Natural bentonite clay (NBC) was activated using nitric acid (HNO3). Characterization techniques including FTIR, SEM, XRD and BET were employed to examine the morphology of NBC and ABC (activated bentonite clay) sorbents. Comparative application of ABC and NBC to remove heavy metals (Fe2+, Zn2+, Ni2+) from pharmaceutical effluents was investigated under various experimental conditions. The maximum proportional removal by ABC was 88.90, 81.80 and 75.50% at pH 8, and 63.90, 59.60, 58.70% at pH 10 for NBC, both for Zn2+, Fe2+ and Ni2+ respectively. The Freundlich multilayer adsorption model and pseudo-second-order kinetics best fit the experimental data, suggesting the formation of multiple adsorption layers via strong ionic and electrostatic interactions. Heavy metals adsorption is more favorable with ABC than NBC, due to the availability of more sorption sites and a larger specific surface. The thermodynamic parameters (ΔH°, ΔS°, and ΔG°) revealed that the adsorption is endothermic and spontaneous in nature for both ABC and NBC.

  • Acid activation of bentonite clay using nitric acid was achieved in this study.

  • Enhanced morphology and physicochemical properties of pristine bentonite was observed.

  • Removal efficiency > 85% of recalcitrant heavy metals was obtained using modified bentonite.

  • Pre-oxidation of contaminated water using H2O2 enhanced clean-up efficiency.

  • Adsorption process was spontaneous, endothermic, and best described by multilayer sorption.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Metal pollution of surface waters is often due to indiscriminate discharge of the metals or wastewater containing the metals into watercourses (Ahmetović et al. 2019). Heavy metals are hazardous to humans and aquatic fauna due to their poor degradability, biomagnification tendencies, and the toxicity of high concentrations (Ore & Adeola 2021). These hazards necessitate legislation against the direct discharge of heavy metals into the environment. Remedial alternatives for heavy metal removal from aqueous solution include adsorption, coagulation, flocculation, electrochemical removal, ion exchange, bioremediation, membrane filtration, chemical precipitation, etc (Burakov et al. 2018; Zhao et al. 2020). However, the limitations associated with most of them include the costs of handling sludge/secondary pollutants, sensitive operating conditions, low efficiency, toxic sludge generation, high energy consumption, and incomplete removal (Kanamarlapudi et al. 2018; Ore & Adeola 2021).

Adsorption has demonstrated high efficiency in water treatment against many contaminants, particularly heavy metals (Abu-Danso et al. 2020). Adsorption techniques can be scaled up, whilst avoiding toxic by-products/metabolite production during treatment, a common problem with photocatalytic- and bio-degradation. The ease of recovery, regeneration, and reusability of many adsorbents also presents a sustainable and cost-effective approach to pollution remediation (Adebiyi et al. 2021). Clay minerals, which are naturally abundant, have has a significant historical impact on civilization (Ismadji et al. 2015). Bentonite is a 2:1 clay mineral composed primarily of montmorillonite (Maged et al. 2020). The distinct physicochemical properties of bentonite – for example, low permeability, low cost, strong absorptive affinity with inorganic and organic substances, large specific surface, small particle size, high porosity, and high cation exchange capacity – make it an effective adsorbent of different kinds of pollutants (Uddin 2017).

Chemical activation, using H2SO4, NaCl, and phosphoric acid, has been used to increase the specific surface of natural bentonite. However, there are few reports of the use of nitric acid for such activation. Bentonite was activated with nitric acid in this study, which was then subject to detailed characterization. Adsorption of Fe2+, Zn2+, and Ni2+ from wastewater from the pharmaceutical industry was studied under various process conditions. The interaction mechanisms between heavy metals and unmodified and/or activated bentonite were also investigated, using established isotherm and kinetic models.

Clay sample collection

Fresh bentonite for use in this study was collected from the Federal Institute of Industrial Research (FIIRO), Oshodi, Lagos State, Nigeria. The 750 g sample was oven-dried (70 ± 1 °C), crushed, ground, and sieved to fine powder (<150 μm).

Pharmaceutical industrial effluents

Pharmaceutical industry wastewater was collected in Ibadan, Nigeria, before discharge to a nearby stream. Sampling bottles were washed with dilute hydrochloric acid, rinsed with de-ionized water and dried for 2 hours at 120 ± 3 °C. At the sampling location they were rinsed with the effluent three times prior to sample collection. The bottles were transported in a cooler box to the laboratory for treatment and analysis (Ibigbami et al. 2016). Sample pH and temperature were taken at the time of collection using a pocket-pen type of pH meter.

Chemicals and equipment

The analytical grade nitric acid (HNO3) used as activating agent for the bentonite; sodium hydroxide (NaOH), hydrogen peroxide 30% w/w (H2O2), sodium chloride (NaCl), sodium acetate (Na-Ac), and acetic acid (98% purity) were purchased from Sigma-Aldrich (Germany).

Sorbent preparation was carried out using a high-temperature resistance furnace (SXL-1200, Zhengzhou Honglang), magnetic stirrer (JJ-4, Changzhou Guowang), microwave oven (MG-5021, Seoul, South Korea), shaking water bath (DK-98-1, Shanghai, China), and centrifuge (TGL-16G, Shanghai, China). Instruments used for sorbent characterization included; UV spectrophotometer (200–1,000 nm, UV-2350, Unico), scanning electron microscope (JSM-6360LV, JEOL), X-ray powder diffractometer (Angle range: 5–120o, Bruker AXS GmbH, Karlsruhe, Germany), and infrared spectrometer (PE-680, PerkinElmer).

Adsorbent preparation

Bentonite pre-treatment

The natural bentonite (NBC) was treated to remove calcite, other carbonates, and organic matter by washing with Milli-Q water. After drying, 25 g were added to 400 mL of 0.1 N Na-Ac solution containing acetic acid to regulate the pH ≈ 5.0. The suspension was stirred for 12 hours at 70 ± 1 °C. Stirring was continued overnight at ambient temperature with gradual addition of 100 mL of H2O2, before the suspension was centrifuged and washed three times with 0.01 N NaCl solution and Milli-Q water, consecutively. The residue was then dried at 105 ± 1 °C for 24 hours in a laboratory oven. The resulting NBC was pulverized, sieved (100 μm mesh-size) and stored in a desiccator.

Bentonite activation

NBC was introduced to 2M HNO3 at a mass:volume ratio of 1:2. The suspension was agitated in a thermostat-controlled water bath at 70 ± 1 °C for 4 hours, after which the residue was washed several times with deionized water. The viscous residue was oven-dried at 60 °C for 24 hours, and the resultant ABC pulverized and sieved (100 μm mesh size).

Sorbent characterization

The morphological distinctions between NBC and ABC were established by analysis using the Brunauer-Emmet-Teller (BET) method, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The specific surface and pore distribution of the powdered samples were determined by comparing N2 gas adsorption onto the surface of the solid with the amount of gas required to form a monolayer on the adsorbent surface. The cation exchange capacity (CEC) of the samples was determined using the copper bis-ethylenediamine complex method (Bergaya & Vayer 1997).

Wastewater sample digestion and treatment

Wet digestion was employed to remove organics from the wastewater and release metals bound to organic matter (Kinuthia et al. 2020). Fe, Zn and Ni adsorption from the wastewater using NBC and ABC was investigated in 40 mL amber vials. The roles of contact time, adsorbent dosage, solution pH, H2O2 oxidation and temperature on treatment efficiency were examined. The data obtained were used to calculate the equilibrium metal uptake capacity using Equation (1):
formula
(1)
formula
(2)
where C0 is the initial concentration of Fe, Zn or Ni (100–500 mg/L); Ce the equilibrium concentration of the ions concerned (mg/L); qe amount of heavy metal ions adsorbed per unit weight of adsorbent (mg/g); V the solution volume (mL); and m the adsorbent dose (mg).

Sorbent characterization

SEM micrographs

The effect of acid activation on the adsorbent surfaces is reflected in the SEM micrographs – Figure 1(a) and 1(b). the ABC surface clearly has relatively larger particles than NBC, which appears to be compact with irregularly shaped particles. The micrographs of ABC show that acid treatment produced distinct spatial structures and disaggregation and reduced the size of the bentonite structure. Khalfa et al. (2016) found the same and suggested that acid activation improves the spatial structure of geo-sorbents.

Figure 1

SEM micrograph of (a) ABC and (b) NBC.

Figure 1

SEM micrograph of (a) ABC and (b) NBC.

Close modal

XRD patterns

The XRD results revealed considerable differences in the symmetry, sharpness, and position of the d(001) plane diffraction peak (Figure 2(a) and 2(b)). The patterns from both NBC and ABC showed the presence of kaolinite, quartz, and montmorillonite, indicating that the smectite phase is present (Holmboe et al. 2012). As can be seen, the primary peaks for ABC shifted to smaller 2θ angles than those for NBC due to the increased interlayer distances. The ABC's basal spacing was reduced from 2.04A0 (2θ = 9.15) to 2.00A0 (2θ = 8.97), and both the octahedral and tetrahedral sites might have been altered.

Figure 2

XRD pattern of (a) ABC and (b) NBC.

Figure 2

XRD pattern of (a) ABC and (b) NBC.

Close modal

FTIR spectra

The bands on the FTIR spectra for both NBC and ABC between 1,200 and 700 cm−1 could be attributed to silicates (Figure 3(a) and 3(b)). Those between 3,589 and 3,500 cm−1 could be attributed to structural hydroxyl groups and water molecules in the bentonite clay layers, as reported by Noyan et al. (2007).The bands at 3,446 cm−1 for NBC are assigned to the O–H stretching vibration of the clay's silanol (Si–OH) groups (coordinated to octahedral Al3+cations) and HO–H vibration of the water molecules adsorbed on the bentonite surface, respectively, while that at 1,600 cm−1 reflects the angular deformation H–O–H bond of interlayer water molecules in the silicate matrix. The bands around 808 and 915 cm−1 are attributed to Al–Mg–OH and Al–Al–OH, respectively (Kumararaja et al. 2017).

Figure 3

FTIR spectra of (a) ABC and (b) NBC.

Figure 3

FTIR spectra of (a) ABC and (b) NBC.

Close modal

Activation of the clay led to penetration of the bentonite layers by the proton (H+) from the acid (HNO3), the penetrating proton becoming attached to the -OH group, resulting in partial dissolution and dihydroxylation of the smectite structure. The changes caused by the acid attack on the absorption bands are shown in Figure 3(b). The band around 3,500 cm−1 for ABC is characteristic of -OH stretching. However, acid attack reduced the OH stretching band intensities at 3,589 cm−1. In general, there was no significant difference between the ABC and NBC FTIR bands. The only observed changes were minor wavenumber shifts and decreased band intensities. This indicated that the bentonite structure was not completely altered (Sdiri et al. 2014).

BET analysis

Figure 4 shows the nitrogen adsorption-desorption isotherms measured on NBC and ABC. Both samples show type IV adsorption isotherms and a large uptake is observed near saturation pressure.

Figure 4

N2-sorption isotherms obtained from BET analysis.

Figure 4

N2-sorption isotherms obtained from BET analysis.

Close modal

The porosity was determined using conventional nitrogen isotherm analysis – Table 1. Both NBC and ABC are mesoporous, their pores having diameters within the range 2 to 50 nm. The adsorbent pores (Dp) of ABC were smaller than those of NBC, while the specific surface (SBET) and total pore volume (VT) of ABC were larger than those of NBC, as also reported by Eloussaief & Benzina (2010). Acid treatment opens the platelet edges, increasing the surface area and pore diameter.

Table 1

Porous properties of NBC and ABC

SampleSBET (m2/g)Vmic (cm3/g)Vmeso+mac (cm3/g)VT (cm3/g)Dp (nm)
NBC 46 0.04 (24%) 0.13 (76%) 0.17 14.78 
ABC 76 0.05 (26%) 0.14 (74%) 0.19 10.00 
SampleSBET (m2/g)Vmic (cm3/g)Vmeso+mac (cm3/g)VT (cm3/g)Dp (nm)
NBC 46 0.04 (24%) 0.13 (76%) 0.17 14.78 
ABC 76 0.05 (26%) 0.14 (74%) 0.19 10.00 

Adsorption isotherm

Adsorption isotherms are mathematical expressions used to evaluate the distribution and interaction between sorbate and sorbents (Ololade et al. 2018). The Freundlich and Langmuir models (Equations (3) & (4), and (5) & (6), respectively) were used to fit the experimental data obtained after equilibration. The magnitude of the regression correlation coefficient (R2) reflects the quality of the isotherm fit to the experimental data.
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
where N (dimensionless) and KF (mg/g) (L/mg)N) are the intensity parameter (a measure of site energy heterogeneity) and Freundlich constant; KL (L/mg) and qmax (mg/g) the Langmuir constant (associated with solute–surface interaction energy) and maximum adsorption capacity, respectively; qmax (mg/g) the maximum adsorption capacity; Ce the liquid phase equilibrium concentration (mg/L), and qe the solid-phase concentration (mg/g) (Ololade et al. 2018).

The model parameters for NBC and ABC are presented in Table 2, and Figure 5 display the plots for Langmuir and Freundlich for Fe2+, Zn2+, Ni2+ adsorption onto the two forms of bentonite. The Freundlich adsorption capacities, KF, and intensity, KL, are highest for ABC, suggesting that the tendency for multilayer adsorption is enhanced by acidified activation, which also improved the natural clay's surface morphology (Figure 1, Table 1). The value of 1/n, between 0.1 and 1, shows good adsorption, confirming the adsorbent's heterogeneity, and heterogeneous adsorption of Fe2+, Zn2+, Ni2+ on the surface of ABC.

Table 2

Summary of Freundlich and Langmuir sorption parameters for Fe2+, Ni2+ and Zn2+ onto NBC and ABC

ModelParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
Freundlich Kf 0.248 0.028 0.057 0.771 0.285 0.482 
N 6.01 2.99 4.00 9.47 6.46 7.19 
R2 0.979 0.973 0.971 0.982 0.995 0.988 
Langmuir qmax (mg/g) 3.65 0.96 0.74 1.65 0.48 0.44 
KL (L/mg) 0.60 0.55 0.42 0.86 0.78 0.67 
R2 0.968 0.943 0.961 0.971 0.963 0.964 
ModelParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
Freundlich Kf 0.248 0.028 0.057 0.771 0.285 0.482 
N 6.01 2.99 4.00 9.47 6.46 7.19 
R2 0.979 0.973 0.971 0.982 0.995 0.988 
Langmuir qmax (mg/g) 3.65 0.96 0.74 1.65 0.48 0.44 
KL (L/mg) 0.60 0.55 0.42 0.86 0.78 0.67 
R2 0.968 0.943 0.961 0.971 0.963 0.964 
Figure 5

Sorption isotherm plots for (a,b) NBC and (c,d) ABC for Fe2+, Ni2+ and Zn2+.

Figure 5

Sorption isotherm plots for (a,b) NBC and (c,d) ABC for Fe2+, Ni2+ and Zn2+.

Close modal

The Langmuir model was efficient in analyzing adsorption data for all ions studied, but the Freundlich multilayer adsorption model best fit the experimental data, with higher R2 values. The qmax values for ion binding onto the adsorbent decreased as Fe2+ > Ni2+ > Zn2+, whereas , which determines the sorption, showed that the ABC sites had greater affinity for Fe2+, Zn2+, and Ni2+ than NBC, and they also decreased in the order Fe2+ > Ni2+ > Zn2+. This is due to the ions' varying electronegativity, which affects the surface ion exchange potential (Igberase et al. 2017).

The electronegativities of Fe, Ni and Zn on the Pauling scale are 1.83, 1.80, and 1.60, respectively, which agrees with the adsorption capacity and affinity of the metal ions to NBC and ABC. Since the values for these ions are higher for ABC than NBC, this indicates more favorable adsorption for the ions using ABC, which is similar to what Igberase et al. (2017) reported for Pb, Zn, Cu, Ni, and Cd adsorption by modified ligand in a single batch experiment.

Adsorption kinetics

The pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models (Equations (8)–(11)) were compared in order to establish the mechanism and rates of the adsorption process (Ololade et al. 2018; Zhao et al. 2020).
formula
(8)
formula
(9)
formula
(10)
formula
(11)
where qe and qt (mg/g) are the amount of adsorbate sorbed per mass of adsorbent at equilibrium and at time (t), respectively; K1 (1/min) and K2 (1/mg/g × min) pseudo-first-order and pseudo-second-order rate constants, respectively; and Kid (mg/g × min1/2) and C (mg/g) the intraparticle diffusion rate constant.

The kinetic parameters from the models are summarized in Table 3 and the kinetic plots displayed in Figure 6. The pseudo-second-order model best fit the kinetic data, with R2 values exceeding 0.99 for both NBC and ABC adsorption, but K2 and Kid suggest that adsorption by NBC is faster.

Table 3

Sorption kinetics coefficients for heavy metal adsorption by NBC and ABC

Kinetic modelParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
Pseudo-first-order qe(exp) 13.80 2.00 1.65 13.80 2.00 1.65 
qe (mg/g) 5.83 1.45 0.59 4.64 1.22 0.48 
K1 (min−1) 0.036 0.057 0.051 0.056 0.045 0.062 
R2 0.564 0.632 0.700 0.675 0.788 0.813 
Pseudo-second-order qe (mg/g) 13.92 2.13 1.70 13.84 2.05 1.67 
K2(g/mg/min) 0.004 0.279 0.303 0.003 0.119 0.171 
R2 0.992 0.994 0.996 0.995 0.997 0.999 
Intraparticle diffusion Kid(mg/gmin1/2) 14.305 2.357 1.724 11.252 1.986 1.593 
R2 0 .999 0.975 0.989 0.931 0.971 0.990 
Kinetic modelParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
Pseudo-first-order qe(exp) 13.80 2.00 1.65 13.80 2.00 1.65 
qe (mg/g) 5.83 1.45 0.59 4.64 1.22 0.48 
K1 (min−1) 0.036 0.057 0.051 0.056 0.045 0.062 
R2 0.564 0.632 0.700 0.675 0.788 0.813 
Pseudo-second-order qe (mg/g) 13.92 2.13 1.70 13.84 2.05 1.67 
K2(g/mg/min) 0.004 0.279 0.303 0.003 0.119 0.171 
R2 0.992 0.994 0.996 0.995 0.997 0.999 
Intraparticle diffusion Kid(mg/gmin1/2) 14.305 2.357 1.724 11.252 1.986 1.593 
R2 0 .999 0.975 0.989 0.931 0.971 0.990 
Figure 6

First-order, second-order and intraparticle diffusion model plots for NBC and ABC.

Figure 6

First-order, second-order and intraparticle diffusion model plots for NBC and ABC.

Close modal

Table 3 shows that the experimental values are close to the calculated values in NBC and ABC for the pseudo-second-order model. This further confirms that the second-order kinetic pathway is correct and suggests that ion-exchange/chemisorption may have occurred (Yu et al. 2015). The R2 values for the intraparticle diffusion model suggest that diffusion occurred for both NBC and ABC adsorption. A significant number of the heavy metal ions may have diffused into the adsorbents' pores before being adsorbed.

Adsorption thermodynamics

The thermodynamics of Fe2+, Zn2+, and Ni2+ ion adsorption was conducted using Van't Hoff plot – Equations (12) and (13) – for different temperatures.
formula
(12)
formula
(13)
where T is the thermodynamic temperature (K), R the gas constant (8.314 J/mol K), the change in entropy (J/mol.K), the change in enthalpy (kJ/mol), and the change in the Gibbs free energy (kJ/mol).

The plots of ln Kd versus 1/T for the adsorption of Fe2+, Zn2+, Ni2+ is presented in Figure 7. The related results given in Table 4 show that the adsorption is endothermic and spontaneous, with positive and negative values of enthalpy and Gibb's free energy, respectively. For all three heavy metals, both adsorption spontaneity and driving force are greater onto ABC than NBC as the temperature increases. A similar finding was reported by Igberase et al. (2017). With increasing temperature, the Gibbs energy magnitude decreases in conformity with the adsorption process' endothermic nature, as an increased energy supply would lead to enhanced adsorption. The enthalpy change (ΔHo) sign associated with sorption will consist of (1) enthalpy change for dehydration (ΔHod), which can be expected to be positive because energy is necessary to break the ion–water and water–water bonding of the hydrated metal ions, and (2) enthalpy change for complexing (ΔHoc), which will make ΔHo more negative because of metal complex formation (Ding et al. 2012). The positive ΔHo value obtained for Fe2+, Zn2+, and Ni2+ adsorption shows that dehydration may be more significant than complexation in the system. The enthalpy changes for adsorption were less than 40 kJ/mol for ABC and NBC, indicating that sorption for the ions studied was controlled by a physical mechanism (Ding et al. 2012). The higher positive entropy change (ΔS°) values for adsorption onto ABC is due to some structural changes of the activated adsorbent. The degree of randomness increased at the solid-liquid interface in ABC than NBC. This has been attributed to a physical adsorption process, and favors complexation and stable interaction (Yang et al. 2010).

Table 4

Thermodynamic parameters for metal ion adsorption onto NBC and ABC

ParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
ΔHo (kJ/mol) 5.59 4.63 4.30 0.77 0.29 0.48 
ΔSo (J/mol.K) 25.16 21.23 20.30 9.47 6.46 7.19 
ΔGo (kJ/mol) 273 K  1.27 −1.14  1.23  3.04  2.34  2.34 
293 K  1.77 −1.57 −1.63 −3.38 −2.84 −3.08 
303 K  2.03 −1.78 −1.84 −3.87 −3.09 −3.47 
313 K  2.28 −1.99  2.04  4.36  3.35  3.86 
323 K  2.53 −2.21  2.24  4.85  3.60  4.25 
333 K  2.78 −2.42  2.45  5.34  3.85  4.64 
ParameterNBC
ABC
Fe2+Ni2+Zn2+Fe2+Ni2+Zn2+
ΔHo (kJ/mol) 5.59 4.63 4.30 0.77 0.29 0.48 
ΔSo (J/mol.K) 25.16 21.23 20.30 9.47 6.46 7.19 
ΔGo (kJ/mol) 273 K  1.27 −1.14  1.23  3.04  2.34  2.34 
293 K  1.77 −1.57 −1.63 −3.38 −2.84 −3.08 
303 K  2.03 −1.78 −1.84 −3.87 −3.09 −3.47 
313 K  2.28 −1.99  2.04  4.36  3.35  3.86 
323 K  2.53 −2.21  2.24  4.85  3.60  4.25 
333 K  2.78 −2.42  2.45  5.34  3.85  4.64 
Figure 7

Van't Hoff plots for Fe2+, Zn2+ and Ni2+ adsorption onto NBC and ABC at 293, 303, 313, 323 and 333 K.

Figure 7

Van't Hoff plots for Fe2+, Zn2+ and Ni2+ adsorption onto NBC and ABC at 293, 303, 313, 323 and 333 K.

Close modal

Thermodynamic data on metal adsorption on clays are limited, with less still Fe2+, Zn2+, and Ni2+ adsorption onto bentonite, and none on acid-activated bentonite. Yavuz et al. (2003) found that ΔHo, ΔSo and ΔGo for Cu2+ adsorption onto Turkish kaolinite are 39.5 kJ/mol, 11.7 J/mol.K and −4.6 kJ/mol, respectively. Echeverria et al. (2003) found that ΔHo, ΔSo and ΔGo for Ni2+ adsorption onto illite have values of +16.8 J/mol, 58 J/mol.K and −1.04 kJ/mol. ΔHo, ΔSo and ΔGo for Cu2+ adsorption on surfactant-modified montmorillonite were reported as 7.05 kJ/mol, 9.09 J/mol.K and − 9.66 kJ/mol, respectively, by Lin & Juang (2002).

Effects of contact time, pH and oxidation using hydrogen peroxide

Figure 8(a) shows the results of wastewater treatment using different equilibration times (30, 45, 60, 90, and 120 minutes). After equilibration with a 5 g bed of either NBC or ABC, the supernatant was analyzed. As can be seen, uptake of, the ions studied was quicker with ABC than NBC, likely due to variation in sorption affinities (KL). The rate of ion uptake from the effluent was also found to increase with increasing contact time, as reported by Murithi et al. (2012). It is believed that the decrease in mass transfer coefficient (Kid) of the diffusion-regulated metal ion/adsorbent reaction was largely responsible for the enhanced removal efficiency (Bhattacharya et al. 2006). Increases in the amount of active sites, porosity and surface area on the ABC, after acid treatment, are responsible for the higher ion removal from the wastewater. For ABC, there was a reduction in Zn2+ concentration from 1.65 to 0.42 mg/L (74.5% removal), Ni2+ removal was from 2.00 to 0.60 mg/L (70.0% removal), and Fe2+ from 13.80 to 5.50 mg/L (60.1%). On the other hand, removal by NBC, starting from the same initial concentrations as ABC, reported Zn2+, Ni2+, Fe2+ final concentrations of 0.83 mg/L (49.7% removal), 1.07 mg/L (46.5%), and 9.25 mg/L (33.0%), respectively.

Figure 8

Influence of process variables on the removal efficiencies of NBC and ABC: (a) contact time, (b) solution pH (c) oxidation using hydrogen peroxide.

Figure 8

Influence of process variables on the removal efficiencies of NBC and ABC: (a) contact time, (b) solution pH (c) oxidation using hydrogen peroxide.

Close modal

pH influences the adsorption chemistry of many aqueous pollutants, and particularly the adsorbent surface characteristics and metal ion speciation (Attahirua et al. 2012). Figure 8(b) clearly shows the effects of varying the pH (pH 4, 6, 8, 10, and 12) on heavy metal ion removal, while keeping other conditions constant. Sorption sites are prone to protonation or deprotonation depending on the solution pH, while the adsorbent surface can also be positively or negatively charged under differing pH conditions (Ololade et al. 2018). At pH 4, when the adsorbent is positively charged, low uptake is observed for the ions studied, for both ABC and NBC. Like charges repel. Furthermore, H+ competes with metal ions in acidic pH. However, as the pH increases, more Fe2+, Zn2+, and Ni2+ are adsorbed, because they exist as FeOH+, ZnOH+ and NiOH+, and the sorbent is deprotonated at basic pH (Ibigbami et al. 2016).

The results show that, as the wastewater pH increases in the presence of ABC and NBC, the amounts of the heavy metals adsorbed increase due to electrostatic attraction. However, the removal efficiency of Fe2+, Zn2+, Ni2+ by ABC is higher than that of NBC, due to its greater surface area and sorption site availability (cavities and/or pores).

Analysis of the treatment results showed that the optimum proportional removal efficiencies occurred at pH 8 for ABC and pH 10 for NBC. In ABC, adsorption capacities are in the order Fe2+ >>> Zn2+ >>> Ni2+. Iron is precipitated, complexed, or adsorbed easily because Fe2+ is oxidized to Fe3+ and precipitates as Fe(OH)3. On the contrary, the adsorption trend onto NBC follows the order, Zn2+ >>> Fe2+ >>> Ni2+ ions, which may be attributed to the smaller ionic size of Zn2+ compared to Fe2+ and Ni2+ (Izidoro et al. 2013).

Figure 8(c) shows the result of varying the H2O2 dose (0.5 to 1.50 v/v%) on heavy metal removal, at 30-minute contact time and using 5 g of NBC or ABC. H2O2 acts an oxidizing agent for heavy metal ions. Peroxide application enhanced the treatment efficiency for the metal ions studied for both adsorbents, possibly due to ionic interaction between the oxidized metals and adsorbents. However, ABC showed more ionic interaction than NBC for all of the metals studied. The formation of hydrophobic/insoluble metal hydroxides like Zn(OH), Ni(OH, Fe(OH) enhances partitioning of the metal pollutants onto the adsorbents' hydrophobic surfaces (Pignatello et al. 1999; Kinuthia et al. 2020).

Comparison with previous studies

Many minerals and plant materials have been used to remove metals from polluted water (Önal & Sarikaya 2007; Bhatnagar et al. 2010; Internò et al. 2015). Adsorption of metal ions using clay minerals such as: palygorskite from Dwaalboom, South Africa for Pb2+, Ni2+, Cr3+, Cu2+ at pH 7 (Potgieter et al. 2006); montmorillonite from India's Karnataka region for Cu2+ at pH 2.5 (Oubagaranadin & Murthy 2010); montmorillonite and kaolinite for Fe2+, Co2+, and Ni2+ at pH 5.7 (Bhattacharyya & Gupta 2008); Ca-bentonite from Almeria, Spain, and an Na-exchanged bentonite from Milos, Greece, for Cr3+ (pH 4), Ni2+ (pH 6), Zn2+ (pH 6), Cu2+ (pH 5) and Cd2+ (pH 6) (Alvarez-Ayuso & Garćia-Sánchez 2003); kaolinite from China's Longyan region for Pb2+ (pH 6), Cu2+ (pH 6.5), Cd2+ (pH 7) and Ni2+ (pH 7) (Jiang et al. 2010); palygorskite for Pb2+ at pH 5 (Fan et al. 2009), illite from Tunisia for Pb2+ at pH 7 (Eloussaief & Benzina 2010); smectite from Tunisia for Pb2+ at pH 4 (Chaari et al. 2008); illite from Tunisia for Cd2+ and Cr3+ at pH 3.5 (Ghorbel-Abid et al. 2010). Although, all of the above are geo-sorbents, they exhibit different heavy metal removal efficiencies due to their differing physicochemical properties. The degree of alteration of the active surface and clay structure porosity produced by acids depends, among other things, on the clay's chemical composition and interlayer cation types, the acid applied, and its concentration, temperature and time of action (Bhattacharyya & Gupta 2008).

The adsorption of Fe2+, Zn2+, Ni2+ from a pharmaceutical effluent was investigated using NBC and ABC. Bentonite is an efficient adsorbent due to its high specific surface and pore volume, negatively charged surface and other physicochemical properties, as found in this study.

In this study, bentonite was activated successfully using nitric acid, as shown by FTIR, CEC, SEM, XRD, and BET studies. It was shown that ABC offered maximum proportional removals of about 89.9, 81.8 and 75.5% for Zn2+, Fe2+ and Ni2+, while NBC's removal capacities were 63.9, 59.6 and 58.7% for the same species. The results indicate that heavy metal removal by bentonite was affected considerably by process conditions, such as solution pH, contact time, and temperature, and can be enhanced by oxidation using hydrogen peroxide.

The experimental data were fitted to various kinetic models, which revealed that the pseudo-second-order model fit best, while it appears that diffusion plays a role in the process. It can be concluded that film and intraparticle diffusion occurred simultaneously during adsorption process considering the strong correlation coefficients (Table 3).

Adsorption isotherm models such as those derived by Langmuir and Freundlich were used to describe the experimental adsorption data, and plots showed that Freundlich's multilayer adsorption model fit best, with better R2 values, showing good adsorption and confirming the adsorbent's heterogeneity. The results show that adsorption was controlled mainly by electrostatic attraction and/or ion exchange. The thermodynamic parameters (ΔHo, ΔSo and ΔGo) for Fe2+, Ni2+, Zn2+ adsorption show that the process behaves endothermically for both ABC and NBC. ABC's ΔHo value exceeds that of NBC, indicating that Fe2+, Zn2+ and Ni2+ are held more strongly by ABC, their magnitude shows moderately stronger bonds between ABC and Fe2+, Zn2+ions than Ni2+. The negative value of ΔG for the ions studied on both ABC and NBC shows the spontaneous and favorable nature of adsorption at various temperatures. ΔGo becomes increasingly negative with increasing temperature, suggesting that adsorption is more favorable.

The study confirmed that modifying bentonite using nitric acid enhanced its specific surface. Finally, it has been shown that ABC, with its improved specific surface compared to NBC, can remove pollutants effectively (especially heavy metals) from pharmaceutical wastewater.

The authors are grateful to the Federal Institute of Industrial Research (FIIRO), Oshodi, Lagos State, Nigeria for the supply of bentonite clay.

The authors declare that they have no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.

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