Role of carboxylic and phenolic groups in NOM adsorption on minerals: a review

2006 ◽  
Vol 6 (6) ◽  
pp. 155-164 ◽  
Author(s):  
X.H. Guan ◽  
D.L. Li ◽  
C. Shang ◽  
G.H. Chen
Keyword(s):  
Organic Matter ◽  
Aluminium Hydroxide ◽  
Interaction Strength ◽  
Outer Sphere ◽  
Hydroxyl Groups ◽  
Aromatic Carboxylates ◽  
Inner Sphere ◽  
Outer Sphere Complexes ◽  
Inner Sphere Complexes

This paper presented the current state of our understanding of the roles of carboxylic and phenolic groups in NOM adsorption and reviewed the contradictory opinions in the literatures. Previous studies carried out by other researchers indicated that aromatic carboxylates were adsorbed onto metal (hydr)oxides via outer-sphere complexes under most conditions and phenolic groups were very crucial for formation of inner-sphere complexes between organic acids and metal (hydr)oxides. Adsorption test with in-situ ATR-FTIR spectroscopic investigation were carried out to verify the role of aromatic carboxylic and phenolic groups in the NOM adsorption onto aluminium hydroxide surfaces by using a series of aromatic carboxylic acids and dihydroxybenzoic acids as the surrogate of NOM. Our studies suggested that the formation of outer-sphere complexes dominated the adsorption of most of the aromatic carboxylates over the pH range of 5–9; inner-sphere complexes were only detected at some pH levels for some aromatic carboxylates adsorption; and the aromatic carboxylates were most likely to be adsorbed to the first surface layer of hydroxyl groups and water molecules without forming coordinative bonds with the aluminium hydroxide surfaces but strong hydrogen bonds were formed in this process. Our study also revealed that (1) the presence of phenolic groups can increase the interaction strength of carboxylate groups with aluminium hydroxide; (2) chelate formation involving a carboxylate oxygen atom and ortho-phenolic-oxygen is important for the adsorption of organic matter on aluminium hydroxide at acidic pH; and 3) the phenolic groups adjacent to each other are more important than the carboxylic groups at alkaline pH for organic matter adsorption.

Surface Complexation of Neptunium(V) with Goethite

2006 ◽  
Vol 985 ◽  
Author(s):  
James L Jerden ◽  
A Jeremy Kropf
Keyword(s):  
Sodium Chloride ◽  
Sodium Silicate ◽  
Outer Sphere ◽  
Distribution Coefficients ◽  
Batch Adsorption ◽  
Inner Sphere ◽  
Show Evidence ◽  
X Ray Absorption ◽  
Outer Sphere Complexes ◽  
Inner Sphere Complexes

AbstractBatch adsorption experiments in which neptunium bearing solutions were reacted with goethite (alpha-FeOOH) have been performed to study uptake mechanisms in sodium chloride and calcium-bearing sodium silicate solutions. This paper presents results identifying and quantifying the mechanisms by which neptunium is adsorbed as a function of pH and reaction time (aging). Also presented are results from tests in which neptunium is reacted with goethite in the presence of other cations (uranyl and calcium) that may compete with neptunium for sorption sites. The desorption of neptunium from goethite has been studied by resuspending the neptunium-loaded goethite samples in solutions containing no neptunium. Selected reacted sorbent samples were analyzed by x-ray absorption spectroscopy (XAS) to determine the oxidation state and molecular speciation of the adsorbed neptunium. Results have been used to establish the pH adsorption edge of neptunium on goethite in sodium chloride and calcium-bearing sodium silicate solutions. The results indicate that neptunium uptake on goethite reaches 95% at a pH of approximately 7 and begins to decrease at pH values greater than 8.5. Distribution coefficients for neptunium sorption range from less than 1000 (moles/kg)sorbed / (moles/kg)solution at pH less than 5.0 to greater than 10,000 (moles/kg)sorbed / (moles/kg)solution at pH greater than 7.0. Distribution coefficients as high as 100,000 (moles/kg)sorbed / (moles/kg)solution were recorded for the tests done in calcite equilibrated sodium silicate solutions. XAS results show that neptunium complexes with the goethite surface mainly as Np(V) (although Np(IV) is prevalent in some of the longer-duration sorption tests). The neptunium adsorbed to goethite shows Np-O bond length of approximately 1.8 angstroms which is representative of the Np-O axial bond in the neptunyl(V) complex. This neptunyl(V) ion is coordinated to 5 or 6 equatorial oxygens with Np-O bond lengths of 2.45 angstroms. The absence of a clearly recognizable Np-Fe interaction for the sodium chloride sorption tests suggests that neptunium in these solutions adsorbs as an outer-sphere complex. XAS results from the calcium-bearing sodium silicate sorption tests show evidence for a neptunyl(V) inner-sphere surface complex with a Np-Fe interaction at 3.5 angstroms. Desorption tests indicate that samples in which neptunium is bound as inner-sphere complexes show significant sorption hysteresis relative to samples in which neptunium is bound largely as outer-sphere complexes.

Adsorption of Organic Matter at Mineral/Water Interfaces. 6. Effect of Inner-Sphere versus Outer-Sphere Adsorption on Colloidal Stability

Langmuir ◽  
2005 ◽  
Vol 21 (14) ◽  
pp. 6356-6365 ◽  
Author(s):  
Stephen B. Johnson ◽  
Gordon E. Brown, ◽  
Thomas W. Healy ◽  
Peter J. Scales
Keyword(s):  
Organic Matter ◽  
Mineral Water ◽  
Colloidal Stability ◽  
Outer Sphere ◽  
Inner Sphere
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