsorption and ion exchange between adsorbent and Congo red.The dosage of MCS and MMCS is too large, which will make the adsorbent agglomerated in the water, so that the specific surface area of the composite material decreases, and all of these factors will result in the difficulty of the adsorption rate of Congo red to be elevated again.

Analysis of the effect of reaction time on the adsorption properties of MCS and MMCS.
As shown in Figure 4, with the adsorption time getting longer, the adsorption rate of both MCS and MMCS on Congo red dye firstly increased continuously and then gradually stabilized after a certain time.The reason that MMCS reached the adsorption equilibrium earlier than MCS may be because the adsorbent increased a part of the adsorption site after the combination of the straw and the magnetism, and the adsorption amount of the Congo red of the MMCS was higher than that of the MCS at the same time, so the same concentration of the Congo red solution MMCS adsorbed faster than MCS.

Analysis of the effect of initial concentration on the adsorption performance of MCS and MMCS.
As shown in Figure 5, the removal of Congo red by MCS and MMCS was similar, both of them showed a rapid decrease in the removal rate when the initial concentration increased from 10 mg/L to 30 mg/L, and both of them reduced the removal rate of Congo red and slowed down the removal rate after 30 mg/L.The maximum adsorption rate of Congo red by MMCS and MCS reached the maximum adsorption rate at the initial concentration of 10 mg/L of Congo red dye, respectively.When the concentration of Congo red continued to increase, the active sites were fully occupied and thus no longer adsorbed excess Congo red ions [9], resulting in a lower adsorption rate.Therefore, both MCS and MMCS are suitable for the adsorption of Congo red solution with lower concentration, and 40 mg/L of Congo red solution was selected because the change in the removal rate of Congo red solution after the initial concentration of 30 mg/L was small.

Conclusion
The main purpose of this experiment is to investigate the structure of citric acid-modified corn stover core and Fe 3 O 4 @straw magnetic composites and to explore the effect on the adsorption properties of both under different conditions.The explored reaction mechanisms and conclusions are as follows: (1) Corn stover cores were modified with citric acid and then prepared into MMCS composite adsorbent materials, which were analyzed and characterized in terms of structure and function.MCS has mainly carboxyl and hydroxyl groups on its surface.MMCS has carboxyl, carbonyl, and metal groups on its surface.The analysis shows that MCS and MMCS successfully make the surface form abundant carboxyl groups, and metal oxide peaks appear on the infrared spectrum of MMCS [10], which means that Fe 3 O 4 @straw magnetic adsorbent composite is successful.
(2) Both MCS and MMCS have good adsorption effects on Congo red, but the pH has a greater effect on MCS, and both of them are suitable for adsorption reactions under acidic conditions.The adsorption process of MCS and MMCS on Congo red was exothermic, and the adsorption was dominated by chemisorption.The adsorption process was fast, and the adsorption equilibrium was reached after 120 min and 90 min for the adsorption of Congo red with MCS and MMCS respectively.The adsorption of Congo red by MCS and MMCS reached equilibrium after 120 min and 90 min, respectively.

Figure 2 .
Figure 2. Effect of pH on the adsorption properties of straw magnetic materials.

Figure 3 .
Figure 3.Effect of dosage on the adsorption performance of straw magnetic material.

Figure 4 .
Figure 4. Effect of reaction time on the adsorption properties of straw magnetic materials.

Figure 5 .
Figure 5.Effect of initial concentration on the adsorption performance of straw magnetic material.