dmi 65

© 2010 Gheorghe Duta

How DMI 65 Granular Catalytic Material Works

Water Treatment

16 November 2010

Gheorghe Duta

Chief Engineer – Technology and Development

Introduction

The purpose of this paper is to provide users of DMI 65, catalytic material with qualitative

information about how the material works, its capabilities and limitations and enable them

to apply the material to water treatment processes, in appropriate manner and with

confidence. The paper avoids the detailed complexity of solid surface electrochemical layers

and colloidal science, quantitative physical and chemical processes and reactions. For

readers having already significant knowledge in this area the paper brings more

understanding of what DMI 65 is and its intended use. Newcomers to this area are provided

with the bases, and perhaps motivation, for directing their more in depth studies as they

might wish.

Background information

DMI 65 is a granular catalytic material boosting oxidation processes in aqueous solutions

(water). The material is part of the broad category of products deriving their physical and

chemical action from the interaction of their metal oxide surface with the water molecules

and ions in solution.

In terms of solid surface interaction with water we distinguish the term adsorption as

attraction - interaction at solid surface level and absorption as transport and retention of

target ions through fine porosity inside the bulk of the material. Interaction of DMI 65 with

water molecules and ions in solution is initiated through adsorption.

Non catalytic type adsorbent materials retain target ions from water until sites available for

adsorption reach a maximum density, saturation, and concentration of target ions in the

treated water attain maximum acceptable concentration. At this point, the adsorbent

material has to be regenerated, to remove or replace the contaminant ions, or the used

material is disposed and new material loaded in the treatment container. When the process

works by swapping one type of ion for target ions from water the process is called ion


exchange. It is appropriate to state that this category of adsorbent and some partly

absorbent materials remove the target ions from water. Obviously, the larger the surface

per volume of material the larger the amount of contaminant target ions which could be

retained. Purely catalytic materials adsorb the reactant ions from solution bringing them in

the proximity of chemical bonding. Then the reaction product moves away from the surface

of catalyst. Strictly speaking catalysts facilitate chemical reactions; they do not implicitly

remove anything. If the reaction product is a solid precipitate, often the product is retained

in the catalytic bed, hence removed by mechanical filtration. Many materials act in a mixed

mode; both ion exchange and catalytic action taking place. For those materials used

primarily for their catalytic action, ion exchange and dissolution of catalytic layer leads to

periodic regeneration or reactivation to correct the matrix of ions at their active surface.

Metals in water may be oxidised, oxidation meaning that the metal loses or shares electrons

with another element. Reduction of a metal ion means that the metal atom acquires one or

more electrons. Reduction and precipitation of metallic elements are usually conducted

outside the scope of water treatment applications.

The activity of hydrogen H+ or hydronium, H3O+ ions, the pH has a strong impact on the

reactions in aqueous solutions. At pH = 7 water is understood to be neutral. At lower pH

water is acidic. Adding a base increases the hydroxide, OH- concentration and the pH, and

water with pH greater than 7 is alkaline.

DMI 65 – Catalytic material

DMI 65 is a granular material of dark brown to black colour. This colour is given by the

manganese oxide in the outer layers of the granules. DMI 65 is a catalytic material in the

true meaning of the word. DMI 65 facilitates or enhances oxidation and does not get

consumed in the reactions. The material does not need regeneration or reactivation and

does not display a decaying capacity to do its catalytic work. Replacement is thought well

before degradation of catalytic capacity takes place. Over 5 to 10 years period, through

many backwashing operations of the bed to remove retained solids, the catalytic layer is

degraded by contact between particles and mechanical abrasion. Then the material has to

be replaced.

Iron, Fe precipitation and removal using DMI 65

The key application of DMI 65 in water treatment is for oxidation, precipitation and removal

of manganese, Mn. Catalytic surface of DMI 65 contains manganese oxide or exposes

manganese and oxygen sites for adsorption of ions in the water. High performance in

oxidation and precipitation of iron, Fe comes in part from the fact that atomic radii of the

two elements, Fe and Mn are almost equal; 127 pm (picometer) for Mn, and 126 pm for Fe.

We discuss first the application of DMI 65 for Fe oxidation and removal because Fe is a more


common contaminant in water than Mn and a number of other metals could be oxidised

and precipitated as metal oxy-hydroxides, whereas manganese is a rather special case.

In the above figure the catalytic surface is presented in a simplified smooth form. Letter “M”

was used to represent a generic metal ion in the lattice of the surface. Letter “O”, in the

centre of circles, represents an oxygen atom. Various ion size and oxygen molecule (blue)

are represented at true relative scale. Except the oxygen molecule, bonded irons are shown

as tangent circles. The interpretation for letters and ions in the figure “Ion oxidation at

catalytic surface” is:

M: generic metal ion in the catalytic surface lattice (Mn+); n = 1, 2…

O: oxygen atom or ion (O-)

Fe: iron atom or ion (Fe2+, Fe3+)

H: hydrogen atom or ion (H+)

OH: hydroxide, or hydroxyl anion (OH-)

H2O, water molecule shown as tangent circles

Fe (OH)2, ferrous hydroxide is shown as tangent circles

Fe (OH)3, ferric hydroxide, shown as tangent circles, brown colour

O2, oxygen molecule, atoms shown at covalent bonding distance, blue colour

M O M O M O M O M O M

H

O

H

H

O H

H

O

Metal oxide catalyst

Fe adsorption Ferric hydroxide

Iron oxidation at catalytic surface.

OH

Fe

OH OH

Fe

OH

H

O

H

O

H

O

H

OH H

OH

Fe

OH

OH

OH

Fe

OH

O

O

O

O

O

O

O

O


Water molecules are an electric dipole and they could be attracted to the ion sites on the

catalytic surface. The following process is called hydroxylation of metal oxide surface. The

oxygen end of the water molecule is first attracted to the metal in the lattice. Subsequently,

oxygen in the neighbouring site attracts the hydrogen and cause splitting of the water

molecule into hydroxide OH- and hydrogen ion H+. This is a dynamic process, not taking

place simultaneously at all sites of the catalyst. Side sliding of the attached matrix of

hydroxide and hydrogen ions, release from adsorption and recombination take place at the

same time.

Dissolved ferrous hydroxide is attracted with the Fe end towards the lattice oxygen of the

catalytic material. This brings the Fe in the proximity of covalent bonding with the hydroxide

ion of a neighbouring site and ferrous hydroxide changes into ferric hydroxide. Electric

charges are better balanced in the ferric hydroxide and the ferric hydroxide could move

easier away from catalyst surface. Moreover, ferric hydroxide is insoluble and precipitates in

crystalline form aggregates of size from 3 nanometre and larger. The aggregates coagulate

in larger flocks and are retained in the catalytic bed.

While ferrous hydroxide is converted into ferric hydroxide, the concentration of ferrous

hydroxide at the catalytic surface decreases. In the bulk of the water, away from catalytic

surface, the concentration of ferrous hydroxide is higher and ferrous hydroxide diffuses

towards the lower concentration according to diffusion law. Diffusion flux is linearly

dependent with concentration gradient over distance.

Dissolved oxygen contributes to production of hydroxide ions through direct oxidation of

hydrogen in combination with Fe splitting the water molecule and by reacting with the

hydrogen at the catalytic surface.

Very important to note is that although a source of oxygen is needed, oxidation and

precipitation of Fe is driven by hydroxide ion. The consequences are that even under

relatively acidic conditions hydroxide ions (very strong anion) are easier available for binding

to Fe than oxygen. Thus, Fe is not very difficult to oxidise and precipitate around neutral pH

condition. In addition, concentration of hydroxyl ions increases with pH value exponentially

and so does the rate of oxidation and precipitation of Fe.

Manganese, Mn precipitation and removal using DMI 65

DMI 65 is a catalytic material specifically tailored to the oxidation and removal of

manganese. The catalytic surface contains manganese oxide for brining into proximity of

covalent bonding manganese and oxygen atoms from water. However, oxidation and

removal of manganese, Mn is vastly different from that of Fe. A major difference is caused

by solubility of manganese hydroxide, Mn (OH)2. Manganese does not precipitate as oxy-

hydroxide but as oxide, MnO2 and higher valency oxides. Presence and concentration of

hydroxide anions does not help much in the precipitation and removal of manganese.

Strong oxidative conditions and oxygen source is needed. To oxidise manganese a strong


source of highly reactive oxygen such as ozone is a possible solution and often used. In the

case of DMI 65 dissolved oxygen level has to be maintained high. Manganese hydroxide will

be attracted with the manganese end to the lattice oxygen. Oxygen molecule has to be

available in the proximity for facilitating oxidation through the oxygen from lattice and

swapping to the lattice with molecular oxygen. Conditions for this to happen are statistically

less probable and reaction is of much slower rate than the oxidation of Fe via hydroxide.

While increase in pH facilitates oxidation and removal of manganese, under anoxic

conditions, although pH could be alkaline, manganese could dissolve back into the water

and also could be mobilised into water from the catalytic surface. Consequently, regardless

of the target contaminant to be removed, anoxic conditions have always to be avoided to

protect the catalytic layer against leaching into water. When oxidising manganese the

recommended pH is close to 8.

Presence of Fe and its oxidation and removal facilitates the removal of manganese.

Similarity of atomic radius facilitates co-precipitation of manganese with Fe as manganese

ion. Backwash sludge formed this way is less stable in the environment because the high

sensitivity of manganese to move in solution.

Manganese oxide has good autocatalytic effect. When backwashing it is better to stop the

process before the water becomes very clear. Manganese oxide residue in the filter bed will

enhance manganese oxidation.

M O M O M O M O M O M

HO

H

H

O H

H

O

Metal oxide catalyst

Mn adsorption ManganeseOxide

Manganese oxidation at catalytic surface.

OH

Mn

OH OH

Mn

OH

H

O

H

O

H

O

H

OH H

Mn

OH

Mn

OH

O

O

O

O

O

O

O

O

O O


Role of sodium hypochlorite, NaOCl

Catalytic surface has to be maintained clean so that ions in water could come in contact

with it. Sodium hypochlorite is injected to prevent bacteria growth and blinding with slime

of catalytic surface. At the same time NaOCl is a source of oxygen more reactive than

molecular. Ideal residual to be maintained downstream of catalytic filter is 0.2 mg/l (0.1 to

0.3) free chlorine. Higher residual of free chlorine and therefore higher sodium hypochlorite

level in the catalytic filter does not always help. It could have an adverse effect due to

venting of chlorine and increase of competing sodium ions, Na+.

Why remove Fe and Mn from water

In the case of drinking water limits maximum contaminant limit for Fe and Mn is limited on

aesthetic considerations. The limits vary from country to country, although is known and

accepted that Fe in excess of 0.3 mg/l and Mn in excess of 0.1 mg/l cause staining of laundry

and plumbing fittings. Fe and Mn are essential elements and deficiency in our diet could

cause serious health problems. Health problems could be caused also by excess of the two

elements in our diet. In terms of limits for public health the limits were unfortunately

established based on adult body weight and water consumption, and it was incorrectly

assumed that children fit the same rules and are ok. Children have higher permeability of

organ and cell membranes and drink about 5 times more water per kilogram of body weight

than adults. Rather, a common sense point, the two elements being essential to our health,

before establishing health limits to include children, we should have looked at the Fe and

Mn content of human breast milk. The Fe content in human breast milk is 0.3 mg/l and Mn

varies across world population from 0.003 to 0.010 mg/l. We could guess that limits in

drinking water a lot higher than in human breast milk could damage the health of children.

By chance in the case of Fe we got things right. In the last few decades, evidence beyond

any doubt has been accumulated that Mn is neurotoxic to children at much lower

concentration than thought before. Mn accumulates in the brain and the strongest

measurable effect is poor memory. The limit set by US EPA of 0.05 mg/l seems acceptable

and should not be exceeded.

In the case of discharge of treated water into aquatic environments, typically concentration

background level should not be exceeded by more than 10%. Local EPA sets the rules.

Aerobic aquatic fauna takes up oxygen from water through membranes exposing an alkaline

protective layer in contact with the water. This is the case of gills and skin. Dissolved Fe and

Mn coming in contact with the respiratory membranes could oxidise and precipitate.

Presence of precipitated Fe and Mn leads to local generation of highly reactive species and

burning of the tissue around. Bleeding of gills, asphyxiation, red fungus attack and other

diseases are the result of excessive dissolved Fe and Mn. Fe is more reactive and dangerous

than Mn.


DMI 65 operating conditions of concern

Treatment processes have to be conducted in such manner so that the catalytic surface of

the material is kept clean and available to ion from water to contact.

Water with large amount of suspended solids has to be clarified before passing it through

the catalytic filter with DMI 65. Acceptable level of suspended solids depends on their

nature. Larger amount of mineral suspended solids then organic suspended solids could be

handled.

Bacteria could grow and deposit slime on DMI 65. Thus disinfectant and oxidation

conditions have to be maintained.

Water containing clays and large organic molecules may result in deposition of such

material on the surface of DMI 65 and blinding of catalytic surface. Treatment for removal

of such contaminant before the catalytic filter is needed.

Polymer flocculent could also stick to the DMI 65 and blind catalytic surface.

Hard, unstable groundwater could cause scale deposition in the catalytic filter and bind the

material in a solid mono bloc. In such case the DMI 65 material in the bed is lost; has to be

replaced. Treatment for stabilizing the water to prevent scale formation in the catalytic filter

has to be carried out.

Both low acidic pH and anoxic conditions could cause dissolution of manganese from

catalytic layer of DMI 65 and loss of its capacity. Excessively high pH means excessive

concentration of hydroxyl ions (corrosive to metals) and could also cause dissolution of

manganese from the catalytic layer.

Do not use demineralized water, distilled water or water known to be strongly corrosive to

metals for initial soaking and activation of DMI 65.

dmi 65

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