Some ongoing research

in the area of humic materials
*Determined by a conductance titration method developed in our laboratory: HA is dissolved in excess NaOH, producing the humate anion (A–) and leaving ~5 µeq of unreacted OH–. When this solution is titrated with standard HCl, the conductance first decreases steeply as OH– (Leq=199.2 S·cm2/eq) is replaced by Cl– (Leq=76.35 S·cm2/eq); this corresponds to region A in the figure above. After the excess OH– is consumed, the humate is progressively protonated, resulting in the replacement of A– by Cl–. This leads to a decrease of the solution conductance, since the ionic conductance of Cl– is higher than that of A– (region B).

Extrapolation of both ends of the curve, and the intercepts with the extrapolated central portion, yield the number of acid equivalents needed to titrate A–. Assuming that the hump in the B-region of the curve is a manifestation of weak- and strong-acid endpoints, a similar extrapolation yields the contents of the two types of acid.

Talanta., 57, 519 - 526 (2002)

Reduction of Arsenates by Humic Materials

Inorganic arsenates are reduced to arsenite by homogeneous aqueous solutions of humic and fulvic acids. The redox potentials of humics is concentration dependent, and As(V) reduction is less likely to happen in concentrated humate solutions. This is especially true in higher pH ranges, and varies with the type of material used. We have developed an ion chromatographic method, validated by ICP/MS, for speciating arsenic after exposure to aqueous humates and fulvates. Reduction of As(V) proceeds in the 20-60% range, depending on the humic material used. The fraction of arsenate reduced initially increases with humic concentration, but levels off as the humic reduction potential increases at higher concentrations. As(III) ns can be re-oxidized by bubbling the solutions with oxygen or air. The reduction capacities of humates, as measured by I2 titration, vary widely.

Environ. Chem, 3, 131-136 (2006)

31P-NMR peak width in humate-phosphate complexes

31P-NMR spectroscopy can be used to determine the mobility of inorganic phosphate (P) attached to solid humic acid (HA) and fulvic acid (FA) via metal anchors. The peak width of the 31P resonance is monitored as an indicator of the degree of attachment of the element to the humic matrix. The concept is demonstrated by contrasting peak widths of thoroughly dry M-HA-P complexes with those that had been allowed to absorb different amounts of moisture. The presence of moisture, which enhances the mobility of P, results in a significant reduction in peak width. This leads to comparisons between dry systems with and without metal anchors;  systems with anchors consisting of different metals;  systems comprising different humates and fulvates;  and systems with different size fractions of a humate.  Both the type of humate/fulvate, and the metal anchor lead to different degrees of mobility within the humic matrix. The effect of metal addition on 31P peak width is greater with fulvates and smaller HA fractions than with the larger HA components.

Talanta, 73, 953-958 (2007)

Interactions between humic materials and uranium in the subsurface environment

Slight momentary heating of aqueous solutions containing humic substances (HS) and uranyl ions (UO22+) yields a humic floc that carries the bulk of the ion down with it. These biphasic systems are at near-neutral pH, and after returning to ambient temperatures they remain intact and are analyzed by determining the free uranyl concentration in the aqueous phase. Phosphate enhanced UO22+ fluorescence is used for this, allowing for the determination of the stability constants of HS-uranyl complexes in the floc. The neutral pH of the systems and the relatively unencumbered nature of the analysis provides a realistic picture of the complex formation in natural environments.

The log K values of the complexes lie in the 5-7 range, and are independent of the acid content of the humates and fulvates. Stability constants of different humic size fractions are generally smaller than those of the whole material, and decrease with molecular size. A comparison of humates and fulvates shows similar trends. The observations suggest that larger HS polyanions, generally having a greater aliphatic content, provided a cage effect and sequester UO22+ more effectively than smaller fractions.

Annals of Environmental Science, 2008, in press

Stability constants of HS-uranyl complexes

Humic Substance

log K

Total acidity of HS (meq g-1)*

LHA (IHSS)

5.84

5.0

LHA (from LHACB)

6.22

4.1

SHA

5.96

6.0

PLVHA

5.07

8.0

PHA

5.89

5.0

SRHA

5.69

3.4

NAHA

6.30

3.0

SRFA

5.22

2.8

NAFA

5.59

4.0