1 Introduction

Coagulation and flocculation are predominant methods used for removing colloidal material during drinking water treatment. Particles removal depends on particles surface, stability and the coagulant type [1]. Flocculation is the process of destabilizing suspended particle systems and bridging the aggregated flocs to form larger agglomerates that settle down under gravity [2]. The coagulation process creates agglomerates or flocs, which can be bridged or linked together by flocculants to form bigger agglomerates that precipitate faster under gravity [3].

Many parameters affect the quality of drinking water. Turbidity is considered the most important parameter for measuring the quality of water. Turbidity is caused by suspended particles, inorganic matter and microscopic organisms. Coagulation/flocculation process is considered the primary process in drinking water treatment steps. Settling and filtration are steps that come after coagulation process [4, 5].

There are many coagulants that are widely used in the coagulation process such as aluminum and iron salts [6]. Mixing speed and water pH, the efficiency of the coagulation process depends on the nature and the dosage of coagulants. Hybrid flocculants are considered another type of flocculant that depends on inorganic salts and organic flocculant as combination [7, 8]. There are many environmental consequences that result from using of these chemicals particularly aluminum such as Alzheimer disease and carcinogenic properties [9].

Nanoremediation has been very important in recent years due to their high adsorption capacity and their large surface area [10]. The unique properties of nanomaterials in water decontamination enable nanomaterials not just to play an important role in water treatment but also allow for a more cost-effective clean-up process [11,12,13,14].

Hematite (α-Fe2O3) colloids have the tendency to form complexes with natural organic matter (NOM) which are precursor compounds that form by-products like hazardous trihalomethanes (THMs) [15]. Thiruvenkatachari et al. reported the application of hematite (α-Fe2O3) colloids as a flocculent in cross-flow microfiltration (CFMF) which is a very useful process for removing colloids and suspended solids in water [15, 16]. Application of iron oxide nanoparticles-based nanomaterials for removal of heavy metals is well-known adsorbents for remediation of water [17]. Free chlorine can react with numerous organic materials such as amino acids, humus, environmental pollutants in raw surface water to yield halogenated organics [18].

In the present study, we report the application of hematite nanoparticles on their own as efficient coagulant in surface water treatment showing the highest turbidity removal as high as 93.8%.

2 Materials and methods

2.1 Description of drinking water treatment plant

Drinking water in Egypt is overwhelmingly provided by the River Nile. In the present study, water samples were collected from River Nile Rosetta Branch at Basyoun City at the uptake site of Basyoun’s Drinking Water Company. The uptake site is located 5 m from canal bank, and samples were collected in the period 3–15 of August.

Sample

Values Prior to treatment

Temperature (°C)

24.1 ± 0.05

Initial turbidity (NTU)

12.19 ± 0.095

Final turbidity (NTU)

9 ± 0.005

Total dissolved solids (mg L−1)

208 ± 0.5

Conduct. (μS cm−1)

354 ± 1

(PO4)−3 (mg L−1)

0.10 ± 0.005

Alkalinity (mg L−1)

134 ± 1

Ammonia (mg L−1)

0.27 ± 0.005

Nitrate (mg L−1)

0.27 ± 0.005

Nitrite (mg L−1)

0.23 ± 0.01

pH

7.55 ± 0.005

Fe2+ (mg L−1)

0.54 ± 0.005

Ca2+ hardness (mg L−1)

81 ± 0.5

Mg2+ hardness (mg L−1)

44 ± 0.5

Total hardness

125 ± 0.5

2.2 Preparation of hematite nanoparticles

Hematite nanoparticles were prepared by using a solution FeCl3 as described before [19]. Briefly, 15 mL from (0.1 M) FeCl3 was added to 100 mL stirred boiling distilled water drop by drop. The solution colour was yellow and then turned to red, and upon excess addition of FeCl3, the final colour of solution changed to dark red. Then this solution was heated to reflux for 30 min. The resulting solution was kept at room temperature to cool [19, 20]. The solution can exist in colloidal state without any of precipitation. Given hematite having trigonal hexagonal crystal structure with cell parameters a = 5.038 Å, c = 13.772 Å; Z = 6 [21], the concentration of as-prepared hematite nanoparticles under the present experimental conditions was 2.98 × 10−6 M.

2.3 Characterization of prepared hematite nanoparticles

The structure of hematite nanoparticles was confirmed by several techniques such as UV–Vis spectra, which was recorded using a CARY Bio 100 spectrophotometer. X-ray diffraction was recorded using a Philips PW 1390 X-ray diffractometer using copper target with nickel filter. FTIR spectra were carried out using FTIR spectrophotometer model 670 (NEXUS) Nicolet in transmittance mode with a resolution of 4 cm−1 and 34 scans min−1 in the 4000–400 cm−1. Transmission electron microscopy (TEM) was applied on transparent samples prepared as suspended solution in distilled water. TEM micrographs were taken with high-resolution transmission electron microscope (JEOL JEM-2100) at an accelerating voltage of 80 kV (Tanta university, Egypt).

2.4 Coagulation flocculation experiments (jar tests)

Fill the beakers with test water. After placing beakers on their position, the paddles should be in their position identically. As soon as the different dosages flocculent was added to beakers as shown in Table 1, mix the solution at 150 rpm for 2 min and then mix slowly the solution at 45 rpm for 30 min. Allow settling to occur for 30 min and then remove the supernatant of the solution from the top by siphoning. Turbidity was analyzed by using turbidity meter (Nephelometers NTU).

Table 1 Effect of hematite nanoparticles dosage on water sample turbidity, alkalinity, pH and total dissolved solids (TDS) according to jar test and settling time of 30 min
Full size table

Originally, five concentrations of flocculants were tested to locate the optimum flocculent dosage. Measurements were then taken in duplicate in two authorized water analysis laboratories; one is Basyoun’s Drinking Water Company Laboratory, and the other is Gharbia Drinking and Waste Water Central Laboratory. The methods used to evaluate TDS, conductivity, (PO4)−3, alkalinity, total hardness, ammonia, nitrate, nitrite and Fe2+ are those adopted by these two professional laboratories according to standard methods. Routine assessment of these contaminants is also carried out on weekly basis.

2.5 Characterization of prepared hematite nanoparticles after agglutination

Scanning electron microscopy (SEM) was applied on samples that prepared as suspended solution in distilled water. It was investigated using JEOL scanning electron microscope, of the model JSM 6510 LV.

3 Results and discussion

3.1 Characterization of iron oxide nanoparticles

UV–Vis spectrum of 9-nm-diameter iron oxide nanoparticles aqueous solutions a characteristic absorption band centered at λmax = 405 nm. The XRD pattern of iron oxide nanoparticles is shown in Fig. 1a. Hematite nanoparticles (α-Fe2O3) showed the hexagonal phase (Fig. 1b) with the peaks around 2θ = 32°, 35°, 40° and 50° [22, 23]. No other phase was detected for prepared hematite nanoparticles, indicating the sample high purity. The FTIR spectrum of as-prepared hematite nanoparticles (Fig. 1c) shows bands at 570 cm−1 and 630 cm−1 due to Fe–O stretching mode of hematite. The band at 468 cm−1 results from lattice mode of FeO6. The band at 3420 cm−1 is due to adsorbed water that also gives rise at 3217 cm−1 and 1640 cm−1 which indicates the hydroxyl stretching mode and the hydroxyl bending mode, respectively [24, 25]. The transmission electron microscopy (TEM) images (Fig. 1d) show a 9-nm-diameter iron oxide nanoparticles.

Fig. 1
figure 1

Structural and spectroscopic data of the as-prepared hematite nanoparticles a UV–Vis spectrum, b XRD patterns, c FTIR spectrum and d transmission electron microscopy (TEM) images for as- prepared hematite nanoparticles

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3.2 Investigation of the optimum coagulant conditions

3.2.1 Effect of hematite nanoparticles dosage

The coagulant dosage is considered the most critical parameters in the coagulation and flocculation processes. Variety of concentration of 0–40 mL of hematite nanoparticles (2.98 × 10−6 M) was examined using jar test at pH = 7.55. According to WHO (World Health Organization) recommendations, turbidity should be less than 5 NTU before water can be adequately cleared. After using different volumes of hematite nanoparticles as coagulant added to 1L water samples, the percentage of turbidity removal ranges from 89.4 ± 0.01 to 93.8 ± 0.005% [26].

The highest turbidity removal of 93.8 ± 0.005% was achieved upon using 30 mL of hematite nanoparticles concentration of 2.98 × 10−6 M per liter of water sample. This is compared to a highest turbidity removal of 95.4% that occurs upon using 2.0 × 10−4 M alum solution as shown in Table 1 and Fig. 2.

Fig. 2
figure 2

a Comparison between turbidity of blank water sample and water samples after coagulation process using alum and hematite coagulants. b Turbidity changes as a function of hematite nanoparticle (2.98 × 10−6 M) dosages according to jar test and 30-min settling time

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3.2.2 Discussions of values of quality tests

The extent of agglutination and subsequent turbidity removal is affected by the volume of coagulants whether hematite nanoparticles or alum. Table 2 shows the turbidity changes as a function of hematite nanoparticle dosages according to jar test following 30-min settling time. The data together with some other parameters such as total dissolved solids (TDS), conductivity, (PO4)3− concentration, alkalinity, ammonia concentration, nitrate concentration, nitrate concentration, pH and Fe2+ concentration are summarized in Tables 2.

Table 2 shows some quality criteria of as-collected and water sample treated by addition of 10 mL of alum solution (0.02 M) to 1 L water sample from Rosetta branch of River Nile in comparison with addition of 30 mL of hematite nanoparticles solution (2.98 × 10−6 M) to 1 L water sample
Full size table

Very slight rise in total dissolved solids concentration, conductivity and alkalinity occurs after using alum and hematite nanoparticles as shown in Fig. 3 and Table 2.

Fig. 3
figure 3

Left: comparison between total dissolved solids, alkalinity, conductivity and total hardness of blank water sample and water samples after coagulation process using alum and hematite coagulants; right: comparison between phosphate concentrations of blank water sample and water samples after coagulation process using alum and hematite coagulants

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Ammonia concentration was measured prior and after addition of 30-mL hematite nanoparticles (2.98 × 10−6 M). The value of ammonia concentration prior to nanoparticle addition was measured as 0.27 mg L−1. The value of ammonia concentration after nanoparticle addition was measured as 0.65 mg L−1. The increased ammonia concentration upon using hematite nanoparticle as coagulant may result from the catalytic action of hematite nanoparticles on nitrite and/or the action of ammonium-oxidizing bacteria resulting in ammonium ion generation [27, 28].

Nitrate concentration was measured prior and after addition of 30-mL hematite nanoparticles (2.98 × 10−6 M). The value of nitrate concentration prior to nanoparticle addition was measured as 0.29 mg L−1. The value of nitrate concentration after nanoparticle addition was measured as 7.45 mg L−1. So, there is an increase in nitrate concentration upon using hematite nanoparticle as coagulant and this may be due to the oxidation of nitrite to nitrate in nanoparticle media.

Phosphate (PO −34 ) ion concentration decreases after treatment either by hematite nanoparticles or by alum as shown in Table 2 and Fig. 3. In case of hematite nanoparticles, the capping Fe3+ ions interact with phosphate ion forming insoluble iron(III) phosphate. This process is of dual purpose; it helps in flocculation by destabilizing the colloidal iron oxide nanoparticles and also reduces the phosphate content in water samples under treatment. Once hematite is released into solution, it rapidly hydrolyzes and precipitates as an oxide. The iron cycle is closely tied to oxygen, carbon, phosphorus, sulfur and heavy metals, as well as biological organisms [29].

3.3 Coagulation mechanism of hematite nanoparticles

A red colloidal system is formed if FeCl3 solution is added slowly to boiling water. This occurs because of a chemical reaction in which the hydrated iron(III) ions lose water and hydrogen ions to form a hydrated oxide: Fe2O3·xH2O. As the particles of Fe2O3 begin to grow, they adsorb Fe3+ ions on their surfaces, which makes them positively charged. Because each of the oxide particles acquires the same electrical charge, they repel each other and the nanoparticle system shows indefinite stability. Hydrated iron(III) oxide particles can be coagulated by electrolytes that are capable of neutralizing the charges on the surfaces of their particles such as phosphate ions [30]. The scanning electron microscopy (SEM) after agglutination is shown in Fig. 4. The agglutination process is associated with the reduction in turbidity.

Fig. 4
figure 4

Scanning electron microscopic (SEM) images for hematite nanoparticles after agglutination

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4 Conclusion

In this study, experiments were conducted on nanoparticle of hematite ability as flocculant for drinking water treatment of raw water. Hematite nanoparticles showed high efficiency in turbidity removal giving percentage turbidity removal as high as 93.8% when used as a coagulant. The efficiency of turbidity removal is highly dependent on hematite nanoparticle dosage. This is compared to a highest turbidity removal by using 2.0 × 10−4 M alum solution. The application of hematite nanoparticles as flocculent/coagulant in surface water treatment avoids the risks of using aluminum or organic derivatives. Aside from turbidity removal upon using hematite nanoparticles in flocculation, other sharp reductions in concentrations of phosphate ion contaminant occur due to reaction with Fe3+ ions capping hematite nanoparticles. Hematite nanoparticles satisfy many crucial requirements including non-toxicity, indefinite stability, low cost and ease of production.