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Labilities of aqueous nanoparticulate metal complexes in environmental speciation analysis

Raewyn M. Town A C and Herman P. van Leeuwen B
+ Author Affiliations
- Author Affiliations

A Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark.

B Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, NL-6703 HB Wageningen, the Netherlands.

C Corresponding author. Email: raewyn.town@sdu.dk

Environmental Chemistry 11(2) 196-205 https://doi.org/10.1071/EN13138
Submitted: 20 July 2013  Accepted: 10 October 2013   Published: 25 March 2014

Environmental context. Sorbing nanoparticles can have a significant effect on the speciation of small ions and molecules in the environment. The reactivity of nanoparticulate-bound species can differ significantly from that of their molecular or colloidal counterparts. We present a conceptual framework that describes the chemodynamics and lability of nanoparticulate metal complexes over a wide range of experimental timescales and environmental conditions.

Abstract. An inherent property of a dispersion of charged nanoparticles is that their charges and reactive sites are spatially confined to the particle body which is at a different potential from that in the bulk medium. This feature has important consequences for the reactivity of nanoparticulate complexants: the diffusive rate of reactant supply is lower as compared to molecular complexants, whereas the local concentration of reactant ions may be enhanced if the particle’s electric field has the opposite charge sign. These effects are most dramatic for soft nanoparticles for which the electrostatic accumulation mechanisms operate on a 3-D level. We show how the interplay of these effects governs the reactivity of charged nanoparticulate metal complexes (M-NPs) at the surface of an analytical speciation sensor. A theoretical framework is presented that describes the lability of M-NP species over a range of effective timescales for different electrochemical and other dynamic speciation analysis techniques. The concepts are illustrated by electrochemical stripping data on metal complexes with natural soft nanoparticles of humic acid.

Additional keywords: humic acid, kinetics, lability, stripping chronopotentiometry.


References

[1]  M. A. Maurer-Jones, I. L. Gunsolus, C. J. Murphy, C. L. Haynes, Toxicity of engineered nanoparticles in the environment. Anal. Chem. 2013, 85, 3036.
Toxicity of engineered nanoparticles in the environment.Crossref | GoogleScholarGoogle Scholar | 23427995PubMed |

[2]  A. Quigg, W.-C. Chin, C.-S. Chen, S. Zhang, Y. Jiang, A.-J. Miao, K. A. Schwehr, C. Xu, P. H. Santschi, Direct and indirect toxic effects of engineered nanoparticles on algae: role of natural organic matter. ACS Sustainable Chem. Eng. 2013, 1, 686.
Direct and indirect toxic effects of engineered nanoparticles on algae: role of natural organic matter.Crossref | GoogleScholarGoogle Scholar |

[3]  P. Miralles, T. L. Church, A. T. Harris, Toxicity, uptake, and translocation of engineered nanomaterials in vascular plants. Environ. Sci. Technol. 2012, 46, 9224.
Toxicity, uptake, and translocation of engineered nanomaterials in vascular plants.Crossref | GoogleScholarGoogle Scholar | 22892035PubMed |

[4]  N. J. Rogers, N. M. Franklin, S. C. Apte, G. E. Batley, B. M. Angel, J. R. Lead, M. Baalousha, Physico-chemical behavior and algal toxicity of nanoparticulate CeO2 in freshwater. Environ. Chem. 2010, 7, 50.
Physico-chemical behavior and algal toxicity of nanoparticulate CeO2 in freshwater.Crossref | GoogleScholarGoogle Scholar |

[5]  J. P. Pinheiro, H. P. van Leeuwen, Metal speciation dynamics and bioavailability. 2. Radial diffusion effects in the microorganism range. Environ. Sci. Technol. 2001, 35, 894.
Metal speciation dynamics and bioavailability. 2. Radial diffusion effects in the microorganism range.Crossref | GoogleScholarGoogle Scholar | 11351532PubMed |

[6]  W. Fan, M. Cui, H. Liu, C. Wang, Z. Shi, C. Tan, X. Yang, Nano-TiO2 enhances the toxicity of copper in natural water to Daphnia magna. Environ. Pollut. 2011, 159, 729.
Nano-TiO2 enhances the toxicity of copper in natural water to Daphnia magna.Crossref | GoogleScholarGoogle Scholar | 21177008PubMed |

[7]  K. L. Plathe, F. von der Kammer, M. Hassellôv, J. Moore, M. Murayama, T. Hofmann, M. F. Hochella, Using FIFFF and a TEM to determine trace metal-nanoparticle associations in riverbed sediment. Environ. Chem. 2010, 7, 82.
Using FIFFF and a TEM to determine trace metal-nanoparticle associations in riverbed sediment.Crossref | GoogleScholarGoogle Scholar |

[8]  J. Buffle, Complexation Reactions in Aquatic Systems: an Analytical Approach 1988 (Ellis Horwood: Chichester, UK).

[9]  A. Tessier, Sorption of trace elements on natural particles in oxic environments, in Environmental Particles, Vol. 1 (Eds J. Buffle, H. P. van Leeuwen) 1992, pp. 425–453 (Lewis Publishers: Boca Raton).

[10]  W. H. van Riemsdijk, L. K. Koopal, Ion binding by natural heterogeneous colloids, in Environmental Particles, Vol. 1 (Eds J. Buffle, H. P. van Leeuwen) 1992, pp. 455–495 (Lewis Publishers: Boca Raton, FL).

[11]  F. J. Doucet, J. R. Lead, P. H. Santschi, Colloid-trace element interactions in aquatic systems, in Environmental Colloids and Particles. Behaviour, Separation and Characterisation (Eds K. J. Wilkinson, J. R. Lead) 2007, pp. 95–157 (Wiley: Chichester, UK).

[12]  R. M. Town, J. Buffle, J. F. L. Duval, H. P. van Leeuwen, Chemodynamics of soft charged nanoparticles in aquatic media: fundamental aspects. J. Phys. Chem. A 2013, 117, 7643.
Chemodynamics of soft charged nanoparticles in aquatic media: fundamental aspects.Crossref | GoogleScholarGoogle Scholar | 23806009PubMed |

[13]  H. P. van Leeuwen, R. M. Town, J. Buffle, R. F. M. J. Cleven, W. Davison, J. Puy, W. H. van Riemsdijk, L. Sigg, Dynamic speciation analysis and bioavailability of metals in aquatic systems. Environ. Sci. Technol. 2005, 39, 8545.
Dynamic speciation analysis and bioavailability of metals in aquatic systems.Crossref | GoogleScholarGoogle Scholar | 16323747PubMed |

[14]  M. Eigen, Fast elementary steps in chemical reaction mechanisms. Pure Appl. Chem. 1963, 6, 97.
Fast elementary steps in chemical reaction mechanisms.Crossref | GoogleScholarGoogle Scholar |

[15]  H. P. van Leeuwen, R. M. Town, J. Buffle, Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges. Langmuir 2011, 27, 4514.
Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges.Crossref | GoogleScholarGoogle Scholar | 21410210PubMed |

[16]  J. F. L. Duval, H. P. van Leeuwen, Rates of ionic reactions with charged nanoparticles in aqueous media. J. Phys. Chem. A 2012, 116, 6443.
Rates of ionic reactions with charged nanoparticles in aqueous media.Crossref | GoogleScholarGoogle Scholar |

[17]  H. P. van Leeuwen, R. M. Town, J. Buffle, Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges. Langmuir 2011, 27, 4514.
Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges.Crossref | GoogleScholarGoogle Scholar | 21410210PubMed |

[18]  H. P. van Leeuwen, J. Buffle, R. M. Town, Electric relaxation processes in chemodynamics of aqueous metal complexes: from simple ligands to soft nanoparticulate complexants. Langmuir 2012, 28, 227.
Electric relaxation processes in chemodynamics of aqueous metal complexes: from simple ligands to soft nanoparticulate complexants.Crossref | GoogleScholarGoogle Scholar | 22126743PubMed |

[19]  H. P. van Leeuwen, R. M. Town, Kinetic limitations in measuring stabilities of metal complexes by competitive ligand exchange-adsorptive stripping voltammetry (CLE-AdSV). Environ. Sci. Technol. 2005, 39, 7217.
Kinetic limitations in measuring stabilities of metal complexes by competitive ligand exchange-adsorptive stripping voltammetry (CLE-AdSV).Crossref | GoogleScholarGoogle Scholar | 16201651PubMed |

[20]  H. P. van Leeuwen, Revisited: the conception of lability of metal complexes. Electroanal. 2001, 13, 826.
Revisited: the conception of lability of metal complexes.Crossref | GoogleScholarGoogle Scholar |

[21]  J. Heyrovský, J. Kuta, Principles of Polarography 1966 (Academic Press: New York).

[22]  Z. Zhang, J. Buffle, H. P. van Leeuwen, Roles of dynamic metal speciation and membrane permeabiity in metal flux through lipophilic membranes: general theory and experimental validation with nonlabile complexes. Langmuir 2007, 23, 5216.
Roles of dynamic metal speciation and membrane permeabiity in metal flux through lipophilic membranes: general theory and experimental validation with nonlabile complexes.Crossref | GoogleScholarGoogle Scholar | 17391055PubMed |

[23]  J. D. Ritchie, E. M. Perdue, Proton-binding study of standard and reference fulvic acid, humic acids, and natural organic matter. Geochim. Cosmochim. Acta 2003, 67, 85.
Proton-binding study of standard and reference fulvic acid, humic acids, and natural organic matter.Crossref | GoogleScholarGoogle Scholar |

[24]  R. M. Town, J. F. L. Duval, J. Buffle, H. P. van Leeuwen, Chemodynamics of metal complexation of natural soft colloids: CuII binding by humic acid. J. Phys. Chem. A 2012, 116, 6489.
Chemodynamics of metal complexation of natural soft colloids: CuII binding by humic acid.Crossref | GoogleScholarGoogle Scholar | 22324832PubMed |

[25]  M. Hosse, K. J. Wilkinson, Determination of electrophoretic mobilities and hydrodynamic radii of three humic substances as a function of pH and ionic strength. Environ. Sci. Technol. 2001, 35, 4301.
Determination of electrophoretic mobilities and hydrodynamic radii of three humic substances as a function of pH and ionic strength.Crossref | GoogleScholarGoogle Scholar | 11718346PubMed |

[26]  R. M. Town, H. P. van Leeuwen, Depletive stripping chronopotentiometry: a major step forward in electrochemical stripping techniques for metal ion speciation analysis. Electroanalysis 2004, 16, 458.
Depletive stripping chronopotentiometry: a major step forward in electrochemical stripping techniques for metal ion speciation analysis.Crossref | GoogleScholarGoogle Scholar |

[27]  R. M. Town, H. P. van Leeuwen, Fundamental features of metal ion determination by stripping chronopotentiometry. J. Electroanal. Chem. 2001, 509, 58.
Fundamental features of metal ion determination by stripping chronopotentiometry.Crossref | GoogleScholarGoogle Scholar |

[28]  J. P. Pinheiro, H. P. van Leeuwen, Scanned stripping chronopotentiometry of metal complexes: lability diagnosis and stability computation. J. Electroanal. Chem. 2004, 570, 69.
Scanned stripping chronopotentiometry of metal complexes: lability diagnosis and stability computation.Crossref | GoogleScholarGoogle Scholar |

[29]  R. F. M. J. Cleven, H. P. van Leeuwen, Electrochemical analysis of the heavy metal–humic acid interaction. Int. J. Environ. Anal. Chem. 1986, 27, 11.
Electrochemical analysis of the heavy metal–humic acid interaction.Crossref | GoogleScholarGoogle Scholar |

[30]  M. O. von Stackelberg, M. Pilgram, V. Toome, Bestimmung von Diffusionskoeffizienten einiger Ionen in wâßriger Lösung in Gegenwart von Fremdelektrolyten. I. Z. Elektrochem. 1953, 57, 342.

[31]  R. S. Altmann, J. Buffle, The use of differential equilibrium functions for interpretation of metal binding in complex ligand systems: its relation to site occupation and site affinity distributions. Geochim. Cosmochim. Acta 1988, 52, 1505.
The use of differential equilibrium functions for interpretation of metal binding in complex ligand systems: its relation to site occupation and site affinity distributions.Crossref | GoogleScholarGoogle Scholar |

[32]  M. Filella, J. Buffle, H. P. van Leeuwen, Effect of physicochemical heterogeneity of natural complexants. Part I. Voltammetry of labile metal–fulvic complexes. Anal. Chim. Acta 1990, 232, 209.
Effect of physicochemical heterogeneity of natural complexants. Part I. Voltammetry of labile metal–fulvic complexes.Crossref | GoogleScholarGoogle Scholar |

[33]  R. M. Town, H. P. van Leeuwen, J. Buffle, Chemodynamics of soft nanoparticulate complexes: CuII and NiII complexes with fulvic acids and aquatic humic acids. Environ. Sci. Technol. 2012, 46, 10 487.
Chemodynamics of soft nanoparticulate complexes: CuII and NiII complexes with fulvic acids and aquatic humic acids.Crossref | GoogleScholarGoogle Scholar |

[34]  H. P. van Leeuwen, J. Buffle, Voltammetry of heterogeneous metal complex systems. Theoretical analysis of the effects of association/dissociation kinetics and the ensuing lability criteria. J. Electroanal. Chem. 1990, 296, 359.
Voltammetry of heterogeneous metal complex systems. Theoretical analysis of the effects of association/dissociation kinetics and the ensuing lability criteria.Crossref | GoogleScholarGoogle Scholar |

[35]  S. Orsetti, J. L. Marco-Brown, E. M. Andrade, F. V. Molina, PbII binding to humic substances: an equilibrium and spectroscopic study. Environ. Sci. Technol. 2013, 47, 8325.
| 23805795PubMed |

[36]  I. Christl, A. Metzger, I. Heidmann, R. Kretzschmar, Effect of humic and fulvic acid concentrations and ionic strength on copper and lead binding. Environ. Sci. Technol. 2005, 39, 5319.
Effect of humic and fulvic acid concentrations and ionic strength on copper and lead binding.Crossref | GoogleScholarGoogle Scholar | 16082962PubMed |

[37]  M. F. Benedetti, C. J. Milne, D. G. Kinniburgh, W. H. van Riemsdijk, L. K. Koopal, Metal ion binding to humic substances: application of the non-ideal competitive adsorption model. Environ. Sci. Technol. 1995, 29, 446.
Metal ion binding to humic substances: application of the non-ideal competitive adsorption model.Crossref | GoogleScholarGoogle Scholar | 22201392PubMed |

[38]  I. Christl, C. J. Milne, D. G. Kinniburgh, R. Kretzschmar, Relating ion binding by fulvic and humic acids to chemical composition and molecular size. 2. Metal binding. Environ. Sci. Technol. 2001, 35, 2512.
Relating ion binding by fulvic and humic acids to chemical composition and molecular size. 2. Metal binding.Crossref | GoogleScholarGoogle Scholar | 11432556PubMed |

[39]  T. Saito, S. Nagasaki, S. Tanaka, L. K. Koopal, Application of the NICA–Donnan model for proton, copper and uranyl binding to humic acid. Radiochim. Acta 2004, 92, 567.
Application of the NICA–Donnan model for proton, copper and uranyl binding to humic acid.Crossref | GoogleScholarGoogle Scholar |

[40]  D. Gondar, A. Iglesias, R. López, S. Fiol, J. M. Antelo, F. Arce, Copper binding by peat fulvic and humic acids extracted from two horizons of an ombrotrophic peat bog. Chemosphere 2006, 63, 82.
Copper binding by peat fulvic and humic acids extracted from two horizons of an ombrotrophic peat bog.Crossref | GoogleScholarGoogle Scholar | 16146645PubMed |

[41]  C. Kolokassidou, I. Pasahlidis, Potentiometric investigations on the interaction of humic acid with CuII and EuIII ions. Radiochim. Acta 2006, 94, 549.
Potentiometric investigations on the interaction of humic acid with CuII and EuIII ions.Crossref | GoogleScholarGoogle Scholar |

[42]  R. Vidali, E. Remoundaki, M. Tsezos, An experimental and modeling study of Cu2+ binding on humic acids at various solution conditions. Application of the NICA–Donnan model. Water Air Soil Pollut. 2011, 218, 487.
An experimental and modeling study of Cu2+ binding on humic acids at various solution conditions. Application of the NICA–Donnan model.Crossref | GoogleScholarGoogle Scholar |

[43]  J. Puy, J. Galceran, C. Huidobro, E. Companys, N. Samper, J. L. Garcés, F. Mas, Conditional affinity spectra of Pb2+–humic acid complexation from data obtained with AGNES. Environ. Sci. Technol. 2008, 42, 9289.
Conditional affinity spectra of Pb2+–humic acid complexation from data obtained with AGNES.Crossref | GoogleScholarGoogle Scholar | 19174906PubMed |

[45]  D. Gondar, R. López, S. Fiol, J. M. Antelo, F. Arce, Cadmium, lead, and copper binding to humic acid and fulvic acid extracted from an ombrotrophic peat bog. Geoderma 2006, 135, 196.
Cadmium, lead, and copper binding to humic acid and fulvic acid extracted from an ombrotrophic peat bog.Crossref | GoogleScholarGoogle Scholar |

[44]  C. J. Milne, D. G. Kinniburgh, J. C. M. de Wit, W. H. van Riemsdijk, L. K. Koopal, Analysis of metal-ion binding by a peat humic acid using a simple electrostatic model. J. Colloid Interface Sci 1995, 175, 448.
Analysis of metal-ion binding by a peat humic acid using a simple electrostatic model.Crossref | GoogleScholarGoogle Scholar |

[46]  A. W. Rate, Sorption of cadmium(II) and copper(II) by soil humic acids: temperature effects and sorption heterogeneity. Chem. Ecol. 2010, 26, 371.
Sorption of cadmium(II) and copper(II) by soil humic acids: temperature effects and sorption heterogeneity.Crossref | GoogleScholarGoogle Scholar |

[47]  D. H. Powell, L. Helm, A. E. Merbach, 17O nuclear magnetic resonance in aqueous solutions of Cu2+: the combined effect of Jahn–Teller inversion and solvent exchange on relaxation rates. J. Chem. Phys. 1991, 95, 9258.
17O nuclear magnetic resonance in aqueous solutions of Cu2+: the combined effect of Jahn–Teller inversion and solvent exchange on relaxation rates.Crossref | GoogleScholarGoogle Scholar |

[48]  R. Poupko, Z. Luz, ESR and NMR in aqueous and methanol solutions of copper(II) solvates. Temperature and magnetic field dependence of electron and nuclear spin relaxation. J. Chem. Phys. 1972, 57, 3311.
ESR and NMR in aqueous and methanol solutions of copper(II) solvates. Temperature and magnetic field dependence of electron and nuclear spin relaxation.Crossref | GoogleScholarGoogle Scholar |

[49]  L. S. W. L. Sokol, T. D. Fink, D. B. Rorabacher, Kinetics of inner-sphere solvent exchange on the aquocopper(II) ion: indirect determination from kinetics of copper(II) reacting with ammonia in aqueous solution. Inorg. Chem. 1980, 19, 1263.
Kinetics of inner-sphere solvent exchange on the aquocopper(II) ion: indirect determination from kinetics of copper(II) reacting with ammonia in aqueous solution.Crossref | GoogleScholarGoogle Scholar |

[50]  W. B. Lewis, M. Alei, Magnetic resonance studies on copper(II) complex ions in solution. I. Temperature dependences of the 17O NMR and copper(II) EPR linewidths of Cu(H2O)62+. J. Chem. Phys. 1966, 44, 2409.
Magnetic resonance studies on copper(II) complex ions in solution. I. Temperature dependences of the 17O NMR and copper(II) EPR linewidths of Cu(H2O)62+.Crossref | GoogleScholarGoogle Scholar |

[51]  F. M. M. Morel, J. G. Hering, Principles and Applications of Aquatic Chemistry 1993 (Wiley: New York).

[52]  J. C. M. de Wit, W. H. van Riemsdijk, L. K. Koopal, Proton binding to humic substances. 1. Electrostatic effects. Environ. Sci. Technol. 1993, 27, 2005.
Proton binding to humic substances. 1. Electrostatic effects.Crossref | GoogleScholarGoogle Scholar |

[53]  S. Scally, H. Zhang, W. Davison, Measurements of lead complexation with organic ligands using DGT. Aust. J. Chem. 2004, 57, 925.
Measurements of lead complexation with organic ligands using DGT.Crossref | GoogleScholarGoogle Scholar |

[54]  H. Zhang, W. Davison, In situ speciation measurements. Using diffusive gradients in thin films (DGT) to determine inorganically and organically complexed metals. Pure Appl. Chem. 2001, 73, 9.
In situ speciation measurements. Using diffusive gradients in thin films (DGT) to determine inorganically and organically complexed metals.Crossref | GoogleScholarGoogle Scholar |

[55]  J. Puy, R. Uribe, S. Mongin, J. Galceran, J. Cecília, J. Levy, H. Zhang, W. Davison, Lability criteria in diffusive gradients in thin films. J. Phys. Chem. A 2012, 116, 6564.
Lability criteria in diffusive gradients in thin films.Crossref | GoogleScholarGoogle Scholar | 22404162PubMed |

[56]  P. L. R. van der Veeken, H. P. van Leeuwen, Gel-water partitioning of soil humics in diffusive gradient in thin film (DGT) analysis of their metal complexes. Environ. Chem. 2012, 9, 24.
Gel-water partitioning of soil humics in diffusive gradient in thin film (DGT) analysis of their metal complexes.Crossref | GoogleScholarGoogle Scholar |

[57]  L. Sigg, F. Black, J. Buffle, J. Cao, R. Cleven, W. Davison, J. Galceran, P. Gunkel, E. Kalis, D. Kistler, M. Martin, S. Noël, Y. Nur, N. Odzak, J. Puy, W. van Riemsdijk, E. Temminghoff, M.-L. Tercier-Waeber, S. Toepperwien, R. M. Town, E. Unsworth, K. W. Warnken, L. Weng, H. Xue, H. Zhang, Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters. Environ. Sci. Technol. 2006, 40, 1934.
Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters.Crossref | GoogleScholarGoogle Scholar | 16570618PubMed |

[58]  M. N. Bravin, C. Garnier, V. Lenoble, F. Gérard, Y. Dudal, P. Hinsinger, Root-induced changes in pH and dissolved organic matter binding capacity affect copper dynamic speciation in the rhizosphere. Geochim. Cosmochim. Acta 2012, 84, 256.
Root-induced changes in pH and dissolved organic matter binding capacity affect copper dynamic speciation in the rhizosphere.Crossref | GoogleScholarGoogle Scholar |

[59]  J. Salvador, J. L. Garcés, E. Companys, J. Cecília, J. Galceran, J. Puy, R. M. Town, Ligand mixture effects in metal complex lability. J. Phys. Chem. A 2007, 111, 4304.
Ligand mixture effects in metal complex lability.Crossref | GoogleScholarGoogle Scholar | 17469809PubMed |

[60]  B. J. Stanley, K. Topper, D. B. Marshall, Analysis of the heterogeneous rate of dissociation of CuII from humic and fulvic acids by statistical deconvolution. Anal. Chim. Acta 1994, 287, 25.
Analysis of the heterogeneous rate of dissociation of CuII from humic and fulvic acids by statistical deconvolution.Crossref | GoogleScholarGoogle Scholar |

[61]  J. A. Lavigne, C. H. Langford, M. K. S. Mak, Kinetic study of speciation of nickel(II) bound to a fulvic acid. Anal. Chem. 1987, 59, 2616.
Kinetic study of speciation of nickel(II) bound to a fulvic acid.Crossref | GoogleScholarGoogle Scholar |

[62]  S. E. Cabaniss, pH and ionic strength effects on nickel-fulvic acid dissociation kinetics. Environ. Sci. Technol. 1990, 24, 583.
pH and ionic strength effects on nickel-fulvic acid dissociation kinetics.Crossref | GoogleScholarGoogle Scholar |

[63]  J. P. Pinheiro, A. M. Mota, M. L. S. S. Gonçalves, H. P. van Leeuwen, The pH effect in the diffusion coefficient of humic matter: influence in speciation studies using voltammetric techniques. Colloids Surf. A 1998, 137, 165.
The pH effect in the diffusion coefficient of humic matter: influence in speciation studies using voltammetric techniques.Crossref | GoogleScholarGoogle Scholar |

[64]  J. R. Lead, K. Starchev, K. J. Wilkinson, Diffusion coefficients of humic substances in agarose gel and in water. Environ. Sci. Technol. 2003, 37, 482.
Diffusion coefficients of humic substances in agarose gel and in water.Crossref | GoogleScholarGoogle Scholar | 12630462PubMed |

[65]  A. Nebbioso, A. Piccolo, Molecular rigidity and diffusivity of Al3+ and Ca2+ humates as revealed by NMR spectroscopy. Environ. Sci. Technol. 2009, 43, 2417.
Molecular rigidity and diffusivity of Al3+ and Ca2+ humates as revealed by NMR spectroscopy.Crossref | GoogleScholarGoogle Scholar | 19455755PubMed |