Abstract

To shorten the ripening period of filter sand, iron–manganese co-oxide filter film (MeOx) was formed quickly on the virgin quartz sand surface by oxidizing Mn2+ and Fe2+ from groundwater using KMnO4 continuously. After the start-up period, we found that Mn2+ could be removed efficiently by MeOx, even if the dissolved oxygen (DO) concentration in the influent was only about 1.0–1.5 mg/L. This means that the removal process of Mn2+ does not need to consume DO. The kinetic experiments for Mn2+ indicated that the adsorption and oxidation kinetics followed pseudo-first-order kinetics. The film (MeOx) was characterized by X-ray photoelectron spectroscopy (XPS). All manganese adsorbed on the surface of the sand was the oxidized form, and the manganese oxide coated onto the sand effectively oxidized Mn2+ to Mn3+ or Mn4+. The binding energy of the observed photoelectron peaks of O(1s) showed the existence of [≡Mn-OH] on the surface of the film by XPS, which might be a key intermediate in the mechanism of Mn2+ oxidation. Finally, a chemical catalytic oxidation mechanism for Mn2+ removal was proposed by the analysis of the oxidation process.

INTRODUCTION

Manganese ion (Mn2+) is detected commonly in natural aquatic environments (Madison et al. 2013), and excessive Mn2+ in the water can affect water quality and human health (Bouchard et al. 2010; Kan et al. 2013). Mn2+ can be removed efficiently by biological or chemical methods. Biological methods involving manganese-oxidizing bacteria can oxidize Mn(II) to Mn(IV) naturally (Pacini et al. 2005; Hoyland et al. 2014). Chemical methods for Mn2+ removal generally include simple adsorption (Mondal et al. 2008; Kamel et al. 2013), oxidation (EI Araby et al. 2009; Phatai et al. 2010) and autocatalytic oxidation processes (Guo et al. 2017; Cheng et al. 2018).

Researchers have shown that the effective removal of Mn2+ could be achieved by manganese-oxide-coated media, and the rate of Mn2+ oxidation onto filter sand is lower than the rate of adsorption; hence, the oxidation of Mn2+ is the rate-limiting step in the process (Sahabi et al. 2009; Kenari & Barbeau 2014). Some media were from ‘aged’ biofilter media using natural start-up strategies (Sahabi et al. 2009; Bruins et al. 2015), other media were prepared in laboratory conditions (acid, alkali, potassium permanganate soaking or high temperature) (Tiwari et al. 2007; Kan et al. 2013). However, we found a catalytic material (MeOx) that could achieve similar removal efficiency of Mn2+ by chemical catalytic oxidation in a pilot-scale filter column. MeOx was formed quickly on the surface of virgin quartz sand by oxidizing Mn2+ and Fe2+ from raw groundwater using potassium permanganate (KMnO4) continuously, and the simultaneous removals of ammonium and manganese could be achieved by MeOx (Guo et al. 2017). Thus, the kinetics and mechanism of Mn2+ removal in this particular condition were different from biological oxidation or the laboratory conditions.

When dissolved oxygen (DO) as oxidant has been used to oxidize Mn2+ in aqueous solution, oxyhydroxide (MnOOH) has been the primary product resulting from manganese autoxidation in aqueous solution (Morgan 2005; Elzinga 2011). However, only a few researchers have studied the oxidation kinetics and mechanism of Mn2+ from real groundwater using MeOx.

The aim of this study was to establish the oxidation kinetics and mechanism of Mn2+ from real groundwater by MeOx in a pilot-scale filter column, rather than laboratory conditions. Ripening quartz sand coated with iron–manganese co-oxides was used as the filter media. KMnO4 oxidizing Mn2+ and Fe2+ from raw groundwater on the surface of virgin quartz sand was adopted as an effective method to shorten the start-up period. The adsorption capacity and kinetic parameters of Mn2+ were determined by batch adsorption experiments, and the oxidation kinetics of Mn2+ were studied using different initial influent Mn2+ concentrations in a pilot-scale filter column. Additionally, the oxidation mechanism of Mn2+ was proposed.

MATERIALS AND METHODS

Configuration of the pilot-scale filter system

The pilot-scale filter system is located in a water source well in Xi'an City, China (34°20′ N, 108°47′ E). The influent was derived from the groundwater supply network, and the quality of the water supply network is shown in Table S1 (available with the online version of this paper). The concentrations of manganese and iron are significantly lower than the drinking water quality standard for raw groundwater (Mn2+ 0.1 mg/L and Fe2+ 0.3 mg/L in GB 5749-2006).

By oxidizing Mn2+ and Fe2+ in raw water using KMnO4 continuously, a iron–manganese co-oxide film was quickly formed on the surface of virgin quartz sand in a pilot-scale filter column (Guo et al. 2017). The filter column consisted of a plexiglas tube with an internal diameter of 0.1 m and height of 3.8 m. The filter bed fixed in the tube was 1 m high, and there was a 0.3-m-high supporting layer of cobblestone (particle size of 2–4 mm) at the bottom of the column. The filtration rate was approximately 7 m/h with an empty bed contact time (EBCT) of 8.6 min. The temperature of the water was 15–21 °C. The concentration of DO in the influent was about 1.0–1.5 mg/L. Influent samples were collected from the top port above the media depth, and effluent samples were collected from the bottom sampling port in the gravel layer. Backwashing and other conditions were as in our previous study, and a schematic of the pilot-scale filter system is presented in our previous study (Guo et al. 2017).

Rapid start-up method

The virgin quartz sand was fixed into the pilot-scale filter column (about 1.5 m depth), and KMnO4 was dosed into the influent to oxidize Mn2+ and Fe2+ from raw water. In the process of preparation for the sand, 20 m polyethylene pipe provided a sufficient reaction time (about 6.8 min) for the oxidation process (Guo et al. 2017). Some methods, such as optimizing the filtration rate, the ratio of iron and manganese concentration, the concentration of KMnO4 in the influent and other process conditions, were taken to further shorten the ripening period of the sand.

The experiment of Mn2+ removal

The influent was derived from groundwater, and the filtration system did not have aeration equipment. So the concentration of DO in the influent was about 1.0–1.5 mg/L. A daily analysis of water samples was performed to determine the concentration of Mn2+ along the filter bed depth.

Kinetic experiments on Mn2+ adsorption and oxidation

The adsorption kinetic experiments were conducted using 2 g (wet weight) of the ripening sand in 200 mL of Mn2+ solution (0.83, 1.69 and 3.87 mg/L) for varying contact times (0.5 to 31.5 hr) at pH 7.8 while shaking at 80 r/min (SHZ-82A, Changzhou Guohua Electric Co., Ltd, China). Mn2+ solution was from analytical grade MnCl2 solution (Sinopharm Chemical Reagent Co., Ltd, China). In the kinetic study, pseudo-first-order and pseudo-second-order equations were used for evaluating the experimental data. The pseudo-first-order (Lagergren) equation can be expressed as Equation (1): 
formula
(1)
where qe (mg/g) and qt (mg/g) are the adsorption capacity at equilibrium and at time t (min), respectively; k1 (min−1) is the pseudo-first-order rate constant (Han et al. 2006).
The linear form of the pseudo-second-order equation is given in Equation (2): 
formula
(2)
where k2 (g/(mg·min)) is the pseudo-second-order rate constant (Dinu & Dragan 2010).

To study the oxidation kinetic experiment of Mn2+, a dosing pump was used to add Mn2+ in different initial concentrations (0.49, 1.15, 1.79, 2.86, 3.81 mg/L). Each Mn2+ initial concentration was maintained in triplicate, and the average of the experimental results was determined. Depletion of Mn2+ concentration in relation to the bed depth at different initial concentrations was discussed, and a linear regression analysis of Mn2+ depletion in relation to the EBCT was performed. The EBCT was calculated according to the described method (Katsoyiannis & Zouboulis 2004).

Characterization of filter sand

The ripening sand was frozen and vacuum-dried using a freeze dryer (FD-1D-50, Beijing Medical Kang Bo Experimental Instrument Co., China), and were kept in sealed vacuum tubes (Zhao et al. 2003). The binding energies of Mn on the surface of the sand were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific-Noran System Six, USA), and the XPS spectra of Mn(3/2p) and O(1s) were analyzed and peak fitted by bundled software (Avantage) (Cheng et al. 2018).

Analytical methods

All the reagents used in the experiment were of analytical grade and the solutions were prepared using deionized water. Concentrations of manganese and iron were determined by conventional spectrophotometric methods (SEPA of China 2002) (HACH, DR5000, USA). DO and pH were determined using a portable instrument (HACH, HQ30d, USA).

RESULTS AND DISCUSSION

Rapid start-up of the pilot-scale filter

The pilot-scale column was monitored continuously for about 3 months (from April 2016 to July 2016), and the ripening process of the filter sand is shown in Figure S1 (available with the online version of this paper). Effective removal of manganese was slower than ammonium removal, so the removal of manganese was an important factor indicating the sand ripening. Compared with our previous research (about 26 days) (Guo et al. 2017), complete removal of manganese was shortened to about 17 days as the start-up period for the ripening sand.

When the concentrations of manganese in the effluent remained less than about 0.1 mg/L, all the sand was coated with dark brown manganese oxides, which was the sign of the sand ripening. KMnO4 was used only for the preparation process of the sand, and after that, it was not dosed anymore. So, KMnO4 was completely consumed in the oxidation process, and it would not have an effect on the environment.

Efficient removal of Mn2+

The ripening sand was used to conduct the experiment of Mn2+ removal. As shown in Figure S2(a) and S2(b) (available online), Mn2+ could be removed efficiently, even if the concentration of DO in the influent was about 1.0–1.5 mg/L. This means that the oxidation process of Mn2+ does not need to consume DO. When the concentration of DO of less than 2 mg/L persisted for about 4 days, the concentration of Mn2+ in the effluent would increase significantly in a biofilter (Bray & Olańczuk-Neyman 2001). However, the concentration of DO of about 1 mg/L persisted for more than 11 days, and the removal efficiency of Mn2+ was still up to 95% in our filter system, so the kinetics and mechanism of Mn2+ removal are different from biological oxidation.

Kinetics of Mn2+ adsorption and oxidation

After the start-up period, the situation of Mn2+ concentration in the effluent exceeding the permitted limits did not occur over the 3 months (Figure S1), and the removal efficiency of Mn2+ continued to be up to 95%. So, Mn2+ was removed through the process of adsorption and subsequent oxidation, rather than only adsorption.

The adsorption capacity of Mn2+ as a function of time was basically the same for different initial Mn2+ concentrations (Figure S3(a), available online). The adsorption capacity increased rapidly over the first 7.5 hr, and reached equilibrium at 19.5 hr. The initial concentration of Mn2+ was 3.87 mg/L, and the maximum equilibrium adsorption capacity was 0.132 mg/g. The zeta potential of the sand surface was about −33 mV (Guo et al. 2017), which indicates that the surface was negatively charged, thereby favoring the adsorption of Mn2+.

The pseudo-first-order (Equation (1)) and pseudo-second-order (Equation (2)) equations were used for evaluating the experimental data, and the results of the calculated kinetic parameters are shown in Table S2 (available online). In all cases, the coefficients of pseudo-first-order kinetics (R2 > 0.98) were higher than those of pseudo-second-order kinetics with different initial concentrations of Mn2+. As shown in Figure S3(b) and S3(c), the pseudo-first-order model was a better fit, indicating that the mechanism of adsorption might be chemical adsorption.

Different initial Mn2+ concentrations were dosed into the influent, and the experimental results are shown in Figure 1.

Figure 1

Removal of Mn2+ using (a) lower and (b) higher initial Mn2+ concentration depletion of Mn2+ concentration in relation to the bed depth; (c) lower and (d) higher initial Mn2+ concentration linear regression analysis of Mn2+ depletion in relation to the empty bed contact time.

Figure 1

Removal of Mn2+ using (a) lower and (b) higher initial Mn2+ concentration depletion of Mn2+ concentration in relation to the bed depth; (c) lower and (d) higher initial Mn2+ concentration linear regression analysis of Mn2+ depletion in relation to the empty bed contact time.

Figure 1(a) and 1(b) indicate the variation in Mn2+ concentration along the filter bed depth. When the Mn2+ concentration of influent was less than 1.79 mg/L, the removal efficiency of Mn2+ was up to 96% at 30 cm depth, and the Mn2+ concentration was not different significantly in the rest of the filter. The entire filter layer worked towards Mn2+ removal when the concentration of Mn2+ in the influent was higher than 2.86 mg/L, and the removal efficiency of Mn2+ was only 53–61% at 30 cm depth.

According to the literature concerning Mn2+ removal kinetics (Katsoyiannis & Zouboulis 2004), keeping the DO concentration and pH value of influent constant ensures that Mn2+ depletion might be pseudo-first-order kinetics: −d[Mn2+]/dt = k [Mn2+].

As shown in Figure 1(c) and 1(d), the value of k for the high initial Mn2+ concentrations (0.14 min−1) was significantly lower than that for the low initial Mn2+ concentrations (0.57 min−1), which was due to the limited number of active sites on the surface of the sand. The plot of log{[Mn2+]t/[Mn2+]o} versus EBCT was linear, confirming that the Mn2+ oxidation followed first-order kinetics.

Characterization of the filter film

The previous study found that the main component of the film was iron manganese co-oxide, with a Mn/Fe ratio of 3.76, and the film on the sand surface was amorphous and that the specific surface area increased from 0.11 to 4.50 m2/g, as well as the film surface comprising many small oxide particles that could play an important role in the removal of Mn2+ (Cheng et al. 2018).

The binding energy of Mn(3/2p) for the ripening sand was characterized by XPS (Figure 2(a)). As shown in Figure 2(a), all manganese adsorbed on the surface of the sand is the oxidized form, and the manganese oxide coated onto the sand effectively oxidizes Mn2+ to Mn3+ or Mn4+. Figure 2(b) shows the binding energy of the observed photoelectron peaks of O(1s). The binding energy of O(1s) may be observed at 529.7 eV, 531.4 eV and 532.9 eV in the O(1s) region respectively, which can be assigned to manganese oxide (Mn-O-Mn), hydroxide [≡Mn-OH], and structural water, respectively (Chigane & Ishikawa 2000).

Figure 2

(a) Mn(3/2p) XPS energy spectra, (b) O(1s) XPS energy spectra of the ripening sands.

Figure 2

(a) Mn(3/2p) XPS energy spectra, (b) O(1s) XPS energy spectra of the ripening sands.

Proposed mechanism for Mn2+ removal

MeOx is coated continuously on the surface of the sand as a catalyst for adsorption and oxidation of Mn2+. Some authors have described the surface reactions of Mn2+ with oxide surfaces (Han et al. 2006; Taffarel & Rubio 2010). The main interactions are summarized as: 
formula
(3)
 
formula
(4)

Figure 2(b) shows the existence of [≡Mn-OH] on the surface of the film. In the continuous catalytic oxidation process of Mn2+, [≡Mn-OH] might be an important intermediate product.

A schematic presentation of the Mn2+ oxidation mechanism was proposed, as shown in Figure 3. The catalytic oxidation process of Mn2+ can be presented as a sequence of four main steps: (1) formation of adsorbed Mn2+ (adsorption on the surface), (2) adsorbed Mn2+ to formation of hydroxide [≡Mn-OH] on the surface of the film, (3) the reaction between adsorbed [≡Mn-OH] and adsorbed Mn2+ to produce a hydrolysis complex [≡MnOMnOH] or [(≡MnO)2Mn] (oxidation of a surface), and (4) a small amount of the complex product forming a new active film, while the remaining oxides formed some loose oxides [≡MnOMnOH] or [(≡MnO)2Mn], which were peeled off by backwashing.

Figure 3

Schematic presentation of the Mn2+ oxidation mechanism.

Figure 3

Schematic presentation of the Mn2+ oxidation mechanism.

CONCLUSION

The ripening period of the sand was further shortened to about 17 days by optimizing process conditions. Mn2+ could be removed efficiently even if the DO concentration in the influent was only about 1.0–1.5 mg/L. Adsorption and oxidation kinetics of Mn2+ were both pseudo-first-order kinetics. Due to the limited number of active sites on the surface of the sand, the rate constant (k) for the high initial Mn2+ concentrations (0.14 min−1) was lower than that for the low initial Mn2+ concentrations (0.57 min−1). The results of XPS characterization showed that the manganese oxide coated onto the sand effectively oxidizes Mn2+ to Mn3+ or Mn4+, and the existence of [≡Mn-OH] on the surface of the film was proved, which was a key intermediate in the mechanism of Mn2+ oxidation.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (Nos 51778521, 21573171), Shaanxi Province Key Research and Development Plan of China (2017GY-121), the special item of Shaanxi Educational Committee of China (Nos 18JK0359, 16JK1344), and the Doctoral Scientific Research Foundation (No. 107020335). Thanks for the anonymous reviewers' invaluable advices.

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Supplementary data