Surface complexation at mineral interfaces: Multisite and Charge Distribution approach
Tjisse Hiemstra
Thesis committee Thesis supervisor Prof. Dr. W.H. Van Riemsdijk Professor of Soil Chemistry and Chemical Soil Quality Wageningen University Other members Prof. Dr. M.A. Cohen Stuart, Wageningen University Prof. Dr. R. Kretzschmar, Swiss Federal Institute of Technology, Zürich, Switzerland Dr. M. L. Machesky, Illinois State Water Survey, Champaign, USA Prof. Dr. S. Sjöberg, University of Umeå, Sweden
Surface complexation at mineral interfaces: Multisite and Charge Distribution approach
Tjisse Hiemstra
Thesis submitted in fulfillment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. dr. M.J. Kropff, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Wednesday 13th October 2010 at 1.30 p.m. in the Aula.
Tjisse Hiemstra Surface complexation at mineral interfaces: Multisite and Charge Distribution approach, 383 pages. Thesis Wageningen University, Wageningen NL (2010) With references, with summary in Dutch and English ISBN 978-90-8585-717-4
Abstract Ion adsorption is highly relevant in science, technology, and the environment. For many elements, the ion concentration is regulated by adsorption and desorption. In nature, three main classes of reactive materials are present, each with a specific characteristic in relation to ion binding. For organic matter, the main keyword is chemical heterogeneity, for clay minerals, it is permanent negative charge with corresponding cation exchange, and for metal (hydr)oxides, it is the dominance of electrostatics in ion binding. This thesis describes a new framework for ion binding to metal (hydr)oxides that allows linking of the microscopic processes of ion binding to macroscopic adsorption phenomena. The novel framework is based on a structural approach of mineral surfaces as well as surface complexes, being described with respectively with the MUlti-SIte Complexation (MUSIC) model and the Charge Distribution (CD) approach. In the MUSIC model, surface groups are distinguished based on their metal coordination that creates differences in charge. This leads to variation in affinity of the surface oxygens for protons. With the MUSIC model, the intrinsic proton affinity of the various types of groups can be derived, which is essential to calculate the overall surface charge of metal (hydr)oxides. For calculation of the particle charge, electrostatic theory is applied since accumulation of protons at the surface will create an electrostatic field that is experienced by the adsorbing protons themselves as well as other ions. The field will strongly affect the ion binding and therefore it is essential to account for this. Ions form complexes at the surface and the structure of the complexes can be elucidated with in-situ spectroscopy and molecular modeling. At the scale of the interface, surface complexes will experience a gradient of repulsion and/or attraction by the electrostatic field. To account for this important phenomenon, a new approach, known as the CD model, has been developed as logical extension of the MUSIC model. The charge distribution is tightly linked to the microscopic structure of the surface complexes. The innovative CD model is able to calculate accurately the electrostatic energy involved in ion binding. This is essential since ion binding at mineral-solution interfaces is dominated by changes in the electrostatic field at variation of solution conditions such as pH, ionic strength, and the concentration of the ion involved as well as of all its competitors. The new ion adsorption framework is able to describe the main macroscopic adsorption phenomena. The CD-MUSIC model has been tested successfully for a series of ions bound in competition to different mineral surfaces, in particular the various Fe and Al hydroxides that play an important role in the environment. To apply the model in field samples, a new methodology has been developed to measure the equivalent reactive surface area of the natural oxide fraction. It reveals that the metal oxide fraction can be considered as a collection of nanoparticles that are covered by and/or embedded in a matrix of natural organic matter. To apply the new framework in soil, a practical solution has been suggested to account for the interaction of oxide particles with natural organic matter.
Content
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Preface and outline of the thesis
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On the relationship between surface structure and ion complexation of oxidesolution interfaces
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Intrinsic proton affinity of reactive groups of metal (hydr)oxide
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The interfacial charging phenomena of Al (hydr)oxides
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A surface structural approach to ion adsorption: The Charge Distribution model
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On the relationship between charge distribution, surface hydration, and the structure of the interface of metal (hydr)oxides
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Interaction of silicic acid with goethite
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Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides
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Nanoparticles in natural systems I: The effective surface area of the natural oxide fraction in field samples
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Nanoparticles in natural systems II: The natural oxide fraction at interaction with natural organic matter and phosphate
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Summary
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Samenvatting
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Dankwoord
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About the author
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Bestaan is een feit, leven een kunst
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Dankwoord enjoyed your enthusiasm to work. Many thanks. In 2005, Mark Barnett has visited our department during his sabbatical leave. He has contributed to the paper on the adsorption of silicic acid that is part of the present thesis. Mark, thanks. Rather recently, Juan Antelo Martinez has been in our lab and together with him, we have further developed a method to estimate the reactive surface area of the soil oxides fraction and the interaction of this fraction with organic matter and phosphate. Juan, I hope you have enjoyed it as much as I did. Thanks for your contributions resulting in the last two papers of this thesis. A very special word is for Moira Ridley. We met a number of years ago at an ACS conference in the USA where you gave a solid and impressive presentation, as you always do. During your sabbatical in Wageningen, we have analyzed your extensive date sets on rutile. I have tremendously enjoyed the work we did together. It was exciting to model your data, revealing systemically that simple electrolyte cations may form innersphere complexes and how this can be extracted from your high-quality data with the CD model. It is a large step forward. Moira, you have done a great job. When you left Wageningen, I was enthusiastic about the results, but I know that you are not easily satisfied. At home, you have critically improved it far above any of my expectations. Almost “too good to be true”. I am proud that I have been part of the work and I look forward to cooperate with you in the future. Many thanks. On a conference in München, I have met André Rossberg. He asked me for cooperation to understand the surface complexation of uranium. The match with you was good and your uncomplicated attitude I enjoy most. We have worked together in your project and made two nice papers. Thank you very much for the pleasant and humorous email contacts we have. Aé, I hope we can continue our work in one way or another. Hoewel mijn directe activiteiten op het laboratorium door de jaren heen relatief beperkt zijn geweest, betekent dit niet dat de bijdragen van de talloze medewerkers van het laboratorium minder belangrijk zouden zijn. Allereerst is er de goede kwaliteit van meten. Betrouwbare en nauwkeurige gegevens zijn de hoeksteen. Ook was er de bereidwilligheid me te helpen met allerlei hand- en spandiensten en om mijn vragen te beantwoorden. Hiervoor allen dank, in het bijzonder Peter Nobels, Gerlinde Vink en Monique Driessen, Jaap Nelemans en Willeke van Tintelen, Arie van de Berg, en heel veel anderen onder wie Kees Koenders. De aanwezigheid van een secretariaat en administratie maakt het werken aangenaam en gemakkelijk. "Tjisse, let je hier of daar even op? Heb je dat al gedaan?" En altijd wel een praatje of een lolletje. Riette, Caroline, Minke, Esther, and Winnie en vele anderen, heel veel dank voor deze zorgzaamheid en vooral jullie geduld met mij. Ook een woord van dank aan alle studenten die bij mij een afstudeervak deden. Ze hebben experimenten gedaan en gegevens verzameld die in een aantal gevallen deel werden van een publicatie. Maar minstens zo belangrijk, ze stelden de juiste en “onjuiste” vragen en waren dan een gewillig oor bij een antwoord dat gestructureerd zou moeten zijn maar dat daarbij kon blootleggen wat nog niet helder werd begrepen. Ook wil ik mijn collega’s bedanken waarmee in het onderwijs heb samengewerkt. Door hun andere manier van kijken kan je veel leren over de betekenis van de bodem in landbouw, ecologie, milieu, en system Earth. Hartelijk dank. 378
Dankwoord Na al die vele woorden zou je kunnen denken waar blijft het bijzondere woord van dank voor de promotor? Beste Willem, waar zou het onderzoek zijn uitgekomen zonder jouw inbreng? Zeker is, een heel andere en veel bescheidener uitkomst. Je heldere manier van denken en je aanwijzingen zijn altijd van groot belang geweest. Je bent optimistisch en bent niet bang voor uitdagingen, mogelijke en schijnbaar onmogelijke én onmogelijke. Het is uiterst inspirerend. Door de jaren heen heb je steeds weer voor mij kansen gecreëerd en stimulerende initiatieven genomen. Daarbij ben je plezierig en gemakkelijk in omgang. Het is altijd fijn om met je te “pingpongen”. Dit is het goede woord. Je doet het samen. In dat spel, houd je de bal op de tafel maar je speelt scherp. Je daagt uit en corrigeert. Ik heb heel veel aan je te danken. Je bent een uitstekend klankbord. Je hebt een fijne neus voor wat wel of niet een goede weg kan zijn. Daarmee bof ik en al de anderen van onze groep. Als er in de kantlijn van het manuscript “hmm~~” staat dan ben ik gewaarschuwd. Mogelijk is hier iets niet in orde, wat ik serieus neem vanwege je bewuste en onbewuste gevoel voor potentiële “nattigheid”. Maar gelukkig duld je tegenspraak. Dat maakt het voor me bijzonder. Eigenwijsheid mág en móet. Ook dank ik jou en Trudy voor al die andere zaken buiten het directe werk. De plezierige reis(z)en die we hebben gemaakt, het lenen van een viool, het aanbod om jullie gárage (je uitspraak ☺) ter beschikking te stellen voor het restaureren van een bootje, je zeillessen, jullie uitnodiging voor een etentje, en het delen van allerlei ervaringen in goede en soms mindere tijden. Heel fijn. Tot slot is er een bijzonder woord van dank voor het thuisfront. Nelly, jij en ik verstaan elkaar vaak al met een half woord. Een man als onderzoeker is wel eens een crime. Nooit klaar met het werk. Vaak te laat. Waar zit je toch met je gedachten? Mijn oprechte excuses. Je geduld met mij en mijn onderzoek is te groot om onder woorden te brengen. Met veel liefs bedank ik je voor jouw vaak onvoorwaardelijke steun die mij en ons hier heeft gebracht. En kids, jullie zijn groot en je vader is trots op zijn dochters. Het is fijn te zien dat jullie allemaal jullie gegeven talenten hebben kunnen én willen benutten. Dat maakt het leven nog extra aangenaam. Niet onvermeld mag blijven mijn dank voor de dierbare zorg en toewijding van mijn ouders.
Jullie aller, Tjisse ☺
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About the author
About the author Tjisse Hiemstra was born as son of Thomas Hiemstra and Neeltje Meidertsma in Zwolle, The Netherlands, at December 3rd, 1954. He did his secondary school (“HBS-B”) at the “Meander College” in Zwolle. He studied Soil Sciences (“Bodemkunde en Bemestingsleer”) at Wageningen University (formerly "Landbouw Hogeschool, LH”), The Netherlands and graduated with honors in 1980. Soil Formation and Field Soil Sciences (“Regionale Bodemkunde”) was his major and he did additionally three minors, i.e. in Clay mineralogy, Soil Chemistry, and Colloid Sciences. This combination of disciplines reflects his profound interest in natural processes based on a fundamental and mechanistic understanding. He started his career at the department Petrology, Mineralogy, Crystallography, Geochemistry, and Soil science of Utrecht University, The Netherlands in May 1980, but returned a short time later to Wageningen where he became staff member in the group ”Soil Physics and Chemistry” of Prof. Dr. Ir. Gerard Bolt and his successor in “Soil Chemistry” Prof. Dr. Willem van Riemsdijk, at the former Department Soil Science and Plant Nutrition, presently known as Soil Quality. He obtained the Certificate of Proficiency of the Netherlands Institute for Catalysis Research after following the post-graduated course in Catalysis in 1994. Tjisse Hiemstra is married with Nelly Vrolijk and they have three daughters, Hannah Eleonore, Anna Louise, and Johanna Maria.
List of publications Bourikas K., Hiemstra T., and Van Riemsdijk W. H. (2001) Adsorption of molybdate monomers and polymers on titania with a multisite approach. J. Phys. Chem. B 105(12), 2393-2403. Bourikas K., Hiemstra T., and Van Riemsdijk W. H. (2001) Ion Pair Formation and Primary Charging behaviour of Titanium Oxide (Anatase and Rutile). Langmuir 17(3), 749756. Filius J. D., Hiemstra T., and Van Riemsdijk W. H. (1997) Adsorption of small weak organic acids on goethite: Modeling of mechanisms. J. Colloid Interf. Sci. 195(2), 368-380. Filius J. D., Hiemstra T., and Van Riemsdijk W. H. (2000) Adsorption of Fulvic Acid on Goethite. Geochim. Cosmochim. Acta 64(1), 51-60. Filius J. D., Meeussen J. C. L., Hiemstra T., and van Riemsdijk W. H. (2001) Modeling the binding of benzenecarboxylates by goethite: The ligand and charge distribution model. J. Colloid Interf. Sci. 244(1), 31-42. Filius J. D., Meeussen J. C. L., Lumsdon D. G., Hiemstra T., and Van Riemsdijk W. H. (2003) Modeling the binding of fulvic acid by goethite: The speciation of adsorbed FA molecules. Geochim. Cosmochim. Acta 67(8), 1463-1474. Geelhoed J. S., Hiemstra T., and Van Riemsdijk W. H. (1997) Phosphate and sulfate adsorption on goethite: Single anion and competitive adsorption. Geochim. Cosmochim. Acta 61, 2389. Geelhoed J. S., Hiemstra T., and Van Riemsdijk W. H. (1998) Competitive Interaction between Phosphate and Citrate on Goethite. Environ. Sci. Technol. 32, 2119-2123.
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About the author Hiemstra T., Antelo J., Rahnemaie R., and van Riemsdijk W. H. (2010) Nanoparticles in natural systems I: The effective reactive surface area of the natural oxide fraction in field samples. Geochim. Cosmoschim. Acta 74(1), 41-58. Hiemstra T., Antelo J., van Rotterdam A. M. D., and van Riemsdijk W. H. (2010) Nanoparticles in natural systems II: The natural oxide fraction at interaction with natural organic matter and phosphate. Geochim. Cosmoschim. Acta 74(1), 59-69. Hiemstra T., Barnett M. O., and Van Riemsdijk W. H. (2007) Interaction of Silicic Acid with Goethite. J. Colloid Interf. Sci. 310, 8-17. Hiemstra T., De Wit J. C. M., and Van Riemsdijk W. H. (1989b) Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach. II. Application to various important (hydr)oxides. J. Colloid Interf. Sci. 133, 105-117. Hiemstra T., Han Yong, and Van Riemsdijk W. H. (1999b) Interfacial Charging Phenomena of Aluminum (Hydr)oxides. Langmuir 15, 5942-5955. Hiemstra T., Rahnemaie R., and Van Riemsdijk W. H. (2004) Surface Complexation of Carbonate on Goethite: IR spectroscopy, Structure and Charge Distribution. J. Colloid Interf. Sci. 278, 282-290. Hiemstra T. and Van Riemsdijk W. H. (1990) The kinetics of dissolution reactions of Al(hydr)oxides: role of surface structure and multisite surface speciation. Trans. 14th Int. Congr. Soil Sci. Kyoto Vol. 2, 28-33. Hiemstra T. and Van Riemsdijk W. H. (1990) Multiple Activated Complex Dissolution of Metal (Hydr)Oxides: A Thermodynamic Approach Applied to Quartz. J. Colloid Interf. Sci. 136(1), 132-150. Hiemstra T. and Van Riemsdijk W. H. (1991) Physical chemical interpretation of primary charging behaviour of metal (hydr)oxides. Colloids Surfaces 59, 7-25. Hiemstra T. and Van Riemsdijk W. H. (1996a) A surface Structural Approach to Ion Adsorption: The Charge Distribution (CD) Model. J. Colloid Interf. Sci. 179, 488-508. Hiemstra T. and Van Riemsdijk W. H. (1999a) Surface structural ion adsorption modeling of competitive binding of oxyanions by metal (hydr)oxides. J. Colloid Interf. Sci. 210, 182-193. Hiemstra T. and Van Riemsdijk W. H. (1999c) Effect of Different Crystal Faces on the Experimental Interaction Force and Aggregation of Hematite. Langmuir 15(23), 80458051. Hiemstra T. and Van Riemsdijk W. H. (2000) Fluoride adsorption on goethite in relation to different types of surface sites. J. Colloid Interf. Sci. 225(1), 94-104. Hiemstra T. and Van Riemsdijk W. H. (2002) On the Relationship between Surface Structure and Ion Complexation of Oxide-Solution Interfaces. In Encyclopaedia of Surface and Colloid Science, pp. 3773-3799. Marcel Dekker, Inc. Hiemstra T. and Van Riemsdijk W. H. (2006) Biogeochemical speciation of Fe in ocean water. Mar. Chem. 102(3-4), 181-197. Hiemstra T. and Van Riemsdijk W. H. (2006) On the relationship between charge distribution, surface hydration and the structure of the interface of metal hydroxides. J. Colloid Interf. Sci. 301, 1-18.
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About the author Hiemstra T. and Van Riemsdijk W. H. (2007) Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides. Geochim. Cosmochim. Acta 71(24), 5913-5933. Hiemstra T. and Van Riemsdijk W. H. (2007) Surface complexation of selenite on goethite: MO/DFT geometry and charge distribution. Croatica Chemica Acta 80(3-4), 313-324. Hiemstra T. and Van Riemsdijk W. H. (2009a) A Surface Structural Model for Ferrihydrite I: Sites related to Primary charge, Molar Mass, and Mass Density. Geochim. Cosmochim. Acta 73, 4423-4436. Hiemstra T., Van Riemsdijk W. H., and Bolt G. H. (1989a) Multisite Proton Adsorption Modeling at the Solid/Solution Interface of (Hydr)oxides: A New Approach. I. Model Description and Evaluation of Intrinsic Reaction Constants. J. Colloid Interf. Sci. 133, 91-104. Hiemstra T., Van Riemsdijk W. H., and Bruggenwert M. G. M. (1987) Proton adsorption mechanism at the Gibbsite and aluminum oxide solid/solution interface. Neth. J. Agric. Sci. 35, 281-293. Hiemstra T., Van Riemsdijk W. H., Rossberg A., and Ulrich K. U. (2009b) A Surface Structural Model for Ferrihydrite II: Adsorption of Uranyl and Carbonate. Geochim. Cosmochim. Acta 73(4437-4451). Hiemstra T., Venema P., and Van Riemsdijk W. H. (1996b) Intrinsic proton affinity of reactive surface groups of metal (hydr)oxides: The bond valence principle. J. Colloid Interf. Sci. 184, 680-692. Meeussen J. C. L., Scheidegger A., Hiemstra T., Van Riemsdijk W. H., and Borkovec M. (1996) Predicting Multicomponent Adsorption and Transport of Fluoride at Variable pH in a Goethite-Silica Sand System. Environ. Sci. Technol. 30, 481-488. Ponthieu M., Juillot F., Hiemstra T., Van Riemsdijk W. H., and Benedetti M. F. (2006) Metal ion binding to iron oxides. Geochim. Cosmoschim. Acta 70(11), 2679-2698. Rahnemaie R., Hiemstra T., and Van Riemsdijk W. H. (2006) Inner- and Outersphere Complexation of Ions at the Goethite-Solution Interface. J. Colloid Interf. Sci. 297, 379-388. Rahnemaie R., Hiemstra T., and Van Riemsdijk W. H. (2006) A new structural approach for outersphere complexation, tracing the location of electrolyte ions. J. Colloid Interf. Sci. 293, 312-321. Rahnemaie R., Hiemstra T., and Van Riemsdijk W. H. (2007) Carbonate adsorption on goethite in competition with phosphate. J. Colloid Interf. Sci. 315(2), 415-425. Rahnemaie R., Hiemstra T., and Van Riemsdijk W. H. (2007) Geometry, Charge Distribution and Surface Speciation of Phosphate on Goethite. Langmuir 23, 3680-3689. Ridley M. K., Hiemstra T., Van Riemsdijk W. H., and Machesky M. L. (2009) Inner-sphere complexation of cations at the rutile-water interface: A concise surface structural interpretation with the CD and MUSIC model. Geochim. Cosmoschim. Acta 73(7), 1841-1856. Rietra R. P. J. J., Hiemstra T., and van Riemsdijk W. H. (1999) Sulfate adsorption on goethite. J. Colloid Interf. Sci. 218(2), 511-521.
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About the author Rietra R. P. J. J., Hiemstra T., and Van Riemsdijk W. H. (1999a) The Relationship between Molecular Structure and Ion Adsorption on Variable Charge Minerals. Geochim. Cosmochim. Acta 63(19/20), 3009-3015. Rietra R. P. J. J., Hiemstra T., and Van Riemsdijk W. H. (1999b) Sulfate Adsorption on Goethite. J. Colloid Interf. Sci. 218, 511-521. Rietra R. P. J. J., Hiemstra T., and Van Riemsdijk W. H. (2000a) Electrolyte Anion Affinity and its Effect on Oxyanion Adsorption on Goethite. J. Colloid Interf. Sci. 229, 199206. Rietra R. P. J. J., Hiemstra T., and Van Riemsdijk W. H. (2001a) Interaction of Calcium and Phosphate Adsorption on Goethite. Environ. Sci. Technol. 35, 3369-3374. Rietra R. P. J. J., Hiemstra T., and Van Riemsdijk W. H. (2001b) Comparison of Selenate and Sulfate Adsorption on Goethite. J. Colloid Interf. Sci. 240, 384-390. Rossberg A., Ulrich K. U., Weiss S., Tsushima S., Hiemstra T., and Scheinost A. C. (2009) Identification of uranyl surface complexes on ferrihydrite: Advanced EXAFS data analysis and CD-MUSIC modelling. Environ. Sci. Technol. 43(5), 1400-1406. Schroder T. J., Hiemstra T., Vink J. P. M., and van der Zee S. (2005) Modeling of the solidsolution partitioning of heavy metals and arsenic in embanked flood plain soils of the rivers Rhine and Meuse. Environ. Sci. Technol. 39(18), 7176-7184. Stachowicz M., Hiemstra T., and Van Riemsdijk W. H. (2006) Surface speciation of As(III) and As(V) adsorption in relation to charge distribution. J. Colloid Interf. Sci. 302, 6275. Stachowicz M., Hiemstra T., and Van Riemsdijk W. H. (2007) The arsenic-bicarbonate interaction on goethite particles. Environ. Sci. Technol. 41(16), 5620-5625. Stachowicz M., Hiemstra T., and Van Riemsdijk W. H. (2008) Multi-competitive interaction of As(III) and As(V) oxyanions with Ca2+, Mg2+, PO43-, and CO32- ions on goethite. J. Colloid Interf. Sci. 320(2), 400-414. Van Riemsdijk W. H. and Hiemstra T. (1999) From Molecular Structure to Ion Adsorption Modelling. In Mineral-Water Interfacial Reactions, Kinetics and Mechanisms, Vol. 715 (ed. D. L. a. G. Sparks, T.J.), pp. 68-86. American Chemical Society. Van Rotterdam A. M. D., Temminghoff E. J. M., Schenkeveld W. D. L., Hiemstra T., and van Riemsdijk W. H. (2009) Phosphorus removal from soil using Fe oxide-impregnated paper: Processes and applications. Geoderma 151(3-4), 282-289. Venema P., Hiemstra T., and Van Riemsdijk W. H. (1996a) Comparison of different site binding models for cation sorption: Description of pH dependency, salt dependency, and cation-proton exchange. J. Colloid Interf. Sci. 181, 45-59. Venema P., Hiemstra T., and Van Riemsdijk W. H. (1996b) Multisite Adsorption of Cadmium on goethite. J. Colloid Interf. Sci. 183, 515-527. Venema P., Hiemstra T., and Van Riemsdijk W. H. (1997) Interaction of Cadmium with Phosphate on Goethite. J. Colloid Interf. Sci. 192, 94-103. Venema P., Hiemstra T., and Van Riemsdijk W. H. (1998) Intrinsic Proton Affinity of Reactive Surface Groups of Metal (Hydr)oxides: Application to Iron (Hydr) oxides. J. Colloid Interf. Sci. 198, 282-295.
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About the author Venema P., Hiemstra T., and Van Riemsdijk W. H. (1996) Multisite adsorption of cadmium on goethite. J. Colloid Interf. Sci. 183(2), 515-527. Venema P., Hiemstra T., and Van Riemsdijk W. H. (1997) Interaction of cadmium with phosphate on goethite. J. Colloid Interf. Sci. 192(1), 94-103. Weidler P. G., Hug S. J., Wetche T. P., and Hiemstra T. (1999) Determination of Growth Rates of 100 and 110 faces of Synthetic Goethite by Scanning Force Microscopy. Geochim. Cosmochim. Acta 62, 3407-3412. Weng L. P., Koopal L. K., Hiemstra T., Meeussen J. C. L., and Van Riemsdijk W. H. (2005) Interactions of calcium and fulvic acid at the goethite-water interface. Geochim. Cosmoschim. Acta 69(2), 325-339. Weng L. P., Van Riemsdijk W. H., and Hiemstra T. (2006) Adsorption free energy of variable-charge nanoparticles to a charged surface in relation to the change of the average chemical state of the particles. Langmuir 22(1), 389-397. Weng L. P., Van Riemsdijk W. H., and Hiemstra T. (2007) Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength. J. Colloid Interf. Sci. 314(1), 107-118. Weng L. P., Van Riemsdijk W. H., and Hiemstra T. (2008) Cu2+ and Ca2+ adsorption to goethite in the presence of fulvic acids. Geochim. Cosmoschim. Acta 72(24), 58575870. Weng L. P., Van Riemsdijk W. H., and Hiemstra T. (2008) Humic Nano-Particles at the Oxide-Water Interface: Interaction with Phosphate Ion Adsorption. Environ. Sci. Technol. 42(23), 8747-8752. Weng L. P., Van Riemsdijk W. H., and Hiemstra T. (2009) Effects of Fulvic and Humic Acids on Arsenate Adsorption to Goethite: Experiments and Modeling. Environ. Sci. Technol. 43(19), 7198-7204. Weng L. P., Van Riemsdijk W. H., Koopal L. K., and Hiemstra T. (2006) Adsorption of humic substances on goethite: Comparison between humic acids and fulvic acids. Environ. Sci. Technol. 40(24), 7494-7500. Weng L. P., Van Riemsdijk W. H., Koopal L. K., and Hiemstra T. (2006) Ligand and Charge Distribution (LCD) model for the description of fulvic acid adsorption to goethite. J. Colloid Interf. Sci. 302(2), 442-457.
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