# Performance of composite mineral adsorbents for removing Cu, Cd, and Pb ions from polluted water

## Introduction

Various researchers9,10,11,12,13 have characterized the chemical, surface, and ion-exchange properties of clinoptilolite. Zamzow et al.11 studied the inorganic cation exchange capacity of clinoptilolite and the selectivity series as follows: Pb2+ > Cd2+ > Cs+ > Cu2+ > Co2+ > Cr3+ > Zn2+ > Ni2+ > Hg2+. Jama and Yücel10 recognized a very high preference of clinoptilolite for ammonium ions over sodium and calcium ions but not over potassium ions. Mier et al.12 identified the interactions of Pb2+, Cd2+, and Cr3+ competing for ion-exchange sites in natural clinoptilolite. Regarding the ion exchange of Pb2+, Cu2+, Fe3+, and Cr3+ on natural clinoptilolite, Inglezakis et al.13 found that equilibrium is favourable for Pb2+, unfavourable for Cu2+, and of sigmoidal shape for Cr3+ and Fe3+. Since commercial clinoptilolite is relatively costly, mixtures of zeolite and other less expensive organic and inorganic materials, such as cement, clays, and polymers, have been formulated for specific pollutants14.

Recently, functionalized adsorbents such as nanocomposite materials have been prepared for harmful heavy metal ions and organic compound adsorption and have been used for diverse applications15,16,17,18,19,20,21,22,23. Moreover, natural mineral-modified absorbents have the advantage that they can be applied not only to the aqueous phase but also to the soil phase24,25,26,27,28,29. This study aimed to reveal the efficiency of the removal of heavy metals by applying modified natural minerals as adsorbents to contaminated groundwater. For this purpose, we evaluated the cation adsorption performance and adsorption equilibrium characteristics of the composite mineral adsorbents CMA1 (zeolite with clinoptilolite of over 20 weight percent and feldspar of ~10 percent, with Portland cement) and CMA2 (zeolite with feldspar of over 15 weight percent and ~9 percent of clinoptilolite, with Portland cement) through adsorption isotherms and kinetics. Specifically, we looked at how CMA1 and CMA2 performed in the removal of Cu, Cd, and Pb ions from polluted water.

## Materials and Methods

### Preparation and characterization of the adsorbents

The preparation procedure of the adsorbents CMA1 and CMA2 is illustrated in Fig. 1. All experiments were conducted in a 10-L polyethylene reactor equipped with a stirrer. To prepare the adsorbents CMA1 and CMA2, clinoptilolite-rich zeolite, slightly weathered feldspars showing a porous structure under a microscope, and normal Portland cement were prepared. Porous feldspar was prepared from weathered feldspar porphyry that was pulverized into 44-µm particles after being heated at 480 °C for 20 min. For the CMA1 adsorbent, a mixture of clinoptilolite-rich zeolite (C) and Portland cement (PC) at a ratio of C:PC = 70:30 wt% was cured for 28 days after adding water and lightweight foam. Finally, the CMA1 adsorbent powder was made by crushing the cured specimen. For the CMA2 adsorbent, a mixture of clinoptilolite-rich zeolite (C), porous feldspar (PF), and Portland cement (PC) at a ratio of C:PF:PC = 40:30:30 wt% was cured for 28 days after adding water and lightweight foam. Eventually, the CMA2 adsorbent powder was produced by crushing the cured specimen.

Crystallization and chemical characterization were performed by X-ray diffraction (XRD, Philips X’Pert-MPD System). XRD patterns of the samples were scanned on a powder diffractometer with Cu Kα radiation (λ = 1.54 Å) at a diffraction angle of 2θ in the range of 5–50° in 0.02° steps (3 s per step). The crystal morphologies of the samples were analysed by using scanning electron microscopy (SEM, Hitachi S-4200), with an accelerating voltage of 15 kV and a magnification of 20,000 times. The samples were coated with a thin layer of platinum and mounted on a copper slab using double-sided tape for the SEM analysis.

### Methodology

Samples were taken at regular intervals and centrifuged (Vision Scientific VS-5000i2, Korea) for 3 min at 3,000 rpm. After centrifugation, the supernatant was filtered, and the Cu, Cd, and Pb ion concentrations were analysed by using an atomic absorption spectrophotometer (Perkin Elmer AAnalyst 100, Germany).

## Theory

The pseudo-first-order (PFO) rate equation for the adsorption kinetics of solutes from a liquid solution that was proposed by Lagergren30 is:

$$\frac{dq}{dt}={k}_{1}({q}_{e}-q)$$
(1)

where q and qe are the amounts of solute adsorbed (mg) per adsorbent (g) at any time and at equilibrium, respectively, and k1 is the PFO rate constant of adsorption. Integrating Eq. (1) for the boundary conditions t = 0 to t and q = 0 to q gives:

$${\rm{l}}{\rm{n}}\,\frac{({q}_{e}-q)}{{q}_{e}}=-{k}_{1}t$$
(2)

The pseudo-second-order (PSO) rate equation for the adsorption kinetics of solutes from a liquid solution proposed by Ho and McKay31 is:

$$\frac{dq}{dt}={k}_{2}{({q}_{e}-q)}^{2}$$
(3)

The integration of Eq. (3) for the boundary conditions t = 0 to t and q = 0 to q gives:

$$\frac{1}{{q}_{e}-q}=\frac{1}{{q}_{e}}+{k}_{2}t$$
(4)

where k2 is the PSO rate constant of adsorption. A linear equation can then be obtained from Eq. (4):

$$\frac{t}{q}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{1}{{q}_{e}}t$$
(5)

The plot of t/q versus t gives a straight line with a slope of 1/qe and an intercept of 1/k2qe2, and then qe and k2 can be evaluated from the slope and intercept, respectively.

According to the formula by Vanderborght and Van Griekenm32, Q, the solute adsorbed (mg) per adsorbent (g), is

$$Q=\frac{V({C}_{i}-{C}_{e})}{W}$$
(6)

$${\rm{ \% }}{\rm{S}}{\rm{o}}{\rm{r}}{\rm{p}}{\rm{t}}{\rm{i}}{\rm{o}}{\rm{n}}=\frac{{C}_{i}-{C}_{e}}{{C}_{i}}\times 100$$
(7)

According to Langmuir, the amount adsorbed, Qe (mg/g), is defined as:

$${Q}_{e}=\frac{{Q}_{m}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
(8)

where Ce is the equilibrium concentration of the adsorbate (mg/L), Qm is the Langmuir constant related to the maximum monolayer adsorption capacity (mg/g), and KL is the Langmuir isotherm coefficient related to the affinity of the sorbate for the binding sites. Equation (8) can be re-arranged into a linear form:

$$\frac{1}{{Q}_{e}}=\frac{1}{{Q}_{m}}+\frac{1}{{Q}_{m}{K}_{L}{C}_{e}}$$
(9)

Using the Freundlich equation, the amount absorbed, Qe (mg/g), is:

$${Q}_{e}={K}_{F}{C}_{e}^{\frac{1}{n}}$$
(10)

Thus, linearizing Eq. (10),

$${\rm{l}}{\rm{n}}\,{Q}_{e}=\,{\rm{l}}{\rm{n}}\,{K}_{F}+\frac{1}{n}\,{\rm{l}}{\rm{n}}\,{C}_{e}$$
(11)

where KF is the Freundlich isotherm coefficient or an approximate indicator of the adsorption capacity (mg/g), 1/n is a function of the strength of the adsorption in the adsorption process, and n is the adsorption intensity35. A value of n = 1 indicates that the partition between the two phases is independent of the concentration, n > 1 indicates normal adsorption, and n < 1 indicates cooperative adsorption36. A value of 1 < n < 10 demonstrates a favourable sorption process37. The constant change of KF and n with an increase in temperature reflects the empirical observation that the quantity adsorbed rises more slowly, such that higher pressures are required to saturate the surface. For the determination of KF and n by data fitting, linear regression is generally used to determine the parameters of the kinetic and isotherm models38. The linear least-squares method and linearly transformed equations have been widely applied to correlate sorption data, where the smaller the 1/n (or heterogeneity parameter), the greater the expected heterogeneity.

## Results

The composite mineral adsorbents, CMA1 and CMA2, are composed of clinoptilolite, feldspars, and Portland cement. XRD analysis showed that CMA1 and CMA2 consisted of albite, calcite, dachiardite, clinoptiloite, and mordenite (Fig. 2). Feldspar is reported to have a heavy metal adsorption capacity39,40,41. Clinoptilolite, with the chemical formula of (Na,K)64Al6Si30O72·nH2O, one of most abundant natural zeolites, is found in sedimentary rocks of volcanic origin and occurs with silicate minerals such as feldspar, quartz, other zeolites (members of the tectosilicates subclass), clays (members of the phyllosilicates subclass), and volcanic glass42. Clinoptilolite, along with heulandite and mordenite, has a high cation capacity43. Its tabular morphology shows an open reticular formation with easy access, formed by open channels of 8- to 10-membered rings. Feldspars, anhydrous aluminosilicates composed of potassium, sodium, and calcium, are structured by silicon and aluminium occupying the centres of the tetrahedrals of SiO4 and AlO4. These tetrahedrals are linked to other tetrahedrals at each corner, forming a 3-D, negatively charged framework. Potassium, sodium, and calcium within the voids of the structure can be exchanged with other cations. Portland cement is a very common solidification and stabilization material and is used as a supplement to clinoptilolite for adsorption purposes44. The SEM images of CMA1 and CMA2 presented amorphous porous particles, which are consistent with the aggregated forms of albite, calcite, dachiardite, clinoptiloite, and mordenite (Fig. 3). Overall, dachiardite, clinoptiloite, and mordenite can be considered major minerals involved in adsorption for heavy metal control45,46,47,48.

### Effect of initial pH on Cu, Cd, and Pb ions

The adsorption of heavy metals is significantly influenced by the initial pH of the solution since the initial pH determines the surface charge of the adsorbent and the degree of speciation and ionization of the adsorbate49,50,51,52,53. The effects of the initial pH value were evaluated by the Cu, Cd, and Pb adsorption capacities in the solutions (Fig. 4). The adsorption capacities of Cu in the solutions were determined to be 71–107.5, 126–144.5, and 395.5–479.5 mg/g at initial pH values of 2.78, 4.94, and 6.50, respectively. The adsorption capacities of Cd in the solutions were 261–279, 302–311, and 198–222 mg/g at initial pH values of 2.80, 4.76, and 6.45, respectively. Additionally, the adsorption capacities of Pb in the solutions were determined to be 453.1–495.4, 570.2–594.6, and 613.8–646.5 mg/g at initial pH values of 2.83, 4.98, and 7.01, respectively. At pH < 5.0, Cu2+, Cd2+, and Pb2+ are the primary species in the Cu, Cd, and Pb solutions, respectively; the species vary with solution pH, and the adsorption of Cu, Cd, and Pb ions mainly involves divalent metal ions54,55,56. Based on the effect of the initial pH on Cu, Cd, and Pb ions, each of adsorption kinetic experiments was conducted at a pH value less than 5.0.

To determine the equilibrium reaction time, the adsorption of Cu, Cd, and Pb ions on the adsorbents CMA1 and CMA2 were plotted as a function of reaction time in Fig. 5. At the same initial concentration and reaction time, the CMA2 demonstrated better adsorption than the CMA1 (Table 2). The Cu, Cd, and Pb ions adsorption on the CMA1 and CMA2 had almost reached equilibrium within 180 min.

PFO and PSO models were applied to the results of the adsorption kinetic experiments for Cu, Cd, and Pb ions on the adsorbents CMA1 and CMA2. The PFO and PSO models demonstrated different determination coefficient (r2) values, between 0.7690–0.9515 and 0.9255–0.9977, respectively, indicating that the latter was more suitable than the former (Fig. 5). The adsorption capacities obtained from the experiments did not agree with the values estimated to reproduce the Cu, Cd, and Pb ions adsorption kinetics with the adsorbents CMA1 and CMA2 from the PFO model. However, the experimental results were similar to the values calculated from the PSO model, and the r2 values were also very close to unity. Therefore, Cu, Cd, and Pb ions adsorption by the adsorbents CMA1 and CMA2 could be more accurately explained by the PSO model than by the PFO model. Additionally, the adsorbent CMA2 was more effective in removing the heavy metals ions of Pb, Cd, and Cu than the CMA1, in decreasing order of their qe cal values.

The adsorption isotherms for Cu, Cd, and Pb ions on the adsorbents CMA1 and CMA2 are plotted in Fig. 6. The isotherm parameters, Qe, KL, n, KF, and r2 obtained from the Langmuir and Freundlich isotherms are given in Table 3. The results of the adsorption isotherm experiment for Cu, Cd, and Pb ions on the adsorbents CMA1 and CMA2 were interpreted by using the Langmuir and Freundlich models. Table 3 and Fig. 6 show that the Langmuir model gave a slightly higher correlation coefficient (r2) than the Freundlich model, indicating that the two models could both be applied to the heavy metal solutions on a spherical monolayer surface with a weak heterogeneity of the surface. The adsorption capacities of Cu, Cd, and Pb ions by the Langmuir model were 145.84–154.71 mg/g, 162.58–177.99 mg/g, and 802.23–932.08 mg/g, respectively, while the adsorption capacities of Cu and Cd ions by Erdem et al.2 and Ok et al.3 were 9.0–23.3 mg/g using clinoptilolite (natural zeolite) and ZeoAds (a mixture of zeolite and Portland cement).

The maximum adsorption capacities (Qm) of Cu, Cd, and Pb ions by using the Langmuir model were determined to be 802.23 mg/g (CMA1) and 932.08 mg/g (CMA2) for Pb, 177.99 mg/g (CMA1) and 162.58 mg/g (CMA2) for Cd, and 145.84 mg/g (CMA1) and 154.71 mg/g (CMA2) for Cu, after dosing the adsorbents with only 1 g/L of the heavy metal solution; and hence, these values are significantly higher than those of the existing adsorbents as presented in Table 1. By comparison, an absorption experiment using a highly porous composite material with immobilization of an organic ligand onto silica monoliths57 that was performed to efficiently remove Pb ions in wastewater resulted in a Qm of 204.34 mg/g, and an absorption experiment using a mesoporous composite material synthesized by the immobilization of an organic ligand onto mesoporous silica for effectively removing Cu ions in aqueous solution obtained a Qm of 197.15 mg/g58.

## Discussion and Conclusions

In this study, the adsorption performance and adsorption equilibrium characteristics of the heavy metal ions were evaluated through adsorption isotherm and adsorption kinetic experiments. We applied the composite adsorbents CMA1 (zeolite with clinoptilolite of over 20 weight percent and feldspar of ~10 percent, with Portland cement) and CMA2 (zeolite with feldspar of over 15 weight percent and ~9 percent clinoptilolite, with Portland cement) to heavy metal (Cu, Cd, and Pb ions) solutions.

The adsorption kinetic experiments results showed that the adsorption of the heavy metal ions almost reached within 180 min. PFO and PSO models for Cu, Cd, and Pb ions on the adsorbents CMA1 and CMA2 resulted in different r2 values of 0.7690–0.9515 and 0.9255–0.9977, respectively, indicating that the PSO model was more suitable than the PFO model. Furthermore, the adsorbent CMA2 was more effective in removing the heavy metal ions of Cu, Cd, and Pb than the CMA1, in decreasing order of their qe cal values.

The adsorption isotherm experiments showed that both the Langmuir model and the Freundlich model were applicable to the heavy metal solutions because the adsorbents had spherical monolayer surfaces with a weak heterogeneity of the surface. The maximum adsorption capacities of Cu, Cd, and Pb ions from the Langmuir model resulted were 802.23 mg/g (CMA1) and 932.08 mg/g (CMA2) for Pb, 177.99 mg/g (CMA1) and 162.58 mg/g (CMA2) for Cd, and 145.84 mg/g (CMA1) and 154.71 mg/g (CMA2) for Cu, dosing the adsorbents at only 1 g/L of the heavy metal solution. These maximum adsorption capacities were significantly higher than the maximum adsorption capacities of Cu, Cd, and Pb ions using other adsorbents, as presented in Table 1.

Furthermore, the adsorbent CMA2, including cost-effective natural feldspar, displayed better adsorption capacity than the CMA1. The results of this study can be applied to effectively remove heavy metal ions in contaminated water and wastewater since both absorbents (CMA1 and CMA2) showed excellent removal efficiency of heavy metal ions in solution. Future research will be focused on revealing the mechanism of the different performances on the heavy metal (Cu, Cd, and Pb) ions by the adsorbents CMA1 and CMA2, which is probably related to the feldspar content.

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## Acknowledgements

This research was funded by National Research Foundation of Korea (NRF) under the Ministry of Science and ICT (NRF- 2017R1A2B2009033) as well as by the Ministry of Oceans and Fisheries, South Korea as a part of the project titled ‘Development on technology for offshore waste final disposal’.

## Author information

S.Y.H., W.R.L. and C.H.L. wrote the manuscript; W.R.L., C.H.L. and H.J.P. conducted the experiment using the composite mineral adsorbents; S.W.K., E.K.C., M.H.O. and S.N.S. developed the composite mineral adsorbents.

Correspondence to Se-Yeong Hamm.

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