Abstract

Manganese oxide coated scoria (MOCS) was prepared as a cost-effective catalytically adsorbent with high permeability to remediate manganese contaminated groundwater. Scanning electron microscope visual expressed that on the relatively smooth surface of raw scoria (RS) a large amount of micro pores and dense bulk-like structures after modification and filtration appeared. The data from Fourier transform infrared showed that the intrinsic scoria structure was unchanged during modification. The X-ray diffraction and X-ray photoelectron spectroscopy instrumental studies revealed that the newborn manganese oxide layer was a mixed-valence of manganese (Mn3+ and Mn4+) which could absorb the Mn2+ and catalytically facilitate oxidation with oxygen. Subsequently, the adsorption capacity of RS and MOCS was demonstrated in adsorption experiments. The kinetics of manganese adsorption by RS and MOCS followed pseudo-second-order with the correlation coefficients of 0.983 and 0.989, respectively. The experimental data were better fitted to Langmuir isotherm than Freundlich isotherm, indicating that the monolayer adsorption process for manganese was acting on the surface of RS and MOCS. The filtration experiment showed high Mn2+ removal efficiency by MOCS in a wide range of hydraulic retention time (15–40 min) in 40 days, which demonstrated that the MOCS is a good potential application prospect for manganese removal from groundwater.

INTRODUCTION

Groundwater has been recognized as a preferred drinking water source when potable surface water is not available in widely ranged arid and semi-arid areas. Drinking water with a high concentration of manganese ions is a critical hazard, which can generate central nervous system, brain tissue and neurological disorder damages in humans (Han et al. 2013). The most notorious example of this is Parkinson's disease (Elsner & Spangler 2005; Roels et al. 2012). There are different standards of manganese intake from drinking water in many countries. In Brazil, India, and China, the optimal recommendation of manganese concentration in drinking water is below 0.1 mg/L (Tekerlekopoulou et al. 2008; Jia et al. 2015). For the United States, the guideline for the upper intake of manganese should be lower than 0.05 mg/L. In addition, in the European Union, legislation under consideration may establish an Environmental Quality standard, which could lower the permissible manganese intake limit of 0.03 mg/L.

Manganese as a common element in groundwater (Esfandiar et al. 2014) and is mainly formed by hydro-geochemical reactions. These reactions lead to manganese release at a slow dissolutive rate. Due to the underground anoxic conditions, manganese generally presents as a divalent ion (Mn2+) and is composed of hydroxides, sulfates, or carbonates, which make it difficult for manganese to decrease to an acceptable level by natural attenuation. Therefore, in order to mitigate manganese damage and to ensure a sustainable drinking water supply, development of manganese removing technologies from groundwater are urgently necessary.

Currently, many treatment methods, such as oxidation (Tekerlekopoulou et al. 2008; Funes et al. 2014; Yang et al. 2014; Wagloehner et al. 2015), membrane filtration (Jia et al. 2015), and adsorption (Lo et al. 1997; Luo et al. 2013; Abdel Salam 2015; Goher et al. 2015) are employed in removing high manganese concentrations from groundwater (Kan et al. 2013). The oxidation method for manganese removal mainly rested with oxidizing agents such as potassium permanganate, chlorine, chlorine dioxide, or ozone. Although these agents can remove the manganese rapidly, the generation of other toxins demonstrates the disadvantages of this method. Membrane filtration has been proven as an effective method for heavy metal removal (Tekerlekopoulou et al. 2008), the same as manganese contaminated groundwater remediation. However, due to the high pH value requirement of filtration, it could increase the resistance of the filtration process, resulting in membrane fouling acceleration, thus limiting the use of membrane filtration for manganese removal from groundwater (Funes et al. 2014). Compared to the above methods, adsorption has appeared to be more economical and has received extensive attention for the removal of manganese from groundwater (Lo et al. 1997; Han et al. 2006a; Ates 2014).

The application of the adsorption method is highly dependent on the adsorptive materials (Zhang et al. 2014a). Recently, the material of manganese oxide as a catalytic adsorbent for manganese removal has been reported (Xu et al. 2009; Taffarel & Rubio 2010; Kan et al. 2013). Studies have indicated that this material has a large surface area, micro porous structure, and high affinity for metal ions, but manganese oxides were available as a powder or were generated in an aqueous suspension only. The difficulty of solid–liquid separation limited the application of this powder formed adsorbent for manganese removal from groundwater. Therefore, in order to overcome these limitations, immobilization of manganese oxide and natural materials like sand (Kan et al. 2013) and zeolite (Taffarel & Rubio 2010; Cai et al. 2015; Li et al. 2015) to prepare a new manganese removal adsorbent has been studied. Although sand is a cost-effective and easily prepared material for manganese removal from groundwater (Kan et al. 2013; Jia et al. 2015), the low porosity and surface areas cause low manganese coating capacity on the surface of sand. Zeolite has the advantage of large surface areas, good chemical stability, strong adsorption capacity, and large amounts of uniform particles (Taffarel & Rubio 2010). However, the weak physical hardness and low permeability make it difficult for the modified zeolite to be widely used. Whatever, utilizing manganese oxide coated onto natural materials seems to be more appropriative and attractive than various methods for manganese removal.

In this study, we attempt to utilize manganese oxide coated raw scoria (RS), a bomb-sized vesicular pyroclastic rock, to develop a new adsorbent (manganese oxide coated scoria (MOCS)). The adsorption capacity of scoria to remove heavy metals such as cadmium, nickel, and copper has been studied and reported, mainly due to its valuable properties: large specific surface area, negatively charged surface, light in weight, low cost, and local availability (Kwon et al. 2010; Zhang et al. 2014a, 2014b). However, few studies are available about the removal of manganese from groundwater by scoria. Therefore, the main objectives of this work are to investigate the ability of manganese removal onto MOCS and to reveal a removal mechanism of manganese from groundwater.

MATERIALS AND METHODS

Materials

Scoria was purchased from Jilin Province, China and the major physical properties are shown in Table 1. The chemical reagents used in this study were all of analytical grade. HCl (99%), KMnO4 (99%), MnSO4 (H2O) (99%), HNO3 (65%), Mn2+ standard solution (1,000 mg/L), K4P2O7·3H2O (99%), and CH3COONa·3H2O (99%) were purchased from Guoyao Co., Shanghai, China. All solutions were prepared by using deionized water from a Millipore system with a resistivity of 18.9 MΩ.

Table 1

Physical properties of RS from Jilin Province

Serial no. Characteristics Value 
Volume weight (kg/m3500–600 
Specific gravity 2.4–2.6 
Total porosity (%) 74–78 
Opening porosity (%) 60–70 
Obturator porosity (%) 10–15 
Stress intensity (kg/cm27.00–8.25 
Specific surface area (cm2/g) 8 × 103–1.5 × 104 
Permeability coefficient (m/d) 1,168 
Filter head loss (cm/m) <12 
10 Intercept miscellaneous quality (kg/m310–13 
Serial no. Characteristics Value 
Volume weight (kg/m3500–600 
Specific gravity 2.4–2.6 
Total porosity (%) 74–78 
Opening porosity (%) 60–70 
Obturator porosity (%) 10–15 
Stress intensity (kg/cm27.00–8.25 
Specific surface area (cm2/g) 8 × 103–1.5 × 104 
Permeability coefficient (m/d) 1,168 
Filter head loss (cm/m) <12 
10 Intercept miscellaneous quality (kg/m310–13 

Preparation of MOCS

The RS was washed with deionized water in an ultrasonic bath and then added into a 13% HCl solution in order to improve the specific surface area. After 24 h, the acid-treated RS was washed with deionized water to a neutral pH, dried in a furnace, kept at 350 °C for 3 h and allowed to cool in the furnace. The standby scoria was impregnated into the potassium permanganate solution with a concentration of 9% for 8 h and processed according to the above washing and drying procedures again. Finally, the MOCS was stored in a polyethylene bottle for the following experiments.

Adsorption experiments

The 2.00 g adsorbents (RS and MOCS) and 80.0 mL manganese ion solution (10.34 and 502.47 mg/L, respectively) were added into 250 mL conical flasks to investigate the adsorption kinetics of RS and MOCS for manganese ion removal. The conical flasks were shaken at 160 rpm and kept at 15 ± 1 °C in an incubator shaker during the whole experiment. At each time interval, the manganese solution was collected and then filtered with 0.45 μm membrane for the following analysis. Two sequences of the initial manganese concentration in the ranges of 2.07–10.34 mg/L and 49.24–502.47 mg/L were employed for the adsorption isotherm experiment, respectively. Residual manganese ion was analyzed by an electron spectrophotometer (Model No. 723PC, SDPDOP, Shanghai, China) and the amount of manganese adsorbed was calculated by the following equation:  
formula
(1)
where qe is the adsorption capacity (mg/g) at equilibrium, C0 and Ce are the initial and equilibrium manganese concentrations (mg/L), respectively, V is the volume (mL) of solution, and W is the mass (g) of adsorbent used.

MOCS filtration experiment

The column filtration experiment was studied in a Perspex column with a length of 1 m and inner width of 2.5 cm, as shown in Figure 1. The filter material of MOCS with a length of 70 cm and particle size of 1.25–2.00 mm was placed in the column. At the inlet and outlet side, two buffer layers (5 cm length each) were constructed to keep the inflow well distributed and prevent the loss of MOCS during the filtration process. The experimental water was prepared to simulate the manganese contaminated groundwater. The procedure of making the experimental water is as follows: groundwater (25 L) obtained from Changchun City, China, was added with 0.17 g MnSO4·H2O to prepare the experimental water with an average manganese concentration of 2.3 mg/L. For the filtration experiment, the prepared experimental water was aerated to achieve dissolved oxygen of 7.5–9.5 mg/L. Then, this water was pumped into the column using a peristaltic pump with hydraulic retention time (HRT) of 10–40 min and the experimental temperature of 15 ± 1 °C. The manganese concentration from the influent and effluent were sampled and tested once a day. The manganese concentration was measured by periodate spectrophotometer measurement (UV-2600, Hengping Inc., China).

Figure 1

Configuration of simulated reactor for manganese removal by MOCS.

Figure 1

Configuration of simulated reactor for manganese removal by MOCS.

Characterization of MOCS

The surface morphology and elemental distribution of RS, MOCS, and filtrated MOCS were observed by a high qualitatively scanning electron microscope (SEM) and energy dispersive analysis (EDS) (JSM-6700 F, JEOL Ltd, Japan). A Fourier transform infrared (FTIR) spectrometer was employed to characterize the functional groups on RS, MOCS, and filtrated MOCS (8400S, Shimadzu Corporation, Japan). The composition of the RS, MOCS, and filtrated MOCS was determined by X-ray diffraction (XRD) analysis (XD-3, Shimadzu Corporation, Japan). The binding energies and atom ratios of the elements of RS, MOCS, and filtrated MOCS were analyzed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, Thermo Corporation, USA). A conventional Al Kα anode radiation source was used as the excitation source. The binding energies were calibrated by the C1s binding energy at 284.8 eV. XPS data processing and peak fitting were performed using a nonlinear least-squares fitting program.

RESULTS AND DISCUSSION

SEM and EDS studies

The SEM images of RS, MOCS, and filtrated MOCS are shown in Figures 2(a), 3(a) and 4(a), respectively. It was observed from Figure 2(a) that the RS formed relatively smooth and shallow depressions. After the manganese coating modifications (Figure 3(a)), MOCS presented a large amount of micro pores and dense bulk-like structures onto the surface. Comparing the images of Figures 3(a) and 4(a), the phenomenon on the rougher surface performed more obviously, which was also observed in other studies (Tiwari et al. 2011; Tizaoui et al. 2012; Jia et al. 2015). EDS of X-ray was done to determine and confirm the elements of RS, MOCS, and filtrated MOCS, as shown in Figures 2(b), 3(b) and 4(b), respectively. Figure 2(b) shows the elements of silica, oxygen, magnesium, calcium, etc. that were observed on the RS. The EDS spectrum of MOCS (Figure 3(b)) showed the presence of manganese ions along with other principal elements. The EDS spectrum of filtrated MOCS is shown in Figure 4(b). It can be concluded that the amount of manganese was apparently increased on the surface of manganese coating scoria after the filtration experiment. Dot mapping can also provide an indication of the qualitative abundance of elements on the surface of these materials. The bright points correspond to the signal of the manganese for MOCS in Figure 3(c). A manganese coating layer with a high density of bright dots in Figure 4(c) indicates that the abundant manganese on the surface of filtrated MOCS was much higher than that of MOCS. The above results demonstrate that manganese was successfully spread over the surface of MOCS. Moreover, the appearance of manganese content increasing on the surface of MOCS after filtration may be due to the catalytic chemical oxidation of manganese through the filtration process (Tiwari et al. 2011; Tizaoui et al. 2012; Jia et al. 2015).

Figure 2

(a) SEM micrograph and (b) EDS spectrogram of RS.

Figure 2

(a) SEM micrograph and (b) EDS spectrogram of RS.

Figure 3

(a) SEM micrograph, (b) EDS spectrogram, and (c) mapping image of MOCS.

Figure 3

(a) SEM micrograph, (b) EDS spectrogram, and (c) mapping image of MOCS.

Figure 4

(a) SEM micrograph, (b) EDS spectrogramm and (c) mapping image of filtered MOCS.

Figure 4

(a) SEM micrograph, (b) EDS spectrogramm and (c) mapping image of filtered MOCS.

FTIR studies

The FTIR spectra of RS, MOCS, and filtrated MOCS are shown in Figure 5(a)5(c). According to the FTIR spectra of RS (Figure 5(a)), the bands located around 3,446.5 cm−1 were typically attributed to the -OH group. The absorption peak at 2,335.6 cm−1 was derived from the existence of CO2 molecule in air. The band around 1,558.3 cm−1 was due to bending vibration of -OH from adsorbed water. Bands around 1,020.3 cm−1 were related to Al–O–Si chain of scoria group. Comparing Figure 5(a) and 5(b), it can be seen that the percent transmittance (%T) values of the peaks did not change obviously, which provided further evidence that the intrinsic scoria structure remains unchanged with the manganese coating. Figure 5(c) shows the FTIR spectra of filtrated MOCS. The appearance of new peaks at 2,304.7 cm−1 and 2,920.0 cm−1 and the peaks in the range of 2,000–1,000 cm−1 were attributed to the binding process of the manganese coating on the surface and associating with carboxyl and hydroxyl groups (Zhu et al. 2015).

Figure 5

FTIR images of (a) RS, (b) MOCS and (c) filtered MOCS.

Figure 5

FTIR images of (a) RS, (b) MOCS and (c) filtered MOCS.

XRD studies

The diffractograms of RS, MOCS, and filtrated MOCS samples were obtained by using XRD, as shown in Figure 6(a)6(c). Figure 6(a) clearly shows that the distinct peaks at the 2θ value of 27.833, 32.146, 35.592, and 39.759 were assigned to (510), (130), (221), and (300) planes of the sample. The above data indicated that the RS was composed of silicas, which were formed by the chemical formula of [Ca,Na,K,Mg]4[Si,Al]24O48·13H2O, [Mg,Fe]2SiO4, Ca[Fe,Mg]SiO6, and Mg7H6Si2O14. After the manganese coating modification, the above characteristic peaks did not change obviously. However, new peaks at 2θ = 30.179, 33.830, 15.161, 35.63, 36.53, 36.62, 38.57, and 56.206 were observed. The positions and relative intensities of these new peaks match well with the (115), (843), (135), (131), (060), (101), (252), and (222) planes of bulk manganic composition (Figure 6(b)). For the peak at 2θ = 21.994, the position and relative intensity matched well with the (101) plane of XRD standard data for the crystal structure of cristobalite. Meanwhile, the XRD pattern of MOCS exhibited new peaks at the angle 2θ = 36.5 and 37.56, which belong to MnO2 with planes of (131) and (211). The new peaks at the angles 2θ = 32.951 (222), 32.920 (222), and 35.597 (110) belong to Mn2O3. After the filtration experiment (Figure 6(c)), new peaks at the 2θ value of 19.112 (111), 21.808 (101), 21.890 (101), and 37.522 (211) similarly belong to MnO2.

Figure 6

XRD patterns of (a) RS, (b) MOCS, and (c) filtered MOCS.

Figure 6

XRD patterns of (a) RS, (b) MOCS, and (c) filtered MOCS.

XPS analysis

The wide scan XPS spectra of MOCS and filtrated MOCS were used to quantitatively estimate the chemical elements (Katsoyiannis & Zouboulis 2004). It can be seen from the spectra (Figure 7(a) and 7(b)) that the elements of manganese, oxygen, carbon, and silicon were present on the materials' surface. From Figure 7(a), survey spectra of MOCS showed that the major element peaks of Mn2p at 654.4 eV (Mn2p1/2) and 642.8 eV (Mn2p3/2) were the characteristics of a mixed-valence manganese system (Mn4+ and Mn3+) (Taffarel & Rubio 2010), which was consistent with the previous results of the SEM and XRD. Moreover, the three peaks at 532.6, 531.6, and 530.5 eV, respectively, belonged to O1s. The peak at 532.6 eV could be assigned to surface adsorbed oxygen in the form of OH (Han et al. 2006b; Taffarel & Rubio 2010) and the peaks at 530.5 and 531.6 eV could be assigned to manganese oxide (Mn-O-Mn) and hydroxide (Mn-OH). After the manganese coating modification (Figure 7(b)), the peaks of Mn2p left shifted 0.1 and 0.2 eV, respectively, and the peaks of O1s right shifted 0.1 eV. However, it was found that the binding energy of MOCS and filtrated MOCS had the same oxidation state. This indicated that the form of manganese ion adsorbed onto the MOCS and filtrated MOCS surface from groundwater did not change. It also can be demonstrated that the surface layer of manganese coated on the scoria effectively oxidized Mn2+ to Mn3+ and Mn4+ from groundwater (Taffarel & Rubio 2010; Jia et al. 2015).

Figure 7

XPS scan of (a) MOCS and (b) filtered MOCS.

Figure 7

XPS scan of (a) MOCS and (b) filtered MOCS.

Kinetic studies

The two sequences of connecting time from 0 to 180 and to 360 min were used to determine the adsorption equilibration times and to investigate the kinetic models. It can be observed from Figure 8(a) and 8(b) that the adsorption capacity of RS and MOCS improved by increasing connecting time at the beginning and then remained steady at 60 and 300 min, respectively. Thus, the suitable adsorption equilibrium times for RS and MOCS were selected as 60 and 300 min, respectively.

Figure 8

Time vs. adsorption capacity of (a) RS and (b) MOCS for manganese removal.

Figure 8

Time vs. adsorption capacity of (a) RS and (b) MOCS for manganese removal.

Pseudo-first-order kinetic model and pseudo-second-order kinetic model were used to describe the kinetics of RS and MOCS for manganese adsorption.

The pseudo-first-order kinetic model of linear form equation is given by the following:  
formula
(2)
The pseudo-second-order kinetic model of linear form equation is given by the following equation:  
formula
(3)
where k1 and k2 are the pseudo-first-order and pseudo-second-order rate constants of adsorption with the units of 1/min and g/(mg/min), respectively. qe and qt are the amounts of manganese adsorbed at equilibrium (mg/g) and at time t (min), respectively. The parameters were calculated from the plots in Figures 9(a) and 9(b) and 10(a) and 10(b) and are shown in Table 2.
Figure 9

Pseudo-first-order kinetic (a) and pseudo-second-order kinetic (b) for the removal of manganese ion by RS.

Figure 9

Pseudo-first-order kinetic (a) and pseudo-second-order kinetic (b) for the removal of manganese ion by RS.

Figure 10

Pseudo-first-order kinetic (a) and pseudo-second-order kinetic (b) for the removal of manganese ion by MOCS.

Figure 10

Pseudo-first-order kinetic (a) and pseudo-second-order kinetic (b) for the removal of manganese ion by MOCS.

Table 2

Kinetic parameters obtained for manganese adsorption onto RS and MOCS

Pseudo-first-order
 
Pseudo-second-order
 
Parameter RS MOCS Parameter RS MOCS 
k1 (L/min) 0.0472 0.0257 k2 (g/mg/min) 0.0128 0.0002 
qe (mg/g) 3.174 83.048 qe (mg/g) 4.395 101.010 
R2 0.979 0.973 R2 0.983 0.989 
Pseudo-first-order
 
Pseudo-second-order
 
Parameter RS MOCS Parameter RS MOCS 
k1 (L/min) 0.0472 0.0257 k2 (g/mg/min) 0.0128 0.0002 
qe (mg/g) 3.174 83.048 qe (mg/g) 4.395 101.010 
R2 0.979 0.973 R2 0.983 0.989 

The values of the rate constants of pseudo-first-order (k1) and pseudo-second-order (k2) usually represent the adsorption rate. The values of k1 and k2 for RS were 0.0472 and 0.0128, and for MOCS were 0.0257 and 0.0002, respectively. The larger k1 values mean a faster adsorption rate of the manganese adsorption process onto RS and MOCS. The calculated adsorption capacities from the pseudo-second-order kinetic model (qe = 3.174 and 83.048 mg/g for RS and MOCS) were also close to the experimental adsorption capacity (qexam = 3.495 and 84.458 mg/g for RS and MOCS). By comparing the correlation coefficient values, pseudo-first-order (R2 = 0.979 and 0.973 for RS and MOCS, respectively) was lower than that of pseudo-second-order (R2 = 0.983 and 0.989 for RS and MOCS, respectively), with the results indicating that the adsorption processes of manganese onto the RS and MOCS were pseudo-second-order rather than pseudo-first-order.

Adsorption isotherms

The Langmuir and Freundlich isotherm equations were applied to analyze manganese adsorption on RS and MOCS in this study.

The Langmuir isotherm model indicated that the adsorption of manganese onto the surface of RS and MOCS was a monolayer adsorption process. The linear form of the Langmuir equation is given as:  
formula
(4)
The Freundlich isotherm model assumes that the manganese adsorption process takes place on the non-uniformity RS and MOCS surfaces. The linear form of the Freundlich isotherm equation is given as:  
formula
(5)
where qe (mg/g) is the amount of manganese adsorbed onto RS and MOCS at equilibrium; ce (mg/L) is the equilibrium concentration of manganese in solution; Qm (mg/g) is the maximum adsorption capacity; Kl is the Langmuir constant; K and n are the Freundlich constants.

The isotherm experiment data are presented in linear form in Figures 11(a) and 11(b) and 12(a) and 12(b). The results were used to determine the most accurately described adsorption process on RS and MOCS. The calculated isotherm parameters of the Langmuir and Freundlich models are listed in Table 3. It can be seen from Table 3 that the correlation coefficients from the Langmuir isotherm model (R2 = 0.997 and 0.989 for RS and MOCS, respectively) were better fitted than that of the Freundlich model (R2 = 0.931 and 0.313 for RS and MOCS, respectively). The Freundlich isotherm model indicates that the manganese adsorption process takes place on the non-uniformity RS surface. However, the adsorption process onto MOCS did not conform with the Freundlich isotherm significantly due to the excessively lower R2 of 0.313. The calculated values of the empirical constant K and n for RS via the Freundlich isotherm model were 0.108 mg/g and 5.959, and for MOCS were 2.372 and 21.276, respectively. Compared with the Freundlich isotherm, the better Langmuir isotherm model fitting indicated that the monolayer adsorption process for manganese was acting on the surface of RS and MOCS. The maximum adsorption capacities (Qm) were 0.143 and 3.355 mg/g for RS and MOCS and coefficient related to the binding energy of the sorption system (Kl) was 5.192 and 0.076 L/g for RS and MOCS, respectively.

Figure 11

Langmuir adsorption isotherm (a) and Freundlich adsorption isotherm (b) for the removal of manganese ion by RS.

Figure 11

Langmuir adsorption isotherm (a) and Freundlich adsorption isotherm (b) for the removal of manganese ion by RS.

Figure 12

Langmuir adsorption isotherm (a) and Freundlich adsorption isotherm (b) for the removal of manganese ion by MOCS.

Figure 12

Langmuir adsorption isotherm (a) and Freundlich adsorption isotherm (b) for the removal of manganese ion by MOCS.

Table 3

The value of parameters for Langmuir and Freundlich isotherm models

Langmuir
 
Freundlich
 
Parameter RS MOCS Parameter RS MOCS 
Qm (mg/g) 0.143 3.355 K (mg/g) 0.108 2.372 
Kl (L/g) 5.192 0.076 n 5.959 21.276 
R2 0.997 0.989 R2 0.931 0.313 
Langmuir
 
Freundlich
 
Parameter RS MOCS Parameter RS MOCS 
Qm (mg/g) 0.143 3.355 K (mg/g) 0.108 2.372 
Kl (L/g) 5.192 0.076 n 5.959 21.276 
R2 0.997 0.989 R2 0.931 0.313 

MOCS filtration for manganese removal

During the 40 days of filter operation, regardless of the variation of Mn2+ concentration from the influent, the effluent concentration of Mn2+ was generally lower than 0.1 mg/L (Figure 13). It indicated that the MOCS exhibited great performance for Mn2+ removal from groundwater. HRT was a significant factor for the effect of the filtration process. It is necessary to investigate the impact of MOCS for Mn2+ removal on HRT. The results from Figure 13 indicate that higher HRT facilitated Mn2+ removal by the MOCS filtration and the concentration of Mn2+ less than 0.1 mg/L from effluent was determined at HRT of 10–40 min. When the HRT was lower than 15 min, the concentration of Mn2+ from effluent increased beyond the WHO standard. Thus, at the end of the filtration process, the HRT was increased to 15 min to maintain good performance for Mn2+ removal from groundwater.

Figure 13

Removal efficiency of Mn2+ by MOCS in the filter experiment.

Figure 13

Removal efficiency of Mn2+ by MOCS in the filter experiment.

CONCLUSION

In this study, a new MOCS filter material was synthesized for manganese ion removal. The newborn manganese oxide layer, a mixed-valence of manganese crystalline, was apparently loaded on the surface of MOCS. This layer catalytically facilitated the adsorption of Mn2+ and effectively oxidized Mn2+ to Mn3+ and Mn4+. From the filtration experiment, the high Mn2+ removal efficiency by MOCS was obtained and remained stable at a wide range of HRT from 15 to 40 min during 40 days. Therefore, it can be concluded that MOCS, a cost-effective manganese ion adsorbent with excellent catalytic oxidation performance, is a good potential application prospect for Mn2+ removal from groundwater.

ACKNOWLEDGEMENTS

We acknowledge the financial support of Major Science and Technology Program for Water Pollution Control and Treatment (2014ZX07201-101).

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