Treatment of Humic Acid in Drinking Water by Combining Potassium Manganate (Mn(VI)),Ferrous Sulfate,and Magnetic Ion Exchange

Abstract

This article summarizes a preliminary investigation of the treatment of humic acid (HA) in drinking water by a novel combination of potassium manganate, ferrous sulfate, and magnetic ion exchange (MIEX) resin. Manganate (MnO₄²⁻) is a stronger oxidant than permanganate, which has received little attention to date as a water treatment chemical, but in combination with ferrous sulfate (FeMnO) can offer a potentially advantageous and more economical means of achieving preoxidation objectives and in-situ coagulation by Fe(III) species. To enhance removal of organic substances, particularly low-molecular-weight and more hydrophilic compounds, subsequent treatment by MIEX was included as part of the overall process. Using model HA solutions of 5 mg/L dissolved organic carbon (DOC), a solution pH of 6.5, and a constant Fe:Mn molar ratio (2:1), the optimal FeMnO dose for organics (DOC) removal was 0.06 mM (as K₂MnO₄). At this optimal dose, and with 10 mL/L of MIEX, the DOC remaining after treatment was only 0.4 mg/L, representing an overall organics removal of 92%. Synchronous fluorescence spectra of solutions during the treatment stages showed a corresponding major reduction in fluorophore peaks with a near complete removal of fluorophore compounds in the ranges, 360 nm<λex<420 nm and λex>420 nm.

Introduction

Presence of elevated natural organic matter (NOM) in surface waters presents treatment challenges relating to taste, odor, color, and disinfection by-products. Since both the quantity and nature of NOM, particularly humic material, in source waters are changing, there is continuing interest in new approaches to treat such material in more efficient and cost-effective ways. The potential use of anion exchange for separating humic substances has been proposed previously (Fu and Symons, 1990) and considerable interest has been focused on the magnetic ion exchange (MIEX^®) resin process. This method of treatment is based on its strong base functionality capable of exchanging weak organic acid ions (phenolic and carboxylic acid groups), and the small particle size of the MIEX resin (150–180 μm) provides a high surface area for solute–resin interaction. The usual arrangement is for MIEX to be used as a pretreatment for, or in combination with, coagulation. Considerable investigation of the process has been reported and the results have shown that MIEX is effective for the removal of a wide range of NOM, particularly the mid-to-low-molecular-weight (MW) fractions (Fearing et al., 2004).

As an intermediate in the industrial synthesis of potassium permanganate (Mn(VII)), potassium manganate (Mn(VI)) is an oxidant with high oxidation strength, as indicated in Table 1. Manganate, like permanganate, can act as both an oxidant and a coagulant/adsorbent arising from the formation of insoluble manganese dioxide (Mn(IV)) from the chemical reduction of MnO₄²⁻. In addition, under conditions typical of water treatment, manganate will disproportionate to permanganate and manganese dioxide, providing similar complementary treatment mechanisms (Zhao et al., 2012). However, in practice, the coagulation/adsorption performance of manganese dioxide produced in-situ is not usually sufficient to achieve the degree of treatment required and the addition of a metal-ion coagulant is required.

Table 1.

Oxidation Reactions of MnO₄²⁻ and MnO₄⁻ Anions (Bard et al., 1985)

Condition	Oxidation reaction	Electrode potential^a, E^o(V)
MnO₄²⁻
pH<7	MnO₄²⁻+4H⁺+2e⁻⇌ MnO₂+2H₂O	2.27
	MnO₄²⁻+8H⁺+4e⁻⇌ Mn²⁺+4H₂O	1.74
pH>7	MnO₄²⁻+2H₂O+2e⁻⇌ MnO₂+4OH⁻	0.62
MnO₄⁻
pH<7	MnO₄⁻+4H⁺+3e⁻⇌ MnO₂+2H₂O	1.70
	MnO₄⁻+8H⁺+5e⁻⇌Mn²⁺+4H₂O	1.51
pH>7	MnO₄⁻+2H₂O+3e⁻⇌MnO₂+4OH⁻	0.60

Standard state: 25°C, 100 kPa.

As a preoxidant, manganate has received little attention in regard to water treatment, but its application with ferrous sulfate offers a potentially economically favorable combination of oxidant and coagulant (the latter produced from the in-situ conversion of Fe(II) to Fe(III)). This article summarizes a preliminary investigation of combining manganate, ferrous sulfate, and MIEX for the removal of humic acid (HA). The principal objectives were to evaluate the overall extent of organics removal and the relative contribution of MIEX.

Materials and Methods

HA and potassium manganate were obtained as commercial reagent grade in solid form from Sigma-Aldrich Company Ltd. Ferrous sulfate was also obtained as commercial reagent grade from BDH. All other reagents and chemicals used in this study were analytical grade, purchased from chemical suppliers in the United Kingdom, and used as supplied. HA solutions were prepared in deionized water to the desired dissolved organic carbon (DOC) concentration of ∼5 mgC/L, as determined by a laboratory TOC instrument (Shimadzu). All treatment experiments were conducted using a Gator Jar, which is a calibrated 2 L square acrylic vessel with paddle stirring, built to a standard design recommended for coagulation testing (AWWA, 2000).

MIEX resin was received in water from Orica Australia Pty Ltd., and used either as received or after washing and regeneration; the latter was carried out according to the supplier's recommended method. Adsorption kinetic tests with MIEX were conducted to determine the effects of resin concentration and contact time on the removal of HA from the test water. In these tests, the Gator jar was filled with l L of test water, dosed with MIEX, and mixed at 150 rpm (G=216 s⁻¹) for 40 min. The volume of wet MIEX resin added was quantified using a measuring cylinder. Samples were withdrawn from the Gator jar at predetermined time intervals and filtered (0.45 μm membrane) before DOC measurement.

Treatment of HA solutions was by the sequential addition of K₂MnO₄, FeSO₄, and MIEX. Initially, K₂MnO₄ reacts with HA either directly or indirectly by MnO₄⁻ and MnO₂ arising from MnO₄²⁻ disproportionation. Subsequently, the addition of FeSO₄ enables the reaction with the remaining Mn oxidants to produce Fe(III) hydrolysis species, such as insoluble Fe(OH)₃, by the following reaction: MnO₄²⁻+2Fe²⁺+4H₂O=MnO₂+2Fe(OH)₃+2H⁺. For convenience, the treatment combination of K₂MnO₄ and FeSO₄ is referred to as FeMnO, and the dosage of FeMnO in the experiments is expressed in mM as K₂MnO₄, based on a theoretical Fe:Mn molar ratio of 2:1 (as per the above reaction). For the treatment tests, an appropriate quantity of K₂MnO₄ (0.03 M) was added with stirring (250 rpm; G=439 s⁻¹) to the HA model water in the Gator jar with HCl added for pH correction. After a reaction time of 40 min, a stoichiometric amount of FeSO₄ (0.06 M, equivalent to Fe:Mn=2:1) was added to the solution in the Gator jar with stirring at 250 rpm for 1 min and at 150 rpm (G=216 s⁻¹) for 4 min, followed by 10 mL/L of MIEX resin added under stirring at 150 rpm for 20 min. The series of treatment tests described in this study were performed in duplicate at pH 6.5 (representative of typical water treatment) and under ambient conditions in an environmentally controlled laboratory (room temperature 20°C±1°C); the variation in duplicate results was within 5%.

Synchronous fluorescence spectroscopy has been used widely for the characterization and identification of NOM from various origins (Matilainen et al., 2011). Selected samples were measured for fluorescence excitation–emission matrices using a JASCO FP-6500 fluorescence spectrometer at ambient pH and room temperature. A constant wavelength difference (Δλ) of 60 nm was maintained between excitation and emission wavelengths over an excitation wavelength range of 200–600 nm. The fluorescence intensity (FI) is expressed in absorbance units.

Results and Discussion

HA removal by MIEX

An initial series of tests were undertaken to establish an optimal quantity of MIEX to use for the subsequent treatment tests. The results are summarized in Fig. 1 and show a systematic reduction in organic carbon with the MIEX concentration, reflecting the increasing surface area (exchange sites) of the MIEX in the solution. The DOC concentration followed nonlinear kinetics with a decreasing rate of change with time, and the extent of adsorption was largely complete by 20 min (∼77–92% of ultimate DOC removal). Since there was relatively little additional benefit in increasing the contact time (an additional 5–15% DOC removal for a further 20 min) and increasing the MIEX concentration from 10 to 12 mL/L, the subsequent tests employed a contact time of 20 min and a constant MIEX concentration of 10 mL/L. These results are consistent with the findings of other studies, such as Humbert et al. (2007) (10 mL/L MIEX) and Singer and Bilyk (2002) (20–30 min contact time), and as used in practice (Slunjski et al., 2000).

FIG. 1.

Effect of MIEX concentration and contact time on DOC removal (pH 6.5). DOC, dissolved organic carbon; MIEX, magnetic ion exchange.

Combined FeMnO and MIEX treatment

A summary of the results showing the treatment performance of FeMnO and MIEX for DOC removal is given in Table 2. It was found that at FeMnO doses of <0.030 mM, there was little detectable change in the DOC by FeMnO treatment, indicating the absence of any significant coagulation by Fe species, and only a modest reduction in the DOC by the MIEX at 0.030 mM. However, at FeMnO doses of 0.06 mM and greater, the reduction in DOC by FeMnO was consistently high, at about 70%, and corresponded to a Fe dose stoichiometry of ∼1.6 mg Fe per mg C removed, which is less than with conventional Fe salts (∼1.8) (Eikebrokk, 1999), indicating a more efficient use of Fe for coagulation. Subsequent MIEX treatment following FeMnO treatment produced further DOC reduction. It was evident that the maximum overall reduction in DOC by the combined FeMnO-MIEX process was ∼92%, corresponding to a final DOC concentration of only 0.4 mg/L. This degree of HA treatment was greater than that found previously by the authors with conventional ferric chloride under more favorable pH conditions (pH 5) (Graham et al., 2010).

Table 2.

Variation of Humic Acid Concentration with FeMnO Dose (and % DOC Reduction) by FeMnO and FeMnO-MIEX Treatments (Initial HA =5 mgC/L)

	After FeMnO		After FeMnO-MIEX
FeMnO dose (mM)^a	DOC (mgC/L)	% Reduction	DOC (mgC/L)	% Reduction
0.03	∼5	0	3.2	36
0.06	1.4	72	0.4	92
0.18	1.5	70	0.8	84
0.27	1.6	68	1.1	78

As K₂MnO₄, where 1 mM FeMnO≡2 mM FeSO₄+1 mM K₂MnO₄.

DOC, dissolved organic carbon; HA, humic acid; MIEX, magnetic ion exchange.

The optimal FeMnO dose of 0.06 mM, corresponding to 11.8 mg/L K₂MnO₄ and 18.2 mg/L FeSO₄, is clearly evident in Table 2. Although higher FeMnO doses achieved approximately the same level of DOC removal, the subsequent removal by MIEX deteriorated with an increasing FeMnO dose. The reason for the deteriorating MIEX performance is unclear, but it is believed to be related to the nature and extent of structural changes in the organics (e.g., MW distribution, hydrophilicity, charge densities) by the FeMnO treatment, which adversely affects the anionic exchange interactions.

Synchronous fluorescence spectral analysis

Synchronous fluorescence spectral analysis was carried out on samples of the HA solution subjected to the combined FeMnO-MIEX treatment at the optimal dose of 0.06 mM, and the results are summarized in Fig. 2. The fluorescence spectrum for the HA solution at λex>300 nm corresponds to the pool of hydrophilic and hydrophobic, humic-type fluorophores, and four main peaks were observed as follows: peak I (λex=330–350 nm), peak II (λex=400–420 nm), peak III (λex=435–450 nm), and peak IV (λex=500–525 nm); peak III had the greatest FI, and the general fluorescence spectrum was similar to those reported previously for soil HA (Trubetskaya et al., 2002).

FIG. 2.

Synchronous fluorescence spectra (Δλ=60 nm; F.I., fluorescence intensity [AU, absorbance units]) at different stages of treatment.

Relative changes in the fluorescence spectra with the sequence of treatment, namely manganate oxidation, Fe(III) coagulation following manganate oxidation, and subsequent MIEX treatment, provide an indication of overall changes to the matrix of organic substances that are sensitive to fluorescence (Fig. 2). First, manganate pretreatment appeared to increase the FI of all peaks (6%, 11%, 11%, and 6%, respectively, for peak I to peak IV), indicating that a modest degree of structural changes had occurred to a broad range of the organic macromolecules. With subsequent coagulation (HA+FeMnO), the FI decreased greatly across the whole range of excitation wavelengths with the FI values of the four peaks decreasing by 20.3%, 62.9%, 52.5%, and 83.1% (relative to untreated HA). Further reduction in FI values was evident after the MIEX treatment with the apparent loss of peaks II and IV. Overall, the total reduction in FI values by the combined FeMnO-MIEX process was 32.7%, 91.4%, 68.7%, and 97.4% (relative to untreated HA), and the results indicated that both steps of the treatment were effective at removing a broad range of fluorophore compounds. Thus, for the fully treated water with a DOC of ∼0.4 mg/L, there was a near complete removal of fluorophore compounds in the ranges, 360 nm<λex<420 nm and λex>420 nm.

Conclusions

The novel combination of manganate (MnO₄²⁻) and ferrous sulfate, FeMnO, provides treatment by oxidation and coagulation, and together with the MIEX process, as a subsequent polishing step, can achieve a high degree of HA removal. The results have shown that at pH 6.5 the removal of organics (as DOC) by FeMnO at an optimal dose (0.06 mM, as K₂MnO₄) was about 70% and corresponded to a Fe dose stoichiometry of ∼1.6 mg Fe per mg C removed; in both measures the performance of FeMnO was comparable or superior to conventional Fe coagulants. With subsequent treatment by 10 mL/L of MIEX, the DOC was reduced further to only 0.4 mg/L, representing an overall organics removal by FeMnO-MIEX of 92%. Associated with the major reductions in the overall organic content were similar reductions in fluorophore compounds in general, and a near complete removal of fluorophore compounds in the excitation wavelength ranges, 360 nm<λex<420 nm and λex>420 nm.

Footnotes

Acknowledgments

The authors acknowledge the support of the China Scholarship Council and the assistance of Orica Australia Pty Ltd.

Author Disclosure Statement

No competing financial interests exist.

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