Chapter 14
Reactions of Phosphonic Acids at the Solid-Water Interface Downloaded by STANFORD UNIV GREEN LIBR on May 11, 2012 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch014
Klaus Fischer Department of Analytical and Ecological Chemistry, FB VI-Faculty of Geography and Geosciences, University of Trier, Trier, Germany
Phosphonic acids contain one or more C-PO(OH) groups and often additional functional groups, i.e. amino or hydroxy groups. Compounds with at least two functional groups have chelating properties, and they are produced for numerous technical and industrial applications, e.g. scale and corrosion inhibitors. Aminopolyphosphonates are often structurally analogous to aminopolycarboxylates, being formally derived from the latter by substitution of carboxylate groups. Reactions at the solid/water-interface are critical for the fate of phosphonates in aquatic environments as well as for their technical use. This review focuses on adsorption-desorption reactions, reflects effects on scale formation, crystal growth and crystal morphology, and considers metal remobilization from sediments. Conclusions concerning the expected environmental behavior of phosphonates were drawn, but the considerable lack of information (e.g. monitoring data, speciation, remobilization of phosphonates from natural sorbents) limits the reliability of such predictions. 2
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© 2005 American Chemical Society In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Introduction Surface reactions are critical for studying the environmental behavior of phosphonates. These reactiosn are also of fondamental importance for modeling predictable environmental concentrations and for the elaboration of an environmental risk assessment for this class of synthetic chelating agents. Several natural phosphonates exist, but so far as known, they do not fonction as "biochelates" (1). Due to their physico-chemical properties (high water solubility, negative log Kow values, very low vapor pressure) and their practical application, most of the released phosphonates, if not eliminated during wastewater treatment, enter aquatic ecosystems. Thus reactive components and physico-chemical conditions that are constitutive for our rivers, lakes and estuarine ecosystems, should be applied in phase transfer experiments. Relevant sorbents are living and dead microorganisms, geochemically generated organic particles, suspended matter, complete sediments and the main sediment constituents, i.e. metal (hydr)oxides, clay minerals, carbonates, silicates and detritus. Viewed by mis standard, the available experimental data are fragmentary and not sufficient to draw sound conclusions. On the other hand, investigations of the application properties of phosphonic acids have built a broad knowledge base which can be exploited for environmental research purposes (2, 3). This applies especially to such effects that belong to reactions at solid/water-interfaces (e.g. inhibition of scale formation (4-6), modification of crystal growth and morphology (7-10), retardation of cement hardening and gypsum settling (11, 12), reduction of the viscosity of clay suspensions and ceramic pastes (2), support of biomineralisation processes (13, 14) and action as dispersants (15)). Thus, this review makes no difference between studies focused on environmental behavior of phosphonates and those engaged in fundamental physico-chemical or technical aspects. Nevertheless it is an open question whether the results of some of the studies attributable to the latter category, e.g. testing the effectiveness of specific phosphonate formulations to inhibit carbonate precipitation (16), are of any significance for chemical reactions in the environment.
Adsorption
General Aspects The majority of the adsorption studies aimed to compare the adsorption behavior of differently structured phosphonic acids and to rank the adsorption capacity of various solids on a rather phenomenoiogical level. Usually, the pH value, the electrolyte concentration, the cation coverage of the sorbent surface, the solid-to-liquid ratio and the reaction time are important reaction parameters.
In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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236 Considering heterogenous solids such as soils or sediments, the different adsorption capacities found were related to their physical properties (specific surface area, grain size distribution, etc.), to their mineralogical composition, and to their organic matter content. Several investigations dealt with mechanistic aspects of the adsorption process. Central questions were the bonding mode and its modification by the reaction conditions (bonding via surface complex formation, electrostatic forces, or hydrogen interactions), the structure of surface complexes formed, and thermodynamic aspects, such as stability of surface complexes, bonding energies and reversibility of the adsorption reaction. Although not explicitly formulated, a few studies (17, 18) intended to formulate qualitative structure-activity relations for the adsorption properties of phosphonates, based on a stringent application of coordination chemistry principles, combined with stereochemical aspects. In this context, the following properties of the ligand functional groups are of high importance: chemical hardness, Lewis basicity, pk value(s), size, and spatial arrangement. At the level of the entire molecule, the maximum denticity of the ligand as well as its dimensions and possible coordination geometries are decisive factors. Consequently the speciation of the solved ligand (i.e. the nature of the coordinated metal ion and the structure of the formed complex), is also regarded as a determinant for the phase distribution of phosphonates (17-21). To elucidate the structure of the formed surface complexes, several spectroscopic techniques were used. Spécifie questions were the differentiation between inner-sphere and outer-sphere surface compiexation, the identification of the binding donor atoms or groups of the ligand, the distinction between mono-, bi- and polydentate surface binding, the distinguishing between monoand binuclear compiexation, the discrimination between "metal-linked" and "ligand-iinked" ternary surface complexes, and the characterization of the surface conformation of the adsorbed phosphonates. For this purpose, various FT-IR techniques, i.e. diffuse reflectance spectroscopy, attenuated total reflectance (ATR) spectroscopy, and cylindrical internal reflectance (CIR) spectroscopy, were predominately used (15, 22-29). Other analytical techniques applied are extended X-ray absorption fine structure (EXAFS) spectroscopy (29, 30), X-ray photoelectron spectroscopy (XPS) (28, 31,32), P and C magic angle spinning (MAS) N M R (22, 33), FT-Raman spectroscopy (23), Auger electron spectroscopy (15) and atomic force microscopy (AFM) (32). Energetic aspects of the interaction between phosphonates and solid surfaces were examined, and corresponding data are completely missing for heterogeneous environmental solids. The reactive chemisorption of methylphosphonic acid at clean and oxidized A l (111) surfaces has been investigated and core binding energies for the molecule units were reported (31). Research on the inhibition of crystal growth and the modification of crystal morphology by phosphonates has paid more attention to energetic reaction terms. For instance binding energies for the docking of several diphosphonates at specific sites, defined for individual crystal faces of barite were calculated using various molecular modeling techniques (34). A similar approach was pursued to compute the energetics of docking of phosphonates on ettringite (11). a
3 ,
l 3
In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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237 Even the reversibility of phosphonate adsorption is rarely investigated. In two studies, the reversibility of the adsorption onto goethite and onto an immobilized humic acid was measured after a rise of the suspension pH (17, 20). In another study desorption of HEDP from several river sediments by differently concentrated salt solutions, diluted nitric acid and natural river water was measured and Freundlich desorption isotherms were calculated (35). In a few examinations differences between the adsorption capacities of aged ("well-developed") and newly generated rough surfaces were highlighted and the distinct adsorption properties of specific crystal faces were described (34, 36). The latter aspect is more recognized in the context of investigations concerned with scale-inhibition reactions and modification of crystal growth and morphology. A synopsis of adsorption studies with aliphatic phosphonic acids is provided in Table I. The elucidation of the adsorption mechanism on a molecular scale creates the prerequisites for the interpretation of resulting phase distribution equilibria and for the rationalization of effects exerted on the sorbents by the adsorbates. Research on the interactions of phosphonates with surfaces of crystalline materials is directed toward the recognition of the critical factors responsible for a specific spatial and electronic matching between adsorbate and substrate. Amongst others, the goodness of this match depends on the correspondence between the dimensions and geometry of the lattice anions and of the binding groups of the adsorbate. A further criterion is the adequacy of the interatomic distances between surface binding sites and between the active functional groups of the adsorbate.
Metal(hydr)oxides Phosphonates adsorb very strongly to almost all mineral surfaces. This behavior is pronounced in the case of metal (hydr)oxides as sorbents.
Goethite (a-FeOOH) Nowack and Stone (17, 18) studied the adsorption of one monophosphonate (MPA), two hydroxyphosphonates (HMP and HEDP) and five aminophosphonates (AMP, IDMP, NTMP, EDTMP and DTPMP) onto goethite as a function of pH. At phosphonate concentrations significantly lower than the total number of available surface sites, nearly 100 % adsorption was observed below pH 8.0. The adsorption approximated to zero percent between pH 9.5 and 12.0, depending on the number of phosphonate groups. This confirms that a certain adsorption capacity is present at pH values above the point of zero charge (8.5). At an excess of surface sites, the adsorption of NTMP as a function of pH was nearly independent from the ionic strength. After a rise of the pH from 7.0 to 12.2, NTMP was completely desorbed within 5 h. At phosphonate concentrations close to the total number of available surface sites, adsorption was highest at lowest pH tested (pH 3.0) and decreased over a broader range in pH. At pH 7.2,
In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Table I: Synopsis of adsorption studies with aliphatic phosphonic acids compound
formula
sorbents'
references
methylphosphonic acid (MP)
CHsCbP
hydroxymethylphosphonic acid (HMP)
CH5O4P
goe goe
17
17,25
aminomethylphosphonic acid (AMP)
CHeNOsP
goe
17,18
N-(phosphonomethyl)glycine' Cglyphosate")
CH
