Natural Organic Matter and Humic Substances Interactions

Humic sol was prepared by dissolution in concentrated NaOH and mixing with 15% (wt,)
solution of HEPES (Aldrich) up to pH = 7, The content of HA in final solution was 14.23 g/L
EDTA was dissolved in the solution of HEPES, Final concentration of EDTA in HEPES
buffer was 5.72 mmol/L (pH = 7).
Ultrasonic spectrometer with high resolution HR-US 102 (Ultrasonic Scientific, Ireland), was
utilized for measurement of basic ultrasonic parameters, The device consists of two
independent cells tempered at 25 °C, Reference cell was filled by HEPES buffer (EDTA
measurement) or HEPES with NaOH (HA measurement), Measuring cell was filled by EDTA
or HA solution prepared by ways described above and titrated by CuCl2 up to saturation of
their acidic groups, Velocity (U) and attenuation (N) in both cells was measured, the resulting
differences ΔU and ΔN between reference and sample cell we can see at graphs, Blank
experiments with NaOH in HEPES, EDTA and HA versus HEPES were carried out to
correction of data obtained from titration experiments, All HRUS measurements were done at
four different frequencies.
3. Results and Discussion
15th IHSS Meeting- Vol. 3
The dependencies of measured ultrasonic velocity and attenuation on degree of saturation of
acidic groups in HA and EDTA by CuCl2 are shown in Figs, 1 and 2, According the results
both compounds are able to bind the bivalent Cu 2+ ions by two functional groups, In case of
EDTA all binding sites can be occupied and saturation is represented by ratio Cu 2+ /H + = 0.5.
Complexation of HA is more complicated, HA contains many various binding sites with
different strength which can be also gradually occupied, On the other hand, each Cu 2+ ion
may not be able to find two suitable acidic groups for its binding for structure and
conformation of HA particles, The steric effects are the reason, why some acidic groups can
remain vacant, A part of Cu 2+ ions can be bonded only by one acidic group but our quantumchemical
computing showed that such arrangement is hardly likely [11], The maximum of
measured ultrasonic parameters then quadrates with ratio Cu 2+ /H + = 0.4.
The differences between complexation behaviour of HA and EDTA are clearly evident also
from slopes of measured dependencies, The slope of decrease of ΔU with degree of saturation
of binding sites is -1.31 ± 0,04 m/s for HA and -0.58 ± 0,01 m/s for EDTA, It shows on more
rigid structure of Cu-HA complexes, which can be caused by formation of inter- and intramolecular
bridges.
Vol. 3 Page - 136 -