Humic acid from Tafila soil in Jordan has been extracted and insolubilized. Fourier transform infrared spectroscopy, thermal gravimetric analysis, elemental analysis, X-ray diffraction, scanning electron microscopy, and differential scanning calorimetry were used to characterize the sodium form of Insolubilized Tafila Humic Acid (INaTAHA). Using the batch technique, the adsorption of Pb(II), Cd(II), and Zn(II) by INaTAHA was investigated at various pH values (4.00, 5.00, and 6.00) and temperatures (25.0, 35.0, and 45.0oC). It was observed that INaTAHA has a maximum uptake as follows: Pb(II) > Zn(II) > Cd(II) and the metal ion uptake increased with pH reaching a peak at pH = 6. Kinetic studies results demonstrated that the maximum uptake level requires an equilibrium time of 24 hours for each metal ion fitting the pseudo-second-order model. Langmuir, Freundlich, and Dubinin-Radushkevich adsorption isotherms fitted the metal ion uptake characteristics of INaTAHA. The activation parameters Ea, ΔG‡, ΔH‡, and ΔS‡ were also calculated. The findings show that physisorption is the mode of adsorption. Using the column technique, the uptake percent with the metal ions is as follows: Pb(II) > Zn(II) > Cd(II). Using 1.0 M HNO3 produced the best percent recovery for the metal ions according to desorption studies.
Research Article
Removal of Pb(II), Zn(II) and Cd(II) ions by insolubilized humic acid
https://doi.org/10.21203/rs.3.rs-3767934/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted
You are reading this latest preprint version
humic acid
soil
characterization
kinetics
adsorption
desorption
Heavy metal pollution is a serious environmental issue that endangers both the health of human populations and natural ecosystems. Significant volumes of heavy metals are released into the atmosphere, surface waters, soil, and plants as a result of industrial and agricultural pollution (Mitra et al. 2022). A class of very hazardous metals known as heavy metals has been connected to mutagenic effects, teratogenicity, cancer, and cytotoxicity. These metals include zinc, lead, mercury, copper, nickel, and zinc. Developing efficient adsorbents for these contaminants is essential to safeguarding and replenishing water and soil contaminated with heavy metals (Arora et al. 2022). Various methods have been suggested in the literature to treat heavy metal-contaminated soils and water. Phytoremediation, soil washing, extraction, isolation, and immobilization are some common methods for cleaning up metal contamination (Shakya et al. 2020). Many studies have been conducted on the use of clay minerals as a natural material to remove heavy metals due to their specific surface area, cation exchange capacity, high removal efficiency, low cost, and adsorption capacity. Clay minerals, both in their natural and altered forms, have remarkable capacity to eliminate various pollutants (Otunola et al. 2020). In contaminated soils, high concentrations of heavy metal ions, including Cd, Pb, and Zn, are frequently found. Although it is well known that metals like Zn are necessary for humans, animals, and plants, their presence in soil can have unfavorable consequences if it surpasses specific threshold levels. Even at low concentrations, other heavy metals that are not necessary for plants or animals, like Cd and Pb, have been shown to be harmful to human health (Alkan et al. 2008). Out of all the significant heavy metals, Pb is said to be the least mobile, while Zn and Cd are significantly more mobile and have a higher propensity to separate from insoluble inorganic and organic compounds. At pH values that are neutral or slightly alkaline, they create soluble ionic species that remain stable (Aigberua and Inengite 2019). The development of inexpensive adsorbents with easy manipulation and regeneration for potential reuse across a larger range of pH values and lower concentrations is the primary goal of the adsorption procedures (Ngulube 2017; Pyrzynska 2019; Yang F. and Yang P. 2022). Numerous natural, organic, and inorganic solids have been used by researchers; these include coir pith, zeolite, activated carbon, cement, alumina, silica, and natural materials like humus (organic matter) and clay minerals (Baker and Khalili 2004; Yusan et al. 2008; Al-Shaybe and Khalili 2009; Salameh and Khalili 2010; Baybas et al. 2011; Pyrzynska 2019; Yang F. and Yang P. 2022). The majority of the organic matter in soils and waters is composed of humic substances (HS), which are a class of naturally occurring high molecular weight compounds. Based on their solubility properties in aqueous media, HS can be divided into three main subclasses: humin, which is the insoluble fraction of humic substances at all pH values; fulvic acids (FAs), which is the soluble fraction of humic substances at all pH values; and humic acids (HAs), which is the soluble fraction of humic substances at high pH values (Choudhry 1984). For a variety of metal cations, HA is an excellent sorbent in terms of capacity and rate of sorption. For this reason, the great capacity of HA to bind metal cations is a major concern when it comes to environmental preservation (Klučáková and Pavlíková 2017).
The primary component of naturally occurring organic matter, humic acid (HA), is a naturally occurring colloid that contains a variety of functional groups that are reactive to chemicals, including carboxyls, phenolic hydroxyls, and aromatic units (Li et al. 2010). HA is produced by the biological process of decomposition from plants and other living organisms (Fig. 1) (Ngah et al. 2011).
Figure 1. Model structure of humic acid (Stevenson,1982).
Humic acid is a common humic material; its non-uniform distribution of functional groups connected by a range of aliphatic and aromatic units makes it impossible to define a precise molecular structure for HA (Baker and Khalili 2003). It was found that HA reacted with metal ions in two different ways: a major reaction in which phenolic hydroxyls and acidic carboxyls participated simultaneously, and a minor reaction in which acidic carboxyls alone were involved. The molecular weight and solubility characteristics of HA vary depending on the extraction process and the source from which it was extracted (Gezici et al. 2005). In aqueous media, HA is generally soluble above pH 2.0. However, HA can be transformed into an insoluble form in aqueous media up to pH 10 through the insolublization of humic acid (IHA) (Celebi et al. 2009). The insolubilization process of HA involves heating the substance along with a few necessary steps; it does not involve complex and time-consuming chemical treatments. HA may rapidly dehydrate and condense between acidic groups through insolubilization or during the heating process (Chen et al. 2008). In comparison to the untreated HA, the amount complimentary of the (-COOH, -OH) groups decreases as a result of heating. IHA possesses interaction abilities with metal ions (Gezici et al. 2005). A variety of publications in the field of adsorption for various cations onto IHA from aqueous solutions are displayed in Supplementary material Appendix, Table S1.
This work aims to prepare and use soil from the Tafila-Abeel forest (south of Jordan) containing insolubilized HA (INaTAHA) as an adsorbent for Pb(II), Cd(II), and Zn(II) ions from aqueous solutions. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), elemental analysis, and scanning electron microscopy (SEM) were used to study and characterize the INaTAHA. Using a batch technique, the adsorption of Pb(II), Cd(II), and Zn(II) onto INaTAHA at various pH and temperature was investigated. The column technique was used to investigate desorption following adsorption. The ultimate objective of this work was to comprehend and elucidate the adsorption behavior through the modeling of adsorption data using Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. Thermodynamic and kinetic parameters of adsorption were computed. Since all of these processes are connected, and since this is the first study on IHA from Tafila soil, it is anticipated to be very beneficial.
Materials
The reagents utilized in this investigation were all of analytical grade. Lead (II) chloride from HandWL; sodium perchlorate (NaClO4) from Acros; zinc nitrate hexahydrate (Zn(NO3)2.6H2O) from Panreac; cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O) from BDH; sodium hydroxide (NaOH) from LOBA; and hydrochloric acid 37% (HCl) and nitric acid 69% (HNO3) from TEDIA. The study's soil sample collected from the Tafila (Abeel forest) in southern Jordan, which is situated 200 kilometers south of Amman; (35.6 degrees latitude, 30.8% longitude). The soil has a darked red color.
Instrumentations
Samples were weighed using an AW120 electronic balance. A METROHM 605 pH meter was used to determine the solutions' pH levels. Using a Thermo Nicolet NEXUS 670 FT-IR Spectrophotometer, the FTIR spectrum of INaAZHA was captured. The instrument used for elemental analyses was the Euro Vector 3000 Elemental Analyzer. The TGA was investigated at a heating rate of 20°C/min using the NETZCH STA 409 PG/PC Thermal Analyzer in the temperature range of 25°C–800°C. With the Shimadzu XRD-7000, the XRD was recorded. Utilizing the FEI-F50 scanning electron microscope (SEM), the shape and surface morphology of the INaTAHA were investigated; elements present at a given point were ascertained through the energy dispersion spectrum (EDS). A thermostat-equipped Clifton shaker was used to shake the samples. Using the Varian Spectra AA-250 pulse atomic absorption spectrometer (AAS), the concentrations of metal ions were measured.
Extraction of humic acid from Tafila soil
200 kilometers south of Amman, in Tafila (Abeel Forest), in southern Jordan, is where soil samples were collected. Before being extracted, the soil samples were crushed, homogenized, and were air-dried. From a Tafila soil sample, humic acid was extracted using the following method: 4.0 liters of 0.1 M NaOH were used to extract 400 g of soil over the course of 24 hours at room temperature. Following the mixture's filtration, the supernatant was acidified with HCl to a pH of 1.5 and allowed to stand at room temperature for a full day. To eliminate chloride ions, the resultant precipitate was repeatedly washed with deionized water; this was tested by AgNO3 solution. The precipitate was dried for four hours at 95°C. Following its insolubilization, which took place at 330°C for 1.5 hours, the solid (ITAHA) was stirred in a 1.0 M NaNO3 solution for 48 hours in order to transform it into its sodium form (INaTAHA). According to Celebi et al. (2009), the resultant solid phase (5.4 g) was dried for 24 hours at 80°C. This procedure yields a 1.88% percentage yield for INaTAHA from soil (18.8 g INaTAHA/kg soil). From a 2.00 kg Tafila soil sample, 37.6 grams were extracted in total.
Characterization of insolubilized Tafila humic Acid
The INaTAHA was subjected to various charactrization techniques such as Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), elemental analysis, thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to study its shape and surface morphology.
Preparation of stock solutions
To maintain ionic strength, a specific amount of each metal salt of Pb(II), Cd(II), and Zn(II) was dissolved in 0.1 M NaClO4 to pepare stock solutions of 2000 ppm for each of the three metal ions. Different concentrations (10, 20, 25, 30, 35, and 40 ppm) were prepared from the stock solutions. 0.1M NaClO4 and 0.1M HClO4 solutions at various pH values of 4.0, 5.0, and 6.0 were used to achieve the dilution.
Sorption experiments
Kinetic Studies
In order to determine the equilibrium time of the adsorption process, 50.0 ± 0.1 mg of INaTAHA was shaken with 50.0 mL (30 mg/L metal ion solution) at pH = 4.00, 5.00, and 6.00 (transferred to Pyrex glass flasks). Contact times at 25.0, 35.0, and 45.0°C ranged from 15 minutes to 72 hours. After the mixture was filtered, AAS was used to measure the concentration the of the metal ions that were still present. By knowing the rate constants of the interaction between metal ions and INaTAHA at various temperatures, we can also determine the activation energy, activation enthalpy, activation entropy, and free energy change from these experiments.
Sorption isotherms
The process of adsorbing Pb(II), Cd(II), and Zn(II) involved taking a known mass of 50.0 ± 0.1 mg of INaTAHA and mixing it with 50.0 mL of metal solutions with concentration variations ranging from (10, 20, 25, 30, 35, and 40) mg/L for each metal, at different temperatures (25.0, 35.0, and 45.0oC) and pH values of 4.00, 5.00, and 6.00. Every sample was shaken for a full day (24 hr).
Metal ion-uptake by the INaTAHA using column experiment
In this experiment, a glass column with a 20 cm length and a 20 mm inner diameter was employed. A 2 g ± 0.1 mg dried INaTAHA sample was packed into the column. The column was filled with a 50.0 mL sample volume containing 1000 mg/L Pb(II) at a flow rate of 0.5 mL/min. AAS was used to measure the metal ion concentration after the eluate was collected in a 100.0 mL conical flask. The determination of Zn(II) and Cd(II) ion uptake was conducted under the same experimental conditions. To ensure the precision and repeatability of the findings, each measurement average was taken. performed three times, and an average was taken.
Desorption studies
The INaTAHA was loaded with each metal ion as specified in Section 2.7 for the Pb(II), Cd(II), and Zn(II) ion desorption, which was performed under column conditions. To recover INaTAHA from adsorbed metal ions, 50.0 mL of three eluting agents: 0.1, 0.5, and 1.0 M HNO3 were utilized, with an elution flow rate of 0.5 mL/min. Five 10.0 mL portions of the eluate were collected, and the concentration of metal ions in each was measured using AAS.
Characterization and analysis of insolubilized Tafila humic acid samples
XRD characterization
In the X-ray diffraction analysis of INaTAHA, a distinct peak emerged at 2θ = 20.11°, which is considered an unusual characteristic associated with aliphatic or saturated ring structures, a phenomenon previously discussed by Khalili (1993) and El-Eswed (2005). Additionally, X-ray diffraction revealed another prominent peak at 2θ = 24.87°, indicating the probable presence of condensed aromatic carbons, as posited by El-Eswed. Furthermore, INaTAHA exhibited an additional peak at 2θ = 26.50°, a feature typically associated with a graphite-like structural arrangement, in line with Khalili's observations in 1993. The existence of an aromatic structure was further corroborated by the presence of peaks around 34.90°, which also suggested the potential presence of graphite-like layers, consistent with Khalili's findings.
Infrared Spectroscopy
The substantial metal ion binding capability exhibited by Humic Acids (HAs) is commonly ascribed to the abundance of oxygen-containing functional groups within HAs, notably including carboxyl (COOH), phenolic (OH), and carbonyl (C = O) groups (Piccolo and Stevenson 1982). In order to elucidate the role of these functional groups in the metal ion binding process, FT–IR spectroscopy was employed to analyze the INaTAHA, as well as their respective complexes (Supplementary material Appendix, Figure S1).
INaTAHA exhibits a notable abundance of aliphatic constituents, as indicated by the pronounced intensity of absorption bands at 2927 cm− 1 and 2837 cm− 1 in its FT-IR spectrum. The broad absorption band observed at 3435 cm− 1, attributed to the stretching vibration of -OH groups, demonstrated a shift to lower frequencies upon metal complexation. This shift implies the involvement of these OH groups in the binding of metal ions to the HA structure. Additionally, a robust band at 1718 cm− 1, corresponding to C = O stretching, was identified in the FT-IR spectrum. Notably, no significant shift in these absorption bands was detected following complexation with metal ions (Fig. 4 and Supplementary material Appendix, Table S2). Two distinctive bands associated with INaTAHA, specifically a sharp band at 1605 cm− 1 and a strong band at 1398 cm− 1, were attributed to the C = O stretching vibration of -COOH groups. These bands are indicative of carboxylate stretching and suggest the presence of sodium ions incorporated into the INaTAHA structure as a consequence of INaTAHA's treatment with 1M NaNO3 for a duration of 48 hrs (Pandey et al. 1999). Moreover, the weak band at 1221 cm− 1 reflects C-O stretching in protonated carboxylic acids within INaTAHA. This band notably diminished to very weak intensities upon complexation with metal ions. The presence of a peak at 1061 cm− 1, within the region corresponding to C-O stretching in alcohols and Si-O stretching in quartz (Supplementary material Appendix, Table S2) did not exhibit significant shifts upon complexation, suggesting that alcohol groups do not participate in metal ion binding (Alberts et al. 1998). The FTIR spectra of IHA from Ajloun and this one are in agreement (Khalili and Al-Banna 2015) and that from Azraq (Khalili et al., 2016).
Thermal properties
Under an inert nitrogen atmosphere, the thermal stability of the INaTAHA was examined using TGA and DSC instruments. Figure 5 shows a TGA curve for INaTAHA with a two-step weight loss. The mass loss was seen in the first step, which took place at temperatures below 150°C, is probably caused by dehydration processes (Chen et al. 2008; Wu et al. 2014).
This initial mass decrease is also associated with the partial decomposition of aliphatic groups, in accordance with findings by Kolokassidou et al. (2007). The weight loss that followed, which was seen between 150°C and 500°C, was caused by both the extensive breakdown of aliphatic structures and the decarboxylation of the humic material. This stage also includes the decomposition of aromatic structures and the degradation of phenolic, amino, and alcoholic groups (Kolokassidou et al. 2007; Al-Banna and Khalili 2015). At 779.4°C, the TGA curve additionally showed a residual mass percentage of 59.25%. According to Khalili et al. (2016), this residual mass could be explained by the presence of sodium in INaAZHA as well as possible contributions from metal oxides and silicate impurities.
Figure 5. TGA thermogram of INaAZHA
In Fig. 6, a prominent endothermic peak is observed at 95.0°C for INaTAHA. This peak represents the lipid fraction of humic acid melting process. In particular, the aliphatic parts of the lipid fraction have properties similar to polymethylene chains, with a semi-crystalline structure resembling polyethylene. The presence of a broad endothermic peak suggests the existence of a more diverse and heterogeneous mixture of compounds (Chilom et al. 2005). Furthermore, the appearance of peaks at 497.5°C can be attributed to the decomposition of aromatic structures within INaTAHA. This decomposition behavior aligns with findings reported in relation to IHA from the Ajloun region (Al-Banna and Khalili 2015) and that from the Azraq region (Khalili et al. 2016).
Elemental Analysis
Quantifying the elemental composition of INaTAHA was done through elemental analysis. The results detailing the percentage of each element present in INaTAHA are presented in Table 1.
|
Site |
% C |
% N |
% H |
Reference |
|---|---|---|---|---|
|
INaTAHA |
25.10 |
2.72 |
1.36 |
(This work) |
|
Azraq IHA |
48.08 |
2.02 |
3.86 |
(Khalili, et al., 2016). |
|
Ajloun IHA |
23.04 |
1.03 |
2.73 |
(Al-Banna and Khalili, 2015) |
The percentages (%) of carbon, nitrogen, and hydrogen found in INaTAHA are notably lower compared to TAHA. This difference in composition can be attributed to the decarboxylation of humic acid and the extensive decomposition of aliphatic structures that occur when subjected to heating at 330°C for 1.5 hours (Kolokassidou et al. 2007). Additionally, the presence of sodium in INaTAHA may contribute to a reduction in the percentage of carbon (C) observed.
Scanning Electron Microscopy (SEM) Screening
Insolubilized Tafila humic acid
Based on the SEM micrograph of INaTAHA, as illustrated in Fig. 7, it is evident that INaTAHA exhibits an irregular morphology characterized by a random distribution of particles. The micrograph further reveals a partially molten and porous structure with a fluffy appearance. In contrast to solid humic acids, INaTAHA displays larger pores.
Figure 7. SEM micrograph for INaTAHA with magnification 20000x.
EDS of INaTAHA with Pb(II), Zn(II) and Cd(II)
EDS analysis was conducted on INaTAHA samples that were loaded with Pb(II), Zn(II), and Cd(II) ions (Supplementary material Appendix, Figure S2). EDS provides valuable information by identifying the types and weight percentages of each element present at specific points within the SEM micrographs. Upon conducting EDS analysis, distinctive peaks corresponding to Pb, Zn, and Cd were observed within the INaTAHA sample, along with the presence of various other elements. Supplementary material Appendix, Table S3 displays each element's percentage composition following normalization. It could be concluded from the results that the percentage of Pb(II) in the INaTAHA-Pb complex surpasses the respective percentages of Zn and Cd in their respective complexes. This observation aligns harmoniously with the adsorption outcomes, emphasizing the superior adsorption capacity of INaTAHA for Pb(II) ions compared to Zn(II) and Cd(II) ions. Additionally, it is noteworthy that the carbon content (% C) within the Pb complex is comparatively lower than that found in the Zn and Cd complexes. This difference can be attributed to the higher quantity of adsorbed Pb(II) relative to Zn(II) and Cd(II), as elucidated earlier.
Kinetics Studies
INaTAHA's metal ion adsorption kinetics were methodically examined at intervals of 15 minutes, 0.5, 1, 2, 4, 6, 8, 12, 16, 18, 24, 48, and 72 hours, with a focus on different contact times. For the three different metal ions: Pb(II), Cd(II), and Zn(II), these investigations were carried out in controlled environments with pH 6.0 and temperatures of 25.0°C, 35.0°C, and 45.0°C. Each metal ion was started at a concentration of 30 mg/L. The experimental data showed that equilibrium was reached after 24 hours of contact time for all three metal ions (Pb(II), Zn(II), and Cd(II)).
The calculated and experimental qe values were compared in order to evaluate the precision of the adsorption predictions, as shown in Table 2. According to this analysis, the pseudo-second-order reaction kinetic model and the adsorption of Pb(II), Zn(II), and Cd(II) onto INaTAHA are well-aligned. Moreover, it is significant that for all three metal ions, the correlation coefficients connected to the pseudo-second-order reaction kinetic model are greater than those connected to the pseudo-first-order reaction kinetic model.
|
Metal ion |
Pb(II) |
Zn(II) |
Cd(II) |
|||||||
|---|---|---|---|---|---|---|---|---|---|---|
|
25.0°C |
35.0°C |
45.0°C |
25.0°C |
35.0°C |
45.0°C |
25.0°C |
35.0°C |
45.0°C |
||
|
pseudo 1st order kinetics |
qe calculated |
3.19 |
5.75 |
5.92 |
3.62 |
7.48 |
6.18 |
2.02 |
1.99 |
4.06 |
|
R2 |
0.9479 |
0.8644 |
0.7484 |
0.9522 |
0.9432 |
0.8741 |
0.2099 |
0.1595 |
0.6941 |
|
|
pseudo 2nd order kinetics |
k2 (g/mg.min) |
0.088 |
0.125 |
0.221 |
0.045 |
0.111 |
0.193 |
0.087 |
0.096 |
0.239 |
|
qe calculated |
17.66 |
20.92 |
21.45 |
11.27 |
17.54 |
15.15 |
2.45 |
3.57 |
6.11 |
|
|
R2 |
0.9999 |
0.9997 |
0.9998 |
0.9997 |
0.9987 |
0.9993 |
0.9935 |
0.9968 |
0.9914 |
|
|
qe experimental (mg/g) |
17.66 |
20.88 |
21.46 |
11.26 |
17.38 |
15.22 |
2.78 |
3.88 |
7.03 |
|
The activation energy (Ea) values provided in Table 3 reveal that the activation energy for Cd(II) is lower than that for Zn(II) and Pd(II), implying that the interaction of Cd(II) is relatively more facile when compared to Zn(II) and Pb(II). These activation parameters play a pivotal role in predicting how the adsorption of Pb(II), Zn(II), and Cd(II) may respond to changes in temperature. Specifically, Ea values recorded were 38.2 kJ/mol for Zn(II), 54.8 kJ/mol for Pb(II), and 34.8 kJ/mol for Cd(II), as detailed in Table 3. These values collectively suggest that the adsorption process is characterized by physisorption, signifying physical interactions.
|
Metal ion |
Ea kJ/mol |
∆H‡ kJ/mol |
∆S‡ J/mol.K |
∆G‡ kJ/mol |
A |
|---|---|---|---|---|---|
|
Pb(II) |
34.77 |
132.2 |
-197.2 |
190.9 |
1.07E + 05 |
|
Zn(II) |
54.80 |
204.1 |
-196.9 |
262.8 |
1.98E + 08 |
|
Cd(II) |
38.16 |
139.8 |
-197.2 |
198.6 |
3.80E + 05 |
Moreover, the fact that the activation enthalpy (∆H‡) is positive indicates that the adsorption process is endothermic and that energy is used up in the process. The fact that the free energy of activation (∆G‡) is positive indicates that energy is required for the adsorption reactions to convert reactants into products. In terms of the frequency of collisions between reactant molecules, the parameter Α, also referred to as the pre-exponential factor, is temperature-dependent (Laidler and Meiser 1995). Ultimately, the fact that the activation entropy (∆S‡) is negative suggests that the adsorption produces more order by forming an activated complex. This implies that a related mechanism underlies the adsorption of Pb(II), Cd(II), and Zn(II) on INaTAHA (Al 2012).
Sorption studies of Pb(II), Zn (II) and Cd(II) by INaTAHA
The adsorption isotherms concerning INaTAHA with Pb(II), Zn(II), and Cd(II) ions exhibit characteristics consistent with the L-type isotherm, as described in the figures (not presented here). This behavior indicates that the ratio of metal ion concentrations remaining in the solution to concentrations of metal ions adsorbed onto the solid decreases as the concentration of the solute (metal ions) increases. This finding implies that, even at lower concentrations, the adsorbent exhibits a greater affinity for the adsorbate (Limousin et al. 2007). However, the adsorbate's affinity decreases as concentration increases. According to Stevenson (1982) and Giles et al. (1960), this phenomenon happens because it gets harder for solute molecules to find available vacant sites as more sites within the substrate (INaTAHA) become occupied. Interestingly, this result is consistent with earlier studies that used these metal ions and Ajloun IHA as reported by Al-Banna and Khalili in 2015.
The adsorption capacity (qm) was calculated using the Langmuir isotherm, and the Langmuir isotherms for Pb(II), Zn(II), and Cd(II) showed good linearity (not shown here). This linearity suggests that the metal ions were adsorbed at sites with uniform binding energies, forming a monolayer on the surface of the adsorbent without inter-adsorbed ion interaction, in accordance with the Langmuir model (Adebowale et al. 2005). The adsorption capacities of INaTAHA for the metal ions were found to be enhanced by higher temperatures and pH levels, as indicated by the qm values obtained from the Langmuir model. These findings support the trend in metal uptake that shows Pb(II) > Zn(II) > Cd(II). It is important to note that the qm values from the Langmuir model and the q/m values from the Dubinin-Radushkevich (D-R) model differ significantly. The two models' different definitions of qm account for this discrepancy. The qm denotes the maximum adsorption at monolayer coverage in the Langmuir model and the maximum adsorption within the sorbent's total specific micropore volume in the D-R model (Xu 2008). Specific values for β, q/m, and E for Pb(II), Zn(II), and Cd(II), respectively are providd in Supplementary material Appendix, Figure S4-S6.
The adsorption of metal ions onto INaTAHA conforms well to the Freundlich model, as evidenced by the excellent linearity observed. This indicates the presence of non-uniform and non-specific adsorption sites, suggesting a complex adsorption process involving multiple mechanisms. The data also fit both the Langmuir and Freundlich isotherm models, implying the existence of both monolayer and multilayer adsorption, as well as homogeneous and heterogeneous sites on the surface. This observation is in accordance with the notion of various possible adsorption sites on the surface of INaTAHA (Jiang et al. 2010). The values of (n) in the Freundlich model represent the favorability of adsorption. Values greater than one indicate favorable adsorption (Unuabonah et al. 2008). the n values for Pb(II) range from 2.050 to 2.142, for Zn(II) from 1.538 to 1.576, and for Cd(II) from 1.503 to 3.108, all indicating favorable adsorption. These results are consistent with previous findings by Al-Banna and Khalili in 2015, which established that the adsorption of metal ions by INaTAHA is a favorable physical process. The (KF) values, representing adsorption constants in the Freundlich model, tend to increase with rising temperature and pH for all three metal ions. These trends align with the metal uptake observations, with Pb(II) > Cd(II) > Zn(II).
The relative selectivity of heavy metals and their sorption capacity can be attributed to various metal properties, including ionic radii, atomic weight, electronegativity, hydrolysis constants, distribution coefficients, and the principles of hard and soft acid-base interactions. The highest sorption capacity for Pb(II) can be explained by the hard and soft acid-base principle, wherein soft ligands containing sulfur provide polarizable sites that attract soft metal ions. Conversely, hard ligands with less polarizable sites, such as those containing oxygen, exhibit affinities for hard metal ions like alkali and alkaline earth metal ions. The chelating group with oxygen as a donor atom is considered a hard ligand. In this context, Pb(II) is categorized as a hard metal, while Zn(II) is soft, and Cd(II) is even softer. The obtained sequences align with the first hydrolysis constants and distribution coefficients of the metal cations (Veeresh et al. 2003).
The Dubinin-Radushkevich (D-R) isotherm model, being more versatile than the Langmuir isotherm, provides insights into the overall adsorption mechanism. As seen in Supplementary material Appendix, Figure S4-S6, the values of E from the D–R isotherm fall below 8.00 kJ/mol, suggesting that physical forces significantly influence the adsorption process (Donat et al. 2005).
Thermodynamics Results
To gain a comprehensive understanding of the underlying mechanisms involved in the metal uptake process, it is imperative to estimate the thermodynamic parameters, namely ΔG° (change in Gibbs free energy), ΔH° (change in enthalpy), and ΔS° (change in entropy). These thermodynamic functions provide insights into the spontaneity, endothermic or exothermic nature, and changes in disorder associated with the adsorption process. Two commonly used equations relate these thermodynamic functions. The first equation (Eq. 1) involves the distribution coefficient (Kd), representing the ratio of metal ions adsorbed by the polymer at equilibrium to those remaining in the solution after adsorption:
The equation also takes into account the absolute temperature (T) and the gas constant (R). The second equation (Eq. 2) allows the estimation of ΔH° and ΔS° through a linear plot of ln Kd versus 1/T:
The slope of this plot provides ΔH°/R, while the intercept yields ΔS°/R. These values can then be substituted into (Eq. 2) to calculate ΔG°. The Ln Kd values were determined from the intercept of Ln (qe/Ce) versus qe plots. The calculated Kd values are summarized in Supplementary material Appendix, Table S7. The Gibbs free energy (ΔG°) is a crucial parameter that indicates the spontaneity of the adsorption process. More negative ΔG° values suggest a highly energetically favorable adsorption process (Dakiky et al. 2002). In this study, the negative ΔG° values (Table 5) confirm the feasibility and spontaneity of the adsorption process. Notably, at pH 4, ΔG° values are small and positive, signifying that adsorption is less favorable at low pH. In this case, adsorption of metal ions by INaTAHA requires a minimal amount of energy to convert reactants into products (Scheckel and Sparks 2001).
According to Table 5, the adsorption of metal ions is endothermic, which means that as temperature rises, so does the adsorption capacity. This is indicated by the positive values of enthalpy change (ΔH°). This observation is in line with the findings for the Dubinin-Radushkevich (D-R) model's metal ion uptake, Langmuir parameters (qm), Freundlich constants (KF), and (q/m) values. The fact that the metal ions are well-solvated is one reason why the heats of adsorption could be endothermic. More energy is needed for the ions to dehydrate during adsorption than for the ions to attach to the surface exothermically (Donat et al. 2005).
During the adsorption process, there is more randomness at the solid/solution interface when ΔS° values are positive. The adsorbent's affinity for the metal ions is indicated by the positive entropy change (Table 5). It is explained by the fact that the adsorbate species displaces the adsorbed water molecules, giving the water molecules more translational energy than the adsorbate ions lose. The system can now be more randomly distributed thanks to this phenomenon. Furthermore, according to Güzel et al. (2008), the dehydration of metal ions adds to the general rise in system randomness. Therefore, the positive entropy change indicates that entropy drives the adsorption process, which in turn promotes disorder in the system. These results are consistent with findings from the adsorption of Th(IV) and U(VI) ions by Ajloun IHA, as reported by Al-Banna and Khalili in 2015.
|
Metal ion |
pH |
ΔG° (kJ/mol) |
ΔH° (kJ/mol) |
ΔS° (J/mol.K) |
|---|---|---|---|---|
|
Pb(II) |
4 |
-0.176 |
31.028 |
105.029 |
|
5 |
-1.031 |
17.669 |
62.571 |
|
|
6 |
-3.771 |
35.137 |
130.679 |
|
|
Zn(II) |
4 |
2.843 |
44.246 |
139.359 |
|
5 |
0.698 |
19.076 |
61.376 |
|
|
6 |
-0.641 |
14.827 |
51.817 |
|
|
Cd(II) |
4 |
4.495 |
83.461 |
265.307 |
|
5 |
1.132 |
53.298 |
175.959 |
|
|
6 |
-4.271 |
5.574 |
33.049 |
Column Experiments
Metal ion uptake by INaTAHA
Column experiments involving Pb(II), Zn(II), and Cd(II) were used to study metal ion uptake by the sodium form of INaTAHA. These experiments were carried out using an initial concentration of 1000 ppm, at room temperature (RT), and a flow rate of 0.50 mL/min, at the corresponding optimal pH levels for each metal ion. The results are shown in Table 6 and are expressed as the column's percentage of metal uptake. The observed uptake capacities for the metal ions follow the order: Pb(II) > Zn(II) > Cd(II). These results align with those obtained from batch experiments. However, it's noteworthy that the percentage uptake values in the column experiments are lower compared to those obtained in batch experiments conducted at pH 6 and 25°C. This disparity can be attributed to the need for an extended contact time to achieve complete saturation in the column experiments. Moreover, the absence of mechanical shaking in the column technique results in decreased metal ion uptake compared to the batch experiments.
|
Metal Ion |
Initial concentration Ci (mg/L) |
Final concentration CF (mg/L) |
Loaded Concentration (mg/L) |
Uptake (mg metal ion /g INaTAHA) |
% uptake |
|---|---|---|---|---|---|
|
Pb(II) |
1000 |
510.2 |
489.8 |
12.2 |
48.9 |
|
Zn(II) |
1000 |
568.4 |
431.6 |
10.8 |
43.2 |
|
Cd(II) |
1000 |
859.6 |
140.4 |
3.51 |
14.0 |
Desorption Studies
The present investigation utilized three distinct concentrations of nitric acid (HNO3) as eluting agents, namely 1M HNO3, 0.5M HNO3, and 0.1M HNO3, with the aim of eliminating metal ions. A steady flow rate of 0.50 mL/min was maintained throughout the elution procedure. Five portions totaling 10.0 mL each were used to collect the eluate, and the results are expressed as a percentage of recovery, as detailed in Table 7. This experimental procedure was conducted to assess the regenerative capability of INaTAHA for subsequent use. Based on the values of the cumulative recovery percentage, the following trend was observed regarding the effectiveness of eluting agents for removing metal ions from INaTAHA:
1M HNO3 > 0.5M HNO3 > 0.1M HNO3.
The increased concentration of HNO3 corresponded to a higher desorption of Pb(II), Zn(II), and Cd(II) ions. This phenomenon can be attributed to the role of nitric acid as a proton-exchanging agent, which promotes physical adsorption rather than chemical adsorption (Salameh and Khalili 2010).
|
Eluting agent: HNO3 |
Pb(II) |
Zn(II) |
Cd(II) |
||||||
|---|---|---|---|---|---|---|---|---|---|
|
1M |
0.5M |
0.1M |
1M |
0.5M |
0.1M |
1M |
0.5M |
0.1M |
|
|
% Recovery 1st portion |
35.2 |
26.8 |
24.4 |
38.1 |
28.5 |
26.5 |
39.9 |
30.5 |
28.6 |
|
% Recovery 2nd portion |
20.6 |
17.5 |
8.3 |
20.2 |
19.9 |
8.9 |
21.6 |
21.3 |
10.2 |
|
% Recovery 3rd portion |
13.0 |
9.3 |
6.6 |
13.8 |
10.5 |
7.5 |
14.7 |
12.1 |
9.2 |
|
% Recovery 4th portion |
9.9 |
8.2 |
4.9 |
11.1 |
9.8 |
5.3 |
12.1 |
10.9 |
7.3 |
|
% Recovery 5th portion |
5.8 |
6.5 |
3.8 |
7.4 |
6.7 |
4.5 |
8.4 |
8.2 |
5.8 |
|
% Cumulative Recovery |
84.5 |
68.3 |
48 |
90.6 |
75.4 |
52.7 |
96.7 |
83 |
61.1 |
This study focused on the extraction, insolubilization, and characterization of Insolubilized Tafila Humic Acid (INaTAHA), followed by an investigation into its adsorption characteristics for Pb(II), Zn(II), and Cd(II) ions. Both batch and column techniques were employed to examine the adsorption behavior under various experimental conditions. The research revealed that INaTAHA exhibited a higher affinity for Pb(II) compared to Zn(II) and Cd(II), with equilibrium reached at 24 hours. The adsorption process was strongly influenced by pH, with maximum uptake occurring at pH 6. Thermodynamic parameters indicated favorable and endothermic adsorption, while activation parameters suggested an associated, physisorption-based mechanism. Adsorption data fitting well with Langmuir, Freundlich, and D–R isotherm models further supported the findings. Additionally, a column packed with INaTAHA demonstrated efficient metal ion uptake, with the order of affinity being Pb(II) > Zn(II) > Cd(II) at pH 6 and room temperature. Metal ion recovery was successful through treatment with 1M HNO3, resulting in a high percentage of recover. These comprehensive findings contribute to a deeper understanding of INaTAHA's potential as an adsorbent for metal ions in aqueous solutions.
Funding
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
Competing Interests
The authors have no relevant financial or non-financial interests to disclose
Author Contribution
FK suggested the idea for this research.FA collected the soil samples and did the extraction and the insolubilization.FA did all the adsorption experiments.OA did all the data analysis, figures, and tables.FK and OA wrote the manuscript.
- Adebowale K, Unuabonah I, Olu-Owolabi B (2005) Adsorption of some heavy metal ions on sulfate- and phosphate-modified kaolin. Appl Clay Sci 29:145–148. https://doi.org/10.1016/j.clay.2004.10.003
- Aigberua A and Inengite A (2019) Chemical speciation and mobility of heavy metal (Pb, Cd, Zn, Mn) pollutants in crude oil contaminated soils of Niger Delta, Nigeria. Elixir Int J 126:52543–52548.
- Alberts J and Filip Z (1998) Metal Binding in Estuarine Humic and Fulvic Acids: FTIR Analysis of Humic Acid-Metal Complexes. Environ Technol 19:923–931. https://doi.org/10.1080/09593331908616750
- Alkan M, Kalay B, Doğan M, Demirbaş Z (2008) Removal of copper ions from aqueous solutions by kaolinite and batch design. J Hazard Mater 153:867–876. https://doi.org/10.1016/j.jhazmat.2007.09.047
- Al-Shaybe M and Khalili F (2009) Adsorption of Thorium(IV) and Uranium(VI) by Tulul al- Shabba Zeolitic Tuff, Jordan. Jordan J Earth Environ Sci 2:108–119.
- Arora P, Paliwal R, Rani N, Chaudhry S (2022) Recent Trends in Bioremediation of Heavy Metals: Challenges and Perspectives. In: Kumar V and Thakur IS (ed), Omics Insights in Environmental Bioremediation,. Springer, Singapore, pp 103–131. https://doi.org/10.1007/978-981-19-4320-1_5
- Baker H and Khalili F (2003) Comparative study of binding strengths and thermodynamic aspects of Cu(II) and Ni(II) with humic acid by Schubert’s ion-exchange method. Anal Chim Acta 497:235–248. https://doi.org/10.1016/j.aca.2003.08.036
- Baker H and Khalili F (2004) Analysis of the removal of lead(II) from aqueous solutions by adsorption onto insolubilized humic acid: temperature and pH dependence. Anal Chim Acta 516:179–186. https://doi.org/10.1016/j.aca.2004.03.068
- Baybaş D and Ulusoy U (2011) The use of polyacrylamide-aluminosilicate composites for thorium adsorption. Appl Clay Sci 51:138–146. https://doi.org/10.1016/j.clay.2010.11.020
- Celebi O, Kilikli A, Erten H (2009) Sorption of radioactive cesium and barium ions onto solid humic acid. J Hazard Mater 168:695–703. https://doi.org/10.1016/j.jhazmat.2009.02.090
- Chen H, Berndtsson R, Ma M, Zhu K (2008) Characterization of insolubilized humic acid and its sorption behaviors. Environ Geol 57:1847–1853. https://doi.org/10.1007/s00254-008-1472-0
- Chilom G and Rice JA (2005) Glass transition and crystallite melting in natural organic matter. Org Geochem 36:1339–1346. https://doi.org/10.1016/j.orggeochem.2005.06.007
- Choudhry GG (1984) Humic Substances. Structural Aspects, and Photophysical, Photochemical and Free Radical Characteristics. In: The Natural Environment and the Biogeochemical Cycles, The Handbook of Environmental Chemistry. Springer, Berlin, Germany, pp 1–24. https://doi.org/10.1007/978-3-540-38829-6_1
- Dai L, Zhao W, Wei B, Zhang K, Han T (2021) Adsorption of Pb2+ by insolubilized humic acid extracted from sewage sludge. J Mater Cycles Waste Manage 23:1037–1047. https://doi.org/10.1007/s10163-021-01193-9
- Dakiky M, Khamis M, Manassra A, Mer’eb M (2002) Selective adsorption of chromium(VI) in industrial wastewater using low-cost abundantly available adsorbents. Adv Environ Res 6:533–540. https://doi.org/10.1016/s1093-0191(01)00079-x
- Donat R, Akdogan A, Erdem E, Cetisli H (2005) Thermodynamics of Pb2+ and Ni2+ adsorption onto natural bentonite from aqueous solutions. J Colloid Interface Sci 286: 43–52. https://doi.org/10.1016/j.jcis.2005.01.045
- El–Eswed B (2005) Adsorption of heavy metals by humic acid from Azraq area. Dissertation, The University of Jordan, Amman, Jordan.
- El-Eswed B and Khalili F (2006) Adsorption of Cu(II) and Ni(II) on solid humic acid from the Azraq area, Jordan. J Colloid Interface Sci 299:497–503. https://doi.org/10.1016/j.jcis.2006.02.048
- Gezici O, Kara H, Ersöz M, Abali Y (2005) The sorption behavior of a nickel-insolubilized humic acid system in a column arrangement. J Colloid Interface Sci 292:381–391. https://doi.org/10.1016/j.jcis.2005.06.009
- Giles CH, MacEwan TH, Nakhwa SN, Smith D (1960) Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J Chem Soc 786:3973. https://doi.org/10.1039/jr9600003973
- Guzel F, Yakut H, Topal G (2008) Determination of kinetic and equilibrium parameters of the batch adsorption of Mn(II), Co(II), Ni(II) and Cu(II) from aqueous solution by black carrot (Daucus carota L.) residues. J Hazard Mater 153:1275–1287. https://doi.org/10.1016/j.jhazmat.2007.09.087
- Jiang MQ, Jin XY, Lu XQ, Chen ZL (2010) Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) onto natural kaolinite clay. Desalination 252:33–39. https://doi.org/10.1016/j.desal.2009.11.005
- Khalili F (1993) Lead (II) complexation by Azraq humic acid. Mu’tah Lil-Buhooth Wa Al-Dirasat 8:121–135.
- Khalili F, Al-Banna G (2015) Adsorption of uranium(VI) and thorium(IV) by insolubilized humic acid from Ajloun soil – Jordan. J Environ Radioact 146:16–26. https://doi.org/10.1016/j.jenvrad.2015.03.035
- Khalili F, Khalifa A, Al-Banna G (2016) Removal of uranium(VI) and thorium(IV) by insolubilized humic acid originated from Azraq soil in Jordan. J Radioanal Nucl Chem 311:1375–1392. https://doi.org/10.1007/s10967-016-5031-y
- Klučáková M and Pavlíková M (2017) Lignitic Humic Acids as Environmentally-Friendly Adsorbent for Heavy Metals. J Chem 2017:1–5. https://doi.org/10.1155/2017/7169019
- Kolokassidou C, Pashalidis I, Costa C, Efstathiou A, Buckau G (2007) Thermal stability of solid and aqueous solutions of humic acid. Thermochim Acta 454:78–83. https://doi.org/10.1016/j.tca.2006.12.022
- Laidler KJ and Meiser JH (1995) Physical Chemistry, (2nd ed). Houghton Mifflin Company, Boston, Massachusetts, USA.
- Li Y, Yue Q, Gao B (2010) Adsorption kinetics and desorption of Cu(II) and Zn(II) from aqueous solution onto humic acid. J Hazard Mater 178:455–461. https://doi.org/10.1016/j.jhazmat.2010.01.103
- Limousin G, Gaudet JP, Charlet L, Szenknect S, Barthès V, Krimissa M (2007) Sorption isotherms: A review on physical bases, modeling and measurement. Appl Geochem 22:249–275. https://doi.org/10.1016/j.apgeochem.2006.09.010
- Mitra S, Chakraborty AJ, Tareq AM, Emran TB, Nainu F, Khusro A, Idris AM, Khandaker MU, Osman H, Alhumaydhi FA, Simal-Gandara J (2022) Impact of heavy metals on the environment and human health: Novel therapeutic insights to counter the toxicity. J King Saud Univ, Sci 34:101865. https://doi.org/10.1016/j.jksus.2022.101865
- Ngah W, Fatinathan S, Yosop N (2011) Isotherm and kinetic studies on the adsorption of humic acid onto chitosan-H2SO4 beads. Desalination 272:293–300. https://doi.org/10.1016/j.desal.2011.01.024
- Ngulube T, Gumbo JR, Masindi V, Maity A (2017) An update on synthetic dyes adsorption onto clay based minerals: A state-of-art review. J Environ Manage 191:35–57. https://doi.org/10.1016/j.jenvman.2016.12.031
- Otunola B and Ololade O (2020) A review on the application of clay minerals as heavy metal adsorbents for remediation purposes. Environ Technol Innovation 18:100692. https://doi.org/10.1016/j.eti.2020.100692
- Pandey AK, Pandey SD, Misra V, Viswanathan PN (1999) Formation of Soluble Complexes of Metals with Humic Acid and Its Environmental Significance. Chem Ecol 16:269–282. https://doi.org/10.1080/02757549908037652
- Petrou AL (2012) The Free Energy of Activation as the critical factor in geochemical processes. Chem Geol 308:50–59. https://doi.org/10.1016/j.chemgeo.2012.03.015
- Piccolo A and Stevenson F (1982) Infrared spectra of Cu2+ Pb2+ and Ca2+ complexes of soil humic substances. Geoderma 27:195–208. https://doi.org/10.1016/0016-7061(82)90030-1
- Pyrzynska K (2019) Removal of cadmium from wastewaters with low-cost adsorbents. J Environ Chem Eng 7:102795. https://doi.org/10.1016/j.jece.2018.11.040
- Salameh N (2010) Adsorption of uranium(VI) and thorium(IV) by Jordanian bentonite. Thesis, The University of Jordan, Amman, Jordan.
- Scheckel KG and Sparks DL (2001) Temperature Effects on Nickel Sorption Kinetics at the Mineral–Water Interface. Soil Sci Soc Am J 65:719–728. https://doi.org/10.2136/sssaj2001.653719x
- Shakya A and Agarwal T (2020) Potential of Biochar for the Remediation of Heavy Metal Contaminated Soil. In: Singh J and Singh C (ed) Biochar Applications in Agriculture and Environment Management. Springer, Cham, Switzerland. https://doi.org/10.1007/978-3-030-40997-5_4
- Stevenson FJ (1982) Humus Chemistry: Genesis, Composition, Reactions, (1st ed). John Wiley and Sons, New York, USA.
- Unuabonah E, Adebowale K, Olu-Owolabi B, Yang L, Kong L (2008) Adsorption of Pb (II) and Cd (II) from aqueous solutions onto sodium tetraborate-modified Kaolinite clay: Equilibrium and thermodynamic studies. Hydrometallurgy 93:1–9. https://doi.org/10.1016/j.hydromet.2008.02.009
- Veeresh H, Tripathy S, Chaudhuri D, Hart BR, Powell MA (2003) Competitive adsorption behavior of selected heavy metals in three soil types of India amended with fly ash and sewage sludge. Environ Geol 44:363–370. https://doi.org/10.1007/s00254-003-0776-3
- Wu J, Ma G, Li P, Ling L, Wang B (2014) Surface modification of nanosilica with acrylsilane-containing tertiary amine structure and their effect on the properties of UV-curable coating. J Coat Technol Res 11:387–395. https://doi.org/10.1007/s11998-013-9552-9
- Xu D (2008) Adsorption of Pb(II) from aqueous solution to MX-80 bentonite: Effect of pH, ionic strength, foreign ions and temperature. Appl Clay Sci 41:37–46. https://doi.org/10.1016/j.clay.2007.09.004
- Yang F and Yang P (2021) Biopolymer‐Based Membrane Adsorber for Removing Contaminants from Aqueous Solution: Progress and Prospects. Macromol Rapid Commun 43. https://doi.org/10.1002/marc.202100669
- Yusan SD and Akyil S (2008) Sorption of uranium(VI) from aqueous solutions by akaganeite. J Hazard Mater 160:388–395. https://doi.org/10.1016/j.jhazmat.2008.03.009
- Zhao W, Ren B, Hursthouse A, Jiang F (2020) The adsorption of Mn(II) by insolubilized humic acid. Water Sci Technol 82:747–758. https://doi.org/10.2166/wst.2020.384
No competing interests reported.