Investigating the hexavalent chromium removal from aqueous solution applying bee carcasses and corpses modified with Polyaniline

There are currently heavy metals in most industrial effluents which are among the most significant environmental pollutants. Hexavalent chromium is one of the most significant heavy metals. In this research for the first time, eliminating the hexavalent chromium from the aqueous medium/aquedia applying bee carcasses and corpses modified with polyethylene was examined. Adsorption experiments were conducted discontinuously on laboratory solutions, including hexavalent chromium. The optimal adsorption conditions such as different pH factors, contact time, initial chromium concentration, and adsorbent value on the adsorption rate were examined at different levels, and adsorption isotherms were plotted. Some adsorbent properties were examined using Field Emission Scanning Electron Microscopy, XRD analysis, Fourier Transform Infrared Spectroscopy, and BET test to study the properties of the synthesized adsorbent. This study indicated that the highest percentage of removal related to polyethylene composite and bee carcasses in the presence of polyethylene glycol was 50.56% among the bee carcasses composites. The parameters effective on the adsorption process for polyethylene composite and bee carcasses and losses in the presence of polyethylene glycol suggested that the adsorption percentage increased for this composite by decreasing the pH, increasing the contact time, and increasing the adsorbent. The highest percentage of adsorption was obtained when the pH was 2, the contact time was 120 min and the adsorbent value was 8 g/L and the initial concentration of chromium was 100 ppm. The most optimal removal percentage was achieved at the pH = 2, the contact time was 30 min, and the adsorbent value was 2 g/L, and the initial chromium concentration was 100 ppm. The results of drawing adsorption isotherms also indicated that higher R2 had a better fit than Langmuir for polyethylene composite and bee carcasses in the polyethylene glycol Freundlich equation.


Synthesis methods.
Preparing bee carcass and waste and polyaniline in the presence of polyethylene glycol. 0.9 g of an oxidizing substance, including potassium iodate as an oxidant and 0.7 g of Polyethylene Glycol (PEG), was mixed in 150 mL of 1 M sulfuric acid solution obtained by combining 53.3 mL of pure sulfuric acid with 947 mL of distilled water in each experiment using a shaker. In another container, 1 g of bee carcasses and wastes were mixed with 1.5 mL of distilled aniline. It was transferred to the solution container and mixed on a shaker at room temperature for 5 h after soaking the contents of this container. The solutions were then removed from the shaker and passed through filter paper. The product was then washed several times with distilled water and dried in an oven at 50 °C for 48 h.
The method to separate the Chromium using composites synthesized with bee carcasses and wastes. We provided a standard solution of 50 ppm chromium after providing the synthesized composites and adsorbents. A complete discontinuous mixing system was adopted to adsorb the heavy metal. Accordingly, 0.1 g of adsorbent (composites synthesized with bee carcasses and wastes) was added to 50 mL of 50 ppm chromium solution, and a shaker was used to mix it for 30 min with a shaker (magnetic stirrer) at a specified speed of 120 rpm. Then, we passed the solution through filter paper and the chromium concentration in aqueous media was determined by using the UV-Vis spectrophotometer 28 . The parameters regarded in this study include the amount of used adsorbent, contact time for perfect mixing, pH, and initial concentration of chromium.
Freundlich and Langmuir adsorption isotherms. Freundlich and Langmuir's isotherms are proposed as follows: www.nature.com/scientificreports/ and: C e is the concentration of metal in the liquid phase after mixing time (mg/L), n and k are Freundlich constants, X is the amount of adsorbed material per unit mass of adsorbent (mg/g), which indicated the intensity and capacity of adsorption, respectively. b is the Langmuir constant, which indicates the absorption energy, and X m is the maximum amount of adsorbed material per unit mass of adsorbent (mg/g). In this study, different amounts of adsorbent were added to a solution of 50 mg/L chromium (VI) at ambient temperature.
Investigating the chromium recovered by Polyaniline. 0.1 g of synthesized polymer (composites synthesized with bee carcasses and wastes) was added to 150 mL of chromium-containing solution at a concentration of 50 ppm, and a shaker (magnetic stirrer) was used to mix it for 30 min at 120 rpm. After mixing, the solution was passed through a sieve, and 0.1 M sulfuric acid of the total adsorbent left on the filter paper was added to 50 mL and mixed on a magnetic stirrer for 30 min. Presently, after mixing, we filtered the solution through a filter paper and measured the concentration of chromium ion in the solution. The total adsorbent left on the filter was then added to 150 mL of 50 ppm chromium in the next step and mixed for 30 min in a shaker (magnetic stirrer). After final mixing, we measure chromium ion as follows.
C 0 is the initial concentration of chromium, C e is the final concentration of chromium after separation, and C t is the chromium concentration in sulfuric acid solution 0.1 M in this equation.

Results and discussion
Investigating the chromium separation using bee carcasses and wastes and their composite with polyaniline. Table 1 shows the results of chromium separation using bee carcasses and wastes and their composites with polyaniline. As Table 1 explains, polyaniline coating has been empowered to affect the removal efficiency of all materials. The highest percentage of removal was related to bee-carcass composites with polyaniline in the presence of polyethylene glycol (4.6 g/L) with a removal percentage of about twice the activated carbon and 50.56% of removal among bee carcasses and wastes provided by polyaniline. Polyaniline coating on bee carcasses and wastes could increase the percentage of chromium removal. Earlier research and studies have explained that polyaniline owns the regenerative property of hexavalent chromium, and the presence of nitrogen (-NH) sites in polyaniline has absorbed chromium 64 . Figure 1 shows the results of field emission scanning electron microscopy for bee carcasses and wastes, and Fig. 2 (parts a and b) explains the results of bee carcasses and wastes with polyaniline. As this figure (parts a and b) show, Aniline is well coated on the surface of bee carcasses and wastes in polyaniline composite and bee carcasses and wastes, and polyaniline particles have been placed in the form of thin, irregular scales on the surface of bee carcasses and wastes.
According to Fig. 2c and d, the polyaniline particles are more uniformly and regularly and spherically covered on the surface of bee carcasses in the composite of polyaniline and bee carcasses and wastes in the presence of polyethylene glycol. Polyethylene glycol is a stabilizing material, and the adsorption of aniline particles has increased and has affected the shape and arrangement of the particles during the polymerization process. The achieved composite is more regular with a higher surface area, and a higher adsorption percentage has occurred. Table 2 shows the results achieved by analyzing the pore size and BET specific surface area for bee composite and wastes and polyaniline, bee composite and wastes, and polyaniline and polyethylene glycol. The results explain that polyaniline and polyethylene glycol have been increased the lateral surface of the composite in the carcass composite and bee wastes. Consequently, the removal efficiency has been increased. Figure 3 shows the FTIR spectrum of bee carcasses and wastes, and Fig. 4 shows the bee carcasses and composites and their polyaniline and polyethylene glycols. The deformation and location of the bands of the FTIR spectrum, with the different molecular environments, can be a helpful guide to the molecular structure of a compound 65  (2) Table 1. Results of chromium separation using bee carcasses and wastes and their composites with polyaniline (contact time 30 min, pH = 6, adsorbent amount 2 g/L). www.nature.com/scientificreports/ Figure 5 shows the XRD results for bee carcasses and wastes. Figure 6 shows the polyaniline composite and bee carcasses and wastes in polyethylene glycol to determine the tested mineral properties of the adsorbents. As the analysis results accomplished on the synthesized composite explained, benzene compounds were observed.
Examining the adsorption test conditions for bee carcass composite and waste/polyaniline/ polyethylene glycol. Investigating the influence of pH of chromium solution on the efficiency of separation  www.nature.com/scientificreports/ by bee carcass and wastes/polyaniline/polyethylene glycol. pH is a factor that can enhance the adsorption efficiency and is one of the most significant and main parameters that determine the adsorption operation 59,66 . The pH influences biomass active site load and heavy metals behavior in solution 67 . Solution pH commonly impacts adsorbent ionization and adsorbent surface properties 68 . Nature of physicochemical interactions between adsorption sites and heavy metal ions to the pH of solution utilized in adsorption approaches, to a great extent 69,70 . in this research, five pHs (2, 4, 6, 8, and 10) were surveyed for chromium isolation and the results were indicated in Fig. 7. Alkaline pHs were avoided to prevent chromium sequestration. The results reveal that the highest adsorption of chromium (VI) occurs at pH = 2 at 78.98%, and the lowest percentage of chromium (VI) adsorption happens at pH = 10 at 9.46%. The study results explained that the pH of the solution highly affects the amount of adsorption so that in most cases, increasing the pH decreases the amount of chromium adsorption. Changes in pH influence chromium adsorption because it determines the type of ionic species of chromium and the charge of the adsorbent surface. The predominant form at pH is 2 for Chromium + 6, and less than it, is as HCrO 4 − and Cr 2 O 7 2− , its most value is HCrO 4 − , which is easily absorbed due to its low free energy. On the other hand, at high pH, the charge on the adsorbent surface is negative. Hence, the inclination to adsorb its anions decreases through the electrostatic process. According to increasing the adsorption at low pH and adding the potassium dichromate salt, chromium ions will be available in the form of Cr 2 O 7 2− in the environment. Consequently, we can explain the decrease in adsorption by increasing pH because the -OH groups on the adsorbent based on Le Chatelier's principle tend to be transferred to the medium at low pH due to the low concentration of -OH ions in solution. Consequently, positive groups are formed on the adsorbent, which leads to adsorption, and -OH groups will no longer tend to be released on the adsorbent. The concentration pH will be increased in the solution by increasing the value of pH. Accordingly, fewer positive environments are created, and consequently, the absorption process occurs to a lower extent.
Examining the contact time. The adsorption rate is frequently very high in the first stage of adsorption processes 71 . Figure 8 shows the effect of contact time of polyethylene composite sorbent and bee wastes in the presence of polyethylene glycol with solutions including chromium (VI) pH = 6. The metal concentration was 50 mg/L. The adsorbent was 2 g/L. Studies and researches conducted in this field explain that the amount and percentage of removal of the ascending path have been increased by increasing the contact time because many active and empty places in the adsorbent surface will be occupied over time 59,72 . And since, even after 120 min, the removal efficiency is still high, and there is no desorption process of the metal ions from the adsorbent at this time; therefore, the lowest amount of chromium removal was 21.16% in 5 min, and the highest amount of chromium removal was 61.64% in 120 min. Presently, the results achieved in the diagram show that the opti- Table 2. Results of BET analysis for bee carcasses and wastes and polyaniline composites.

Composite type
Special surface area (m 2 /g of adsorbent) Bee carcasses and wastes/polyaniline composite 11.91 Bee carcasses and wastes/polyaniline/polyethylene glycol composite 16.72 Figure 3. FTIR spectrum, related to bee carcasses. www.nature.com/scientificreports/ mal contact time is 30 min, which is 50.56% for the removal rate, and from this time on, there is no significant removal rate.
Examining the amount of adsorbent. The amount of adsorbent applied in removing metal ions is highly significant [73][74][75][76][77] . At this stage, we added different amounts of adsorbent (1, 2,4,6,8 g/L) to the chromium solution at pH = 6 and optimal contact time (30 min). As the results of Fig. 9 show, the removal efficiency was increased by increasing the amount of adsorbent. Polyaniline composite with bee carcass and waste in the presence of polyethylene-glycol could purify up to 81.1% of the solution, including 50 mg/L chromium in an adsorbent amount of 8 g/L. It is because of an increase in available places for the adsorption of metals, which has been performed by increasing the amount of adsorbent 46 . The results achieved in the diagram show that the optimal amount of adsorbent is 2 g per liter, which is a 50.56% removal rate, and the removal rate is then increased with a lower slope.  www.nature.com/scientificreports/ Examining the effect of initial chromium concentration. A driving force is created in an aqueous solution with the initial concentration of sorbate, which helps to transfer Cr (VI) ions to the adsorbent surface 78 . At this stage, we examined the effect of initial concentrations of chromium (at four levels of 5, 10, 50, and 100 ppm) on the separation process by the selected composite. As the results of Fig. 10 show, polyethylene composite and bee carcasses and wastes in the presence of polyethylene glycol could remove the chromium optimally in the range of various concentrations of water and wastewater. And increasing the initial concentration of chromium from 5 to 100 ppm, the percentage of chromium removal from 45.63 to 57.59%, explains that the amount of adsorption was increased by increasing the initial concentration of chromium in the effluent. As the concentration of the solution increases, the density of ions in the solution increases. As a result, as the ions approach the surface of the particles, the adsorption percentage increases, consequently, the adsorption percentage is increased as the ions approach the surface of the particles, and consequently, the possibility of adsorption saturation of the metal ions increases 23, 79 .
Langmuir and Freundlich adsorption isotherms for bee carcass composites and wastes/ polyaniline/polyethylene glycol. The Langmuir and Freundlich adsorption models are usually used www.nature.com/scientificreports/ for absorption data 80 . We tested absorption data by Langmuir 81 and Freundlich 82 equations. At this stage, the adsorption isotherms were examined according to the values obtained before. Accordingly, Figs. 11 and 12 show the fit of the results with Langmuir and Freundlich adsorption models for chromium (VI). Table 3 also shows the parameters and constant coefficients of these models. Chromium metal fitted with both models due to the proximity of two R 2 s to each other, but it had higher compatibility in the Langmuir model due to higher R 2 (R 2 = 0.9598). The coefficient n of the Freundlich model, which is an indicator of adsorption intensity, was calculated at 1.19 for the adsorption of chromium by the adsorbent. The slope is increased by increasing the adsorbent concentration in this isothermal model but is eventually decreased to zero by filling the empty adsorption sites. The coefficient K is a measure of the absorption power in the Freundlich model, which is equal to 0.739 L per gram. In the Langmuir model, the R L constant defines an important characteristic of this isotherm, named the equilibrium parameter. We calculated R L values achieved for chromium adsorption by 0.611, which is between 0 and 1, and indicates the optimal adsorption to interpret metal adsorption by adsorbent 83 . The maximum adsorption capacity for chromium metal (X m ) was 45.25 mg/g of adsorbent copper in the Langmuir model, and b (constant of adsorption tendency) was determined by 0.0127 L/mg.
Recreation and reuse of bee composite carcass and waste/polyaniline/polyethylene glycol. Reutilizing multiple adsorbents to remove water contaminants is ordinarily a great improvement because www.nature.com/scientificreports/ it can decrease the cost of synthesizing and purchasing the chemicals required to make or modify some adsorbents. Moreover, reusability restricts the disposal of adsorbents after a single-use 84 . As previous research explains, we can reuse some chromium adsorbents after a simple treatment with dilute solutions of sodium hydroxide, hydrochloric acid, or sulfuric acid [85][86][87][88] . According to the obtained results, for the initial solution of 50 ppm chromium, after the recovery of 10.58 ppm chromium was returned, which means about 41.85% desorption occurred. The recycled adsorbent was then re-used to remove chromium and increased the initial concentration of 50 mg/L chromium to 34.22 in similar conditions, and about 31.56% was removed.

Conclusion
This study tested bee carcass composites and wastes and polyaniline with polyethylene glycol additive to remove hexavalent chromium from an aqueous medium. Ultimately, we recognized that the highest percentage of removal related to the polyethylene composite and bee carcass and waste in the presence of polyethylene glycol was 50.56% among the carcass and bee waste composites, which we then selected as the best adsorbent. The parameters influencing the adsorption process for polyethylene composite and bee carcass and waste in the presence of polyethylene glycol in the next step, which had the adsorption percentage increased for this composite by decreasing the pH, increasing the contact time, and increasing the adsorbent, and increasing the initial concentration of chromium. The highest percentage of adsorption was obtained when the pH was 2, the contact time was 120 min and the adsorbent value was 8 g/L and the initial concentration of chromium was 100 ppm. The most optimal removal percentage was achieved at the pH = 2, the contact time was 30 min, and the adsorbent value was 2 g /L, and the initial chromium concentration was 100 ppm. The outlining of the adsorption isotherms also revealed that the Langmuir equation had a better fit than Freundlich because of the higher R 2 . Ultimately, the results were achieved by reduction and desorption affirmed that the adsorbent could absorb continuously, and recreation and recovery performed.  www.nature.com/scientificreports/