The persistence, toxicity and bioaccumulative nature of these pollutants have made the heavy metals poisoning of aquatic environments a major worldwide environmental and public health concern [1].  Mercury (Hg) is one of the most hazardous metals because of its high mobility in aquatic environment and its detrimental effects on living creatures [2]. Mercury is released into the environment via industrial activities such as electroplating, battery manufacture, pigment production, mining, metal processing and fertiliser production [3].

Mercury may cause major health issues including kidney damage, liver damage, bone marrow disease, lung illness and many cancerogenic effects [4]. Mercury is non-biodegradable and may be deposited in biological tissues, hence its removal from contaminated water is particularly crucial for environment [5].

Heavy metals may be removed by a number of processes including chemical precipitation [6], ion exchange, membrane filtration, electrochemical treatment, coagulation-flocculation and adsorption [7]. Of these approaches, adsorption has gained much attention because of its simplicity, high efficiency, cheap cost of operation and capacity to remove pollutants at low concentration levels [8] .

In the recent years, the agricultural waste and lignocellulosic biomaterials have been recognised as potentially efficient adsorbents because of their availability [9], renewability, ecologically benign nature and diversified surface chemistry . Flaxseed hull is an agricultural by-product of flaxseed production. Cellulose, hemicellulose, lignin and pectin are the components and they include diverse functional groups such as hydroxyl, carboxyl and phenolic groups that may interact with metal ions [10].

Ultrasonic assisted adsorption has recently attracted much attention as a promising way to enhance adsorption performance. Ultrasonic irradiation produces acoustic cavitation events which improve the surface accessibility, accelerate the adsorption rates and enhance the mass transfer . The bursting of the tiny bubbles generates local turbulence and micro-mixing processes that improve the transit of adsorbate molecules towards active adsorption sites [11].

However, biosorption technology has attracted a great deal of attention, although the information available on the adsorption of Mercury ions on flaxseed husk biomass in ultrasonic environment is rare. We present the features of Mercury ion adsorption by flaxseed husk biomass and the influence of ultrasonication on the adsorption efficiency. Furthermore, adsorption kinetics, equilibrium isotherms and thermodynamic parameters were explored to describe the adsorption process and practicality of the approach [10].

MATERIALS AND METHODS

2.1. Material

Mercury nitrate was used as a source of Mercury ions in aqueous solution. The pH was adjusted using hydrochloric acid (HCl) and sodium hydroxide (NaOH). All experimental methods used deionised water. The biosorbent used was flaxseed husk obtained from a local agricultural processing business.

2.2. Production of the biomass of flaxseed husk

The recovered flaxseed husks were thoroughly washed with distilled water to get rid of dust, soluble contaminants and surface defects. The cleaned biomass was dried in an oven at 70 °C for 24 h. The material was then dried, crushed and sieved to produce a standardised particle size of roughly 250 μm. The produced adsorbent was kept in a closed container till its use.

2.3 Preparation of the Mercury solution

A stock solution of Mercury was prepared by dissolving known weights of Mercury nitrate in de-ionized water. To prepare working solutions, the stock solution was diluted to the desired concentrations. Solutions were freshly made before each experiment.

2.4. Determination of pHPZC

pHPZC was determined using pH drift method. 0.01 M NaCl solutions were prepared at initial pH values from 2 to 10. In each solution, a fixed amount of flaxseed husk biomass was added and equilibrated for 24 h. The computed final pH values were shown as a function of original pH values. At ΔpH = 0 the pH value is specified as pH PZC .

2.5. Batch adsorption experiment

Batch adsorption tests were performed using 100 cc Mercury solution. The quantity of adsorbent employed was 1.3 g and the suspension was agitated at a rate of 200 rpm. Samples were obtained at predefined time intervals and residual Mercury was measured using Atomic Absorption Spectroscopy (AAS).

2.6 Ultrasonic enhanced adsorption

Ultrasound aided adsorption investigations were performed using an ultrasonic bath of frequency 60 kHz and power output 150 W . In the studies using adsorption tank ultrasonic bath was utilised. For the sake of a meaningful comparison, all operating parameters were kept as they are often used in traditional adsorption research.

2.7 Assay of adsorption

The adsorption efficiency was studied in detail with respect to contact time, pH, adsorbent dose, initial Mercury concentration and temperature. The adsorption capacity (qe) and the removal efficiency (%) were estimated using the adsorption standard equations.

2.8. Models dynamic 

Pseudo first and second order kinetic models were used to study the kinetics of adsorption. The performance of each model was evaluated using the correlation coefficient (R2) of the linear regression analysis.

2.9. Isotherm of adsorption

Langmuir and Freundlich isotherm models were used to study the equilibrium data of adsorption to understand the adsorption behaviour and adsorption properties of the flaxseed husk biomass.

2.10 Thermodynamics analysis

Thermodynamic parameters were calculated at different temperatures (298-333 k). The distribution coefficient (Kd) was calculated and the standard enthalpy (ΔH°) and entropy change (ΔS°) of the adsorption process were calculated using Van’t Hoff equation.

2.11. Statistical analysis

All studies were performed in triplicate and data are shown as mean ± standard deviation. Statistical significance was evaluated by one way analysis of variance (ANOVA) and the level of significance was set at p < 0.05.

RESULTS AND DISCUSSION

3.1 Effect of Contact Time

Effect of contact time (10-100 min) on the adsorption of Hg (II) ions by flax seed husk biomass was investigated. Table 2 indicates the removal efficiency improved from 33% at 10 min to 83% at 100 min, while the adsorption capacity (Qt) increased from 2.538 to 6.385 mg/g.

The faster adsorption process was seen in the first contact stages because of the increased availability of empty adsorption sites in the surface of the adsorbent. rise in contact time caused rise in adsorptive capacity and removal efficiency with continuous adsorption of Mercury ions on active sites [12].

The rate of adsorption was decreased after 60 min suggesting the increasing saturation of available adsorption sites and the competition of Mercury ions for the remaining active binding sites. Little change in adsorption capacity was seen at 90-100 min, when equilibrium was attained [13]. 

Table 1. The effect of contact time on Hg sorption.

The adsorption capacities (Qt) from contact time studies were maximum at 6.385 mg g-1 . But the first concentration test reached the equilibrium adsorption capacity (Qe) of 3.538 mg g-1.

Figure1: Effect of contact duration on adsorption of Mercury

Results revealed the adsorption equilibrium was reached in around 90-100 min. For future analysis, the equilibrium contact time was taken as 100 min contact duration.

3.2 Effect of Initial Concentration of Mercury

The biosorption of Mercury on flaxseed husk biomass was examined in the initial concentration range of 10-100mg/L. From Table 3 it can be observed that the adsorption capacity (Qe) rises from 0.385 mg/g to 3.538 mg/g as the initial level of Mercury increases from 10 to 100 mg/L.

Table 2. Equilibrium Adsorption Data

Figure 2 shows the relationship between initial concentration and adsorption capacity.

The increase in the adsorption capacity is attributed to the significant concentration gradient between the solution and the adsorbent surface which promotes the mass transfer and permits Mercury ions to reach the free adsorption sites. At high concentrations, the chances of the Mercury ions interacting with the functional groups on the surface of the biomass also rise [14].

The greatest adsorption capacity was 3.538 mg/g at an initial concentration of 100 mg/L. This suggests that flaxseed husk biomass has a high affinity for Mercury ions and may be an efficient adsorbent for large concentrations of metal ions in aqueous solutions.Table 2  Figure 2 demonstrates the relationship between the starting concentration and the adsorption capacity obtained from the equilibrium adsorption data.

The increase in adsorption capacity shows that flaxseed husk biomass has the ability to adsorb more Mercury ions at greater concentration.

3.3 Effect of pH

The effect of pH on Mercury adsorption was studied at pH 2-7 using the flaxseed husk biomass. The results revealed that the adsorption capacity and removal efficiency increased with increasing pH. The adsorption capacity was enhanced from 2.308 mg g-1 at pH 2 to 5.769 mg g-1 at pH 7 and the removal efficiency was enhanced from 30% to 75%.

Figure 3: Effect of solution pH on the adsorption of Mercury ions onto flaxseed husk biomass

The low pH value indicates the presence of large concentration of H+ ions in the solution, which compete with Mercury ions for the few available sites thereby reducing the efficiency of adsorption. The increase in pH decreased the competition of hydrogen ions and increased the availability of negative functional groups on the adsorbent surface which promoted the adsorption of Mercury. Therefore, pH 7 was selected as the optimal pH for Mercury adsorption under the experimental circumstances [15].

Figure 3: Effect of solution pH on Mercury ions adsorbed by flaxseed husk biomass

3.4 Effect of Amount of Adsorbent

The influence of adsorbent dose on Mercury removal by flaxseed husk biomass was studied within the range of 0.1–1.3 g. The adsorption capacity and removal efficiency were improved by the increase of adsorbent dosage. The elimination efficiency improved from 35% to 82% and the adsorption capacity increased from 2.692 to 6.308 mg g-1 with increasing amount of adsorbent from 0.1 g to 1.3 g.

Figure 4. Effect of adsorbent dosage on the removal of Mercury ions using flaxseed husk biomass.

The enhanced adsorption efficiency at higher adsorbent dose was related to the increased number of adsorption sites and surface area for Mercury absorption. At low dosages, the amount of active sites accessible decreases, therefore lowering the efficiency of drug clearance [16]. The adsorption efficiency increases with the adsorbent dosage because more binding sites are accessible for interaction with the Mercury ions. However, the increase was small (1.0 g) indicating the increasing approach to adsorption equilibrium. Figure 4 shows that the optimum adsorbent dose was 1.3 g under the conditions tested. Effect of adsorbent dosage on removal of Mercury ions by flaxseed husk biomass [17].

3.5 Kinetics of the Adsorption

The adsorption process was examined using pseudo-first-order and pseudo-second-order kinetic models.

Table 3. Kinetic parameters

The pseudo-first-order model had a low correlation value (R2 = 0.769), but the pseudo-second-order model showed an excellent agreement with the real data (R2 = 0.982).

Figure 5. Pseudo first order kinetic plot for adsorption of Hg(II) onto flaxseed husk biomass.

Figure 6 Pseudo second order kinetic curve for adsorption of Mercury ions on flaxseed husk biomass

Refer to Table 3. The correlation coefficient R2 for faux first order model was 0.769 whereas pseudo second order model was superior (R2 = 0.982). The pseudo-second-order model exhibited a better fitting which suggested that the adsorption of Mercury on flaxseed husk biomass was mostly chemisorption, including the interaction between Mercury ions and the active functional groups on the surface of the adsorbent. Therefore, the pseudo-second-order model was the best model to describe the adsorption kinetics in the present investigation [18].

3.6 Adsorption Isotherm

The equilibrium adsorption data were analyzed using the Langmuir and Freundlich isotherm models to evaluate the adsorption behavior of Mercury ions onto flaxseed husk biomass. The calculated isotherm parameters are presented in Table 6.

Table 4. Isotherm Parameters for Mercury Adsorption onto Flaxseed Husk Biomass

The Freundlich model showed a higher correlation coefficient (R² = 0.909) than the Langmuir model (R² = 0.048), indicating that the Freundlich equation provided a better description of the equilibrium adsorption data.

The Freundlich constants KF and n were determined to be 0.0677 and 1.058, respectively. The value of n greater than one indicates favorable adsorption and a good affinity between Mercury ions and the adsorption sites available on the flaxseed husk biomass surface.

In contrast, the Langmuir model exhibited poor agreement with the experimental data. The calculated Langmuir parameters resulted in non-physical negative values, indicating that the assumptions of monolayer adsorption on a homogeneous surface are not applicable to the present adsorption system.

The better fitting of the Freundlich model suggests that Mercury adsorption occurred on a heterogeneous surface containing adsorption sites with different energy levels. Therefore, the Freundlich isotherm was found to be the most suitable model for describing Mercury adsorption onto flaxseed husk biomass under the investigated experimental conditions [19].

Figure 7. Langmuir Isotherm Plot for Mercury Adsorption onto Flaxseed Husk Biomass.

Figure 8. Freundlich Isotherm Plot for Mercury Adsorption onto Flaxseed Husk Biomass

3.7 Thermodynamic Studies

The thermodynamic parameters were determined using adsorption experiments conducted at temperatures between 298 and 333 K.

Table 5. Thermodynamic Data

The increase in Kd values with temperature indicates enhanced adsorption performance at elevated temperatures.

Table 6. Thermodynamic Parameters

The positive value of ΔH° indicates that Mercury adsorption is an endothermic process. The positive value of ΔS° suggests increased randomness at the adsorbent–solution interface during adsorption [20].

Figure 9. Van’t Hoff plot for the thermodynamic analysis of Mercury adsorption onto flaxseed husk biomass.

The thermodynamic parameters (ΔH° and ΔS°) were calculated from the Van’t Hoff plots over the temperature range of 298–333 K using the distribution coefficient (Kd). The obtained values of the distribution coefficient (Kd) and lnKd are presented in Table 5, while the calculated thermodynamic parameters are summarized in Table 6.

The results showed a gradual increase in Kd values with increasing temperature, indicating that Mercury adsorption onto flaxseed husk biomass became more favorable at higher temperatures. The increase in adsorption capacity with temperature suggests that additional thermal energy enhances the mobility of Mercury ions in solution and facilitates their interaction with the active adsorption sites available on the adsorbent surface.

The positive enthalpy change (Delta H = +25.7 kJ mol-1) confirms that the adsorption process is endothermic in nature. This finding indicates that energy is required for Mercury ions to interact with the adsorption sites of the biomass. The magnitude of Delta H further suggests that the adsorption process may involve strong interactions between Mercury ions and surface functional groups, supporting the kinetic results that favored the pseudo-second-order model [21].

The positive entropy change (Delta S = +63.8 J mol-1 K-1) indicates an increase in randomness at the solid-solution interface during adsorption. This behavior may be attributed to structural rearrangements occurring at the adsorbent surface and the displacement of water molecules previously associated with Mercury ions in solution. The increase in disorder suggests a favorable affinity between Mercury ions and the adsorption sites of flaxseed husk biomass [22].

Furthermore, the increase in adsorption performance with temperature confirms the beneficial effect of thermal energy on Mercury uptake and supports the applicability of the adsorption process under a wide range of operating conditions. The linear relationship obtained from the Van't Hoff plot demonstrates the reliability of the calculated thermodynamic parameters and confirms the thermodynamic consistency of the adsorption system [23].

Overall, the thermodynamic results indicate that Mercury adsorption onto flaxseed husk biomass is an endothermic and favorable process, with adsorption efficiency improving as temperature increases

Table 7. Thermodynamic Parameters for Mercury Adsorption onto Flaxseed Husk Biomass

3.8 Effect of Ultrasonication

The influence of ultrasonic irradiation on Mercury adsorption was investigated under identical experimental conditions.

Table 8. Comparison Between Conventional and Ultrasonic-Assisted Mercury Adsorption

Figure 10. Comparison between conventional adsorption and ultrasonic-assisted adsorption for Mercury removal

The effect of ultrasonic irradiation on adsorption of Mercury using flaxseed husk biomass was studied by comparing conventional and ultrasonic assisted adsorption under optimal operating conditions. This may be seen from Table 7 and Figure 8.

The Mercury removal efficiency was significantly increased by ultrasonic irradiation under all the adsorption conditions studied. The removal efficiency increased from 40% (traditional adsorption) to 75% with ultrasonic irradiation with optimisation of pH. The removal efficiency increased from 35% to 83% with increasing contact time. The initial concentration of Mercury was increased, the removal efficiency increased from 20% to 54%. Removal efficiency was increased from 25-82% with increase in adsorbent dosage  [24].

The ultrasonic pulses create the acoustic cavitation effect, resulting in an improved adsorption efficiency. Small bubbles are formed and collapsed to generate local turbulence and to induce intense micro-mixing in the solution. It improves mass transfer and facilitates the transport of Mercury ions to the surface of the adsorbent [25].

Ultrasonication also reduces diffusion constraints and enhances the availability of active adsorption sites in the flaxseed husk biomass. The surface renewal of the adsorbent increases the interaction between the Mercury ions and the surface functional groups, always leading to higher adsorption capacities and removal efficiencies than the classic adsorption approaches [26].

The increase of the adsorption efficiency shows that the ultrasonic irradiation has a positive effect on the acceleration of the adsorption process and on the improvement of the removal of Mercury from aqueous solutions . Therefore, ultrasonic-assisted adsorption is a potential and feasible technique for improving the efficiency of flaxseed husk biomass as a low-cost biosorbent.

CONCLUSIONS 

The current study deals with adsorption of Mercury ions from aqueous solutions by employing flaxseed husk biomass using conventional and ultrasound assisted approaches. Results indicated that flaxseed husk is an effective, economical and environmentally friendly biosorbent for Mercury removal.

The operational factors including contact time, initial Mercury content and temperature had a significant role in the adsorption process. The adsorption capacity also increased with increase in contact time and equilibrium was achieved within 90-100 min. Maximum adsorption capacity found in the testing conditions was 6.385 mg g-1 and Mercury removal efficiency was 83%.

The kinetic research results revealed that pseudo-second order model (R2 = 0.982) was more suitable to explain the experimental data than pseudo-first order model (R2 = 0.769) indicating that the adsorption process was following the chemisorption mechanism.

The equilibrium investigations demonstrated that the adsorption behaviour was more well explained by the Freundlich isotherm model (R2 = 0.909) than the Langmuir model (R2 = 0.048). The data suggest a heterogeneous surface for Hg adsorption with locations of varying energies and perhaps multilayer adsorption.

Thermodynamic investigation showed that the adsorption process was endothermic with positive enthalpy change (ΔH° = +25.7 kJ mol−1). The positive value of entropy change (ΔS° = +63.8 J mol−1 K-1) is due to the rise in the disorder at the adsorbent-solution interface during the adsorption. The favourable effect of thermal energy on the adsorption of Mercury was confirmed by the increase in adsorption efficiency with temperature.

The ultrasonic irradiation boosted the ultrasonic adsorption process owing to the increased effects of acoustic cavitation, therefore the improved mass transfer and availability of the active adsorption sites. The ultrasonic-assisted adsorption was more effective than the standard adsorption methods.

Use of flaxseed husk biomass as a very efficient and eco-friendly adsorbent for removal of Mercury ions from polluted water. Ultrasonic technology is an efficient and environmentally benign approach for wastewater biosorption and might be an inexpensive alternative to the traditional methods. The findings show that flaxseed husk biomass is an effective, affordable and renewable biosorbent for the removal of Mercury ions from aqueous solutions notably under ultrasonic irradiation.

REFERENCES

1. Ali, H., E. Khan, and I. Ilahi, Environmental chemistry and ecotoxicology of hazardous heavy metals: environmental persistence, toxicity, and bioaccumulation. Journal of chemistry, 2019. 2019(1): p. 6730305.
2. Edo, G.I., et al., Environmental persistence, bioaccumulation, and ecotoxicology of heavy metals. Chemistry and Ecology, 2024. 40(3): p. 322–349.
3. Fishbein, L., Sources, transport and alterations of metal compounds: an overview. I. Arsenic, beryllium, Mercury, chromium, and nickel. Environmental health perspectives, 1981. 40: p. 43.
4. Alshebly, S.A.K., et al. Serum levels of lead, Mercury and silver in patients with breast cancer compared with healthy females in Iraq. in AIP Conference Proceedings. 2019. AIP Publishing LLC.
5. Thévenod, F. and W.-K. Lee, Toxicology of Mercury and its damage to mammalian organs. Mercury: from toxicity to essentiality, 2012: p. 415–490.
6. Azimi, A., et al., Removal of heavy metals from industrial wastewaters: a review. ChemBioEng Reviews, 2017. 4(1): p. 37–59.
7. Alkhadra, M.A., et al., Electrochemical methods for water purification, ion separations, and energy conversion. Chemical reviews, 2022. 122(16): p. 13547–13635.
8. Rashid, R., et al., A state-of-the-art review on wastewater treatment techniques: the effectiveness of adsorption method. Environmental Science and Pollution Research, 2021. 28(8): p. 9050–9066.
9. Alaboodi, A.S., et al. Estimation of the radiation hazard indices in most types of Pasta spread in the Iraqi markets. in Journal of Physics: Conference Series. 2020. IOP Publishing.
10. Bhardwaj, A., et al., Lignocellulose biosorbents: Unlocking the potential for sustainable environmental cleanup. International Journal of Biological Macromolecules, 2025. 294: p. 139497.
11. Çalışkan, B. and E. Şayan, A brief overview of the effects of ultrasound on the adsorption/desorption process: a review. International Journal of Environmental Analytical Chemistry, 2024. 104(16): p. 3821–3851.
12. Toor, M. and B. Jin, Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing diazo dye. Chemical Engineering Journal, 2012. 187: p. 79–88.
13. Melo, B.C., et al., Cellulose nanowhiskers improve the methylene blue adsorption capacity of chitosan-g-poly (acrylic acid) hydrogel. Carbohydrate polymers, 2018. 181: p. 358–367.
14. Annane, K., W. Lemlikchi, and S. Tingry, Efficiency of eggshell as a low-cost adsorbent for removal of Mercury: Kinetic and isotherm studies. Biomass Conversion and Biorefinery, 2023. 13(7): p. 6163–6174.
15. Boparai, H.K., M. Joseph, and D.M. O’Carroll, Mercury (Hg2+) removal by nano zerovalent iron: surface analysis, effects of solution chemistry and surface complexation modeling. Environmental Science and Pollution Research, 2013. 20(9): p. 6210–6221.
16. Kadhim, B.A., et al., Study the kinetic of Acridine orange dye adsorption by ZnO/GO nanocomposite. J. Iraqi Al-Khwarizmi Soc, 2020. 4: p. 85–94.
17. Nadeem, M., et al., Sorption of Mercury from aqueous solution by surfactant-modified carbon adsorbents. Chemical Engineering Journal, 2009. 148(2-3): p. 365–370.
18. Melia, P.M., et al., Agricultural wastes from wheat, barley, flax and grape for the efficient removal of Hg from contaminated water. RSC advances, 2018. 8(70): p. 40378–40386.
19. Terdputtakun, A., et al., Adsorption isotherm models and error analysis for single and binary adsorption of Hg (II) and Zn (II) using leonardite as adsorbent. Environmental Earth Sciences, 2017. 76(22): p. 777.
20. Pashai Gatabi, M., H. Milani Moghaddam, and M. Ghorbani, Efficient removal of Mercury using magnetic multiwalled carbon nanotube nanoadsorbents: equilibrium, kinetic, and thermodynamic study. Journal of Nanoparticle Research, 2016. 18(7): p. 189.
21. Daneshfozoun, S., et al., Kinetics, isotherms and thermodynamics of heavy metal ions sorption onto raw and agro-based magnetic biosorbent. Research and advances: environmental sciences, 2018. 2018(1): p. 27–42.
22. Aigbe, U.O., et al., Applications of magnetic nanomaterials for wastewater treatment, in Magnetic Nanomaterials: synthesis, characterization and applications. 2023, Springer. p. 129–169.
23. Guesmi, A., et al., Sustainable β-cyclodextrin/polyethylenimine-encapsulated activated algae hydrogel beads for high-capacity Hg (ii) removal: adsorption performance, mechanism, thermodynamics, and Box–Behnken optimization. RSC Advances, 2026. 16(20): p. 17918–17945.
24. Lindi, A.M., et al., Optimization of fenugreek seed mucilage extraction for the synthesis of a novel bio-nano composite for efficient removal of Mercury ions from aqueous environments. International Journal of Biological Macromolecules, 2024. 261: p. 129882.
25. Siddique, M., R. Rashid, and A. Ali, Fundamentals of acoustic cavitation, ultrasound-assisted processes, and sonochemistry, in Modeling and Simulation of Sono-Processes. 2025, Elsevier. p. 3–17.
26. Abiodun, O.-A.O., et al., Remediation of heavy metals using biomass-based adsorbents: adsorption kinetics and isotherm models. Clean Technologies, 2023. 5(3): p. 934–960