Engineering Chemistry, ISSN: 2813-6535, Vol. 7, pp 61-70

doi: 10.4028/p-UqJ3In

© 2024 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2023-12-13

Revised:
2023-12-29

Accepted:
2024-01-02

Online:
2024-06-26

Utilization of Adsorbent Based on Rice Straw (Oryza Sativa) for Cr(VI) Ions Reduction in Aqueous Solutions

The original version of this paper is available on https://www.scientific.net/EC.7.61.pdf

Harmami , Ita Ulfin , Baswara Gilang Ramadhan and Kartika A. Madurani

Chemistry Department, Faculty of Science and Data Analytics, Institut Teknologi Sepuluh Nopember, Indonesia

a*harmami@chem.its.ac.id, itau@chem.its.ac.id, baswara.18012@mhs.its.ac.id, kartika.madurani@its.ac.id

Abstract

In this study, we utilized an adsorbent based on rice straw for reducing hexavalent chromium ions ( ) in an aqueous solution. The rice straw as adsorbent raw material was washed, dried, and powdered. Rice straw powder was heated at for 2 hours to obtain rice straw adsorbent. The adsorbent was activated with for 4 hours. Characterization of the adsorbent was done using Fourier Transform Infra-Red (FTIR) method. FTIR spectra showed the presence of hydroxy, carboxylic, aromatic, and ether groups on the surface of the rice straw and the made adsorbent. The reduction of ions in aqueous solutions was carried out using the adsorption batch method. The adsorption process was conducted in various the solutions pH for 1-5 and variations in contact time for 5-720 minutes. The highest percentage reduction of reached . It has occurred at pH 2 and equilibrium at 600 minutes of contact time.

Introduction

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Chromium ( Cr ) are famous compounds in various industries such as dyes, coating, catalysts, corrosion inhibitors, etc. [1]. This condition makes the main industrial waste contain Cr compounds. Cr compounds in wastewaters is primarily encountered in oxidation stages III and +6 [Cr(VI)]. Recent research suggests that Cr (VI) compounds are more toxic dan reactive than Cr (III) [2]-[4]. These properties combined with sufficient solubility and mobility of VI in the environment, makes the issue of utilization and disposal of chromium-containing waste relevant to many countries. To cover this problem, the possible thing to do is improve the technology for its removal from industrial waste. The most common technology for removal is adsorption. Adsorption is cost-effective and easy operating method [2]. The important part of adsorption method is the adsorbent used [3]. The conventional choice for adsorbent is commercial activated carbon. Unfortunately, this adsorbent is not cheap due to the high-cost raw materials for production. Therefore, developing low-cost activated carbon from inexpensive and sustainable raw materials is important.

Rice straw is byproducts of rice harvest. Rice straws reached 415,970.61 ton in East Java, Indonesia, in 2018 [5]. Besides being used for material production and as a source of renewable energy, rice straw is often utilized as an adsorbent for reducing heavy metals in aqueous solutions. There are various types of adsorbents that can be obtained from rice straw, such as bio sorbents, nanomaterials, and activated carbon [6]-[9]. Adel et al. made an adsorbent based on heated rice straw at temperatures ranging from 300 to for 30 to 120 minutes, followed by activation with 1 M HCl and NaOH [10]. The results showed that heating at for 120 minutes produced adsorbents with the highest adsorption capacity for reducing Pb (II) in aqueous solutions. The heating temperature can affect the functional groups on the adsorbent. Heating above can increase the amount of aromatic compounds and reduce the polarity of the adsorbent, which has a negative impact on the heavy metals adsorption [11], [12]. Rice straw, which is a lignocellulosic compound, is a suitable raw material for the production of carbon-based adsorbents.

Zhou et al. [13] used activated carbon derived from rice straw, activated with HCl and NaOH , as an adsorbent for the removal of from aqueous solutions. They found that the optimum condition for adsorption was pH 2 , with equilibrium reached after a contact time of 1440 minutes, resulting in a reduction of and for HCl and NaOH activators, respectively. Farahmand et al. utilized activated carbon from rice straw with an activator for the removal of Ce (IV) [14]. The optimum pH for Ce (IV) adsorption occurred at pH 4 , with equilibrium achieved after 500 minutes of contact time. The maximum adsorption capacity for Ce (IV) obtained was . In the research conducted by Sakhiya et al., carbon adsorbent from rice straw was used for the reduction of Pb (II) and Zn (II) metal ions in aqueous solutions [9]. The optimum pH for adsorption of both metals was pH 6 , with equilibrium achieved after a contact time of 150 minutes. The adsorption capacity for Pb (II) was found to be , and for Zn (II), it was .

Research on the adsorption of from aqueous solutions continues to be a topic of investigation. Fan et al. used a mixed adsorbent of rice straw and sewage sludge activated with to reduce in aqueous solutions [15]. The optimum pH for adsorption was pH 2 , and equilibrium was reached after a contact time of 1440 minutes. The adsorption capacity for VI obtained was . Gao et al. employed rice straw bio sorbent activated with for the reduction of in aqueous solutions [16]. The optimum pH for adsorption was 2, with a contact time ranging from 0 to 3600 minutes. The highest percentage reduction of occurred at a contact time of 2880 minutes. The study of Ma et al. reported that activated carbon from corn straw, activated with KOH, was used to adsorb at a capacity of with a contact time of 480 minutes [17]. Despite various studies investigating the reduction of from aqueous solutions using straw-based adsorbents activated with different agents, it appears that has not yet been used as an activator.

The use of as an activator can indeed enhance the adsorption capacity of activated carbon obtained from various raw materials. plays a crucial role in the pyrolytic transformation of the raw material and contributes to the formation of cross-linked carbonaceous structures [18], [19]. In this study, rice straw adsorbents were made and activated with for the reduction of in aqueous solutions. The research aims to determine the optimum pH and equilibrium contact time for the adsorption process.

Materials and Methods

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Materials. The semi-dried rice straw was obtained from local paddy fields in Malang, East Java, Indonesia. Phosphoric acid , chloride acid , and sodium hydroxide were purchased from local market. Demineralized water was bought from a local market and used for cleaning and chemical preparation. All chemicals were used without any purification. 1 M solution as adsorbent activator and 1 M HCl solution were prepared in Laboratory of Instrumentation and Analytical Sciences, Chemistry Department, Faculty of Sciences and Data Analytics, Institut Teknologi Sepuluh Nopember. The standard solutions of in various concentration (10-50 ) were prepared by diluting potassium dichromate (VI) with demineralized water. The standard solutions were measured by Atomic Absorption Spectrophotometry (AAS, Thermo Fisher) at 357.9 nm to obtain calibration curve. The working solution is wastewater that containing . The working solution was prepared from dilution of potassium dichromate (VI) and demineralized water.

Preparation of Rice Straw Adsorbent. Rice straw was cut into pieces and washed until the rinsed water became clear. The straw was then air-dried at room temperature for approximately 24 hours before being heated in an oven at for 24 hours. The dried straw was finely ground to obtain a powder, which was subsequently heated to with a retention time of 2 hours and a heating rate of minute, resulting in the rice straw adsorbent [9], [20]. The rice straw adsorbent was washed with 1 M HCl and deionized water until it reached a pH of 6-7, and then it was dried in
an oven at for 12 hours. Prior to use, the rice straw adsorbent was sieved through a 120 -mesh sieve to obtain uniform particles with a size below and stored in the desiccator [21].

The activated rice straw adsorbent was prepared by mixing it ( 5 g ) with 100 mL of activator solution. The mixture was stirred for 4 hours [10]. Subsequently, it was filtered and washed with demineralized water to obtain a pH of 6-7. Then, it was dried in an oven at for 12 hours until a constant weight. The activated adsorbent was sieved through a 120 -mesh sieve and stored in the desiccator.

Batch Experiments. The batch adsorption method was employed to determine the optimum pH and contact time using an adsorbent dosage of [22]. A total of 0.1 g of rice straw activated carbon was mixed with 25 mL of a wastewater containing of . The mixture was stirred for a specified period [15]. The mixture was filtered, and the resulting filtrate was analyzed using AAS technique. The removal percentage was calculated using Eq. 1, where refers to the initial concentration of the adsorbate in the solution, and represents the concentration of the adsorbate after the adsorption process has taken place.
Removal Percent

The testing of the adsorbate's pH influence was conducted using a working solution adjusted to pH values of , and 5 , with each test having a contact time of 100 minutes [23]. The pH variation that resulted in the highest percentage reduction was determined as the optimum pH and will be used in the following experiments. The effect of contact time was conducted by varying the contact time between the adsorbent and adsorbate, starting from 5 minutes and continuing until a constant percentage reduction was obtained. This was done using a working solution containing at a concentration of , each at the previously determined optimum pH .

FTIR Characterization. The KBr pellet method was employed for FT-IR characterization. The samples were mixed with KBr at a ratio of 1:100 and finely ground and pulverized. The mixture of powders was then placed into a press apparatus to form pellets, which were subsequently loaded into a sample holder for FT-IR analysis. This analysis was carried out for the following samples: dried rice straw powder, rice straw activated carbon, -activated rice straw adsorbent, and activated rice straw adsorbent after adsorption.

Results and Discussions

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Fabrication of Rice Straw Adsorbent. Fig. 1A shows the cut and washed rice straw and the powdered rice straw is depicted in Fig. 1B. The rice straw powder displayed a brown color throughout, indicating the degradation of chlorophyll during the heating process [24]. The heating of the rice straw powder was carried out in an electric furnace at for 2 hours with a heating rate of minute to obtain rice straw adsorbent. The result of this heating process showed a color change in the powder to black, with a small amount of ash on the surface. The formation of ash is attributed to the rice straw powder reacting with oxygen inside the crucible, resulting in complete combustion. The appearance of the rice straw powder after heating can be seen in Fig. 2A. The obtained rice straw adsorbent was washed with 1 M HCl and demineralized water until pH of 6-7 to remove residual ash from carbonization. After activating and sieving, there were no significant differences observed in the rice straw adsorbent except that it appeared darker in color and had uniform particle size, as shown in Fig. 2B.

Fig. 1. Dried rice straw (A) and rice straw powder (B).

Fig. 1. Dried rice straw (A) and rice straw powder (B).

Fig. 2. Rice straw adsorbent (A) and the activated rice straw adsorbent (B).

Fig. 2. Rice straw adsorbent (A) and the activated rice straw adsorbent (B).

FTIR Characterization. The rice straw powder, rice straw carbon, -activated rice straw carbon before and after adsorption were characterized its functional groups using FT-IR. Table 1 provides information about the vibrational bands corresponding to the peaks in the spectra. The main peaks that appear indicate the presence of oxygen functional groups such as ether, ketone, phenolic, and alcohol groups. There are changes in the peaks in the spectra of the powder and adsorbent, specifically the disappearance of peaks at wavenumbers 1375 and . The absence of the peak at is due to dehydration and aromatization resulting from the decomposition and condensation of volatile compounds [25]. Meanwhile, no peak of methylene at is attributed to the decomposition of bonds during heating to form more stable aromatic bonds at high temperatures. Conversely, a new peak is formed in the adsorbent spectrum at wavenumber , indicating the oxidation of surface functional groups of the adsorbent. The increased intensity of the carboxylate peak in the spectrum of the -activated adsorbent is caused by the protonation of carboxylate groups by the acid activator [13].

Table 1. FT-IR vibration peak.

Wavenumber
(cm⁻¹)
Vibration
Peak

Rice straw powder
Rice straw activated carbon
H₃PO₄-activated rice straw adsorbent
H₃PO₄-activated rice straw adsorbent after adsorption

468
Si-O-Si stretch
✓
✓
✓
✓

800
C-H aromatic
✓
✓
✓
✓

1101
C-O/ C-O-C
✓
✓
✓
✓

1375
C-H aliphatic/ O-H phenolic
✓
✗
✗
✗

1639
C═C aromatic
✓
✓
✓
✓

1695
C═O
✗
✓
✓
✓

2060
Overtone aromatic
✓
✓
✓
✓

2920
C-H methylene
✓
✗
✗
✗

3420
O-H stretch
✓
✓
✓
✓

Calibration Curve. The calibration curve is depicted in Fig. 3. The calibration curve exhibits a linear equation, i.e. , with an value of 0.9968 . The value obtained is above 0.995 , which indicates that the calibration curve is acceptable and can be used to calculate the concentration of in the following tests.

Fig. 3. Calibration curve of the standard solution.

Fig. 3. Calibration curve of the standard solution.

Effect of pH. The testing of pH variations in the adsorbate was carried out to determine the optimum pH , using a range of pH values: , and 5 . The results of the removal percentage of can be found in Fig. 4. The removal percentage of increases with the acidity of the adsorbate solution, and the highest percentage is achieved at a pH of . This is followed by a decrease at pH 1 . This can be attributed to the fact that in an acidic environment, the surface of the activated carbon is surrounded by ions, which facilitate the binding of anionic, through electrostatic attraction. The more positive the surface of the activated carbon, the easier it is to bind ions [26]. In addition to electrostatic attraction, the removal mechanism of in the solution can be attributed to the reduction of ions to ions. In an acidic environment, ions exist in the form of and , which can be directly reduced to ions by electron-donating groups on the activated carbon [13], [27].

Fig. 4. Graph of the effect of varying pH on the removal percent of ions.

Fig. 4. Graph of the effect of varying pH on the removal percent of ions.

The reduction reaction of Cr (VI) ions can be seen in Fig. 5. Cr (III) ions, which carry a positive charge, can then be released back into the solution due to electrostatic repulsion from the surface of the activated carbon or adsorbed by the acidic active sites of the activated carbon through cation exchange [12], [15], [28]. At extremely acidic pH levels, the Cr (III) ions released back into the solution may have difficulty reabsorbing onto the surface of the activated carbon due to the excess ions present on both the carbon surface and in the solution. The results obtained align with the findings of Parlayici and Pehlivan [29], where a decrease in the removal percentage was observed at pH values below 2.

Fig. 5. Adsorption mechanism of ions in activated carbon surface.

Fig. 5. Adsorption mechanism of ions in activated carbon surface.

Effect of Contact Time. The effect of contact time was conducted by varying the duration of stirring between the adsorbent and adsorbate in the batch adsorption process. Various contact times of 5, 30, , and 720 minutes were used. The results of removal percentage by contact time variations is presented in Fig. 6. The graph indicates that the removal percentage of ions at a contact time of 5 minutes is , followed by a increasing the removal percentage at a contact time of 30 minutes. This is then followed by a gradual increase until equilibrium is reached at a contact time of 600 minutes, with an adsorption percentage of . A similar trend was observed in the research conducted by [17]. This phenomenon is attributed to the high availability of active sites on the -activated rice straw adsorbent at the beginning of the adsorption process, allowing VI to be readily bound to the adsorbent surface. One of the proposed mechanisms is the adsorption of by functional groups containing oxygen, either through ion exchange or complexation [17]. This mechanism may underlie the adsorption process in this study, as evidenced by the high presence of oxygen-containing functional groups in the FT-IR spectra obtained. The reduction of ions can be visually observed because ion solutions are yellow in color and will turn orange as the concentration of ions increases [30]. Fig. 7 illustrates the color change of the solution before and after the adsorption process.

Fig. 6. Graph of the effect of varying contact time on the removal percent of ions.

Fig. 6. Graph of the effect of varying contact time on the removal percent of ions.

Fig. 7. Comparison of working solution before (working solution) and after adsorption with contact time of 60 and 360 minutes.

Fig. 7. Comparison of working solution before (working solution) and after adsorption with contact time of 60 and 360 minutes.

Conclusions

The original version of this paper is available on https://www.scientific.net/EC.7.61.pdf

An adsorbent based on carbon derived from rice straw was successfully prepared, heated to for 2 hours, and activated with for the reduction of in aqueous solutions. The obtained results revealed an optimum pH at pH 2 and equilibrium at a contact time of 600 minutes, with a reduction percentage of 66.90%.

Acknowledgment

The original version of this paper is available on https://www.scientific.net/EC.7.61.pdf

The authors gratefully acknowledge financial support from the Institut Teknologi Sepuluh Nopember for this work, under project scheme of the Publication Writing and IPR Incentive Program (PPHKI) 2023.

References

The original version of this paper is available on https://www.scientific.net/EC.7.61.pdf

D. P. Ordinartsev, N. V. Pechishcheva, S. Kh. Estemirova, A. V. Kim, and K. Yu. Shunyaev, 'Removal of from wastewater by modified montmorillonite in combination with zero-valent iron', Hydrometallurgy, vol. 208, p.105813, Feb. 2022, doi: 10.1016/j.hydromet.2021.105813.

L. Fan et al., 'Adsorption Removal of with Activated Carbon Prepared by Co-pyrolysis of Rice Straw and Sewage Sludge with ZnCl2 Activation', Water. Air. Soil Pollut., vol. 230, no. 10, p. 233, Oct. 2019, doi: 10.1007/s11270-019-4305-8.

E. Vaiopoulou and P. Gikas, 'Effects of chromium on activated sludge and on the performance of wastewater treatment plants: A review', Water Res., vol. 46, no. 3, pp. 549-570, Mar. 2012, doi: 10.1016/j.watres.2011.11.024.

A. M. Sulaiman, S. Mas'ud, A. N. Daroini, and P. Purnami, 'THE EFFECT OF ELECTRODE COATING FROM BISPHENOL-A-POLYCARBONATE CD-R WASTE FOR HYDROGEN PRODUCTION', Int. J. Mech. Eng. Technol. Appl., vol. 4, no. 1, pp. 10-21, Jan. 2023, doi: 10.21776/MECHTA.2023.004.01.2.

E. I. Rhofita, R. Rachmat, M. Mayer, and L. Montastruc, 'An Energy Potential Estimation of Rice Residue in Indonesia: A Case Study in East Java', IOP Conf. Ser. Earth Environ. Sci., vol. 1024, no. 1, p. 012029, May 2022, doi: 10.1088/1755-1315/1024/1/012029.

E. S. Elmolla, W. Hamdy, A. Kassem, and A. Abdel Hady, 'Comparison of different rice straw based adsorbents for chromium removal from aqueous solutions', Desalination Water Treat., vol. 57, no. 15, pp. 6991-6999, Mar. 2016, doi: 10.1080/19443994.2015.1015175.

X. Huang, X. Sheng, Y. Guo, Y. Sun, P. Fatehi, and H. Shi, 'Rice straw derived adsorbent for fast and efficient phosphate elimination from aqueous solution', Ind. Crops Prod., vol. 184, p. 115105, Sep. 2022, doi: 10.1016/j.indcrop.2022.115105.

H. R. Robles-Jimarez et al., 'New silica based adsorbent material from rice straw and its inflow application to nitrate reduction in waters: Process sustainability and scale-up possibilities', Sci. Total Environ., vol. 805, p. 150317, Jan. 2022, doi: 10.1016/j.scitotenv.2021.150317.

A. K. Sakhiya, V. K. Vijay, and P. Kaushal, 'Efficacy of rice straw derived biochar for removal of and from aqueous: Adsorption, thermodynamic and cost analysis', Bioresour. Technol. Rep., vol. 17, p. 100920, Feb. 2022, doi: 10.1016/j.biteb.2021.100920.

A. M. Adel, S. Kamel, and M. El-Sakhawy, 'Rice straw charcoal: Characterization and adsorption of from aqueous solution', Environ. Sci., p. 10, 2013.

Z. Chen, X. Xiao, B. Chen, and L. Zhu, 'Quantification of Chemical States, Dissociation Constants and Contents of Oxygen-containing Groups on the Surface of Biochars Produced at Different Temperatures', Environ. Sci. Technol., vol. 49, no. 1, pp. 309-317, Jan. 2015, doi: 10.1021/es5043468.

X. Xiao, B. Chen, and L. Zhu, 'Transformation, Morphology, and Dissolution of Silicon and Carbon in Rice Straw-Derived Biochars under Different Pyrolytic Temperatures', Environ. Sci. Technol., vol. 48, no. 6, pp. 3411-3419, Mar. 2014, doi: 10.1021/es405676h.

J. Zhou, H. Chen, R. W. Thring, and J. M. Arocena, 'Chemical Pretreatment of Rice Straw Biochar: Effect on Biochar Properties and Hexavalent Chromium Adsorption', Int. J. Environ. Res., vol. 13, no. 1, pp. 91-105, Feb. 2019, doi: 10.1007/s41742-018-0156-1.

E. Farahmand, 'Adsorption of Cerium (IV) from Aqueous Solutions Using Activated Carbon Developed from Rice Straw', Open J. Geol., vol. 06, no. 03, pp. 189-200, 2016, doi: 10.4236/ojg.2016.63017.

L. Fan et al., 'Adsorption Removal of with Activated Carbon Prepared by Co-pyrolysis of Rice Straw and Sewage Sludge with ZnCl2 Activation', Water. Air. Soil Pollut., vol. 230, no. 10, p. 233, Oct. 2019, doi: 10.1007/s11270-019-4305-8.

H. Gao, Y. Liu, G. Zeng, W. Xu, T. Li, and W. Xia, 'Characterization of Cr(VI) removal from aqueous solutions by a surplus agricultural waste-Rice straw', J. Hazard. Mater., vol. 150, no. 2, pp. 446-452, Jan. 2008, doi: 10.1016/j.jhazmat.2007.04.126.

H. Ma, J. Yang, X. Gao, Z. Liu, X. Liu, and Z. Xu, 'Removal of chromium (VI) from water by porous carbon derived from corn straw: Influencing factors, regeneration and mechanism', J. Hazard. Mater., vol. 369, pp. 550-560, May 2019, doi: 10.1016/j.jhazmat.2019.02.063.

Z. Wang, J. Wu, T. He, and J. Wu, 'Corn stalks char from fast pyrolysis as precursor material for preparation of activated carbon in fluidized bed reactor', Bioresour. Technol., vol. 167, pp. 551-554, Sep. 2014, doi: 10.1016/j.biortech.2014.05.123.

M. Zięzio, B. Charmas, K. Jedynak, M. Hawryluk, and K. Kucio, 'Preparation and characterization of activated carbons obtained from the waste materials impregnated with phosphoric acid(V)', Appl. Nanosci., vol. 10, no. 12, pp. 4703-4716, Dec. 2020, doi: 10.1007/s13204-020-01419-6.

V. Fierro, G. Muñiz, A. H. Basta, H. El-Saied, and A. Celzard, 'Rice straw as precursor of activated carbons: Activation with ortho-phosphoric acid', J. Hazard. Mater., vol. 181, no. 13, pp. 27-34, Sep. 2010, doi: 10.1016/j.jhazmat.2010.04.062.

S. Mashhadi et al., 'Rapid removal of Hg (II) from aqueous solution by rice straw activated carbon prepared by microwave-assisted H 2 SO 4 activation: Kinetic, isotherm and thermodynamic studies', J. Mol. Liq., vol. 215, pp. 144-153, Mar. 2016, doi: 10.1016/j.molliq.2015.12.040.

W.-T. Tan, H. Zhou, S.-F. Tang, P. Zeng, J.-F. Gu, and B.-H. Liao, 'Enhancing Cd(II) adsorption on rice straw biochar by modification of iron and manganese oxides', Environ. Pollut., vol. 300, p. 118899, May 2022, doi: 10.1016/j.envpol.2022.118899.

R. Kumar, D. K. Arya, N. Singh, and H. K. Vats, 'Removal of Cr (VI) Using Low Cost Activated Carbon Developed By Agricultural Waste', IOSR J. Appl. Chem., vol. 10, no. 01, pp. 76-79, Jan. 2017, doi: 10.9790/5736-1001017679.

C. A. Weemaes, V. Ooms, A. M. Van Loey, and M. E. Hendrickx, 'Kinetics of Chlorophyll Degradation and Color Loss in Heated Broccoli Juice', J. Agric. Food Chem., vol. 47, no. 6, pp. 2404-2409, Jun. 1999, doi: 10.1021/jf980663o.

M. J. Saad et al., 'Physical and Chemical Properties of the Rice Straw Activated Carbon Produced from Carbonization and KOH Activation Processes', Sains Malays., vol. 48, no. 2, pp. 385-391, Feb. 2019, doi: 10.17576/jsm-2019-4802-16.

T. Lin et al., 'Synergistic effect of iron doped ordered mesoporous carbon on adsorptioncoupled reduction of hexavalent chromium and the relative mechanism study.', Chem. Eng. J., vol. 239, pp. 114-122, 2014.

L. Anah and N. Astrini, 'Influence of pH on ions removal from aqueous solutions using carboxymethyl cellulose-based hydrogel as adsorbent', IOP Conf. Ser. Earth Environ. Sci., vol. 60, p. 012010, Mar. 2017, doi: 10.1088/1755-1315/60/1/012010.

J. Valentín-Reyes, R. B. García-Reyes, A. García-González, E. Soto-Regalado, and F. CerinoCórdova, 'Adsorption mechanisms of hexavalent chromium from aqueous solutions on modified activated carbons', J. Environ. Manage., vol. 236, pp. 815-822, Apr. 2019, doi: 10.1016/j.jenvman.2019.02.014.

Ş. Parlayici and E. Pehlivan, 'Comparative study of removal by bio-waste adsorbents: equilibrium, kinetics, and thermodynamic', J. Anal. Sci. Technol., vol. 10, no. 1, p. 15, Dec. 2019, doi: 10.1186/s40543-019-0175-3.

H. P. Singh, P. Mahajan, S. Kaur, D. R. Batish, and R. K. Kohli, 'Chromium toxicity and tolerance in plants', Environ. Chem. Lett., vol. 11, no. 3, pp. 229-254, Sep. 2013, doi: .

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