A simple and efficient alternative treatment for different drinking water Open Access

Most conventional urban drinking water treatments using surface freshwater sources adopt a long treatment train principally comprising grille screens, coagulation, sedimentation, filtration, and chlorine disinfection (Edzwald 2011; Crittenden John et al. 2012), and pre- and/or post-treatment steps need to be considered into the conventional treatment train such as the addition of activated carbon adsorption (Katsigiannis et al. 2015), chemical oxidation (Ikehata et al. 2008), ion exchange (Jiang et al. 2015) to cope with deficiencies of poorly removing many persistent or new pollutants. However, the sequential and multiple treatment steps increase complexity of treatment design and operation and typically require a large space to accommodate these different reactors. Besides, all the added treatment steps designed for specific purposes further increase the system complexity and treatment costs. Another challenge for traditional water treatment technologies is disinfection byproducts (DBPs) (Sedlak & von Gunten 2011), because the disinfectant (e.g. chlorine and ozone) tends to react with certain water matrix constituents (e.g. natural organic matter (NOM) and bromide) to produce more DBPs and bromate in finished water (Sedlak & von Gunten 2011; Wu et al. 2019). Therefore, the development of alternative drinking water treatment approaches need to be urgently developed as a high priority, which can address both traditional and emerging contaminants, produce less DBPs, require a small space, and simplify the system design and operation.

In recent years, Ferrate (Fe(VI)), as a multi–functional agent with efficient oxidation and coagulation, has been applied with minimal operation and has thus provided a new candidate for drinking water treatment (Jiang & Lloyd 2002; Jiang 2014). It was reported that Fe(VI) can damage cell membranes (Ramseier et al. 2011; Zhou et al. 2014) and likely destroy bacteriophage genome (Hu et al. 2012). The presence of total coliform and Escherichia coli in drinking water has been frequently associated with disease outbreaks which are widely used for assessment of the effectiveness of water disinfection in scientific investigations (Li et al. 2017; Zhang et al. 2020). Better Fe(VI) disinfection efficiencies are achieved with a greater Fe(VI) dosage, which results in greater exposure to the oxidant. Previous studies have shown that Fe(VI) treatments are effective for drinking water in organic pollutants degradation, sludge stabilization (Ye et al. 2012), waste plastics separation (Wang et al. 2020), and disinfection (Yang & Ying 2013; Liu et al. 2020). Meanwhile, as the end product of Fe(VI) decomposition, Fe(III) solids are non–toxic, making Fe(VI) an ‘environmentally friendly’ water treatment with potential for subsequent water treatment processes such as adsorption and coagulation (Lee et al. 2004).

Although the application of Fe(VI) has been extensively studied for various water remediation scenarios (Zheng et al. 2020), significant opportunities and challenges coexist towards the practical applications. The two major mechanisms in drinking water treatment with ferrate(VI) are coagulation and chemical oxidation for addressing particulate matters, humic substances (Graham et al. 2010; Lv et al. 2018), algal cells (Deng et al. 2017) and organic pollutants (Aiken et al. 2011). Dissolved organic matter (DOM) consists of a complex, heterogeneous continuum of low– to high– molecular weight species, exhibiting a range of water solubility and reactivity, which plays a critical role in the biogeochemical cycling of trace metals and the mobility of colloidal particles in aquatic environments (Aiken et al. 2011). The ferrate(VI)-driven oxidation proceeds via one or two-electron transfer mechanisms (Sharma 2010), which has proven effective for destruction of natural organic matter (NOM) (Jiang et al. 2016; Song et al. 2016). Most studies conducted to date have only considered removal of one target contaminant with a single controllable water environment (Lim & Kim 2009), whereas real contaminated water bodies may contain a complex matrix of coliform, turbidity, DOC and solid particles. There is relatively little literature concerning the effects of different water types. The effect of these factors on the stability and performance of Fe-based treatment approaches for real water environmental needs to be better investigated.

As the main sources of drinking water are reservoirs and rivers in this work, different surface waters of the reservoir (slower flow and poorer water quality) and river water (faster flow and better water quality) were studied. The objectives of this study include: (1) evaluation of Fe(VI) and Fe(III) effect on the behaviors of turbidity, DOC and disinfection for natural water treatment; (2) conditions determination that benefit Fe(VI) and Fe(III) oxidation, coagulation, adsorption and disinfection; (3) examination of the relationship between Fe(VI) decay and NOM decomposition. This work highlights the significance of the application of different ferrites for different natural water remediation.

All the reagents used were at least analytical grade, except where noted. Potassium ferrate (K₂FeO₄), ferric chloride hexahydrate (FeCl₃·6H₂O), and 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All the other chemicals were purchased from Fisher Scientific (Fair Lawn, NJ, USA).

Water was sampled from the Passaic River which is influent of the Little Falls Water Treatment Plant in Totowa, New Jersey and the reservoir in Short Hills, New Jersey. Once collected, the samples were delivered to the Montclair State University's water treatment laboratory and stored at 4 °C in a cool room until use. Basic water quality parameters for the reservoir and river water are, respectively, initial turbidity of 11.44 and 5.61 NTU (nephelometric turbidity units), UV₂₅₄ of 0.142 and 0.116 cm⁻¹, DOC of 5.66 and 4.03 mg L⁻¹ and SUVA of 2.51 and 2.88 L mg⁻¹ m⁻¹.

All Fe(VI) and Fe(III) tests were performed in 1 L beakers with 400 mL raw water on a six–paddle programmable jar tester (Phipps & Bird –7790–950). If needed, the initial water pH was adjusted with 0.1 or 1.0 N NaOH or H₂SO₄ solution. On introduction of K₂FeO₄ and/or FeCl₃·6H₂O the reaction was initiated. For the first minute, rapid mixing (150 rpm) was applied to mix the added Fe. In the following 120 min, slow mixing (30 rpm) was used to ensure that the solution was continuously agitated. pH was monitored (Thermo Scientific Orion 5–Star Plus) and where appropriate residual Fe(VI) was periodically measured, and to determine the Fe(VI) dosage in aqueous solutions the ABTS method (Lee et al. 2005) was applied. Supernatant was collected for analysis after settling for 30 min. All experiments were carried out in triplicate.

A portion of the treated supernatant (150 mL) was filtered through 0.45 μm membrane filters, and was then isolated into hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions using the absorbent resins Supelite DAX–8 and Amberlite XAD–4 (Carroll et al. 2000; Song et al. 2016). From the same sample batch a further 150 mL was sequentially fractionated in terms of molecular weight (MW) using a stirred cell (Millipore, Model 8200) with 100, 10, and 1 kDa UF membranes. Prior to use, the UF membrane filters were used to filter 2 L Milli–Q water (18.2 M U cm⁻¹). New filters were used in each separation to ensure that no filter cakes were formed on the membrane restricting sample filtration.

As shown in Figure 1, while Fe dosage of 3.0 mg L⁻¹ and pH 8.0, pure Fe(III) treatment (0:3) was significantly more effective at turbidity and DOC removal than pure Fe(VI) treatment (3:0). Overall, at pH 8.0, the removals of turbidity, DOC and total coliform were better when the mass ratio of Fe(VI): Fe(III) was 1:2 regardless of the water source (Figure 1). In natural water, Fe(VI) addition at pH 8.0 produces significantly more nanoparticles than Fe(III), these particles have a negative surface charge, resulting in a stable colloidal suspension (Goodwill et al. 2015), further resulting in poorer turbidity removal on addition Fe(VI) than Fe(III). At alkaline pH, the impact of charge neutralization decreases and the coagulation of Fe(VI) is associated with the formation of sweep-flocs (Lv et al. 2018), but the total coliform was removed in the presence of Fe (VI).

As Fe(III) is much less expensive than Fe(VI), so it is advantageous to optimize the impact of Fe(III) flocculation to reduce the need for Fe(VI) addition for disinfection. In the two treatment mechanisms of Fe(VI) (i.e. chemical oxidation and coagulation), chemical oxidation plays the dominant role in the disinfection performance. The mechanisms of treatment by Fe(III), such as coagulation and adsorption, make the dominant contribution to the removal of turbidity and DOC. The application of the optimal ratio can maximize the advantages of both Fe(VI) and Fe(III), and enables the maximum purification effectiveness at minimum dosage and cost.

After union treatment of Fe(VI) and Fe(III), the removal of turbidity both in the reservoir and river waters was achieved when pH was increased from 4.0 to 9.0. The residual turbidity increased from 1.69 to 2.41 NTU and then decreased to 1.97 NTU for the reservoir water and increased from 1.01 to 2.73 NTU for the river water (Figure 2), on the one hand, due to the presence of colloidal particles resulting from the formation of fine iron (hydroxyl) oxide particles for Fe(VI) (Goodwill et al. 2015). Under acidic conditions, the oxidation–reduction potential of Fe(VI) is greater than most commonly used water treatment oxidants (Sharma 2002). Meanwhile, for Fe(III), the higher the pH of aqueous solution, the better the electric neutralization effect and the better the coagulation sedimentation, so the turbidity removal effect is better. Therefore, when they are used together, the turbidity will rise first and then decrease.

Regardless of the raw water source, Fe(VI) and Fe(III) removal of DOC became less effective with increasing of pH, which indicates that oxidation of organic matter is maximum in acid conditions (pH 4.0) while it is minimal under alkaline conditions. These results are consistent with the pH–dependent reduction potential of Fe(VI) which decreases with increasing pH (Graham et al. 2010), and the effect of electric neutralization of Fe(III) is minimal under acidic conditions while maximum in alkaline conditions, but Fe(III) is favorable for adsorption of DOC under acidic condition.

A greater pH favors the disinfection performance on Fe(VI) addition despite having reduced reactivity (pH 8.0) due to the increased stability and longer lifetime of Fe(VI) at alkaline pH as compared to acidic pH, thereby achieving greater exposure to Fe(VI). Hence, although Fe(VI) is highly reactive at low pH, low exposure leads to poor disinfection efficiency (Zhang et al. 2020).

In summary, both oxidation and neutralization by Fe(VI) were better under acidic conditions than alkaline conditions. Zeta potential analysis suggested that the primary role of Fe(III) was the neutralization of flocculants and precipitation of pollutants. Fe(VI) addition resulted in oxidation but with reduced neutralization.

It has been observed that at any specific Fe(VI) dosage, k_obs is greater at initial pH 5.8 than at pH 7.8 (Song et al. 2016). It has also been demonstrated that HFeO₄⁻ (pH 3.5–7.2) is more reactive but more unstable than FeO₄²⁻ (pH > 7.2) due to reactions between Fe(VI) species (Lee et al. 2005). Fe(VI) decomposition in the presence of NOM exhibits a three-stage kinetic pattern, consisting of an initial Fe(VI) loss, a 2nd order homogenous Fe(VI) self-decay and a 1st order surface-catalyzed Fe(VI) decomposition in the absence of NOM (Deng et al. 2018). Therefore, it is not surprising to observe the difference in the Fe(VI) degradation rates at fixed initial pH 8.0 being similar for the two water types. Fe(VI) self–decay is accompanied by the release of OH⁻ and O₂ gas, thereby leading to pH increase (Lee et al. 2004).

A decrease was seen in dissolved organic carbon and in the hydrophobic and hydrophilic fractions of NOM when the interaction of Fe(VI) and NOMs was independently examined at pH 7.0 (Gan et al. 2015). However, overall DOC and SUVA reduction was very limited for Fe(VI) treatment at pH 8.0 for any of the three NOM fractions in Figure 5. Fe(VI) treatment at pH 4.0 was more reactive with the NOM. It was reported that ferrate oxidation increases the hydrophilic and electronegative nature of the humic acid (HA) leading to an extended region of charge neutralisation (Graham et al. 2010), organic macromolecules within water have been found to be cleaved into smaller (HPO < TPI < HPI), more hydrophilic fractions due to oxidation by ferrate (Chen et al. 2019). Fe(III) treatments, at both pH 4.0 and 8.0, tended to be intermediate in their effectiveness between the Fe(VI) treatments (Fe(VI)-4 < Fe(III)-4 < Fe(III)-8 < Fe(VI)-8). Consequently, these (NOM fractions of HPI) were more difficult to remove by coagulation, requiring a greater dosage of Fe species (Graham et al. 2010). In the process of oxidation and degradation, Fe(VI) is reduced to Fe(III) or Fe(II) through one or two electron transfers (Sharma 2008; Jain et al. 2009). The specific reaction path depends on the characteristics of the pollutant.

The DOC and SUVA distributions for the two waters across the MW fractions is very different. From analysis of hydrophobic, transphilic and hydrophilic components and different molecular weight fractions, it is found that for reservoir water, oxidation due to the addition of Fe(VI) is greatly influenced by pH and there is a shift in the distribution of the natural organic matter toward smaller (<1 K), while for river water there is a shift in the distribution of the natural organic matter toward smaller (1–10 K), more oxygenated compounds due to the oxidation process. The smaller compounds typically have a greater negative charge, thus a more reactive coagulant surface (such as ferric chloride and alum) is needed to obtain the same total organic carbon removal (Graham et al. 2010). Therefore, under acidic conditions, it was conducive to the removal of smaller NOM (MW < 10 K) of DOC, and it played the efficient role of coagulation and oxidation with the union use of Fe (VI) and Fe (IIII).

The standard redox potentials of Fe(VI) under acidic and alkaline conditions are 2.20 and 0.72 V, respectively. As a powerful oxidant, Fe(VI) is stable in dry environments but extremely unstable in acidic aqueous solutions. Fe(VI) is quickly reduced by water to Fe(III), releasing O₂. With increasing pH, the stability of Fe(VI) gradually increases, and decomposition is slower in the pH range of 11.5–13.5 than at lower pH (Sharma 2008). With increasing of pH, the concentration of HFeO₄⁻ and H₂FeO₄ (strongly oxidizing) gradually decrease, and the concentration of FeO₄²⁻ gradually increases, leading to reduced oxidation potential. It is worth noting that Fe(III) introduction in the one-step Fe(III) salt addition is dissimilar to Fe(III) formation during Fe(VI) addition. One-step ferric salt addition directly inputs the entire Fe(III) dosage at the onset of treatment. In contrast, Fe(III) formation is due to Fe(VI) reduction and Fe(III) is released in-situ.

In conclusion, under acidic conditions, the union use of Fe (VI) and Fe (III) played a leading role in the removal of DOC and SUVA and the removal of HPO, TPI of DOC and smaller NOM (MW < 10 K) of DOC. Meanwhile coagulation and oxidation benefited. However, it was better for disinfection in the alkaline conditions than that in the acidic conditions from the single factor experiment.

This study demonstrated that the union use of Fe(VI) and Fe(III) treatment has their own advantages. Fe(III) treatment was better than Fe(VI) treatment for the removal of turbidity, natural organic matter (NOM) and specific ultraviolet absorbance (SUVA). However, Fe(VI) has the unique advantage of inactivation of pathogens with oxidation, flocculation and disinfection being accomplished with little production of disinfection by-products. The overall treatment of removal of coliform, turbidity and DOC was better with the mass ratio of Fe(VI): Fe(III) of 1:2 at pH 8.0 regardless of reservoir or river water. The addition of Fe(III) enable lower Fe(VI) dosage and enhance coagulation effect, therefore lower overall treatment expenses.

It was concluded that Fe(VI) used in combination with Fe(III), at optimal ratio, has the potential to serve as a simple and effective emergency technology for drinking water treatment designs, integrating oxidation, flocculation and disinfection, without the need for separate sterilization and disinfection processes, depending on raw water quality and treatment goals. In addition, it can be considered to make it into a solid tablet for easy storage and carrying as an emergency treatment such as wilderness survival or work, or in extreme weather or environment conditions such as heavy rainstorm or pandemic virus causing a safe drinking water shortage, using for deep purification and disinfection in families and individuals, to obtain temporary, relatively safe drinking water.

This project was supported by the National Natural Science Foundation of China (No. 51808202 and 51909082), Innovation and entrepreneurship training program for college students (No. S202110500060).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.