ADSORPTION AND OXIDATION OF MICROPOLLUATANTS BY MANGANESE OXIDE

Conclusion ...81 Chapter V: Oxidative transformation of triketone herbicide, sulcotrione, by manganese oxide: kinetic, transformation products and impact of natural organic matter ...83

THESE

Pour l’obtention du Grade de DOCTEUR DE L’UNIVERSITE DE POITIERS (ECOLE NATIONALE SUPERIEURE d’INGENIEURS de POITIERS)

(Diplơme National - Arrêté du 7 aỏt 2006) Ecole Doctorale : Sciences pour l’Environnement Gay Lussac

Secteur de Recherche : CHIMIE ET MICROBIOLOGIE DE L’EAU

ACKNOWLEDGEMENTS

I wish to thanks my advisors Hervé GALLARD and Jérôme LABANOWSKI for directing their attentions forward to my work and welfare, and for allowing me freedom and flexibility in research Working with them is my honor Their advices helped me to develop scientific skills and will be useful for my future career

I would also like to thank those who encouraged me to enter this work: my father, my mother,

my wife and my little daughter, my whole family Their supports are always the best thing for me

A number of fellow students and associates contributed to this work trough their supports, advice and friendship: Alice TAWK, Virginie SIMON, Pamela ABDALLAH, Rose-Michelle SMITH, Amal YOUSSOUF IBRAHIM, and many others

I wish to thank Philippe BEHRA, Emmanuel GUILLON, Patrick MAZELLIER, Sylvain OUILLON They kindly served on my examining committees Their comments and enthusiasm were appreciated

The staff of equip E1-Eaux Géochimie Santé and Platform APTEN went to great lengths to assist with whatever problems arose, especially Audrey ALLAVENA, Sylvie LIU, Marie DEBORDE

Financial support from Vietnamese project 911 is gratefully appreciated

CONTENTS

List of Figures vi

List of Tables x

General introduction 1

Chapter I: Literature review 4

I.1 Fate of organic micropollutants in the environment 4

I.2 Geochemistry and Manganese oxides mineralogy 6

I.2.1 Tunnel structure 8

I.2.2 Layer structure 9

I.2.2.1.Birnessite 9

I.2.2.2.Others layer structures 11

I.2.3 Other Mn oxides minerals 11

I.3 Interactions between Manganese oxides and organic compounds 12

I.3.1 Reaction Models 13

I.3.1.1.Electron-transfer with bond-formation between metal sites and organic reductant .14

I.3.1.2.Electron-transfer through formation of outer-sphere complex between metal sites and organic reductant 14

I.3.2 Transformation reactions 15

I.3.2.1.Reactions with model organic compounds 15

I.3.2.1.1.Phenols 16

I.3.2.1.2.Anilines 17

I.3.2.1.3.Low molecular weight carboxylic acids 19

I.3.2.2.Reactions with organic contaminants 19

I.3.2.2.1.Endocrine discruptors 19

I.3.2.2.2.Antibacterial agents and antibiotics 21

I.3.2.2.3.Other pharmaceuticals and industrial contaminants 25

I.3.2.3.Reactions with natural organic matter 26

I.3.3 Kinetic aspects and influence of reaction conditions 28

I.3.3.1.Kinetics and reaction order 28

I.3.3.2.Effect of pH 30

I.3.3.3.Effect of mineral constituents 31

I.3.4 The role of NOM in NOM – micropollutants – MnO2 system 32

I.3.4.1.Interactions between NOM and micropollutants 32

I.3.4.2.Effect of NOM on reaction of micropollutants by MnO2 34

I.4 Application in water treatment and decontamination of polluted sites 36

Chapter II: Material and methods and preliminary study 39

II.1 Manganese dioxide and reagents 39

II.1.1 Preparation of Manganese dioxide and characterization 39

II.1.2 Natural organic matters and micropollutants 40

II.2 Protocols 43

II.2.1 Kinetic experiments with MnO2 suspensions 43

II.2.2 Sorption experiments of NIS by NOM 44

II.3 Analysis of micropollutants 44

II.4 Identification of transformation products 45

II.5 Natural Organic matter characterization by Dissolved organic carbon and UV absorbance analysis and high-pressure size exclusion chromatography 46

II.6 Results from preliminary study 49

Chapter III: Adsorption and oxidation of the anthelmintic drug Niclosamide by birnessite 50 III.1 Introduction 50

III.2 Results and discussion 52

III.3 Conclusion 68

Chapter IV: Sorption and transformation of Pyrantel pamoate by synthetic birnessite70 IV.1 Introduction 70

IV.2 Results and discussion 71

IV.3 Conclusion 81

Chapter V: Oxidative transformation of triketone herbicide, sulcotrione, by manganese oxide: kinetic, transformation products and impact of natural organic matter 83

V.1 Introduction 83

V.2 Results and discussion 85

V.3 Conclusion 99

Chapter VI: Degradation of Sulfamethoxypyridazine and cross-coupling reactions mediated by MnO2 100

VI.1 Introduction 100

VI.2 Results and discussion 102

VI.3 Conclusion 118

GENERAL CONCLUSION 120

List of Figures Chapter I

Figure I.1 – Schematic diagram highlighting potential sources and pathways for groundwater

pollution by micropollutants Adapted from [18] 4

Figure I.2 – Cristalline structure of (A) Pyrolusite, (B) Ramsdellite, (C) Hollandite, (D) Romanecdite, and (E) Todorokite [42] 8

Figure I.3 – Cristal structure of (A) Lithiophorite, (B) Chalcophanite, (C) Na-rich Birnessite like [42] 11

Figure I.4 – General view of transformation of organic compound by MnO2 13

Figure I.5 – Major reactions involved in the oxidation of phenols by δ-MnO2 and accumulation of reduced Mn species on the mineral surface The final aqueous products are shown in blue [32] 16

Figure I.6 – Postulated mechanism for oxidative coupling of aniline by reactions with manganese oxide The reaction proceeds from a cation radical through coupling products, which then undergo further oxidation Adapted from [77] 18

Figure I.7 – Transformation of citrate by MnO2 19

Figure I.8 – Proposed reaction pathway of oxidation of triclosan by MnO2 [63] 21

Figure I.9 – Proposed Reaction Scheme for Oxidation of FQs by MnO2 [106] 23

Figure I.10 – Proposed mechanism for the oxidative transformation of DCF by manganese oxide [112] 26

Figure I.11 – Cross-coupling of sulfamethazine with syringic acid [41] 27

Figure I.12 – Impact of NOM on oxidation and hydrolysis of micropollutant by MnO2 Adapted from [119] 34

Chapter II

Figure II.1 – Zeta potential of Manganese oxide suspension for pH range 2.5 – 6.0 and ionic

strengths of 1 and 10 mM NaNO3 40 Figure II.2 – HPSEC calibration curve obtained with polystyrene sulfonate standards 47 Figure II.3 – Typical HPSEC chromatograms for the three different NOM extracts 48

Chapter III

Figure III.1 – Concentration profiles of NIS after centrifugation or reduction by ascorbic acid

([NIS]o = 130 nM , 10 mM acetate buffer pH 5.0) 53 Figure III.2 – Proposed pathway for the catalytic hydrolysis of NIS by MnO2 55 Figure III.3 – Linear adsorption isotherms of Niclosamide at pH 4.0, 4.5, 5.0 and 5.5 57 Figure III.4 – First-order kinetic representation of NIS transformation for different MnO2

concentrations (pH 5.0, 10 mM acetate buffer) 59 Figure III.5 – First-order dependence with respect to MnO2 at pH 5.0 60 Figure III.6 – Dependence of k with respect to H+ (10 mM acetate buffer, 130 nM [NIS]0 and 100

µM [MnO2]0) 62 Figure III.7 – Evolution of NIS concentration in the absence and presence of CR-NOM 64 Figure III.8 – Effect of CR-NOM concentration on (a) observed rate constants of NIS transformation and (b) adsorption isotherms (pH 5.0, 175 µM MnO2 and 130 nM NIS) 65 Figure III.9 – Changes in 2D fluorescence spectra of CR-NOM with increasing NIS concentration 66 Figure III.10 – Application of Stern-Volmer model for interaction between NIS and NOM (pH 5.0±0.1 and 22±1 °C, 2.2 mg-C L-1, 255/400 nm excitation/emission wavelengths) 67 Figure III.11 – HPSEC/UV chromatograms of 2.0 mgC L-1 NOM before and after contact with 500

µM MnO2 at pH 5.0 68

Chapter IV

Figure IV.1 – Behaviour of Pyrantel Pamoate in presence of birnessite ([MnO]2 = 500 µM, [PMA]0

= 260 nM, [PYR]0 = 260 nM 10 mM acetate buffer pH 5.0) 72

Figure IV.2 – Influence of initial concentrations of MnO2 on removal of PMA ([PMA]0 = 260 nM, pH 5.0, 10 mM acetate buffer) 73

Figure IV.3 – First-order dependence with respect to MnO2 73

Figure IV.4 – Dependence of kobs with respect to H+ concentration 75

Figure IV.5 – Effect of SR-NOM on oxidation of PMA by MnO2 76

Figure IV.6 – Proposed reaction pathway for the degradation of PMA by MnO2 77

Figure IV.7 – Concentration profile of PYR in presence of MnO2 after quenching by ascorbic acid or centrifugation 79

Figure IV.8 – Sorption isotherm of PYR by MnO2 at pH 5.0 80

Figure IV.9 – Effect of NOM on sorption kinetic of PYR by MnO2 ([MnO2]0 = 1 mM, pH 5.0, [NOM]0 = 0.5 mgC L-1) 81

Chapter V Figure V.1 – Degradation kinetics of triketone herbicides tembotrione and sulcotrione by MnO2 85

Figure V.2 – Influence of quenching mode on kinetics of SCT oxidation by MnO2 ([MnO2]o = 217 µM; [SCT]o = 6.3 µM; pH 5.0 10 mM acetate buffer) 86

Figure V.3 – First order representation of SCT degradation for different MnO2 concentrations ([SCT]o = 6.3 µM; pH 5.0 10 mM acetate buffer) 87

Figure V.4 – First-order dependence with respect to MnO2 89

Figure V.5 – Dependence of k with respect to H+ 90

Figure V.6 – Effect of NOMs isolates on SCT transformation by MnO2 91

Figure V.7 – Effect of NOM concentration on observed rate constants of SCT transformation 92

Figure V.9 – Reaction pathway for SCT transformation by MnO2 98

Chapter VI Figure VI.1 – Influence of quenching mode on the time profile of SMP in presence of MnO2 ([MnO2]0 = 250 µM; [SCT]0 = 1.2 µM; pH 5.0 10 mM acetate buffer) 102

Figure VI.2 – Logarithmic representation of SMP transformation by birnessite showing deviation from first order ([SMP]o = 1.1 µM; pH 5.0 10 mM acetate buffer) 103

Figure VI.3 – Determination of the order of the initial reaction rate of SMP degradation 106

Figure VI.4 – Retarded model fit for reaction of SMP oxidation by MnO2 109

Figure VI.5 – Effect of different concentrations of CR-NOM on transformation of SMP by MnO2 (The curves are the retarded model fit) 111

Figure VI.6 – Effect of syringic additions on SMP degradation by MnO2 113

Figure VI.7 – Proposed pathway for SMP transformation by MnO2 117

Figure VI.8 – Cross-coupling reaction between SMP and SYR mediated by MnO2 118

List of Tables

Chapter I

Table I.1 – Nomenclature and chemical formula of some manganese oxides [42] 7 Table I.2 – Proposed methods for synthesis of Birnessite 10 Table I.3 – Summary of model phenols, anilines and low molecular weight (LMW) acids used to dissolve manganese oxides [33], [75]–[77] 15 Table I.4 – Reaction order for the transformation of various organic compounds by MnO2 29

Chapter II

Table II.1 – Physico-chemical properties of micropollutants tested in this study 42 Table II.2 – Experimental conditions used in this study 43 Table II.3 – Conditions used for the analysis of micropollutants by HPLC/UV 45

Chapter III

Table III.1 – MS spectra of NIS and its transformation products 56 Table III.2 – Pseudo-First order kinetic model constant for the transformation of NIS by MnO2 in 10mM acetate buffer 60

Chapter IV

Table IV.1 – Pseudo-first order kinetic rate constants for the transformation of PMA by MnO2 in 10

mM acetate buffer at pH 5.0 74 Table IV.2 – MS spectra of PMA and its transformation products 78

Chapter V

Table V.1 – Pseudo-first order kinetic constant for the transformation of SCT by MnO2 in 10 mM

acetate buffer 88

Table V.2 – Effect of NOM on pseudo-first order kinetic model constant for the transformation of SCT by MnO2 at pH 5.0 (430 µM MnO2, 6.3 µM SCT) 92

Table V.3 – DOC values in mgC L-1 of NOM solutions before and after incubation with MnO2 (pH 5.0, 500 µM MnO2) 94

Table V.4 – Tranformation products formed from degradation of SCT by MnO2 96

Chapter VI Table VI.1 – Initial rate for the transformation of SMP by MnO2 in 10 mM acetate buffer 107

Table VI.2 – Retarded model parameters for the reaction of SMP with MnO2 110

Table VI.3 – Retarded model parameters for the reaction of SMP with MnO2 111

Table VI.4 – Tranformation products formed by SMP oxidation by MnO2 114

Table VI.5 – Cross–coupling products formed from reaction between SMP and SYR mediated by MnO2 115

General introduction

Manganese (Mn) oxides are naturally present in soils and sediments These oxides play an important role in controlling biogeochemical cycles Interactions of manganese oxides with organic and inorganic compounds have been the subject of a number of environmental studies Their adsorptive and oxidizing properties allow interactions with organic compounds Environmental studies conducted with synthetic manganese oxides show that these manganese oxides can react with model organic compounds such as phenols and anilines Recent studies have shown that manganese oxides can contribute to the elimination of organic contaminants such as pesticides, hormones or antibacterial agents in the environment These oxidative processes involving Mn oxides may constitute an important abiotic degradative pathway for organic compounds in subsurface environment Additionally some authors have discussed the use of manganese oxides as a solution for the treatment of emerging contaminants present in waste water

or in polluted sites

A large number of synthetic organic compounds are used by modern society for a wide variety of purposes including the production and preservation of food, industrial manufacturing processes, and human and animal healthcare The presence of these organic compounds has been widely reported in many ecosystems and their reactivity in the environment has been the subject of many studies Because of most of these compounds are environmentally present at low concentration ranges from pg L-1 to ng L-1, they are often named micropollutant The presence and the concentrations of organic micropollutants are important in the quality evaluation of surface and ground waters These pollutants can be persistent, bioresistant compounds or they may undergo biotransformation, photo-oxidation or/and hydrolysis In some cases, transformation products can be more toxic than the parent compound Similar studies have been conducted to evaluate their behaviour during water treatment The use of strong oxidants such as ozone and adsorbents such as activated carbon can remove micropollutant residues during production of

cannot always ensure a complete removal of these compounds Effluent discharges from waste water treatment plants, therefore, are the main source of contamination by micropollutants residues Tertiary treatments are currently being studied to reduce this contamination Potential treatments are ozone, activated carbon or membrane processes such as nanofiltration or reverse osmosis when water reuse is considered The implementation of these treatments is costly and is not considered without new regulations for waste water discharges Alternative methods are also being investigated like the use of advanced oxidation processes or oxidants such as ferrate Some studies also suggested the use of manganese oxides, alone or in combination with biological process

The main objective of this thesis is to investigate the interactions of selected micropollutants towards synthetic manganese oxide birnessite The influence of experimental conditions such as the presence of natural organic matter (NOM) was also evaluated The interactions between natural organic matter and oxides were studied because NOM are ubiquitous in the environment and are present in higher concentrations than micropollutants A preliminary study was performed

in order to select molecules that will react with manganese oxides Four molecules were then choosen for kinetic study and identification of transformation products

The manuscript is organized into six chapters

The first chapter is a literature review This section presents the geochemistry of manganese oxides and the state of the art describing the transformation of organic compounds by manganese oxide with a focus on organic micropollutants

The second chapter described the analytical and experimental methods employed over the study

The next four chapters are written as projects of publication

The third chapter focuses on the adsorption and degradation of the hydrophobic antihelminthic drug Niclosamide (NIS) by MnO2 The aim was to investigate the role of NOM as competitor for oxidation and adsorption but also the ability of NOM as sorbent of hydrophobic micropollutants in presence of a mineral oxide

The fouth chapter presents the reactivity of birnessite towards the deworming agent, pyrantel pamoate

The fifth chapter presents study on the transformation of triketone herbicide, Sulcotrione (SCT) by manganese oxide The oxidation kinetics of SCT was followed for different initial conditions and transformation products were identified by LC/MSn Degradation mechanism of SCT has been proposed

The sixth chapter deals with the transformation of sulfonamide antibiotic, sulfamethoxypyridazine (SMP), by manganese oxide Transformation products were identified by LC/MSn Degradation mechanism of SMP has been proposed A retarded kinetic model including the influence of the natural organic matter acting as a competitor in the system is presented The cross-coupling reaction between SMP and Syringic acid, a model NOM constituent, mediated by MnO2 was also presented

Finally, this paper ends with a general conclusion summarizing the essential advances obtained in our study

Chapter I: Literature review I.1 Fate of organic micropollutants in the environment

In the last few decades, the occurrence and fate of organic micropollutants in water bodies including veterinary drugs, endocrine disrupting chemicals, pharmaceuticals, personal care products and antibiotics have been extensively studied due to their possible adverse effects to wildlife and humans [1]–[5] It is now established that these compounds enter the environment from a number of sources: livestock activities including waste lagoons and manure application to soil [6]–[8]; subsurface storage of household and industrial wastes [9] [10], as well as indirectly through the process of groundwater–surface water exchange [11]; wastewater effluents from municipal treatment plants [12]–[15]; septic tanks [16], [17]; hospital effluents [1] All described pathways were illustrated in Figure I.1

Figure I.1 – Schematic diagram highlighting potential sources and pathways for groundwater

pollution by micropollutants Adapted from [18]

Once a micropollutant is released into the environment, key physico-chemical properties – such as water solubility and volatility – will influence its behavior In surface water, the main elimination processes are hydrolysis, biodegradation, sorption, and photodegradation

Organic micropollutants can be degraded through biotic transformations in soils and water Generally, these processes reduce the potency of the drugs However, some transformation products have similar toxicity to their parent compound [19], [20] Bacteria and fungi are the two major groups of microorganisms responsible of organic compounds biotransformation Fungi are particularly important in soils, but do not usually play an important role in the aquatic environment Therefore, in sewage treatment plants, surface and ground water bacteria are assumed to be responsible for most of the biodegradation processes Even though microbial degradation is enhanced in waste water treatment systems due to the higher bacterial density, recalcitrant micropollutants were detected in secondary waste water effluents and polishing treatments such as ozone and activated carbon are proposed to fully eliminate these compounds and reduced the toxicity of effluents [3] For example, more than 20 antibiotics representing the most important groups of antibiotics were found not to be readily biodegradable [1], [21], [22] Sorption may also have an impact on the spread and bio-availability of micropollutants in the environment [23]–[29] Sorption coefficients (Kd) for micropollutants in soils and sediments range from 0.2 for chloramphenicol in marine sediment to 5610 L kg-1 for enrofloxacin in soil [6] Values vary considerably for a given compound in different soils [6] Mechanisms other than hydrophobic partitioning, such as cation exchange, cation bridging at clay surfaces, surface complexation and hydrogen bonding play a role in the sorption of organic micropollutants [25] Also, the sorption of selected micropollutants may depend heavily on pH and ionic strength [26]

The highest values of Kd,solid ranged between 70 and 5000 L kg-1 for tetracycline and quinolone carboxylic acid antibiotics According to a classification of pesticide mobility in soil [25], these

micropollutants can be considered to be immobile Intermediate Kd,solid for avermectin, tylosin, and efrotomycin ranged between 7 and 300 L kg-1 Low values of Kd,solid ranging between 0.2 and 2 L

sulfamethazine, sulfathiazole, metronidazole, chloramphenicol The latter two groups of micropollutants can be considered to be low to slightly mobile (50 L kg-1 > Kd,solid > 5 L kg-1) and

medium to highly mobile (Kd,solid < 5 L kg-1), respectively [25]

Many micropollutants are also expected to be photoactive because many of these compounds feature aromatic rings, heteroatoms, and other functional groups that can either absorb solar radiation or react with photogenerated transient species in natural waters Thus, photolysis and photochemical processes should be considered as important removal mechanisms of micropollutants in surface waters [30], [31]

In recent years, the sorption and abiotic transformation of organic micropollutants by metal oxides has been in the scope of several studies [32]–[34] The ability of manganese oxides to participate in the fate of micropollutants in the environment has been extensively studied [35]–[37] The use of manganese oxides in water treatment to remove organic micropollutants [38], [39] and to decontaminate polluted soil [40], [41] has then be proposed From this point of view, the reactivity between manganese oxides and organic micropollutants will be the object of this literature review General information on the geochemistry and mineralogy of manganese oxides will first be given in the next paragraphs

I.2 Geochemistry and Manganese oxides mineralogy

Manganese is one of the most abundant elements in the earth and the second transition metal after iron present in the earth’s crust [42]–[44] Humanity has started using manganese for thousand years as pigment in cave and to clarify glass Nowadays manganese is mostly used as catalysts and for the production of battery [45]

There are more than 30 Mn oxides/hydroxide minerals, and many of them occur abundantly

in a wide variety of geological settings In addition to being important as ores of Mn metal, they also play an active role in the environmental geochemistry at the Earth’s surface Manganese oxides and hydroxides are important constituents of the soil and sediments, and because they are highly chemically active and strong scavengers of heavy metals, they exert considerable

influences on the composition and chemical behavior of the sediments, soils and associated aqueous systems They participate in numerous chemical reactions with constituents of the soil and/or groundwater On the other hand, manganese is an essential nutrient for plants and animals Manganese participates in the synthesis of enzyme and is a cofactor involved in carbohydrate and nitrogen metabolism [46] It was reported that at high concentrations manganese is toxic to plant growth [47]

Many studies were carried out to understand the crystallography of mineral phases and the geochemistry of manganese [42]–[44], [48] In the environment, manganese exists in three different oxidation states: +2, +3, +4 The oxides can be regrouped in three types: tunnel, layer structure and others The basic structural units for Mn oxides are MnO6 octahedra Listed in Table I.1 are the informations for some of the most important Mn oxide minerals and their chemical formula

Table I.1 – Nomenclature and chemical formula of some manganese oxides [42]

Lithiophorite LiAl2(Mn24+,Mn3+)O6(OH)6 Layer

ρ-MnO2

Romanechite Ba0.66Mn4+3.68Mn3+1.32O10x1.34H2O Tunnel

Todorokite (Na,Ca,K)x(Mn4+,Mn3+)6O12.3.5H2O Tunnel

I.2.1 Tunnel structure

The tunnel Mn oxides are constructed of single, double, triple chains of edge-sharing MnO6octahedra, and the chains share corners with each other to produce framework that have tunnel with square or rectangular cross section The larger tunnels are partially filled with water molecules and/or cations Pyrolusite, Ramsdellite, Hollandite, Romanecdite, and Todorokite belongs to this group of Mn oxides Figure I.2 illustrated different types of crystalline tunnel structure

Figure I.2 – Cristalline structure of (A) Pyrolusite, (B) Ramsdellite, (C) Hollandite, (D)

Romanecdite, and (E) Todorokite [42]

Hollandite group includes hollandite, cryptomelane, coronadite, and manjiroite This group

is constructed of double chains of edge-sharing MnO6 octahedra, but they are linked in such a way

as to form tunnels with square cross sections and two octahedra for each side (Figure I.2) The tunnels are partially filled with large uni- or divalent cations and, in some cases, water molecule These manganese oxides differ by the presence of cation K, Ba and Pb inside the structure [49] Cryptomelane is generally synthesized by dropwise addition of HCl to KMnO4 Then the solution

is washed, dried, and iginited at 400 oC for 60 hours, and again washed with water This gave cryptomelane containing 7.2% K, with a surface area of 58 m2 g-1 [50]

Pyrolusite is the most stable form of Mn oxide minerals with a tunnel structure In pyrolusite (β-MnO2), single chains of edge-sharing MnO6 octahedra share corners with neighboring chains to form a framework structure containing tunnels with square cross sections Each square cross section is one by one octahedron (1x1) on a side (Figure I.2) [42]

The ramsdellite β-MnO2 is one rare structure of MnO2 In the ramsdellite structure the MnO6

octahedra are linked into double chains, each consisting of two adjacent single chains that share octahedral edges The double chains, in turn, link corners with each other to form a framework having tunnels with rectangular shaped cross sections that are 1x2 octahedra on a side (Figure I.2) [42], [44], [50]

Romanechite is a valuable ore of manganese, which is used in steelmaking The romanechite structure is constructed of double and triple chains of edge-sharing MnO6 octahedra that link to form large tunnels with rectangular cross sections, measuring two by three octahedra (Figure I.2) The tunnels are filled with Ba cations and water molecules in a 1:2 ratio, and the charges on the tunnel cations are balanced by substitution of Mn3+ for some of the Mn4+ [42], [51], [52]

Todorokite is a rare complex hydrous manganese oxide mineral Todorokite is made up of MnO6 octahedra that share edges to form triple chains Nsutite (γ-MnO2) is an important cathodic material for use in dry-cell batteries [42] Although classified as a mineral, nsutite is actually an intergrowth between pyrolusite and ramsdellite.

I.2.2 Layer structure

Layer Manganese oxides as birnessite consist of stacked sheets of edge-sharing Mn octahedral

I.2.2.1 Birnessite

Birnessite was first described as a natural phase from Birness [42], and since then it has been

settings It is a major phase in many soils and an important component in desert varnishes and other coatings and in ocean Mn nodules [53] It readily participates in oxidation-reduction and cation-exchange reactions and therefore plays a significant role in soil and groundwater chemistry Birnessite has a two-dimensional layered structure that consists of edgeshared MnO6

octahedra with water molecules and alkali metal cations or protons occupying the interlayer space The interlayer spacing is typically about 0.7 nm [54], [55] The heterovalent Mn cations (i.e., Mn3+and Mn4+) result in a net negative charge for the MnO2 basal layers, which is balanced by the interlayer cations The chemical composition of birnessite can be expressed by the general formula

AxMnO2 · yH2O, where A is H+ or a metal cation such as K+, Na+, and Ca2+[56] Birnessite materials have received increasing attention in recent years, owing to their wide range of applications as ion-exchangers, selective adsorbents, catalysts, electrode materials [56] Table I.2 presented several methods to synthesize birnessite in laboratory conditions

Table I.2 – Proposed methods for synthesis of Birnessite

m2g-1

described

by McKenzie [50]

This gave a brown birnessite with a potassium content of 9.5% K, and a surface area of 32 m2g-1

described

by McKenzie [50]

Reduction of

potassium

permanganate

Eight mmoles of MnO4- and 16 mmoles of NaOH were added

in 3 litre deionized water, followed by a dropwise addition of 1.5 litre MnCl2 solution (12 mM) The MnO2 particles were collected by decantation, centrifugation and the supernatant was replaced by deionized water several times until the conductivity was significantly reduced This birnessite has

pHzpc of 2.25 with surface area value of 270 m2g-1

described

by Murray [53]

I.2.2.2 Others layer structures

The lithiophorite structure consists of a stack of sheets of MnO6 octahedra alternating with sheets of Al(OH)6 octahedra in which one-third of the octahedral sites is vacant (Figure I.3) In the ideal formula, Li cations fill the vacant sites in the Al layer, and charge balance is maintained by substitution of an equal number of Mn3+ for Mn4+ cations [42] The layers are cross-linked by hydrogen bonds between hydrogen of the hydroxyl groups on the Al/Li layer and O atoms in the

Mn sheet

Chalcophanite is a common weathering product in many Mn-bearing base metal deposits Its structure consists of sheets of edge-sharing MnO6 octahedra that alternate with layers of Zn cations and water molecules (Figure I.3) One of the seven octahedral sites in the Mn layer is vacant, and the Zn cations are above and below the vacancies [42] The water molecules form a hexagonal close-packed layer with one of the seven molecules absent

Figure I.3 – Cristal structure of (A) Lithiophorite, (B) Chalcophanite, (C) Na-rich Birnessite like

[42]

I.2.3 Other Mn oxides minerals

Hausmannite [Mn2+Mn3+2O4] has a spinel-like structure with Mn2+ in the tetrahedral and

Mn3+ in the octahedral sites Hausmannite and bixbyite [(Mn,Fe)2O3] typically are found in

bixbyite structure is a function of temperature [57], and therefore the mineral is an important geothermometer in some ore deposits The crystal structure of pyrochroite [Mn(OH)2] consists of stacked sheets of Mn2+(OH)6 octahedra, and manganosite (MnO) is isostructural with halite Both minerals are relatively rare, typically occurring in low-temperature hydrothermal veins in Mn-rich deposits[50].

I.3 Interactions between Manganese oxides and organic compounds

Manganese oxides rank among the strongest natural oxidant in soil and sediments [32], [53] The standard reduction potential of MnO2 at pH 7 and 25°C is 1.29 V [53] (equation (I.1)) With large surface area up to 270 m2 g-1 (Table I.2), manganese oxides can sorb and further transform organic micropollutants via direct oxidation [33] and/or surface catalysis [58]

The sorption capacity of manganese oxides without further transformation was documented for several organic compounds (chlorpheniramine [59], ciprofloxacin [60] and benzoic acid [61]) Both adsorption and transformation of macrolides [62], triclosan and chlorophen [63], tetracycline [64], [65] were also reported Reduction of manganese oxides by ascorbic or oxalic acids and separation by centrifugation or filtration are used to determine the amount of organic compounds adsorbed onto MnO2 surface and transformed by MnO2 The reduction of MnO2 allows determining the total amount of organic compounds i.e in solution and sorbed onto MnO2 surface Centrifugation allows determining the amount of organic compound in solution The difference between both methods gives the organic amount sorbed onto MnO2 surface Compared to

centrifugation, reduction of MnO2 by ascorbic acid releases the adsorbed organic compound from oxide surface and is thus a strategy to figure out the rate-limiting step Electron transfer can be considered as the rate-limiting step if unreacted sorbed organic compounds are detected on MnO2surface This behaviour was reported for transformation of triclosan [63], tetrabromobisphenol [66], N-oxides [67] Conversely, if unreacted sorbed organic is determined with no detectable amount, then adsorption is the rate-limiting step Transformation of lincosamide [68], tetracycline [69] showed this behaviour

Figure I.4 – General view of transformation of organic compound by MnO2

I.3.1 Reaction Models

Two mechanisms are usually considered to describe the oxidation of organics by metal oxides: bonded and nonbonded electron transfer [33]

I.3.1.1 Electron-transfer with bond-formation between metal sites and organic reductant

This mechanism involves direct bonding between the metal center and the organic reductant i.e inner-sphere complex formation prior to electron transfer Electron transfer at the oxide surface complex is represented by the following reactions:

1 1

≡MnA + H2O (I.2) ≡MnA →k2

HA is the organic reductant, ≡MnOH is a free oxide surface site, and ≡MnA an inner-sphere surface complex When this mechanism occurs, relative rates of reaction with different organic substrates will reflect, in part, bonding requirements of Mn oxide surface sites An organic reductant with high affinity for surface sites will react more quickly than one with low affinity, providing that rates of electron transfer are similar This mechanism implies that the organic reductant is chemically bonded to reactive surface sites in the precursor complex In previous studies catechol [70], pentachlorophenol [71], glyphosate [72] were reported to form inner-sphere complex with birnessite In the inner sphere complex, the organic is partially dehydrated and directly bound to the surface, whereas the outer sphere complex, the organic retains its hydradation sphere and attaches to the surface [73]

I.3.1.2 Electron-transfer through formation of outer-sphere complex between metal sites and organic reductant

Reaction via a non-bonded mechanism, in contrast, involved outer-sphere complex formation prior to electron transfer, and no direct bond between oxidant and reductant is formed For example acid orange 7 [74] , glyoxylic acid [75] would form outer-sphere complex with manganese oxides In the following scheme, (≡MnOH, HA) represents the outer-sphere precursor complex

I.3.2 Transformation reactions

I.3.2.1 Reactions with model organic compounds

The reaction between Mn oxides and model organic compounds was well investigated and documented in the previous two decades [33], [75]–[82] Table I.3 gives a list of references where reductive dissolution of manganese oxides by phenols, anilines and low molecular weight carbonyl compounds was studied

Table I.3 – Summary of model phenols, anilines and low molecular weight (LMW) acids used to

dissolve manganese oxides [33], [75]–[77]

Chlorophenols Acid birnessite, birnessite Dibromophenols Acid birnessite, birnessite, δ-MnO2

Glyoxylic acid Birnessite, manganite

I.3.2.1.1 Phenols

Phenols were used firstly as organic compounds to investigate the reactivity of Mn oxides [33], [78], [79] Mn oxides were well dissolved with an excess amount of phenols The reaction mechanism of the oxidation of phenols by manganese oxides is illustrated by Figure I.5 This mechanism was first described by Stone et al [33], [76], [78] and would proceed according to the following steps: (i) first step is diffusion of organics into the boundary layer (reaction (1) in Figure I.5), following by (ii) the formation of an inner surface complex between organics and Mn oxides (reaction (2)), (iii) the phenolate anion (ArO-) is oxidized via a one-electron transfer inside the complex to form phenoxy radical (ArO.; reaction (3)); (iv) The phenoxy radical can diffuse away from the surface (reaction (4)) and react with a second phenoxy radical to form polymeric products (reaction (5)) Alternatively, a second electron transfer may occur within the surface complex to form Mn2+ and a phenoxenium ion (ArO+; reaction (6)) Both ArO+ and Mn2+ diffuse from the surface (reaction (7)), where ArO+ can also undergo hydrolysis to form a benzoquinone (reaction (8))

Figure I.5 – Major reactions involved in the oxidation of phenols by δ-MnO2 and accumulation of reduced Mn species on the mineral surface The final aqueous products are shown in blue [32]

Previous studies indicated that the formation of the surface complex or the electron transfer may be the rate-limiting step [33], [76] The formation of the surface complex depends on the molecular properties of the organic compound (e.g., charge due to protonation or deprotonation and hydrophobic character) and on the solution conditions (e.g., pH)[34], [63], [78].The electron

transfer process is possibly governed by the potential of the redox reaction, which is also pH dependent [76], [80], [81] The surface complex formation would be rate limiting in the case of catechol [35], while electron transfer is rate limiting in the case of hydroquinone [33] and dihydrodiol polynuclear aromatic compounds [81]

Phenols produce phenoxyl radicals by reacting with Mn oxides Then phenoxy coupling forms polymeric products Hydroquinone, resorcinol, phenol, catechol, and bromophenols were shown to undergo oxidative polymerization [82]–[85] Mn oxides were then used to accelerate the formation of colored organic polymers from phenolic compound [86] The polymerization was monitored by measuring the absorbance at 360 nm Commercial grade Mn dioxide was the most effective reagent for accelerating the color development from catechol, increasing the extent of polymerization by a factor of 30 compared to purified clays

radicals-The nature of the substituent on the aromatic ring is important for kinetic At pH 4, the dissolution of Mn oxide suspension by 10-4 M substituted phenols follows the order: p-methylphenol > p-ethylphenol > m-methylphenol > p-chlorophenol > phenol > m-chlorophenol > p-hydroxybenzoate > o-hydroxybenzoate > 4’-hydroxyacetophenone > p-nitrophenol [76], [87] Electron-donating substituents such as methyl group accelerate the rate of dissolution whereas electron-withdrawing substituents lead to a decrease The extent of reductive dissolution of the oxide can be monitored by quantification of Mn2+ in solution [33], [76], [87] although Mn2+ may remain adsorbed to the oxide surface under some conditions [78]

Similar phenol oxidation reactions may occur in soils containing manganese oxides [88], [89] This reaction is postulated to be a source of humic materials in the environment [82], [90], [91] and is more likely to occur when higher concentrations of phenolic materials are present [84], [92] Such reactions may also participate to the resilience of polluted site by phenolic compounds [93], [94]

I.3.2.1.2 Anilines

The oxidation mechanism of anilines by manganese oxides is similar to that of phenols Aniline loses one electron to produce a cation radical, which can undergo head-to-tail, tail-to-tail,

or head-to-head coupling to form polymeric products [77], [95], [97] (Figure I.6) The radical may undergo a second oxidation, resulting in an overall two electron transfer [77] The major oxidation products of aniline and p-toluidine by δ-MnO2 at pH 4 are symmetrical azobenzene and 4,4’-dimethylazobenzene, respectively [77] 4-Chloroaniline produced 4,4’-dichloroazobenzene and 4-chloro-4’-hydroxydiphenylamine, while 3,4-dichloroaniline produced 3,3’,4,4’-tetrachloroazobenzene and 3’,4’-dichloro-3-chloro-4-hydroxydiphenylamine [95] The significance of the processes whereby Mn dioxide induced oxidative coupling of aniline and substituted anilines may serve as an elimination pathway for anilines

NH2

H N NH

+ 2H+

+ 2H+

+ 2H+

N HN

N N

-Resonance structure of cation radical which

undergo coupling reactions

Final productCation radical

I.3.2.1.3 Low molecular weight carboxylic acids

Several low molecular weight carboxylic acids such as oxalic and pyruvic acids undergo oxidation in the presence of Mn oxides (Table I.3) Reductive dissolution of mixed MnIII,IV oxide

by oxalate was faster than by pyruvate [76] The surface area-normalized reductive dissolution rate of γ-MnOOH by oxalate was faster than that of β-MnO2 at all pH values studied [98] Phosphonoformic acid, glyoxylic acid, pyruvic acid are transformed into orthophosphate ion, formic acid, and acetic acid respectively by reaction with birnessite and γ-MnOOH [75] Citrate is oxidized by birnessite to yield acetoacetate and 3- ketoglutarate [75] (Figure I.7) The reaction occurs by two reaction pathways: (1) free citrate is oxidized by surface bound Mn oxides to yield

Mn2+ and citrate oxidation products and (2) electron transfer from Mn2+–citrate to surface bound

Mn oxides, generating Mn3+–citrate and Mn2+, followed by electron transfer from citrate to Mn oxides [75]

MnIII/IV

Figure I.7 – Transformation of citrate by MnO2

I.3.2.2 Reactions with organic contaminants

I.3.2.2.1 Endocrine discruptors

Like other phenols, bisphenol A (BPA) undergoes oxidation in the presence of Mn oxides Comparison between filtration and reduction of MnO2 by ascorbic acid showed there is no specific amount of BPA adsorbed onto oxide surface [99] The half-life values of BPA were 70, 38 and 16 min respectively in presence of 15, 30 and 60 g L-1 manganese oxide-coated sand (MOCS) More

µM BPA [99] Pure aqueous MnO2 suspension showed faster degradation of BPA than MOCS The oxidation of BPA by MnO2 formed ten oxidation products [99] Radical coupling produced dimers and further oxidation of the phenoxy radical formed hydroquinone For example, 4-(2-(4-(4-hydroxyphenoxy)phenyl)propan-2-yl)phenol was detected as an example of dimer products [99] Similar products were later reported in studies using MOCS In a recent study, 4-hydroxycumyl alcohol (HCA) was identified as major product of BPA transformation by MnO2[100] Up to 60% of BPA was transformed into HCA and HCA was also degraded by MnO2 into

CO2 as final product

Bisphenol F (BPF) is also oxidized by δ-MnO2 under environmentally relevant conditions 90% of initial BPF was taken up after 20 minutes of reaction with 100 µM MnO2 [101] The oxidation rate of bisphenol F was slower than the oxidation rate of bisphenol A under the same conditions; but it reacted faster than the bis(2-hydroxyphenyl) methane The 2,2′-bisphenol, 3,4-hydroxybenzyl alcohol, hydroquinone, and 4-hydroxybenzaldehyde were identified as main products via cleavage of the BPF molecule and radical coupling

Tetrabromobisphenol A, a flame retardant, was oxidized by δ-MnO2 and seven by-products were identified Two products can be detected by HPLC-UV systems Retarded model was proposed to describe the reaction rate over the experiment course[66]

The estrogenic compounds 17α-ethynylestradiol (EE2), 17β-estradiol (E2), estrone (E1) and estriol (E3) are also susceptible to oxidation by manganese oxides EE2 underwent oxidation by granular MnO2 in upstream bioreactors Almost 81.7% of EE2 (15 µ g L-1) was removed after 40 days of treatment [102] δ-MnO2 was also capable of degrading E2, E1 and E3 at similar rates of reaction compared to EE2 [103] The oxidation of E2 by δ-MnO2 was also investigated in more detail in two additional studies [104], [105] More than 90% of 0.037 mM E2 was degraded within

8 hours at pH 6.8 in presence of 0.37 mM birnessite [105] E1 and 2-hydroxyestradiol were identified as the reaction products The estrogenic activities of all four solutions were nearly eliminated after reaction with δ-MnO2

I.3.2.2.2 Antibacterial agents and antibiotics

Two antibacterial agents triclosan and chlorophene were rapidly eliminated by adsorption and oxidation at pH 5 by MnO2 [34] About 55 and 10% of triclosan (initial concentration 10 µM) was adsorbed at pH 5 and pH 8 in MnO2 suspension (100 µM) respectively Both antibacterial agents were oxidized by MnO2 to form quinone transformation products (Figure I.8) 2,4-dichlorophenol, 2-chloro-5-(2,4-dichlorophenoxy)-[1,4]benzoquinone and 2-chloro-5-(2,4-dichlorophenoxy)benzene-1,4-diol p-quinone were identified as by-products of triclosan oxidation 2-benzyl-[1,4]benzoquinone and 2-benzyl-benzene-1,4-diol are chlorophene oxidation by-products Radical coupling played an important role in the formation of by-products (Figure I.8)

OH

Cl

O Cl

Cl

Cl

O Cl

Cl

O

Cl

O Cl

Cl

O

Cl

O Cl

Cl O

Other coupling products

X-ArO

The reactivity of triclosan and chlorophene with MnO2 was shown to be controlled by electronic effect, steric hindrance and hydrophobicity In terms of electronic influence, o-

dichlorophenoxy group provides strong resonance and weak electron-withdrawing effects, thus leading to activating effect The m-Cl substituent exerts an electron-withdrawing effect The o-

benzyl group of chlorophene is electron-donating, and the p-Cl substituent is weakly

electron-withdrawing Besides electronic effect, steric hindrance likely occurs for triclosan, chlorophene, methyl-4-chlorophenol, and 2,4-dichorophenol because of o-substitution of the phenol ring The

2-steric hindrance is probably greater for triclosan and chlorophene due to their larger o-substituents

Furthermore, the higher hydrophobicity of triclosan and chlorophene may contribute to higher adsorption to manganese oxide than the related substituted phenols since previous work has shown that adsorption of chlorophenols to manganese oxide increases as the compound's Kow increases The higher adsorption to manganese oxide may lead to more surface precursor complex formation and thus faster reaction rate [63]

A series of fluoroquinolone (FQ) antibacterial agents were oxidized by δ-MnO2 [106] [40] [76] The experiments were performed with  [FQ]0 = 1.5 µM, [MnO2]0 = 100 µM, 0.01 M MOPS buffer, pH 6, 0.01 M NaCl and 22 °C According to kinit (h-1) values 0.81, 1.11, 1.4, 0.91, 0.54, 0.12; the rate of oxidation decreases in the order: ciprofloxacin ≈ enrofloxacin ≈ norfloxacin ≈ ofloxacin > lomefloxacin > pipemidic acid The relative reaction order followed the ability of the compounds to adsorb onto the oxide surface These zwitterionic compounds adsorb more strongly and are oxidized faster under acidic conditions [34] The piperazine moiety is involved in the adsorption and oxidation reactions The reaction mechanism involves (1) formation of a surface complex, (2) oxidation at the aromatic N1 atom of the piperazine moiety to produce an anilinyl radical, (3) N-dealklyation, C-hydroxylation, and possible radical coupling (Figure I.9) [34] Kinetic modeling indicated that electron transfer was the rate-limiting step for the oxidation of fluoroquinolone antibacterial agents by δ-MnO2 [107]

Figure I.9 – Proposed Reaction Scheme for Oxidation of FQs by MnO2 [106]

Ar = aromatic ring

A recent study showed that the transformation rates of human-used macrolide antibacterial agents (clarithromycin (CLA) and roxithromycin (ROX)) by δ-MnO2 are slow [62] The initial rate constants kinit for 10 mM MnO2 suspension were 3.2 (± 0.1) x 10-14 and 2.9 (± 0.2) x 10-14 M s-

1

m-2 for CLA (1.34 µM) and ROX (1.19 µM), respectively Both compounds were strongly adsorbed onto the oxide (i.e., >90% under experimental conditions) Sorption was attributed to complexation, rather than hydrophobic interactions The adsorption of macrolides onto Mn oxides fitted well by using a Freundlich model The values of the Freundlich affinity constants Kf (in

µM1-n Ln m-2) were 0.145 and 1.612 for clarithromycin and roxithromycin, respectively The major transformation product was formed via hydrolysis of the cladinose sugar Two additional minor products were formed via hydrolysis of the lactone ring and oxidative N-dealklylation of the amino sugar through a one electron transfer

Tetracycline antibiotics include a class of broad spectrum antimicrobial agents with

Oxytetracycline (OT) was rapidly oxidized by MnO2 [65] The MnO2 reactivity toward oxytetracycline decreased with time A retarded rate equation was then used to describe oxytetracycline transformation by MnO2 under declining rate conditions A series of tetracycline antibiotics were oxidized by δ-MnO2 and reaction rates followed the order: rolitetracycline ≈ oxytetracycline < tetracycline ≈ meclocycline ≈ chlortetracycline [65] Chlorine at carbon position

7 plays important role in the reactivity of tetracycline with MnO2 Both ring opening and coupling products were identified [69] Surface complex formation was proposed as the rate-limiting step for the oxidation of tetracycline, chlortetracycline, and oxytetracycline by δ-MnO2 Tetracycline underwent isomerization at the C ring to form iso-tetracycline, while the phenolic-diketone and tricarbonylamide groups are oxidized Oxytetracycline primarily formed N-demethylated products Chlortetracycline transformation products were formed by isomerization, dehydration, cyclization, ketonization, and N-demethylation[69]

The lincosamide antibacterial agents clindamycin and lincomycin were also decomposed by δ-MnO2 [68] Adsorption was determined to be negligible However, electrostatic attraction of pyrrolidine ring of lincosamide to MnO2 surface was proposed to facilitate the association More than 95% of clindamycin was eliminated within 5 hours for 13.6 µM clindamycin and 1.0 mM MnO2 at pH 4.8 The formation of the lincosamide-oxide complex and subsequent reaction occurs

at the pyranose ring The reaction occurs via cleavage of the ether linkage in the pyranose ring

N-oxides such as Carbadox and Olaquindox were reported to be susceptible to oxidation by δ-MnO2 [34] The initial reaction rates of N-oxides with MnO2 followed the order: quindoxin > quinoxaline N-oxide ≈ quinoline N-oxide (QNO) ≈ carbadox Comparison between centrifugation and ascorbic acid addition showed that < 14% of unreacted N-oxides remained onto MnO2

surface Authors indicated that N-oxide functional group plays a dominant role in the oxidation with MnO2 Oxidation of QNO by MnO2 formed 2-hydroxyquinoline as the main product (>95%)

I.3.2.2.3 Other pharmaceuticals and industrial contaminants

Pesticides, such as atrazine, glyphosate, 2-mercaptobenzothiazole (MBT) were reported to

be transformed by manganese oxides [58], [108]–[110] Atrazine was transformed by MnO2 via N-dealkylation and hydrolysis to form mono- and didealkyl atrazine, ammeline and cyanuric acid [111] For glyphosate, C-P and C-N bond cleavage occur to form orthophosphate and sarcosine Aniline-sulfonic acid, benzothiazole-2-sulfite, benzothiazole-2-sulfonate were identified as products of 2-mercaptobenzothiazole oxidation by MnO2 About 94% of MBT was degraded after

180 min reaction time in the presence of 1.0 g L-1 β-MnO2 Low-molecular-weight (LMW) carboxylic acids have inhibitive effect on MBT degradation by MnO2 in waste water effluent The kinetic constant (kinit) of MBT degradation decreased from 5.43 × 10−2 mM−0.51 min−1 in the absence of carboxylic acid to 4.64 × 10−2, 3.69 × 10−2, 4.63 × 10−2 or 4.88 × 10−2 mM−0.51 min−1 in the presence of 1.0 mM oxalic acid, citric acid, tartaric acid or malic acid, respectively This can

be explained by the competive occupation of MnO2 sites by carboxylic acids

In recent studies, paracetamol (PRC) and diclofenac (DCF) were well removed by Mn oxide bed filter [39], [112], [113] Paracetamol was transformed into 1,4-benzoquinone 3-morpholinopropane-1-sulfonic acid (MOPs), an organic buffer, exhibited inhibitive effect toward the oxidation of PRC and DCF Oxidative decarboxylation, iminoquinone formation and dimerization were proposed as reaction pathways for DCF oxidation by MnO2 (Figure I.10) 5-hydroxydiclofenac, 5-iminoquinone and a dimer product were reported as the main transformation products of DCF [112], [113]

H N

Cl

O

H N

I.3.2.3 Reactions with natural organic matter

The sorption and oxidation of Natural Organic Matter (NOM) by manganese oxides were first reported by Stone et al and Tipping et al [33], [114] Adsorption studies of NOM by Mn oxides is complicated by the distribution of molecular size, functional groups, and charge within NOM Adsorption of humic substances in the absence of specifically adsorbing ions decreased with increasing pH [115] [114] This finding, as well as infra-red studies by Parfitt et al (1977), indicates that functional groups having low pKa value (such as carboxyl groups) are responsible for adsorption Addition of Ca2+ and Mg2+ enhances the adsorption of humic substances on Mn oxide [114] The divalent cations may enhance adsorption by partially neutralizing the negative charge on the humic molecules and by making bridge between negatively charged Mn oxide and organics

NOM can be sorbed and oxidized on Mn oxides surface [116]–[119] Birnessite retained low amounts of organic carbon (<2 g-C kg-1) but exhibited the highest capacity for oxidative transformation of NOM in comparison with goethite and smectite [117] Authors also indicated that two processes, including uptake of aromatic constituents and oxidative transformation occur

during the reaction between NOM and birnessite Marine humic substances were shown to facilitate the photoreduction of Mn oxides [33], [116] Final reaction rate in sunlight were 7 to 8 times faster than in the dark The dissolution was faster at lower pH values, most likely caused by greater adsorption of fulvic acid on the oxide surface

Acid humic substances are sorbed onto manganese oxides surface by ligand exchange and/or formation of surface complex [114], [120] The hydrophobic interactions were also proposed to explain the adsorption of NOM onto manganese oxides [120]

The adsorption of NOM is the initial stage followed by the reduction of manganese oxides The electron transfer is accompanied by the formation of low-molecular-weight (LMW) organics such as pyruvate, formaldehyde, acetaldehyde and acetone [116] The formation of formic, acetic, propionic, oxalic and malonic acids was confirmed by HPLC and FTIR [117] Thus, the oxidative cleavage of NOM by Mn oxides represents a mechanism by which biologically refractory organic matter can be converted into a suite of LMW compounds that can be used as microbial substrates.Phenolic compounds, considered as NOM constituents, were oxidized by Mn oxides to produce phenoxy radicals which further may self-couple to form oligomers and/or polymers or cross-couple with aromatic amines [41], [121], [122] The coupling reaction and polymerization of phenols mediated by manganese oxides were then proposed as a process contributing to the formation of humic substances [33] The cross-coupling reactions between micropollutants such as sulfonamide antibacterial agents with NOM constituents mediated by Manganese oxides were also considered as a detoxification pathway in the environment ([41], [123], [124])

OH OCH3

N

H3C

CH3

O OCH3

OCH3

+

Figure I.11 – Cross-coupling of sulfamethazine with syringic acid [41]

I.3.3 Kinetic aspects and influence of reaction conditions

I.3.3.1 Kinetics and reaction order

The reaction of organic compounds with MnO2 suspension can be written by the following general reaction, taking into account the role of H+:

aOrganic + bMnO2 + cH+→ products (I.6)

Then the initial reaction rate r init is given by the following equation:

The reaction order a with respect to the reductant concentration varies from 0.5 to 1.0 for different organic compounds (Table I.4) but is usually close or equal to 1.0 [77], [125] Difference between reaction orders was attributed to difference in adsorption extent onto the oxide surface [34]

Usually, the reaction is first order with respect to the manganese oxide concentration (b) especially when phenols and anilines were used as reductants [77], [125] According to Table I.4,

b value range from 0.2 to 1.3 for different organic contaminants Non-integer reaction orders are characteristic of nonelementary reactions (i.e., reactions in which the concentration term is not directly related to stoichiometry) and may indicate that precursor complex formation is limited by weak adsorption [65] Zhang and Huang [34] suggested that noninteger rate laws may reflect weak adsorption limiting precursor complex formation

Table I.4 – Reaction order for the transformation of various organic compounds by MnO2

where kobs is the observed rate constant depending on the concentrations of MnO2 and H+.