Environmental chemistry of phosphonates

Environmental chemistry of phosphonates

ARTICLE IN PRESS

Water Research ] (]]]]) ]]]–]]]

Review Environmental chemistry of phosphonates

Bernd Nowack*

Institute of Terrestrial Ecology (IT O), Swiss Federal Institute of Technology Z urich (ETH), Grabenstrasse 3,

CH-Schlieren 8952, Switzerland Received 16 August 2002; received in revised form 21 January 2003; accepted 31 January 2003

Abstract

Phosphonates are anthropogenic complexing agents containing one or more C–PO(OH)2groups They are used in numerous technical and industrial applications as chelating agents and scale inhibitors Phosphonates have properties that differentiate them from other chelating agents and that greatly affect their environmental behavior Phosphonates have a very strong interaction with surfaces, which results in a significant removal in technical and natural systems Due

to this strong adsorption, little or no remobilization of metals is expected No biodegradation of phosphonates during water treatment is observed but photodegradation of the Fe(III)-complexes is rapid Aminopolyphosphonates are also rapidly oxidized in the presence of Mn(II) and oxygen and stable breakdown products are formed that have been detected in wastewater The lack of information about phosphonates in the environment is linked to analytical problems of their determination at trace concentrations in natural waters Further method development is urgently needed in this area, including speciation of these compounds With the current knowledge on speciation, we can conclude that phosphonates are mainly present as Ca and Mg-complexes in natural waters and therefore do not affect metal speciation or transport

r2003 Elsevier Science Ltd All rights reserved

Keywords: Phosphonates; Chelating agents; Adsorption; Heavy metals; Degradation; Speciation

Contents

1 Introduction 2

2 Properties 2

3 Analysis of phosphonates 4

3.1 Analytical methods 4

3.2 Concentrations in the environment 5

4 Surface reactions 5

4.1 Adsorption 5

4.2 Dissolution of minerals 6

4.3 Remobilization of metals 6

4.4 Precipitation 7

4.5 Inhibition of dissolution and precipitation 7

*Tel.: +41-1-633-61-60; fax: +41-1-633-11-23.

E-mail address: nowack@ito.umnw.ethz.ch (B Nowack).

0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd All rights reserved.

doi:10.1016/S0043-1354(03)00079-4

1 Introduction

Phosphonic acids, compounds containing the Lewis

acid moiety R-CP(O)(OH)2, are characterized by a

stable, covalent carbon to phosphorous bond The

corresponding anions of the phosphonic acids are called

phosphonates The most commonly used phosphonates

are structural analogues to the well-known

aminopoly-carboxylates such as ethylenediaminetetra acetate

(EDTA) and nitrilotriacetate (NTA) The environmental

fate of these aminopolycarboxylate chelating agents has

received considerable attention [1–5] Much less is

known about the fate and behavior of the corresponding

phosphonates in the environment [4,6,7] The existing

reviews are either several years old and therefore do not

cover the newest literature[6]or focus on toxicology and

risk assessment based on the limited data that were

available at that time[7] What is missing is an overview

of the chemistry of these compounds which can help us

to understand and predict the environmental behavior

of these compounds more accurately and that can be the

basis for a refined risk assessment The aim of this

review is therefore to provide an overview of the current

knowledge of the environmental chemistry of

phospho-nates It concentrates on polyphosphonates, compounds

containing more than one phosphonic acid group, and

especially aminopolyphosphonates, compounds

contain-ing several phosphonate and one or more amine groups

Glyphosate, a herbicide containing a phosphonate, a

carboxylate and an amine functional group, is not

discussed in detail in this review There is, however,

much information available about the environmental

chemistry and behavior of this compound[8–10]

This review starts with a short description of the

properties of phosphonates and their analysis

Phos-phonates have a very strong interaction with surfaces

and the section discussing the surface reaction follows:

adsorption, dissolution of minerals, remobilization of

metals, precipitation of phosphonates and inhibition of

precipitation of minerals are covered In the degradation

section biodegradation, photodegradation, chemical degradation and degradation during oxidation processes are discussed The speciation of phosphonates in the environment covers the next section, which is followed

by a discussion of their environmental behavior This section contains a summary of the data on measured concentrations of phosphonates and their behavior during wastewater treatment

2 Properties Table 1 lists the abbreviations, names and structures

of the phosphonates discussed in this review These compounds are known under many different abbrevia-tions that vary between the disciplines and countries and have changed with time Phosphonates are effective chelating agents according to the IUPAC definition that chelation involves coordination of more than one sigma-electron pair donor group from the same ligand to the same central atom Phosphonates are used as chelating agents in many applications, e.g in pulp, paper and textile industry to complex heavy metals in chlorine-free bleaching solutions that could inactivate the peroxide In medicine phosphonates are used to chelate radionuclides for bone cancer treatments[11]

A recent IUPAC Technical Report [12] critically evaluates the available experimental data on stability constants of proton and metal complexes for phospho-nic acids It presents high-quality data as ‘‘recom-mended’’ or ‘‘provisional’’ constants while for example, all constants for DTPMP have been rejected due to insufficient purity of the parent compound This report will be of great use for all future speciation calculations and should be the sole source of stability constants when ever possible

The stability of the metal complexes increases with increasing number of phosphonic acid groups Fig 1 shows that the monophosphonate aminomethylpho-sphonic acid (AMPA) has the lowest stability constants

5 Degradation 7

5.1 Biodegradation 7

5.2 Photodegradation 8

5.3 Chemical degradation 8

5.4 Degradation during oxidation processes 9

6 Speciation 9

7 Behavior during wastewater treatment 10

8 Conclusions 11

Acknowledgements 11

References 11

Table 1

Abbreviations, names, and structures of the phosphonates covered in this review

C

PO (OH )2

H3C OH

PO (OH )2

NTMP ATMP, NTP, NTPH, NTPO Nitrilotris(methylenephosphonic acid)

N (H O) 2 OP

(H O) 2 OP PO (OH) 2

EDTMP EDTP, EDTPH, ENTMP, EDTMPO, EDTMPA 1,2-Diaminoethanetetrakis (methylenephosphonic acid)

N

N P O (OH )2

PO (OH )2

(H O)2OP

(H O)2OP

DTPMP DETPMP, DTPPH, DETPMPA, DETPMPO Diethylenetriaminepentakis (methylenephosphonic acid)

N N

(H O)2OP

PO (OH )2

PO(OH )2

C

CO OH

PO (OH ) 2

H OOC

CO OH

and EDTMP with 4 phosphonic aid groups the highest.

The log K values of the different transition metal

complexes follow the Irving–Williams series Mn2+

o-Fe2+oCo2+oNi2+oCu2+>Zn2+

Fig 2shows a speciation diagram for the system

Zn-EDTMP calculated with the constants from[12]with the

speciation program ChemEQL [13] This calculation

shows that in the pH range found in technical

applications and in natural waters a large number of

possible complexes with different degree of protonation

and charge exist At pH 6 the species H4EDTMP4
,

ZnH3EDTMP3
, ZnH2EDTMP4
and ZnHEDTMP5

occur at a percentage of more than 5% of total

EDTMP Complexation of other metals by other

phosphonates is similar and at each pH value several

species coexist

Phosphonates are not only chelating agents but also

very potent inhibitors of mineral precipitation and

growth This effect works at concentrations well below

the amount needed to chelate all metals An important

industrial use of phosphonates is in cooling waters,

desalination systems and in oil fields to inhibit scale

formation, e.g barium sulfate or calcium carbonate

precipitation Phosphonates are also used in medicine to

treat various bone and calcium metabolism diseases[14]

In detergents phosphonates are used as a combination of

chelating agent, scale inhibitor and bleach stabilizer[15]

Phosphonates are highly water-soluble while the

phosphonic acids are only sparingly soluble

Phospho-nates are not volatile and poorly soluble in organic

solvents More detailed data on the

physico-chemical properties of the phosphonates can be found

in reference[7]

The consumption of phosphonates was 56,000 tons

worldwide in 1998 [16] and 16,000 tons in Europe in

1999[4] Data about the distribution among the various

phosphonates are available for Europe and the US[6],

for the Netherlands[7]and for Germany[4] HEDP and

DTPMP are the most important phosphonates based on

the used volumes

The toxicity of phosphonates to aquatic organisms is low[6,7,17] Reported values for 48 h LC50 values for fish are between 0.1 and 1.1 mM [18,19] Also the bioconcentration factor for fish is very low [20,21] Phosphonates are poorly absorbed in the gastro-intestinal tract and most of the absorbed dose was rapidly excreted by the kidneys[22] Human toxicity is also low which can be seen in the fact that phosphonates are used to treat various diseases[14,23]

3 Analysis of phosphonates 3.1 Analytical methods The absence of a reliable trace analytical method for phosphonates results in a lack of detailed information about the environmental behavior of phosphonates Most of the current methods for phosphonate determi-nation have detection limits above the expected natural concentrations or suffer from interferences in natural samples

The standard method for the determination of phosphonates is ion-chromatography followed by post-column reaction with Fe(III) and detection of the Fe(III)-complexes at 300–330 nm[24–26] This method has a detection limit of about 2–10 mM Other methods have been developed based on post-column oxidation of the phosphonate to phosphate and detection of phos-phate with the molybdenum blue method [27] Ion-chromatography with pulsed amperometric detection of amine-containing phosphonates [28], ion-chromatogra-phy with indirect photometric detection [29] and capillary electrophoresis with indirect photometric detection have also been described[30] These methods all have high detection limits of 1 mM or more and are therefore not suitable for natural systems

A very powerful method is the derivatization of the phosphonic acid group with diazomethane and

0

2 10 -7

4 10 -7

6 10 -7

8 10 -7

1 10 -6

pH

H

3-H

7-ZnH

4-ZnH

3-H

4 EDTMP

4-H

-Fig 2 Speciation of 1 mM EDTMP in the presence of 1 mM Zn The diagram has been calculated using the constants from [12]

0

5

10

15

20

EDTMP

AMPA

IDMP

Fig 1 Stability constants of 1:1 complexes (M+HL) with

transition metals of AMPA, IDMP, HEDP, NTMP and

EDTMP (Irving–Williams series) with data from [12]

separation and detection of the derivatives by

HPLC-MS [31] This method is however, not applicable to

natural waters due to interference by the major cations

and anions of the water matrix The only method with a

low enough detection limit in natural samples is an

ion-pair HPLC method with precolumn formation of the

Fe(III)-complexes [32] The phosphonates can be

measured with a detection limit of 0.05 mM in natural

waters and wastewaters The method, however, is not

able to quantify bisphosphonic acids such as HEDP at

low concentrations This is a major drawback because

HEDP is one of the most used phosphonates[4,6]

The breakdown products of the Mn(II)-catalyzed

degradation of NTMP[33],

iminodimethylenephospho-nic acid (IDMP) and

N-formyl-iminodimethylenephos-phonic acid (FIDMP), can be detected after derivatization

of the aldehyde group in FIDMP by

2,4-dinitrophenylhy-drazine and derivatization of the imine-group in IDMP by

9-fluorenyl methylchloroformate[34] A detection limit of

0.01 mM FIDMP and 0.02 mM IDMP has been achieved

Anion-exchange chromatography coupled to ICP-MS

is able is a very promising method for chelating agent

analysis [35,36] The method is also applicable to

phosphonates and it has been shown that CuEDTMP

can be determined with a very low detection limit in the

nanomolar range

Preconcentration of phosphonates from natural water

samples using different adsorbents has been tested[37]

It was found that the investigated phosphonates HEDP,

NTMP, and EDTMP differed so much in their chemical

behavior that a simultaneous enrichment from natural

samples cannot be achieved Successful

preconcentra-tion of the phosphonates NTMP, EDTMP and DTPMP

from natural waters or wastewaters was achieved using

freshly precipitated CaCO3[32] Recoveries at the 1 mM

level were 95–102% for an influent sample of a

wastewater treatment plant

3.2 Concentrations in the environment

No measurements of phosphonates in natural samples

have been reported and only data for wastewaters are

available This is mainly due to the fact that most

analytical methods are not able to quantify

phospho-nates in natural waters at low concentrations

Phospho-nates have been measured in Swiss wastewater treatment

plants (WWTP)[38] The concentrations of NTMP were

between o0.05 and 0.85 mM, of EDTMP between

o0.05 and 0.15 mM and of DTPMP between o0.05

and 1.7 mM The highest concentration of DTPMP was

found in a WWTP influenced by textile industry

Effluent samples from all investigated WWTP were with

the exception of one case always below the detection

limit Another WWTP influenced by textile industry

contained NTMP concentrations in the influent between

0.2 and 1.1 mM[39]

The oxidative breakdown products of NTMP, IDMP and FIDMP, have been detected in two WWTPs receiving water from textile industry at concentrations

of 0.08 and 0.015 mM FIDMP and 0.49 and 0.3 mM IDMP in the influent[34]

The expected concentrations in rivers are maximal 0.1 mM and with adsorption/photodegradation included about 1–4 nM for NTMP and 25 nM for HEDP [6,7] Locally higher concentrations can be expected because of intermitted discharge of cooling tower water

4 Surface reactions 4.1 Adsorption Phosphonates adsorb very strongly onto almost all mineral surfaces This behavior distinguishes them from the corresponding aminocarboxylates, which exhibit much weaker interaction with mineral surfaces, espe-cially near neutral pH [40] Some of the investigated adsorbents for phosphonates are calcite [41], clays [42,43], aluminum oxides [44–46], iron oxides [47–49], zinc oxide[49], hydroxyapatite [50,51] and barite[52] For all those compounds very strong adsorption is observed in the pH range of natural waters Natural materials are also very potent adsorbents for phospho-nates, for example sewage sludge [20,21,39,53,54], sediments [54] and soils [55] Most of these studies, however, have not considered that metal ions might significantly alter the adsorption of a chelating agent [56] However, no influence of Fe(III), Zn, and Cu(II) on phosphonate adsorption onto goethite was observed [49] This was explained by the very strong adsorption of the uncomplexed phosphonate, which resulted in a dissociation of the complex at the surface and separate adsorption of the metal and the phosphonate onto different surface sites Fig 3 shows the adsorption of NTMP and the NTMP complexes with Zn, Cuand Fe(III) Complete adsorption is observed up to a pH of 8 and no influence of the complexed metal on the shape of the adsorption edge can be seen In the pH range of natural waters adsorption is therefore very strong Other phosphonates, e.g HEDP, EDTMP and DTPMP, adsorb in a similar manner to NTMP

Ca has a very strong positive effect on phosphonate adsorption [49] In the presence of mM Ca concentra-tions, phosphonates were completely adsorbed up to pH

of 12 The maximum surface concentration of phospho-nates was also greatly enhanced in the presence of Ca This effect can be explained by the formation of ternary surface-phosphonate-Ca complexes Precipitation of Ca-phosphonates on the surface can be ruled out[49] When evaluating the adsorptive capacity of a surface towards phosphonates in a natural system, it is therefore

ARTICLE IN PRESS

necessary to conduct the adsorption experiments under

natural Ca concentrations

4.2 Dissolution of minerals

Dissolution of a mineral phase by chelating agents can

be explained in terms of a ligand exchange process and is

related to the concentration of surface bound ligands

The ligands weaken the metal–oxygen bonds on the

surface and enhance the release of metal ions from the

surface into the adjacent solution [57] Reactions with

iron oxides are especially of great importance regarding

the speciation of the ligand in solution due to the very

strong Fe(III)-complexes Reactions like this have been

observed in subsurface systems and have a pronounced

influence on the mobility of heavy metals[58,59]

Very little is known about the dissolution of iron

oxides by phosphonates It was observed that HEDP

significantly mobilized Fe from natural sediments but no

information was given about the pH value of the

experiments [60] No enhanced solubilization of Fe

from river sediment was observed at pH 3 by 0.01 M

NTMP[61]

The concentration of the Fe(III)-complex in the

presence of an iron oxide phase can be calculated when

the stability constants of the Fe(III)-complexes are

known Fig 4 shows the calculated Fe(III)NTMP

concentration in a system with NTMP and hydrous

ferric oxide (HFO) The speciation has been calculated

with the published stability constants for metal-NTMP

complexes [12] and the NTMP-Fe(III) stability

con-stants from[62]using the program ChemEQL[13] The

formation of Fe(III)NTMP is important at pH values

below 6 in the absence of other metal ions At pH above

7 NTMP is present as uncomplexed ligand 1 mM Ca

Fe(III)NTMP slightly Cuwhich forms the strongest complexes with NTMP has the largest influence on Fe(III)NTMP formation We can therefore conclude that dissolution reactions are able to occur at low pH However, the strong adsorption of phosphonates, especially at low pH, will limit the formation of dissolved Fe(III)NTMP complexes and therefore no dissolution will occur at low, environmentally relevant concentrations

4.3 Remobilization of metals Metals adsorbed onto a mineral surface can be solubilized by chelating agents This process has always been mentioned as one of the most adverse effects of elevated chelating agent concentrations in the environ-ment[63]

Metal adsorption in the presence of phosphonates has been studied There is an increase in Cu adsorption in the presence of phosphonates at low pH, which is caused

by electrostatic effects [49] At high pH there is a mobilization of Cudue to the formation of dissolved Cu-phosphonate complexes.Fig 5shows as an example the influence of EDTMP on Cu adsorption onto goethite Overall, the influence of phosphonates on metal adsorption in the natural pH range from 4 to 8 is weak We can therefore expect that phosphonates have only a slight influence on metal remobilization in natural systems This was actually found during the study of metal mobilization from river sediments by the phos-phonate HEDP[60] The only metal to be remobilized was Fe whereas Zn, Cr, Ni, Cu, Pb and Cd were not increased compared to a blank sample and only dissolution of iron oxides was observed Remobilization

of Cu, Cd, and Pb from river sediment was only observed at NTMP concentrations above 0.1 mM[61]

0

20

40

60

80

100

NTMP ZnNTMP CuNTMP Fe(III)NTMP NTMP/ 1 mM Ca

pH

Fig 3 Adsorption of 10 mM NTMP onto goethite in the

absence and presence of equimolar Zn, Cu, and Fe(III) and

1 mM Ca Reprinted with permission from [49] Copyright

(1999) American Chemical Society.

0

2 10-7

4 10-7

6 10 -7

8 10-7

1 10-6

4 4.5 5 5.5 6 6.5 7 7.5 8

NTMP

2 mM Ca

1 µM Zn

1 µM Cu

pH

Fig 4 Speciation of 1 mM NTMP in the presence of HFO and

in the absence and presence of 1 mM Zn and Cuand 2 mM Ca without considering adsorption of the phosphonate Log K values from [12] and [62]

We can therefore conclude that phosphonates probably

have only a marginal influence on metal mobilization in

the environment

4.4 Precipitation

In many applications, phosphonates are added to

waters containing high concentrations of dissolved ions

to prevent the formation of precipitates However, due

to the insolubility of some metal-phosphonates, the

phosphonates itself can precipitate This phenomenon

often occurs in oil field applications when phosphonates

are injected into the subsurface and are left to interact

with calcium-containing formation waters[64,65]

The solubility of precipitates of NTMP with divalent

metals increases in the order CaoBaoSroMg[66] The

insoluble Ca precipitates of DTPMP [67,68], NTMP

[69], and HEDP[70]and the precipitates of NTMP with

Fe(II) [71] and Fe(III) [72] have been investigated in

detail Insoluble products of HEDP are also formed with

heavy metals such as Pb and Cd[73]

The precipitates are important in oil field applications

or in technical systems where high phosphonate and

high ion concentration occur simultaneously In natural

waters or wastewaters, the phosphonate or Ca

concen-trations are far too low to exert any influence on

phosphonate concentrations The solubility of NTMP in

the presence of 1 and 5 mM Ca is always above 200 mM

[49] In natural waters precipitation reactions are

there-fore not important

4.5 Inhibition of dissolution and precipitation

Scale formation, e.g precipitation of calcium

carbo-nate or calcium sulfate, is a significant problem in

commercial water treatment processes including cooling

water technology, desalination and oil field applications

This scale formation can be alleviated by the use of

chemical water treatment additives, known as ‘‘thresh-old inhibitors’’ Phosphonic acids are among the most potent scale inhibitors next to the polyphosphates They poison the crystal growth at concentrations far below stoichiometric amounts of the reactive cations Models for this poisoning include inhibition of nucleation, adsorption onto growth sites, distortion of the crystal lattice, changes in surface charge and association with precursors of crystal formation[74,75]

The morphology of crystals formed in the presence of phosphonates is markedly different from those in the absence of phosphonates [76] Phosphonates limit the size of the growing crystals and produce a lag phase in which crystal growth is greatly reduced [77] It was found that the ability of different phosphonates to inhibit crystal growth can be interpreted in terms of the Langmuir adsorption model with the strongest inhibi-tory effect from compounds that adsorb most strongly [78]

Due to their inhibitory effect on crystal growth it has been argued that phosphonates may have an adverse effect on phosphate elimination by precipitation with iron or aluminum salts during wastewater treatment [79,80] It was found that the phosphonates had an influence on flocculation but it was possible to compensate for it by increased addition of flocculating agent The resulting particulate precipitation products were stabilized by the dispersing action of the phospho-nates and not retained in the sand filter Another study, however, found no influence of HEDP on phosphate elimination[60]

5 Degradation 5.1 Biodegradation Phosphonates are similar to phosphates except that they have a carbon–phosphorous (C–P) bond in place of the carbon–oxygen–phosphorous (C–O–P) linkage Due

to their structural similarity to phosphate esters, phosphonates often act as inhibitors of enzymes due in part to the high stability of the C–P bond[81] In nature bacteria play a major role in phosphonate biodegrada-tion The first phosphonate to be identified to occur naturally was 2-aminoethylphosphonic acid [82] It is found in plants and many animals, mostly in mem-branes Phosphonates are quite common among differ-ent organisms, from prokaryotes to eubacteria and fungi, mollusks, insects and others but the biological role of the natural phosphonates is still poorly under-stood[83] Due to the presence of natural phosphonates

in the environment, bacteria have evolved the ability to metabolize phosphonates as nutrient sources Those bacteria able of cleaving the C–P bond are able to use phosphonates as a phosphorous source for growth

ARTICLE IN PRESS

Cu alone

Cu with NTMP

pH

0

20

40

60

80

100

Fig 5 Adsorption of 10 mM Cuonto goethite in the absence

and presence of 10 mM NTMP Reprinted with permission from

[49] Copyright (1999) American Chemical Society.

Aminophosphonates can also be used as sole nitrogen

source by some bacteria[84]

The polyphosphonate chelating agents discussed here

differ greatly from natural phosphonates such as

2-aminoethylphosphonic acid, because they are much

larger, carry a high negative charge and are complexed

with metals Biodegradation tests with sludge from

municipal sewage treatment plants with HEDP and

NTMP showed no indication for any degradation based

on CO2 formation [18,20,21] An investigation of

HEDP, NTMP, EDTMP and DTPMP in standard

biodegradation tests also failed to identify any

biode-gradation[53] It was noted, however, that in some tests

due to the high sludge to phosphonate ratio, removal of

the test substance from solution observed as loss of

DOC was observed This was attributed to adsorption

rather than biodegradation because no accompanying

increase in CO2was observed

However, bacterial strains capable of degrading

aminopolyphosphonates and HEDP under P-limited

conditions have been isolated from soils, lakes,

waste-water, activated sludge and compost[85] The

phospho-nate phosphonobutane-tricarboxylic acid (PBTC) was

also rapidly degraded by microbial enrichment cultures

from a variety of ecosystems under conditions of low

phosphate availability[86]

The effects of other more accessible P sources on

phosphonate uptake and degradation are of great

environmental importance Many environments such

as activated sludge, sediments and soils that act as a sink

for phosphonates are not characterized by a lack of P

most of the time Because phosphonates are utilized

almost exclusively as P-source, little biodegradation can

be expected under these conditions It has been

demonstrated, however, that simultaneous phosphate

and phosphonate utilization by bacteria can occur[87]

Adsorption of chelating agents by surfaces has been

shown to decrease the biodegradability The easily

biodegradable NTA for example is much slower

degraded when adsorbed to mineral surfaces [88] It

can be expected that phosphonates with their higher

affinity to surfaces are much slower degraded in a

heterogeneous compared to a homogeneous system

This was found to be the case for

N-phosphonomethyl-glycine, the phosphonate-containing herbicide

glypho-sate[89]

Phosphonates are therefore similar to EDTA[3,90]in

that little or no biodegradation is observed in natural

systems but that microorganisms have been isolated

from these environments capable of degrading the

compound

5.2 Photodegradation

Photodegradation of the Fe(III)-complexes is an

important pathway of aminopolycarboxylate elimination

in the environment [91] Phosphonates have a similar reactivity In distilled water and in the presence of Ca no photodegradation of HEDP was observed but the addition of Fe(III) and Cu(II) resulted in rapid photo-degradation [20,92] The mechanism of Fe(III)EDTMP photodegradation [93] is equivalent to the photodegra-dation of Fe(III)EDTA[94] Fe(III)EDTMP is degraded

in a stepwise process from the parent compound through ethylenediaminetrimethylenephosphonate and ethylene-diaminedimethylenephosphonate to ethylenediaminemo-nomethylenephosphonate which is stable in the presence

of Fe(III) and light For EDTA the photodegradation of the Fe(III)-complexes is the major elimination pathway

in natural waters [91] We can therefore expect that photodegradation is also very important for the fate of dissolved phosphonates in surface waters The photo-degradation products of Fe(III)EDTA are readily biodegradable, but this is not the case for phosphonates [95]

5.3 Chemical degradation Phosphonates are very stable and breakdown of uncomplexed phosphonates requires long timescales and severe chemical conditions At temperatures above

200C free NTMP decomposes to various breakdown products[96,97] These conditions are important for the fate of the chelating agents in technical systems at elevated temperatures, e.g in cooling waters of power plants, but not for natural waters One study performed

at room temperature within the pH range of 2–10 reported that over a several month period, EDTMP hydrolyzed under formation of phosphate, phosphite and hydroxymethylphosphonate (HMP) [98] Other phosphonate-containing breakdown products were pre-sent but were not identified No information on the kinetics or the percentage degraded was given

In natural waters chelating agents and therefore the phosphonates always occur in the form of metal complexes Studies on the chemical degradation of phosphonates should therefore always include the presence of metals Degradation of the amine

DTPMP was negligible in metal-ion free oxygenated solutions, but Ca, Mg, and Fe(II) brought about conversion to free phosphate at a rate of approximately

1 percent per day [99] Although the degradation was classified as hydrolysis, the conversion rate dropped to negligible levels in the absence of O2, indicating that redox reactions play a role HEDP, which does not contain an amine linkage, degrades approximately 20-times more slowly

A loss of NTMP in different natural waters (river waters, groundwaters) and appearance of the degrada-tion products has been observed[21] The conversion of NTMP into iminodimethylenephosphonate (IDMP) and

HMP was attributed to abiotic hydrolysis and the

subsequent conversion to aminomethylphosphonate

(AMPA) and CO2 to microbial degradation The

authors performed a follow-up study in a medium that

was free of microorganisms, but contained mM levels of

Ca, Mg, K, and Na and trace levels (o1 mM) of Fe(III),

Cu(II), Mn(II), and Zn Complete conversion of NTMP

to IDMP, HMP and AMPA occurred within 32 h

Because multiple metal ions were present in these

investigations [21,99], it was not possible to identify

the catalytic agent

A systematic study on the influence of metal ions on

phosphonate breakdown has been reported [33] No

breakdown of NTMP was observed in metal-free

systems and in the presence of Ca, Mg, Zn, Cu(II) and

Fe(III) which disagrees to previous results where

degradation of NTMP was observed in the presence of

Ca or Mg [21] Very rapid degradation of

aminopoly-phosphonates occurred in the presence of Mn(II) and

molecular oxygen[33] The half-life for the reaction of

NTMP in the presence of equimolar Mn(II) and in

equilibrium with 0.21 atm O2was 10 min at pH 6.5 The

reaction occurs more slowly under more alkaline or

acidic conditions In the absence of oxygen no reaction

took place, indicating that an oxidation step was

involved The presence of other cations such as Ca,

Zn, and Cu(II) can considerably slow down the reaction

by competing with Mn(II) for NTMP (Fig 6) Catalytic

Mn(II) is regenerated by oxygen in cyclic fashion as the

reaction takes place The hypothesized pathway is that

Mn(II)-phosphonate is oxidized by molecular oxygen to

the Mn(III)-phosphonate In an intramolecular

redox-reaction the Mn(III) oxidizes the phosphonic acid and is

in turn reduced to Mn(II)

Formate, orthophosphate, IDMP and FIDMP

break-down products have been identified Breakbreak-down also

occurs in oxygen-free suspension of the Mn(III) containing mineral manganite (MnOOH) and with MnOOH in the presence of oxygen [100] EDTMP and DTPMP are also degraded in the presence of Mn(II) and oxygen, although at a slower rate, but not the amine-free HEDP[33] Two of the breakdown products

of NTMP, IDMP and FIDMP, have been detected

in WWTP [34] This indicates that manganese-catalyzed oxidation of aminopolyphosphonate is likely

to be an important degradation mechanism in natural waters

5.4 Degradation during oxidation processes Phosphonates present in natural waters may be subject to oxidation and disinfection processes during drinking water treatment No information on the behavior of phosphonates during chlorination is avail-able Ozonation of NTMP, EDTMP, and DTPMP resulted in the rapid disappearance of the parent compound in less than a minute[101] 60–70% of the degraded phosphonate was found as phosphate; AMPA and phosphonoformic acid were also detected The amine-free HEDP was degraded much more slowly with only 15% degradation after 30 min The reaction path-way of EDTMP during ozonation is equivalent to that

of EDTA [102] The herbicide glyphosate was formed during ozonation of EDTMP with concentration of up

to 10 nM [103] The environmental fate, behavior and analysis of both AMPA and glyphosate has received considerable attention [10] and the formation of these compounds during ozonation of an aminopolypho-sphonate may change the risk analysis of these compounds considerably

6 Speciation The speciation of chelating agents in the environment can be calculated based on the known stability constants

of the metal–ligand complexes and the measured total concentrations of metals and chelating agents This approach has been used to predict the speciation of EDTMP in Rhine water[6] The simulated speciation was dominated by CuEDTMP and ZnEDTMP HEDP was predicted to be mainly complexed with Ca and NTMP with Cuand Zn [104,105] But how accurate are such calculations? There are several points to consider: In speciation calculations it is always assumed that equili-brium has been reached in the system This is not always the case Some metal complexes of aminocarboxy-lates have very slow exchange kinetics[106] It has been found for example that Fe(III)EDTA is not in equilibrium with other metals in river water due to slow exchange kinetics of Fe(III)EDTA [107] Almost nothing is known about the exchange kinetics of metal-phosphonate

ARTICLE IN PRESS

0

20

40

60

80

100

120

no oxygen only Mn(II) Mn(II)/ 0.5 mM Ca Mn(II)/ 10 µM Zn

time (minutes)

Fig 6 Oxidation of 10 mM NTMP in the presence of 10 mM

Mn(II) in the presence and absence of dissolved oxygen and

competing metal ions at pH 7.0 Reprinted with permission

from [33] Copyright (2000) American Chemical Society.

complexes and therefore all equilibrium calculations have

to be treated with care

Most calculations also do not consider that besides

the chelating agent of interest other chelating agents

and natural ligands are present in the water and

com-pete for available metals The interaction between

the binding properties of phosphonates and fulvic

acids is weak [108] but it has been shown that

considering the natural ligands for Cu and Zn is critical

for obtaining an accurate speciation of chelating agents

[109]

In the following section a speciation model for three

phosphonates is developed, based on a river water

sample from Switzerland with well-known composition

of metals, anthropogenic and natural ligands [110]

These ligands compete with the phosphonates for the

same metals and have to be included in the speciation

calculation The concentration of the phosphonates in

the calculations was set to 20 nM, comparable to EDTA

at that location

The speciation was calculated for HEDP and NTMP

with the constants from the IUPAC report[12]and for

DTPMP with the constants from [111] If only total

metals and the phosphonates are taken into

considera-tion, speciation is dominated by Cufor DTPMP, Ca for

HEDP, and Ca, Mg, Zn and Cufor NTMP Including

EDTA and NTA does not change the speciation

significantly; however, as soon as the natural ligands

for Cuand Zn are considered, the calculated speciation

for NTMP and DTPMP changes drastically For

NTMP the Cuand Zn complexes disappear totally due

to the very strong binding of Cuto the natural ligands

and CaNTMP and MgNTMP are dominant For

DTPMP the Ca and Mg complexes also become very

important with more than 60% of the DTPMP

complexed by these metals CuDTPMP is only a minor

species under these conditions For HEDP the alkaline

earth metals Ca and Mg are the major bound metals

under all conditions The fraction of other metal

complexes is never above 0.1% It can be concluded

that phosphonates are most probably complexed to

alkaline earth metals in natural waters This calculation

shows that considering the natural ligands is crucial for

obtaining a reasonable result for phosphonate speciation

(Table 2)

Analytical methods have been developed to determine

directly the speciation of aminocarboxylate chelating

agents[112–114] In principle these methods should also

be applicable to phosphonates A recent very promising

method uses anion-exchange chromatography coupled

to ICP-MS for the separation of metal-chelating agent

complexes [35,36] The method is also applicable to

phosphonates and it has been shown that the

CuEDTMP complex can be determined The use of

these methods to determine the speciation of

phospho-nates in natural waters is needed

7 Behavior during wastewater treatment The studies about the behavior of phosphonates during wastewater treatment can be divided into two groups: field studies with the addition of elevated concentrations of phosphonates to the influent of the treatment plant and investigations at ambient concen-trations

The elimination of phosphonates during wastewater treatment was found to be very high, even with high concentrations of added phosphonates of about 10 mM Elimination of 9.7 mM HEDP in a field experiment was about 60% during the sedimentation and 90–97.5% during the biological step with simultaneous FeCl3 precipitation[60] Lower removal rates of 50–60% were found with the addition of 5–10 mM HEDP and 3–7 mM NTMP to a WWTP without iron-addition [115] The behavior of 4.5–12 mM DTPMP was followed through the different treatment steps[39] It was found that the DTPMP removal in the biological step was 95% After the precipitation step with aluminum sulfate about 97%

of the added DTPMP had been removed This investigation has shown that even without simultaneous addition of iron or aluminum salts, very good removal

in the biological step can be achieved

The second group of studies investigated the fate of phosphonates that are already present in the influent of the WWTP For a 13-day field study a total amount of

117 mol of DTPMP was found in the influent of the WWTP compared to an effluent load of 17 mol, meaning that the removal efficiency was 85%[38]

Elimination of NTMP and EDTMP from another WWTP was at least 80% and 70%, respectively [38] Because the concentration in the effluent was below the detection limit, this removal efficiency is the lower limit

Table 2 Calculated species distribution of HEDP, NTMP, and DTPMP

in river water Conditions: 20 nM phosphonates, 29.4 nM EDTA, 8.6 nM NTA and natural ligands for Cu, Zn and Ni

% of total phosphonate HEDP

With EDTA, NTA, natural ligands

NTMP

With EDTA, NTA, natural ligands

DTPMP

With EDTA, NTA, natural ligands