Characterisation of Mg-Al Hydrotalcite and Surfactant-Modified Bentonite Nano Clays for the Treatment of Acid Mine Drainage

Abstract

The presence of acid mine drainage (AMD) poses health risks to humans and other living organisms. While much research on AMD has been conducted, the absence of a widely accepted AMD treatment technology makes it an ongoing topic for further exploration. The present study explored the potential of nano-clay adsorbents for the removal of iron and manganese from AMD. The two nano-clay adsorbents used in this study were synthetic hydrotalcite (HT) and modified bentonite (MB) clays. The adsorption media were characterised before and after adsorption using X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET), and Scanning Electron Microscope (SEM) to explain the adsorption mechanisms. To investigate the efficiency of the clays, batch adsorption studies were carried out by varying the concentration, pH, and time. To establish the kind of adsorption mechanism that occurred, Langmuir and Freundlich isotherms were applied. It was found from characterisation results that HT and MB contained hydroxyl and carbonyl functional groups responsible for surface complexation mechanisms; XRD showed that isomorphic substitution and precipitation also occurred in adsorption using HT. The specific surface area for modified bentonite and hydrotalcite as determined by BET was 3.13 and 44.7 m²/g respectively. The XRD and the SEM results showed that HT was crystalline while MB was amorphous, probably due to the irregular stacking of the surfactant. It was observed that the adsorbent deprotonated as pH increased, resulting in an increase in metal removal. The Langmuir isotherm provided the best monolayer adsorption capacity with R_L < 1 and correlation coefficients ranged from 0.95 to 0.99 for both adsorbents.

1. Introduction

Many industrial processes generate wastewater that is detrimental to the environment and human life. However, there seems to be a consensus that Acid Mine Drainage, AMD, is caused by a process where sulfidic minerals, such as pyrite (FeS₂), are oxidised to sulfates then form metallic hydroxides [1]. It is characterised as having high acidity, a high concentration of sulfate, and containing a wide range of heavy metal ions [2].

Numerous approaches, such as electrochemical methods, chemical precipitation, chemical coagulation, membrane filtration, ion exchange, bioremediation, and adsorption, have been applied for the removal of heavy metals [3]. However, adsorption technologies offer several designs, operation, and efficiency advantages over other wastewater remediation technologies [4]. Adsorption is a physical treatment process that removes contaminants such as heavy metals by mass transfer through its adsorbent, which has oppositely charged binding sites for the contaminants to latch onto [5].

Activated Carbon is considered one of the best adsorbents for most contaminants as it has a high adsorption capacity due to its large surface area [6]. However it is still expensive relative to other low-cost adsorbents such as clay minerals [7]. The clay minerals are very widely distributed and abundant in soils, which make them the promising environmental adsorbents that can be used in industrial processes [6]. Nanoclay is one of the lowest-cost nanomaterials which has properties such as being non-toxic and environmentally friendly, having a specific surface area, and it also has high adsorption capacity [8].

The adsorption of heavy metal ions is associated with the formation of anion–metal complexes and hydroxide precipitation on surfaces through chemical bonding with the hydroxyl groups of clay [9]. The process of chemical bonding with the hydroxyl group is called specific adsorption. Functional surface groups can be silanol groups, inorganic hydroxyl groups, or organic functional groups. Specific adsorption is based upon adsorption reactions at OH-groups at the clay surfaces and edges, which are negatively charged at high pH. The adsorbing cation bonds directly by an inner-sphere mechanism to atoms at the surface. As a consequence, the properties of the surface and the nature of the metal constituting the adsorption site influence the tendency for adsorption [10]. It will be of benefit in understanding the sorption mechanisms and developing new technologies for water treatment to investigate the sorption of metal cations on clay.

Clays can be modified chemically with acids, bases, cationic surfactants, and certain poly hydroxyl cations to improve their efficiency [11]. Surfactant-modified clays provide different d-spacing for the clay and improve the efficiency of adsorbing pollutants [12]. In addition, nanomaterials, used as adsorbents to remove heavy metal ions from wastewater, have received significant attention owing to their high specific surface area [13], which make nano-clays the most promising adsorbent. Clays may be divided into two broad groups: cationic clays, widespread in nature, and anionic clays, rarer but relatively simple and inexpensive to synthesise.

Hydrotalcite materials, also known as layered double hydroxides (LDH, generally formulated as [M_1−x²⁺M_x³⁺(OH)₂A_x/nⁿ⁻·mH₂O]), a type of anionic clay, are a large group of natural and synthetic layered materials [14]. Synthetic clays may have advantages over natural clay because variables such as purity, composition, and reproducibility can often be controlled better than when using natural clay specimens, which, among other drawbacks, typically contain impurities [15]. They are the perfect material for adsorption with a large surface area per unit of mass, large porosity, etc., i.e., they satisfy all the requisite properties of adsorption [16].

A study on the removal of halides (F⁻, Cl⁻, Br⁻, I⁻) and oxyanions (NO³⁻, SO₄²⁻, PO₄³⁻, AsO₄³⁻) and heavy metals on LDH has been performed [17], but there is a need for an extensive study on the adsorption mechanism responsible for the removal of heavy metals. Jawad, A. et al. [18] used FeMgAl derived from layered double hydroxides (LDH) to remove heavy metals, and it was found that the precipitation mechanism dominated the removed heavy metals as metal-hydroxide or metal-carbonate. A Li-Al hydrotalcite-like compound (Li-Al HTlc) was used to adsorb Cu²⁺ and Zn²⁺ for investigating the adsorption characteristics of heavy metal cations; the adsorption mechanism denoted that the adsorption process was controlled using two main mechanisms, i.e., surface complexation and isomorphic substitution [19].

Previous studies have investigated the use of organo-clays, which are clay particles modified with organic cations, to improve the adsorption ability of organic contaminants. Studies such as the one by Roca, Y. B. and Fuentes, W. S. [20] used hexadecyltrimethylammonium-modified montmorillonite nanoclay for removal of Cu²⁺ from acid mines, and it was found that the modified nanoclay removed up 99% of Cu²⁺. Almasri, D. A. et al. [7] carried out a study to remove arsenite (As (III)) using raw montmorillonite and HyFe-MMT by adsorption experiments conducted under various conditions and it was found that the adsorption capacity of raw MMT was increased more than five times after iron modification. Natural clay has been considered absorbent to treat wastewater. Nonetheless, the effective application of clay for water treatment is limited due to some of its characteristics, leading to a low adsorption capacity [21]. The clay minerals modified with surfactant alter the structure of the clay then improve the adsorption capacity [22]. In this view, a highly effective adsorbent leads to high productivity and, hence, economic benefits.

The solution pH also plays a vital role in the adsorption process and research has shown that it is one of the critical parameters. It influences the solubility of heavy metals and affects the concentration of the counter ions on the functional groups. The level of pH has a great impact on the reactions since, in lower pH solutions, there is high competition to bind with the hydroxyl group [10]. When the pH of the solution is increased after adsorption, it favours the formation of metal hydroxides in the surface of hydrotalcite [9].

Furthermore, the adsorption capacity and the mechanism for adsorption can be analysed by using adsorption isotherms. The two most common adsorption isotherms employed for heavy metal adsorption are the Langmuir isotherm and Freundlich isotherm [23]. The Langmuir adsorption model describes the adsorption due to chemical interactions and it is centred on the adsorption on a homogeneous surface by monolayer sorption without interaction between adsorbed species and all adsorption sites are equal for each layer of the adsorbent. Therefore, the adsorption capacity is limited to the formation of monolayer due to the available active sites.

The model is described by the equation

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{Q}}_0{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}}$$

(1)

And can be linearised as

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{Q}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{L}}}$$

(2)

where K_L (L/mg) is the Langmuir adsorption constant and Q_L (mg/g) is the monolayer adsorption capacity of the adsorbent. Therefore, a plot of C_e/q_e versus C_e provides a straight line of slope 1/Q_L and intercepts 1/Q_LK_L.

The adsorption process can be evaluated to see whether it is favourable using a Langmuir dimensionless separation factor R_L defined as:

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_0}$$

(3)

where C₀ (mg/L) is the initial metal ion concentration in solution. The favourability of this isotherm is determined by the value of R_L. If it is less than 1.0, the adsorption is said to be favourable, if it is greater than 1.0 than Langmuir is unfavourable.

The Freundlich adsorption isotherm is related to surface heterogeneity and exponential distribution of the active sites and their energies. The surface contains adsorption sites with different attractions. The model is described by the equation

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}{}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}$$

(4)

When linearised it comes as

$${\mathrm{logq}}_{\mathrm{e}}={\mathrm{logK}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}{\mathrm{logC}}_{\mathrm{e}}$$

(5)

The Freundlich constant K_F indicates the adsorption capacity of the adsorbent and n is a measure of the deviation from linearity of the adsorption.

This work aimed to propose the use of Arquad surfactant-modified nano clays as potential adsorbents for the removal of manganese and iron from acid mine drainage and to compare the results with hydrotalcite. There have been studies on using anionic surfactant but there have not been studies on the use of this commercially available cationic surfactant Arquad surfactant. The adsorption mechanisms were identified from the characterisation of the adsorbents before and after adsorption and from batch experiments by varying pH and concentration. Furthermore, Langmuir and Isotherm isotherms were applied.

2. Materials and Methods

2.1. Preparation of the Adsorbents

The adsorbents in this study were surfactant-modified bentonite and hydrotalcite donated by the Council for Scientific and Industrial Research (CSIR). The materials were produced in the CSIR’s Nanomaterials Industrial Development Facility (NIDF) South Africa, Pretoria. Arquad 2HT-75 was used as a surfactant to modify Ca-based bentonite at a ratio of 2:1 for the removal of manganese and iron. Arquad is one of the commonly used surfactants when modifying clays. It is traded as Arquad 2HT-75, and its composition is as follows: di (hydrogenated tallow) dimethyl ammonium chloride with 2-propanol and water.

2.2. Adsorption Experiments

While the work was carried out using synthetic AMD, real AMD was necessary to determine the real concentration of metals from mines. Samples were obtained from three different mines, i.e., two from Johannesburg (AMD 1 and AMD 2) and one in Mpumalanga (AMD 3), South Africa as shown in

Table 1. The concentrations of AMD from different sources.

	Source	Fe (II) (mg/L)	Mn (II) (mg/L)
AMD 1	Johannesburg	864	83
AMD 2	Johannesburg	407.8	61
AMD 3	Mpumalanga	154.4	55.3

. AMD samples were collected in pre-cleaned high-density polyethylene containers sealed with airtight lids. In all the three samples, iron and manganese were found to be the dominant heavy metals, and hence these two metals formed the basis of this study.

Synthetic solutions were used in this study. A dosage of 0.5 g of iron sulphate (FeSO₄·7H₂O) was mixed with 1000 mL of distilled water in a flask. This solution yielded a concentration of 800 mg/L of iron. To make 50 mg/L of manganese, 0.307 g of anhydrous manganese sulphate (MnSO₄·H₂O) was mixed to 1000 mL with deionised water. The solution pH was adjusted using sulphuric acid and sodium hydroxide solutions. Synthetic AMD solution prepared as such was kept in the refrigerator at 4° before use. All working solutions were prepared from the stock solutions. Batch adsorption studies were then performed using 1 g of clay adsorbent added to a fixed volume (50 mL) of the metal ion solutions at 25 °C, and shaker speed of 150 rpm. After 24 h in a shaker, the solutions were filtered, and the remaining metal ion concentrations were analysed using Inductively Coupled Plasma Mass Spectrometry. The filter papers were dried in the oven overnight and the clay solids were taken for characterisation using SEM, BET, XRD, and FTIR

2.3. Effect of Concentration

When determining the effect of concentration, a 50 mL of metal test solution of concentrations from 100–800 mg/L for Fe and 10 to 50 mg/L for Mn (II) was dosed with 0.1 g of adsorbent and mixed using a shaker at 25 °C for 24 h at a fixed speed of 150 rpm. H₂SO₄ was added to reduce the pH to 2. After 24 h passed, the solution was filtered and the concentration of the metal ions remaining in the solution was analysed by using the ICP.

The percentage removal and quantity adsorbed were calculated as follows:

$$\%\mathrm{removal}=100\times \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}$$

(6)

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{V}\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{m}}$$

(7)

where C₀ (mg/L) is the initial metal ion concentration, C_e (mg/L) is the equilibrium metal ion concentration in solution, q_e (mg/g) is the quantity of metal ions adsorbed per unit mass of adsorbent, V (L) is the volume of solution used, and m (g) is the mass of the adsorbent.

2.4. Effect of pH

The effect of pH on metal removal from AMD was explored by varying the initial solution pH from 2–6. A dosage of 0.1 g for each adsorbent was contacted with 50 mL of metal ion-rich solutions at 25 °C and 150 rpm. The solution was filtered and the concentration of the metal ions remaining in the solution was analysed by using the ICP.

2.5. Characterisation Techniques

X-ray diffraction (XRD) measurements were obtained using a Bruker multipurpose powder diffractometer (D8 Advance), powered with 40 kV, current of 40 mA, and irradiation Cu Kσ (λ = 1.5406 nm). FTIR spectra were recorded using Spectrometer Perkin Elmer, Frontier model over the wavelength range of 4000–400 cm⁻¹. Surface area and porosity were measured using a TriStar 3000 V 6.08 A. The Nova Nano SEM scanning electron microscope with EDT detector and TLD detector was used to determine the morphology and elementary composition of the adsorbent materials.

3. Results and Discussion

3.1. X-ray Diffraction (XRD)

The XRD patterns enable the interpretation of basal spacing (d-spacing) of clay before and after adsorption of metals and provide some indications of any change in crystallographic structure. The adsorbents were analysed after adsorption to investigate if heavy metals intercalated within clay sheets. The change in cationic composition after adsorption is an indication of adsorption taking place in clay. This change may be seen from the change in peak intensity. The peaks become broad and their intensity decreases depending on the size of the metal being adsorbed [24]. Figure 1 and Figure 2 show the XRD patterns of hydrotalcite and modified bentonite, respectively.

Experimental pattern HT shows the XRD pattern for the raw hydrotalcite (before adsorption), experimental pattern HT FE MN shows the XRD pattern after the adsorption of Fe (II) and Mn (II), experimental pattern HT FE shows the XRD pattern after the adsorption of Fe (II)), and experimental pattern HT MN shows the XRD pattern after the adsorption of Mn (II). The sample exhibited a typical layered double hydroxides structure as illustrated by its well-defined XRD pattern, with the high intensity peak (003) at low degree (2θ = 11.73°) followed by two weaker peaks (006, 2θ = 23.6° and 009, 2θ = 39.42°) [22]. The sharpest diffraction peak for the (003), (006), and (009) planes proves that it had crystallinity and was essentially maintained after adsorption. The peak at 11.73$°$ 2$\mathsf{\theta }$ for all samples of hydrotalcite is attributed to the reflections from (003) of crystallographic planes [25]. This peak is also described as a characteristic of divalent cations occupying the interlayer sites; see Figure 2 for more description of the peak [26].

Figure 3 shows the change in intensity on a peak at 11.73 2θ. HT shows the peak before adsorption at 8563, HT Mn shows the peak after adsorption of Mn (II) at 14,441, HTFe shows the peak after adsorption of Fe at 10,773, and HTFeMn shows the peak after adsorption of Fe (II) and Mn (II) at 11,483. The intensity at peak 11.73 slightly shifted to lower 2θ values, indicating a slightly larger interlayer space. This indicates that Fe²⁺ and Mn²⁺ ions were intercalated into the HT via isomorphic substitution with Mg²⁺. During isomorphic substitution, the gallery height expanded to fit the larger-ionic-size metals [24]. Mn²⁺ had ionic radii of 0.083 and Fe had ionic radii of 0.078 nm, whereas the substituent (Mg²⁺) had ionic radii of 0.066. This may explain the peak expansion at 11.73 2θ. Yue, X., et al. obtained similar results using Cl-LDH for the adsorption of Cu ²⁺ [13].

Figure 4 shows the XRD image for modified bentonite before and after the adsorption of singular Fe and Mn and the binary mixture of Fe and Mn. Experimental pattern MB shows the XRD pattern for MB (before adsorption), experimental pattern MB FE + MN shows the XRD pattern after the adsorption of Fe (II) and Mn (II), experimental pattern MB FE shows the XRD pattern after the adsorption of Fe (II), and experimental pattern MB MN shows the XRD pattern after the adsorption of Mn (II). The XRD pattern of modified bentonite show poor crystallinity, broad and less intense peaks due to the presence of layers of surfactant, or an irregular stacking of surfactant layers, and thus the structure of the clay became amorphous to XRD. The X-ray diffractometer of modified bentonite samples showed that the main constituent was quartz, with moderate to small amounts of some other minerals.

3.2. FTIR Analysis

3.2.1. For Hydrotalcite

The FTIR analysis was utilised to determine the functional groups (

Table 2. Functional groups found in hydrotalcite at different wavelengths.

Functional Group	Wavelength cm−1
O–H for AlOH, SiOH	3420
H–O–H	1740
CO32−	1363
Si–O quartz	770
Mg–O–Al, Si–O–Al	548
Si–O–Si bending	442

) on the surface of clay responsible for adsorption. Comparing the FTIR spectra before and after adsorption as shown in Figure 5, there was a slight band shift and transmittance percentage increase, suggesting a decrease in intensity.

In Figure 5, HT shows the FTIR pattern for the raw hydrotalcite (before adsorption), HT Fe + Mn shows the FTIR pattern after the adsorption of Fe (II) and Mn (II), HT Fe shows the FTIR pattern after the adsorption of Fe (II), and HT Mn shows the FTIR pattern after the adsorption of Mn (II). The broad peak at 3420 cm⁻¹ denotes the vibrational stretching of OH groups of adsorbed water. It can be observed that the band intensity of the modified bentonite decreased, which could be attributed to the replacement of cations of the initial nano-clay with iron cations during the exchange process.

Almasri, D.A. et al. [7] had similar observations on the characterisation of modified MMT after As (III) adsorption at a pH of 6. Another band at 1740 cm⁻¹ was due to the bending vibration of H–O–H and it should be assigned to the adsorbed water molecule in the interlayer. An intense absorption band at about 1363 cm⁻¹ and 680 cm⁻¹ was assigned to the asymmetric stretching of CO₃⁻² in the interlayer, indicating that some CO₃⁻² ions existed in the gallery of HT [13,27,28]. The absorption at 548 cm⁻¹ might be related to the lattice vibrations, such as Mg–O–Al or O–Mg–O vibration. This sample shows, a spectrum similar to that reported by [29]. The most remarkable difference amongst the peaks was at 3420 cm⁻¹, representing hydroxyl (OH) stretching. After the HT was loaded with Fe and Mn, some differences in the locations of the absorbance peaks were observed. The band shifted and peak intensity decreased. The reduction of the OH⁻ group suggests that the water molecules in the gallery were replaced with Fe and Mn ions. As described by ref. [30], this is a surface complexation mechanism where the chemical or electrostatic binding reaction between the metal ions and the functional groups occurs.

3.2.2. For Modified Bentonite

The FTIR analysis of modified bentonite was also carried out to identify the functional groups as seen in

Table 3. Functional groups of the modified bentonite at different wavelengths.

Functional Group	Wavelength cm−1
O–H for Al–OH, Si–OH	3628
C–H stretching	2920
C–H stretching	2850
H–O–H bending of water	1633
C–H stretching	1470
Si–O stretching	1000
OH, deformation, linked to Al3−, Mg2−	800
Fe–O, Fe2O3 Si–O–Al	510
Si–O–Si bending	420

and the change in transmittance was observed and is shown in Figure 6.

The examination of the FTIR spectra (4000–400 cm⁻¹) provides information on fundamental vibrational modes of the constituent units of these materials. MB shows the FTIR pattern for MB (before adsorption), MB Fe Mn shows the FTIR pattern after the adsorption of Fe (II) and Mn (II), MB Fe shows the FTIR after the adsorption of Fe (II), and MB Mn shows the FTIR after the adsorption of Mn (II) OH stretching, and bending vibrations occurred in the spectral regions of 3750–3500 and 950–600 cm⁻¹, respectively. Si–O and Al–O stretching modes were found in the 1200–700 cm⁻¹ range, while Si–O and Al–O bending modes dominated the 600–400 cm⁻¹ region [31].

Table 3 shows the list of functional groups that were found in modified bentonite. Slight changes in intensities after adsorption were observed in the corresponding vibration absorption bands of the reactive functional groups as shown in Figure 6. The FTIR spectra showed that the aluminol and silanol groups are major sites for the binding of metals to MB. Similar functional groups were found in alkaline-modified montmorillonite by ref. [32] and the adsorbent was proven to be efficient for the removal of Mn (II) and Ni (II).

3.3. BET Analysis

Surface area and pore size of adsorbents are among the important parameters that describe the quality of adsorbents, as they affect directly their analyte retention abilities [33]. Bentonite clay can have the inter-lamellar expanded and the insertion of larger organic cations, increasing the spacing between the tetrahedral sheets. This behaviour may lead to an increase or decrease in surface area, depending on the arrangement, and thus the packing of organic cations in the space among the alumina-silicate sheets. Larger surfactant cations may result in compact packing in the inter-lamellar layer and result in more serious pore-blocking that inhibits the passage of nitrogen molecules during BET analysis [34].

Table 4. Surface properties of hydrotalcite and modified bentonite analysed with BET.

Adsorbent	Surface Area (m2/g)	Pore Size (Å)	Pore Volume (cm3/g)
MB before ads.	3.13	508.6	0.036
MB after Mn ads.	4.93	275.6	0.029
MB after Fe ads.	8.55	168.5	0.041
MB after Fe + Mn ads.	7.87	184.5	0.041
HT before ads.	44.71	647.4	0.72
HT after Mn ads.	25.29	145.9	0.11
HT after Fe ads.	21.61	189.6	0.11
HT after Fe + Mn ads.	24.25	173.2	0.11

shows that the specific surface areas for modified bentonite and hydrotalcite are 3.13 m²/g, and 44.7 m²/g, respectively as measured by the BET method. Normally the specific surface area of raw bentonite clay is 8.9 m²/g. The BET surface area for modified bentonite is smaller than the unmodified bentonite, which indicates the impregnation of surfactant onto bentonite. The specific surface area decreased, probably due to clay particle surface covered by the surfactant. This sort of reduction in surface area usually occurs in organically modified clays, as reported previously by Wahab N. et al. and Saeed M. et al. [35,36].

Results showed that the surface area of MB increased from 3.13 m²/g to 4.93 m²/g, 8.55 m²/g, and 7.87 m²/g after the adsorption of Mn, Fe, and a mixture of FE and Mn. This may be because sulphuric acid was added to the solution and removed the impurities. It was proven by ref. [37] that the activation treatment using sulphuric acid can be used to increase the surface area of the adsorbent.

3.4. SEM Analysis for HT and MB

The scanning electron microscope (SEM) was used for the analysis of the microstructure morphology and chemical composition characterisations. The porous nature of clay is desirable as the metal ions can easily penetrate the pores of clay for efficient adsorption.

The micrographs which were obtained are shown in Figure 7 and Figure 8. The HT consists of very thin rod fibres, confirming the crystallinity (Figure 8a), while the granules of MB had smooth surfaces (Figure 7a); however, after adsorption, the porous structure was observed due to the removal of impurities that might have taken place. The micrographs results correspond with the XRD results, which confirmed that HT is crystalline and MB is amorphous. The smoother surface and smaller BET surface area might be due to the intercalation of surfactant in the interlayer of MB and surfactant entering the pores on the surface of MB. The particle distribution is not homogeneous, as the samples have different-sized grains.

MB and HT qualitative elemental analysis is shown in

Table 5. Elementary composition of MB, before and after adsorption analysed using EDS.

Elements	Before Ads (%)	After Ads (Mn) (%)	After Ads (Fe) (%)	After Ads (Fe + Mn) (%)
C	43	48.33	42.99	42.91
O	37.70	36.79	40.13	38.27
Mg	0.79	0.92	0.97	0.94
Al	4.19	3.57	3.9	4.24
Si	10.16	9.07	10.68	11.94
Ca	2.89	1.32	0	0
Fe	1.26	0	1.32	1.7

and

Table 6. Elementary composition of HT, before and after adsorption analysed using EDS.

Elements	Before Ads (%)	After Ads (Mn) (%)	After Ads (Fe) (%)	After Ads (Fe + Mn) (%)
C	34.97	28.98	15.28	21.92
O	50.55	54.54	62.85	58.7
Na	0.37	0.22	0	0
Mg	8.26	9.35	13.85	10.82
Al	2.79	4.96	7.27	6.18
Si	3.05	1.24	0	0.52
S	0	0.2	0	0
Ca	0	0.54	0	0.94
Fe	0	0	0.75	0.92

, respectively. The common components of these samples are Si and Al, the main components of clay minerals. The presence of Ca and Mg can also be observed. Mg and Ca contents indicate variable cation amounts, which are suitable for the adsorption process.

In this case, Ca have been exchanged with Fe by isomorphic substitution since the composition of Fe increases after adsorption of Fe in a singular and in the binary adsorption systems.

Identifying the elemental information by EDS confirms the metal ion adsorbed onto HT as shown in Table 6. Fe was detected after the adsorption of Fe and after adsorption of Fe + Mn, confirming the adsorption of Fe (II). In addition, the results of EDX analyses showed that both adsorbents could better adsorb the Fe (II) ion rather than the Mn (II) ion, which is in accord with the experimental results and the maximum adsorption capacity. Depletion of some cations (C, Na, and Si) was observed in HT, signifying that the leaching of such ions occurred as well as loss of the affected ions in form of oxides and carbonates during adsorption.

3.5. Effect of Initial Concentration

The effect of concentration was investigated by varying Fe (II) concentration from 800 mg/L to 100 mg/L and Mn (II) from 50 mg/L to 10 mg/L while all other parameters were kept constant, i.e., pH was at 2, adsorbent mass was 0.1 g, and temperature was 25 °C. It was necessary to investigate the effect of concentration in adsorption because normally wastewater contains different ion concentrations depending on the source as seen from samples collected in different mines.

The percentage removal of Fe (II) and Mn (II) showed a decrease with an increase in ion concentration for both HT and MB. The percentage removal for Mn decreased from 60% to 18% and from 40% to 18% using HT and MB, respectively, as shown in Figure 9a.

For the removal of Fe in Figure 9b, percentage removal for Fe also decreased from 100% to 35% (HT) and from 44% to 30% (MB) binary solution of concentration of 100 mg/L to 800 mg/L. The decrease is because all adsorbents have a fixed number of active sites and at higher concentrations, the active sites become saturated.

3.6. Effect of pH on the Adsorption of Fe (II) and Mn (II)

Adsorption experiments were performed in the pH range of 2–6 (error of less than 0.3).

The effect of pH on the removal of Fe (II) and Mn (II) from synthetic AMD using HT and MB is shown in Figure 10 for the binary solution. It is noted in both figures that the adsorption uptake of Mn (II) was enhanced from 9% to 65% using MB and from 23.6% to 81% using HT. The adsorption uptake of Fe (II) was enhanced from 33.5% to 56.5% using MB and from 50% to 73.9% using HT with increasing pH value from 2 to 6. HT contains hydroxyl functional group and MB contains the aluminol and silanol as analysed by the FTIR, which are active sites for the metal binding. At low pH these functional groups are more protonated and, hence, they are less available to retain the metals [38]. As a result, the adsorption of Mn and Fe decreases. At a lower pH, the positive-charged ion (Fe (II) and Mn (II)) species may compete with H+ and be adsorbed at the surface of the adsorbent by ion exchange mechanism [39]. At high pH values, the quantity of H⁺ ion is reduced, while most active sites on the adsorbent are de-protonated. This enhances metal uptake [40]. For an example, Setshedi et al. performed an investigation on the effect of pH for the adsorption of Pb²⁺ in hydrotalcite; it was found the removal of Pb²⁺, increases with an increase in pH. This was due to hydrogen ions competing with Pb²⁺ ions on the functional groups of hydroxyls. As the pH increases, the amount of hydrogen ions decreases, Pb²⁺ is converted to PbOH⁺, and the active site of the adsorbent become deprotonated, meaning the amount of negatively charged active sites increases, hence improving the adsorption capacity [10,41].

3.7. Isotherm Models

The adsorption isotherms were used to compare the adsorption capacity of adsorbents for Fe (II) and Mn (II). Adsorption can take place by physical and chemical adsorption. In the case of physical adsorption, the interactions between the adsorbate and the adsorbent are electrostatic as reported by numerous researchers. Chemical adsorption involves specific forces, such as those that are operative in the formation of chemical bonds [23]. The common adsorption isotherms employed to represent the adsorption data are Langmuir isotherm and Freundlich isotherm.

The isotherm models of Mn (II) and Fe (II) removal were studied by different initial concentrations ranging from 10 to 50 mg/L for Mn (II) and 100 to 800 for Fe (II). Langmuir constants q and K were obtained from the slope of C_e/q_e vs. C_e and are shown in Figure 11. While the isotherm constants K and n are the Freundlich constants, which represent sorption capacity and sorption intensity, respectively and were evaluated from the intercept and slope of the linear plot of log qe versus log Ce and given in Figure 12.

To analyse the adsorption data, Langmuir and Freundlich isotherms (Figure 11 and Figure 12) were fitted, and the parameters were obtained as follows (

Table 7. Langmuir isotherm.

Nano-Clay	Heavy Metal	Langmuir Parameters
		q (mg/g)	KF (L/mg)	r2	RL
MB	Mn	5.36	8.65	0.99	1.41 × 10−3
	Fe	232.56	564.6	0.95	2 × 10−6
HT	Mn	4.67	4.27	0.97	4.7 × 10−3
	Fe	120.48	34.7	0.98	3 × 10−5

and

Table 8. Freundlich isotherm.

Nano-Clay	Heavy Metal	Freundlich Parameters
		K (mg/g)	n	r2
MB	Mn	1.4	2.54	0.89
	Fe	1.55	1.49	0.94
HT	Mn	0.67	6.5	0.85
	Fe	45.55	6.1	0.84

):

From the analysis on the table, the correlation co-efficient (r²) values indicate if the isotherm is the best fit to the experimental data. It was found that Langmuir isotherm had r² values ranging from of 0.97 and 0.99 and Freundlich ranged from 0.84 to 0.94. Therefore, the Langmuir isotherm was the best fit in both adsorbents. A similar trend was observed by Masindi et al., 2015 [17] on the adsorption of Fe (II) and Mn (II) using raw bentonite; the adsorption isotherms indicated that removal of metals fitted the Langmuir adsorption isotherm for Fe (II). Since the Langmuir model was the best fit in HT and MB for both ions, this means the adsorption occurs on a homogeneous surface by monolayer coverage, with uniform binding sites and no interactions between adsorbed species. This means that the whole surface of HT has identical adsorption activity and therefore the adsorbed manganese or iron ions do not interact, and they are adsorbed by forming almost complete monolayer coverage of particles, as was also observed by ref. [24].

This work proposed the use of hydrotalcite nanoparticles and surfactant-modified nano clays as potential adsorbents for the removal of manganese and iron from acid-mine drainage. Similarly, this work demonstrated that:

• Nano-clays can adsorb metals from AMD owing to their intrinsic physicochemical characteristics that can trap contaminants.
• This study will go a long way in providing an efficient AMD treatment technology with a special focus on the removal of heavy metals.

4. Conclusions

From the characterisation results by FTIR, it was found that HT and MB contain functional groups that are generally responsible for surface complexation mechanism adsorption. Additionally, HT contains a CO₃²⁻ FTIR peak which showed a slight decrease in intensity after the adsorption of Fe (II) and Mn (II) may be attributed to precipitation. Both MB and HT contain Mg²⁺, as analysed by EDS, which could be responsible for isomorphic substitution with Fe²⁺ and Mn²⁺ and it was shown from XRD results that an isomorphic mechanism may have occurred. Through the analysis of the BET, HT had a higher surface area than MB, which may contribute to the higher adsorption capacity in HT than in MB. This finding is in accordance with the theory by ref. [38], which specifies that the adsorption mechanisms of heavy metal ions involve precipitation, surface complexation, isomorphic substitution, and chelation. Based on the characterisation results, it can be concluded that both nano-clays investigated may be used to remove heavy metals from acid mine drainage.

The experimental results clearly showed that the adsorbent performance was highly dependent on the initial solution concentration, with percentage removals decreasing with increase in initial solution concentration over the studied concentration ranges. The effect of pH on percentage removal was confirmed with higher pH resulting in enhanced metal removal. Further, it was noted that HT had a higher pH buffering effect that raised the solution pH more than MB. This observation is good because it eliminates the need to adjust pH after treatment. The increase in percentage removal with the pH also confirms that indeed the hydroxyl and silanol functional groups are responsible for adsorption.

The batch equilibrium data for both HT and MB were best described by the Langmuir model, and it was observed that both adsorbents had more affinity for iron than manganese. This observation confirms that the adsorption of heavy metals onto nano-clay adsorbents is by chemisorption through the sharing or exchange of electrons between sorbent and sorbate.

The adsorption capacity of hydrotalcite and surfactant modified clay was well investigated in this study and it can be confirmed that it can be used to treat acid mine drainage laden with heavy metals. After the adsorption process, an adsorbent carries heavy metal and disposes it into the environment, generating secondary pollution from chemicals and the adsorbent itself. Disposal of the adsorbent with heavy metals results in the contamination of soil, affecting the environment and human beings; therefore, it is necessary to remove heavy metals before disposing the adsorbent waste. Adsorbents may need to be assessed further by reusing them in at least three successive adsorption–desorption cycles. This will go a long way in enhancing the technology’s economic feasibility.