Removal of hexavalent chromium from industrial wastewater using modified imperata cylindrica biosorbent and method validation

Khansaa Al-Essa; Mohamed A Habib; Ethar M. Al-Essa

doi:10.25259/AJC_440_2025

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Original Article

:19;

4402025

doi:

10.25259/AJC_440_2025

Removal of hexavalent chromium from industrial wastewater using modified imperata cylindrica biosorbent and method validation

Khansaa Al-Essa^a,*, Mohamed A Habib^b, Ethar M. Al-Essa^c,

aDepartment of Chemistry, Faculty of Science, Jerash University, Jerash, Jordan

bDepartment of Chemistry, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia

cDepartment of Civil Engineering, Isra University, Amman, Jordan

*Corresponding author: E-mail address: k.essa@jpu.edu.jo, (K. Al-Essa)

Received: 2025-04-27, Accepted: 2025-09-24, Epub ahead of print: 2026-02-19, Published: 2026-04-02

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Abstract

Hexavalent chromium ions are recognized for their extreme toxicity and environmental impact. Industrial effluents frequently contribute substantial amounts of these ions to ecosystems, causing a significant threat to environmental and human health. To address this issue, this study investigates the conversion of abundant parasitic plants, specifically Imperata cylindrica, into an efficient and cost-effective biosorbent for chromium ion removal. The adsorption performance of the biosorbent was enhanced through chemical modification using phosphoric acid and sodium hydroxide as acid and base treatments, respectively. Results demonstrated that acid-treated biosorbent achieved a remarkable chromium ion removal efficiency of 87.2% under optimal conditions. The optimal conditions were identified as pH 2, a contact time of 20 h, and a biosorbent dosage of 0.1 g. The untreated and base-treated biosorbents showed significantly lower efficiencies, with removal rates of 10.4% and 67.7%, respectively. The kinetic analysis confirmed that the adsorption process followed a quasi-secondary model, as evidenced by a strong agreement between experimental and calculated maximum adsorption capacity values (with a difference of 0.7121 for the untreated sample). The Freundlich isotherm model provided an excellent fit to the adsorption data, with correlation coefficients exceeding 0.96 for all biosorbent types. These findings highlight that acid treatment significantly enhances biosorption properties, making the treated biosorbent an effective and economical solution for chromium ion removal from contaminated water sources. Additionally, this study optimized the 1,5-diphenylcarbazide method using ultraviolet-visible (UV-Vis) spectroscopy for efficient and cost-effective chromium ion concentration measurement. For validation, results were compared with inductively coupled plasma mass spectrometry (ICP-MS), showing a strong correlation (R² > 0.93) and excellent agreement between the two methods. This confirmed that UV-Vis spectroscopy is a reliable and accurate technique for assessing chromium ion concentrations, offering a simplified and cost-effective alternative to ICP-MS analysis.

Keywords

Agriculture by-products

Hexavalent chromium ions

Kinetic and isotherm adsorption characteristics

Method validation

Modified Imperata cylindrica biosorbent

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1. Introduction

Factories around the world discharge vast amounts of polluted water into the surrounding environment. Industrial waste streams often contain heavy metals, dyes, and other organic pollutants that pose serious risks to human health and the broader ecosystem [1]. Heavy metals are particularly concerning because they can bioaccumulate and biomagnify through food chains, leading to toxic effects such as organ damage, neurological disorders, and developmental issues in both humans and wildlife [2].

Industries are major contributors to heavy metal contamination in wastewater. Metals such as chromium, cadmium, lead, mercury, nickel, and copper are non-biodegradable and tend to persist in the environment, accumulating over time in soil and aquatic ecosystems [3]. Due to their persistence and toxicity, controlling heavy metal pollution is essential for preserving biodiversity, ensuring food safety, and preventing long-term public health threats [4]. Among these metals, chromium is particularly prominent in industrial effluents, especially from sectors such as leather tanning, electroplating, animal hide processing, paints and pigment production, pulp and paper processing, wood preservation, corrosion inhibition, and steel manufacturing [5,6]. Therefore, their removal from industrial drains before entering freshwater sources is an urgent issue. Traditional methods for heavy metal removal are costly due to the expenses associated with chemicals and energy. Developing an effective and low-cost solution is paramount.

Various methods have been employed for the removal of heavy metal ions, including precipitation, flotation, ion exchange, electrochemical deposition, and adsorption [7]. Precipitation involves the formation of insoluble metal hydroxides; it requires large chemical inputs and generates sludge, which poses disposal challenges. Ion exchange resins, though explored, are limited by their sensitivity to pH, poor selectivity, and vulnerability to fouling in complex wastewater. Electrolytic recovery can effectively remove heavy metals but is hindered by high costs and short electrode lifespans [8].

Among these methods, adsorption has gained significant attention due to its efficiency and simplicity. It occurs in three main stages: (i) migration of ions to the adsorbent surface, (ii) surface adsorption, and (iii) interaction and binding with the adsorbent structure [9,10]. Heavy metal removal mechanisms include chemical interactions, particularly chelation, where functional groups bind metal ions, as well as ion exchange, precipitation, π-interactions from electron-rich functional groups like aromatic rings and C-O groups, and electrostatic interaction [11].

Zeolites, both natural and synthetic, have been investigated as adsorbents [12,13], but their high cost and limited resources hinder widespread adoption. Recently, modified agricultural by-products have emerged as promising biosorbents.

Agricultural biomass, which is rich in cellulose and lignin containing hydroxyl (-OH) groups, can chelate and adsorb metal ions. Additionally, physical adsorption through Van der Waals forces and hydrogen bonding can complement these chemical interactions.

Simple chemical treatments can enhance the surface area, porosity, and exposure of functional groups without destroying the biomass structure, thereby improving metal ion uptake. Examples of such biosorbents include rice husks, maize cobs, and pecan shells [14,15]. Converting biomass into activated carbon can further enhance adsorption capacity, but the high energy input required limits its sustainability.

This study explores the use of parasitic plants, commonly removed and discarded in farming operations, as a low-cost, sustainable biosorbent. These weeds compete with crops for nutrients, water, and sunlight, reducing yields and raising production costs [16]. Utilizing them as adsorbents provides an eco-friendly solution while addressing waste management challenges.

One of the most prevalent and readily available parasitic plants is Haifa (Imperata cylindrica) (Plants of the World Online, https://powo.science.kew.org [17]), a perennial and vigorous grass in the Poaceae family. It possesses an extensive and deeply penetrating rhizome system, with shoots reaching a height of 1 m [18]. Imperata cylindrical is widely distributed across Asia (including temperate, western, and tropical regions), Africa, southern Europe, Australia, and America [19].

Numerous studies have documented crop yield and production losses due to weed infestations, both in cultivated and neglected farms. For instance, in the palm plantations of Al-Hassa Oasis in Saudi Arabia, Imperata cylindrical is present in 60% of cultivated farms and 40% of neglected farms, with an average cover of about 84% [20]. Given that Saudi Arabia has more than 31 million palm trees, effective weed management is urgently needed [21].

Although previous studies such as Hanafiah et al., [22-25] have examined the use of Imperata cylindrical for the removal of metal ions like Ni²⁺, Cd²⁺, Cu²⁺, and Pb²⁺. These investigations were primarily limited to unmodified biomass and did not address the removal of hexavalent chromium (Cr(VI)). Furthermore, those studies lacked assessments under realistic industrial wastewater conditions. To address these gaps, the present study explores the use of chemically activated Imperata cylindrica for the removal of Cr(VI) ions from wastewater. This work evaluates the biosorbent’s removal efficiency under varying pH levels and biosorbent doses and employs multiple adsorption isotherms and kinetic models to characterize the mechanism and optimize the conditions for Cr(VI) removal. By enhancing the adsorptive capacity and stability of Imperata cylindrica through chemical modification and validating its performance under more practical conditions, this study offers a novel and application-oriented contribution to the field of biosorption-based wastewater treatment. Additionally, this study aims to optimize 1,5-diphenylcarbazide (DPC) method for low-cost and straightforward incorporation into wastewater treatment processes. Laboratory-scale performance testing and validation of the method have also been conducted as part of this novel approach.

2. Materials and Methods

2.1. Chemicals

Chemicals such as sodium hydroxide (NaOH, 98%, Acros), sulphuric acid (H₂SO₄, 98%, Acros), orthophosphoric acid (H₃PO₄, 98%, Sigma-Aldrich), potassium dichromate (K₂Cr₂O₇, 98%, ACS), 1,5-diphenylcarbazide (C₆H₅NHNHCONHNHC₆H₅, 99%, Sigma-Aldrich), and deionized water were used. All chemicals were analytical grade. The industrial wastewater was obtained from a textile factory in Al Hassan Industrial City-Irbid-Jordan (G22H+9X8). All the plots presented were generated using OriginPro 2025 software, www.OriginLab.com, (28 November 2024).

2.2. Agro-waste

The green weeds of Imperata cylindrica (WIC) were collected from the olive trees farm in the Jerash University district, located in Jerash, Jordan, at coordinates (32.2522677865495°N, 35.897914514656364°E),

2.3. Instrumentation

The concentrations of Cr(VI) ions were determined using a UV-spectrophotometer (Shimadzu), inductively coupled plasma-mass spectrometer (ICP-MS), Agilent 7500A, ICP-MS2-B7.2, (Shield Torch System), Fourier transform infrared (FTIR) spectroscopy (Thermo Nicolet NEXUS 670 Spectrophotometer), and X-ray diffraction (XRD) (Philips X pert pro). The pH was measured using a METROHM 605 pH meter.

2.4. Modification of the WIC samples

The agriculture waste from the WIC samples was washed with distilled water several times to remove dirt and impurities. The cleaned WIC samples were then dried in sunlight for several days. Next, the samples were ground using an ordinary stainless-steel blender and sieved to achieve a particle size of less than 100 μm. Then, 5g of WIC samples were impregnated overnight with excess amounts of 0.1M sodium hydroxide, while the other 5g samples were treated with 0.1M phosphoric acid solutions at room temperature.

The selection of 0.1 M phosphoric acid and sodium hydroxide for the chemical treatment of Imperata cylindrica was guided by both literature [25] and preliminary experiments. This concentration was found to be optimal in enhancing surface functional groups, such as hydroxyl and carboxyl, without compromising the structural integrity of the biosorbent. Higher concentrations led to partial degradation of the biomass and poor recovery after washing and drying, whereas lower concentrations had minimal effect on adsorption capacity. Thus, 0.1 M was chosen as a balanced, effective concentration for chemical activation.

All samples were filtered and washed multiple times with distilled water until a clear filtrate solution was obtained. Subsequently, all samples were dried in an oven at 70°C until a constant weight was reached. The modified WIC masses were stored in sealed containers, as shown in Figure 1.

Visual appearance of raw (untreated), acid-treated, and base-treated WIC biosorbent.

Figure 1 reveals clear visual differences that indicate structural and compositional changes in the biosorbent. The raw sample is olive green with a coarse texture, reflecting natural pigments, waxes, and organic impurities. The acid-treated sample darkens considerably and forms clumps, suggesting partial breakdown of hemicellulose and lignin, along with removal of colored organic matter. These changes typically enhance surface reactivity and expose more functional groups. In contrast, the base-treated sample appears pale with a finer texture, indicating effective lignin removal and increased surface accessibility. These visual changes correspond to chemical and structural modifications that are expected to affect the biosorbent’s adsorption properties.

2.5. Determination of the point of zero charge of WIC biosorbent samples

The salt addition method was employed to determine the point of zero charge (pHpzc) of WIC samples (untreated, acid-treated, and base-treated). In a 50 mL Erlenmeyer flask, 0.05 g of WIC sample was added to 10 mL of 0.01 M NaCl solution with varying pH values (2, 4, 6, 8, 10, and 12). The initial pH of each flask was adjusted using 0.1 M NaOH/HCl. Subsequently, all flasks were shaken for 24 h at room temperature, and the final pH values were measured. The difference between the initial and final pH values was calculated and plotted against the initial pH values. The point of intersection with the pH axis indicates the pH_pzc [26-28].

2.6. Preparation of standard hexavalent chromium solutions

Potassium dichromate (K₂Cr₂O₇) is used as a source of hexavalent chromium ions (Cr(VI)). A stock solution of 10 mg/L of Cr(VI) is prepared by dissolving 0.0141g of potassium dichromate in 500 mL of distilled water. The solution is diluted as required to obtain 50 mL of a 2 mg/L working standard solution. A series of different standard solutions of Cr(VI) is prepared as follows (0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 2 mg/L). The stock solutions are stored at 4°C to minimize degradation.

2.7. Batch adsorption experiments for the Cr(VI) ions removal

Various important parameters affecting the removal efficiency have been investigated. pH, contact time, and biosorbent dose were studied to maximize removal efficiency. Additionally, to evaluate the performance of the biosorbent on a larger scale and to assess the applicability of the method, an initial Cr(VI) concentration of 50 ppm was selected for the adsorption experiments. This concentration was chosen due to its relevance to typical contamination levels found in industrial effluents. According to previous reports, Cr(VI) concentrations in wastewater from industries such as leather tanning, hardware manufacturing, and electroplating commonly range from 10 to over 100 ppm [29]. Therefore, 50 ppm represents a moderate and realistic concentration within this range, allowing for a practical assessment of the biosorbent’s removal efficiency under industrially relevant conditions. For each experiment, 15 mL of the 50 ppm Cr(VI) solution was measured into a 250 mL Erlenmeyer flask and mixed with the designated biosorbent dose. The solution was stirred at a constant rate (500 cycles/minute) for the desired time at room temperature. Prior to analysis, the sample solution was filtered using 0.2 μm syringe filters and stored in a brown bottle.

The filtrate from the sample was spectrophotometrically analyzed for Cr(VI) ions removal by measuring the absorbance of the solution before and after the adsorption process.

To attain reliable results with good reproducibility and accuracy, each measurement was conducted three times, and then the average was considered. The removal efficiency was calculated as follows (Eq. 1):

(1)

% o f r e m o v a l = (\frac{C o - C e}{C o}) X 100

Where,

$Co$ is the initial concentration of Cr(VI) ions in the solution (ppm), $Ce$ is the equilibrium concentration of Cr(VI) ions in the solution after adsorption (ppm) [30].

2.8. Determination the concentration of Cr(VI) ions

2.8.1. UV-Vis spectrophotometry

The concentration of the Cr(VI) ions in the water sample was measured using a UV-spectrophotometer, by developing a dark red-violet color with 1,5-diphenylcarbazide (DPC) in an acidic condition solution as a complexing agent. Firstly, the following reagent solutions were prepared: 0.1 M sulfuric acid by adding 1mL of concentrated sulfuric acid to deionized water and diluting to 100 mL. Then, 250 mg of DPC (chromophoric dye) was dissolved in 50 mL of acetone and stored in a brown bottle.

2.8.1.1. Blank reagent

A mixture of 3 mL of 0.1 M of sulphuric acid and 6 mL of the prepared solution of DPC solution.

2.8.1.2. Calibration reference solutions

Then, 4 mL of each Cr(VI) standard solution was taken in a proper vial, then its pH was adjusted to ∼2 with 0.1 M sulfuric acid, and then 2 mL of DPC solution was added for complexation. The mixture was gently shaken and left for 5 min for full color development. The absorbance of the red-violet colored complex was measured at 540 nm against the reagent blank, at which the maximum absorbance of the Cr(VI)-diphenylcarbazide complex. The scan range was set between 400-600 nm, with a slit width of 1 nm and quartz cuvettes of 1 cm path length. A calibration curve was plotted from eight standard solutions of concentrations between (0.1 and 2 mg/L), with a correlation coefficient, R², of > 0.99, for the quantitation of Cr(VI) in samples.

2.8.1.3. Sample analysis

The sample was prepared as follows: 24 μL of the Cr(VI) sample was transferred to a glass vial and diluted to 4 mL. The pH was adjusted to ∼2 with 0.1 M sulfuric acid, followed by adding 2 mL of DPC solution. The mixture was gently shaken and left for 5 min for full color development. The absorbance of the red-violet colored complex was measured at 540 nm against a reagent blank [31,32].

2.8.2. ICP-MS analysis

The concentrations of Cr were measured using ICP-MS. The ICP-MS was operated with an RF power of 1550 W, plasma gas flow rate of 15 L/min, auxiliary gas flow of 1.0 L/min, and nebulizer gas flow of 1.05 L/min. The sample uptake rate was 1.0 mL/min, and measurements were acquired in standard mode with a dwell time of 0.5 s per isotope. Calibration was carried out using multi-element standard solutions traceable to NIST.

3. Results and Discussion

3.1. WIC biosorbents characterization

3.1.1. FTIR analysis

FTIR was performed on the three WIC samples to identify the major functional groups responsible for the main adsorption sites. Spectra were represented as transmittance.

Spectra were recorded in the range of 400-4000 cm^-1 with a resolution of 4 cm^-1. The samples were prepared as KBr pellets and scanned at room temperature.

Figure 2 shows the IR spectra of untreated, acid-treated, and base-treated WIC biosorbent. The most characteristic broad and strong intensity peak observed on untreated WIC in the range 3395 -3456 cm⁻¹ corresponds to the OH intramolecular and intermolecular stretching owned by lignin, cellulose, and hemicellulose. Prominent narrow peaks at 2849 and 2916 cm⁻¹ for the CH stretching of CH₂, CH₃, and CH₃O group, and CH vibration of the aromatic methoxyl group, respectively, indicate the presence of lignin, cellulose, and hemicellulose. Small peaks at about 2355 cm^-1 indicate the presence of the C-H group in cellulose. Peaks at 1600-1650 cm⁻¹ refer to the C=C stretching of the aromatic ring, indicating lignin, and at 1170 cm⁻¹ for C–O stretching of the alcohol primary group, –CH₂OH, attributed to cellulose. The peak at 1030 cm⁻¹ is assigned to the C–O stretching at holocellulose and lignin [33,34].

FTIR spectra comparison of untreated, acid-treated, and base-treated WIC biosorbent.

After acid/base modification, noticeable shifts in the positions of the bands occurred, especially during base treatment (1170 to 1157 and 1070 cm⁻¹, 1650 to 1637 and 1452 cm⁻¹, and 3456 to 3445 and 3395 cm⁻¹ for acid and base treatment, respectively).

Additionally, the peak intensity related to O-H groups increased and became sharper (at 3445 and 3395 cm⁻¹), indicating the presence of free OH sites on the modified WIC biosorbent. Moreover, narrow peaks of C=C and C-H stretching bonds of 1637 and 1452 cm⁻¹, 987 and 864 cm⁻¹ become sharper. On the other hand, a relative decrease in the intensity of the peaks in the region of 2849 and 2916 cm^-1, which represent the alkyl R-group, indicates that acid/base causes semi-breaking of the WIC lignin into its phenylpropane monomer [35]. Additionally, two distinguished peaks appeared in the range of 2322-2470 cm⁻¹, detected in base-treated WIC biosorbent, confirming the modification process. These changes effectively enhance the absorption characteristics. The spectroscopic results suggest that the modification process did not collapse the basic structure of the WIC biosorbent [36].

Furthermore, these several notable shifts in absorption peaks indicate enhanced adsorption of Cr(VI) ions through the involvement of specific functional groups in the binding process. For example, the peak at 1170 cm⁻¹ in the raw biosorbent shifted to 1157 cm⁻¹ after adsorption. This red shift suggests the formation of coordination bonds or strong hydrogen bonding between Cr(VI) ions and oxygen-containing functional groups. Such interactions likely increase adsorption efficiency by providing active sites for electrostatic attraction or ligand exchange. Additionally, changes in the O–H stretching region (around 3445 and 3395 cm⁻¹), including peak sharpening and shifting, further support the involvement of hydroxyl groups in Cr(VI) complexation.

These spectroscopic changes are widely recognized as strong evidence of surface chemistry alterations and structural modifications in biosorbent surfaces, especially in the absence of SEM/Brunauer-Emmett-Teller (BET) data [37-39].

3.1.2. Powder X-ray diffraction (XRD) analysis

The XRD pattern of WIC biosorbent samples before and after modification has been shown in Figure 3. The peaks of untreated WIC exhibit a broad peak at 21.5°, 16°, and 35°, which correspond to the characteristic of amorphous cellulose, and peaks at 10° and 40.5°, which correspond to sodium carbonate hydrate [40,41].

XRD spectra of untreated, acid-treated, and base-treated WIC biosorbent.

During acid and base modification, the whole skeleton of the WIC biosorbent samples remained stable and did not deteriorate. Additionally, the amorphous phase decreased, and the structure became more crystalline as the number of intramolecular and intermolecular hydrogen bonds increased.

In the spectra of acid-treated WIC biosorbent, a broad peak at 21.5° indicates that amorphous cellulose is the main phase. The intensities of some peaks at 10° and 40.5° become smaller, and sometimes disappear, indicating a trace of Na₂CO₃.xH₂O. These alterations in the spectrum confirm the acid modification process.

Meanwhile, distinct changes are noticeable during base modification. For instance, the intensity of the main peak at 21.5° and 16° becomes sharper and stronger. Overall, these main changes confirm the chemical modification and are highly consistent with the results shown in Figure 2. Moreover, increased crystallinity can affect adsorption behavior in multiple ways. On one hand, a more crystalline structure enhances the thermal and mechanical stability of the biosorbent, improving its durability, reusability, and structural integrity during the adsorption process. On the other hand, crystallinity may promote the exposure of new functional groups and the formation of additional pores as a result of acid or base modification, thereby enhancing the overall adsorption capacity by increasing the number of active binding sites.

Although SEM and BET analyses provide direct insights into surface morphology and surface area, XRD is also a well-established tool for assessing chemical and structural changes in biosorbents. Alterations in XRD patterns suggest modifications in crystallinity or the formation of new complexes. Such indirect evidence has been employed effectively in prior studies to support adsorption mechanisms in the absence of SEM/BET data (e.g., [42,43]).

Data were collected with Cu Kα radiation (λ = 1.5406 Å), operating at 40 kV and 40 mA. Scans were performed over a 2θ range of 5°-80° at a step size of 0.02°.

3.2. Batch adsorption

In this section, results and analysis of the adsorption efficiency based on various parameters have been presented. Different graphical figures were built to figure out the optimal conditions. pH, contact time, and biosorbent dose are studied and analyzed for maximum removal efficiency.

The experiments were conducted at room temperature (25°C), which is the preferred temperature for industrial processes, primarily for its economic and environmental benefits.

It is acknowledged that a blank control experiment was not conducted in this study. While the experimental conditions were carefully controlled to minimize external influences, the absence of a blank control may allow minor contributions from non-adsorptive removal mechanisms such as photolysis, volatilization, or adsorption onto the container surfaces. Although these effects are considered negligible, their potential presence may slightly influence the interpretation of Cr(VI) removal efficiency. Future studies will incorporate appropriate blank control experiments to ensure greater accuracy and reliability of the adsorption results.

The obtained data were examined using pseudo-first-order and quasi-secondary models to investigate the kinetic adsorption characteristics. Additionally, adsorption isotherm studies were conducted using Langmuir and Freundlich models.

3.2.1. Effect of pH

The variation in the pH of the medium significantly affects the binding on the surface of the biosorbent, thereby impacting the removal efficiency of the Cr(VI) ions.

The measurement of the pH_pzc is necessary since the pH of the solution controls the surface charge of the adsorbent and the magnitude of electrostatic charges [44]. Therefore, the pH_pzc of the biosorbent was measured, and they were found to be 6.5, 4, and 7.5 for untreated, acid-treated, and base-treated WIC samples, respectively (Figure 4), further confirming the modification of the WIC samples via acid and base treatments.

Point of zero charge (pHpzc) of WIC biosorbent.

The effect of pH was tested for the three biosorbent types using batch experiments. For all samples, the pH of the Cr(VI) ion solution was adjusted to different values, all less than the pH_pzc, by adding the required amount of 0.1 M NaOH/HCl solution.

For the untreated sample, 0.1 g of adsorbent was used with 15 mL of 50 ppm of Cr(VI) for 24 h, at different pH values of 2, 4, and 6. For the acid-treated and base-treated samples, the conditions were as follows: 0.01g biosorbent dose with 10 mL of 50 ppm of Cr(VI) for 24 h of contact time, with different pH values of 2, 3, and 4 for the acid-treated samples, while the pH values for the base-treated samples were 3, 5, and 7.

The pH value at which the maximum Cr(VI) ions were removed was determined. The quantitation of Cr(VI) in samples was calculated from the calibration equation, which showed good linearity and an excellent correlation coefficient (R²), where y is absorbance and x is concentration in ppm (Table 1).

Table 1. RSD%, LOD, and LOQ of the UV-Spectroscopy for method validation (n=9).

Type of sample	Linear equation	Correlation coefficient (R²)	RSD%	LOD, ppm	LOQ, ppm
Untreated WIC	y = 0.4347x - 0.0047	0.9914	1.0812	0.0357	0.1081
Acid-Treated WIC	y = 0.4443x + 0.0113	0.9984	2.5433	0.0839	0.2543
Base-Treated WIC	y = 0.4612x + 0.0093	0.9964	2.0165	0.0665	0.2016

Figure 5 shows the variation of the ion removal with changing pH values. The data show that the best removal efficiency can be reached between 2-3 for the three types. This can be explained by the concept of pH_pzc. At pH values below the pH_pzc, the surface of the adsorbent will be mainly positively charged, while a net negative charge would exist when the solution pH is above the pH_pzc. Based on that, at higher pH, the competition between hydroxyl groups and the anion part in the chromate ion complex is increased, leading to a reduction in the adsorption of metallic ions [45]. Furthermore, the remarkable adsorption occurs at acidic media when the pH values are smaller than the PZC; pH values of 2 and 3 seem to be optimum values for acid- and base-treated WIC biosorbent, respectively. At these pH values, chromates are the major species in the solution, and the surface of biosorbent has a net positive charge; its active sites are fully protonated and easily bonded to anionic species via electrostatic forces, resulting in an increase in surface potential. As is well known, aqueous speciation of Cr(VI) varies with pH and Cr(VI) concentration [46]. The dominant species between pH of 0 and 6 is HCrO₄^-1 at Cr(VI) concentrations below approximately 0.003 mol/L (160 mg/L) and Cr₂O₇^2- at higher Cr(VI) concentrations, so the adsorption becomes more favorable at lower pH values [47]. This fact is in line with the findings of cited references [48]. On the other hand, the reduction of Cr(VI) to Cr(III) in an aqueous solution is promoted by low pH [49,50]. Furthermore, in the presence of oxygen, Cr(III) is thermodynamically unstable. Additionally, the concentrations of Cr(III) in aqueous systems are limited by the low solubility of the formed precipitate of Cr(OH)₃ [51].

Effect of pH on the % removal of Cr(VI) ions onto different types of WIC biosorbent.

However, the data in Figure 5 showed that the acid-treated biosorbent exhibits the best removal efficiency compared to base-treated and untreated biosorbent; moreover, the base-treated one is still better than the untreated samples. These differences may be attributed to several factors associated with acid modification. Acid treatment effectively removes surface impurities, including minerals and amorphous components like hemicellulose and lignin, which not only cleans the biosorbent surface but also increases the exposure of functional groups, such as carboxyl, hydroxyl, and phenolic moieties; key contributors to metal ion binding. Additionally, acid treatment often promotes partial hydrolysis of the biomass structure, leading to an increase in porosity and surface roughness, which improves the accessibility of active sites for Cr(VI) adsorption. Compared to base-treated biosorbents, which may suffer from pore collapse or excessive swelling of cellulose fibers, acid-treated materials typically maintain a more open and stable porous structure. This structural integrity, along with a greater density of protonated functional groups under low pH conditions, facilitates electrostatic attraction and potential ligand exchange with anionic Cr(VI) species such as HCrO₄⁻ and Cr₂O₇²⁻. Thus, the combination of increased functional group availability, improved pore characteristics, and favorable surface charge conditions explains the superior Cr(VI) adsorption observed in acid-treated biosorbents.

3.2.2. Effect of contact time

The effect of contact time was studied over time periods reaching 24 h. The batch process was carried out at an initial concentration of 50 ppm Cr(VI) ions, pH of 2, and 3 for acid and base-treated WIC biosorbent, respectively, with a biosorbent dose of 0.01g at ambient temperature. Figure 6 illustrates the effect of different time periods on removal efficiency. The best removal efficiency can be observed up to 6 h for both untreated and base-treated WIC, and around 20 h for the acid-treated samples. A further increase in contact time had a negligible effect on adsorption; beyond a certain contact time, the adsorption capacity shows a negligible increase, indicating that equilibrium has been reached. This plateau occurs because most of the available active sites on the biosorbent surface become saturated, limiting further uptake of Cr(VI) ions.

Effect of contact time on the % removal of Cr(VI) ions onto different types of WIC biosorbent.

It is evident that the efficiency of removal increases with the contact time. This result can be explained based on the limited number of available sites on the surface of the biosorbent at higher contact times.

The removal efficiency is fast at the beginning of Cr(VI) ion adsorption due to the large concentration gradient between the fluid bulk and the available pore sites. However, as the adsorption sites become saturated over time, their ability to capture ions is reduced [52]. Therefore, considering the optimal contact time of 20 h is essential. Furthermore, the acid-treated biosorbent exhibits the best removal efficiency compared to the other biosorbent samples.

The equilibrium time is achieved after approximately 20 h under batch conditions; it may be considered lengthy for practical industrial applications, particularly in continuous treatment systems.

To improve the feasibility of the biosorption process for real-world wastewater treatment, several strategies could be employed to reduce the required contact time. These include reducing the particle size of the biosorbent to increase the surface area and enhance diffusion rates, increasing the biosorbent dosage to provide more active sites within a shorter time frame, and improving mixing or agitation to accelerate mass transfer. Additionally, further surface modification or chemical activation of the biosorbent could increase its affinity toward Cr(VI) ions, thereby improving adsorption kinetics.

Future studies should focus on kinetic modeling under continuous flow conditions and on optimizing these parameters to ensure the process is scalable and efficient for industrial implementation.

3.2.3. Effect of biosorbent dose

The effect of the biosorbent dose has been evaluated, considering its significance from both an economic point of view and in terms of the degree of removal of Cr(VI) ions. The process was conducted at different doses (0.01, 0.03, 0.05, 0.07, 0.09, 0.1 g/L), with 10 mL of 50 ppm Cr(VI) ions concentration, pH 2, and 3 for acid- and base-treated WIC biosorbent, respectively. The contact time was 6 h for both untreated and base-treated WIC, and around 20 h for the acid-treated samples. The effect of the biosorbent dose has been illustrated in Figure 7. The data show that the maximum removal efficiency can be observed at 0.1g biosorbent doses, and there was a significant change at doses greater than this concentration for both treated WIC samples. The increase in the amount of biosorbent dose significantly enhances the removal efficiency. However, there was no significant removal of Cr(VI) ions at high biosorbent doses due to the decrease in the available Cr(VI) ions for adsorption, even in the presence of more biosorbent sites [53].

Effect of dose on the % removal of Cr(VI) ions onto different types of WIC biosorbent.

3.3. Method validation for determination of Cr ion content by UV-spectroscopy

The limit of detection (LOD) and limit of quantitation (LOQ) were calculated to validate the analysis method. The LOD is the lowest possible concentration at which the method can detect the analyte (but not quantify) within the matrix with certain reliability, while the LOQ is the lowest level at which an analyte can be quantified with an acceptable degree of certainty, precision, trueness, and reproducibility [54].

The LOD was estimated from the calibration function according to the formula: LOD = 3.3(SD/S), and LOQ was similarly defined according to the formula: LOQ = 10(SD/S), where (SD) = standard deviation of the response based on the standard deviation of the y-intercepts of the regression lines. (S) = slope of the calibration curve. In addition, the relative standard deviation (RSD%) was calculated according to the formula = (S/X!) × 100 Where X! is the mean of the observations [55]. The results have been summarized in Table 1.

The calibration curves for nine calibration standard solutions were found to be linear in the range of 0.1-2.0 ppm. Each concentration level was measured in triplicate, and the average absorbance values were plotted against Cr(VI) concentration. Error bars representing standard deviations were included for each point. The calibration curves for the three bio-adsorbent samples showed excellent linearity with correlation coefficient values being better than 0.990, which suggested that the data obtained by the UV-Spectroscopy method has high stability, reproducibility, and reliability.

Results showed that the LOQ for all bio-adsorbents ranged from 0.047 to 0.113 ppm. Therefore, this method is sensitive enough to quantify Cr content in water samples. On the other hand, RSD% values obtained for all bio-adsorbents were less than 3%, which indicates good precision [56,57].

The data were recorded with an instrument accuracy of ±0.3 nm for wavelength and ±0.3% T for photometric measurements.

3.3.1. Comparison of Cr ion content determined by UV-spectroscopy and ICP-MS using regression analysis

Regression analysis was implemented to further assess the relationships among the values obtained by UV-Spectroscopy and ICP-MS. Twelve samples of industrial water were analyzed by the two instruments, and the responses were recorded.

The results showed that the regression coefficient (R²) was 0.9375 (Figure 8). A high value of R² shows a stronger relationship between these two methods, and suggests a good agreement between the results of Cr ion concentrations determined by UV-Spectroscopy and ICP-MS. Consequently, it is concluded that the UV-Spectroscopy method is remarkably good and accurate when compared with ICP-MS.

Regression analysis of Cr ions content in industrial water samples measured by UV-Spectroscopy and ICP-MS.

Furthermore, it was observed that the values measured by UV-Spectroscopy were lower than the data measured by ICP-MS, but they were very close to each other in the range under 35 ppm concentrations of Cr ions content, and there was a linear relationship between them. Moreover, the linear regression analysis results reinforce the findings presented in Table 1, indicating that the data collected through both methods are highly stable and reliable. This consistency highlights the robustness of the results and aligns with the current state-of-the-art practices in the field.

Practically, the instrument for ICP-MS determination is more expensive than that of UV-Spectroscopy. Therefore, it is a practical choice to choose UV-Spectroscopy, especially considering its LOD less than 0.7729 ppm, even though it may consume more time and sometimes requires dilution of the solution.

To further validate the consistency between UV-Vis and ICP-MS techniques, Table 2 presents the actual Cr ion concentrations measured by both methods before and after adsorption by acid-treated WIC. The results show strong agreement, with RSD% values obtained at less than 3%, which indicates good precision between the two techniques. This close correlation supports the reliability of UV-Vis spectrophotometry as a practical and accurate method for Cr ion quantification in routine analyses, particularly when ICP-MS is not readily available.

Table 2. Initial actual Cr ions concentrations in industrial water samples measured by UV-Vis and ICP-MS.

Initial Cr ions (ppm)	Final Cr ions (ppm) UV-Vis	Final Cr ions (ppm) ICP-MS	RSD%	% Removal using UV-Vis
119	16.41	17.63	0.86	86.21
180	23.55	23.96	0.29	86.92
182	33.78	34.12	0.24	81.44
278	35.05	35.62	0.40	87.39
189	36.01	38.16	1.52	80.95
380	37.51	35.24	1.61	90.13
235	38.15	37.16	0.70	83.77
195	38.64	41.28	1.87	80.18
220	40.47	44.96	3.17	81.60
215	40.79	42.70	1.35	81.03
387	42.64	46.50	2.73	88.98
234	43.14	41.24	1.34	81.56

Measurements were taken under optimum conditions post-adsorption by acid-treated WIC, and all values represent the average of triplicate measurements.

3.4. Kinetics studies of the adsorption process

Experimental data were simulated using both pseudo-first-order and quasi-secondary models, with their corresponding equations described previously [58]. The experimental data and the simulated results have been presented in Figure 9. The results indicated that the quasi-secondary model provided a significantly better fit to the data than the pseudo-first-order model, with higher correlation coefficients (R² > 0.99). Therefore, only the kinetic parameters of the quasi-secondary model have been summarized in Table 3, as they offer a more accurate explanation of the experimental data. This is supported by the comparison of the experimental (Q_{m, exp} (mg.g^-1)) values (where Q_m is the maximum value) with the calculated ones (Q_{m, cal} (mg.g⁻¹)), and by comparing the correlation coefficients R² for each model. Additionally, a good agreement was found between the experimental and calculated values of Q_m, indicating that one of the mechanisms or rate-determining steps of the adsorption process may be by chemisorption [59]. Moreover, the values of Q_{m, exp}, Q_{m, cal}, and k₂ follow the order:

$\begin{matrix} Q m \exp A c i d - T r e a t e d > Q m \exp B a s e - T r e a t e d > Q m \exp U n t r e a t e d W I C \\ Q m c a l A c i d - T r e a t e d > Q m c a l B a s e - T r e a t e d > Q m c a l U n t r e a t e d W I C \\ k 2 U n t r e a t e d W I C > k 2 B a s e - T r e a t e d > k 2 A c i d - T r e a t e d \end{matrix}$

Plots of (a) Pseudo-first-order, and (b) quasi-secondary models for the adsorption of Cr(VI) ions onto different types of WIC. 10mL of Cr(VI) of 50ppm Cr(VI) ion concentration were conducted at optimum values of contact time, pH, and adsorbent dose.

Table 3. Quasi-secondary model kinetic parameters for the adsorption of Cr(VI) ions onto different type of WIC biosorbent.

WIC biosorbent sample

Q_{m, exp}

(mg.g⁻¹)

Q_{m, cal}

(mg.g⁻¹)

Difference

(Q_{m, cal –}Q_{m, exp)}

k₂

(g.mg⁻¹ min⁻¹)

R²

Untreated

8.6600

9.3721

0.7121

0.00098

0.9953

Base-Treated

38.8430

39.8406

0.9976

0.00064

0.9998

Acid-Treated

58.0366

60.9756

2.9390

0.00013

0.9943

It is worth noting that these results are consistent with other findings.

3.5. Adsorption isotherms studies of adsorption process

An adsorption isotherm reveals the mathematical relationship between the equilibrium concentration of an adsorbent and its adsorption capacity at a constant temperature. The most common types of isotherms are Langmuir and Freundlich models; their corresponding equations are described previously [60]. The experimental data and the simulated results have been given in Figure 10, while Table 4 summarizes the predicted Freundlich model parameters. It can be noted that the adsorption of Cr(VI) ions onto all types of WIC fit the Freundlich isotherm model better than the Langmuir isotherm model, with high correlation coefficient values (R²). Thus, multilayers coverage of Cr(VI) ions was adsorbed onto heterogeneous WIC surfaces. K_F and n are constants indicating the adsorption capacity and the adsorption intensity, respectively. In general, as the K_F value increases, the adsorption capacity of the adsorbent increases [61]. Therefore, the acid-treated method is the best for enhancing the adsorption capacity of WIC. This result is consistent with the other previous results.

Plots of (a) Langmuir, and (b) Freundlich isotherm models for the adsorption of Cr(VI) ions onto different types of WIC. 10 mL of 50 ppm Cr(VI) ions concentration was conducted at optimum values of contact time, pH, and adsorbent dose.

Table 4. Freundlich isotherm parameters for the adsorption of Cr(VI) ions onto different type of WIC.

	n (mg/g)	K_F (mg/g)	R²
Untreated WIC	1.4935	0.1463	0.9646
Base-Treated WIC	1.0607	0.4403	0.9809
Acid-Treated WIC	2.3543	3.0568	0.9978

3.6. Comparison of adsorption ability of Imperata cylindrica towards heavy metal ion

The maximum adsorption capacity of the WIC biosorbent towards Cr(VI) was also compared with previously studied cases involving different heavy metal ions, as shown in Table 5 [22-25, 29, 35, 62-64]. As seen in this table, the WIC biosorbent was readily utilized for the uptake of various heavy metal ions from wastewater. Additionally, the adsorption capacity of Cr(VI) by the WIC used in this study is higher than that of other heavy metal ions such as Pb(II) and Cd(II).

Table 5. A Comparison of the maximum adsorption capacity of Imperata Cylindrica biosorbent towards different heavy metal ions based on the quasi-secondary model.

Heavy metal ion	Adsorption capacity Q_m (mg.g⁻¹)	Reference
Ni(II)	5.0056 (H₂SO₄ -treated)	[29]
Ni(II)	5.0304 (H₃PO₄ -treated)
Ni(II)	4.550	[25]
Cd(II)	0.067	[22]
Cu(II)	5.890	[24]
Pb(II)	3.600	[62]
Pb(II)	0.978	[23]
Ni⁽II)	16.469 (H₂SO₄ -treated) 7.7570 (H₃PO₄ -treated)	[35]
Zn(II)	To be determined	[63]
Fe(II)	Not specified	[64]
Cr(VI)	58.0366 (Acid-treated)	This work

Overall, it is considered that the properties of WIC have significant effects on its efficiencies, such as its habitat, and experimental conditions also affect the results. Moreover, WIC biosorbents are easily accessible and readily available, confirming that activated WIC could be a promising candidate for the uptake of some heavy metal ions from wastewater.

As shown in Table 6 [65-74], different biosorbents have been investigated for the removal of Cr(VI) ions due to their low cost, convenient accessibility, recyclability, and biodegradability. This suggests that WIC may work well and hold promise as a biosorbent for the removal of Cr(VI) ions from contaminated water. Enhancing its adsorption efficiency could be achieved by optimizing all factors to the optimum condition, such as the equilibrium contact time, pH, and biosorbent dosage. Furthermore, activation of its performance through different approaches could be explored. Particularly, WIC biosorbent could decrease toxic Cr(VI) ions.

Table 6. A comparison of the intensity of adsorption for Cr(VI) ions by different biosorbents based on the Freundlich isotherm model.

Biosorbent type	Intensity of adsorption n (mg/g)	Reference
Allium sativum L (Garlic stem) Aesculus hippocastanum (Horse chesnut shell)	1.86 2.23	[65]
Ficus carica	1.72	[66]
Date palm empty fruit bunch	1.68	[67]
Pongamia pinnata shells (sulphuric acid-activated)	1.60	[68]
Phanera vahlii biochar	1.92	[69]
Dalbergia sissoo pods	0.0016 (Hydrated beads) 0.0017 (Carbonized form)	[70]
Heinsia crinita seed coat	0.91	[71]
Activated Carbon (WD-ekstra) Activated WD(HCl)	0.972 1.18	[72]
Leucaena leucocephala	2.13	[73]
Almond green hull	2.10	[74]
WIC	2.35 (Acid-treated)	This work

4. Conclusions

Chemical activation by an inorganic base or acid of WIC creates an efficient biosorbent for the hexavalent chromium ions from aqueous solutions. Treated WIC biosorbent exhibits a greater affinity for Cr(VI) ions than untreated ones. Furthermore, a significant increase in the percentage removal was observed for the acid-treated WIC rather than the base-treated ones. The best results were obtained under optimum conditions of 20 h of contact time in acidic media. The experimental data show that the quasi-secondary model appropriately explains the adsorption mechanism. Moreover, the Freundlich model adequately describes the adsorption data. Consequently, this method is successful and environmentally friendly, especially suitable for operation in developing countries. Therefore, this work successfully transforms abundant renewable agricultural by-products into useful and low-cost biosorbents for Cr(VI) ions via simple processing. Additionally, the DPC method is a reliable method to quantify Cr(VI) ions in aqueous solutions because of the intense color development at the 540 nm region, and good validation results using UV-Vis and ICP-MS techniques.

While the results are promising, practical challenges must be addressed for real-world application. These include the need to reduce contact time, improve biosorbent regeneration efficiency, and ensure cost-effective scalability of the preparation process. Moreover, the presence of competing ions in real wastewater may influence adsorption performance, requiring further investigation.

Future research should focus on applying this biosorbent to real industrial wastewater samples, exploring continuous-flow systems for process optimization, and evaluating its performance in multi-metal systems. Special attention should be given to the effects of coexisting ions commonly found in textile wastewater—such as inorganic anions (e.g., sulfate, chloride, phosphate, and nitrate), heavy metal ions like Pb (II), Cd (II), and Zn (II), and metal-containing dyes. These substances may compete with Cr(VI) for active adsorption sites and influence overall removal efficiency, as supported by relevant literature. Enhancing regeneration cycles, reusability, and integrating biosorption with other treatment technologies could further improve the viability of this approach in large-scale water treatment applications. Future studies will focus on these aspects to enhance the long-term performance and cost-e of the biosorbent system.

Acknowledgment

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2602).

CRediT authorship contribution statement

Khansaa Al-Essa, M. A. Habib, Ethar M. Al-Essa; Methodology: Khansaa Al-Essa, M. A. Habib; Formal analysis and investigation: Khansaa Al-Essa, M. A. Habib, Ethar M. Al-Essa; Writing—original draft preparation: Khansaa Al-Essa, M. A. Habib; Writing—review and editing: Khansaa Al-Essa, Ethar M. Al-Essa; Funding acquisition: Khansaa Al-Essa, M. A. Habib, Ethar M. Al-Essa; Resources: Khansaa Al-Essa, M. A. Habib; Supervision: Khansaa Al-Essa, M. A. Habib.

Declaration of competing interest

The authors declare no conflicts of interest for this work.

Data availability

The experimental data of this study is available upon request.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Materials and Methods

2.1. Chemicals
2.2. Agro-waste
2.3. Instrumentation
2.4. Modification of the WIC samples
2.5. Determination of the point of zero charge of WIC biosorbent samples
2.6. Preparation of standard hexavalent chromium solutions
2.7. Batch adsorption experiments for the Cr(VI) ions removal
2.8. Determination the concentration of Cr(VI) ions

3. Results and Discussion

3.1. WIC biosorbents characterization
3.2. Batch adsorption
3.3. Method validation for determination of Cr ion content by UV-spectroscopy
3.4. Kinetics studies of the adsorption process
3.5. Adsorption isotherms studies of adsorption process
3.6. Comparison of adsorption ability of Imperata cylindrica towards heavy metal ion

4. Conclusions

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Removal of hexavalent chromium from industrial wastewater using modified imperata cylindrica biosorbent and method validation

Abstract

Keywords

Agriculture by-products

Hexavalent chromium ions

Kinetic and isotherm adsorption characteristics

Method validation

Modified Imperata cylindrica biosorbent

1. Introduction

2. Materials and Methods

2.1. Chemicals

2.2. Agro-waste

2.3. Instrumentation

2.4. Modification of the WIC samples

2.5. Determination of the point of zero charge of WIC biosorbent samples

2.6. Preparation of standard hexavalent chromium solutions

2.7. Batch adsorption experiments for the Cr(VI) ions removal

2.8. Determination the concentration of Cr(VI) ions

2.8.1. UV-Vis spectrophotometry

2.8.1.1. Blank reagent

2.8.1.2. Calibration reference solutions

2.8.1.3. Sample analysis

2.8.2. ICP-MS analysis

3. Results and Discussion

3.1. WIC biosorbents characterization

3.1.1. FTIR analysis

3.1.2. Powder X-ray diffraction (XRD) analysis

3.2. Batch adsorption

3.2.1. Effect of pH

3.2.2. Effect of contact time

3.2.3. Effect of biosorbent dose

3.3. Method validation for determination of Cr ion content by UV-spectroscopy

3.3.1. Comparison of Cr ion content determined by UV-spectroscopy and ICP-MS using regression analysis

3.4. Kinetics studies of the adsorption process

3.5. Adsorption isotherms studies of adsorption process

3.6. Comparison of adsorption ability of Imperata cylindrica towards heavy metal ion

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of generative AI and AI-assisted technologies in the writing process

References

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