Monitoring the kinetics of reactions between natural organic matter and Al(III) ions using differential absorbance spectra

This study examined the kinetics of the binding of Al(III) ions by natural organic matter (NOM) exemplified by Suwannee River humic acid (SRHA). This processes was studied for a 5–8 pH range and environmentally relevant concentrations of the system components. Al(III)-NOM interactions were quantified using differential absorbance spectra whose intensity and shape depended on pH and reaction time. In all cases the differential spectra had four bands with maxima located at 245, 275, 320, 380 nm. These bands were assigned to the engagement of the carboxylic-like and/or phenolic-like groups, as well as electrostatic gel in NOM. Several parameters of the absorbance spectra (e.g., spectral slopes of log-transformed spectra in wavelength range 260–270 and 350–400 nm, ΔS260-270 and ΔS350-400 respectively) were linearly correlated (R2 = 0.98) with concentrations of carboxylic-like groups and total NOM-bound Al(III) ions predicted based on the NICA-Donnan model. The binding of Al(III) ion by NOM at all pHs was modeled assuming the presence of three kinetically distinct sites. This study demonstrates that differential absorbance spectroscopy can be used to quantify the kinetics and mechanisms of NOM-metal ions interactions and monitor them in practically important system including water treatment operations.