Natural diatomite as an effective adsorbent for heavy metals in water and wastewater treatment (a batch study)

Peer review under responsibility of National Water Research Center.

Abstract

This study presents an evaluation of Egyptian diatomite as a low cost adsorbent for the removal of heavy metal under different conditions (pH, weight of diatomite and contact time). The adsorption of heavy metals was investigated under various pH values ranged from 2 to 8 at 25 °C. The obtained results indicate that at low pH (2–4), the removal efficiency of diatomite for heavy metal increased slightly as the pH, adsorbent dose and contact time increased, while at pH > 4, the percentage of metal ions adsorbed decreased with increasing pH due to precipitation of heavy metals. At pH equal 4, with using 2 g L⁻¹ of diatomite and 75 min as contact time, the maximum adsorption capacity of diatomite was obtained. The high adsorption capacity of diatomite makes it a suitable low cost material for the removal of different heavy metals from aqueous solutions.

Keywords:

• Diatomite clay
• Heavy metals treatment
• Adsorption
• Removal efficiency

1 Introduction

The removal of heavy metals ions from the environment is a big concern due to their association with health hazards for all living organisms due to its toxicity and non-biodegradable nature (El-Bayaa et al., 2009; Xue et al., 2009). The municipal and industrial solid wastes are another important source of pollution, which are considered a main source of the heavy metals like Hg, Cd, Fe, Mn, Pb and the treatment of these wastes must be agreed with the environmental standard before it is reused or returned to surface water (Ali and El-Sayed, 2017). There are many techniques have been developed such as chemical precipitation, reverse osmosis, ion exchange, and adsorption to remove heavy metals and other hazard materials from wastewaters (Shi et al., 2009). Among all of the previous techniques, the adsorption on clay and other chemical substances is considered to be a particularly more effective process specially by using low cost and environment friendly martial for the removal of heavy metals from industrial wastewaters and aqueous solutions (Lin and Juang, 2002; Swayampakula et al., 2009).

Table 4 Removal efficiency of different heavy metals concentrations using diatomite clay (1 g L⁻¹).

No.	Element	Removal efficiency (%) at different initial concentrations and contact times
	Time and conc.	1 h			2 h			3 h			4 h
		0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)
1	Aluminum (Al^+3)	79.2	76.0	80.0	85.1	94.2	92.2	83.1	90.3	90.0	89.1	90.0	90.0
2	Barium (Ba^+2)	86.0	90.1	911	99.2	95.2	90.5	99.5	90.1	90.0	99.0	90.0	90.0
3	Cadmium (Cd^+2)	99.0	97.2	96.9	98.9	99.0	99.0	99.0	98.2	97.4	89.1	95.0	94.8
4	Chromium (Cr^+3)	98.1	89.1	94.0	99.1	99.0	98.2	99.1	96.5	98.0	98.3	97.0	97.0
5	Copper (Cu^+2)	98.2	94.1	96.4	99.1	99.0	98.2	99.0	98.10	98.0	98.5	98.0	96.2
6	Iron (Fe^+2)	81.3	90.1	92.1	86.8	91.1	90.3	79.0	90.0	90.0	78.6	89.3	89.1
7	Lead (Pb^+2)	99.2	95.0	95.5	99.3	99.1	98.2	99.0	98.2	98.0	99.0	98.0	98.0
8	Manganese (Mn^+2)	99.2	98.2	99.1	99.3	99.1	99.2	99.1	98.0	97.6	98.6	97.0	97.0
9	Nickel (Ni^+2)	99.2	93.2	92.5	99.3	98.0	98.0	99.0	98.0	98.0	99.0	96.0	96.0
10	Zinc (Zn^+2)	98.2	94.7	94.3	99.4	99.0	98.2	99.1	93.0	90.1	98.4	90.0	90.0

The most common sorption methods are using of natural clay minerals and activated carbon which has high metal adsorption capacity (Ayala et al., 2008). Although activated carbon is effective in the removal of metal ions from wastewater, it is expensive and requires chelating agents to enhance its performance, thus increasing treatment cost (Oliveira et al., 2005). Some researchers have focused on using low cost, highly efficient sorbents for pollutants, and the sorption behavior of several natural materials and chemical products has also been studied (Ijagbemi et al., 2009). Some of these materials are clay minerals (Solener et al., 2008), agricultural bi-products, some aquatic plants, and microorganisms (Ijagbemi et al., 2009). Most of these investigations have shown that natural materials can act as good sorbents for hazard materials including heavy metals (Ali and El-Sayed, 2017), while the types and availability of clay minerals also are important, because they are involved in choosing which pollutants will be treated. Egypt has huge reserves of diatomite deposits in Gebel Elow El Masakheet at south west El-Fayoum Governorate. These deposits vary in the quality grade from high, moderate to low (Ibrahim and Selim, 2011). The diatomite deposits of the El-Fayoum area are fine and regular in lamination developed through seasonal fluctuation of sedimentation. The deposits are overlain in some parts by detrital materials such as blown sands and semi-consolidated sandy clay. Because diatomaceous deposits occur in the form of several disconnected outcrops extending over about 30 km from Qasr Elsagha to Kom Oshim in the east, some authors concluded that they were formed in separate lakes.

Diatomite is a siliceous sedimentary rock composed of an amorphous form of silica (SiO₂·nH₂O) containing a small amount of microcrystalline material. It has a unique combination of physical and chemical properties such as high porosity, high permeability, small particle size, large surface area, and low thermal conductivity. Due to these properties the diatomite has been successfully used for cleaning drinking water or industrial and sewage waste water. Sometimes it is blended with aluminum chloride or ferric chloride to enhance the filtration processes (Marquez et al., 2004; Qdais and Moussa, 2004).

The removal of hazardous contaminants from wastewater using natural zeolite was performed by Ali et al. (2017). Their results indicate that, the uptake of clinoptilolite for ammonium was ranged from 70 to 92%, while it ranged from 70 to 99% for heavy metals at various conditions. The removal efficiency of zinc from aqueous solution with different adsorbents was investigated by Bhattacharya et al. (2006), the obtained results showed that the adsorption percentage of zinc increased by increasing the concentration of the adsorbents to reach maximum uptake at 98% and pH between 5 and 7.

This study aims to investigate and evaluate the removal capacities of diatomite for some metal ions such as aluminum (Al⁺³), barium (Ba⁺²), cadmium (Cd⁺²), chromium (Cr⁺³), copper (Cu⁺²), iron (Fe⁺²), lead (Pd⁺²), manganese (Mn⁺²), nickel (Ni⁺²), and zinc (Zn⁺²). The adsorption experiments were performed in laboratory, using metal ions initial concentration, pH of solution, and contact time as variables.

2 Materials and methods

2.1 Identification of clay

Diatomite sample was obtained by the Egyptian Mineral Resources Authority from Kom Oshim in the east of El-Fayoum Governorate (Fig. 1). The diatomite sample was crushed using roller mills and the small sizes were selected to be prepared for the batch study, while the large size was used for physical and chemical identification. The characterizations of diatomite were performed using X-ray (model X Pert PRO). All the previous analysis were done including electroscaning microscope (ESM) at the Laboratories of The Egyptian Mineral Resources Authority.

Fig. 1 The diatomite deposits in ElFayoum Depression (Kom Oshim), Egypt.

2.2 Diatomite preparation for the study

A representative sample of diatomite was prepared for the batch experiment by washing with distilled water to remove any non-adhesive impurities and then, dried in oven at 80 °C for 24 h to remove any moisture. The diatomite sample was grinded to 0.25–0.5 mm size to be used for the batch study (Reddy and Dastgheibi, 2014).

2.3 Heavy metals preparation

Metals stock standard of multi element (Al, Ba, Cd, Cr, Cu, Fe, Pb, Mn, Ni and Zn, 1000 mg L⁻¹ Merck Brand, product ID: 1.11355.0100-100 mL), has been used to prepare various concentrations of heavy metals (0.5, 1, and l0 mg L⁻¹) with deionized water (APHA, 2012). The desired pH varying from 2 to 8 were obtained by adding negligible drops of sodium hydroxide (NaOH) or hydrochloric acid (HCl) to the solution.

2.4 Batch study

The experiments were performed in duplicate, while the blank and other concentrations of heavy metals (1.0, and 10 mg L⁻¹) were measured as the quality control samples.

The equilibrium state for removing of heavy metals by diatomite clay was obtained by adding the constant weight of absorbent (0.5, 1.0 and 2.0 g) to 100 mL of initial concentrations of prepared aqueous solution of heavy metals (0.5, 1, and 10 mg L⁻¹), with constant agitation shaking at100 rpm and 25 °C to enhance the interaction of the diatomite particles and heavy metals solution (Lasheen et al., 2017). It was observed that the equilibrium between absorbed heavy metals ions and adsorbent was after 2 h (the heavy metals concentrations were measured for all samples after every 15 min until reach to equilibrium state at which the heavy metals uptake is constant with insignificant changes). However, to ensure that the absorption process reached to complete equilibrium the experiment was kept for 4 h. After that the samples were filtered using filter paper and the concentration of different heavy metals ions were measured by ICP-MS (model NexIon 300D), and the removal was calculated using Eq. (1(1) $\text{Removal efficiency}(\%)=\frac{\left(C_0-C_e\right)}{C_0}\times 100$(1) ): (1) $\text{Removal efficiency}(\%)=\frac{\left(C_0-C_e\right)}{C_0}\times 100$(1)

where C₀ is the initial concentration of heavy metals ions (mg L⁻¹) and C_e is the equilibrium concentration of the metal ions (mg L⁻¹).

3 Results and discussion

3.1 Characterizations of Egyptian diatomite

The chemical composition of diatomite plays an important role in metals removal. In this study, the physical properties of the Egyptian diatomite is presented in Table 1, while the X-ray diffraction analysis indicates that diatomite contains calcite, quartz and clay minerals as shown in Fig. 2 and Table 2. Fig. 3 presents the electroscaning microscope (ESM) for the studied diatomite particles.

Fig. 2 XRD pattern of diatomite (C = calcite and Q = quartarz).

Fig. 3 Diatomite under electron microscope (ESM).

Table 1 The physicochemical properties of Kom Oshim diatomite at El Fayoum Governorate.

Parameters	Value
Moisture content, %	81.5
Specific gravity, g mL^−1	1.94
pH	8.34

Table 2 The chemical characteristics of the natural sorbent.

Ref. code	Mineral name	Chemical formula
96-900-1298	Calcite	Ca5.62 Mg0.38 C6.00 O18.00
96-901-3322	Quartz	Si3.00 O6.00
96-100-8758	Anorhtitesodian	Na1.92 Ca2.08 Si10.00 Al6.00 O32.00

The presence of calcite gives an advantage to diatomite as adsorbent because, it is widely used as sorbent material by formation of metal hydroxide or metal carbonate as reported by Al-Wakeel (2009), and also quartz is one of the most common minerals found in the earth's crust. Its structure shows unique physicochemical characteristics therefore it can be used as sorbent substance (Ibrahim and Selim, 2011). In this study, it was found that the formation of metal hydroxide and silicone dioxide due to presence of calcite and quartz respectively leads to increase the metals uptake properties of diatomite.

3.2 Effect of pH

The adsorption capacity of heavy metals on the adsorbents surface is controlled by the pH of aqueous solution. Therefore, the effect of various pH values at 25 °C was investigated in the current study. The batch experiment was conducted in acidic conditions (pH = 2) to ensure adsorption between diatomite particles and pollutants, due to in alkaline media heavy metals start to precipitate (forming metal oxides and hydroxides), and also in higher pH conditions the diatomite particles is slightly unstable as a results of presence of silica that can be precipitate or dissolve in alkaline condition. So the batch experiments were conducted at pH equal to 2, to keep and preserves the chemical stability of the diatomite. The pH of the solution influence the chemical groups on to the adsorbent surfaces, where the following equations describe the process (Sud et al., 2008): (2) ${\text{M}}^{2+}+n{\text{H}}_2\text{O}\to \text{M}{({\text{H}}_2\text{O})}_n^{2+}$(2) (3) $\text{M}{({\text{H}}_2\text{O})}_n^{2+}\to \text{M}{({\text{H}}_2\text{O})}^{n-1}+{\text{H}}^{+}$(3) (4) $n{\text{M}}^{2+}+m{\text{H}}_2\text{O}\to \text{M}{\left(\text{OH}\right)}_m^{\left(2n-m\right)}+m{\text{H}}^{+}$(4)

In the current study, the removal of metal ions as a function of pH is illustrated in Figs. 4–13 and Tables 3–5. The figures displayed a general trend of increasing heavy metals removal with increasing pH values until maximized at pH = 4. The results indicate that the slow metal uptake shown by diatomite at low pH values (pH = 2), which can be explained by increasing the positive charge (protons) density on the surface sites and thus, electrostatic repulsion occurred between the heavy metal ions (M²⁺) and the edge groups with positive charge (Si–OH²⁺) on the surface of diatomite, which is agreed with by Johnson and Hallberg (2005), while when pH of the solution varies from 2.0 to 4.0, the adsorption efficiency is increased, because the heavy metals solution is exposed to the negative charges on the surface of diatomite and strongly attracting the metal ions. The highest adsorption percentage for heavy metal ions was observed at pH equal to 4. According to the obtained data, diatomite is the efficient clay with the selectivity sequence given as Pb > Mn > Ni > Zn > Cr > Fe > Cu > Cd > Ba > Al, due to the correlation between the ionic radii of heavy metals with external and internal pore diameters of diatomite. The obtained results were agreed with the study performed by Yusan et al. (2012) for using diatomite as adsorbent for heavy metals.

Fig. 4 Effect of pH for the removal of Al using 2 g of adsorbent.

Fig. 5 Effect of pH for the removal of Ba using 2 g of adsorbent.

Fig. 6 Effect of pH for the removal of Cd using 2 g of adsorbent.

Fig. 7 Effect of pH for the removal of Cr using 2 g of adsorbent.

Fig. 8 Effect of pH for the removal of Cu using 2 g of adsorbent.

Fig. 9 Effect of pH for the removal of Fe using 2 g of adsorbent.

Fig. 10 Effect of pH for the removal of Pb using 2 g of adsorbent.

Fig. 11 Effect of pH for the removal of Mn using 2 g of adsorbent.

Fig. 12 Effect of pH for the removal of Ni using 2 g of adsorbent.

Fig. 13 Effect of pH for the removal of Zn using 2 g of adsorbent.

Table 3 Removal efficiency of different heavy metals concentrations using diatomite clay (2 g L⁻¹).

No.	Element	Removal efficiency (%) at different initial concentrations and contact times
	Time and conc.	1 h			2 h			3 h			4 h
		0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)
1	Aluminum (Al^+3)	80.6	78.8	80.2	95.8	95.4	95.1	91.5	90.8	90.2	91.0	90.1	90.2
2	Barium (Ba^+2)	99.1	90.9	92.8	99.5	97.2	98.5	99.5	90.3	92.2	99.2	90.1	91.7
3	Cadmium (Cd^+2)	99.1	98.9	96.1	99.5	99.3	99.1	99.0	98.7	98.0	99.0	97.2	95.1
4	Chromium (Cr^+3)	98.0	95.3	95.2	99.7	99.5	99.2	99.23	97.3	98.3	99.0	97.1	97.3
5	Copper (Cu^+2)	98.5	95.3	98.1	99.5	99.2	99.0	99.1	98.2	98.5	99.0	96.2	98.1
6	Iron (Fe^+2)	96.3	93.4	93.2	99.0	95.3	97.8	97.5	95.1	95.4	96.0	94.4	95.0
7	Lead (Pb^+2)	99.7	95.5	99.2	99.7	99.5	99.3	99.0	98.4	98.2	99.0	98.2	98.0
8	Manganese (Mn^+2)	99.5	99.0	99.0	99.5	99.5	99.5	99.3	98.5	98.0	99.2	97.3	97.0
9	Nickel (Ni^+2)	99.4	94.1	93.2	99.5	98.2	98.0	99.3	98.0	97.6	99.0	96.2	96.0
10	Zinc (Zn^+2)	99.0	95.0	95.0	99.7	99.0	99.0	99.2	90.4	90.2	99.0	90.1	90.0

Table 5 Removal efficiency of different heavy metals concentrations using diatomite clay (0.5 g L⁻¹).

No.	Element	Removal efficiency (%) at different initial concentrations and contact times
	Time and conc.	1 h			2 h			3 h			4 h
		0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)	0.5 (mg L^−1)	1.0 (mg L^−1)	10.0 (mg L^−1)
1	Aluminum (Al^+3)	55.2	78.2	74.3	78.1	94.0	92.0	78.4	90.0	88.2	80.0	82.0	86.2
2	Barium (Ba^+2)	78.1	90.0	89.1	99.1	95.1	90.0	99.2	90.0	84.1	95.0	89.2	84.0
3	Cadmium (Cd^+2)	93.9	96.1	95.9	98.2	99.0	98.9	98.6	96.1	96.5	99.9	94.3	92.2
4	Chromium (Cr^+3)	98.0	93.4	88.0	89.8	89.4	98.0	99.0	96.0	97.0	98.9	96.2	96.4
5	Copper (Cu^+2)	93.6	93.6	94.1	97.4	99.0	98.0	97.0	97.2	97.3	94.9	97.0	96.0
6	Iron (Fe^+2)	72.0	89.4	89.2	82.1	90.5	89.1	90.0	89.3	89.8	72.5	89.0	87.3
7	Lead (Pb^+2)	99.1	93.1	94.1	99.0	98.5	98.0	99.0	97.1	97.0	99.0	96.0	97.0
8	Manganese (Mn^+2)	99.0	98.0	98.0	99.1	99.0	99.0	99.0	98.1	97.2	98.0	96.8	96.0
9	Nickel (Ni^+2)	99.0	90.0	92.1	99.0	97.0	97.0	99.0	97.0	97.0	99.0	96.0	96.0
10	Zinc (Zn^+2)	98.2	94.1	94.0	99.0	99.0	98.0	99.0	92.2	90.0	98.2	87.3	90.0

3.3 Effect of adsorbent dosage

Adsorbent dosage is an important parameter because of its strong effect on the capacity of the diatomite at a given initial concentration of the pollutants. The effect of various doses of diatomite (0.5, 1.0 and 2 g L⁻¹) on heavy metals removal was studied in the current study at 25 °C and the results are listed in Tables 3–5. The obtained data regarding removal efficiency of heavy metals is increased from 55.2% to 99.7% by increasing the amount of the adsorbent between 0.5 and 2 g L⁻¹, this is because at low diatomite content, all surface sites are exposed for metal ions and the surface reaches saturation faster, which means that the adsorption of metals had the highest values using 2 g L⁻¹ of the diatomite. Regarding the data, the removal efficiency was recorded 99.7% for Cr, Pb and Zn, while the lowest value was recorded for Fe 99% using 2 g L⁻¹ of the adsorbent. Therefore, the amount of 2 g L⁻¹ was selected as the optimum dosage of the used diatomite under study. Bilgin and Tulun (2015) studied the removal of lead ions from wastewater using diatomite. The maximum obtained removal efficiency was 98.09% by using 1.5 g L⁻¹ diatomite for 20 min, which is close to the results of the current study.

3.4 Effect of contact time

The contact time is important factor that influence the adsorption of heavy metal ions onto diatomite. During the current study, the batch experiment investigate the effect of contact time on interaction between diatomite particles and metal ions at 25 °C. The results are presented in Tables 3–5. The obtained results indicate that the removal percentage started rapid and then became slower which can be explained by, at the beginning of the batch experiment, the large surface area of diatomite is ready to interact with heavy metals (the adsorption capacity was high because all sites on the adsorbent were vacant and heavy metals concentration was high) however with time the uptake rate was slowed due to reduction in sorption sites. As soon as the surface of adsorption sites became filled with metal ions, the uptake rate slightly decreased and controlled by the ability of metal ions can moved from outside to inside sites of the diatomite particles (Yu et al., 2000).

In this study, removal efficiency for Ba, Cd, Pb, Mn, Ni and Zn indicates that it remained almost unchanged during the experiment when the time increased from 15 to 70 min and recorded (99%–99.7%). The results revealed that the optimum contact time for heavy metals removal is at 75 min, after that the adsorption percentage of metal ions gradually decreased with increase in contact time. The obtained results are agreed with that found by Riza et al. (2010); Sheng et al. (2009) and Varank et al. (2014) which investigate the removal of heavy metal ions from aqueous solutions by natural adsorbant.

4 Conclusions

Natural diatomite collected from El-Fayoum area has been investigated for the removal of different metal ions from aqueous solutions. The batch study was preformed at various conditions such as pH, adsorbent dose and contact time. The maximum percent removal of many heavy metal ions were observed at pH 4 and significantly decreased at higher pH values, while metal ions removal is positively influenced by an increase in the adsorbent dosage and contact time.

5 Recommendation

Diatomite rocks are already found in governorate of El-Fayoum, South West of Cairo with large amount, while the present study give an evidence that using of diatomite as low cost adsorbents for hazard material uptake from wastewater is efficient, so it is recommended to use diatomite particles on large scale as a single unit in a treatment plant for industrial wastewater, and then the treated water must be compared with the international guidelines to check its ability to be reused.

Acknowledgements

This research was performed through using of ICP-MS (model NexIon 300D) funded by Science & Technology Development Fund (STDF), project code 5186/2013.

Special appreciation and thanks to Dr. Mohamed Ramadan (The Egyptian Mineral Resources Authority) who bring the diatomite from El-Fayoum Governorate and helped me well to prepare it for this study. I would especially like to thank Prof. Dr. Maha Ali (Deputy Director and Operation Manager of CLEQM) you have been a tremendous mentor for me.

Notes

Peer review under responsibility of National Water Research Center.