Keywords

1 Introduction

Zeta potential measurement plays a crucial role in determining the stability of colloidal suspensions. The stability of dispersions relies on the magnitude of zeta potential, influencing particle aggregation. These studies are closely linked to the coagulation-flocculation process, wherein the isoelectric point (IEP) becomes a critical indicator of colloidal system stability [1]. Effective coagulation processes hinge on an understanding of IEP, where the potential energy barrier opposing coagulation disappears. Previous studies have indicated the suitability of Moringa oleifera seed/gum coagulant and composites for removing turbidity, fluoride, and heavy metals from water and wastewater [2,3,4,5,6,7,8].

Despite these individual studies, there exists a noticeable research gap in comprehensively understanding the zeta potential characteristics of Moringa oleifera-derived coagulants, their interactions with other coagulants, and their practical applications in heavy metal removal. This gap highlights the necessity for a holistic investigation that integrates these key elements to address the complexities of water purification.

This research significantly contributes to bridging the existing gap in the literature. The study not only explores the zeta potential characteristics of Moringa oleifera seed, gum powder, bentonite clay, and clay-polymer composites but also delves into their interactions within the colloidal system. Furthermore, the systematic examination of the application of these coagulants in heavy metal removal enhances our understanding. This holistic approach provides valuable insights that can inform more effective and sustainable water purification practices.

2 Methodologies

The Zeta potential analyzer, Malvern Zetasizer Ver. 7.01, analyzed Zeta potentials of coagulants at various pH conditions for heavy metal removal. pH significantly influences Zeta potential, with a positive curve at low pH and a lower or negative curve at high pH. The point where the curve crosses zero is the isoelectric point, crucial for stability. Zeta potential measurements, conducted on Moringa oleifera seed (MOS) cake coagulant, Moringa oleifera gum (MOG) powder coagulant, and bentonite clay (BC) and its composite Moringa oleifera seed composite (MOSC), Moringa oleifera gum composite (MOGC) in water at room temperature, help optimize coagulant dosage in water treatment.

Electrophoresis, measuring electrophoretic mobility (EM) using Henry’s Equation, determined Zeta potential for each coagulant. Electrophoresis involved injecting 25 ml of aqueous dispersions into the Zeta potential instrument’s cell at room temperature. The electrophoresis cell, designed for microscopy, comprises two electrode chambers connected by an optically polished tube. Zeta potential was determined by directly measuring electrophoretic mobility using Henry’s Equation [9].

$$ \mu = \frac{{2\varepsilon_{0} \varepsilon_{r} \zeta f\left( {\kappa r} \right)}}{3\eta } $$
(1)

where, μ = electrophoretic mobility, η = viscosity of medium, □ = permittivity of a vacuum, Æ’(κr) = Henry’s function, ε0 = permittivity of a vacuum, εr = medium dielectric constant (or permittivity), ζ = zeta potential, κ = Debye-HĂ¼ckle parameter and r = hydrodynamic radius of particle.

Previous studies in 2020, Ravikumar and Udayakumar investigate a green clay-polymer nanocomposite for the removal of heavy metals [7] and in 2021, Ravikumar and Udayakumar introduce Moringa oleifera gum composite as a novel material for heavy metals removal [8]. This research delves into the preparation and characterization of the nanocomposite, showcasing its potential in addressing heavy metal pollution. Figures 1 and 2 show the isoelectric pH and apparent pH of MOS, MOG and BC.

Fig. 1.
figure 1

Apparent zeta potential values of Moringa oleifera seed and gum

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Fig. 2.
figure 2

Apparent zeta potential of bentonite

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The modified coagulants, MOSC and MOGC, demonstrated varying zeta potential values at pH 5.5. The clay-polymer composite media exhibited enhanced heavy metal removal efficiency compared to natural clay, emphasizing the significance of the composite in coagulo-adsorption processes.

The measurement of the zeta potential is also a method that provides us with insight into the character of the particle surface itself and the processes occurring on this surface (e.g., adsorption, ion exchange, modification).

Figure 3 show that the apparent zeta potential of MOSC (3.78 mV) was less positive than that of MOGC (−1.21 mV) at a pH of 5.5.

Fig. 3.
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Apparent zeta potential of MOSC and MOGC

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Table 1 shows the isoelectric pH and apparent pH of BC, MOS and MOG, MOSC, and MOGC. The isoelectric point indicates that at the experimental pH > pHpzc, heavy metal species are attracted to the surface sites of the coagulant, which are negative. To avoid the possible precipitation of metals, pH studies were not performed at pH > 8. The comprehensive influences of all functional groups determine pHpzc (point of zero charge) of a coagulant and the pH at which the charge on the adsorbent surface is zero.

Table 1. Isoelectric pH and apparent pH of MOSC and MOGC
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3 Result and Discussion

The pH influenced the adsorption process for metal ions. This observation was attributed to the surface charge of MOSC/MOGC, which could be modified by changing the pH of the solution.

It could be mentioned that both MOSC and MOGC particles have a higher adsorption affinity to adsorb metal cations (cadmium and lead) at high pH values. As the pH increases and the balance between H3O+ and OH− becomes equal, more of the positively charged metal ions in the solution are adsorbed on the negative nano clay-polymer composite surface and thus, the removal percentage of the metal cations (Cd (II) and Pb (II)) increases.

When the pH decreased toward acidic conditions, the zeta potentials of MOSC and MOGC decreased and converted to positive values caused by the protonation of the carboxylic and hydroxide ions on the surface. The results of zeta potential measurements indicated that the isoelectric point (pHzpc) of the MOSC and MOGC nanocomposite was 5.7 and 5.3 and the surface charge of the composite was positive at pH < 5. This positively charged surface at pH < 5 favours the retention of anionic contaminants. With the increase of pH, the zeta potential value decreased, which will in turn reduce the Cr (VI) removal efficiency.

Fig. 4.
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Influence of pH in the removal process of cadmium, chromium and lead by MOSC coagulo-adsorbent

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Figures 4 and 5 show the influence of pH in the heavy metal’s removal efficiency by coagulation/flocculation with 5 g/l of MOSC (at 40 rpm of rapid mixing and 15 rpm of slow mixing) and MOGC (at 45 rpm of rapid mixing and 20 rpm of slow mixing) composite coagulo-adsorbent with an initial heavy metal concentration of 6 mg/l.

The clay-polymer coagulo-adsorption studies with MOSC confirmed that optimum condition for metal ions removal were pH 6–8 for cadmium, pH 2–4 for chromium and pH 5–7 for lead. At very low pH values metal uptake has been found very less in the case of cadmium and lead. But chromium has more removal efficiency at very low pH values.

The adsorption studies with MOGC confirmed that optimum condition for metal ions removal were pH 7–8 for cadmium, pH 1–4 for chromium and pH 6–7 for lead.

Fig. 5.
figure 5

Influence of pH in the removal process of cadmium, chromium and lead by MOGC coagulo-adsorbent

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MOSC and MOGC have a tendency to chelate with metal ions like Cd, Cr, and Pb, etc. The functional groups present in the clay-polymer chain are strongly active with metal ions, as follows:

$$ {\text{M}}^{{{\text{n}} + }} + {\text{ RNH}}_{{2}} \to {\text{M}}\left( {{\text{RNH}}_{{2}} } \right)^{{{\text{n}} + }} $$
(2)

The amino group forms coordinate bonds with the metal ions by donating free electrons present on nitrogen and oxygen in the amino groups and hydroxyl groups, respectively, to the vacant orbitals of the metals. The metal binding efficiency of the clay-polymer depends on the availability of the amino groups for interaction with metal ions, chain length, and the extent of inter/intra-molecular hydrogen bonding, etc.

Solution pH determines the level of electrostatic or molecular interaction between the adsorption surface and adsorbate owing to charge distribution on the materials. The zero point charge or isoelectric point (pHzpc) of the MOS was about 6.2 and then shifted to 5.7 after being modified by bentonite clay. Similarly, in the case of MOG, it shifted from 1.5 to 5.3. The pHzpc of MOSC and MOGC was obtained at pH = 5.7 and 5.3. At pH below 5.3, more amino groups in the MOSC and MOGC were protonated (i.e., from −NH2 to −NH3 +). From pH 5.7 to 10, the negative zeta potential of MOSC and MOGC, the amino group in the composite media was not deprotonated under this pH condition (i.e., from −NH2 to −NH-). Moreover, the adsorption capacity of composite media highly depended on the positive charge at pH < 5.3.

Clay-polymer composites were found to be much more effective than natural clay for the removal of heavy metals. If a bare particle may have a high zeta potential value, a polymer is adsorbed, giving an increase in the adsorbed layer thickness; the zeta potential would be reduced to some extent. The adsorbed clay particles with Moringa oleifera shift the net surface charge of clay from slightly positive to negative in the case of MOSC; the point of zero charge of clay shifted from 6.2 to 5.7. The adsorbed clay particles with Moringa oleifera shift the net surface charge of clay from negative to positive from 1.5 and 5.3 in the case of MOGC. The heavy metal ion adsorption on composite media was found to be affected by the ionic attraction between the protonated surface groups on Moringa oleifera and the heavy metal ions. But, the heavy metal ions adsorption on natural clay is governed by the positively charged clay particle edges formed by broken bonds of Al–O and Si–O.

Moringa oleifera seed/gum composite presents interesting adsorption capacities because it contains deprotonated hydroxyl groups and is able to give electrostatic interaction with heavy metals, which mainly include complexing with non-binding doublets on nitrogen and oxygen atoms. The intercalation part of Moringa chains in the clay minerals and the porous network of the composite ensured a good transport of the heavy metal solution to the remaining reactive sites of the bentonite.

Fig. 6.
figure 6

TEM image in 50nm scale (a) MOSC and MOGC before heavy metal removal (b) Cd loaded MOSC and MOGC (c) Cr loaded MOSC and MOGC (d) Pb loaded MOSC and MOGC

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TEM images of MOSC/ MOGC clay-polymer metal complexes indicate the formation of nano structure. TEM images of MOSC/MOGC before and after coagulo-adsorption (Cd, Cr and Pb) was undertaken in order to locate the active adsorptive sites of the composite coagulants to form its metal complexes. In each figure (a) is pure MOSC/MOGC, (b) MOSC/MOGC-Cd complex, (c) MOSC/MOGC-Cr complex and (d) is MOSC/MOGC-Pb complex. TEM images with 50 nm magnification the adsorbed heavy metal ions assume more clarity in their shapes in the form of dark dots as shown in Fig. 6.

Another study [10] contributes to this research by investigating a composite of kaolinite clay and moringa seedcake, which effectively removes methylene blue and acid orange-7 dyes. This investigation includes a comprehensive examination involving batch and column tests, chemical modifications, and characterization techniques. The isoelectric pH values of kaolinite clay, Moringa seedcake, and their composite are presented in Table 2.

Table 2. Isoelectric pH of kaolinite clay, Moringa seedcake, and its composite
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The study achieved optimum removal rates of 86% and 94% at pH 2 and 10, respectively, for acid orange-7 (AO-7) and methylene blue (MB). These results were obtained using a 1 g/L adsorbent dose and 50 mg/L initial dye concentration. This study underscores the potential of the moringa-based clay-polymer composite for dye removal.

4 Conclusion

In conclusion, the zeta potential analysis revealed distinctive characteristics of Moringa oleifera seed (MOS) and gum (MOG) coagulants at pH 6.5, with MOS exhibiting a zeta potential of −0.516 mV and MOG showing −38.4 mV. Notably, MOS demonstrated superior coagulant activity at pH 7.0, making it more effective for cadmium removal within the pH range of 7–8. Bentonite, with an apparent zeta potential of −15.5 mV at pH 6.5, demonstrated a pH-sensitive zeta potential, underscoring its role in system stability. The study revealed that heavy metal ion adsorption on the clay surface altered bentonite’s zeta potential, highlighting its significance in the removal process.

Furthermore, the modified coagulants, MOSC and MOGC, exhibited varying zeta potential values at pH 5.5. The clay-polymer composite media, particularly MOSC/MOGC, demonstrated enhanced heavy metal removal efficiency compared to natural clay, emphasizing the composite’s pivotal role in adsorption processes. Transmission electron microscopy (TEM) images of MOSC/MOGC composite coagulant confirmed the mono-dispersed and spherical nature of nanoparticles, highlighting their uniform distribution and absence of agglomeration. The visualization of adsorbed heavy metal ions in the clay-polymer nanocomposites at 50 nm elucidated their shape and size.

In essence, this comprehensive analysis of zeta potential characteristics in the context of heavy metal removal using Moringa oleifera-based coagulants provides valuable insights into the underlying mechanisms. The findings not only advance our understanding of coagulant behavior but also carry practical implications for the development of effective and sustainable water purification methods. Exploring the scalability and long-term efficacy of these coagulants in real-world scenarios would be instrumental in realizing their potential for widespread application in water treatment processes.