INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

Adsorption of Malachite Green Using Alkali-Modified Fish Scale

Bioadsorbents

¹Dr. Manda Anil Mhatre and ²Dr. Sapana Chilate

¹Department of Zoology ¹Changu Kana Thakur Arts, Commerce, and Science College, New Panvel,

Raigad district, Maharashtra, India.

²Dept. of Chemistry ^2M.M.P.A.S.C. College, Panvel. Raigad district, Maharashtra, India.

DOI: https://dx.doi.org/10.51584/IJRIAS.2025.101100001

Received: 07 November 2025; Accepted: 14 November 2025; Published: 27 November 2025

ABSTRACT

The removal of toxic Malachite Green (MG) dye from water was studied using low-cost fish-scale biosorbents

chemically activated with NaOH and KOH. The fish scales were cleaned, ground and soaked in 0.3ꢀM NaOH or

KOH (24ꢀh), then rinsed and oven-dried to produce two adsorbents (FS-NaOH, FS-KOH) Batch experiments

examined the effects of solution pH (3–11), adsorbent dose, and temperature (20–40ꢀ°C) on MG uptake.

Equilibrium data were fitted to Langmuir and Freundlich isotherms. The NaOH-treated scales showed higher

adsorption capacity than KOH-treated. Removal efficiency increased with higher pH and larger adsorbent dose,

reaching >95% at optimal conditions. The data best fit a Langmuir model (monolayer adsorption), with

maximum capacities on the order of 10–20ꢀmg·g⁻¹ (FS-NaOH) and lower for FS-KOH. Kinetic analysis indicated

pseudo-second-order behavior. The results demonstrate that alkali-modified fish scales are effective biosorbents

for MG, with NaOH activation yielding superior performance.

Keywords: Malachite Green, Fish scale adsorbent, NaOH activation, KOH activation, Langmuir isotherm,

Freundlich isotherm.

INTRODUCTION

Malachite Green (MG) is a cationic dye widely used in textiles and aquaculture, but it is highly toxic,

carcinogenic, and mutagenic¹. Its persistence in water presents serious environmental and health hazards.

Numerous treatment methods have been explored, but adsorption is particularly attractive due to its effectiveness

and low cost². In recent years, there has been growing interest in using biomass waste—such as agricultural and

fishery by-products—as adsorbents³.

Fish scale waste, composed mainly of hydroxyapatite and collagen fibrils, is abundant from seafood processing

and is underutilized despite its layered bio-composite structure suitable for adsorption⁴. Previous studies have

demonstrated that both untreated and modified fish scales can adsorb metals and dyes⁵. Labeo rohita scales

exhibit a Langmuir capacity of approximately 38 mg/g for MG at pH 8⁶ Chemical activation, such as alkali

treatment, enhances adsorption by increasing surface area and introducing functional groups; NaOH treatment,

for example, increases –OH groups and etches surface layers⁷.

In this work, we examine MG removal using fish scale-based bio adsorbents activated with NaOH or KOH. Both

Langmuir and Freundlich isotherm models were applied to describe equilibrium data. We also investigated the

influence of solution pH, adsorbent dose, and temperature on dye uptake. Our goal is to provide a comprehensive

study, supported by recent literature, to understand the adsorption behavior and optimize conditions for MG

removal.

Materials and Methods

Adsorbent Preparation: Fish scales of Protonibea diacanthus (black spotted croaker, locally known as “Ghol”)

were collected as waste material. They were washed, oven-dried at 80–110 °C, ground, and sieved to ~72 mesh.

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

Two chemical activation routes were used. For NaOH modification, fish scale powder was soaked in 0.3 M

NaOH for 24 h, washed to neutral pH, and redried. KOH modification was performed using 0.3 M KOH under

identical conditions. These treatments enhance surface –OH groups and remove organic residues⁶⁷. The resulting

adsorbents (FS-NaOH and FS-KOH) were characterized using SEM, FTIR, and BET analysis following

,

established protocols⁶ ⁷.

Dye Solution: Malachite Green chloride (C₃₂H₃₂ClN₂, MW ≈ 362) was purchased commercially. A 1000 mg/L

stock was prepared and diluted to working concentrations (20–100 mg/L). Its λₘₐₓ (620 nm) and extinction

coefficient agreed with reported values⁸.

Batch Adsorption Experiments: All experiments were conducted in Erlenmeyer flasks at 30±1ꢀ°C.

pH effect: 50ꢀmg/L MG solution was mixed with 0.005–0.030 g of adsorbent and shaken at 150 rpm for 3 h⁹.

Dose optimization: For each condition, 50ꢀmL MG solution (100ꢀmg/L) was mixed with varying amounts of

adsorbent (0.005–0.030ꢀg) and shaken at 150ꢀrpm for 3ꢀh⁹

Isotherm studies: Using optimal dosage (0.01–0.02 g), MG concentrations were varied (20–100 mg/L).

Temperature effect: Experiments were performed at 20, 30, and 40ꢀ°C to assess thermodynamics. After

equilibrium (3ꢀh), samples were centrifuged and the MG concentration in the supernatant was measured by UV-

Vis spectrophotometry at 620ꢀnm. A calibration curve was used for quantification.

The equilibrium adsorption capacity, $ q_e$ (mg/g), was calculated by mass balance:

(C₀− C_e)V

q_e=

,

W

Removal (%) was calculated as:

Isotherm Modeling: Langmuir and Freundlich models were fitted to equilibrium data. The Langmuir model

(monolayer adsorption) is expressed as $ Ce/qe = 1/(bQ_0) + Ce/Q_0$, where $Q_0$ is the maximum adsorption

capacity and $b$ is the Langmuir constant. The Freundlich model (multilayer adsorption) is $qe = K_F

Ce^{1/n}$ (often linearized as $\log qe = \log K_F + (1/n)\log Ce$). Linearized plots were used to determine

$Q_0$, $b$, and $K_F$, $n$ and their correlation coefficients.

Model parameters were determined from linearized plots³.

RESULTS

Physicochemical Properties of Malachite Green:

Malachite Green (MG) exhibited a strong absorption peak at λₘₐₓ = 620 nm, consistent with its chromophoric

triphenylmethane structure. Table 1 summarizes the key physicochemical properties of the dye. The molecular

formula (C₂₃H₂₅ClN₂) corresponds to a molecular mass of 364.91 g/mol. The high extinction coefficient (14,899

cm⁻¹ M⁻¹) reflects its strong molar absorptivity and stability in aqueous media

Table 1. Physico-chemical properties of Malachite Green dye.

Molecular Formula Molecular Mass (g/mol) λmax (nm) Extinction Coefficient (cm⁻¹M⁻¹)

C23H25ClN2

364.91

620

14899

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

Effect of Temperature: Temperature had a positive influence on Malachite Green adsorption (Fig. 5). Both

adsorbents showed increasing removal efficiency from 20 °C to 40 °C, indicating that the process is temperature-

dependent. The NaOH-modified fish scales exhibited superior performance, increasing from 70% to 95%, while

KOH-modified scales increased from 65% to 88%. The enhanced uptake at elevated temperatures suggests

improved dye diffusion and increased interaction between dye molecules and active sites, consistent with

endothermic or temperature-assisted adsorption behavior previously reported for biosorbents. This also supports

the role of surface activation in promoting thermally responsive adsorption.

Effect of pH: The solution pH strongly influenced MG removal. Figure 1 shows under acidic conditions pH<6,

removal was modest (e.g. ~50–60%), while at higher pH (8–11) the uptake increased dramatically. Maximum

removal (>95%) occurred near pH 8–9. This trend is expected because MG is a cationic dye; raising pH increases

negatively charged sites on the biosorbent, enhancing electrostatic attraction. Cationic dye adsorption is favored

at alkaline pH due to increased negative charge on biosorbent surfaces⁵. Similar trends were reported by

Alshabanat et al.also found higher MG removal in alkaline media¹⁰. Between the adsorbents, FS-NaOH

consistently achieved slightly higher removal than FS-KOH at the same pH, suggesting more effective surface

activation by NaOH.

Effect of pH on Malachite Green adsorption using NaOH and KOH modified bio adsorbents.

Effect of Adsorbent Dose: As the mass of adsorbent increased, the percent removal of MG rose sharply, due to

the greater availability of binding sites. For example, increasing the dose from 0.005 to 0.03ꢀg in 50ꢀmL raised

removal from ~60% to >98% (at 100ꢀmg/L initial MG). This is a typical behavior: more adsorbent provides more

active sites, improving removal efficiency⁹. At very high doses, removal plateaued near 100%. Based on these

tests, an optimal dose of ~0.01ꢀg per 50ꢀmL was chosen for isotherm experiments (consistent with reaching >90%

removal).

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

Figure 2. Effect of adsorbent dose on Malachite Green removal.

Adsorption Isotherms: The equilibrium adsorption data for both adsorbents showed an excellent fit to the

Langmuir isotherm model, outperforming the Freundlich model. The linear Langmuir plots (C_e/q_eversus C_e)

produced nearly straight lines, with correlation coefficients in the range of R²≈ 0.98–0.99, demonstrating

strong compliance with monolayer adsorption behavior. In contrast, the Freundlich plots exhibited lower

correlation values (R²≈ 0.90–0.95), indicating that the heterogeneous multilayer model was less suitable for

describing the interaction between Malachite Green and the modified fish-scale adsorbents.

The calculated maximum monolayer adsorption capacities (Q₀) further confirmed the superior performance of

the Langmuir model. For FS-NaOH, the Langmuir fitting yielded a Q₀value of approximately 18 mg/g, whereas

FS-KOH exhibited a comparatively lower capacity of around 12 mg/g. This trend suggests that alkali

modification with NaOH enhances the availability of active binding sites more effectively than KOH treatment.

Although the Freundlich model indicated favorable adsorption, as reflected by 1/ values between 0.3–0.5, its

weaker fit relative to Langmuir reaffirms that monolayer adsorption on a homogenous surface is the dominant

mechanism governing dye uptake in this system.

Table 2. Langmuir and Freundlich adsorption constants for NaOH and KOH modified bio adsorbents

Adsorbent

Langmuir

Q₀ B

Freundlich Kf

1/n

(mg/g)

NaOH

modified

8.70

75.61

199.07

0.398

0.255

KOH modified 3.69

46.16

204.17

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

Figure 3. Langmuir isotherm plot for Malachite Green adsorption.

Figure 4. Freundlich isotherm plot for Malachite Green adsorption.

The superior Langmuir fit suggests monolayer coverage on a relatively homogeneous fish-scale surface. Indeed,

a recent review notes that fish-scale adsorption data are “most adequately described by the Langmuir model”.

Kinetics and Thermodynamics: The adsorption followed pseudo-second-order kinetics (in line with many

dye–biosorbent systems suggesting chemisorption as the rate-limiting step. Thermodynamically, removal was

found to decrease with increasing temperature – e.g. MG uptake at 20ꢀ°C was higher than at 40ꢀ°C. This indicates

an exothermic adsorption process. Correspondingly, the calculated enthalpy change (ΔH°) was negative, and

ΔG° became less negative at higher T, confirming that adsorption is spontaneous but less favorable at elevated

temperature.

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

3.7 Comparative Performance of Modified Adsorbents: Overall, FS-NaOH exhibited the best performance. Its

Langmuir capacity and removal efficiency were higher than FS-KOH under comparable conditions. This

suggests that NaOH activation produced more favorable surface properties for MG binding.

DISCUSSION

The adsorption patterns observed in this study are consistent with adsorption theory and previously reported

trends. The strong pH dependence follows well-established electrostatic principles: at low pH, the surface of fish

scales—composed of collagen and calcium-based minerals—remains protonated, which repels the cationic

Malachite Green. Under alkaline conditions, surface deprotonation generates negatively charged sites,

significantly enhancing dye uptake, consistent with previous reports on alkaline-treated biosorbents ¹⁹. Similarly,

the increase in dye removal with increasing adsorbent dose reflects the greater number of available binding sites,

although the per-gram adsorption capacity generally decreases at higher dosages due to particle aggregation and

reduced surface accessibility ¹⁵.

The strong agreement of the equilibrium data with the Langmuir isotherm indicates monolayer adsorption on

energetically uniform sites with negligible lateral interactions among adsorbed molecules, in line with classical

Langmuir theory ²⁰ and mechanistic analyses of fish-scale biosorbents ¹⁰. In contrast, the Freundlich model,

which assumes multilayer adsorption on heterogeneous surfaces, provided a weaker fit, consistent with its

empirical nature ²¹. Prior studies on fish-scale adsorbents also report Langmuir-type behaviour and pseudo-

second-order kinetics, supporting the present findings ¹⁰˒¹⁶. The adsorption capacities obtained (tens of mg/g)

are comparable to other bioadsorbents; for example, raw Labeo rohita scales showed ~38 mg/g uptake in the

study by Chowdhury et al. ⁶.

The superior performance of NaOH-modified scales over KOH-modified scales can be attributed to their

differing chemical effects on biomass. NaOH effectively dissolves amorphous mineral phases and exposes new

–OH functional groups, enhancing surface reactivity ¹³. It also generates silanol and aluminol groups under

moderate treatment conditions. In comparison, KOH—due to the larger radius of K⁺—reacts more slowly and

typically requires harsher conditions for equivalent modification ¹³˒²¹. Comparative studies indicate that NaOH

activation produces a uniformly etched surface, whereas KOH tends to increase microporosity without

necessarily improving accessibility of adsorption sites ²¹. In this study, NaOH treatment likely produced more

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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)

ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025

favourable surface chemistry and accessible active sites, resulting in superior Malachite Green adsorption

compared to KOH treatment.

CONCLUSION

The study demonstrates that alkali-modified fish scales are highly effective, low-cost bioadsorbents for the

removal of Malachite Green from aqueous solutions. Adsorption efficiency was strongly influenced by pH,

adsorbent dosage, contact time and initial dye concentration, reflecting well-established principles of

electrostatic attraction and surface chemistry. Among the treatments tested, NaOH-modified fish scales showed

the highest adsorption capacity, primarily due to increased surface deprotonation and exposure of reactive

functional groups.

The adsorption equilibrium followed the Langmuir isotherm, indicating monolayer coverage on homogeneous

active sites, while kinetic analysis supported pseudo-second-order behaviour, suggesting chemisorption. Overall,

the adsorption capacities observed were comparable to previously reported values for similar biosorbents.

Given their availability, biodegradability, and effective dye removal performance, alkali-modified fish scales

represent a sustainable and economical alternative for wastewater treatment, particularly for communities

dependent on fisheries. Further studies on regeneration, reusability, and scale-up potential will enhance their

practical application in real-world effluent systems.

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