The morphology and surface characteristics of manganese dioxide (MnO2) formed in situ, which was prepared through the oxidation of MnSO4 using KMnO4, were studied. The effects of factors including the form of MnO2, dosage, pH, dosing sequence of in situ MnO2 on the enhanced coagulation were systematically evaluated. The results of analysis by the UV254 and permanganate index CODMn methods indicated that humic acid removal increased from 9.2 and 2.5% to 55.0 and 38.9%, when 10 mg/L of the in situ MnO2 was added in the presence of 2 mg/L of polyaluminum sulfate. The studies of orthogonal experiment revealed that coagulation was most affected by the pH, whereas the dosage of in situ MnO2 and slow stirring duration exhibited a weaker effect. At a pH value of 4.0, in situ MnO2 dosage of 10 mg/L, slow stir over 40 min, and the total solids content was 20 mg/L, the humic acid removal by UV254 and CODMn methods reached 71.2 and 61.2%. These results indicated that the presence of in situ MnO2 enhanced the coagulation and removal of humic acid from water.

INTRODUCTION

Humic acid, which is a main organic constituent, accounts for 60–90% of the total organic content of natural waters (Traversa et al. 2014; Wang et al. 2014). The functional groups of humic acid, such as the carboxyl group, hydroxyl group, quinone, and ether, can react with various types of cations or organic reactive groups in water, particularly polar organic compounds to form complexes. These reactions can increase the solubility of insoluble toxic compounds in water; therefore, the existence of humic acid is one of the primary factors that create potential safety hazards in water (El-Rehaili & Weber 1987; McKnight & Aiken 1998; Chai et al. 2013).

Humic acid treatments can be divided into physical, physico-chemical and biological methods (Brattebø et al. 1987; Ghernaout et al. 2009; Leodopoulos et al. 2012). Fang et al. (2008) demonstrated an efficiency of 95.2% for humic acid removal by adsorption onto modified zeolites. Humic acid removal using modified ultrafiltration membranes was investigated and demonstrated that the humic acid removal of regenerated cellulose membranes was very promising (Song 2012). A humic acid degradation rate of 40% was achieved through a biological fluidized bed treatment, which utilized cultivated and domesticated microbes and operated under optimized conditions (such as hydraulic load, pH, temperature, and aeration) (Huang 1999). Fenton's peroxidation was used as an advanced oxidation technique to treat high concentrations of humic acid in simulated wastewater, and the mechanism consists of both oxidation and coagulation simultaneously (Wu et al. 2010).

Enhanced coagulation is a method that is based on conventional coagulation. By adjusting the pH and increasing the coagulant dosage, the efficacy of the coagulation effect is enhanced. The development of MnO2 originates from the use of potassium permanganate for pre-treatment of wastewater. During the reduction of potassium permanganate, in situ-state MnO2 is formed; this compound possesses greater reactivity, specific surface area, and absorption capacity than aged MnO2 (Liang et al. 2005; Fan et al. 2010). At present, the use of in situ MnO2 in research is primarily focused on its role as both an oxidant and absorbent; more specifically, the latter use is removal of heavy metals, such as cadmium, strontium, and lead, from water (Xu et al. 2013; Wan et al. 2014). In contrast, relatively few studies have reported using in situ MnO2 as an enhanced coagulant to remove natural organic matter from waters. This paper reports the enhanced coagulation effect of in situ-state MnO2 formed by reacting potassium permanganate and manganese sulphate, which has value for removal of organic pollutants from water.

METHODS

Reagents and water sources

All the reagents used were analytical grade. The concentration of stock solution KMnO4 and MnSO4 was 4.6 mmol/L and 6.9 mmol/L, respectively. Commercial polyaluminium sulfate (PAS) was purchased from a purification-reagent company in Henan province, China, and used in the experiment as the coagulant. The flocculant polyacrylamide (PAM) used in jar tests were provided from other companies. The raw water from East Lake of Wuhan, China was used in this investigation. The initial concentration of humic acid in micro-polluted water by the UV254 and permanganate index CODMn methods was in the range of 0.20–0.80 and 4 mg/L–20 mg/L, respectively.

Experimental methods

Jar tests: the enhanced coagulation test was conducted in a jar filled with 1 L of micro-polluted lake water, and a programmable jar testing apparatus was used with the following procedure: addition of coagulant PAS following 1 min of rapid mixing at 250 rpm, followed by addition of MnO2 in different forms, rapid mixing at 250 rpm for 1 min. Then, 1 mg/L of PAM was added to the solution, which was subsequently stirred at 150 rpm for 30 min and settling. After settling for 30 min was completed, the samples (1 cm below) were collected and filtered with 0.45 μm cellulose acetate membrane for measurement of CODMn and UV254.

The effects of different factors on enhanced coagulation were systematically studied. These tests included the comparison between PAS and PAS + MnO2, the comparison between in situ MnO2 and commercial MnO2; the effects of pH of the water, temperature, dosage of in situ MnO2, and ratio of KMnO4/MnSO4, dosing sequence, dosing methods, and mixing time.

Orthogonal experiments: to obtain the optimal enhanced coagulation parameters, a standard orthogonal array matrix L9 (34) was constructed with four major factors and three levels. The four factors and three levels are as follows: (1) pH: 4, 6, 8; (2) dosage of in situ MnO2: 6, 10, 15 mg/L; (3) slow stirring time: 20, 30, 40 min; (4) particulate concentration: 10, 20, 30 mg/L. Results of humic acid removal were analyzed and optimized statistically to identify significant factors and to evaluate optimal values.

The percentage of humic acid removal (%) by UV254 and CODMn measurements was considered as 
formula
1
where C0 is the initial concentration of humic acid by UV254 and CODMn methods, and Ct is the concentration of humic acid by UV254 and CODMn methods after completing coagulation.

The Brunauer Emmett Teller (BET) specific surface area of in situ MnO2 was determined using nitrogen adsorption-desorption analysis (Ladavos et al. 2012). A total of 0.0443 g of sample was used, with nitrogen gas as the adsorbate, in the adsorption measurement under liquid nitrogen at 77.31 K. The specific surface areas of the samples were determined by applying the BET equation to the data.

The zero point of charge (pHZPC) of the in situ MnO2 was determined using a Malvern Zetasizer at 25 °C. The same mole ratio of KMnO4 and MnSO4 were added into 100 mL of deionized water. 1 M NaOH and 1 M HCl solution was used to adjust the pH values to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0. The zeta potential at different pH conditions was measured.

The permanganate index was calculated as the chemical oxygen consumption using potassium permanganate solution as the oxidant, which is denoted as CODMn, in accordance with the standards (Standard Methods for the Examination of Water and Wastewater 1998). The UV254 absorbance was defined as the absorbance of some organic matter in water in the presence of ultraviolet light with a wavelength of 254 nm. UV254 can reflect the content of natural organic matter such as humic acid, molecules that consists of conjugated double bonds, and aromatic compounds with C = O functional groups.

Instruments

X-ray diffraction (XRD) was performed to confirm the crystal structure and identity using X-ray diffractometer (Rigaku D/MAX-RB, Japan) with Cu Kα radiation in the 2θ ranges of 5–80 ° at a scan rate of 1 °/min. The surface structure of samples was determined using scanning electron microscopy (SEM, JSM-5610LV, Japan). The characterizations of the samples were carried out at their optimal working conditions. A glass pH electrode (PHS-25, China) was used for pH measurement. The instrument used in BET specific surface area was a Micromeritics (Shanghai) automated ASAP 2020 accelerated specific surface area and porosimetry analyzer. The UV254 absorbance was measured using a UV-1600 UV/Vis spectrophotometer.

RESULTS AND DISCUSSION

XRD and SEM

The XRD pattern of the in situ MnO2 is shown in Figure 1. The diffraction peak of the in situ MnO2 is relatively weak and wide, which indicates low crystalline structure. Analyzing the XRD pattern as a whole, only one peak is apparent, which suggests that the purity of the in situ MnO2 was relatively high. Given that the diffraction peak is at 2θ = 37.17 °, a comparison with powder diffraction patterns indicates that the nature of the manganese dioxide is γ-MnO2 (Fathy et al. 2013).

Figure 1

XRD pattern of the in situ MnO2.

Figure 1

XRD pattern of the in situ MnO2.

The SEM image of the in situ MnO2 is shown in Figure 2. It can be observed that the in situ MnO2 particles are amorphous in nature, with a maximum diameter of approximately 120 nm. With an aging time of 30 min, the in situ MnO2 is observed to coagulate into clusters. There is a tendency of increasing aggregation with increasing concentration of the in situ MnO2.

Figure 2

SEM images of the in situ MnO2.

Figure 2

SEM images of the in situ MnO2.

BET specific surface area and pHZPC

The specific surface area directly governs the contact interfacial area that is available for adsorption. Generally speaking, the adsorption capacity increases with increasing specific surface area. The N2 adsorption/desorption isotherm of MnO2 and the fitted curve of the BET equation are shown in Figures 3(a) and 3(b). According to the classification of gas adsorption isotherms, the nitrogen adsorption isotherm of MnO2 exhibits a steep straight line at low pressures, whereas at high pressures, the adsorption isotherm plateaus at a fixed value. The calculated specific surface area of the MnO2 is 99.31 m2/g. The specific surface area of in situ MnO2 is dependent on the preparation method and conditions: Zhang & Ma (2008) used equivalents of KMnO4 to oxidize MnSO4 (small amounts of NaCO3 were added to neutralize the H+ produced during the reaction), and the specific surface area of the produced in situ MnO2 was 184 m2/g.

Figure 3

(a) Adsorption isotherm of N2 on MnO2; (b) specific surface area of N2 on MnO2.

Figure 3

(a) Adsorption isotherm of N2 on MnO2; (b) specific surface area of N2 on MnO2.

It can be observed from Figure 4 that the zeta potential of in situ MnO2 varies within the tested pH range of 2.0–10.0. The pHZPC of in situ MnO2 is in the pH range of 2.5–5.0. When the pH of the solution is greater than this value, the surface of the in situ MnO2 is negatively charged, i.e., the zeta potential is negative on the surface of in situ MnO2. Conversely, when the pH is less than the isoelectric point, the zeta potential of the in situ MnO2 is positive (Savaji et al. 2014; Kosmulski 2014).

Figure 4

pHZPC of the in situ MnO2.

Figure 4

pHZPC of the in situ MnO2.

Effect of the form of the manganese dioxide on enhanced coagulation

Figures 5(a) and 5(b) show a comparison between the effects of in situ MnO2 and commercial MnO2 on the efficiency of enhanced coagulation. It was observed that with increasing coagulant dosage, both UV254 removal and CODMn removal increased dramatically with in situ MnO2. This enhanced coagulation effect was even more apparent at low coagulant dosage. When the dosage of PAS was 2 mg/L, the removal of UV254 and CODMn were 9.2% and 2.5%, respectively. When 10 mg/L of in situ MnO2 was added, the removal of UV254 and CODMn increased to 55.0% and 38.9%, respectively. In contrast, the UV254 removal remained unchanged and the CODMn removal increased to a far lesser extent when commercially available MnO2 particles were used. The reason for this difference is that in situ MnO2 has strong adsorption capacity at low coagulant dosage. Therefore, adding a certain amount of in situ MnO2 can effectively enhance the removal of organic matter from water. In contrast, commercially available MnO2 has weak adsorption capacity, and its enhanced coagulation efficacy is not significant.

Figure 5

Effect of the form of MnO2 on enhanced coagulation.

Figure 5

Effect of the form of MnO2 on enhanced coagulation.

Effect of pH value on enhanced coagulation

The pH value is an important factor that affects the coagulation efficiency. The pH value can impact both the reaction and the product of coagulant in water, and it also influences the amount of organic matter in water (Xie et al. 2012).

As shown in Figures 6(a) and 6(b), at pH ≤ 4.0, humic acid removal was higher than under alkaline conditions. Specifically, the humic acid removal by UV254 method reached 65.9% at pH ≤ 4.0, whereas the humic acid removal by CODMn method was 32.2%. In alkaline conditions, humic acid removal by UV254 and CODMn methods were 40.0% and 25.0%, respectively. This difference can be attributed to the following: at pH = 4.0, PAS hydrolyses in water, forming Al8(OH)204+. pH 4.0 is close to pHZPC of in situ MnO2, which causes MnO2 to aggregate with hydrolysate of PAS through electrostatic attraction. This generates a manganese-dioxide aluminum network that precipitates readily and facilitates the removal of humic acid molecules by adsorption from the aqueous solution. At pH = 2.0, the hydrolysate of PAS is present in the form [Al(H2O)n]3+ (n = 6–10). The degree of polymerization is less than that for pH = 4.0. At pH > 8.0, PAS takes the forms of Al(OH) and Al(OH)262−, and the zeta potential of the in situ MnO2 is less than −8.0 mV; this results in an electrostatic repulsion that negatively impacts adsorption and bridging, so resulting in less of an coagulation enhancement.

Figure 6

Effect of pH on enhanced coagulation.

Figure 6

Effect of pH on enhanced coagulation.

Effect of in situ manganese dioxide dosage on the enhanced coagulation

As shown in Figure 7, humic acid removal by UV254 and CODMn methods increased rapidly when the dosage of in situ MnO2 was increased from 2 to 10 mg/L; the UV254 removal increased from 5.8 to 26.0%, whereas the CODMn removal increased from 6.1 to 25.9%. When the dosage of in situ MnO2 was continuously increased from 10 to 20 mg/L, both the UV254 and CODMn removal decreased slightly. Because this study used micro-polluted water, which exhibits low turbidity, this decrease resulted in an increased interparticle distance within the aqueous environment. The addition of in situ MnO2 causes high efficiency of adsorbing organic contaminants and concomitantly increases the probability of interparticle collision, thus way causing large aggregates that precipitate out of the water. Consequently, both the UV254 and CODMn removal increased with increasing dosage of in situ MnO2. With a further increase in the dosage of in situ MnO2, the resultant increase in concentration caused auto-aggregation between MnO2 species, accordingly reducing its total adsorption capacity. In addition, aggregated MnO2 clusters reduced the interparticle collision frequency, thereby causing a reduction in coagulation effects.

Figure 7

Effect of in situ MnO2 dosage on enhanced coagulation.

Figure 7

Effect of in situ MnO2 dosage on enhanced coagulation.

Effect of the KMnO4/MnSO4 dosing ratio on enhanced coagulation

The effect of the KMnO4/MnSO4 dosing ratio on the enhanced coagulation is shown in Figure 8. As the KMnO4/MnSO4 dosing ratio was decreased from 5 to 0.33, the UV254 removal first exhibited an increase, which was followed by a decrease. When the KMnO4/MnSO4 dosing ratio was greater than 1, the UV254 removal increased with decreasing KMnO4/MnSO4 ratio. This result occurred because as the KMnO4/MnSO4 dosing ratio became greater than 1, the concentration of KMnO4 was 10 mg/L, whereas that of MnSO4 was less than 10 mg/L. As a result, when the KMnO4/MnSO4 dosing ratio was greater than 1, MnSO4 became the limiting reagent in the production of in situ MnO2. The concentration of in situ MnO2 increased with decreasing KMnO4/MnSO4 dosing ratio (increasing MnSO4 concentration), hence increasing the enhanced coagulation effect. When the KMnO4/MnSO4 dosing ratio was less than 1, the concentration of in situ MnO2 was 10 mg/L, with an excess of MnSO4 in solution. The concentration of excess MnSO4 increased with decreasing KMnO4/MnSO4 dosing ratio, resulting in an increase of Mn2+ species in solution, which hindered the adsorption of small organic molecules, thus reducing the UV254 removal efficiency.

Figure 8

Effect of KMnO4/MnSO4 dosing ratio on enhanced coagulation.

Figure 8

Effect of KMnO4/MnSO4 dosing ratio on enhanced coagulation.

Effect of dosing sequence and dosing methods on enhanced coagulation

Six types of dosing method were used: (1) in situ MnO2 (online) + PAS + PAM; (2) PAS + in situ MnO2 (online) + PAM; (3) MnO2 (commercial source) + PAS + PAM; (4) in situ MnO2 (mixed first then added) + PAS + PAM; (5) PAS + MnO2 (commercial source) + PAM; and (6) PAS + PAM.

The effect of the dosing sequence and methods on coagulation is shown in Figure 9. The UV254 removal was consistent among all methods, with a value of approximately 40%, which indicates that the differences in dosing methods and sequence had little effect on the UV254 removal. However, method 2 resulted in the optimal CODMn removal of 21.6%. In comparison with method 6, the CODMn removal was double in method 2. The addition of in situ MnO2 resulted in better CODMn removal compared with commercially available MnO2. In addition, the removal efficiency was greater when the in situ MnO2 was added after the addition of PAS compared with that when the in situ MnO2 was added before the addition of PAS. This result can be attributed to the fact that in situ MnO2 is not yet aggregated, in contrast with commercially available MnO2. Therefore, in situ MnO2 possesses a greater adsorption capacity. Furthermore, addition of in situ MnO2 can enhance coagulation through adsorption bridging, which leads to a stronger coagulation effect. Addition of in situ MnO2 after the coagulant facilitates the formation of aggregates of PAS with stronger coagulation efficiency and [Al(OH)3]m precipitate, thus enhancing humic acid adsorption in water.

Figure 9

Effect of dosing sequence and method on enhanced coagulation.

Figure 9

Effect of dosing sequence and method on enhanced coagulation.

Orthogonal array analysis of the enhanced coagulation parameters

The results of orthogonal test are summarized in Tables 1 and 2. In Table 2, Ki (i = 1, 2, 3) is the sum of humic acid removal by UV254 and CODMn methods at the same codified factor I; ki (i = 1, 2, 3) is the average value of humic acid removal at the same codified factor i; R is the difference value between the maximum of ki and the minimum ki, R = max (ki) − min (ki), which can identify the order of significant factors.

Table 1

A standard L9 (34) matrix

  Factors
 
 Codified factors
 
Numerical values
 
Experiments pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) 
20 10 
10 30 20 
15 40 30 
30 30 
10 40 10 
15 20 20 
40 20 
10 20 30 
15 30 10 
  Factors
 
 Codified factors
 
Numerical values
 
Experiments pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) 
20 10 
10 30 20 
15 40 30 
30 30 
10 40 10 
15 20 20 
40 20 
10 20 30 
15 30 10 
Table 2

A standard L9 (34) matrix analysis and humic acid removal

  Experiments Factors
 
Codified factors
 
Humic acid removal (%)
 
pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) UV254 CODMn 
 67.2 63.3 
 71.2 61.2 
 71.2 57.1 
 18.4 20.4 
 22.4 24.5 
 17.6 28.5 
 32.8 44.9 
 31.6 42.8 
 29.6 38.7 
Sum of Ki and average of ki by UV254 method K1 209.6 118.4 116.4 119.2   
K2 58.4 125.2 119.2 121.6   
K3 94.0 118.4 126.4 121.6   
k1 69.9 39.5 38.8 39.7   
k2 19.5 41.7 39.7 40.5   
k3 31.3 39.5 42.1 40.5   
 R 50.4 2.3 3.3 0.8   
Sum of Ki and average of ki by CODMn method K1 181.6 128.5 134.6 126.4   
K2 73.4 128.5 120.3 134.6   
K3 126.4 124.4 126.4 120.3   
k1 60.5 42.8 44.9 42.2   
k2 24.5 42.8 40.1 44.9   
k3 42.2 41.5 42.2 40.1   
 R 36.1 1.4 4.8 4.8   
  Experiments Factors
 
Codified factors
 
Humic acid removal (%)
 
pH In situ MnO2 dosage (mg/L) Slow stirring time (min) Particulate concentration (mg/L) UV254 CODMn 
 67.2 63.3 
 71.2 61.2 
 71.2 57.1 
 18.4 20.4 
 22.4 24.5 
 17.6 28.5 
 32.8 44.9 
 31.6 42.8 
 29.6 38.7 
Sum of Ki and average of ki by UV254 method K1 209.6 118.4 116.4 119.2   
K2 58.4 125.2 119.2 121.6   
K3 94.0 118.4 126.4 121.6   
k1 69.9 39.5 38.8 39.7   
k2 19.5 41.7 39.7 40.5   
k3 31.3 39.5 42.1 40.5   
 R 50.4 2.3 3.3 0.8   
Sum of Ki and average of ki by CODMn method K1 181.6 128.5 134.6 126.4   
K2 73.4 128.5 120.3 134.6   
K3 126.4 124.4 126.4 120.3   
k1 60.5 42.8 44.9 42.2   
k2 24.5 42.8 40.1 44.9   
k3 42.2 41.5 42.2 40.1   
 R 36.1 1.4 4.8 4.8   

It can be found from Table 2 that for the UV254 removal, the pH of the water was the greatest contributing factor, followed by the slow stirring time, then the in situ MnO2 dosage, and finally the concentration of particulates. The R value was 50.4 for the pH factor, which is significantly greater than the R values for the factors of in situ MnO2 dosage, slow speed stirring time, and concentration of particulates. This result indicates that pH is the dominant determinant of the UV254 removal efficiency. For the CODMn removal, the pH was also found to be the dominant factor, and the rest of the parameters are ranked in order of decreasing significance as follows: slow stirring time > particulate concentration > manganese dioxide dosage. Comparing the R values for each parameter, the R value for pH factor is high for both UV254 and CODMn removal. Furthermore, at pH = 4, the removal of UV254 and CODMn was far greater than at pH = 6.0 and pH = 8.0. This result is consistent with the results obtained through single-factor analysis. By comparing the K values of the orthogonal test, the following can be deduced: for maximal UV254 and CODMn removal, a pH of 4.0, in situ MnO2 dosage of 10 mg/L, slow stirring time of 20 or 40 min, and particulate concentration at 20 or 30 mg/L should be used.

CONCLUSIONS

In situ MnO2-enhanced coagulation can significantly improve humic acid removal efficiencies of PAS. At pH = 4, the enhancement of coagulation was most apparent when the dosage of PAS was 2 mg/L and the dosage of in situ MnO2 was 10 mg/L. Humic acid removal by UV254 and CODMn methods increased from 9.2% and 2.5% to 55.0% and 38.9%, respectively, upon the addition of in situ MnO2.

The effect of in situ MnO2 on coagulation enhancement is most affected by pH, whereas the effects of the other parameters rank in order of decreasing significance as follows: slow stirring time > particulate concentration > MnO2 dosage.

The use of in situ MnO2 as a coagulant additive enhances the coagulation effect of PAS, which can effectively improve the removal of humic acid. The dissolved aluminum and manganese content after treatment met the corresponding water standards, and there was no secondary pollution. This method is easy to use and economically sound. This work provides a scientific basis and reference for the removal of humic acid through enhanced coagulation.

ACKNOWLEDGEMENT

The authors thank the planning project on innovation and entrepreneurship training of China University for financial support.

REFERENCES

REFERENCES
Brattebø
H.
Ødegaard
H.
Halle
O.
1987
Ion exchange for the removal of humic acids in water treatment
.
Water Research
21
(
9
),
1045
1052
.
Fan
C. Z.
Lu
A. H.
Li
Y.
Wang
C. Q.
2010
Pretreatment of actual high-strength phenolic wastewater by manganese oxide method
.
Chemical Engineering Journal
160
(
1
),
20
26
.
Fang
J. P.
Zhang
P. Y.
Zeng
G. M.
2008
Research on humic acid adsorption by modified C linoptilolite
.
Chinawater & Wastewater
24
(
23
),
48
51
(in Chinese).
Fathy
N. A.
El-Shafey
S. E.
El-Shafey
O. I.
Mohamed
W. S.
2013
Oxidative degradation of RB19 dye by a novel γ-MnO2/MWCNT nanocomposite catalyst with H2O2
.
Journal of Environmental Chemical Engineering
1
(
4
),
858
864
.
Huang
T. L.
1999
DBPs precursor removal by enhanced coagulation in water purification process
.
ACTA Scientiae Circumstantiae
19
(
4
),
399
404
(
in Chinese
).
Kosmulski
M.
2014
The pH dependent surface charging and points of zero charge. VI. Update
.
Journal of Colloid and Interface Science
426
(
15
),
209
212
.
Ladavos
A. K.
Katsoulidis
A. P.
Iosifidis
A.
Triantafyllidis
K. S.
Pinnavaia
T. J.
Pomonis
P. J.
2012
The BET equation, the inflection points of N2 adsorption isotherms and the estimation of specific surface area of porous solids
.
Microporous and Mesoporous Materials
151
(
15
),
126
133
.
Leodopoulos
Ch
Doulia
D.
Gimouhopoulos
K.
Triantis
T. M.
2012
Single and simultaneous adsorption of methyl orange and humic acid onto bentonite
.
Applied Clay Science
70
(
12
),
84
90
.
Liang
H. F.
Ma
Z. C.
Zhang
J.
Hu
Z. J.
2005
Study on the removal of As(III) in water by the in situ manganese dioxide
.
Environmental Pollution Control
27
(
3
),
168
170
(in Chinese).
McKnight
D. M.
Aiken
G. R.
1998
Sources and age of aquatic humic
. In:
Aquatic Humic Substances: Ecology and Biogeochemistry
(
Hessen
D. O.
Tranvik
L. J.
, eds).
Ecological studies
,
133, 9–39
.
Savaji
K. V.
Niitsoo
O.
Couzis
A.
2014
Influence of particle/solid surface zeta potential on particle adsorption kinetics
.
Journal of Colloid and Interface Science
431
(
10
),
165
175
.
Song
H. C.
2012
Removal of Natural Organic Matter in Water with Modified Ultrafiltration Membrane
.
Shanghai JiaoTong University
,
Shanghai
(in Chinese)
.
Standard Methods for the Examination of Water and Wastewater
1998
20th edn
,
American Public Health Association/American Water Works Association/Water Environment Federation
.
Washington, DC
,
USA
.
Traversa
A.
Orazio
V. D.
Mezzapesa
G. N.
Bonifacio
E.
Farrag Senesi
K. N.
Brunetti
G.
2014
Chemical and spectroscopic characteristics of humic acids and dissolved organic matter along two Alfisol profiles
.
Chemosphere
111
(
9
),
184
194
.
Wan
S. L.
Ma
M. H.
Lv
L.
Qian
L. P.
Xu
S. Y.
Xue
Y.
Ma
Z. Z.
2014
Selective capture of thallium(I) ion from aqueous solutions by amorphous hydrous manganese dioxide
.
Chemical Engineering Journal
239
(
3
),
200
206
.
Wang
W. D.
Wang
W.
Fan
Q. H.
Wang
Y. B.
Qiao
Z. X.
Wang
X. C.
2014
Effects of UV radiation on humic acid coagulation characteristics in drinking water treatment processes
.
Chemical Engineering Journal
256
(
11
),
137
143
.
Wu
Y. Y.
Zhou
S. Q.
Qin
F. H.
2010
Removal of humic acids by oxidation and coagulation during Fenton treatment
.
Environmental Science
31
(
4
),
996
1000
(in Chinese).
Xie
J. K.
Wang
D. S.
Leeuwen
J.
Zhao
Y. M.
Xing
L. N.
Chow
C. W. K.
2012
pH modeling for maximum dissolved organic matter removal by enhanced coagulation
.
Journal of Environmental Sciences
24
(
2),
276
283
.
Xu
M.
Wang
H. J.
Lei
D.
Qu
D.
Zhai
Y. J.
Wang
Y. L.
2013
Removal of Pb(II) from aqueous solution by hydrous manganese dioxide: adsorption behavior and mechanism
.
Journal of Environmental Sciences
25
(
3
),
479
486
.